An engineer emailed me this question yesterday,,,
My setup technician believes gas entrapment is the cause for poor surface finish on a particular process. When we clean the vents, the appearance improves, but does this make sense, or is there something else causing this?
Although most people associate burning or dieseling with gas entrapment, there are many other defects that can be cause by gas entrapment. Poor surface finish, flash, short shots, poor weld line strength and appearance can also result. In the case of surface finish, If the gas does not escape, it can collect at the end of the fill or it may become distributed between the mold surface and the polymer melt. When it gets between the melt and the mold surface, it typically prevents the melt from filling each contour of the mold surface resulting in a poor reproduction of the mold surface in the final molded part.
One easy way to test this is to reduce the clamp tonnage and determine if the improved parting line venting improves the surface finish.