Many mixed silicone rubbers (A+B) are thick and have a high viscosity. Silicones with a viscosity above 15,000 centipoise should be vacuumed to avoid air entrapment and bubbles in the finished mold.  If you mix and pour silicone rubber that has not been vacuumed, air may be trapped in the mixture as it turns from liquid to solid and these bubbles may show up on the working surface of the mold. These bubbles will be transferred to your finished casting which is unsightly and undesirable.

What Are Your Options?

1. Buy a vacuum pump and vacuum chamber –  If you are using a high viscosity silicone regularly, you may consider investing in a vacuum pump and vacuum chamber.  Approximate cost; $800 – $1,000.  Click here to view vacuum chamber. 

Choosing a Vacuum Pump – Vacuum pumps vary in terms of size, capability and expense.  From experience, we can tell you that the most expensive is not the best.  Do your homework before you buy.

For vacuuming silicone and urethane mold rubber or resins (urethane, epoxy, etc.) we can suggest the following minimum vacuum pump specifications;  Rotary vane vacuum pump with a minimum airflow of 8 CFM (ft3/min) or 13.6 Cubic Meters/hour (m3/h).
Pump must pull
 29 inches of mercury.  If the pump does not pull 29 inches of mercury, you are wasting your money and time. 

2. Low Viscosity Silicone – Smooth-On does have silicone rubber products that do not require vacuum degassing (or a weighing scale).  Mold Star 15, 16 and 30 are all low viscosity platinum catalyzed silicones suitable for a variety of applications.  Tear strength is very good and these products will perform for many applications.  OOMOO silicone is a tin-catalyzed silicone that also does not require vacuum or a weighing scale.  It is also very inexpensive.  Tear strength, however, is minimal and this product is good for making one- or two-piece block molds only.

3. Urethane Mold Rubber – you can also choose to use a urethane rubber instead of silicone rubber.  Again, no vacuum or a weighing scale is necessary.  However, using a sealing agent and release agent to prepare your model in advance is usually required.  Read the technical bulletin before you buy urethane mold rubber.