Given their excellent chemical inertness, toughness, ample operating range, flexibility, and wide range of available durometers, silicone rubbers find uses in a wide variety of industries. One industry with a growing use of these elastomers is medical device manufacturing, where their low toxicity, great biocompatibility, and ability to repeatedly withstand steam, gamma ray, EtO, and UV sterilization only add to their appeal.

Medical device companies still face a tough choice even when they do choose this class of materials for their new products, namely, “which type of silicone rubber should we use, high consistency rubber (HCR) or liquid silicone rubber (LSR)?”.

Although the performance and mechanical characteristics of both types are nearly identical, choosing between HCR and LSR boils down to how the part will be made.

High Consistency Rubber

HCR is produced as gummy, high viscosity sheets of various thicknesses that are partially vulcanized. This form makes HCR a natural fit for compression and transfer molding as well as extrusion (it’s the go-to material for flexible components like rubber tubing). Fabricating parts out of HCR requires many steps that are labor-intensive, albeit simple (relative the injection molding process used with LSR): mill softening, preform preparation, extrusion/molding, vulcanization, and finishing.

Although HCR can in theory be injection molded, the material’s high viscosity and long cure times result in cycle times that are often too long to be practical, which is why the compression and transfer molding methods are used for anything that’s not tubing or cording. HCR’s reliance with these fabrication methods means producing a part out of HCR generates substantial waste material, incurs high labor costs, and requires much floor space, tools, and equipment to accommodate the many required steps. It should be noted, however, that those equipment costs are less than the steep design and machining costs required by LSR’s injection molding process.

Liquid Silicone Rubber: Perfect for Injection Molding

LSR, by contrast, starts out as a 2-part liquid that cures into a solid form when mixed together. Mixing is performed by a metered mixer that precisely combines the two parts in a 1:1 ratio (mixing in additives if needed) right before pumping the fluid into the mold, which is heated to accelerate the vulcanization process.

The mixed LSR is pushed into the mold under pressure. LSR’s low viscosity results in a quick mold fill and pack time, while the elevated mold temperature ensures a short cure time. Since all curing takes place inside the mold, there is less wasted material compared to HCR. The use of “cold drop” or “cold runner” tooling reduces this waste even further by the keeping the LSR cool inside the sprue and runners, which mean vulcanization only occurs inside the hot part cavities, resulting in no LSR lost to sprue and runner volume and no additional step of trimming these sections from the molded part.

LSR’s injection molding process produces consistent parts, cycle to cycle. Another advantage to the LSR process is the ability to reproduce intricate and complex shapes, as LSR’s low viscosity permits the fluid to fill even the tiniest of spaces.

Because this is such a highly automated process, once the LSR process is up and running, very little labor is required to produce large quantities of parts, making LSR the dominant choice for high-volume production.

LSR is also gaining popularity among medical device OEMs and their contract manufacturers, since the widespread availability of all-electric injection molding machines means that part production can take place inside a clean room. With no hydraulic fluid to potentially contaminate the parts or other objects inside the clean room, high levels of product sterility and quality can achieved, both of which are requirements for implantables.

Making parts via injection molding with LSR does require specific expertise, including mold design, mold performance analysis via simulation software, and very high precision machining of the mold materials. In fact, most of the cost of making silicone parts this way is incurred by the design and production of the tooling itself.

Scale is Key to Selection?

Deciding whether HCR or LSR injection molding is the better choice for your project largely depends on the production volume you will require. Generally, HCR’s methods are better suited for smaller production runs, while LSR is a better fit when making hundreds or thousands of parts. We at ProMed are experts at both types of silicone rubber manufacturing, and can use our expertise to help you make right decision for your next project.

HCR vs. LSR Injection Molding: Which is Right for Your Project?
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HCR vs. LSR Injection Molding: Which is Right for Your Project?
Which type of silicone rubber should we use, high consistency rubber (HCR) or liquid silicone rubber (LSR)? choosing between HCR and LSR boils down to how the part will be made.
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ProMed Molding
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