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Temperature Effects

Because of the cross-linked chain structure of polymers, the tensile strength of polymers tends to degrade with increasing temperature.
As the temperature rises, the polymer's:

Modulus (tensile, flexural) values drop.
Tensile strength drops.
Elongation increases.
Creep effects increase.
Stress relaxation increases.
Impact strength (toughness) increases.
Skin Effects
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. This fact makes prototyping (prior to hard tooling) difficult.
Effects of Additives

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. The standards, listed in descending order from the highest flame resistance, are V-0, V-1, V-2, and HB.
  - Flammability requirements could dictate wall thickness.
- Increasing flammability requirements cuts down on available material choices.
- The end-product must be tested to get UL approval.
Stabilizers are added to inhibit degradation caused due to exposure to Oxygen, sunlight (UV exposure), heat, and water.
Colorants are added to give certain desired colors to the polymer.
Flowing agents are added to improve flow characteristics during processing.
Release agents are added to improve mold release characteristics.
Lubricity agents are added to lower the surface coefficient of friction of the finished product.