Plastic Product Material and Process Selection Handbook Part 16 ppsx

5 1 0 Plastic Product Material and Process Selection Handbook wettability, increasing its critical surface tension.. Also used is the injection-compression cycle where after a prcforming

Because the pressures of injection arc low at approximately 25 to 50 psi (172 to 345 kPa) very fragile inserts can be molded and mold wear is at

a minimum Some formulations for L I M also may be molded at temperatures as low as 200F (93C) which permit the encapsulation of some heat-sensitive electronic components that do not lend themselves

to encapsulation at conventional transfer molding temperatures of 300F (149C) or higher

Vacuum Assisted LIM

The vacuum assisted liquid molding process has been used for the manufacture of large composite parts In this process, a preform is placed in an open mold and a plastic vacuum bag placed on top of the mold A vacuum is created in the mold using a vacuum pump A resin source is connected to the mold As vacuum is drawn through the mold, resin infuses into the preform Application includes the fabri- cation of large products with complex geometry such as panels of all- composite buses, railroad cars, and vehicle components

Impregnation

This method has been popular impregnating liquid plastic in products such as electrical coils and transformers The liquid plastic is forced by pressure, vacuum, or their combination into the interstices of the component A related process is trickle impregnation It uses reactive (polymcrizable) plastics with a low viscosity, first catalyzing them followed with dripping them onto a transformer coil or similar device with small openings (Chapter 1) Capillary action draws the liquid into the openings at a rate slow enough to allow escape of the air displaced

by the liquid When the device is fully impregnated exposing it to heat cures the plastic system

Chemical etching

This is the exposure of certain plastic surfaces to a solution of reactive chemical compounds Solutions are oxidizing chemicals, such as sulfuric and chromic acids, or metallic sodium in naphthalene and tetra- hydydrofuran solutions Such solutions arc highly corrosive; thus, require special handling and disposal procedures This treatment causes

a chemical surface change, such as oxidation, thereby improving surface

5 1 0 Plastic Product Material and Process Selection Handbook

wettability, increasing its critical surface tension It may also remove some material, introducing a micro-roughness to the surface

Chemical etching requires immersion of the part into a bath for a period of time, then rinsing and drying This process is more expensive than most other surface treatments, such as flame treatment, thus it is used when other methods are not sufficiently effective Fluoroplastics arc often etched chemically because they do not respond to other treatments, ABS are usually etched for metallic plating, and so on

Twin screw injection molding extruder

Glass fiber reinforcements are added to plastics in order to improve mechanical and physical properties of the plastic The traditional route

to producing fiber reinforcement involves blending the fibers into plastic in a twin-screw extruder followed by pelletization (Chapter 5) The pellets are then molded using an injection molding machine (IMM) to form the fabricated products (Chapter 4) This action results

Melt compression molding

Melt compression molding identifies in-mold laminating and in-line molding of carriers, decorations, etc The basic technique has been used for over a century There has been an increased application of textile cover stock and leather substitutes both preferably with a soft touch This type development was primarily initiated by the automotive industry with the objective to be prepared for future trends Other industries such as furniture and packaging manufacturers use this process

Different methods arc used such as back injection including the injection-compression molding and mclt flow compression molding

Mold design is a decisive factor for the molding success such as dimensioning and location of the sprue gates, dimensioning of shear edges, flow aids, cooling and ejector techniques, etc

With backpressure the process is performed in conventional injection molding machines (IMMs) (Chapter 4) The cover stock is inserted and located in an open mold A shear edge mold permits draw-in of the cover stock during the closing cycle to avoid wrinkles and damage by stretching of the fabric Molds require special attention They generally use a hot runner system with its shut-off nozzle(s) All mold elements such as ejector, core pulls, and slides have to be on the injection side mold half

Also used is the injection-compression cycle where after a prcforming stroke for the cover stock, the carrier material is injected in a partially open mold (Chapter 4) By closing the gap the part is formed and laminated The mold corresponds to a back injection mold The method has similarities with melt flow compression molding

Melt flow molding is performed on vertical clamping IMMs The cover stock is inserted into an open mold followed with the mold partially closed The carrier stock is injected from below through a hot runner system and several gates with actuated control needle shut-off nozzles The final melt shot from the gates is compression formed into the part

by closing the remaining mold gap Shear cdgc molds with hot runner systems similar to those for back injection arc used

