In injection molding it may happen, that the air can’t escape or that it moves only very slowly out of the cavity.
An air trap will occur if the air is surrounded by the melt completely or if it is at the end of the flowpath (missing or insufficient venting). Therefore, in this situation burn marks may occur.
The air trap can be located in the inside or at the surface of the part.
The later causes an incomplete filling.
With a good venting system and a mainly uniform wall thickness, air traps can be avoided.