The influence of molecular weight and mould temperature on the skin-core morphology in injection-moulded polypropylene parts containing weld lines

• Published in: Journal of materials science 1996-05, Vol.31 (9), p.2487-2493
• Main Authors: WENIG, W, STOLZENBERGER, C
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Injection moulding; Machinery and processing; Moulding; Plastics; Polymer industry, paints, wood; Technology of polymers
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/BF01152966

The sandwich structure of injection-moulded polypropylene parts with and without weld lines has been investigated using optical microscopy, x-ray scattering, and differential scanning calorimetry. It was found that the skin-layer thickness is strongly dependent on whether the mould is injected through one gate or through two gates. Samples containing a weld line show a much lower skin-layer thickness than samples without a weld line. This difference, however, depends strongly on the molecular weight of the polypropylene. The skin-layer thickness also varies along the flow path and decreases with increasing mould temperature. While most of the polypropylene crystallizes in the crystallographic alpha -modification, in the textured skin layer, some beta -modification occurs. The beta -crystallinity depends on the molecular weight but disappears in the weld line. X-ray investigations exhibited a bimodal crystal orientation in the skin layer, one with the c-axis oriented parallel to the flow direction, the second one with the a exp * -axis oriented parallel to the flow direction. The core exhibited only a weak c-axis orientation. In the skin, the metastable alpha sub 1 -modification was found.