Monitoring of the thermal deformations on polymer parts using a vision system

Dimensional measurements in production environment are affected by non‐controlled temperature conditions. In the case of polymer parts the high thermal expansion coefficient leads to significant dimensional changes. In order to achieve high accuracy in dimensional measurements, thermal deformations must be monitored and the measurements compensated. In this investigation thermal deformations on polymer parts are monitored using a vision system consisting of a camera equipped with telecentriclenses focused on the surface of the part. The magnification of the optics and an axial illumination allow appreciating the surface texture and surface details on the parts. A set of images is acquired at varying temperature. Digital image correlation with subpixel resolution is performed on images to estimate the displacement of the surface features. The effectiveness of the calculation is related to the quality of the surface features caught by the camera. Experimental tests are performed on a commercial ABS (Acrylonitrile Butadiene Styrene) part. Two series of pictures are acquired in different locations of the part during a cooling period of 10 minutes. Traceability of the method is established through a calibrated artefact for optical microscopes. Displacement measurement uncertainties lower than 0.5 μm have been documented.