Back compression is a process based on compression molding (Chapter 14) of a melt strip deposited in an open mold It describes the process during which a cover stock cutting is placed on a melt strip for simultaneous compression molding and lamination of parts Melt strip deposition also includes fiber reinforced thermoplastic stock with subsequent compression molding of non-laminated structural parts

or they are used in secondary operations such as cutting dies, stamping sheet dies, etc These tools fabricate or shape products In this chapter injection molds and extrusion dies are primarily reviewed because they represent over 95% of all tools made for the plastic industry This chapter also includes information applicable to other molds and dies used in the other processes; some of the other chapters too provide information applicable to their tools

Mold and die tools are used in processing many different materials with many of them having common assembly and operating parts (pre- engineered since the 1940s) with the target to have the tool's opening

or cavity designed to form desired final shapes and sizes They can comprise of many moving parts requiring high quality metals and precision machining 3~ As an example with certain processes to capitalize on advantages, molds may incorporate many cavities, adding further to its complexity Most tools have to be handled very carefully and must be properly maintained to ensure their proper operation They are generally very expensive and can be very sophisticated 31~ Tools of all types can represent upward to one-third of the companies manufacturing investment 282 Metals, specifically steels, are the most common materials of construction for the rigid parts of tools Some mold and die tools cost more than the primary processing machinery with the

most common approaching half the cost of the primary machine About

5 to 15% of tool costs are for the material used in their manufacture, design about 5 to 10%, tool building hours about 50 to 70%, and profit

at about 5 to 15%

There are standards for materials of construction such as those from the American Iron and Steel Institute (AISI) and German Werkstoff The proper choice of materials for their cavities (openings) is paramount to quality, performance, and longevity (number or length of products to

be processed) of tools Desirable properties are good machinability of component metal parts, material that will accept the desired finish (polished, etc.), ability with most molds or dies to transfer heat rapidly and evenly, capability of sustained production without constant main- tenance, etc (Table 17.1) As the technology of tool enhancements continues to evolve, tool manufacturers have increasingly turned to them to gain performance/cost advantages

There are now a wide variety of enhancement methods and suppliers, each making their own claims on the benefits of their products With so many suppliers offering so many products, the decision on which tech- nology to try can be time consuming There are toolmakers that do not have the resources to devote to a detailed study of all of these options

In many cases they treat tools with methods that have worked for them

in the past, even though the current application may have different demands and newer methods have been developed What can help is to determine what capabilities and features are needed such as hardness, corrosion resistance, lubricity, thermal conductivity, thermal expansion, polishing, coating, and repairing This type of information is available

on hard copies and software 452, 4s3

There are many tool metals such as D2 steel that are occasionally used

in their natural state (soft) when their carbon content is 1.40 to 1.60wt% Tool metals such as P20 are generally used in a pre- toughened state (not fully hardened)

By increasing hardness longer tool life can often be achieved Increased wear properties are especially critical when fabricating with abrasive glass- and mineral-reinforced plastics This is important in high-volume applications and high-wear surfaces such as mold gates inserts and die orifices Some plastic materials release corrosive chemicals as a natural byproduct during fabrication For example hydrochloric ( H C I ) acid is released during the tooling of PVC These chemicals can cause pitting and erosion of untreated tools' surfaces Mso, untreated surfaces may rust and oxidize from water in the plastic and humidity and other contaminants in the air

Table 1 7.1 Ex~rroles cf th~ properties,~f different :oc-I materials

Cescr~Jon Pc Temp ('F} Te~'T~ ('F} Treatab~'~ S~e.r~g~ Resistance Resistance Toughness Machinability PoCishabitit~ Weldabitity Conductivity

¢

0 "

0

Polishing and coating tools permit meeting product surface require- ments Improved release characteristics of fabricated products are a common advantage of tool coatings and surface treatments 3 This can

be critical in applications with long cores, low draft angles, or plastics that tend to stick on hot steel in hard-to-cool areas Coatings developed

to meet this need may contain PTFE (Chapter 2) Mso used are metals such as chrome, tungsten, or clcctroless nickel that provide inherent lubricity

