Analysis on factors affecting the demolding of micro-structured devices in the injection molding process

Barbara Strohmayer

Research output: ThesisMaster's Thesis

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Abstract

The increasing demand for disposable 'point-of-care' systems poses on-going challenges for health care industries to provide improvements in diagnosis and fabrication techniques. Regarding injection molding of thermoplastic polymers as one possible mass manufacturing process, the demolding stage is most critical for success, since the structures are prone to deformation during demolding. As a consequence, a concept for acquiring demolding energies in the fabrication process was developed to obtain a quantitative value that describes the demolding stage. Based on a literature study and theoretical considerations regarding the demolding behavior of micro-structured devices in the injection molding process, the main influencing factors were grouped into four categories: polymer, machine parameters, micro-structure design and tool surface. Subsequently, a number of factors affecting the demolding energy were attributed to each of those categories and examined systematically. The aim of this work was to investigate and understand the four main influencing factors as well as their interaction. A systematic study allowed for the determination of the parameters, that led to low stress and deformation in the polymer, thereby achieving successful demolding by lowering demolding energies. In the category polymer a semi-crystalline material (PP), two amorphous materials (COP and PMMA) and a thermoplastic COC based elastomer were investigated. It was found that the demolding energies were rather determined by the elasticity of the polymer than the shrinkage. To investigate the machine parameters, the mold temperature and variothermal heating were analyzed. A critical demolding temperature T_dcr, which had been mentioned in other reports, was confirmed for amorphous polymers. When the variothermal heating was applied, the demolding energies were decreased by up to 50 %, which was most likely caused by relaxation mechanisms as well as limitations of the measurement system. The investigation on the arrangement of the micro-structures confirmed a favorable placement close to the gate and design in flow direction. Still, their influence was very low, revealing a low impact of shrinkage on the demolding energy. In addition, since the efficiency of a TiN coating had been proven in other reports, it was found to lower demolding forces in a confined temperature range compared to the Ni surface. In general, TiN is advantageous at higher temperatures, although a thorough investigation for a specific polymer-coating-combination is necessary. Summing up, the demolding temperature was deduced to be the most important parameter affecting demolding energies, since polymer properties are strongly temperature dependent.
Translated title of the contributionUntersuchung der Einflussfaktoren auf den Entformungsprozess beim Spritzgießen von mikrostrukturierten Teilen
Original languageEnglish
QualificationDipl.-Ing.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Holzer, Clemens, Supervisor (internal)
  • Lucyshyn, Thomas, Co-Supervisor (internal)
Award date26 Jun 2015
Publication statusPublished - 2015

Bibliographical note

embargoed until null

Keywords

  • injection molding
  • demolding of micro-structures
  • demolding energy

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