Mold Designs

Developments. A variety of process modifications aimed at improving surface finish or weld line integrity have been described. They include gas assisted, co-injection, fusible core, multiple Hve feed, and push—pull injection mol ding (46,47). An important development includes computer-aided design (CAD) methods, wherein a proposed mold design is simulated by a computer and the melt flow through it is analy2ed (48). 

MoldShrinka.ge. Mold shrinkage of unreinforced crystalline plastics is high. This is critical in part and mold design, especially for large... 

MOLDEST Provides product design, mold design, and injection molding process control by Fujitsu Ltd., Tokyo, Japan. 

Many injection molded products will influence the final product s performance, dimensions, and other characteristics. The mold includes the cavity shape, gating, parting line, vents, undercuts, ribs, hinges, and so on (Table 3-17). The mold designer must take all these factors into account to eliminate problems. At times, to provide the best design... 

As reviewed in other chapters, different plastics have different melt and flow characteristics. What is used in a mold design for a specific material may thus require a completely different type of mold for another material. These two materials might, for instance, be of the same plastic but use different proportions of additives and reinforcements. This situation is no different than that of other materials like steel, ceramics, and aluminum. Each material will require its own cavity shapes and possibly have its own runner system. 

These possible sources of problems in a molded part should be marked on the product drawing and explained to the mold designer for corrective action or creating an awareness of possible product defects. This is a necessary step in the chain of events in which the aim is to produce a tool that will provide useful products. Even if the mold s design, workmanship, and operation are carried out to the highest degree of quality, they cannot overcome a built-in weakness due to the product design. 

Undercut Whether external or internal, undercuts should be avoided if possible to reduce mold cost (by about 25 to 30%) and simplify melt flow during molding. However many molds use external and/or internal undercuts. In cases where it is essential to incorporate them in a design, appropriate mold design is required. The mold will include action such as sliding components on tapered surfaces, split cavity cam actions to produce the needed undercut, etc. (Chapter 8, TOOLING)... 

Thin to large wall Designing around TP problems is the joint responsibility of the product and mold designers. For example, one way to handle the problem of thin to large area walls is by the inclusion of long ribs into the product in the direction of plastic flow. These ribs are not a functional requirement of the product but they act as auxiliary runners attached to the product to facilitate plastic flow in difficult to fill areas. In some instances the ribs may be used as a surface decoration like a corrugation or they may be on the concealed side of the product where they are stiffeners. 

Jetting Jetting is a condition that results when the mold design has no immediate impediment to flow and the plastics is ejected into a relatively large open volume. This jetted material becomes a weak point on the product and a surface blemish that is difficult to conceal. 

By contrast with ICM, a compression mold design is used where male plug fits into a female cavity rather than the usual flat surface parting line mold halves for IM (Fig. 8-27). The melt is injected into the cavity as a short shot thereby not filling the cavity. The melt in the cavity is literally stress-free it is literally poured into the cavity. Prior to receiving melt, the mold is slightly opened so that a closed cavity exists the male and female parts... 

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