How to identify molding silicone rubber

The molding silicone rubber is mainly used for plaster molds and resin craft molds.   　　To make plaster moulds ,that requires the silicone rubber must have good tensile and tear strength,otherwise the silicone molds will be in a weak and flimsy manner.In order to make the molds for resin crafts,you must choose the silicone rubber which has heat-resistant,favorable tension and hardness.because the unsaturated resin produces great heat when added the curing agent.But how to distinguish good or bad molding silicone rubber?   　　1.Such a high purity molding silicone rubber its brightness is good.The black one has the impurity of products is made by the dealer to reduce cost and add a few dollars a kilo of powder.   　　2.When with the same weight,which volume is larger,it is the better.Small volume of products with high density.The main reason is to add the high density and low powder.   　　3.After a certain period of time.The silicone rubber appear statified phenomenon,the above looks like oil,the following is the powder,this molding silicone rubber is more inferior.