Meaning
A separation event occurs during injection molding when the solidified outer layer of a plastic part pulls away from the mold cavity wall before cooling is complete. In precision molding, skin detachment is caused by the volumetric shrinkage of the cooling polymer core overcoming the holding pressure. Once this contact is lost, the heat transfer rate between the part and the tool drops dramatically.
This delays the cooling of the core and can introduce surface defects.
Thermal Impact
Air has a much lower thermal conductivity than the steel mold wall, acting as an insulating blanket. This sudden reduction in cooling efficiency means the core remains hot for longer, extending the required cooling time. If the part is ejected too early, the soft core will cause the part to warp or swell.
Accurate packing phase control is needed to minimize this premature separation.
Pressure Relationship
Maintaining high packing pressures for an extended duration keeps the polymer skin pressed against the mold steel. This ensures rapid, uniform cooling throughout the solidifying layer. Once pressure is released, the polymer naturally shrinks.
Defect Avoidance
Early separation can cause localized cosmetic defects such as sink marks or uneven gloss. Parts with thick ribs or bosses are particularly sensitive to these flaws. Controlling skin contact is critical for high-quality surface finishes.
