Effects of Aluminum Alloy and Steel Molds on the Productivity, Mechanical Properties, and Surface Quality of Semicrystalline PP Molded Parts

Aluminum alloy mold (abbreviated as Al mold) is gradually replacing steel mold as an important development direction in the injection mold industry due to its light weight and efficient heat transfer. However, the disparities in the heat transfer performance of molds exert distinct influences on the crystallization behavior of the polymers. To evaluate the potential of an Al mold for molding crystalline polymer, this study carried out the injection molding of semicrystalline polypropylene (PP) by a steel mold and an Al mold, respectively. The differences in the cooling and crystallization behaviors of PP in the two molds were elucidated through multidimensional characterization and numerical simulation. The results demonstrate that the low volumetric heat capacity and high thermal conductivity of the aluminum alloy minimize heat storage, enhancing both the heat exchange efficiency and the cavity temperature uniformity of the mold. Compared to the steel mold, the cavity temperature uniformity of the Al mold is improved by approximately 75%, and the temperature fluctuation is reduced by about 52%. The Al mold augments the temperature gradient and shear rate gradient in the melt thickness direction, which enhances the crystal orientation and molecular chain entanglement density at the shear region of the molded parts. Consequently, the tensile, flexural, and impact strengths of the PP molded parts are improved by about 3.33%, 7.78%, and 12.5%, respectively. Meanwhile, the high cooling rate of the Al mold can significantly increase the thickness of the freezing layer and reduce the uneven shrinkage of the surface layer of the PP molded part, which makes the replication accuracy of the PP polymer on the cavity surface of the Al mold better than that on the cavity surface of the steel mold. In addition, the Al mold offers additional advantages of shorter processing time, reduced wear of equipment, and low manufacturing costs.