Stress Relaxation in Poly(methyl methacrylate) (PMMA) At Large Strains During The Process Of Hot Embossing
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Date
2013-12
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The Ohio State University
Abstract
Poly(methyl methacrylate) is a strong and lightweight polymer which has a number of applications, hot embossing being one of them. The output of an embossing process, a surface profile, involves monitoring a number of factors such as strain rate, total strain, embossing temperature and spring back. Due to its glass transition of around 107˚C, PMMA is an ideal candidate for this process and is used in multiple manufacturing techniques. Previous experimental work in this area had led to the development of a mechanical model which inaccurately predicted stress relaxation in the polymer at higher temperatures. The purpose of this project was to improve this model by performing experiments incorporating certain factors that were not included before, such as cooling the sample and spring back. For this, a number of samples were tested at temperatures above their glass transition temperature using an Instron testing machine. The samples were compressed between two compression plates, and held at a constant strain level for a specific period of time. During this period, the stress in the material was recorded by measuring the force exerted by the sample on the compression plate. Similar tests were carried out by varying the test parameters and the differences in stress relaxation behavior were observed. At the end of a test, spring back in the sample was recorded after a period of one hour. Percent spring back was calculated based on these values, and along with data obtained from the experiments, was incorporated in the mechanical model. Simulations from this mechanical model were compared with the experimental data and it was found that this model fits the new stress relaxation with cooling data reasonably well. By further improving this mechanical model, PMMA’s behavior under complex loading conditions can be determined which will ultimately reduce trial and error in the mold design procedure, and benefit the industry by saving time and money.
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Keywords
Polymer, Stress relaxation, Hot Embossing, PMMA, mechanical model