Défauts d’aspect des pièces moulées par injection : exemple de l’influence de la surface du moule sur les “défauts de vagues”

Flow marks in injection molding of polypropylene : influence of processing conditions, mold’s surface and formation in fountain flow

¹ École Supérieure de Plasturgie, Laboratoire de Recherche Pluridisciplinaire en Plasturgie, Oyonnax
² Pôle Européen de Plasturgie, Bellignat
³ Centre de Mise en Forme des Matériaux, École des Mines de Paris, Sophia-Antipolis

Résumé

Cette étude expérimentale porte sur la caractérisation et la propagation des défauts de vagues sur des plaques injectées en polypropylène. Nous avons mis en évidence deux sortes de bandes brillantes (très orientées et non orientées). Le front de matière des pièces incomplètes est déséquilibré alternativement d’un coté et de l’autre du moule. La périodicité correspond aux différentes bandes orientées ou non. Nous avons montré que les conditions de contact entre le moule et le polymère fondu influence fortement l’initiation ou non de ce défaut. Nous comparons nos résultats avec quelques modèles théoriques proposés dans la littérature.

Abstract

Aspect of injection molded part is one of most important criterion for quality in polymer processing industry. The injection molding of a polypropylene in a polished rectangular mold leads to parts with a surface defect appearing as successive bright and dull bands. These bands are known as flow marks. Their appearance mainly depends on processing conditions and on polymer. An ethylene-propylene copolymer with carbon black and a natural homopolymer polypropylene lead to the same flow marks which are more obvious with the copolymer.

These marks result from a difference of reflectivity as measured with a micro-glossmeter. They can result from a change of composition or change of orientation. Micro-infrared spectroscopy in ATR mode with dichroism was performed on these sucessive bands on the surface and on a cut in the mid-plane of the sample. No change of composition or crystallinity was observed, these bands differ by a large difference of molecular orientation. The bright bands are highly or not oriented while dull bands are slightly oriented.

This superficial orientation is the result of local differences appearing during the filling stage. An analysis of the material front on incomplete molded plates shows a periodically unbalanced front. This periodicity corresponds to the change in the surface orientation and the resulting flow marks. The shape of this front is the result of the fountain flow on which is applied a periodical fluctuation. We shall discuss of this observation in touch with the boundary condition. The mold surfaces were modified with an adhesive film of teflon. When the two mold surfaces are covered with the teflon, the material front are balanced and the flow marks are missing (contrary to Heuzey [7], We shall also discuss the hypothesis considering the stick-slip with a critical shear-stress, or the stagnation point as described by Schmidt [8], These are open points.

Financial supports granted from Peugeot SA, Renault, Sommer Allibert Industrie, ECIA, Plastic-Omniurn, Peguform, Appryl and Montell, are gratefully acknowledged.