Because of the cross-linked chain structure of polymers, the tensile strength of polymers tends to degrade with increasing temperature. Tensile strength drops. Creep effects increase. Stress relaxation increases. Skin typically formed because of thermal and viscous boundary layer effects upon injection of molten material into a mold.
Skin consists of highly directional, often fiber-depleted and sometimes crystalline material.
Skin is typically highly influential to the properties of a molded part. Flame-retardants are added to minimize the chance of a material igniting. For a material to be considered flame-retardant, it needs to meet the UL flammability standards, UL 94. Colorants are added to give certain desired colors to the polymer. Flowing agents are added to improve flow characteristics during processing.