Asking the company to do a test shot on an obsolete mold of similar size to your project is an easy way to see plastic shrinkage first hand. Many injection molders will have a large supply of obsolete molds. ICOMold will always recommend performing test shots prior to injecting parts that are large, parts that have tight tolerances, or using a new or unusual material. This can be done by manipulating how hard the cavity is packed or by extending the cooling period in the mold. The molder can fine-tune the shrinkage of the parts by adjusting the density of the material.
This will ensure that the final parts meet the model specifications. It is critical to factor in the shrinkage rate at the tooling design stage for any parts with tight tolerances. during validation and inspection, the shrink average along those walls is expected to be 0.006 in. The material supplier will usually provide a material data sheet that specifies a shrinking range for the material (e.g. Plastic injection molding shrinkage will also vary with wall thickness. In other words, ICOMold compensates for plastic shrinkage in the mold design so that the part specs match the model. At ICOMold, the mold shrinkage calculation takes place during the mold building stage, so the stabilized part dimensions should align with the CAD model specs and part prints. This is done by multiplying every number on the drawing by 1.00X. In pre-CAD days, the engineer would compensate for mold shrinkage by enlarging the part. When considering mold shrinkage calculations, the tooling engineer simply scales the mold tooling by 1.00X. Mold - Compensating for Shrinkage in Both Any dimensional inspection should wait at least a day after part ejection to maintain consistency and suitable part stabilization The part may continue to shrink slightly for several hours or even days until the temperature and moisture content stabilize to match the surrounding environment. The majority of excess heat has been already dissipated and the most of shrinkage has occurred before the part is ejected from the mold. This is especially true for plastic materials like Delrin or Polyoxymethylene. Most of the shrinkage occurs while the part is still within the molding tool during the cooling stage, but a small amount of plastic shrinkage may occur after ejection, as the part continues to cool. Mold shrinkage is usually stated as a percentage. Mold shrinkage is the dimensional change that occurs in a molded part as it cools after injection. Designers must take into account the shrinkage and contraction rate of the injected material and how that will affect the shape of the part. The two terms may be used interchangeably but it is important to note that mold shrinkage refers to the dimensional change of the injection molded object as it cools, while plastic shrinkage can be used to refer to the characteristics of a specific plastic. Plastic shrinkage or mold shrinkage is an important consideration when designing any part for injection molding.
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