Analysis of Factors Affecting the Demolding of Micro-Structured Devices in the Injection Molding Process

摘要

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 the fabrication in an injection molding process, the demolding stage is critical for success. For an in-depth investigation a tool for acquiring the demolding energies during the injection molding process was developed. This quantitative value in addition to injection molding simulations was used to describe the demolding stage. For easier classification the relevant parameters were grouped into four main influencing factors: polymer, machine parameters, microstructure design and the tool surface. Subsequently, several parameters affecting the demolding energy were attributed to each of those categories and examined systematically. One semi-crystalline polymer (PP), two amorphous polymers (COP and PMMA) and a thermoplastic COC-elastomer were investigated. It was found that the demolding energies were determined strongly by the polymer elasticity and less by its shrinkage. To investigate the machine parameters, the mold temperature and variothermal heating were analyzed. In this study a critical demolding temperature T_(dcr), as mentioned in literature, was confirmed for amorphous polymers. The investigation on the design of the micro-structures confirmed a favorable placement close to the gate and a preferable structure orientation in flow direction. However, a number of demolding effects that can distort the measurement had to be considered. These include part failure for brittle materials as well as strong deformation of elastic materials during the demolding stage. For the tool surface a TiN coating was found to lower demolding energies in a confined temperature range compared to the Ni surface. In general, TiN is advantageous at higher temperatures, although a thorough investigation for specific polymer-coating-combinations is necessary.