Material of construction

Materials of construction can be of a simple design made from wood such as generally used in RP bag molding (Chapter 15) For the more sophisticated processes such as injection molding, extrusion, and blow molding (Chapters 4: to 6) it can comprise of many parts requiring high quality metals and precision metal machining

The choices range from computer-generated tools that use specialty alloys or pure carbide tooling usually made from steels Everyone from purchasing agents to shop personnel must consider the ramifications of tool performance requirements One may consider the sorest tool that will do the job because it is usually the least expensive to build but requires special/careful handling with limited life

Different materials of construction principally use different grades of steels; others include types such as aluminum, beryllium copper alloy, brass, ldrksitc, sintercd metal, steel powdered filled epoxy plastic, silicone, metal spray, porous metal, plaster of Paris, reinforced plastic, sand, wood, and flexible plastic Commonly used is P20 steel, a high grade of forged tool steel relatively free of defects and it is available in a prehardened steel

It can be textured or polished to almost any desired finish and it is a tough mold material H-13 is usually the next most popular mold steel used Stainless steel, such as 420 SS, is the best choice for optimum polishing and corrosion resistance Other steels and materials are also used to meet specific requirements in mold life and cost The choice of steel is often limited by the available sizes of blocks or plates that arc required for the large molds 3,163,278,299,309,317

Somc of thc tool materials incorporatc different special metals pro- viding improvements in heat transfer, wear resistance of mating mold halves, etc These special metals include beryllium copper alloy, brass, aluminum, kirksitc, and sintered metal

51 6 Plastic Product Material and Process Selection Handbook

Photochemical machining has several distinct advantages over these other processes Low tooling costs associated with the photographic process, quick turnaround times, and the intricacy of the designs that can be achieved by the process are some of the advantages, as are high productivity and the ability to manufacture burr-free and stress-free parts Of paramount importance in using this process are the cost savings associated with generating prototypes

The advantages of using fully hardening tool steels rather than case- hardening steels for the manufacture of tools, arc primarily the simpler heat treatment and the possibility of making corrections to the cavity at

a later time without a new heat treatment However, the greater risk of cracking is a disadvantage, particularly for tools with a larger cavity depth, because tools from these steels do not have a tough core More- over, the tougher steels with a carbon content of about 0.4% do not attain the high surface hardness of about 60 HRC which is desirable with respect to wear and polish

Sometimes the mechanical action of the tool may require certain steel selections so as to permit steel on steel sliding without galling Tooling surfaces of precision optics will need steel that can be polished to a mirror finish If the inserts will receive coatings to further enhance performance, then steel characteristics to receive coating or endure a coating process must be considered (coating application temperature

vs tempering temperature) H o t runner mold components often use hot work steel because of their superior properties at elevated temperatures Very large molds a n d / o r short run molds may use pre- hardened steel (270 to 350 Brinell) to eliminate the need for additional heat treatment

When tool steels of high hardness are used they arc supplied in the soft annealed condition (hardened mold inserts for cores, cavities, other molding surfaces and gibs, wedge locks, etc are typically hardened to a

range of 48 to 62 RC They arc then rough machined, stress relieved, finish machined and go to heat treatment for hardening and tempering

to desired hardness After this heat treatment, the core or cavity typically must then be finish ground a n d / o r polished In some applications, there will be additional coatings or textures to further treat the tool surfaces

When processing particularly highly abrasive plastics, the wear can still

be too high even when using high-carbon, high-chromium steels Metallurgical melting cannot produce steels with even higher amounts

of carbides In such cases hard material alloys, produced by powder metallurgy, are available as a tool material These alloys contain about 33wt% of titanium carbide, which offers high wear resistance because of its very high hardness

Like other tool steels, hard material alloys are supplied in the soft- annealed condition where they can be machined After the subsequent heat treatment, which should if possible be carried out in vacuum- hardening furnaces, the hard materials attain a hardness of about 70

H R C Because of the high carbide content dimensional changes after the heat treatment are only about half as great as those in steels produced by the metallurgical melting processes

In machining as well as in non-cutting shaping processes stresses develop chiefly as a result of the solidification of surface layers near the edge These stresses may already exceed the yield point of the respective material at room temperature and consequently lead to metallic plastic deformations Since the yield point decreases with increasing temper- ature additional stresses can be relieved by plastic deformation during the subsequent heat treatment In order to avoid unnecessary, ex- pensive remachining it is advisable to eliminate these stresses by stress- relief annealing

