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.