Volume 15 - Casting Part 9 ppsx

G 122a Nonmetallic inclusions generally impregnated with gas and accompanied by blowholes B 113 Slag blowhole defect G 130: Nonmetallic inclusions; mold or core materials Inclusions

F 232 Deformation with respect to drawing proportional for casting

and mold; pattern conforms to drawing

Deformed mold, mold creep, springback

F 233 Casting deformed with respect to drawing; pattern and mold

Metallic inclusions whose appearance, chemical analysis or

structural examination show to be caused by an element

foreign to the alloy

Metallic inclusions

G

112(a)

Metallic inclusions of the same chemical composition as the

base metal; generally spherical and often coated with oxide

Cold shot

G 113 Spherical metallic inclusions inside blowholes or other

cavities or in surface depressions (see A 311) Composition

approximates that of the alloy cast but nearer to that of a

eutectic

Internal sweating, phosphide sweat

G 120: Nonmetallic inclusions; slag, dross, flux

G

121(a)

Nonmetallic inclusions whose appearance or analysis shows

they arise from melting slags, products of metal treatment or

fluxes

Slag, dross or flux inclusions, ceroxides

G

122(a)

Nonmetallic inclusions generally impregnated with gas and

accompanied by blowholes (B 113)

Slag blowhole defect

G 130: Nonmetallic inclusions; mold or core materials

Inclusions of mold blacking or dressing, generally very close

to the casting surface

Blacking or refractory coating inclusions

G 140: Nonmetallic inclusions; oxides and reaction products

G 141 Clearly defined, irregular black spots on the fractured surface

of ductile cast iron

Common Inspection Procedures

Inspection of castings is most often limited to visual and dimensional inspections, weight testing, and hardness testing However, for castings that are to be used in critical applications, such as in aerospace components, additional methods of nondestructive inspection are used to determine and to control casting quality

Visual inspection of each casting ensures that none of its features has been omitted or malformed by molding errors, short running, or mistakes in cleaning Most surface defects and roughness can be observed at this stage

Initial sample castings from new pattern equipment should be carefully inspected for obvious defects Liquid penetrant inspection can be used to detect surface defects Such casting imperfections as shrinks, cracks, blows, or dross usually indicate the need for adjustment in the gating or foundry techniques If the casting appears to be satisfactory upon visual inspection, internal quality can be checked by radiographic and ultrasonic inspection

The first visual inspection operation on the production casting is usually performed immediately after shakeout or knockout of the casting This ensures that major visible imperfections are detected as quickly as possible This information, promptly relayed to the foundry, permits early corrective action to be taken with a minimum of scrap loss The size and complexity of some sand castings require that the gates and risers be removed to permit proper inspection of the casting Many castings that contain numerous internal cores or have close dimensional tolerances require a rapid but fairly accurate check of critical wall dimensions In some cases, an indicating-type caliper gage is suitable for this work, and special types are available for casting shapes that do not lend themselves to the standard types Ultrasonic inspection

is also used to determine wall thickness in such components as cored turbine blades made by investment casting (see the article "Investment Casting" in this Volume)

Dimensional Inspection. Dimensional deviations on machined surfaces are relatively simple to evaluate and can be accurately specified However, it is not so simple to determine the acceptability of dimensions that involve one or more unmachined surfaces Dimensional inspection can be carried out with the aid of gages, jigs, and templates

Most initial machining operations on castings use a cast surface as a datum; the exceptions are those large castings that are laid out, before machining, to give the required datum Therefore, it is important that the cast surface used as a datum

be reasonably true and that it be in the correct position relative to other critical machined or unmachined surfaces on the same casting, within clearly defined limits

The cast surface used as a datum can be a mold surface, and variations can occur because of mold movement The cast surface can be produced by a core; movement of cores is a frequent cause of casting inaccuracy Errors involving these surfaces can produce consequential errors or inadequate machining stock elsewhere on the casting

Where dimensional errors are detected in relation to general drawing tolerances, their true significance must be determined A particular dimension may be of vital importance, but may have been included in blanket tolerances This situation stresses the desirability of stating functional dimensions on drawings so that tolerances are not restricted unnecessarily

Weight Testing. Many intricately cored castings are extremely difficult to measure accurately, particularly the internal sections It is important to ensure that these sections are correct in thickness for three main reasons:

• There should be no additional weight that would make the finished product heavier than permissible

• Sections must not be thinner than designed to prevent detracting from the strength of the casting

• If hollow cavities have been reduced in area by increasing the metal thickness of the sections, any flow

of liquid or gases is reduced

A ready means of testing for these discrepancies is by accurately weighing each casting or by measuring the displacement caused by immersing the casting in a liquid-filled measuring jar or vessel In certain cases in which extreme accuracy is demanded, a tolerance of only ±1% of a given weight may be allowed

Hardness testing is often used to verify the effectiveness of heat treatment applied to actual castings Its general correlation with the tensile strength of many ferrous alloys enables a rough prediction of tensile strength to be made

The Brinell hardness test is most frequently used for casting alloys A combination of large-diameter ball (5 or 10 mm) and heavy load (500 to 3000 kgf) is preferred for the most effective representation because a deep impression minimizes the influence of the immediate surface layer and of the relatively coarse microstructure The Brinell hardness test is unsuitable for use at high hardness levels (above 600 HB), because distortion of the ball indenter can affect the shape of the indentation

Either the Rockwell or the Vickers (136° diamond pyramid) hardness test is used for alloys of extreme hardness or for high-quality and precision castings in which the large Brinell indentation cannot be tolerated Because of the very small indentations produced in Rockwell and Vickers tests, which use loads of 150 kg or less, results must be based on the average of a number of determinations Portable hardness testers or ultrasonic microhardness testers can be used on large castings that cannot be placed on the platform of a bench-type machine More detailed information on hardness testing is

available in Mechanical Testing, Volume 8 of ASM Handbook, formerly 9th Edition Metals Handbook

The hardness of ferrous castings can be related to the sonic velocity of the metal and determined from it if all other test conditions remain constant This has been demonstrated on chilled rolls in determining the average hardness of the core

Liquid Penetrant Inspection

Liquid penetrant inspection essentially involves a liquid wetting the surface of a workpiece, flowing over that surface to form a continuous and uniform coating, and migrating into cracks or cavities that are open to the surface After a few minutes, the liquid coating is washed off the surface of the casting and a developer is placed on the surface The developer

is stained by the liquid penetrant as it is drawn out of the cracks and cavities Liquid penetrants will highlight surface defects so that detection is more certain

Liquid penetrant inspection should not be confined to as-cast surfaces For example, it is not unusual for castings of various alloys to exhibit cracks, frequently intergranular, on machined surfaces A pattern of cracks of this type may be the result of intergranular cracking throughout the material because of an error in composition or heat treatment, or the cracks may be on the surface only as a result of machining or grinding Surface cracking may result from insufficient machining allowance, which does not allow for complete removal of imperfections produced on the as-cast surface, or it may result from faulty machining techniques If imperfections of this type are detected by visual inspection, liquid penetrant inspection will show the full extent of such imperfections, will give some indication of the depth and size of the defect below the surface by the amount of penetrant absorbed, and will indicate whether cracking is present throughout the section

Magnetic Particle Inspection

Magnetic particle inspection is a highly effective and sensitive technique for revealing cracks and similar defects at or just beneath the surface of castings made of ferromagnetic metals The capability of detecting discontinuities just beneath the surface is important because such cleaning methods as shot or abrasive blasting tend to close a surface break that might

go undetected in visual or liquid penetrant inspection

When a magnetic field is generated in and around a casting made of a ferromagnetic metal and the lines of magnetic flux are intersected by a defect such as a crack, magnetic poles are induced on either side of the defect The resulting local flux disturbance can be detected by its effect on the particles of a ferromagnetic material, which become attracted to the region

of the defect as they are dusted on the casting Maximum sensitivity of indication is obtained when a defect is oriented in

a direction perpendicular to the applied magnetic field and when the strength of this field is just enough to saturate the casting being inspected

Equipment for magnetic particle inspection uses direct or alternating current to generate the necessary magnetic fields The current can be applied in a variety of ways to control the direction and magnitude of the magnetic field

In one method of magnetization, a heavy current is passed directly through the casting placed between two solid contacts The induced magnetic field then runs in the transverse or circumferential direction, producing conditions favorable to the detection of longitudinally oriented defects A coil encircling the casting will induce a magnetic field that runs in the longitudinal direction, producing conditions favorable to the detection of circumferentially (or transversely) oriented defects Alternatively, a longitudinal magnetic field can be conveniently generated by passing current through a flexible cable conductor, which can be coiled around any metal section This method is particularly adaptable to castings of irregular shape Circumferential magnetic fields can be induced in hollow cylindrical castings by using an axially disposed central conductor threaded through the casting

Small castings can be magnetic particle inspected directly on bench-type equipment that incorporates both coils and solid contacts Critical regions of larger castings can be inspected by the use of yokes, coils, or contact probes carried on flexible cables connected to the source of current this setup enables most regions of castings to be inspected

Eddy Current Inspection

Eddy current inspection consists of observing the interaction between electromagnetic fields and metals In a basic system, currents are induced to flow in the testpiece by a coil of wire that carries an alternating current As the part enters the coil, or as the coil in the form of a probe or yoke is placed on the testpiece, electromagnetic energy produced by the coils is partly absorbed and converted into heat by the effects of resistivity and hysteresis Part of the remaining energy is reflected back to the test coil, its electrical characteristics having been changed in a manner determined by the properties

of the testpiece Consequently, the currents flowing through the probe coil are the source of information describing the characteristics of the testpiece These currents can be analyzed and compared with currents flowing through a reference specimen

Eddy current methods of inspection are effective with both ferromagnetic and nonferromagnetic metals Eddy current methods are not as sensitive to small, open defects as liquid penetrant or magnetic particle methods are Because of the skin effect, eddy current inspection is generally restricted to depths less than 6 mm (1

