These silicone prototype inserts fit a representative production mold base, allowing exact production simulation.

Although liquid silicone rubber (LSR) has been commonly used in Europe for a long time, some North American OEMs still view it as a “black magic” material because of a lack of internal expertise as compared to thermoplastics. But the performance advantages that LSR provides in medical devices and disposables are great. Therefore, leaving more about the material’s proper use and how to select the best molding method and molder is important. Critical factors for a successful silicone molding program include:

Material selection. Engineering groups can take the path of least resistance and use existing approved materials already in their database. They really can’t be faulted for doing this because it ensures the selection of a qualified and validated material, greatly reducing the risk of potential program delays or problems with biocompatibility.

However, LSR material providers have significantly improved their formulations over the last 10 years, so it doesn’t make sense to rely only on familiar materials and potentially overlook performance gains, processing stability, and lower prices. The latest LSRs have improved upon original formulations and have closed the gap on the mechanical properties of high consistency silicone (HCR) — a material typically compression or transfer molded. LSRs can provide a better choice than HCRs because its platinum cure system takes significantly shorter cure times.

Also, LSRs come in sealed drums from material suppliers, reducing the risk of contamination. Materials typically have more batchto- batch consistency because they are produced on a larger scale at the raw-material manufacturer and not on a two-roll open mill at a molder or custom mixer. Needless to say, in medical molding, material consistency is critical, based on the requirement to operate the process within a tightly validated window.

In addition, LSRs better suit automated production because of the material consistency and the large-drum delivery systems allow the extended runs without stoppage. HCR can have good tear resistance, compression set, and abrasion resistance, but an alternative and suitable LSR grade is typically available for most applications.

The 2 + 2 mold handles a plastic substrate and a silicone overmold.

Another issue with material selection is a tendency for engineering groups to want to use thermoplastic elastomers instead of silicone. While thermoplastic elastomer (TPE) or thermoplastic vulcanizate (TPV) materials have improved over the years, silicone is still a better choice when it comes to sealing applications. TPEs don’t reach liquid silicone rubber’s clarity, biocompatibility, compression set, and temperature resistance. In addition, the raw-material pricing between LSR and a specialty grade TPE is not that different. It becomes even less of a factor when working with a silicone molder that uses the latest in flashless and automatic molding.

Vendor selection. Because of a lack of expertise in silicone at the OEM level, many molding programs are sourced based on an Internet search, a past relationship, or what company is closest or cheapest. But just because a company owns a silicone machine does not make it a qualified and experienced silicone molder. Silicone molding is much different from thermoplastics and can pose many unique problems. For example, strict silicone tooling requirements mean gating, venting, and demolding parts is always a challenge. So silicone experience counts a lot in part design, mold design, and the molding process.

Additionally, there is a wide variety of methods for molding silicone. OEMs should ask their suppliers if they use flash free tooling and automated molding or if they rely on human operators and secondary flash removal. Each approach has its own merits, but operators and secondary operations add more variables and costs into an already complicated process.

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