Liquid silicone rubber (LSR) and high consistency rubber (HCR) both support injection molding. They have similar physical properties but very different processing requirements. By understanding how LSR injection molding and HCR injection molding work, designers can specify the best manufacturing method for their silicone parts and products.
What is LSR Injection Molding?
LSR injection molding uses a base material and a catalyst, a vulcanizing agent that turns liquid silicone rubber into a solid material. Most LSRs use a platinum-based curing system where an addition reaction occurs. Heat or pressure can be applied to accelerate curing, but LSRs cure more quickly than HCRs. Yet the advantages of LSR injection molding don’t end there.
Platinum or addition curing doesn’t produce significant levels of byproducts, such as volatile organic compounds (VOCs), that require removal. This eliminates the need for post-baking in industrial ovens, which saves time and money during silicone parts production. If post-curing is used, it’s generally to stabilize or enhance an LSR’s properties for an especially demanding application.
LSR injection molding starts with meter-mixing. A pump dispenses the ingredients and a static mixer combines the components. This mixture is then transferred to a cooled section of the injection molding machine. Because the material is homogeneous, LSRs support consistency throughout individual parts and from part to part. LSR injection molding also supports automated operations for greater efficiency.
When LSR curing occurs, there’s no loss of material in the feed lines for reduced material waste. Stainless steel molds are more expensive, but molds that are properly tooled produce silicone parts with minimal flash that eliminate the need for trimming. Because LSRs have a low viscosity or resistance to flow, they also support the use of complex, intricate molds in applications such as medical devices.
What is HCR Injection Molding?
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.
Choose the Best Molding Method
Extreme Molding specializes in silicone injection molding and understands the many important processing differences between liquid silicone rubber (LSR) and high consistency rubber (HCR). At our Made in America manufacturing facility in Watervliet, New York, we also offer LSR compression molding and HCR transfer molding. To request a quote or discuss your specify molding application, please contact us.