A void, or bubble, is defect resulting in a hollow region within a part. This can result from either a trapped gas (bubble) or from insufficient packing. Trapped air will result in a pressurized void whereas insufficient pressure will result in a negative pressure in the void.
Pressurized Void: During plastification and injection, any gas in the melt may be kept to a small volume while under high pressure. When pressure is relieved, the compressed gas in the melt can expand and create a void. In this case, as the plastic freezes around the void, the gas may remain under pressure for some time until the gas is allowed to migrate out.
Negative Pressure Void: After a cavity is filled with plastic, the material will begin to cool and shrink. To compensate for the shrinkage, the molding machine will try to feed in additional material during pack phase of the molding cycle. If the packing pressure is removed too early, the continued shrinkage between the frozen outer skin of the plastic part will result in a void. There will be a vacuum within the void which try to pull in the frozen surfaces of the part. If the frozen surfaces are pliable enough, this will result in a depression or “sink” on the parts surface. In some cases the sink may be sufficient to relieve any internal negative pressure sufficiently that a void will not develop.
Packing pressure may be lost to the location having a void due to the operator prematurely cutting off pack pressure or from some region of the runner, gate or cavity freezing off and preventing a pack pressure being transfered to a location in the part where the void is occurring. Often voids and sinks will occur in thicker regions of a part.