Injection molding limitations

Husky Newsletter, (Feb. 1978), "Stack Mold Developments", Husky Injection Molding Limited, P.0. Box 1000, Bolton, Ontario, Canada LOP 1A0. 

Injection molding of styrene-based plastics is usually carried out at 200—300°C. For ABS polymers, the upper limit may be somewhat less, because these polymers tend to yellow somewhat if too high a temperature and/or too long a residence time are imposed. 

Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

The results of viscosity versus shear rate are reported in Fig. 11 for the two pure components and their blend, respectively. The temperatures were the same for the viscosity measurements and for the injection molding. At temperatures of 280°C and 320°C, the viscosities of the blend are found to be values between the limits of the two pure components. In both cases, the TLCP still... 

Range of materials that can be molded Similar to thermoplastic injection molding Unlimited cost depends on performance requirements Limited higher cost... 

The low thermal stability of many poly(iminocarbonates) limits the use of melt fabrication techniques such as injection molding or extrusion. For example, among all six polymers tested, only poly-(Dat-Tyr-Hex) and poly(CTTH) had low enough softening points to be compression moldable without a significant degree of thermal decomposition. ... 

Fiber reinforcement of polymers allows us to use them in engineering applications that were previously off limits, We use fiber reinforced injection moldings in many under-the-hood automotive applications, such as rocker box covers and front end modules, which support the radiator, cooling fan, and hood latch, that are subjected to a wide range of temperatures. 

Figure 16.8 Injection molding. (Source Wittcoff and Reuben II and Reuben and Burstall 1974, Reprinted by permission of Pearson Education Limited.)...

Foams have limited use for these purposes. Rigid cellular PVC is good as a thermal barrier but not for structural parts. Doors and frames of structural molded foam, eg, foamed high impact polystyrene, can be made by injection molding, with recesses for hinges, striker plates, and miter corners. Solid polystyrene and structural foam-molded polyurethane have been molded for door frames. 

Amino resins are fabricated principally by transfer and compression molding. Injection molding and extrusion are used on a limited scale. Urea resins are not recommended for outdoor exposure. The resins show rather high mold shrinkage and some shrinkage with age. The melamines are superior to the ureas insofar as resistance to heat and boiling water, acids, and alkalis is concerned. 

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