Investigation of wave-like flow marks in injection molding: Flow visualization and micro-geometry

The wave-like flow mark phenomenon is one of the surface defects that can o ccur during the injection stage of the injection molding process. The gener ating mechanism of this phenomenon is not yet well understood. In this pape r the flow mark generation mechanism is investigated by (i) directly observ ing the behavior of the polymer in the flow front and contact line areas un der molding conditions when flow marks are generated, and (ii) studying the micro-geometry of the flow marks after molding. Specific attention was giv en to (i) the-point of origin of flow marks (relative to the moving contact line, as well as the history along the flow path), and (ii) the effect of thermal boundary conditions on the micro-geometry of flow marks. These find ings and their significance in relation-to the flow mark generation mechani sm are discussed. It is proposed that the flow marks are caused by nonunifo rm thermal contraction in the solidifying layer immediately behind the cont act line.