McGraw-Hill Machining and Metalworking Handbook 3rd ed - R. Walsh_ D. Cormier (McGraw-Hill 2006) WW Part 12 potx

avail-able from the plastic manufacturers, whose catalogs may be obtained through such trade magazines as The American Machinist, Machine Design, Product Design and Development, Modern M

■ High ductility

■ Corrosion resistance

irons and cast steels

ASTM A159-83 (R1988), Standard Specification for Automotive Gray Iron Castings See Fig 12.14 for mechanical properties.

The grades of gray cast iron consist of the following:

Cast-lists the tensile strength properties.

ASTM A 297/A 297M-89, Standard Specification for Steel ings, Iron-Chromium, Iron-Chromium-Nickel, Heat Resistant, for General Applications Figure 12.17 lists the physical properties.

ASTM A 352/A 352M-89, Standard Specification for Steel ings, Ferritic and Martensitic, for Pressure Containing Parts, Suitable for Low-Temperature Service Table 12.1 lists the chem-

Cast-ical, tensile, and impact properties

ASTM A 436-84, Standard Specification for Austenitic Gray Iron Castings Figure 12.18 lists the chemical and mechanical require-

Cast-heat-treatment and mechanical requirements

ASTM A 743/743M-89, Standard Specification for Castings, Chromium, Iron-Chromium-Nickel, Corrosion Resistant, for Gen- eral Applications Tables 12.4 and 12.5 list the heat-treatment and

Iron-mechanical requirements

Brinell hardness measurements for castings and other applications.

Calculation of the Brinell hardness number (BHN) can be performed

using the equation shown in Sec 4.8 of this Handbook Table 12.6 is

a table of Brinell hardness numbers as determined by the diameter

of the indentation of a 10-mm ball at applied loads of 500, 1500, and

3000 kgf (kilogram-force) The table may be used for determining thehardness of most metals and alloys.

12.5 Plastic Moldings

There are two classifications of plastics and their moldings: moplastics and thermoset plastics Thermoplastics are basicallythe same chemically after molding as they were in the raw form.This means that once molded, they may be reused, in most cases,

ther-by chopping the parts into small pieces and remelting Thermoset

Figure 12.14

plastics, once molded, cannot be remolded or reprocessed becausethey have a one-way chemistry that alters their “as molded” char-acteristics from their raw constituents.

Types of thermoplastics include

castings (Reprinted with permission from the Annual Book of

ASTM Standards, copyright 1992, American Society for Testing

and Materials.)

Figure 12.16

■ Allyl (diallyl phthalate)

■ Amino (urea, melamine)

■ Epoxy (including cycloaliphatic)

and heat-resisting castings (Reprinted with permission from the

Annual Book of ASTM Standards, copyright 1992, American

Soci-ety for Testing and Materials.)

(Text continued on page 793.)

a For each reduction of 0.01% below the specified maximum carbon content, an increase of 0.04% manganese above the

b Specified Residual Elements—The total content of these elements is 1.00% maximum.

c See 1.2.

d Determine by either 0.2% offset method or 0.5% extension-under

e When ICI test bars are used in tensile testing as provided for in Specification A 703/A 703M, the gauge length to

f See Appendix X1. SOURCE

779

Figure 12.19

782 Chapter Twelve

Austenitizing Tempering temperature, Quenching cool temperature, Grade Class min, °F (°C) Media* below, °F (°C) °F (°C) †

¶ Air cool to below 200 °F (95°C) after first temper.

copy-right 1992, American Society of Testing and Materials.

so as to develop acceptable corrosion resistance, or (2) As cast if corrosion resistance is acceptable.

† For low ferrite or nonmagnetic castings of this grade, the following values shall apply; tensile strength, min, 65 ksi

‡ These mechanical properties apply only when heat treatment (1) has been used.

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

Diameter of indentation, mm500-kgf load1500-kgf load3000-kgf load

materials such as glass, carbon, and mineral fibers for added strength.Thermoset plastics usually are more dimensionally stable and heatresistant and have better electrical properties than thermoplastics.Complex molded shapes of the plastics are analyzed today usingthe advanced finite-element analysis (FEA) techniques availablefor the personal computer and engineering design stations Figure12.20 shows a typical intricate plastic part that must be dimen-sionally accurate as well as chemical resistant.

