Optical measurement system for characterizing plastic surfaces

Abstract

Injection molded plastic parts are often influenced with the surface defect tiger stripes, which dramatically reduce the visual quality. Tiger stripes are known as alternating bands of bright and dull regions normally to the molded flow direction. This defect highly depends on the injection time and on the formation of the plastic compound. In the last years, the intensity of the tiger stripes defect was controlled visually. For quantifying the tiger strip defect a new, efficient, repeatable, reliable and nondestructive optical measurement system is proposed. To evaluate the dependency of the injection time, a number of five DIN-A5 plastic specimens are molded. Each of the five plates consists of the same material but they have different injection times. For the measurement, one specimen is put into the specimen holder, which is placed on the drawer of a closed cabinet. In this inside black painted cabinet a LED light source and a CCD Camera are mounted. The beams of the LED light are diffuse reflected on the surface of the specimen. To catch only parallel beams by the lens of the camera a large distance between specimen and camera is realized by two justified mirrors in the cabinet. The bright and dull regions of the tiger stripe defect have different diffuse reflection parameters. Thus in a picture of defined brightness the visibility of this defect is very good. To enhance the repeatability the failure of the camera noise and of the light oscillation is reduced by mends of averaging multiple images. Next, the surface structure is filtered out of the image and a representing number of horizontal grey-value lines are extracted. The so called tiger line signal is the difference between the grey line and a calculated polynomial function (degree of 6) and shows the surface defect of each line oscillating on the zero x-axis. For each tiger line signal the mean squared error is evaluated. To calculate a quantitative value of the whole surface, all line errors are averaged to the so called MSE-value. Measurements and comparisons show, that this MSE-value represents surface defects and especially the intensity of tiger stripes very good. The repeating error is lower than 1 %. Experiments for showing unknown effects of normal and of accelerated aging and weathering of plastic surfaces were done successfully.

Experimental Verification of Geometric Changes Caused by the Release of Residual Stresses for Large-Scale Welded Frames

“…The second group of measurement methods are non-contact methods, in which there is no contact between the measuring instrument and the object being measured [26,27]. This allows for accurate measurements of objects with complex shapes, of both small and large size [28][29][30][31], as well as those made of flexible materials such as sponges, foams, or plastics [32,33] or made by additive technologies [34][35][36]; the same applies to metal elements manufactured by wire arc additive manufacturing [37]. For such large components as the welded frame under study, the use of a photogrammetric method is both reasonable and preferable.…”

Experimental Verification of Geometric Changes Caused by the Release of Residual Stresses for Large-Scale Welded Frames

“…The second group of measurement methods are non-contact methods in which there is no contact between the measuring instrument and the object being measured [26,27]. This allows for accurate measurements of objects with complex shapes, both small and large size [28][29][30][31], as well as those made of flexible materials such as sponges, foams or plastics [32,33] or made by additive technologies [34][35][36] also for metal elements manufactured by Wire Arc Additive Manufacturing [37]. For such large components as the welded frame under study, the use of a photogrammetric method is preferred as well as reasonable.…”