Prediction of post-molding shrinkage of plastic parts by laser interferometry

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Prediction of post-molding shrinkage of plastic parts by laser interferometry

 

Author: Lau, Fuk-Sun
Title: Prediction of post-molding shrinkage of plastic parts by laser interferometry
Degree: M.Sc.
Year: 1994
Subject: Laser interferometers -- Industrial applications
Plastics -- Molding
Hong Kong Polytechnic -- Dissertations
Department: Dept. of Mechanical and Marine Engineering
Pages: 86 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1140653
URI: http://theses.lib.polyu.edu.hk/handle/200/3594
Abstract: Post-molding shrinkage of thermal plastics material has been a major issue for determining the final dimension of plastics parts which require precise dimension and shape for fitting with other parts. Holographic and Speckle interferometry enables measurement of displacement to be made on optically rough surfaces at sensitivity of the order of the wavelength of light. It is therefore suitable for the study of the profile of minute post-molding shrinkage of a plastic part. As the prediction of post-molding shrinkage is an on-line process, Electronic Speckle Pattern Interferometry (ESPI) was adopted as the measurement instrumentation with the consideration of its fast measurement speed and ease of handling. The objectives of the experiment are to develop and utilise the Set-up of ESPI arrangement for the measurement of slow but continuous in-plane displacement; to evaluate the measured data and derive a model for the prediction of long-term shrinkage of plastics parts. In the experiment, 'Derlin' acetal resins parts were chosen as the study medium as these crystalline polymers possess relatively high post-molding shrinkage but respond less to environmental changes such as humidity and temperature. Four types of variation of displacement fringe patterns were observed. By comparing the ESPI data obtained, it was found that the specimens have nearly the same shrinkage coefficient and it was deduced that the difference in shrinkage after 10 years is less than 5um. In other words, if the molding conditions are properly controlled, the plastics part shrinkage behaviour at a given exposure temperature will be the same. A fix model was therefore deduced for the determination of post-molding shrinkage.

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