4 in.) The results of inspecting ferromagnetic materials can be obscured by changes in the magnetic permeability of the testpiece Changes in temperature must be avoided to prevent erroneous results if electrical conductivity or other properties, including metallurgical properties, are being determined

Applications of eddy current and electromagnetic methods of inspection to castings can be divided into the following three categories:

• Detecting near-surface flaws such as cracks, voids, inclusions, blowholes, and pinholes (eddy current inspection)

• Sorting according to alloy, temper, electrical conductivity, hardness, and other metallurgical factors (primarily electromagnetic inspection)

• Gaging according to size, shape, plating thickness, or insulation thickness (eddy current or electromagnetic inspection)

Radiographic Inspection**

Radiographic inspection is a process of testing materials using penetrating radiation from an x-ray generator or a radioactive source and an imaging medium, such as x-ray film or an electronic device In passing through the material, some of the radiation is attenuated, depending on the thickness and the radiographic density of the material, while the radiation that passes through the material forms an image The radiographic image is generated by variations in the intensity of the emerging beam

Internal flaws, such as gas entrapment or nonmetallic inclusions, have a direct effect on the attenuation These flaws create variations in material thickness, resulting in localized dark or light spots on the image

The term radiography usually implies a radiographic process that produces a permanent image on film (conventional radiography) or paper (paper radiography or xeroradiography), although in a broad sense it refers to all forms of radiographic inspection When inspection involves viewing an image on a fluorescent screen or image intensifier, the radiographic process is termed filmless or real time inspection (Fig 1) When electronic nonimaging instruments are used

to measure the intensity of radiation, the process is termed radiation gaging Tomography, a radiation inspection method adapted from the medical computerized axial tomography scanner, provides a cross-sectional view of a testpiece All of the above terms are primarily used in connection with inspection that involves penetrating electromagnetic radiation in the form of x-rays or γ-rays Neutron radiography refers to radiographic inspection using neutrons rather than electromagnetic radiation

Fig 1 Schematic of real time x-ray inspection system Source: TFI Corporation

The sensitivity, or the ability to detect flaws, of radiographic inspection depends on close control of the inspection technique, including the geometric relationships among the point of x-ray emission, the casting, and the x-ray imaging plane The smallest detectable variation in metal thickness lies between 0.5 and 2.0% of the total section thickness Narrow flaws, such as cracks, must lie in a plane approximately parallel to the emergent x-ray beam to be imaged; this requires multiple exposures for x-ray film techniques and a remote control parts manipulator for a real time system

Real time systems have eliminated the need for multiple exposures of the same casting by dynamically inspecting parts on

a manipulator, with the capability of changing the x-ray energy for changes in total material thickness These capabilities have significantly improved productivity and have reduced costs, thus enabling higher percentages of castings to be inspected and providing instant feedback after repair procedures

Advances. Several advances have been made to assist the industrial radiographer These include the computerization of the radiographic standard shooting sketch, which graphically shows areas to be x-rayed and the viewing direction or angle

at which the shot is to be taken, and the development of microprocessor-controlled x-ray systems capable of storing different x-ray exposure parameters for rapid retrieval and automatic warm-up of the system prior to use The advent of digital image processing systems and microfocus x-ray sources (near point source), producing energies capable of penetrating thick material sections, have made real time inspection capable of producing images equal to, and in some cases superior to, x-ray film images by employing geometric relations previously unattainable with macrofocus x-ray systems The near point source of the microfocus x-ray system virtually eliminates the edge unsharpness associated with larger focus devices

Digital image processing can be used to enhance imagery by multiple video frame integration and averaging techniques that improve the signal-to-noise ratio of the image This enables the radiographer to digitally adjust the contrast of the image and to perform various edge enhancements to increase the conspicuity of many linear indications

Interpretation of the radiographic image requires a skilled specialist who can establish the correct method of exposing the castings with regard to x-ray energies, geometric relationships, and casting orientation and can take all of these factors into account to achieve an acceptable, interpretable image Interpretation of the image must be performed to establish standards in the form of written or photographic instructions The inspector must also be capable of determining if the localized indication is a spurious indication, a film artifact, a video aberration, or a surface irregularity

Note cited in this section

** This section was prepared by Frederick A Morrow, TFI Corporation

Ultrasonic Inspection

Ultrasonic inspection is a nondestructive method in which beams of high-frequency acoustic energy are introduced into the material under evaluation to detect surface and subsurface flaws and to measure the thickness of the material or the distance to a flaw An ultrasonic beam will travel through a material until it strikes an interface or defect Interfaces and defects interrupt the beam and reflect a portion of the incident acoustic energy The amount of energy reflected is a function of the nature and orientation of the interface or flaw as well as the acoustic impedance of such a reflector Energy reflected from various interfaces or defects can be used to define the presence and locations of defects, the thickness of the material, or the depth of a defect beneath a surface

The advantages of ultrasonic tests are as follows:

• High sensitivity, which permits the detection of minute cracks

• Great penetrating power, which allows the examination of extremely thick sections

• Accuracy in measurement of flaw position and estimation of defect size

Ultrasonic tests have the following limitations:

• Size-contour complexity and unfavorable discontinuity orientation can pose problems in interpretation

of the echo pattern

• Undesirable internal structure for example, grain size, structure, porosity, inclusion content, or fine dispersed precipitates can similarly hinder interpretation

• Reference standards are required

Ultrasonic inspection is more commonly used for wrought and welded products than for castings It should be noted, however, that slag, porosity, cold shuts, tears, shrinkage cracks, and inclusions can be detected, particularly in castings that are not complex in shape Wall thickness examination of cored castings is also conducted by ultrasonic inspection

Leak Testing

Castings that are intended to withstand pressures can be leak tested in the foundry Various methods are used, according

to the type of metal being tested One method consists of pumping air at a specified pressure into the inside of the casting and then submerging the casting in water at a given temperature Any leaks through the casting become apparent by the release of bubbles of air through the faulty portions An alternative method is to fill the cavities of a casting with paraffin

at a specified pressure Paraffin, which will penetrate the smallest of crevices, will rapidly find any defect, such as porosity, and will show quickly as an oily or moist patch at the position of the fault Liquid penetrants can be poured into areas of apparent porosity and time allowed for the liquid to seep through the casting wall

Pressure testing of rough (unmachined) castings at the foundry may not reveal any leaks, but it must be recognized that subsequent machining operations on the casting may cut into porous areas and cause the casting to leak after machining Minor seepage leaks can be sealed by impregnation of the casting with liquid or filled sodium silicate, a synthetic resin, or other suitable substance As-cast parts can be impregnated at the foundry to seal leaks if there is to be little machining or

if experience has shown that machining does not affect the pressure tightness However, it is usually preferable to impregnate after final machining of the casting

Inspection of Ferrous Castings

Ferrous castings can be inspected by most of the nondestructive inspection methods Magnetic particle inspection can be applied to ferrous metals with excellent sensitivity, although a crack in a ferrous casting can often be seen by visual inspection Magnetic particle inspection provides good crack delineation, but the method should not be used to detect other defects Irrelevant magnetic particle indications occasionally occur on ferrous castings, especially with a strong magnetic field For example, a properly fused-in steel chaplet can be indicated as a defect because of the difference in

magnetic response between low-carbon steel and cast iron Even the graphite in cast iron, which is nonmagnetic, can cause an irrelevant indication

Standard x-ray and radioactive-source techniques can be used to make radiographs of ferrous castings, but the typical complexity of shape and varying section thicknesses of the castings may require complex procedures Radiography is sometimes used to inspect critical production castings that will be subjected to high service stresses, but it is more often used to evaluate design or casting procedures

Ultrasonic inspection for both thickness and defects is practical with most ferrous castings except for the high-carbon gray iron castings, which have a high damping capacity and absorb much of the input energy The measurement of resonant frequency is a good method for inspecting some ductile iron castings for soundness and graphite shape Electromagnetic testing can be used to distinguish metallurgical differences between castings The criteria for separating acceptable from unacceptable castings must be established empirically for each casting lot

Gray Iron Castings

Gray iron castings are susceptible to most of the imperfections generally associated with castings, with additional problems resulting from the relatively high pouring temperatures These additional problems result in a higher incidence

of gas entrapment, inclusions, poor metal structure, interrupted metal walls, and mold wall deficiencies

Gas entrapment is a direct result of gas being trapped in the casting wall during solidification This gas may be in the metal prior to pouring, may be generated from aspiration during pouring, or may be generated from core and mold materials Internal defects of this type are best detected by radiography, but ultrasonic and eddy current methods of inspection are useful when the defect is large enough to be detected by these methods

Inclusions are casting defects in which solid foreign materials are trapped in the casting wall The inclusion material can

be slag generated in the melting process, or it can be fragments of refractory, mold sand, core aggregate, or other materials used in the casting process Inclusions appear most often on the casting surface and are usually detected by visual inspection; however, in many cases, the internal walls of castings contain inclusions that cannot be visually detected The internal inclusions can be detected by eddy current, radiographic, or ultrasonic inspection; radiography is usually the most reliable method

Poor Metal Structure. Many casting defects resulting from metal structure are related to shrinkage, which is either a cavity or a spongy area lined with dendrites or is a depression in the casting surface This type of defect arises from varying rates of contraction while the metal is changing from a liquid to a solid Other casting defects resulting from varying rates of contraction during solidification include carbide formation, hardness variations, and microporosity

Internal shrinkage defects are best detected by radiography, although eddy current or ultrasonic inspection can be used Soft or hard gray iron castings are usually detected by Brinell hardness testing; electromagnetic methods have proved useful on some castings

Interrupted Metal Walls. Included in this category are such flaws as hot tears, cold shuts, and casting cracks Cracking of castings is often a major problem in gray iron foundries as a result of the combination of casting designs and high production rates Visual inspection, or an aided visual method such as liquid penetrant or magnetic particle inspection, is used to detect cracks and cracklike flaws in castings

