Improved Processing Speed and Dimensional Stability of Polypropylene in Injection Molding Applications

摘要

Polypropylene-based materials are used extensively in the automotive industry in injection molding applications. This paper will review a nucleation technology for polypropylene that gives the fastest processing speed and promotes the most dimensional stability in the final part. Nucleating agents increase the temperature at which the polymer begins to crystallize upon cooling and increase the overall rate of crystallization. These effects result in shorter part cooling times in injection molding processes, which translate into shorter molding times and increased productivity. Crystallization temperatures and rates are commonly measured by Differential Scanning Calorimetry (DSC). DSC data for commercial nucleating agents will be presented and correlations to actual productivity trials in commercial parts will be drawn. Nucleating agents also have an effect on part shrinkage and warpage. As described in the Injection Molding Handbook, 'warpage is caused by variations in shrinkage throughout the part. These variations may appear from region to region through the thickness, and as different shrinkages in the directions parallel and perpendicular to the plastic orientation.' Differential shrinkage is measured by dividing the flow direction shrinkage by the transverse direction shrinkage, using a rectangular mold that would be typically gated in the center of the shortest side of the rectangle. Differential shrinkage data for commercial nucleating agents will be presented and correlations to warpage in actual commercial parts will be made.