A sampling shows medical parts possible with LSR.

In a competitive medical-molding market, companies that bring integrated solutions using costeffective techniques stand a better chance of success. One way to gain an edge is to add processing capabilities for liquid silicone rubber (LSR), an increasingly widespread material that is well suited for a variety of medical devices.

What are some of the properties of LSR that make it especially attractive for medical devices? Compared to high-consistency silicone rubber (HCR) and other elastomers, LSR provides greater clarity, chemical resistance, better economics for highprecision parts, and resistance to high temperatures. It works particularly well for parts with intricate geometries and smooth surfaces that require high precision, including O-rings, stoppers, closures, liquid feeding bottles, and catheters.

LSR comes in a variety of forms, including self-adhesives; fastercuring grades for larger, thicker parts; self-lubricating formulas to reduce friction on surfaces that need to be slippery; and liquid fluorosilicone elastomers. Advantages to using LSR include minimal waste; faster cycle times — for every millimeter there is 5 sec. of curing; flashless technology; no secondary operations — less handling; increased process control; and limited likelihood of cross-contamination.

Faced with stringent regulations and shortened product-development timelines, medical-device manufacturers and suppliers need systematic and repeatable process quality control. LSRs significantly increase quality manufacturing while wasting less material. However, companies can only reap these rewards by using the right equipment.

The demand for LSR for medical application in North America is expected to keep rising.
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LSRs are designed to be used in liquid injection-molding operations, but many rubber molders have modified existing equipment with less-than-ideal results. The lack of standardization for modified equipment can mean long cycle and setting times and an inability to replace parts. Also, older equipment may not have pneumatic controls, a necessity since the silicone rubber must remain at a consistent temperature and not heat up. Further, equipment not intended to accommodate LSR can result in high material costs due to flashing and contamination.

How do LSRs work?

Silicone rubber is a thermoset elastomer that alternates silicone and oxygen atoms along with methyl or vinyl side groups. The solidification process of all heat-activated cure thermosets, including silicone rubber, is caused by a chemical reaction called vulcanization, or cure. LSRs have the same structure as solid silicone rubbers. However, the chain length of the polydimethylsiloxane used for LSR is lower by a factor of about six. Therefore, the viscosity of the polymer is reduced by a factor of about 1,000.

Unlike plastics, which are injected at high temperatures and cooled inside the mold, elastomers are injected at lower temperatures and cured by heating the mold. With injection molding, the plastic typically packs into the mold during the final part fill. In contrast, in LSR molding, the final fill must accommodate the material’s thermal expansion. Packing as performed in plastics molding would make defective parts with LSR.

Also, in plastics, processors can remove and recycle runners (strips that connect parts) that result from molding multiple pieces in a single mold, or they can eliminate this issue by using hot runner systems. Conversely, in LSR molding, runners are waste because their state is permanently changed when they’re cured. The use of cold runner systems solves this problem.

Liquid silicone molding has a much tighter tolerance than parts made with plastics. Normal plastics flash in 0.002 in. (less than a human hair), while silicone can flash in 0.0001in. The tool temperature for an elastomer tool can rise to 356°F, compared to a tool temperature range of 68ºF to 284°F for plastics. The material temperature for an elastomer tool is 68ºF to 176°F, compared to temperatures of 356ºF to 716°F for plastics.