Silicone Curing Procedures
Vacuum Blanket Silicone
The silicone compounds which Mosites Rubber Company normally recommend and use for high strength vacuum blankets contain peroxide curatives. The curing chamber in our plant uses high pressure saturated steam to provide temperature and external pressure during vulcanization. An added benefit of the steam atmosphere is the ability of the silicone surface to cure without the necessity of being protected and sealed with bagging film. If the silicone surface is left uncovered in a dry heat autoclave or oven, the surface tends to be uncured. This is due to the volatility of the catalyst and the inhibiting effect of air.
Most of our customers who purchase uncured or “B” stage silicone do not have steam autoclaves. These customers use inert gas atmospheres such as Nitrogen or film bag and cure in hot air autoclaves or ovens. Regardless of the method used, the cure times and temperatures remain the same.
Mosites Rubber Company recommends that our products receive an initial cure of thirty minutes at 300°F. Time commences after the tooling reaches 300°F. After the initial cure time has elapsed, cool the part to room temperature and remove it from the tool.
Lay the initial cured silicone on clean fiberglass and place it in an air circulating oven for post-curing. Folds or cavities in the bag can be packed with clean fiberglass to prevent adjacent surfaces from touching during post cure. The post cure cycle consists of three hours at 400°F.
Molding Compound Silicone
The peroxide catalyst system used to vulcanize Mosites calendered sheet silicone is not recommended for thick section (over 0.5 inches thick) molded articles. These applications require a peroxide that offers good mold fill / flow time, prior to crosslink formation and must not generate acidic cure by products that would cause reversion during subsequent high temperature exposure in service. The organic peroxides used in these applications require closed metal molds in order to effectively cure the silicone polymer. Normally, the molds are placed in presses for the most efficient productivity. A press is not necessary and ovens or autoclaves can be used, but the mold must be securely bolted or held together to withstand the internal pressures exerted by the silicone polymer as it expands during the vulcanization process. A procedure for curing a thick section molded silicone article is shown below.
- Clean the mold thoroughly and apply a uniform coating of mold release. (Teflon Spray or dilute detergent soap solution )
- Calculate the weight or size of the silicone preform required to completely fill the mold cavity and add 3% for optimum mold fill. Calendered sheet is not required for these applications and uncured mill slab is a much more economical choice.
- The mold can be room temperature if the assembly process is time consuming. If molding is performed in a press, where the process has a rapid turn around time, a hot or pre-heated mold is recommended.
- Our recommendation for cure time and temperature is based on the thickness of the rubber part. Cure for 30 minutes at 300 degrees F for the first 1/8 inch of thickness. Cure an additional 5 minutes at 300 F for each extra 1/8 inch of thickness. The cure time should commence after the mold reaches 300 degrees F, if it is loaded at room temperature.
- After the initial vulcanization is complete, remove the part from the mold; trim and inspect. For most Aerospace applications a post cure of 3 hours at 400 degrees F in an air circulating oven is required. Less critical applications may not require the post cure due to the absence of acidic by-products.