Mold wall deficiencies are common problems in gray iron castings They result in surface flaws such as scabs, rattails,

cuts, washes, buckles, drops, and excessive metal penetration into space between sand grains These flaws are generally detected by visual inspection

Malleable Iron Castings

Blowholes and spikes are defects that are often found in malleable iron castings Spikes are a form of surface shrinkage not normally visible to the naked eye but appear as a multitude of short discontinuous surface cracks when subjected to fluorescent magnetic particle inspection Unlike true fractures, spikes do not propagate, but they are not acceptable where cyclic loading could result in fatigue failure Spikes are usually seen as short indications about 1.6 mm ( 1

16 in.) long or less and never more than 75 μm (0.003 in.) deep These defects do not have a preferred orientation but a random pattern

that may or may not follow the direction of solidification Shrinkage or open structure in the gated area is a defect often found in malleable iron castings that may be overlooked by visual inspection, although it is readily detected by either liquid penetrant or magnetic particle inspection

Ductile Iron Castings

Ductile iron is cast iron in which the graphite is present in tiny balls or spherulites instead of flakes (as in gray iron) or compacted aggregates (as in malleable iron) The spheroidal graphite structure is produced by the addition of one or more elements to the molten metal

Casting defects associated with foundry practice that is, shrinkage, voids from entrapped gas, nonmetallic inclusions, and failure to fill the mold shape are essentially the same for ductile iron as for gray iron Carbonnodule segregation occurs when the carbon equivalent (CE) of ductile iron [CE = % total carbon + 0.3 (%Si + %P)] is incorrect for the section thickness of the casting

Subsurface inclusions arise from the formation of nonmetallic compounds (mainly sulfides) following inoculation of the molten iron Slag inclusions form in ductile iron in appreciable amounts upon inoculation with magnesium because some

of the magnesium ignites in the molten metal Desulfurization also promotes slag formation

Inspection of Aluminum Alloy Castings

Effective quality control is needed at every step in the production of an aluminum alloy casting, from selection of the casting method, casting design, and alloy to mold production, foundry technique, machining, finishing, and inspection Visual methods, such as visual inspection, pressure testing, liquid penetrant inspection, ultrasonic inspection, radiographic inspection, and metallographic examination, can be used to inspect for casting quality The inspection procedure used should be geared toward the specified level of quality

Stages of Inspection. Inspections can be divided into three stages: preliminary, intermediate, and final After tests are conducted on the melt for hydrogen content, for adequacy of silicon modification, and for degree of grain refinement, preliminary inspection may consist of the inspection and testing of test bars cast with the molten alloy at the same time the production castings are poured These test bars are used to check the quality of the alloy and effectiveness of the heat treatment Preliminary inspection also includes chemical or spectrographic analysis of the casting, thus ensuring that the melting and pouring operations have resulted in an alloy of the desired composition

Intermediate inspection, or hot inspection, is performed on the casting as it is taken from the mold This step is essential

so that castings that are obviously defective can be discarded at this stage of production Castings that are judged unacceptable at this stage can then be considered for salvage by impregnation, welding, or other methods, depending on the type of flaw present and the end use of the casting More complex castings usually undergo visual and dimensional inspection after the removal of gates and risers

Final inspection establishes the quality of the finished casting, using any of the methods previously mentioned Visual inspection also includes the final measurement and comparison of specified and actual dimensions Dimensions of castings from a large production run can be checked using gages, jigs, fixtures, or coordinate-measuring systems (described later)

Liquid penetrant inspection is extensively used as a visual aid for detecting surface flaws in aluminum alloy castings Liquid penetrant inspection is applicable to castings made from all the aluminum casting alloys as well as to castings produced by all methods One of its most useful applications, however, is for inspecting small castings produced

in permanent molds from alloys such as 296.0, which are characteristically susceptible to hot cracking For example, in cast connecting rods, hot shortness may result in fine cracks in the shank sections Such cracks are virtually undetectable

by unaided visual inspection, but are readily detectable by liquid penetrant inspection

All of the well-known liquid penetrant systems (that is, water-washable, postemulsifiable, and solvent-removable) are applicable to inspection of aluminum alloy castings In some cases, especially for certain high-integrity castings, more than one system can be used Selection of the system is primarily based on the size and shape of the castings, surface roughness, production quantities, sensitivity level desired, and available inspection facilities

Pressure testing is used for castings that must be leaktight Cored-out passages and internal cavities are first sealed off with special fixtures having air inlets These inlets are used to build up the air pressure on the inside of the casting The entire casting is then immersed in a tank of water, or it is covered by a soap solution Bubbles will mark any point of air leakage

Radiographic inspection is a very effective means of detecting such conditions as cold shuts, internal shrinkage, porosity, core shifts, and inclusions in aluminum alloy castings Radiography can also be used to measure the thickness of specific sections Aluminum alloy castings are ideally suited to examination by radiography because of their relatively low density; a given thickness of aluminum alloy can be penetrated with about one-third the power required for penetrating the same thickness of steel

Aluminum alloy castings are most often radiographed by an x-ray machine, using film to record the results Real time radiography is also widely used, particularly for examining large numbers of relatively small castings, and is best suited

to detecting shrinkage, porosity, and core shift Gamma-ray radiography is also satisfactory for detecting specific conditions in aluminum castings Although the -ray method is used to a lesser extent than the x-ray method, it is about equally as effective for detecting flaws or measuring specific conditions Aluminum alloy castings are most often radiographed to detect about the same types of flaws that may exist in other types of castings, that is, conditions such as porosity or shrinkage, which register as low-density spots or areas and appear blacker on the film or fluoroscopic screen than the areas of sound metal

Aluminum ingots may contain hidden internal cracks of varying dimensions Depending on size and location, these cracks may cause an ingot to split during mechanical working and thermal treatment, or they may show up as a discontinuity in the final wrought product Once the size and location of such cracks are determined, an ingot can be scrapped, or sections free from cracks can be sawed out and processed further Because the major dimensions of the cracks are along the casting direction, they present good reflecting surfaces for sound waves traveling perpendicular to the casting direction Thus, ultrasonic methods using a wave frequency that gives adequate penetration into the ingot provide excellent sensitivity for 100% inspection of that part of the ingot that contains critical cracks Because of ingot thickness (up to 406

mm, or 16 in.) and the small metal separation across the crack, radiographic methods are impractical for inspection

Ultrasonic Inspection. Aluminum alloy castings are sometimes inspected by ultrasonic methods to evaluate internal soundness or wall thickness The principal uses of ultrasonic inspection for aluminum alloy castings include detection of porosity in castings and internal cracks in ingots

Inspection of Copper and Copper Alloy Castings

Inspection of copper and copper alloy castings is generally limited to visual and liquid penetrant inspection of the surface, along with radiographic inspection for internal discontinuities In specific cases, electrical conductivity tests and ultrasonic inspection can be applied, although the usual relatively large cast grain size could prevent a successful ultrasonic inspection

Visual inspection is simple yet informative A visual inspection would include significant dimensional measurements as well as general appearance Surface discontinuities often indicate that internal discontinuities are also present

For small castings produced in reasonable volume, a destructive metallographic inspection on randomly selected samples

is practical and economical This is especially true on a new casting for which foundry practice has not been optimized and a satisfactory repeat-ability level has not been achieved

For castings of some of the harder and stronger alloys, a hardness test is a good means of estimating the level of mechanical properties Hardness tests are of less value for the softer tin bronze alloys because hardness tests do not reflect casting soundness and integrity

Because copper alloys are nonmagnetic, magnetic particle inspection cannot be used to detect surface cracks Instead, liquid penetrant inspection is recommended Ordinarily, liquid penetrant inspection requires some prior cleaning of the casting to highlight the full detail

For the detection of internal defects, radiographic inspection is recommended Radiographic methods and standards are well established for some copper alloy castings (for example, ASTM specifications E 272 and E 310)

As a general rule, the methods of inspection applied to some of the first castings made from a new pattern should include all those that provide a basis for judgment of the acceptability of the casting for the application intended Any deficiencies

or defects should be reviewed and the degree of perfection defined This procedure can be repeated on successive production runs until repeatability has been ensured

Gas Porosity. Copper and many copper alloys have a high affinity for hydrogen, with an increasing solubility as the temperature of the molten bath is increased Conversely, as the metal cools in the mold, most of this hydrogen is rejected from the metal Because all the gas does not necessarily escape to the atmosphere and may become trapped by the solidifying process, gas porosity may be found in the casting

In most alloys, gas porosity is identified by the presence of voids that are relatively spherical and are bright and shiny inside Visible upon sectioning or by radiography, they may either be small, numerous, and rather widely dispersed or fewer in number and relatively large Regardless of size, they are seldom interconnected except in some of the tin bronze alloys, which solidify in a very dendritic mode In these alloys, the gas porosity tends to be distributed in the interstices between the dendrities

Shrinkage voids caused by the change in volume from liquid to solid in copper alloys are different only in degree and possibly shape from those found in other metals and alloys All nonferrous metals exhibit this volume shrinkage when solidifying from the molten condition

Shrinkage voids may be open to the air when near or exposed to the surface, or they may be deep inside the thicker sections of the casting They are usually irregular in shape, compared to gas-generated defects, in that their shape frequently reflects the internal temperature gradients induced by the external shape of the casting

Hot Tearing. The tin bronzes as a class, as well as a few of the leaded yellow brasses, are susceptible to hot shortness; that is, they lack ductility and strength at elevated temperature This is significant in that tearing and cracking can take place during cooling in the mold because of mold or core restraint In aggravated instances, the resulting hot tears in the part appear as readily visible cracks Sometimes, however, the cracks are not visible externally and are not detectable until after machining In extreme cases, the cracks become evident only through field failure because the tearing was deep inside the casting

Nonmetallic inclusions in copper alloys, as with all molten alloys, are normally the result of improper melting and/or pouring conditions In the melting operation, the use of dirty remelt or dirty crucibles, poor furnace linings, or dirty stirring rods can introduce nonmetallic inclusions into the melt Similarly, poor gating design and pouring practice can produce turbulence and can generate nonmetallic inclusions Sand inclusions may also be evident as the result of improper sand and core practice All commercial metals, by the nature of available commercial melting and molding processes, usually contain very minor amounts of small nonmetallic inclusions These have little or no effect on the casting Inclusions of significant size or number are considered detrimental

