Packing injection molding

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. As an example, the injection mold cavity pressure profile is a parameter that is easily influenced by variations in the materials. Related to this parameter are four groups of variables that when put together influences the profile (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plastica-tor. TTius material variations may be directly related to the cavity pressure variation. Details on EQUIPMENT/PROCESSING VARIABLE are in Chapter 8. 

In 2002, the world production of polymers (not including synthetic libers and rubbers) was ca. 190 million metric tons. Of these, the combined production of poly(ethylene terephthalate), low- and high-density polyethyelene, polypropylene, poly(vinyl chloride), polystyrene, and polyurethane was 152.3 milhon metric tons [1]. These synthetic, petroleum-based polymers are used, inter alia, as engineering plastics, for packing, in the construction-, car-, truck- and food-industry. They are chemically very stable, and can be processed by injection molding, and by extrusion from the melt in a variety of forms. These attractive features, however, are associated with two main problems ... 

We can explain the injection molding process with reference to the molding cycle illustrated in Fig. 13.2. The cycle consists of four stages injection, packing, cooling, and ejection. 

As-spun fibers of Ph-HQ/HQ/BB with m/n = 50/502 exhibited a much higher modulus (100 GPa) than Ph-HQ/BB (40GPa) due to the increased F-value and rigidity of the polymer chain. Injection molded specimens of Ph-HQ/HQ/BB with m/n = 50/50 exhibited a much higher modulus than those of Ph-HQ/BB due to the increased rigidity and packing density of the polymer chain upon copolymerization with HQ. However, the F-value of injection molded specimens of Ph-HQ/HQ/BB (m/n = 50/50) was not much higher than those of Ph-HQ/BB, as shown in Table 19.4. 

Thus, both the rigidity and packing density of the polymer chain seem to be more influential factors than the F-values in achieving a high modulus of injection molded specimens. Figure 19.10 shows the variation of the flexural moduli as a function of the F-values for various substituted-HQs/BB and substituted HQs/BB modified with DHB, HQ, 2,6-dihydroxynaphthalene (DHN), NDA, Cl-PEC and TA [31,32], We could find no relationship between them. 

Thus, the influential factors in obtaining high moduli for injection molded specimens are as follows (1) the stability of the liquid crystalline state, (2) the rigidity of the polymer chain, and (3) the packing density of the polymer chain. 

A typical injection pack/hold pressure during injection molding of polyamide 66 components is 1,000 bar and the injection temperature is 280°C. The gate freezes shut when the average temperature inside the mold reaches 225°C. 

Injection-molded parts such as electrical connectors and sockets, distillation column plates and packings, valve bodies pipe and fitting linings are easily made because ETFE exhibits a low shear sensitivity and wide processing window 42... 

The part taken from the mold is, in most cases, a finished product ready to be packed and shipped or ready to be used as a part of an assembled unit. In contrast to metal forming, there is very little if any wasted material in injection molding. For cold runner TP systems most runners and sprues are reground and reused. By using hot runner molds, the sprue and runner systems remain in a melted state in the mold and become part of the next finished part (Chapter 17). The hot runners can be thought of as an extension of the plasticizing chamber. 

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