Shrinkage behavior and optimization of injection molded parts studied by the Taguchi method

Shrinkage behavior of a plastic plays a critical role in determining the fi nal dimensions of an injection-molded part. It is well known that process c onditions affect many properties of plastic parts, including shrinkage. Thi s study applies the Taguchi method to systematically investigate the effect s of process conditions on the shrinkage (along- and across-the-flow direct ions) of three plastics: high-density polyethylene (HDPE), general-purpose polystyrene (GPS), and acrylonitrile-butadiene-styrene (ABS). The results s how that HDPE, a semicrystalline plastic, shrinks more than GPS and ABS, tw o amorphous materials. The extent of anisotropic shrinkage in the along-the -flow and across-the-flow directions for HDPE is different from GPS and ABS . More shrinkage occurs in the across-the-flow direction of HDPE than in it s along-the-flow direction. The reverse is true for GPS and ABS. Mold and m elt temperatures, along with holding pressure and holding time, are the mos t significant influences on the shrinkage behaviors of three materials, alt hough the importance of each is different for each plastic. The optimal con ditions for reducing shrinkage identified by the Taguchi method are experim entally verified and validated by t-statistic tests. The prediction matches very well with the experimental value for the along-the-flow shrinkage of GPS.