Ethylene-propylene rubber modified polypropylene (R-PP) has been often used for automotive exterior parts, but it has been difficult to apply R-PP to large size parts because of the poor dimensional stability. Recently a low coefficient of linear thermal expansion (CLTE) is required to reduce the gap between exterior parts.
In PP/EPR/Talc blends, a reduction of the CLTE in the flow direction of injection molding was tried by changing the flowability of both PP and EPR and the PP/EPR ratio. The flowability showed small effect on CLTE. The lowest CLTE was obtained around a PP/EPR ratio of 30/45 at which the blend showed a co-continuous two-phase structure of EPR and PP phases highly elongated in the flow direction.

抄録全体を表示

The melt flow patterns inside an injection mold have been approximated by utilizing the short shot method. However, whether the shape of a short shot front corresponds to that of the flow front shape during the filling process has not been investigated. It is necessary, therefore, to understand the factors which influence the behavior of the flow front during the short shot process.
In this paper, by using the “Glass-Inserted Mold” (which one of the coauthors and his research group have developed), we observed the behavior of the flow front during the short shot process, and analyzed several phenomena occurring near the flow front. In conclusion, we obtained the following results.
The flow front of the short shot is observed to move gradually during the cooling process after the end of injection. This movement is classified into the following three patterns:
(1) Advancement caused by pressure relaxation of the resin compressed in the sprue and runner.
(2) Regression caused by molding shrinkage of the resin during the cooling process.
(3) Advancement caused by the local transformation near the flow front of some crystalline polymer.
The advnacement (1) accounts for more than 60% of the total, and this advancement depends on the residual pressure inside the runner and the method of screw control. The regression (2) depends on the molding shrinkages. The advancement (3) depends on the filling time inside the mold cavity and the cavity thickness. The advancement (3) of polypropylene reaches 0.1 to 0.2 times the cavity thickness, and that of polyacetal reaches 1.0 to 1.5 times the cavity thickness.

抄録全体を表示

The formation of weld-line or skin-core structure in injection molded materials reduces their strength. A new technique which is called “micro-cutting test” has been performed on injection molded specimens of crystalline polymers. This method can give quantitative information on shear yield strength in the region near weld-line or in the subsurface layer so that inhomogenous mechanical properties of the weld-line and the skin-core structure can be evaluated. The following results have been obtained:
(1) There is a significant difference in shear yield strength between the flow and transverse directions in a liquid crystalline polymer (LCP) specimen. This difference attained a minimum value at the weld-line.
(2) The shear yield strength obtained from cutting along the weld-interface decreased with approach to the weld-line. The region which was affected by the presence of the weld-line was within 6-8mm from the center of the weld line.
(3) The fracture toughness of the LCP weld specimen was about 25% of that of the non-weld material. The reduction of fracture toughness has also been observed for the other crystalline polymers.
(4) The skin-core structure of HDPE consisted of three different layers. The shear yield strength of the skin-layer increased with increasing mold temperature.

抄録全体を表示