Mold and Temperature Control
Mold shrinkage is the percentage by which the part's dimension, measured 16 hours after it leaves the mold, is smaller than the mold cavity dimension (DIN 16901). Shrinkage that continues after this time is post-shrinkage. A low mold temperature reduces mold shrinkage but increases post-shrinkage; total shrinkage is greater at a high mold temperature but more stable. A measurement taken before 16 hours have passed is a measurement of a part that is still shrinking.
Amorphous materials (PS, PC, ABS) shrink by about 0.3–0.8% with little dependence on direction; semi-crystalline materials (PP, PA, POM) shrink by 1–2.5% depending on the flow direction. Glass fiber reinforcement lowers shrinkage greatly in the fiber direction and only slightly in the perpendicular direction; this difference is the main source of warpage.
Holding pressure and holding time reduce shrinkage in one direction only; mold temperature increases it; melt temperature and injection speed act in both directions, and the flow path ÷ wall thickness ratio decides which. What holds the dimension is not the average value but stability from cycle to cycle.
Because of post-shrinkage. In a semi-crystalline material crystallization continues after the part leaves the mold; at a low mold temperature this extends over days. Decisions on mold corrections are not made on measurements taken before 16 hours.