Distinct shrink rates for general plastic gears A good method to determine shrink requires to two-step approach. Shrink factors are estimated for the gear in question. After the tool is made and the first gears are molded, they are then profile inspected for exact involute geometry. The individual shrink rates are then determined, a new cavity is made to the measured shrink and the final gear geometry is properly sized. Only profile inspection will be able to accurately determine involute shrinkage. Gear roll testing may give some idea of shrinkage anomalies, but it can also give misleading indications. Sometimes heavily glass filled material is selected for gears due to its low shrink rate. Shrinkage then becomes less of an issue in mold design. This approach can also cause its own problems. Unfilled engineering resins such as nylon and acetal mold into very precise shapes, albeit with shrinkage. Glass filled materials will have knit lines where injection flow fronts merge. These knit lines can cause distortion at the tooth surface as well as localized weak spots on the gear.
Distinct shrink rates for general plastic gears A good method to determine shrink requires to two-step approach. Shrink factors are estimated for the gear in question. After the tool is made and the first gears are molded, they are then profile inspected for exact involute geometry. The individual shrink rates are then determined, a new cavity is made to the measured shrink and the final gear geometry is properly sized. Only profile inspection will be able to accurately determine involute shrinkage. Gear roll testing may give some idea of shrinkage anomalies, but it can also give misleading indications. Sometimes heavily glass filled material is selected for gears due to its low shrink rate. Shrinkage then becomes less of an issue in mold design. This approach can also cause its own problems. Unfilled engineering resins such as nylon and acetal mold into very precise shapes, albeit with shrinkage. Glass filled materials will have knit lines where injection flow fronts merge. These knit lines can cause distortion at the tooth surface as well as localized weak spots on the gear.
247102386-Fabrication-of-Gear-Type-Injection-Moulding-Achine.docx