Shrinkage is inherent in the injection molding process. Shrinkage occurs when the density of polymer varies from the processing temperature to the ambient temperature. During injection molding, the variation in shrinkage, both globally and through the cross-section of a part, creates internal stresses. These residual stresses act on a part with effects similar to externally applied stresses in the injection molding of thermoplastic plastics. It is possible to obtain a molded product with the desired dimensions using the mold shrinkage phenomenon. What does this all mean and what are the causes and fixes? Can SOLIDWORKS Plastics help with this problem? Let’s take a look and see If issues are controllable.

The range of the basic shrinkage factor is determined by the type of plastic material used. However, there will be fine differences depending on the material manufacturer and the grade of the material. SOLIDWORKS Plastics gives the ability to select different manufacturers of plastics and different grades of the material in the class used. SOLIDWORKS Plastics also allows the creation of custom materials. The user database is a great place to start troubleshooting.

The molding shrinkage factor varies depending on the cavity surface temperature during injection molding. In general, the shrinkage factor tends to be large when the temperature is high. With SOLIDWORKS Plastics, you can set cavity surface temperature along with overall cavity block temperature. Using the plastics measure tool, you can check both mold cavities and the plastics part to make the needed changes.

The molding shrinkage factor varies depending on the magnitude of the pressure maintained after plastic injection and the time of maintaining that pressure. In general, there is a trend in the shrinkage factor becoming smaller when the maintained pressure is high and the pressure maintenance time is long. You have the ability, not only to set time pressure is held, but the amount of pressure applied.

The shrinkage factor also varies depending on the wall thickness of the molded item. There is a trend in the shrinkage becoming larger as the wall thickness increases.  Not only will catching this early in part design save excessive shrinkage, but it will also allow you to view any sink marks from the wall thickness. SOLIDWORKS and SOLIDWORKS Plastics can catch bad wall thickness.

The shrinkage factor varies depending on the gate shape and the gate size. In general, the shrinkage factor becomes smaller as the cross-sectional area of the gate becomes larger. There is also a trend in the shrinkage becoming smaller in the case of a side gate rather than in the case of a pinpoint gate or a submarine gate. SOLIDWORKS Plastics gives the ability use any geometry for the gate detail. Changing the size is very simple and intuitive.

Mold shrinkage is caught early in the design process with SOLIDWORKS Plastics.  You control melt temps, cooling temps, pressures and hold pressures, injection speeds, gate location and part geometry. All materials have a specific shrinkage rate range that provides a guideline to use ideas based on best molding practice.. The further you deviate from specified parameters, the further you will deviate in a normal shrinkage of the part geometry. SOLIDWORKS Plastics will not only catch the problem but will also give you the functionality to minimize it.

Jeff Osman

Jeff Osman has more than 23 years of experience in the mechanical CAD industry. As Senior Technical Sales Specialist Plastics NA, he is responsible for all technical Sales of SolidWorks products, focusing on SolidWorks Plastics, for North America and has been with SolidWorks for 19 years. Prior to joining SolidWorks, Jeff was a senior technical manager with Microcadam, a division of IBM. In addition, he has held several manufacturing positions with companies Processed Plastics, Plano Molding and Furnas/Siemens Electric.