| We have investigated structure development of various polyolefins in the injection molding process, which include high density polyethylene (PE), isotactic polypropylene (PP), isotactic polybutene-1 (PB1) and isotactic poly(4-methyl pentene-1) (P4MP1). Orientation, morphology and crystallization variation along thickness direction of molded part were studied under varying processing parameters: volumetric injection rate or injection pressure, packing pressure, and thermal condition based on mold and inlet melt temperatures.; The effort to account for these variations on structure development was combined with considerations of quiescent crystallization kinetics and rheological properties of raw materials. The order of crystallization rate from fast to slow was found out to be PE > P4MP1 > PP > PB1. The relaxation behavior of each polymer chain was studied to consider the amount of chain relaxation before crystallization from melt state and explain resulting orientation development.; In order to make objective and quantitative comparisons in structure development of different materials or even the same material with different crystallization and rheological behavior, it was necessary to apply equivalent stress since crystalline orientation and morphology are determined by applied stress fields. From the effort to provide equivalent mechanical stress during molding process, we could find out that the level of orientation during mold filling is highest in P4MP1 under same level of stress applied, then PB1 > PP > PE, which is consistent with the earlier Ph.D. dissertation of C.H. Choi for melt-spinning study. A mathematical model was developed to represent the relationship between the levels as well as the modes of orientation in melt state and in the molded parts after crystallization. The orientation state is represented by a second order tensor, Oij. Different crystalline orientation states in crystallized polymers as compared to the melt are represented by a 4th-order tensor, Sijkl which varies with the materials. |
