Plastic mold shrinkage varies due to many factors including, but not limited to:
Plastic supplier/ material batch
Mold Temperature
Plastic melt temperature
Injection and hold pressures
Gate and runner size
Part wall thickness
Colorant/ Additives
List of Shrink rates from various sources.
You should of course, do your own sampling and make your own judgements based on your plastic parts and molding equipment and setups. This information is provided as a reference only. We will not be responsible for any use of this information for manufacturing processes.
ABS .004/.006
Acetal (POM) .020/.035
Acrylic .002/.010
HDPE .015/.030
LDPE .015/.035
Nylon (PA) .007/.025
PBT .008/.010
Moldmakers and molders often have problems deciding who is responsible for deciding upon the shrink rate for manufacturing a mold. Unless the moldmaker has molding presses and can sample and control the molding parameters they should not be responsible for choosing the shrinkage for the china mould manufacturer. As the wide ranges of shrink values shown above, how could the molder know what exact material is purchased and how it will be processes. We work closely with the molders for the molds we manufacture. We get the exact designation of the material and run moldflow simulation on the part to predict the shrinkage at various processing settings. With this information in hand, we discuss and decide jointly what shrinkage will be used. In some cases we build a steel safe preliminary (prototype) blank to test for shrink. Then we rework the mold to add the details that had tight tolerances. In other cases, we will manufacture the mold and make minor changes due to molding shrink variation. We have discussed and decided PRIOR to mold manufacture who will be responsible for the cost of various modifications. As the mold builder, we always provide one round of gate and vent modifications with our build. Dimensional modifications on a couple of core pins would be included, but not reworking major part detail.