Related Injection Molding Processes

Although most injection molding processes are covered by the conventional process description discussed earlier in this chapter, there are several important molding variations including  

In contrast to multi-color and multi-component injection molding, co-injection molding uses the same gate and runner system. Here, the component that forms the outer skin of the part is injected first, followed by the core component. The core component displaces the first and a combination of the no-slip condition between polymer and mold and the freezing of the melt creates a sandwiched structure as depicted in Fig. 3.51. 

In principle, the gas-assisted injection molding process is similar to co-injection molding. Here, the second or core component is nitrogen, which is injected through a needle into the polymer melt, blowing the melt out of the way and depositing it against the mold surfaces. 

Injection-compression molding first injects the material into a partially opened mold, and then squeezes the material by closing the mold. Injection-compression molding is used for polymer products that require a high quality surface finish, such as compact discs and other optically demanding components because it practically eliminates tangential molecular orientation. 

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There are no generalized models that include all these variables for thermosetting polymers. However, extensive work has been done on the viscosity of polyurethanes [9, 10] used in the reaction injection molding process. An empirical relation which models the viscosity of these mixing-activated polymers, given as a function of temperature and degree of cure, is written as... 

Gunes, l.S. and Ersoy, O.G. (2006) Effects of injection molding processing parameters on appearance related optical properties of polycarbonate. J. Polym. Eng., 26, 743. 

Injection molding processing involves both a molten flow phase and a solidification phase. The major challenges of constitutive modeling of the liquid-solid transition involve two related topics. First, one needs to eonsider how the flow and thermal history influence the structure of the fluid. Seeondly, one needs to understand how the ehanges in the internal strueture stiffen the material. There are some investigations on the topics for semicrystaUine materials and a variety of approaches or models have been proposed by different authors. Most of these results are reviewed by Tanner and Qi (2005) and Pantani et al. (2005). However,... 

The injection molding process includes both molten polymer and solidification phase, where flow and crystallization may take place simultaneously. Therefore, for modeling injection molding, a rheological relation connecting the relative crystallinity a to flow parameters is needed. 

As with injection molding, process problems can be solved by altering process variables. Table 9-4 presents a chart that relates problems to solutions. 

Injection Molding. In iajection mol ding a molten thermoplastic is iajected under high pressure iato a steel mold. After the plastic solidifies, the mold is opeaed and a part ia the shape of the mold cavity is removed. Geaeral descriptioas of the process and related equipment are given ia Refereaces 29—34. 

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. 

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