Many mixed silicone rubbers (A+B) tend to be viscous and possess a high viscosity. Silicones with a viscosity exceeding 15,000 centipoise should be subjected to vacuuming to prevent the entrapment of air and the formation of bubbles in the final mould. If you blend and pour silicone rubber without vacuuming it, air may become trapped within the mixture during its transition from liquid to solid, resulting in the appearance of these bubbles on the working surface of the mould. These bubbles will be transferred to your finished casting, which is unsightly and undesirable.

What Are Your Options? Purchase a vacuum pump and vacuum chamber – If you regularly use high-viscosity silicone, you may want to consider investing in a vacuum pump and vacuum chamber. Estimated cost: £600 - £800.

Selecting a Vacuum Pump – Vacuum pumps differ in size, capability, and cost. From our experience, we can tell you that the most expensive option is not necessarily the best. Do your research before making a purchase.

For degassing silicone and urethane mould rubber or resins (urethane, epoxy, etc.), we recommend the following minimum vacuum pump specifications: A rotary vane vacuum pump with a minimum airflow of 8 CFM (cubic feet per minute) or 13.6 cubic meters per hour (m³/h).

The pump must achieve a vacuum of 29 inches of mercury. – If the pump does not reach 29 inches of mercury, you will be wasting your money and time. (related FAQ: My silicone takes a long time to degas, and I still end up with some bubbles in my silicone rubber. Is this normal?)

Low-Viscosity Silicone – Smooth-On offers silicone rubber products that do not require vacuum degassing (or a weighing scale). The Mold Star® Series silicones are all low-viscosity platinum-catalysed silicones suitable for various applications. They have excellent tear strength and perform well in many applications. The OOMOO® Series silicones are tin-catalysed and also do not require vacuuming or a weighing scale. They are cost-effective; however, their tear strength is limited, making them suitable for creating one- or two-piece block moulds only.

Urethane Mould Rubber – You can also opt to use urethane rubber instead of silicone rubber. Again, vacuuming or the use of a weighing scale is unnecessary. However, it usually requires the use of a sealing agent and release agent to prepare your model in advance. Be sure to read the technical bulletin before purchasing urethane mould rubber.