Do temperature and humidity affect the rubber processing process? Processing silicone or polyurethane rubber, for example for making molds, can indeed influence the processing of the materials. Both warmer and cold temperatures than the “room temperature” will alter the curing as prescribed. Colder temperatures will slow down the curing process (possibly with some rubbers not curing at all < 15 degrees °C) and warmer temperatures will speed up the entire process, just like heatable moulds. Not only will the pot life and curing time change, but the viscosity and fluidity of the material may also change. As a rule, the lower temperatures will cause a higher viscosity. As a result, the substance of both components is thicker and at higher temperatures the liquids have a lower viscosity and are therefore thinner and more fluid. The mixing of the materials and the flow of the materials may therefore be different at the different temperatures.
High humidity can also affect the curing process.
Tin cured silicones, the condensation silicones like our SG C-Sil are condensation cured, so high humidity accelerates curing on the surface exposed to air, while keeping the internal part of the mold liquid. So keep in mind that the entire mold may not have cured yet. It may be that the surface is already hard but the core of the silicone rubber is still sticky or even liquid.
Addition and thus Platinum-cured silicones such as our Platsil and Eurosil are generally not affected by high humidity. This A-quality silicone is less or not affected by humidity.
Polyurethanes are very sensitive to humidity. It can cause air bubbles or foaming in the molded part, often you can quickly see this at the top of the mold. In addition, the containers with liquid part can be comp. A and comp. B accumulate moisture over time due to high humidity. Therefore it is important to keep the containers sealed when not in use or to re-seal with Poly purge. Part A will actually cure from the moisture in the air and form a hard crystallized material around the lid or as a top layer in the package. Part B may look good, but it may still have moisture in it. If you see many air bubbles in a polyurethane cast part that are not caused by air trapped in the mold undercuts, or air being knocked in from mixing too quickly, it is likely due to moisture.
Please note that all data in the Technical Bulletins is based on curing at room temperature (21 – 25°C / 70° – 75°F), unless otherwise stated.
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