Detection of visually perceptible sink marks on high gloss injection molded parts by phase measuring deflectometry

Phase Measuring Deflectometry is used as a means to detect visually perceptible sink marks on the surfaces of injection molded parts. Discrete orthonormal polynomials are used for the regularized estimation and numerical differentiation of the measurement results which contain the first derivative of the part's surface. Since sink marks are perceived as local disturbances of surface curvature, image processing methods are used for the detection of sink marks. The numerical evaluation of the sink marks was achieved by the application of a model for the sink mark intensity. For the verification of the proposed method, several series of an injection molded part were manufactured for experimentation. Beginning with process parameters which gave rise to good parts, the holding pressure was incrementally decreased for each series, while other processing parameters were kept constant. The results obtained with the proposed method showed good correlation between decreasing holding pressure and the expected increase of sink mark perceptibility.