Computer-Aided Dimensional Inspection

The use of computer equipment in foundry inspection operations is finding more acceptance as the power and usefulness

of available hardware and software increase The computerization of operations can reduce the man-hours required for inspection tasks, can increase accuracy, and can allow the analysis of data in ways that are not possible or practical with manual operations Perhaps the best example of this, given the currently available equipment, is provided by the application of computer technology to the dimensional inspection of castings

Importance of Dimensional Inspection

One of the most critical determinants of casting quality in the eyes of the casting buyer is dimensional accuracy (see the article "Dimensional Tolerances and Allowances" in this Volume) Parts that are within dimensional tolerances, given the absence of other casting defects, can be machined, assembled, and used for their intended functions with testing and inspection costs minimized Major casting buyers are therefore demanding statistical evidence that dimensional tolerances are being maintained In addition, the statistical analysis of in-house processes has been demonstrated to be effective in keeping those processes under control, thus reducing scrap and rework costs

The application of computer equipment to the collection and analysis of dimensional inspection data can increase the amount of inspection that can be performed and decrease the time required to record and analyze the results This

furnishes control information for making adjustments to tooling on the foundry floor and statistical information for reporting to customers on the dimensional accuracy of parts

Typical Equipment

A typical equipment installation for the dimensional inspection of castings consists of an electronic coordinate-measuring machine, a microcomputer interfaced to the coordinate-measuring machine controller with a data transfer cable, and a software system for the microcomputer This equipment is illustrated in Fig 2 The software system should be capable of controlling the functions and storing of the coordinate-measuring machine as well as recalling and analyzing the data it collects The software serves as the main control element for the dimensional inspection and statistical reporting of results Such software can be purchased or, if the expertise is available, developed in-house where requirements are highly specialized

Fig 2 Equipment used in a typical installation for the computer-aided dimensional inspection of castings

showing a coordinate-measuring machine and microcomputer

Coordinate-measuring machines typically record dimensions along three axes from datum points specified by the user Depending on the sophistication of the controller, such functions as center and diameter finds for circular features and electronic rotation of measurement planes can be performed Complex geometric constructions, such as intersection points of lines and planes and out-of-roundness measurements, are typically off-loaded for calculation into the microcomputer The contact probe of the coordinate-measuring machine can be manipulated manually, or in the case of direct computer controlled machines, the probe can be driven by servomotors to perform the part measurement with little operator intervention

The Measurement Process

Figure 3 illustrates the general procedure that is followed in applying semiautomatic dimensional inspection to a given part The first step is to identify the critical part dimensions that are to be measured and tracked Nominal dimensions and tolerances are normally taken from the customer's specifications and blueprints Dimensions that are useful in controlling the foundry process can also be selected A data base file, including a description and tolerance limits for each dimension

to be checked, is then created using the microcomputer software system

Fig 3 Flowchart showing typical sequence of operations for computer-aided dimensional inspection

The next step in the setup process is to develop a set of instructions for measuring the part with the coordinate-measuring machine The instructions consist of commands that the coordinate-measuring machine uses to establish reference planes and to measure such features as center points of circular holes

This measurement program can be entered in either of two ways Using the first method, the operator simply types in a list of commands that he wants the coordinate-measuring machine to execute and that give the required dimensions as defined in the part data base The second method uses a teach mode; the operator actually places a part on the worktable

of the coordinate-measuring machine and checks it in the proper sequence, while the computer monitors the process and stores the sequence of commands used In either case, the result is a measurement program stored on the microcomputer that defines in precise detail how the part is to be measured Special commands can also be included in the measurement program to display operator instructions on the computer screen while the part is being measured

In developing the measurement program, consideration must be given to the particular requirements of the part being measured Customer prints will normally show datum planes from which measurements are to be made When using a cast surface to establish a datum plane, it is good practice to probe a number of points on the surface and allow the computer to establish a best-fit plane through the points Similarly, center points of cast holes can best be found by probing multiple points around the circumference of the hole Machined features can generally be measured with fewer probe contacts When measuring complex castings, maximum use should be made of the ability of the coordinate-measuring machine to electronically rotate measurement planes without physically moving the part; unclamping and turning a part will lower the accuracy of the overall layout

Once the setup process is complete, dimensional inspection of parts from the foundry begins Based on statistical considerations, a sampling procedure and frequency must be developed Parts are then selected at random from the process according to the agreed-upon frequency The parts are brought to the coordinate-measuring machine, and the operator calls up the measurement program for that part and executes it As the part is measured, the dimensions are sent from the coordinate-measuring machine to the data base on the microcomputer Once the measurement process is complete, information such as mold number, shift, date, or serial number should be entered by the operator so that this particular set of dimensions can be identified later A layout report can then be generated to show how well the measured part checked out relative to specified dimensions and tolerances Figure 4 shows a sample report in the form of a bar graph, in which any deviation from print tolerance appears as a line of dashes to the left or right of center A deviation outside of tolerance limits displays asterisks to flag its condition Such a report is useful in that it gives a quick visual indication of the measurement of one casting

Fig 4 Example layout report showing all dimensions measured on a single casting, with visual indication of

deviations from print mean Note out-of-tolerance condition indicated by asterisks

Statistical Analysis

The use of statistical analysis permits the mathematical prediction of the characteristics of all the parts produced by measuring only a sample of those parts All processes are subject to some amount of natural variation; in most processes, this variation follows a normal distribution, the familiar bell-shaped curve, when the probability of occurrence is plotted against the range of possible values Standard deviation, a measure of the distance from center on the probability curve, is the principal means of expressing the range of measured values For example, a spread of six standard deviations (plus or minus three standard deviations on either side of the measured mean) represents the range within which one would expect

to find 99.73% of observed measurements for a normal process This allows the natural variation inherent in the process

to be quantified

Control Charts. With statistical software incorporated into the microcomputer system, the results of numerous measurements of the same part can be analyzed to determine, first, how well the process is staying in control, that is, whether the natural variations occurring in a given measurement are within control limits and whether any identifiable trends are occurring This is done by using a control chart (Fig 5), which displays the average values and ranges of groups of measurements plotted against time Single-value charts with a moving range can also be helpful The control limits can also be calculated and displayed With the computer, this type of graph can be generated within seconds Analysis of the graph may show a developing trend that can be corrected by adjusting the tooling before out-of-tolerance parts are made

Fig 5 An example control chart with average of groups of measurements (X values) plotted above and ranges

within the groups (R) plotted below Control limits have been calculated and placed on the chart by the

computer

Statistical Summary Report. The second type of analysis shows the capability of the process, that is, how the range

of natural variation (as measured by a specified multiple of the standard deviation) compares with the tolerance range specified for a given dimension An example of a useful report of this type is shown in Fig 6 This information is of great interest both to the customer and the process engineer because it indicates whether or not the process being used to produce the part can hold the dimensions within the required tolerance limits The user must be aware that different methods of capability analysis are used by different casting buyers, so the software should be flexible enough to accommodate the various methods of calculation that might be required

Fig 6 An example statistical summary report showing the mean of measured observations, the blueprint

specification for the mean, difference between specified and measured means, the tolerance, the standard deviation of the measured dimensions, and the capability of the process These calculations are performed for all measured dimensions on the part

Histograms. An alternative method of assessing capability involves the use of a histogram, or frequency plot This is a graph that plots the number of occurrences within successive, equally spaced ranges of a given measured dimension Figure 7 shows an example output report of this type A graph such as this, which has superimposed upon it the tolerance limits for the dimension being analyzed, allows a quick, qualitative evaluation of the variation and capability of the process It also allows the normality of the process to be judged through comparison with the expected bell-shaped curve

of a normal process

Fig 7 A frequency plot for one measured dimension showing the distribution of the measurements

Other Applications for Computer-Aided Inspection

The general sequence described for semi-automatic dimensional inspection can be applied to a number of other inspection criteria Examples would include pressure testing or defect detection by electronic vision systems The statistical analysis

of scrap by defect types is also very helpful in identifying problem areas In some cases, direct data input to a computer may not be feasible, but the benefits of entering data manually into a statistical analysis program should not be overlooked The computer allows rapid analysis of large amounts of data so that statistically significant trends can be detected and proper attention paid to appropriate areas for improvement The benefits and costs of each anticipated application of automation to a particular situation, as well as the feasibility of applying state-of-the-art equipment, need to

be studied as thoroughly as possible prior to implementation

Note cited in this section

This section was prepared by Lawrence E Smiley, Reliable Castings Corporation

alloys is available in Surface Engineering, Volume 5 of the ASM Handbook

Cleaning

Cleaning of the surface is the most important prerequisite of any coating process Suitable levels of cleanliness and surface roughness are established by various mechanical and chemical methods Foundries deliver castings that have been shot or grit blasted (see the article "Blast Cleaning of Castings" in this Volume) Supplementary nonmechanical cleaning may be needed to reach interior passages or to remove heat-treating scale or machining oil

The choice of cleaning process depends not only on the types of soils to be removed but also on the characteristics of the coating to be applied The cleaning process must leave the surface in a condition that is compatible with the coating process For example, if a casting is to be treated with phosphate and then painted, the cleaning process must remove all oils and oxide scale because these inhibit good phosphating

If castings are heat treated before they are coated, the choice of heat treatment conditions can influence the properties of the coating, particularly a metallic or conversion coating In most cases, heat treatment should be done in an atmosphere that is not oxidizing Oxides and silicates formed during heat treating must be removed before most coating processes

Molten salt baths are excellent for cleaning complex interior passages in castings In one electrolytic, molten salt cleaning process, the electrode potential is changed so that the salt bath is alternately oxidizing and reducing Scale and graphite are easily removed with reducing and oxidizing baths, respectively Molten salt baths are fast compared to other nonmechanical methods, but castings may crack if they are still hot when salt residues are rinsed off with water

