THE OCCURRENCE OF FLOW MARKS DURING INJECTION-MOLDING OF LINEAR POLYETHYLENE

Injection molded parts often show several types of surface defect. It has been hypothesized that wall slip is associated with some of these defects. Wall slip of molten plastics has been observed above a certai n critical shear stress in rheological measurements. The objectives of the proposed research were to analyze the occurrence of flow marks du ring the injection molding of linear polyethylene and evaluate its pos sible relation to wall slip. Various variables were studied in terms o f their influence on flow mark formation: mold thickness, mold tempera ture, melt temperature, gate, injection speed and Teflon(TM) and silic one oil coatings. It was found that injection speed is the controlling factor for the generation of flow marks during injection molding of t he linear polyethylenes studied. Since one of the resin studied had sh own non tendency to slip in capillary flow experiments, and since a Te flon(TM) coating on the mold walls did not affect the occurrence of fl ow marks, we conclude that there is no relationship between wall slip and the generation of flow marks. Microscopic observation of molded su rfaces suggests instead that flow marks result from the filamentaton a nd stretching of semi-solidified material in the neighborhood of the t hree-phase contact line.