Molds for injection molding

The locator ring helps fit the injection mold into a press clamp. The sprue bushing is a channel by which the plastic flows from the injection 

Support plate Ejector pins Ejector retainer plate Ejector plate 

FIGURE A.6 Typical mold components for injection molding (not to scale). 

FIGURE A.7 Plastic fountain flow inside cavity of injection molds. 

Key design guidelines for building injection molding molds are listed below. 

---

Figure 7.75 Schematic diagram of eight-cavity mold for injection molding operations. The molded parts of interest are indicated by the shaded regions.

BPA/DC-based composition served as a polymer matrix in carbon fiber composites [24]. Metal powder or fiber filled molding compounds were also described. The molding compounds, which can contain, moreover, numerous other components, bismaleimides (cf. Sect. 5) in particular, are used for the manufacturing of heat conductive molds for injection molding [25],... 

Most molds for injection molding are unique in design, which depends on application, fluoroelastomer compound, and feed system (hot or cold runners). Standard systems are distinguished as two plate, three plate, or stack molds [58],... 

A few considerations are listed to help understand what is needed in designing molds for injection molding. Even though there are many factors to be considered that include its operation, designing the mold is not complex. What is needed is a thorough understanding of the requirements of the literature. 

Molds are used with injection molding to form plastic resin into a shape that is depicted in the mold. Molds for injection molding are typically made from tool steel, for example, P-20, HI3, S-4. The part features are produced with the cavity (A-side) and core (B-side). Figure A.6 provides a schematic representation of common items in an injection mold. 

Manufacture of Synthetic Resin Molds for Injection Molding... 

Processing. SAN copolymers may be processed using the conventional fabrication methods of extmsion, blow mol ding, injection molding, thermoforming, and casting. Small amounts of additives, such as antioxidants, lubricants, and colorants, may also be used. Typical temperature profiles for injection mol ding and extmsion of predried SAN resins are as follows (101). 

Hexafluoropropylene—tetrafluoroethylene copolymers are available in low melt viscosity, extmsion grade, intermediate viscosity, high melt viscosity, and as dispersions. The low melt viscosity (MV) resin can be injection molded by conventional thermoplastic molding techniques. It is more suitable for injection molding than other FEP resins (51). 

At the same time, a decrease in orientation- and stress-birefringence for the modified BPA-PC could be achieved by optimizing the processing conditions (184,187,189,190,193) this lowers birefringence below 20 nm/mm (single path) for injection-printed injection molded CDs and substrate disks (Fig. 24) (191). The typical characteristics of a modified BPA-PC especially developed for CD production are Hsted in Table 6 (187,193). 

Guide for Injection Molding Technical Bulletin AP-017, Himont, Inc., Wilmington, Del., May 1992. 

Because of low injection pressure, some cost savings are possible in mold and press constmction. Mol ding cycles are somewhat longer than for injection molding. The part must be cooled in the mold long enough to be able to resist swelling from internal gas pressure. In stmctural foam parts there is almost a total absence of sink marks, even in the case of unequal section thickness. Stmctural foam has replaced wood, concrete, sohd plastics, and metals in a variety of appHcations. 

PEN film for audio- and videotape and various electronic appHcations and blow molded PEN containers for hot-fill appHcations are already being marketed in Japan. NDA is unlikely to ever become as inexpensive as terephthaUc acid but novel NDA-based polyesters will become available if a market need exists. One example could be the experimental polyester PBN (Celanese Corp.) this is the NDA analogue of PBT, poly(l,4-butylene naphthalene-2,6-dicarboxylate) [28779-82-0]. It has a high rate of crystallization, faster even than that of PBT, and its combination of physical properties is weU-suited for injection molding. 

The major use of 4-cumylphenol is as a chain terminator for polycarbonates. Its use in place of phenol gives a polycarbonate with superior properties (33). Eor a low molecular weight polycarbonate used for injection-molding appHcations, the use of 4-cumylphenol as a chain terminator significantly lowers the volatiHty of the resin. Other uses of 4-cumylphenol include the production of phenoHc resins, some of which have appHcations in the electronics industry (34). Another appHcation of 4-cumylphenol involves its reaction with ethylene oxide to form a specialty surfactant. 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

Table 16. Properties of Resin for Injection-Molding Applications ...

Extrusion Resins. Extmsion of VDC—VC copolymers is the main fabrication technique for filaments, films, rods, and tubing or pipe, and involves the same concerns for thermal degradation, streamlined flow, and noncatalytic materials of constmction as described for injection-molding resins (84,122). The plastic leaves the extmsion die in a completely amorphous condition and is maintained in this state by quenching in a water bath to about 10°C, thereby inhibiting recrystallization. In this state, the plastic is soft, weak, and pHable. If it is allowed to remain at room temperature, it hardens gradually and recrystallizes partially at a slow rate with a random crystal arrangement. Heat treatment can be used to recrystallize at controlled rates. 

Pellets for injection-molding, extmsion, and blow-molding appHcations include mineral-filled, glass-filled, pigmented, and flame-retarded grades. 

PPS resins are chiefly used for injection mol ding. The melt flow of the glass-fiUed resins is very stiff, and high injection pressures are required. Mold surface wear is heavier than for most other engineering plastics. Mol ding melt temperatures are near 330°C for optimum surface gloss and impact strength, mold temperatures of 130°C should be used. The resins are brown to brown-black. 

An alternative approach for injection molding is the use of a rotary table injection machine. This type of machine uses the multistation concept, each station having a single cavity mold generally using a point gating/ injection point as previously mentioned. Consider a rotary table machine with eight stations, station one would be the injection station and station seven would be the unload and load metal insert station. This type of process has been successfully used for different types of lip type rotary shaft seals where more expensive elastomers are used and waste needs to be kept to a minimum. 

Table 3-6 Examples of dimensional tolerances for injection molded TP products... 

An easy method for estimating shrink allowance for injection molding is as follows ... 

Note Values are typical for injection molding and most extrusion operations. Extrusion coating is done at higher temperatures (i.e., about 600 F for LDPE). 

Molds are used in many plastic processes with many of the molds having common assembly parts (Fig. 8-11). Many molds, particularly for injection molding, have been preengineered as standardized products that can be used to include cavities, different runner systems, cooling lines, unscrewing mechanisms, etc. (Table 3-17). 

Most computer-integrated systems have been developed for injection molding, since a much bigger market exists with IM. Other computer systems are available for the other processes. 

Design Considerations for Injection Molded Parts (Parting lines, draft angles, wall thickness, fillets and radii, bosses, ribs, opening formations, shrinkage, gating, vents, potential knit lines)... 

Malloy, R. A., Plastic Part Design for Injection Molding, Hanser, 1994. 

---