Material flow influence on the quality of molded parts

Abstract

This paper focuses on a new occurrence mechanism of the flow marks, which appear on the surface of injection-molded parts, through characterization of the part surface structure. Injection-molded parts involving long flow lengths can exhibit visible flow defects characterized by alternating glossy and dull bands on the polymer surface, which have approximately the same direction with the material flow front. These flow defects are considered to have strong negative effect on the appearance of the molded component and thus on the perceived quality. The reasons for the appearance of these stripes are not entirely clear. Defect flow marks (shiny areas) were cut from the molded part and analyzed using optical microscopy, Fourier transform infrared spectroscopy, laser Raman spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray (SEM/EDX) spectroscopy. Raman spectra detected differences between spectra of defect area and of the reference one. The EDX analysis revealed that the defect area contains slightly more quantity of talc (Mg and Si). This happened because of material flow morphology in the areas where the molded part presents ribs. The flow of melted resin through an enlargement (rib) results in a decrease in flow velocity, and consequently, a pressure rise will occur and it also involves a vortex development. Therefore, the shear stress in this area is higher than upstream and downstream ones, and it will cause a delay of the flow stream, a premature solidification of the melted resin and ribs flow marks occurrence.