Compounds for Injection Molding


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Quality control for injection molding compounds has to be tighter than for those that are compression molded because the injection-molding process itself can be more precisely controlled and is therefore more sensitive to variations in compound properties. Compounds must have sufficient processing safety (scorch safety) to flow through the nozzle, runners, and gates without scorching, but still cure rapidly in the mold. Thus, the balance of viscoelastic and curing characteristics of the compounds are extremely important.
Designing rubber compounds for injection molding has often been a trial and error process because the processor usually does not have data on the behavior of such compounds at the temperatures, pressures, and shear rates involved in injection molding. In fact, most rubber compounds that will compression mold can be satisfactorily injection molded provided that they flow well enough and are not too scorch sensitive, and that the machine controls are properly adjusted.
However, for maximum productivity, the compound has to be injected rapidly into the mold at near vulcanization temperature. It is in this area of optimization that laboratory tests, properly interpreted, can be of value in delineating an operating window of time, temperature, and pressure, within which a particular material will flow well and cure effectively without danger of scorching.
The three major areas in which data on a compound are required are rheological behavior, rate of vulcanization, and heat flow into and through the compound.

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