Residual stress measurement study on injection molded Polyoxymethylene specimens

Theresia Schrank

Research output: ThesisMaster's Thesis

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Abstract

Polyoxymethylene is an engineering thermoplastic, which can reach a high degree of crystallinity and shows good mechanical properties. These mechanical properties are on the one hand dependent on the polymer’s morphology and on the other hand on residual stresses. Residual stresses develop as a consequence of non-uniform cooling conditions and deformations during the processing. In this thesis, injection molded tensile specimens were examined regarding their residual stress distribution throughout their thickness. These specimens were produced in March 2017 within the framework of a former study. A Design of Experiments was carried out at that time: the holding pressure, the temperature profile along the barrel and the mold temperature were varied. Previously, the specimens were analyzed by tensile tests, fracture tests, differential scanning calorimetry (DSC), microscopy and X-ray diffraction. Now, within this thesis, three different methods for the evaluation of the residual stresses and stress profiles were conducted: layer removal technique (LRT), wide angle X-ray diffraction (WAXD) and nanoindentation. One aim was to obtain residual stress distributions in dependence of the injection molding conditions and to compare them to each other. For this, layer removal technique was chosen, as this method has been widely used in polymer science. Furthermore, basic feasibility studies on WAXD and nanoindentation for their capability in the determination of residual stress profiles were of interest. With layer removal technique, plausible results regarding residual stress distributions were obtained. Compressive stresses were found in the near surface regions and tensile stresses in the core. The influence of the different processing parameters also met the expectations. Nevertheless, high standard deviations in the near surface stress values were observed, which arose as a consequence of the milling process and the curvature determination at big bending radii. Therefore, no exact statements could be made on this areas. WAXD also gave plausible stress distributions, which were in good accordance with the ones obtained by LRT. However, due to assumptions made on the stress free state and the so-called X-ray elastic constants during stress calculation, no quantitative comparison with other methods could be made. For nanoindentation, two models for the stress calculation based on literature were used. Samples were not milled as in the two other methods, but the surfaces of the tensile specimens were analyzed. This was conducted along the thickness, width and the cross-section. Neither of the two models led to satisfying results. Geometrical issues as well as rough surfaces made it difficult to conduct valid measurements. In general, LRT is considered to give the best results, but WAXD measurements do also yield potential. Nanoindentation is at the present considered to be not suitable for the residual stress determination of Polyoxymethylene.
Translated title of the contributionAnalyse von Messmethoden zur Bestimmung von Eigenspannungsprofilen in spritzgegossenen Proben aus Polyoxymethylen
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Berer, Michael, Supervisor (external)
  • Pinter, Gerald, Supervisor (internal)
Award date12 Apr 2019
Publication statusPublished - 2019

Bibliographical note

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Keywords

  • Polyoxymethylene
  • Residual Stress
  • Layer Removal Technique
  • WAXD
  • Nanoindentation

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