In order to explore the coupling effects of second shear flow and temperature fields on the formation as well as the evolution of crystal morphologies of high-density polyethylene (HDPE)

the crystal morphologies of the parts obtained by multi-fluid multi-injection molding (MFMIM) were investigated.The main feature of MFMIM is that the polymer melts in two different injection units are injected sequentially into the cavity to form an encapsulated sandwichstructure product.Polarizing light microscopy (PLM) results showed that some peculiar microstructures (e.g.cylindritic structures and ring-banded spherulites) were generated in the skin layer of the MFMIM parts.With the aid of scanning electronic microscopy (SEM)

a detailed description of cylindritic structure was obtained

which showed that the cylindritic structure consisted of some ring-banded spherulites.For comparison

the crystalline morphology of the sample molded by conventional injection molding(CIM) was also investigated.Different from the MFMIM parts

there were not observed any ringbanded spherulites or cylindritic structures in CIM parts except the common spherulites.And there was not a significant skin-core structure formed due to the higher temperature of the mold.Based on the comparison between the experimental results of the MFMIM parts and those of the CIM counterparts

it was concluded that the main reasons for ring-banded spherulites and cylindritic structures arising in the skin layer of MFMIM parts were the shearing and the complex thermo-mechanical fields provided by the secondary melt penetration process.Therefore

on the basis of the previous results

the mechanisms of ring-banded spherulites and cylindritic structures formation were discussed.