When silicones are used to manufacture commercial products, they are often mixed with pigments for aesthetic or functional purposes (e.g., barium sulfate for radiological opacity), additives such as rheology modifiers and adhesion promoters, or even active pharmaceutical ingredients for drug delivery.  Whether they are being mixed neat or modified in some way, they must be mixed thoroughly, at the right ratio, and without voids to achieve the best possible physical properties.  As with most reactive materials, controlling temperature during the mixing process will help to maximize working time.  Vacuum mixing is also helpful in many cases to remove difficult voids as well as entrapped air and gasses.  It is one of the best ways to avoid bubbles which can be formed by expanding gasses during a heat accelerated cure.