During the process of injection molding, due to the poor exhausting condition of molds, the water vapor in the melt, the air in cavity, the volatile gas from volatile substance in raw materials as well as the material decomposition gas either remain inside the products to form air bubble or are stored in between the melt and surface wall of cavity to form air cavitation which causes inner concave on the surface of products.

Sometimes, at parts with thick walls, gas may also exist in the shrinkage porosity caused by insufficient shrinkage compensation. The occurrence of air cavitation and air bubble may further result in inadequate filling in the cavity and insufficient dwell, and may even lead to the rising of gas temperature due to the compression of melt, thereby occurring such phenomenon as degradation, discoloration or scorching of plastics around the air cavitation or air bubble.

As for injection raw material. If the raw material contains too much water, additive of decomposition gas and recycled materials may occur therein and thereby forming gas sources. Hence, the raw material of plastics should be strictly dried and barrels with exhausting function should be applied to the injection machine; preferably reduce or replace the additives easy to decompose, and reduce the use of remnant materials or recycled materials.