Pickling in an acid bath is usually done prior to hot dip coating or electroplating Overpickling should be avoided because

a graphite smudge can be formed on the surface Cast iron contains silicon; therefore, a film of silica can form on the surface as a result of heavy pickling This can be avoided by adding hydrofluoric acid to the pickling bath Special safety and environmental protection regulations must be met when using pickling

Chemical cleaning is different from pickling because, in chemical cleaning, the cleaners attack only the surface contaminants, not the iron substrate Many chemical cleaners are proprietary formulations, but in general they are alkaline solutions, organic solvents, or emulsifiers Alkaline cleaners must penetrate contaminants and wet the surface in order to

be effective

Organic solvents that were commonly used in the past include naphtha, benzene, methanol, toluene, and carbon tetrachloride These have been largely replaced by chlorinated solvents, such as those used for vapor degreasing Solvents effectively remove lubricants, cutting oils, and coolants, but are ineffective against such inorganic compounds as oxides

or salts Emulsion cleaners are solvents combined with surfactants; they disperse contaminants and solids by emulsification Emulsion cleaners are most effective against heavy oils, greases, slushes, and solids entrained in hydrocarbon films They are relatively ineffective against adherent solids such as oxide scale

After wet cleaning, short-term rust prevention is accomplished by the use of an alkaline rinse This can be followed by mineral oils, solvents combined with inhibitors and film formers, emulsions of petroleum-base coatings and water, and waxes

Electroplating

Table 1 lists the common electroplated metals Other metals for example, gold, silver, and alloys such as brass and bronze are used for special decorative effects

Table 1 Properties and characteristics of conventional electroplated metal coatings

Thickness Metal Coating

hardness

Appearance

μm mil

Characteristics and uses

Cadmium 30-50 HV Bright white 3-10 0.15-0.5 Pleasing appearance for indoor applications; less likely to darken

than zinc; anodic to ferrous substrate

Chromium 900-1100

HV

White can be varied

0.2-1(a)1-300(b)

0.06(a)0.05- 12.0(b)

0.01-Excellent resistance to wear, abrasion, and corrosion; low friction and high reflectance

0.1-Resistant to many chemicals and corrosive atmospheres; often used in conjunction with copper and chromium; can be applied

0.1-Easily applied; high corrosion resistance; anodic to ferrous substrate

(a) Decorative

(b) Hard

(c) Wear applications

(d) Corrosion applications

Iron castings are electroplated to impart corrosion resistance or to provide a pleasing appearance Typical applications include many different types of interior hardware, machine parts such as printing cylinders, decorative trim, and casings Iron castings are also electroplated to enhance their wear resistance; for example, hard chromium plating is applied to the wear surfaces of piston rings Areas not requiring a plated surface can be masked or stopped off to prevent coverage Plating can be applied as a very thin layer for applications requiring only a pleasing appearance or mild corrosion resistance; thicker plating can be applied for more wear resistance, longer corrosion resistance, or to replace lost metal (Table 1)

Electrodeposition is done by making clean iron castings cathodic in an aqueous solution containing a salt of the coating metal and then passing a direct electrical current through the solution The effective metal content in the plating bath can

be replenished by using anodes made of the coating metal to complete the electrical circuit Variations in the properties of the deposited coating are influenced by the composition, temperature, pH, and agitation of the bath and by current density In addition, further variations in the coating can result from the design of the casting, the distance of the casting from the anode, and the preparation of the surface before plating

Nickel is unique among plated coatings because it can be applied without using an impressed electrical current Table 2 lists non-electrolytic methods The high-temperature treatments produce partly diffused coatings containing both nickel and phosphorus Because no impressed electrical current is involved, the thickness of an electroless nickel plate is exceptionally uniform regardless of the shape of the casting Preheating large castings prior to immersion prevents a delay

in the start of nickel deposition

Table 2 Nonelectrolytic nickel plating methods and solution constituents

Temperature Plating rate Method Solution constituents

°C °F μm/h mil/h

Special considerations

Immersion Nickel chloride, sodium

hypophosphite

70 160 1.2 0.05 Porous coating with moderate adhesion

can be improved with postheating at 650

Selective coverage by reduction of

mixture at high temperature

Electroless acid Nickel chloride, sodium

hypophosphite, sodium citrate

95 200 12 0.5 Postplating treatment not necessary for

iron castings

Alkaline Nickel chloride, sodium

hypophosphite, ammonium

95 200 8 0.3 Solutions are more difficult to maintain

than acid baths

Electroless nickel is used not only for its uniformity in coating thickness but also for its ability to be heat treated to increase its hardness Typically, an as-plated hardness of 49 to 50 HRC can be increased to 67 to 68 HRC by proper heat treatment More detailed information on electroplating is available in the Section "Plating and Electropolishing" in

Surface Engineering, Volume 5 of the ASM Handbook and in the articles "Electroplated Coatings" and "Electroplated Hard Chromium" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Hot Dip Coatings

Hot dip coating consists of immersing the casting in a bath of molten metal A flux-coated and/or chemically cleaned surface is necessary to achieve satisfactory results Aluminum, tin, zinc, and their alloys can be applied from a molten bath Hot dip coatings are preferred because they are thicker than electroplates and because an alloy layer is formed between the coating metal and the iron This provides additional durability and adhesion Castings of complex shape are easily coated by these processes, although air may become trapped in blind holes unless the castings are rotated

Hot dip zinc coating (galvanizing) is widely used on iron castings, particularly pipe, valves, and fittings The uniform and adherent coating provides a barrier against corrosive attack and will further protect an iron casting by acting as a sacrificial anode or by undergoing preferential corrosion Successful galvanizing depends on surface preparation Pickling followed by dipping in a bath of zinc ammonium chloride or other flux is done prior to dipping in molten zinc Excess zinc may be drained or centrifuged from the castings before quenching Quenching improves the brightness of the coating Iron castings of any type and any composition can be hot dip galvanized

Hot dip tin coating (hot tinning) provides a protective, decorative, and nontoxic coating for food equipment, a bonding layer for babbitted bearings, or a precoated surface of soldering Surface preparation is particularly important, and when maximum adherence is desired, such as when tinning is used to prepare a casting for the application of babbitt, electrolytic cleaning in a molten salt is preferred

For the hot dip lead coating of iron castings, lead-base alloys are preferred over pure lead; with pure lead, bonding is mechanical rather than metallurgical Tin is the element most widely used to enhance bonding Lead coatings are noted for their resistance to fumes from sulfuric and sulfurous acids

The aluminum coating (aluminizing) of iron castings imparts resistance to corrosion and heat The coating oxidizes rapidly, thus passivating the surface The resultant aluminum oxide is refractory in nature; it seals the surface and resists degradation at high temperatures An aluminized surface has limited resistance to sulfur fumes, organic acids, salts, and compounds of nitrate-phosphate chemicals More detailed information on hot dip coating (continuous and batch

processes) is available in the articles "Hot Dip Coatings" and "Corrosion of Zinc" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Hardfacing

Hardfacing can be used when a casting requires an unusually hard and wear-resistant surface and when it is impractical to produce a hard surface in the casting process or by selective heat treatment Frequently, hardfacing is used to repair worn castings by building up an overlay of new material

Hardfacing is basically a welding operation in which an alloy is fused to the base metal Both gas and arc methods are used Ferrous alloys can be either austenitic or hardenable types Metallic carbides are the chief intermetallic materials used for hardfacing Carbides are not melted or fused into the surface layer, but are bonded in place by an enveloping

metal such as cobalt The articles "Hardfacing, Weld Cladding, and Dissimilar Metal Joining" in Welding, Brazing, and Soldering, Volume 6, and "Metal and Alloy Powders for Welding, Hardfacing, Brazing, and Soldering" in Powder Metal Technologies and Applications, Volume 7, ASM Handbook contain detailed information on hardfacing materials,

hardfacing alloy selection, and hardfacing process selection

Thermal Sprayed Metals and Ceramics

Thermal spraying originally consisted of propelling small molten globules of metal through a flame onto the workpiece Technological improvements in the process have permitted the development of methods for applying both metallic and nonmetallic materials Nonmetallics include ceramics, refractories, and carbides

The thermal spraying of low-carbon iron or steel can be used to build up worn or mismachined surfaces Nickel combined with other elements can be thermal sprayed to form a hardfaced surface Zinc, aluminum, and lead can impart general corrosion resistance and can be deposited at very high rates Babbitt alloys can be thermal sprayed onto bearings Copper and bronzes are used for electrical and decorative purposes Thermal barriers in the form of oxides or silicides are generally applied by plasma methods to provide either heat shielding or resistance to attack by molten metals Wear resistance is enhanced by the application of carbides or borides

Thermal spraying uses oxyacetylene, oxyhydrogen, detonation plasma, and electric arc techniques to impact molten or semimolten particles onto a substrate casting and to build up the desired thickness The sprayed coating resembles the

source material in composition, but its physical and mechanical properties are different from those of the source -more closely resembling the properties of sintered metals or refractories Bonding of the coating to the casting surface occurs principally by mechanical adhesion and only partly by metallurgical bonding Rough machining, arc spattering, or coarse abrasive blasting prior to thermal spraying improves the mechanical bonding Selection of the appropriate method

material-of application depends on the form material-of the source material (wire, rod, or powder), the stability material-of the coating material at the application temperature, and the temperature required to achieve satisfactory adhesion Temperatures within the nozzle range from 2760 °C (5000 °F) in the oxyacetylene process to 16,500 °C (30,000 °F) in plasma spraying More detailed

information is available in the article "Thermal Spray Coatings" in Surface Engineering, Volume 5 of the ASM Handbook

on the surface of the casting Diffusion coating and case-hardening processes are discussed in "Diffusion Coatings" in

Surface Engineering, Volume 5, and "Carbonitriding" in Heat Treating, Volume 4 of the ASM Handbook