Electric-discharge machining (EDM), also called spark erosion, is a method involving electrical discharges between graphite or copper anode and a cathode of tool steel or other tooling material in a dielectric medium The discharges are controlled in such a way that erosion of the workpiece takes place developing the required contours The positively charged ions strike the cathode so that the temperature

in the outermost layer of the steel rises so high as to cause the steel layer

to melt or vaporize, forming tiny drops of molten metal that are flushed out as chippings into the dielectric

E D M is a widely utilized method of producing cavity and core stock removal Electrodes fabricated from materials that are electrically conductive are turned, milled, ground, and developed in a large variety

51 8 Plastic Product Material and Process Selection Handbook

of shapes, which duplicate the configuration of the stock to be removed The electrode materials include graphite, copper, tungsten, copper-tungsten, and other electrically conductive materials Special forms of E D M can now be used to polish tool cavities, produce under- cuts, and make conical holes from cylindrical electrodes

The electroforming process is used for the production of single or low numbers of cavities, as opposed to others requiring many cavities The process deposits metal on a master in a plating bath Many proprietary processes exist The master can be constructed of such materials as plastic, reinforced plastic, plaster, or concrete that is coated with silver

to provide a conductive coating The coated master is placed in a plating tank and nickel or nickel-cobalt is deposited to the desired thickness of

up to about 0.64 cm (0.25 in.) With this method, a hardness of up to

46 RC is obtainable To reinforce the nickel shell it is backed up with different materials (copper, plastic, etc.) to meet different applications A sufficient thickness of copper allows for machining a flat surface to enable the cavity to be mounted into a cavity pocket

Tooling surfaces such as mold cavities and die openings require meeting certain surface finishes Rather than identifying the required finish as dull, vapor-honed satin, shiny, etc., there are standards such as a diamond polishing compound, SPI (originally SPI/SPE) Mold Standard Finish, and American Association's standard B46.1 Surface Texture (extremely accurate surface measurements; a near-perfect system) that are used This ASA B46.1 corresponds to the Canadian standard CSA B 95 and British standard BS 1134

A general requirement for all tools is that they have a high polish where the plastic melt contacts the tools 316, 317 Other parts of the tools may require a degree of polishing (smooth) permitting parts to fit with precision and eliminating melt leaks in the tools A large part of tool cost is polishing, which can represent from 5 to 30% of the tool cost Polishing can damage the tool material unless it is properly done An example of a c o m m o n defect is orange-peel It is a surface wa W effect that results when the metal is stretched beyond its yield point by over polishing and takes a permanent set Further polishing will only make matters worse with small particles breaking away from the surface The harder the steel, the higher the yield point and therefore the less chance

of orange-peel Hard carburized or nitrided surfaces are much less prone to this problem To avoid orange-peel, polish the tool by hand With powered polishing equipment, it is easier to exceed the yield point

of the metal If power polishing is done, use light passes to avoid over- stressing

Protective coating/plating

There is a distinction between platings and coatings Generally, thin layers

of metals applied to the surface of tool components are considered platings The application of alloys, fluorocarbons, or fluoropolymers [such as PTFE) (polytetrafluoroethylcne (Chapter 2)], or dry lubricants is considered a coating With few exceptions, treatments involve processes and chemicals that should not be used anywhere near a fabricating machine (because of corrosiveness), and they are best handled by custom plating and treating shops that specialize in their use

Tool coatings/platings are typically used to enhance tool performance

in one or more of the following areas" wear resistance, corrosion resistance, improved tool release, resizes components, a n d / o r their combination No single treatment is ideal for solving all these problems Treatments are used that resist the corrosion damage inflicted by chemicals such as hydrochloric acid when processing PVC, formic acid

or formaldehyde with acetals, and oxidation caused by interaction between tools and moisture in the plant atmosphere Release problems require treatments that decrease friction and increase lubricity in mold cavities 3

Tools can be subjected to sweating and moisture condensation particularly during the summer months This can lead to corrosion and rust, and in turn, to poor finishes and inferior quality fabricated products By keeping the air in the plant or around the tool dry, you can not only eliminate rust but also improve product quality and increase your production rate

