Optical measurement system for characterizing plastic surfaces

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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.

Original languageEnglish
Title of host publicationImage Processing
Subtitle of host publicationMachine Vision Applications
DOIs
Publication statusPublished - 2008
EventImage Processing: Machine Vision Applications - San Jose, CA, United States
Duration: 29 Jan 200831 Jan 2008

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
Volume6813
ISSN (Print)0277-786X

Conference

ConferenceImage Processing: Machine Vision Applications
CountryUnited States
CitySan Jose, CA
Period29.01.200831.01.2008

Keywords

  • Computer vision
  • MSE
  • Surface quantification
  • Tiger stripes

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