Mold Best Assurance Company Limited
This is an interesting question, because the lineal mold shrinkage is characteristic percent shrinkage of a plastic between melt processing and the final cooled article. Specifying materials of low shrinkage will make it easier to avoid problems of warpage and to increase dimensional precision of injection moldings.
Hire a thermal imaging camera, as I've done on many occasions, including in the last fortnight. Though the semi-crystallinity will accentuate warpage tendencies compared to amorphous materials, just see how temperatures vary across all surfaces of the part on ejection.
Warpage is usually caused by molded-in stresses. The issue of warpage is related to being a semi crystalline material, HDPE, being semi crystalline polymer, will have differential shrinkage along and across the flow leading to warpage.
Not just the specifics of thermodynamics, but also a more fundamental one of Mankind (sorry, Personkind!) collectively learning from experience and recording that experience. Often, that condensed experience will end up in... that right, TEXTBOOKS.
One issue that can frustrate toolmakers is damage from plastic flakes building up on the parting-line shutoffs. This is typically a result of bad gating, undercuts on sprue-puller pins, or stringers from the sprue or hot drop. The flaking or stringing issues should be resolved to prevent mold damage.
In my job I have had to address cases of running hours of scrap after preventive maintenance—that is, when the mold is completely disassembled, cleaned, and greased with new lubricants. I have found that some greases are more prone to bleeding if not applied properly...
Not only injection mold temperature determines melt temperature as it moves quickly through the mold. The melt temperature itself as it starts its journey through the injection mold (self-evidently) is the reference point. Speed of injection, particularly in thin sections, is of huge influence, as it will determine the degree of shear heating.
Many engineering materials like PBT (Polybutylene terephthalate) have a tendency of out-gassing. The best practices that I have to date are: Add 10mm wide vents spaced 30mm from each other all the way around parting line. Always follow the recommendations of the material company for vent depth.
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