Meaning
Asymmetrical shift in the boundary region of an interference pattern occurs during the measurement of thin film thickness or surface profiles. Optical sensors detecting the gap between a mould surface and a polymer melt layer experience fringe envelope skew when the light path encounters refractive index gradients. This shift introduces errors in the calculated distance between surfaces.
Optical Interference
Light waves reflecting from different depths create a constructive and destructive pattern known as a fringe. Thermal fluctuations in the melt stream produce fringe envelope skew by altering the local density of the polymer. Measurement precision depends on a stable, symmetrical distribution of these wave peaks.
Signal Distortion
Variations in pigment concentration or filler distribution scatter the incoming laser light. If the intensity of the return signal drops unevenly across the detector array, fringe envelope skew misrepresents the actual part geometry. Software algorithms must compensate for this lack of symmetry to maintain tolerances in precision moulding.
Accurate readings require the sensor to identify the peak of the interference signal. This process becomes difficult when the signal is stretched or warped by material opacity.
Differing Cooling
Rates across the part surface create density shifts that affect light transmission. Because the refractive index is temperature dependent, fringe envelope skew appears more frequently in semi-crystalline resins like polypropylene than in amorphous materials. Accurate mapping of the part wall requires a stable fringe center.
