As for molding technics. Air bubble or air cavitation may occur in the plastic parts due to improperly-controlled material temperature and mold temperature, overfast injection speed, overlow injection pressure, too large or too small quantity of injection, insufficient dwell, unbalanced or insufficient cooling etc. Therefore, the injection rate of melt should be properly reduced to ensure sufficient time for gas in the cavity to be discharged; properly improve injection pressure and mold temperature, keep appropriate molding pressure in the cavity and properly prolong dwell time; dosage of injection should be kept in a proper amount; reinforce the effect of uniform cooling; improve temperature at pre-warming section of barrel; increase length of pre-warming section, reinforce dehumidifying and exhausting effect of plastics in the barrel; properly reduce temperature of plastic melt and the time of storing at high-temperature section to avoid degradation which will produce decomposition gas.