Table 3 Types of diffusion coatings and their characteristics

Calorized Metallic aluminum introduced into

surface layer, forming aluminum-iron alloy

High-temperature oxidation resistance

Chemical processes, steam superheaters, and heat transfer

Chromized Chromium carbide case formed on

Chemical process pipe and fittings

Nitrided Nitrogen introduced into surface by

contact of ammonia or other nitrogenous material

Wear and corrosion resistance at elevated temperatures

Same as for carbonitrided

Sheradized Zinc introduced into surface Corrosion resistance Atmospheric-corrosion

resistance

Conversion Coatings

Chemical reactions at casting surfaces can produce iron-containing compounds that provide wear resistance or an attractive appearance or that serve as excellent bonding agents for subsequent organic coatings Table 4 details common conversion coatings and their useful properties (chromate and phosphate conversion coatings are discussed in greater

detail in Surface Engineering, Volume 5, and Corrosion, Volume 13, of the ASM Handbook) Most of the successful

processes are proprietary, and reproducibility of consistently good finishes is one of the important features

Table 4 Chemical conversion coatings, structures, and characteristics

Type Coating structure Properties Uses

Chromate Nonporous film acts as moisture barrier High corrosion resistance;

inhibits corrosion if surface is broken; can be colored

Marine applications; can be decorative; nonporous bond layer for paint

Oxide Ferric oxide formed from iron Inhibits formation of ferrous

oxide; highly absorbent; some wear resistance

Decorative blue-black coating; readily absorbs overlays of wax or oil

Phosphate Iron zinc or manganese phosphates are

crystalline structures formed on the surface

by deposition from chemical solution

Chemically neutral and high adherence to iron surfaces;

highly absorbent

Excellent for bonding paint to iron; prevents abnormal wear or seizing during break-in

Gray, ductile, and malleable iron castings all lend themselves readily to phosphating The ability of a cast iron to accept a phosphate coating is not affected by alloy content, but hinges primarily on two requirements: a clean surface, and a metal temperature approximately equal to that of the phosphating bath Dry machined surfaces need no further cleaning; cast surfaces can be prepared by blasting or other cleaning methods to remove scale and sand

Porcelain Enameling

Porcelain enamels are inorganic vitreous coatings that are matured by heat The inherently good heat transfer, thermal stability, and rigidity of iron at firing temperatures, coupled with the excellent adherence of vitreous ceramic frits as they fuse onto the cast surface, make the combination of porcelain on iron an excellent product General corrosion resistance

or resistance to specific chemicals can be obtained by selecting the proper porcelain enamel The scratch resistance and hardness of the enamel coating, which allow the surface to resist abrasion, are almost equal in importance to the corrosion resistance

Four processes are used to apply enamels: dry, wet, thermal spray (or plasma spray), and electrostatic precipitation The latter two methods are seldom used on iron castings Preparations for enameling start by blasting with sand, steel shot, or iron grit

Dry coating methods use formulations that are mainly silica; these formulations generate a surface with the hardness and abrasion resistance of glass Fluxes and opacifiers are mixed into the silica, and the mixture is then melted, quenched, and ground to make frit (Frit is the term applied to the basic coating materials.) After application of a bonding ground coat, relatively heavy, smooth coatings (such as those on sinks and bathtubs) can be obtained by multiple firing Each firing is followed by hot dusting with additional powdered frit, until the desired finish has been achieved

Wet methods produce thinner coatings The powdered frit is suspended in a solution of electrolytes and water or in an organic solvent and is applied by spraying or dipping over the ground coat The sprayed or dipped coating must be dried prior to firing

Ground coats that wet iron readily, adhere well, and are compatible with the cover coat are essential to the enameling process Good ground coats promote adhesion between the enamel layer and the substrate, seal and smooth the irregularities of the surface, and prevent oxidation of the iron casting at firing temperatures Top coats (cover coats) must provide the desired appearance and must be compatible with the ground coat Formulation of frits requires the judgment and experience of a frit manufacturer to ensure that the coating provides successful results

Organic Coatings

Organic coatings have a wide variety of properties, but their primary uses require corrosion resistance combined with a pleasing colored appearance An organic-base film is often resistant to certain environmental substances but not to others, and so must be chosen for a specific set of well-defined service conditions For example, a vinyl paint might be used on a pump casing that must operate in contact with acidic industrial waters However, if the same casing is expected to contact

hydrocarbons such as gasoline or solvents, a styrene, epoxy, or phenolic coating would most likely provide superior protection

The term paint was once commonly used to designate all liquid organic coatings, but it is considered inadequate to describe modern liquid organic coatings, which in general are subdivided into enamels, lacquers, aqueous mixtures, suspensions, bituminous substances, and rubber-base products Resins dispersed in a vehicle for example, enamels or lacquers cure to relatively hard gels by polymerization, oxidation, or solvent evaporation A comparison of the chemical and environmental resistance of common resins is given in Table 5

Table 5 Properties of organic coatings on iron castings

method Resin

Hydrocarbons Solvents Acids Alkalies Salts Water Weathering Heat Cold Abrasion Need

for primer

Spray Dip Fluidized

bed

Air dry Bake

Typical

applications

Low cost

finishes Moderate cost

Vinyl

chloride

equipment

Abrasion-resistant coatings

purpose, primers

polyether equipment

equipment, nonstick surfaces

Heat-resistant finishes

(a) E, excellent; G, good; F, fair; P, poor

Enamels consist of milled pigments and other additives dispersed in resins and solvents and are converted from liquids to hard films by oxidation or polymerization Lacquers are thermoplastic resins dissolved in organic solvents that dry rapidly

by evaporation In aqueous coatings, water is the principal vehicle or reducer The advantages of water-base paints are nominal cost, nonflammability, true odorlessness, and nontoxicity The disadvantages are difficulties in wettability, flow, and drying Rubber-base coatings are noted for their mechanical properties and corrosion resistance rather than their decorative effects Bituminous paints are black materials in which coal tar is dissolved in a solvent that evaporates The major uses of bituminous paints are those that require extremely low permeability and high resistance to water Unusual protection against chemical solutions, or special decorative effects, can be obtained by the use of asphaltic coatings or those produced by japanning, both of which are also considered bituminous coatings

Fluorocarbon coatings produce an unusual combination of properties They are tough, stain resistant, and nonsticking, and have a very low coefficient of friction Fluorocarbon coatings resist all common industrial acids and temperatures to 300

°C (570 °F) Domestic cookware and chemical-processing equipment are two major applications of fluorocarbon coatings

on iron castings Fluorocarbon coatings are sprayed as emulsions of proprietary products onto a primed surface and then fused at temperatures of 385 to 425 °C (725 to 800 °F)

Organic coatings are applied by spraying, dipping, flow coating, fluidized bed coating, electrostatic deposition, and electrophoresis (electrocoating)

Spraying is adaptable to both low-volume and high-volume workloads It is done by propelling the coating material toward the workpiece by compressed air, hot spraying, hydraulic-airless, and airless-electrostatic methods Overspraying

is most troublesome with compressed air methods and least troublesome with electrostatic methods

Dipping has been used for centuries; modern refinements include flow coating and electrophoresis Not all shapes can be painted by dipping Pockets can exclude paint from some surfaces The shape of the casting should allow easy draining after dipping The coating should be selected to inhibit sagging or the formation of droplets on edges The dipping process

is easily automated and can be very efficient in use of materials A thorough review of organic coatings and their

applications, advantages, and limitations is available in the article "Organic Coatings and Linings" in Corrosion, Volume

13 of ASM Handbook, formerly 9th Edition Metals Handbook

Fused Dry-Resin Coatings

Dry-resin polymers can be applied (by fusion bonding) to iron castings for many of the same applications for which liquid organic coatings are used Generally, the fused coatings are thick and can be applied very rapidly often in minutes; in contrast, several hours is required for the drying and curing of a liquid organic coating

The fusion bonding of polymers on iron castings can be readily accomplished by the application of dry solvent-free powder by fluidized bed coating or electrostatic deposition The advantages of the process include the use of resins that are insoluble in ordinary solvents, the elimination of carrier solvents, and the ability to combine various plastics in the coating for maximum effectiveness Sandblasted castings are excellent bases for this process The plastic films can be easily machined, which contributes to flexibility in manufacturing Two disadvantages are that thin films are not easily applied and that finding a suitable holding point on a part to be coated in a fluidized bed may be difficult The six basic types of plastic fusion-bonded finishes, as well as a comparison of their characteristics, are given in Table 6

Table 6 Relative effectiveness of fusion-bonding resin coatings

Effectiveness(a)

Vinyl Cellulose Epoxy Nylon Chlorinated

(a) E Excellent; VG, very good; G, good F, fair; P, poor

(b) Ranges from excellent to poor depending on composition

In a fluidized bed, castings heated to 175 to 310 °C (350 to 590 °F) are placed in a chamber containing resin powder suspended by upward-moving air The dry resin floats around the casting, adhering to all the surfaces regardless of the complexity of the shape Heating fuses the coating into a continuous film This method produces a uniform coating that covers sharp corners, edges, and projections and that can be applied in a wide range of thicknesses (up to 1.5 mm, or 0.06 in.) in a single application

In electrostatic deposition, the powdered resin is conveyed to a gun, in which it is given an electrostatic charge The casting has the opposite charge The charged powder is attracted to the surface, where it is deposited evenly The electrostatic process is especially useful for applying thinner coatings because the residual charge on the workpiece is discharged by the powder, or leaks off, thus limiting the amount of powder that can be deposited Preheating of the casting permits thicker coatings because the initial powder layer fuses as it is applied Curing is done by reheating the casting to fuse the resin coating Primers may be needed for some types of polymers (such as butyrates and vinyls) in order to achieve adequate bonding

Foundry Automation

Ronald L Lewis, and Yeou-Li Chu, The Ohio State University

Introduction

AUTOMATION in the foundry is an integrated technology concerned with the application of both complex hardware (robots, computers, and electronic controllers) and software (management information software, statistical process control, and material process planning techniques) to control various foundry production activities Automation applications in the foundry can be divided into the following categories:

• Foundry robotic applications

• Cell applications

• Automatic pouring systems

• Automatic sorting and inspection systems

• Automatic storage and retrieval systems

• Computer-aided design and manufacture

Each of these will be briefly reviewed in the following sections

Foundry Robotic Applications

A robot is a programmable, multifunctional manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks (Ref 1, 2) Depending on the sensory requirement of the task to be performed, robots can be divided into different classes One classification scheme is given below (Ref 3):

this level is the pick-and-place device It is merely capable of being fed movements that are repeated indefinitely These devices are limited to assembly tasks involving smooth parts Foundry applications include spraying mold wash and extracting castings from a die casting machine

• Level 1: Capable of monitoring contact between two surfaces through force and tactile sensors, these

robots provide feedback to allow the mating of pieces without jamming However, they are restricted to what they feel They must be taught the location of the parts to be assembled, and the design of the parts cannot change during an assembly run An example of such a foundry application would be robots attached to coremaking machines

they can select pieces from a moving conveyor line and then inspect and orient each workpiece for assembly Examples would include adaptively controlled grinding and torch cutting operations

Different foundry applications will dictate different levels of sophistication and cost Robots may range in price from

$5000 for a simple pick-and-place unit to over $200,000 for a large, highly programmable and flexible multiaxis robot (1988 dollars) In addition to this capital investment, the foundry must be prepared to incur other costs, such as maintenance and education Therefore, a number of benefits must be achieved to offset these costs and to provide a beneficial cost position These benefits include (Ref 3, 4, 5, 6, 7, 8, 9, 10):

robots do not require coffee breaks, lunch periods, and so on

• Reduced costs: Direct labor costs and labor overhead costs such as parking spaces, pensions, health

care, wash rooms, and so on, are saved

hazardous conditions

production function repeatedly, rapidly, and consistently without the variations inherent in human handcraft

accuracies than any human operator Repeatability accuracies for some robot applications approach 0.013 mm (0.0005 in.), but because of the nature of most foundry applications, these higher accuracies are not achieved Movement speeds of 1270 mm/s (50 in./s) are also possible

dissimilar tasks

as high as 98% in long-run high-production situations

• Inflation resistance: The hourly rate for a robot installed today will be the same in 5 years, with no

slowdowns or strikes

parts, materials, and maintenance support

installed to perform unpopular foundry tasks

Robotic applications in the foundry industry include cleaning, riser cutting, pick and place, mold venting, and mold spraying

Cleaning Operations

Cleaning (fettling) operations are traditionally a very labor intensive component of the cost of a casting, often exceeding 60% of the total casting cost (Ref 11) A very high percentage, some estimate over 90%, of all cleaning operations are still performed manually (Ref 12) A number of robotic installations have been completed both in Europe and in the United States Foundry technical associations and universities such as BCIRA (Ref 13), the Steel Castings Research and Trade Association (Ref 14), Svenska Gjuteriforeningen (Ref 15), the Fraunhofer-Institute in Stuttgart (Ref 16), the University of Aachen, the University of Leuven, and the University of Rhode Island have been conducting research in robotic foundry applications

Cleaning operations can be divided into two categories: fixed and variable (Ref 17) Fixed grinding is the removal of the material that is present on every casting in a fixed position (that is, feeder pads, flash, gates, and so on) Variable cleaning can occur anywhere on the casting and usually results from defects in the mold or cores To date, there are no applications

of automated variable cleaning In addition, if an automatic cleaning operation is implemented, some form of inspection is necessary to detect variable cleaning requirements and to ensure that they do not interfere with the automatic cleaning operation

For high-volume production, automatic fixed-stop machines have been specifically designed to grind castings Many were designed to allow for the simultaneous grinding of many casting faces Other machines are rotary in design with multiple heads Through the use of template-driven machines, a complex casting periphery can be ground Unfortunately, all of these systems, although automated, are not usually programmable; therefore, they cannot be easily adjusted for widely varying casting shapes or geometries In addition, they cannot adapt to significant differences in casting irregularities

Truly programmable cleaning operations are normally conducted by robots Figure 1 shows a successful robotic installation The cell consists of a robot and four specially designed grinding wheels The wheels are hydraulically driven from a central unit Wheel wear compensation is automatic such that the working point of the wheel is at the exact same location from cycle to cycle Before entering the cell, castings are introduced to a dedicated press, where the main riser is removed and specific contour locations are stamped that will serve as reference areas for positioning The casting is picked up by the robot and moved to each grinding machine The system is now operating three shifts a day with an 89% uptime The casting finish weight is 18 kg (40 lb) The complete cycle time is 3 min per casting The average production rate, sampled over a 4-month period, is over 400 castings per day

Fig 1 Schematic of a first-generation robotic system designed to perform fettling operations at a rate of one

completely machined casting every 3 min

Depending on casting size, the robot can either hold the casting and move it to the grinding machine or it can hold the grinding tools and apply them to the casting The advantages of using a robot to manipulate a casting include the following (Ref 17):

• The robot can pick up the casting from a fixed point that can be magazine fed or from several predetermined positions if the castings are palletized

• The robot can pick up castings within the limits of its capacity that would in many cases be too heavy for a manual operation

• The robot can move the casting to a succession of different tools to perform the cleaning operation to the required standard in the minimum amount of time by using the most efficient tool available at each operation These sequential operations can be carried out without having to stop the cycle to change tools

• The robot can put the casting down on a fixture and regrip it to allow access to other surfaces of the casting For example, the robot can turn the casting around or completely over

• Upon completion of cleaning, the robot can place the casting in a fixture, drop it in a bin, or place it on any predetermined position on a pallet

When the casting weight is high or when a variety of grinding wheels are necessary, it is more feasible to allow the robot

to handle the tool rather than the casting Industrial manipulators, although not programmable, are useful for this application (Ref 18) Tool change downtime is obviously very important with such a system

A number of problems are inherent in any system, regardless of the type used These problems include the following (Ref 17):

• Grinding develops varying load conditions, which cause inaccuracies in robot positioning This can be overcome by either an adaptive control system or by a template-driven system such as that shown in Fig 2

• Imprecision in casting geometries creates inaccuracies in location

• Corrections must be made for wheel wear; simple light beam sensors are often used to sense the location

of the edge of the wheel

Fig 2 Guidance template used to control the movement of the robotic arm

Riser Cutting

For the reasons previously outlined, robotic flame cutting for riser and gate removal is now possible This is particularly important for heavy section steel castings, where risers cannot be removed economically by grinding, impact, and so on Unlike cleaning, however, the torch must be held by the robot As in cleaning, such systems range from semiautomatic to fully programmable robotized systems Installations can use both conventional and gantry-style robots (Ref 19, 20) Steel thicknesses in excess of 610 mm (24 in.) have been successfully cut To perform the flame cutting operation, the operator, using the control panel, moves the torch into the general position of the riser The unit is preprogrammed for six riser contact sizes The operator then selects the proper size, and the cut is executed

A more automatic system uses a six-axis robot for complete automation of the cutting operation (Ref 19) Specially designed locating blocks or pads are cast into each casting Contact location principles are utilized to provide alignment between the robot and the casting The robot is programmed using a teach pendant device In addition to position and path, the operator programming the robot must also define the travel speed between each point, preheat delay, and cutting oxygen pressure for each cut Results indicate a savings of 120 to 180%, depending on the casting to be cut (Ref 19, 20, 21)

The following factors must be taken into account when considering an installation or application (Ref 21):

• Fixturing: Distortion of gate position due to rough handling, mold inaccuracy, and so on, all lead to

errors in position Some means of sensing the positions of the gates and risers is necessary

casting, an adaptive control system that accounts for this would be very beneficial (Ref 22)

(Ref 19) An infrared sensing measurement is made to the computer controlling the robot If the signal indicates kindling temperature, oxygen is employed and the cut is continued

Pick and Place Operations

The most common application of robots in any manufacturing environment consists of pick-and-place tasks This is also true in the foundry industry Some applications are given below:

• The first application of robotics in the foundry industry was in the die casting process (Ref 21, 23, 24, 25) Robots were used for such operations as extracting the casting from the die and loading and unloading the casting to firm presses

• Most attribute the second application of robotics in the foundry to investment casting (Ref 21, 26) In this application, robots are used to dip the pattern assembly into the ceramic slurry to obtain a precise and uniform distribution of the coating This process has been quite successful because it has greatly increased the consistency as compared to manual dipping

• More recently, robots have been used in the evaporative foam process Here robots have been used to remove the expandable polystyrene patterns from the mold and then assemble them into a complete pattern, to position the patterns into flasks, and to remove them after pouring As in investment casting, robots have also been used to dip the castings into mold washes

• Core handling applications can also be found, especially for shell core handling

Mold Venting

To produce sound, high-quality castings, one of the most important manufacturing steps is to vent the mold properly In green sand molding, this is accomplished either by pressing a metal pin into the mold or by drilling through the mold in precise locations

A robot can be programmed by a teach pendant device to perform this operation The program is then stored and retrieved

when the pattern is produced again The venting function requires a robot capable of only an X-Y positioning and an

up-and-down motion (Ref 27) Off-line programming functions for the robot have even been incorporated into this operation

Mold Spraying

One of the most common applications of robotics in manufacturing is paint spraying Similarly, in sand molding, robots have been used to spray molds and cores with a wash before drying with gas torch flames One application is shown in Fig 3

Fig 3 Robots stationed along two indexing lines to spray a coating on each cope and drag, followed by drying

of the foundry mold components prior to assembly in the turnover machine

One advantage of such a system is the consistency of spray and the reduction of overspraying Robotic-controlled drying has been found to reduce fuel consumption by 67% In addition, after each spray of the mold wash, the spray gun can be cleaned for the next mold, thus eliminating the downtime that would have been spent to fix clogged spray heads

Another application of robotic spraying systems is found in die casting (Ref 24, 28, 29, 30) Robots are used to spray both water and die lubricants on the die during each cycle

