Plastic Product Material and Process Selection Handbook Part 16 ppsx

16. Other processes 509 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 LIM 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 510 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 in fiber attrition. The twin-screw injection molding extruder is an injection molding machine that is capable of both blending/compounding and extrusion in one step. Because it is a one step process, the fibers never go through the entire extrusion process as well as the pelletization that limits the fiber size, but are blended into the molten plastic before injection. The screw part of this machine is based on a non-intermeshing, counter- rotating twin-screw extruder (Chapter 5). One of the screws in this machine is capable of axial movement and has a non-return valve on the end. This action enables the screw to inject and mold parts. 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. 16 9 Other processes 51 1 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. MOLD AND DI E TOOLI NG Overview When processing plastics some type of tooling is usually required. Tools include molds, dies, mandrels, jigs, fixtures, punch dies, perforated forms, etc. The terms for tools are virtually synonymous in the sense that they have some type of female and/or negative cavity into or through which a molten plastic moves usually under heat and pressure 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 17 9 Mold and die tooling 513 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 (HCI) 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 AISi t~signation Hardness Hardening Tempe~nq liP.at C, ompm~sive ~ion Wear Thermal Cescr~Jon Pc Temp ('F} Te~'T~ ('F} Treatab~'~ S~e.r~g~ Resistance Resistance Toughness Machinability PoCishabitit~ Weldabitity Conductivity 4140 30-36 150{) 1200 10 4 I 2 8 6 5 4 5 P20 30-36 1600 1100 10 4 2 2 9 8 8 4 5 420SS 35-,40 1865 1;050 10 4 6 3 9 4 9 4 2 P5 59-61 1575 450 6 6 2 8 8 10 7 9 3 P8 58-60 1475 425 8 6 3 8 7 10 7 8 3 420SS 50-52 1885 480 8 8 7 6 8 7 10 6 2 440SS 56.58 1900 425 7 8 8 8 3 8 9 4 2 BECU 36 42 625 NR 7 2 6 1 I 10 9 7 9 U'I 4:= o u~ "0 ¢ f'3 t~ u,t -¢ '-r- Q,1 ¢ 0" 0 0 17 9 Mold and die tooling 515 ~ 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 Manufacturing Different conventional metal cutting methods are used to meet require- ments based on type of material used and the configuration of the tool. As an example, the process of photochemical machining (PCM) is recognized by the metalworking industry as one of several effective methods for metal parts fabrication. The technique, also called photo- etching, chemical etching, and chemical blanking, competes with stamping, laser cutting, and electric-discharge machining (EDM). It uses chemicals, rather than mechanical or electrical power or heat, to cut and blank metal. 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). Hot runner mold components often use hot work steel because of their superior properties at elevated temperatures. Very large molds and/or 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 17 9 Mold and die tooling 517 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 and/or 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 HRC. 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. EDM 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 EDM 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 common 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. [...]... the potential to burn (degradation) the plastic exiting With microprocessor-based extruders and process lines, die temperature control can easily be accomplished without discrete controllers 536 Plastic Product Material and Process Selection Handbook Microprocessor control generally results in less operator attention required, higher levels of reliability, and ease of changing groups of set points... Cubes eae O O i, LQ O"1 ~0 ~d 538 Plastic Product Material and Process Selection Handbook 1 flange fitting with a clamp ring on the barrel and a fixed flange on the die, 2 flanges on the barrel and die with tapered links and two bolted halfclamps, or a ring clamp hinged at one side and bolted to the other side, and 3 swing-bolt flange connection between the barrel flange and a die flange Flat Die The flat... different pipe die inline and crosshead designs 542 Plastic Product Material and Process Selection Handbook Foam Die Spider dies are used to a large extent because of their low cost and for many applications in thermoforming, the spider lines can be aligned with edges and center material, which is trimmed and recycled Spiral dies are used when spider marks are unacceptable Its center mandrel normally is adjusted... moves the melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3, 32,326-332,490 526 Plastic Product Material and Process Selection Handbook In the past perhaps the least-understood and least well applied factor is the inclusion of cooling channels to meet proper heat transfer from the plastic melt to the cooling liquid (for thermoplastics) Usually, insufficient... right job To 528 Plastic Product Material and Process Selection Handbook assist, the Moldmakers of the SPI provides industry with an updated directory of its members and their special capabilities The SPI Moldmaker members are in constant contact with the plastics industry and its ever-changing technology The directory lists moldmakers as contract or custom services and in turn by type of process mold... certain plastics, such as PVC and PVDC, can degrade with temperature and time to produce acids that will corrode plated alloy steels High-nickel alloy steels provide good corrosion resistance without plating and simplify manufacturing, cleaning, and repair Stainless steel also is used with degradable materials Profile, pipe, blown film, and wire coating dies 530 Plastic Product Material and Process Selection. .. equipment /-;~// / C / / / ( z / / / ~ //'/'///if' '~./~/~.,~,"/~ LANDLENGTH 9 ::::;;,'::~::~- ~::::::,,i:!,'-.:'~':-:-:-:,:,:,:-:,',' 9 ':" //f//-/~////~////] Melt swell LAND LENGTH l _] Oie !baDe s' s s Part ~al:e Part ~haOe Figure 17~7 Examplesof melt flow behavior from pull roils 532 Plastic Product Material and Process Selection Handbook The approach used for shaping orifices in the dies is important... 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 520 Plastic Product Material and Process Selection Handbook makes for... symmetrical flow channels, particularly tube and pipe heads, circular rod and monofilament dies, 2 angled dies particularly crossheads and angular heads for wire and cable coveting, crossheads and offset heads for tube and pipe, and film blowing heads, 3 profile dies that include slot dies for flat film and sheet, and multiorifice heads for monofilaments; 4 dies for special products such as netting The... 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 . plating, particularly chrome plating, will not make it correct. A poorly prepared surface 520 Plastic Product Material and Process Selection Handbook makes for poor adhesion between treatment and. melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3, 32,326-332,490 526 Plastic Product Material and Process Selection Handbook In the past perhaps. thereby improving surface 510 Plastic Product Material and Process Selection Handbook wettability, increasing its critical surface tension. It may also remove some material, introducing a micro-roughness