Building a prototype plastic part is a compromise because the part

is usually machined from plastic blocks and slabs and will not cate the exact performance of the finished part, which is made in amold The closest duplicate to a plastic production part is made bymolding a prototype in mild-steel molds made specifically for theprototype This method is expensive but is the closest method to use

dupli-if you wish to avoid expensive rework or changes to a finished duction mold This approach will give the most accurate test resultsprior to building the actual production mold

12.5.2 Properties and characteristics

of modern plastics

Widely used plastics are discussed in Chap 4, “Materials and TheirUses.” The applications of the modern plastics are given there, aswell as the trade names and families of all plastics manufacturedtoday

Final selection of plastic type and part configuration or design should

be reviewed and coordinated with the mold maker and plastic partmanufacturer before the final design drawings are made Moldmakers and molded-part manufacturers can alert the designer tothe many problem areas that are prevalent on preliminary designsfor plastic parts

Plastic part design handbooks are available from all the leadingproducers of plastic materials, such as DuPont, General Electric,Monsanto, etc These manuals or handbooks cover detail design,appropriate calculation techniques, and complete chemical, physical,and electrical properties of the materials The design handbooksmay be secured by writing directly to the plastics sections of thelarge suppliers or their distributors

A great many machines are made for molding plastic parts, most ofwhich are expensive and require specialized techniques for opera-tion Figures 12.21 and 12.22 show a typical group of machinesrequired for producing parts made of cycloaliphatic epoxy This class

of thermoset plastic has gained wide recognition in the electricalpower distribution industry for parts that support or brace high-voltage current-carrying busses and parts of switching devices such

as breakers and switches of all classes up to 34.5 kV Figure 12.21shows the mixing and dispensing machine, and Fig 12.22 showsthe complete group of equipment, with the mold-clamping machine

at the front of the photograph

Because of the high injection pressures developed in this plasticmolding process, the clamping machine must exert a high load onthe mold to prevent it from opening during the injection process Inthis process, the hot, molten plastic is injected under pressure intothe mold, where it is held for the “curing” or setting time prior tobeing released from the mold In the molding of cycloaliphatic epoxyafter the proper cure, the plastic part is removed from the mold,

Figure 12.21 Mixing and dispensing sections of epoxy casting equipment.

(center) die-clamping machine, (far right) dispensing section, (center)

back-ground process control panel (CNC)

placed in an oven set to a preselected temperature, and baked for apredetermined time interval Cycloaliphatic epoxy is being used inthe electrical industries as a substitute for wet-process porcelain,which for many years was one of the few materials available for thistype of service Glass and polyester-glass thermosets are also used

in electrical applications, as are other electrical-grade epoxies

The tonnage and size of the clamping machine for injection ing plastics are determined by the volume of plastic to be injected

mold-at one time (a single shot) A machine thmold-at handles parts up to 20

in3 would be considered of moderate size, whereas a machinerequired to injection mold parts with a volume of 60 in3would beconsidered large Clamping machines are made in sizes up to hun-dreds of tons of clamping capacity, and these machines usually arefound at the larger plastic molding manufacturers

Aluminum alloys are particularly well suited for the extrusionprocesses Very large, high-tonnage extrusion presses are requiredfor the extrusion of aluminum and its alloys Generally, two classes

or grades of aluminum extrusions are widely available: structuralaluminum and architectural aluminum shapes These two classes

of extrusions are available in the following alloys and tempers:

Class Shape Alloy, temper, and specification

Aluminum alloy extrusions are also available in the followingstandard shapes:

A table of distances across flats for squares, hexagons, andoctagons is given in Fig 12.24 This will prove useful and time sav-ing when these dimensions need to be known and you do not wish

to calculate them by using the figures shown in Chap 2 of this

Handbook.

avail-able from the plastic manufacturers, whose catalogs may be obtained

through such trade magazines as The American Machinist, Machine Design, Product Design and Development, Modern Machine Shop,

and others If you work with or need plastic-extruded shapes, keep a

798 Chapter Twelve

Figure 12.24 Dimensions of squares, hexagons, and octagons.

series of these catalogs in your reference files The plastic-extrudedshapes not only make the design job easier, but they also enhance theappearance of the final product and facilitate assembly in many cases.

