Abstract
Weldlines represent potentially fatal sources of weakness in injection molded thermoplastic parts. This is particularly true for filled and reinforced composi tions where usual remedies consisting in a modification of processing conditions such as increase of melt temperature, injection speed, and pressure do not always work. This paper deals with weldline strength of injection molded polypropylene containing spherical and irregularly shaped fillers (glass spheres, calcium carbonate), or reinforcements such as glass flakes and fibers, talc, and mica. Two types of weldline will be considered: weldlines formed when two melt streams meet head-on and those formed as a result of flow around an insert. It will be shown that the loss of strength is greatest in compositions reinforced with flakes and fibers. This is due to flow induced filler orientation in the weldline zone. Approaches to reduce the loss of strength which make use of known flow behavior of suspensions of fibers and flakes will be illustrated.
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