Tool wear cannot be prevented This wear should be observed, acknowledged during maintenance check-up, and dealt with at intervals

in the tool's useful life; otherwise, the tool could be allowed to wear past the point of economical repair Periodic checks of how platings and coatings are holding up will allow the fabricator to have a tool resurfaced before damage is done to the tool A poorly finished tool that is being used for the first time, its heat, pressure, and exposure to plastic are actually reworking its surface Fragmented metal is pulled out

of the metal fissures, and plastic forced into them While the fissures are plugged with plastic, the fabricator may actually be processing plastic against plastic

Starting up a tool that has a poor finish can damage the tool without proper presurfacing If the tool surface is unsound (no prior treatment was used although required), a thin layer of metal plating, particularly chrome plating, will not make it correct A poorly prepared surface

5 2 0 Plastic Product Material and Process Selection Handbook

makes for poor adhesion between treatment and the base metal The effectiveness of a surface treatment depends on not only the material being applied, but also the process by which it is applied For any plating or coating to adhere to the surface of a tool component, it has

to bond to the surface The bonding may bc relatively superficial, or a chemical/molecular bond may accomplish it The nature and strength

of the bond directly affect the endurance and wear characteristics of the plating or coating The experience of the plater is an important factor in applications where cut-and-dried or standard procedures have not been developed.453,483

Mold

Following the product design, a relevant tool (mold or die) needs to be produced Figures 17.1 and 17.2 provides an introduction to layouts, configurations, and actions of molds (Chapter 4) Alignment of mold halves during their opening and closing actions requires precision mold parts to fabricate quality parts When possible mold cavity walls are tapered to permit ease of separating molded parts from the cavity Operations of tools vary from fabricating solid to foamed products such

Mold Runner - 1 -

G a l e ~ Cavity

Plasticator Hopper

l//L////J~ =Front Clamping Plate

~ ~ ~ l l ~ ~ ~,~ -F,ont Car Retainer PL

~NXIL~ I ~ I~ ~,N] ~ / ~ W a t e t Ch Is

~.,

i

[Figure 1 7.2 Sequence of mold operations

as using a steam chest for producing expandable polystyrene foams (Chapter 8 )

There are approaches to simplifying mold design and its action (Figure 17.3) There are different approaches used to mold threaded parts such

as bottle caps, medical components, mechanical and electrical connectors, etc To date most of these molds use mechanical a n d / o r hydraulic (toothed racks, spur-type gears, etc.) unscrewing drive systems To meet more precise dimensions, more compactness, faster cycle, no oil con- tamination, and save space unscrewing cores are driven electrically with servomotors These Programmable Electric Rotating Core (PERC) systems use small motors mounted on the mold 322

A mold is an efficient heat exchanger If not properly designed, handled, and maintained, it will not be an efficient operating device

H o t melt, under pressure, moves rapidly through the mold Air is released from the mold cavity(s) to prevent the melt from burning, prevent voids in the product, a n d / o r prevent other defects including the molded products service operating performances 3 In order to solidify the TP, hot melt water or some other media circulates in the mold to remove heat from TPs or higher heat is used with TSs

The melt flow is largely governed by the shape and dimensions of the product and the location and size of the gate(s) A good flow will ensure uniform mold filling and prevent the formation of layers Jetting

of the plastic into the mold cavity may give rise to surface defects, flow lines, variations in structure, and air entrapment This flow effect may occur if a fairly large cavity is filled through a narrow gate, especially if a plastic of low melt viscosity is used 487

522 Plastic Product Material and Process Selection Handbook

Figure 17.3 Examples to simplify mold design and action

The hot TP melt entering the cavity solidifies immediately upon contact with the relatively colder cavity wall The solid outer layer thus formed will remain in situ and forms basically a tube through which the melt flows on to fill the rest of the cavity This accounts for the fact that a rough cavity wall adds only marginally to flow resistance during mold filling Practice has shown that only very rough cavity walls (sandblasted surfaces) add considerably to flow rcsistancc 487

