The strength properties of weldline part have been studied in fiber reinforced polypropylene injection molded plaques. Mechanical tests were carried out for two types of weldline, such as (1) weldline formed by counter flows from double gates, and (2) weldline formed by concurrent flows divided by an insert core in a cavity. Thin plaques were injection molded using either the double gate mold or the single gate mold with a circle insert core. Cylinder temperature and mold temperature were kept constant, and injection speed and holding pressure were varied within the appropriate range. The effects of “counter flow” type weldline on the strength depended on molding conditions, injection speed and holding pressure, while the “concurrent flow” type weldline was independent of the molding conditions. As the strengths of specimens with weldline, weld strengths, were almost same for both types of weldline, injection molding can be designed based on the weld strength using the double gate dumbbell tensile bars. Further the concurrent flow weld strength was influenced by the specimen geometry. The weld strength with a half of core hole contour reduced more remarkably than the one without it. Because the contour affected as a notch and induced the stress concentration. And near the hole, the difference of orientation angle along weldline, the local anisotropy, was larger, when the core diameter was wider. Thus, the weld strength of 20mm hole specimen was lower than that of 10mm hole specimens. These results indicate that geometries must be taken into consideration in order to estimate the properties of weldline.
