The flow curve of molten high-density polyethylene (HDPE) has the discontinuous point characterized by the critical shear stress. In this paper we propose a method for measuring the critical shear stress and discusse the relationship between the critical shear stress and molecular weight or molecular weight distribution of HDPE.
During the experiment at a constant shear rate, the pressure increases with maximum peak in the early stage of the extrusion. This peak appeares at initial flow and the critical shear stress is calculated from the peak. The critical shear stress decreases when increasing the molecular weight and molecular weight distribution. It is clarified by GPC measurements that molecular weight of strand surface is smaller than that of the whole strand and this tendency becomes remarkable in accordance with decreasing the critical shear stress. Therefore it is concluded that the decrease of the critical shear stress is caused by the low molecular weight ingredient at the surface.

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For the evaluation of the quality of injection molded products, the surface properties is important. It has been shown that a spectrum analysis quantitatively evaluates the surface properties of injection molded products with emboss. In this research, the surface properties showed that the GF content of the injection molded product of a single cavity, molding conditions, and thickness influenced. Moreover, the influence of the resin temperature and pressure was evaluated by using a numerical flow analysis.
The result of a new evaluation method in the spectrum analysis proposed by this research corresponded to the observational result for one cavity. Moreover, the resin temperature and the pressure obtained from the numerical analysis showed a good agreement with the tendency to the evaluation results of various conditions.

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