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HCRs are pre-mixed compounds, master batches, or partially vulcanized sheet materials with a higher viscosity index, a different curing system, and longer cure times than LSRs. Although HCRs can be injection molded, compression molding and transfer molding are often used for products other than silicone tubes and cords. Yet HCR molding offers lower design, machining, and equipment costs than the LSR process.
HCR injection molding and LSR injection molding have some similarities, but the process for high consistency rubber requires several more steps. During production, HCR silicones are fed into a heated barrel and injected into a mold where vulcanization occurs. HCR cure time is determined by parameters such as part cross-section thickness, heat transfer, and mold temperature.
Most HCRs use peroxide curing, a reaction that leaves acidic residues in the cured elastomer. If these acids aren’t removed, they may produce a powder on the surface of a part that’s known as “bloom”. Post-curing in an oven can remove these byproducts, but post-baking adds time and money to injection molding projects. Plus, if a mold is removed before curing is complete, it’s too late to remove peroxide residues.
Higher mold temperatures can reduce the time needed for peroxide curing, but there’s a trade-off since these HCR parts can tear when removed from the mold. Unlike LSR injection molding, loading and de-molding is performed by an operator. Labor costs are also incurred during trimming since HCR molded parts may require surface improvements.
Compare HCR Injection Molding to LSR Injection Molding
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