The principal types arc two-plate, three-plate, and stack molds Others include the family mold that has multiple cavities of different shapes in one mold 3, 324 A further distinction concerns the feed system that can

be either the cold or hot type These classifications overlap Three-plate molds will usually have a cold runner feed system, and a stack mold will have a hot runner system Two-plate molds can have either feed system

are known as the fixed or injection half that is attached to the machine fixed platen, and the moving or ejection half that is attached to the moving platen 3 This is the simplest type of injection mold and can be adapted to almost any type of molding The cavities and cores that define the shape of the molding are so arranged that when the mold opens at the parting line (PL), the molding remains on the ejection half

of the mold 325 In the simplest case, this is determined by shrinkage that causes the molding to grip on the core Sometimes it may be necessary to adopt positive measures such as undercut features or cavity air blast to ensure that the molding remains in the ejection half of the mold

machine clamp opens (Figure 17.4) As well as the fixed and moving parts equating to the 2-plate mold there is an intermediate floating cavity plate The feed system is housed between the fixed injection half and the floating cavity plate When the mold opens it is extracted from the first daylight formed by these plates parting The cavity and core is housed between the other side of the floating cavity plate and the moving ejection part of the mold Moldings are extracted from the

5 2 4 Plastic Product Material and Process Selection Handbook

second daylight when these plates separate The mold needs separate ejection systems for the feed system and the moldings Motive power and opening time for the feed system ejector and the movement of the floating cavity plate is derived from the clamp-opening stroke position

by a variety of linkage devices

The 3-plate mold is normally used when it is necessary to inject multiple cavities in central rather than edge positions a n d / o r to increase the production rate This is done for flow reasons, to avoid gas traps, ovality caused by differential shrinkage, or core deflection caused by unbalanced flow This type of mold also has the advantage of auto- matically removing (degating) the feed system from the molding The disadvantages are that the volume of the feed system is greater than that

of a 2-plate mold for the same component, and that the mold construction

is more complicated and costly

Stack mold also features two or more daylights in the open position

Two daylights are the normal form (Figure 17.5) but up to four are also used; more could be used The purpose of the stack mold is to increase the number of cavities in the mold without increasing the projected area so the clamp force required from the IMM remains the same Similar cavities and cores between each of the daylights are normally used

Figure 17.5 Examples of stacked molds

PL PL

Microscale mold fabrication continues to expand its capability from recent developments in electronic signal sensing, part measurement, and process control These improvements allow m o l d makers to pro- duce molds with extremely small cavities and hold tolerances of _+10 n m while cutting mold steel (Chapter 4).46, 150,444

To make tiny features, mold makers can use an unconventional tech- nique such as reactive ion etching, developed at the Georgia Institute of Technology in Atlanta The mold makers use reactive ions to knock metal atoms out of a mold surface Mold makers can use lasers to create extremely small features such as small holes that can not be made with a conventional electronic discharge machine (EDM)

New technologies in manufacturing micromolds continue As an example there is the LIGA It is a lithography/electroplating technique developed in Germany Companies are producing LIGA structures that could be converted into molding cavities This technology allows molds

to be minuscule To date high-volume molding operations using 64- cavity molds limits control of the individual cavities so the parts are not the same Use is made of two- or four-cavity molds that produce more identical parts

nozzle of the I M M and the mold cavity (Figure 17.1) This feature has

a considerable effect on both the quality and economy of the molding process The fccd system must conduct the plastics melt to the cavity via a sprue, runner, andgate at the correct t e m p e r a t u r e / p r e s s u r e / t i m e period, must not impose an excessive pressure drop or shear input, and should not result in non-uniform conditions at the cavities of multi- impression molds

The feed system is an unwanted by-product of the molding process, so a further requirement is to keep the mass of the feed system at a m i n i m u m

to reduce the amount of plastic used This last consideration is a major point of difference between cold and hot runner systems The cold runner feed system is maintained at the same temperature as the rest of the mold

In other words, it is cold with respect to the melt temperature The cold runner solidifies along with the molding and is ejected with it as a waste product in every cycle The hot runner system is maintained at melt temperature as a separate thermal system within the cool mold Plastic material within the hot runner system remains as a melt t h r o u g h o u t the cycle, and is eventually used on the next cycle Consequently, there is little or no feed system waste with a hot runner system Effectively, a hot runner system moves the melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3, 32,326-332,490