As a silicone molder and extruder, Sil-Pro uses a variety of silicones from multiple manufacturers, including Dow Corning, Nusil, Momentive, and Wacker, to name a few. All of these companies offer materials that are approved for contact with body fluids or for use in the mouth, and all would have FDA compliance/approval for use. However, since I am not aware of UMC Corp. or its glow color products, I am not familiar with the company’s certifications. Sil-Pro uses pigments from Nusil and other companies that are approved for use in short-term implants or for applications that are placed in the mouth.

I don’t have much experience with the interaction between silicone and polyurethane, and without knowing what silicone you are using (platinum, peroxide cured, RTV, HCR, LSR, etc.), it is difficult to say. I guess the first thing I would try is to postcure the silicone mold at 350°F for four hours. Next, try to clean it well with 99% isopropyl alcohol and allow it to dry thoroughly. Then, give it a shot and see if you have better results.

There are surface coatings that are commonly used on tooling cavities or mold cores to allow for better release and reduced sticking. Nickel-Teflon is one coating I know of that is used most commonly on many tools that produce parts used in medical applications. This coating, in conjunction with a dry powder mix of Texapon K-12 and DI water applied to the cores or cavities, seems to offer the best results. Another technique used is air assist during part removal. This method uses compressed air to aid in removing the part.

Generally speaking, parylene is difficult to get a good chemical bond with. Its lubricous nature does not allow for a strong interaction between the silicone and the substrate, resulting in poor cohesion. You could try priming the surface prior to the overmold or try bonding to room-temperature vulcanizing (RTV) silicone. A strong primer that’s approved for medical use is Nusil MED 6-161. Good luck—it sounds like a bit of a challenge.

Generally speaking, one of the biggest differences in physical properties between LSR and HCR is probably elongation. This would likely be the largest difference in a balloon application between HCR and LSR. Commonly, a midrange durometer HCR has an elongation of 900% or more, while an equivalent LSR may only have approximately 600%.

If your application can live with the lower elongation, LSR generally costs less and provides more-repeatable results in high-volume production than HCR. If your application requires higher elongation, HCR may likely be your best bet.

By prototyping a few parts from both materials, you can test each one and come to a final conclusion about what your application requires.

A wide variety of factors will determine where to begin on the silicone material selection and ultimately much of it will depend on how your device is being used. With material suppliers, much will depend on requirements of the application: long-term implant, short-term implant (less than 29 days), body-fluid contact, etc. This will determine what classification of material may be used or which material company’s silicones will be allowed for specific applications or be restricted from its use. Once this is determined, a durometer and likely an LSR or HCR will be reviewed as a good candidate to start. Depending on the application, there are advantages and disadvantages to both LSRs and HCRs, and higher and lower durometers advantages and disadvantages from a molding perspective, as well as sealing, insertion/withdrawal force, etc. I would recommend, depending on its exact use, to start with a quick-turn prototype tool and begin with short run of 50-durometer LSR and sample a couple different materials (relatively quickly and inexpensively) to see what would perform best in your application.