12.7 Powder-Metal Technology

Powder-metal parts are made by compressing a highly purifiedmetallic powder in a set of dies under extremely high pressure andthen fusing the particles in an oven under controlled high temper-ature This compression process compacts the powder metal untilthe part is approximately 90 percent the density of the solid metal

or alloy The density of the part can be controlled to an extent thatthe open pores in the powder-metal part, after fusion, may be impreg-nated with lubricants or filler resins and binders The well-knownsintered-bronze journal bearings with impregnated lubrication areprime examples of powder-metal technology

Powder-metal parts may be made of aluminum alloys, copperalloys, steels, and stainless steels Other metals and alloys also can

be used in powder-metal processing

The modern powder-metal part may be impregnated with resinsand binders, which make possible the application of electroplatedfinishes such as copper, zinc, nickel, and chromium If the part ismade of one of the corrosion-resistant stainless steels, the platingprocess may be eliminated

Parts may be produced using powder metal that normally would

be difficult to machine An example of powder-metal parts is shown

in Fig 12.25 Here are shown a group of metal parts, most of whichwould be difficult to make using other processes The parts showncould be made using the investment casting process or machiningtechniques but would be costly and time-consuming to produce.Figure 12.26 is a closer view of the three small three-pointed partsthat shows a minute hole in the center of the parts Each division

on the scale shown in Fig 12.26 is equal to 0.020 in (1/50 inch) One

of the parts shown in Fig 12.26 had secondary machining operationsperformed on it (threading) All the parts shown in both figures areeasy and economical to produce using powder-metal technology

powder-metal parts are not difficult if the basic rules of powder-metalpart design are followed and if the powder-metal part manufacturer

is consulted during the design stages The part manufacturer willadvise you if the part can be produced as designed and what remedial

Figure 12.27 Typical properties of powder-metal parts.

actions to take or design changes are needed to produce the part Themanufacturer also will advise you of the availability of the differentmetals and alloys for producing the part.

Basic rules for powder-metal part design

■ Keep the outline elements of the part on lines parallel to the partaxis or in the direction of compression of the male die

■ Do not design parts with reverse angles along the axis (the partcannot be extracted from the dies if this occurs)

■ Keep wall sections or webs as thick as possible

■ Single tapered holes in the part are possible when the direction

of taper allows extraction from the dies after compression of thepowder metal

■ Limit the size or volume of the part to that which may be producedwith available machinery

■ Do not specify electroplated finishes unless necessary (the partmay be produced in a corrosion-resistant alloy if necessary)

■ Holes in the part may be controlled to close tolerances

■ Check with the part manufacturer to ascertain if the part cansustain the imposed stress loads anticipated

■ The outline accuracy of parts can be closely controlled

Some of the powder-metal part producers can provide designmanuals or brochures to the design engineer that outline in moredetail the design procedures for powder-metal parts

Small-part design always should be reviewed to see if the designrequirements can be met using powder-metal technology Powder-metal technology is much further advanced today than in the past;then it was used mainly to produce journal bearings with impreg-nated lubrication

Powder-metal technology has advanced greatly over the past 10years Although powder-metal parts have been in use for many years

in the form of sintered bronze bearings, the technology now is able toproduce parts with a drastic increase in compositions as well asstrengths Figure 12.27 (see previous page) shows some of the types

of materials now available through the powder-metal processes andtheir relative strengths for use in various types of parts.

Standards for powder metal parts are available from the MetalPowder Industries Federation (MPIF) MPIF Standard 35 lists thecompositions and physical properties of available powder metals

the left side are not acceptable; parts shown on the right side embody gooddesign and are acceptable

For those interested in obtaining the full standards listings, theaddress of the MPIF is shown in Chap 16.

The design of powder-metal parts is limited by the process usedfor compression of the different powder metals Some examples ofdesign considerations are shown in Fig 12.28 The figures on theleft are not recommended; the figures on the right are recommended.Notice that generous radiuses are required for powder-metal partsdesign For more detailed information on the design of powder-metal parts, consult powder-metal manufacturers or the MPIF

Plating Practices and Finishes for Metal

Commercial products and equipment in all classifications requirefinishes of one type or other These finishes range from basic oxidecoatings to the various paints and plastics to electrodeposited metalssuch as copper, chromium, nickel, etc

The finish or plating used on any particular part should contribute

to the engineering qualities of the final product and not merely toits cosmetic appeal The desired finish could include weather pro-tection, resistance to corrosive chemicals, heat resistance, electricalconductivity, wear resistance, and improved lubrication qualities

It is the design engineer’s responsibility to specify the finishcharacteristics and specifications on a part or assembly Designersshould be aware of the types of finishes and plating processes thatare available commercially and how to specify them on the designand detail part drawings Arbitrary selection of a finish or platingand its thickness range can lead to many design problems relating

to corrosion, cost, and dimensional interference

This chapter will familiarize the designer, engineer, and otherpersonnel in the metalworking industries with the common finish-ing processes, procedures for specifying thicknesses of platings,and the appropriate industrial standard specifications that controlthese finishes

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