References cited in this section

1 R Asfahl, Robots and Manufacturing Automation, John Wiley & Sons, 1985

2 W.R Tanner, Industrial Robots, Society of Manufacturing Engineers, 1979

3 C.W Meyers and J.T Berry, The Impact of Robotics on the Foundry Industry, Paper 30, Trans AFS, 1979,

p 107-112

4 G.N Booth, High Tech in a Smokestack Industry, Foundry Mgmt Technol., April 1985

5 C.F James and J.G Sylvia, The Robot's Role in Foundry Mechanization, Mod Cast., May 1982

6 H.J Heine, New Ideas for the Cleaning Room Part I, Foundry Mgmt Technol., Aug 1983

7 J.C Miske, Improving Cleaning Room Productivity, Foundry Mgmt Technol., Oct 1984

8 E Ford, Automating Britain's Foundries, Mod Cast., June 1982

9 D Williamson, Automating Castings: What GM Gets for $214 Million, Mfg Eng., Aug 1985

10 Foundry Management and Technology Data Book, 1988

11 Costs and Methods for Fettling Castings, IVF Result 76639, Svenska Gjuteriforeningen, 1976

12 R.G Godding, Fettling in the Light of Recent Developments, Br Foundryman, Vol 76, Dec 1983

13 R.A Wragg, Practical Experiences of Using a Robot to Fettle Casting, Seminar Proceedings, BCIRA

14 B.J Sims and R Wallis, The Role of Automation in Foundry Operation, in Proceedings of the SCRATA

Research Annual Conference, Steel Castings Research and Trade Association, 1984

15 Experiments With Robots in Foundries, Mekanresultat 76009, Svenska Gjuteriforeningen, Nov 1976

16 W Sturz and D Boley, Development in Using Industrial Robots for Deburring and Fettling of Castings,

Fraunhofer Institute

17 R.G Godding, Fettling in the Light of Recent Developments, Br Foundryman, Vol 76, Dec 1983

18 W.T Hickman, Cleaning Casting With an Industrial Manipulator, Mod Cast., Sept 1984

19 M.D Schneider and R.R Petersen, Production Applications of Manipulators and Robots for Riser Cutting,

Paper 161, Trans AFS, 1986

20 M.D Schneider, Automatic Riser Removal at Rockwell International, in Proceedings of the 39th Annual

Technical and Operating Conference, 12 Nov 1984, Steel Founders' Society of America

21 G.E Munson, Foundries, Robots and Productivity, Unimation Inc., 1978

22 R.T Hughes, S Lepak, and R Scholz, Demonstration of Control Technology for Torch Cutting, Trans

AFS, Vol 59, 1985

23 V.G Parodi, Robots in the Automated Production of Aluminium-Alloy Pressure Diecasting, Foundry Trade

J Int., Dec 1978

24 N.W Rhea, Robots Improve a Die Casting Shop, Tool Prod., Vol 43, March 1978

25 Die Casting With Robot, in Die Casting and Metal Molding, Cutlands Press, p 16-20

26 Robots at Work: Unimation Designs Automated Investment Casting System, Robotics Today, Fall 1981

27 A.L Carr and W.P O'Neil, Computerized Off-Line Programming for Robotic Mold Venting, Paper 106,

Trans AFS, 1984

28 R.C Rodgers, Robots 9 Show and Conference Highlights Vision Systems, Foundry Mgmt Technol., Sept

1985

29 W.A Wiesmueller, Robots in the Real World of Die Cast Foundries, Robotics Eng., March 1986

30 J Canner, Automated Die Casting A Concept Comes Full Circle, Die Cast Eng., March/April 1986

Cell Applications

Many traditional manufacturing processes use cell concepts to improve productivity, control, and quality (Ref 31) Figure

4 shows a die casting cell Such cells incorporate many individual automation concepts and techniques into an integral production system (Ref 32, 33) One important feature of cells is that they are usually under the supervision of a computer control This concept of cell automation is being used in the foundry industry Examples can be found in both the die casting and the evaporative foam processing areas (Ref 7, 21, 29, 30) The die casting cell consists of a number of components, including the following:

temperature

the gating system

remove the casting from the machine and to load the casting parts into a trim press, cooling tower, quench tank, or pallet Second, robotic ladling systems handle the liquid metal Finally, robots are used

to spray lubricants on the die surface itself

complete die casting cell Many die casting process variables are controlled and/or monitored by computer controls This computer control allows the die casting machine to regulate the die casting process more precisely, thus producing higher-quality and more consistent castings

Fig 4 A typical highly automated cell used in a die casting operation It includes a die casting machine, a

furnace, a trim press, two robots, and a computer control system to coordinate the movement of each cell component

The results of implementing such cells are significant A savings of $120,000 of direct labor (1986 dollars) cost per year per cell for a three-shift operation has been realized over manual operations (Ref 29) The hourly production of parts has been found to increase by 48.7% (Ref 23)

A totally automatic computer-integrated manufacturing (CIM) system for the Replicast process is currently under development (Ref 33) The proposed layout is shown in Fig 5 As can be seen, the system is totally automated The proposed system will use existing technology in each subportion of the cell The integration of these techniques is the problem The products to be produced are steel valve castings The expected production level is 18 Mg (20 tons) of finished steel castings per week The loop conveyor system consists of 72 flasks; the proposed cycle time is 165 to 190 min per flask

Fig 5 Schematic of a layout for a CIM system that uses the Replicast ceramic shell process to produce steel

valve castings The loop conveyor system consists of four separate tracks (B-B in a U-shaped configuration along the perimeter of the system and A-A, C-C, and D-D enclosed in a ladder array within B-B) that contain 3 sets of 24 flasks each for a total of 72 flasks Each track performs the following function in the production routing A-A is the storage area for empty flasks B-B transfers empty flasks to the flask filling station, where the thin ceramic shell is placed in the flask with its pouring cup at the center and the flasks vibrated to compact the sand that supports the shell; conveys flasks to liquid-metal refining (LMR) station turntable, where the molten metal is then poured into the mold (either under atmospheric pressure or a vacuum) C-C and D-D are cooling stations where the molten metal solidifies B-B conveys flasks to a device that separates the casting from the mold and then transports the flasks to a discharge station, where the sand is dumped into hopper to

be screened, cooled, and classified Empty flasks in A-A await reuse to repeat the cycle of operations described

A second proposed prototype casting cell is shown in Fig 6 The proposed system uses well-established techniques for the automatic transport of expanded polystyrene type patterns, ceramic shells, and flasks The system requires two to three robots, depending on the level of automation required The first would be used to produce the molding material, while the second would position the pattern into the flask and then remove the casting Robotic-controlled flame cutting stations and automatic cleaning facilities could also be installed

Fig 6 Schematic of an integrated casting system that incorporates existing installations and technology to

demonstrate the operation of a totally automated system Up to three robots could be employed The first is used to produce the ceramic shells widely used in investment casting operations; the second, to position shells

in molding flasks and to remove castings from the mold after the molten metal has been poured; and the third,

to flame cut or abrasive cut the casting in an automated fettling cell

References cited in this section

7 J.C Miske, Improving Cleaning Room Productivity, Foundry Mgmt Technol., Oct 1984

21 G.E Munson, Foundries, Robots and Productivity, Unimation Inc., 1978

23 V.G Parodi, Robots in the Automated Production of Aluminium-Alloy Pressure Diecasting, Foundry Trade

J Int., Dec 1978

29 W.A Wiesmueller, Robots in the Real World of Die Cast Foundries, Robotics Eng., March 1986

30 J Canner, Automated Die Casting A Concept Comes Full Circle, Die Cast Eng., March/April 1986

31 P Ranky, The Design and Operation of FMS, IFS Publications, 1983

32 M.P Groover, Automation, Production Systems and Computer Aided Manufacturing, Prentice-Hall, 1983

33 C Lewis, State of the Technology in Die Casting, Ohio State University, 1987

Automatic Pouring Systems

The pouring control system for any automatic foundry cell is critical for achieving maximum yield per run or shift while still maintaining high quality standards in the castings produced (Ref 35) A number of automatic pouring systems are available; some are discussed below Additional information can be found in the article "Automatic Pouring Systems" in this Volume

Robotic System. The die casting industry uses an automatic pouring ladle with robot-like movements that is controlled

by a programmable controller The cycle begins at the holding furnace, the ladle dips precise amounts of molten aluminum from a dipwell, carries it to a position directly over the mold without spillage within 3 to 6 s, and pours it quickly into the mold

A simple adjustment on the controller determines the amount of liquid metal dipped by the ladle A sensor detects the level of the liquid-metal surface to ensure accurate shot repeatability throughout the metal-level height range

Bottom-Pour System. The automatic system combines microprocessor control, fiber optics, and infrared sensing to control bottom-pour ladles A pulsed infrared beam is bounced off the mold surface to direct and control the hydraulic alignment of the mold with the ladle stopper Two fiber optic cables sense the leading and trailing edges of the pouring cup This reduces metal loss due to spillage and results in higher-quality castings The microprocessor controls the time the stopper is open by computing the diminishing height of metal during the pouring process (Ref 36)

Laser Level Measurement. It is feasible to combine a laser light source with a solid-state electronic camera and a process control computer to control the amount of molten iron dispersed into special pouring ladles This control has been installed on holding furnaces that supply predetermined quantities of molten iron to rotary mechanical pouring machines Figure 7 shows a basic schematic of the system

Fig 7 Laser light source and a solid-state electronic camera interfaced with a process control computer to

monitor the furnace level in a state-of-the-art rotary mechanical pourer

The laser beam controls the rate at which each ladle is filled based on the heat of metal in the holding furnace The laser beam is focused on the surface of the molten iron A video camera picks up the laser beam reflection and transmits the information to a process control computer Decisions are then made as to the amount of metal poured It is estimated that

5 to 14 kg (10 to 30 lb) per mold of iron is saved by this system Payback is estimated to be 1 to 3 months

References cited in this section