The reason of air traps of a molding product:

1.Poor venting of the mold

2.The plastic raw material break down
3.Mold design is unreasonable(such as sprue is too small)
4.Injection speed is too fast

How to improve air traps:
1.Improve the mold venting 
2.Lower down the hopper temperature
3.Improve mold design (like enlarge the sprue)
4.Lower down the injection speed