Question: What is the difference in cycle time for molding medical silicone parts vs cycle time for molding medical thermal plastic parts?

Answer: The processing conditions can be optimized to match the tool design, part geometry, and material but each presents limitations. Injection process times are driven by material. Silicone parts can often be released with no draft angle or even undercuts due to the high elongation and low modulus that save time on actions but knock out pins may often damage parts. Silicone part removal is often done by automated brush, hand pull, compressed air, or another way that may take additional time compared to plastic.

Curing time can be reduced in silicone by increasing processing temperature until filling fails or surface quality diminishes. Plastic cooling rates may be more limited by internal stresses causing warping or property changes from rapid cooling. Other contributions to the cycle time include heating and cooling rates, curing or solidification time of the material, injection time, mold travel time, and other smaller contributions increase the cycle time.

Direct comparisons between cycle times of medical plastics and medical silicones are not readily available. Your molder may be able to go into detail for your application if cycle time is critical.

Question: When molding medical silicone parts in the same mold will changing the durometer change the dimensions of the molded part?

Answer: The final part dimensions will depend on inherent shrink rate and processing conditions. Different materials molded in the same mold with the same shrink rate should produce the same size parts.

Durometer changes require different materials or additives which often changes the shrink rates. Higher durometer materials often have a greater raw viscosity and may achieve greater cavity pressures that can sometimes produce slightly larger parts compared to lower durometer materials. The difference has been shown to be less than 1% for a 40 durometer change without resetting the optimal processing conditions. Generally, changing durometer in the same series for most suppliers will not significantly affect the final dimensions but a first article is recommended. If you are considering trying multiple materials then I would recommend letting your molder know as early as possible.

Question: Why are medical silicone parts post cured?

Answer: Post curing or post baking is a simple heating process that takes place after silicone parts have been molded.  Although post baking parameters will vary from material to material, the post baking process typically involves heating parts to 300 – 400°F for 2 – 4 hours.  This process is performed for a variety of different reasons.

The first reason parts were post baked was due to the initial chemical composition of the earlier HCR (High Consistency Rubber) materials.  HCR materials typically cure as the result of a peroxide  based chemical mechanism (where as the more commonly used LSR (Liquid Silicone Rubber) materials cure as the result of a platinum based chemical reaction).  A peroxide based cured silicone will leave undesirable, low molecular weight byproducts that must be cooked out of the silicone matrix, typically these byproducts are various forms of benzoic acid.  For this reason, silicone vendors required that all HCR molded parts must be post baked prior to shipping.

The other reasons to post bake, which were likely discovered later, are the completion of silicone matrix cross-linking, and standardization of part size and physical and mechanical properties, namely compression set.

All silicone parts will shrink (slightly, typically 1-4% on overall size), while they may not all come out of the mold with the same degree of shrinkage, after a post baking process, all of the parts will reflect the same degree of shrinking.  Most molders factor in a value for shrink based on requested material data when designing the mold initially, this could mean that a shrink value is called out on the material of 2% but only approximately 1% of the shrink occurs in the molding process, were parts not post baked prior to shipping, the customer would receive parts that were roughly 1% too big.

The same is true of other physical and mechanical properties, compression set and percent elongation have been witnessed to fluctuate drastically if the parts are not post baked.  For this reason design engineers should always inquire with silicone molders as to whether or not they perform a post baking process, prior to embarking on large production  runs with parts that have critical physical property requirements.

Question: How is compression testing done for liquid silicone?

Answer: Compression set measures the permanent deformation of parts that have been subjected to high temperatures for some predetermined amount of time. One standard for compression set used in supplier literature is ASTM D395 A or B. Basically a standard puck is compressed between two plates and subjected to temperatures in the range of 250F at constant force or constant deflection. The sample is allowed to recover for 30 minutes and then measurements are taken. Alternative tests like ISO 815-B heat for 22 hours at 175 degrees C. The test method dictates the calculation but basically the compression set is a measurement of how well the puck returns to the initial thickness, so lower values indicate higher resistance to permanent deformation.

Question: I recently completed a silicone molding project and took it into production. The production has produced less than stellar yield. The main issue is the medical liquid silicone sticking to the tool. The tooling is fairly complex with some thin material undercuts. However, my question is: What methods for silicone molding do you recommend or material formulation to help eliminate material (silicone) being stuck in the tool?

Answer: Sticking can be reduced by changing liquid silicone molding materials, changing the tool surface, adding a release, or seasoning the tool. Changing medical silicone molding material with regard to the issue may improve release. For example, undercuts may be dealt with by finding a softer material with higher tear strength. Rougher surface finishes tend to promote less sticking compared to highly polished tools for liquid silicone molding. Alternatively some plating companies offer nonstick coatings that for some materials may be effective in improving release. Some molds will improve with increased number of cycles. The quick and easy option may be to use a release agent which many suppliers offer. Some releases may be specified for your liquid silicone material or for general use and Soap may be an alternative for those trying to control contamination risks. Releases usually only last some number of cycles before the effect diminishes and some may build up and require periodic cleaning.