Core melt temperature effects on cylindritic structures of co-injection molded polypropylene parts

The co-injection process offers the inherent flexibility to use the optimal properties of each material to reduce the material cost and part weight. The micro structural characteristic of co-injection molded isotactic polypropylene (iPP) products is given. Polarized light microscopy (PLM) is used for comparison co-injection molded products' microstructure of different locations (near gate, middle location, and core melt flow front) and different core melt temperatures. It is shown that a cylindritic layer occurs on the boundary of the skin and core components or the mold-skin melt interface at middle and far gate locations along filling direction. The relations between processing parameters and cylindritic structures formation are discussed by numerical simulated temperature field which considering the latent heat of crystallization. Furthermore, the energy per unit volume is used as a parameter to indicate the formation of cylindritic structure. It is found that the energy of cylindritic structures almost in the range of [39, 108] J/m3 under the given process parameters.