Mechanism-based wear models for plastic injection moulds

Injection moulding is a single-step manufacturing process for plastic parts that require very precise dimensions, geometries and low part roughness (Ra 0.2–0.025 μm). The high cost of moulds (typ. > 100,000 €) and unacceptable wear of their cavities can limit the cost competitiveness of the process. Mould cavity surfaces are deteriorated by different wear mechanisms which may drastically reduce the quality of injection moulded parts, interrupt their production, raise maintenance and repair costs, or delay delivery. Therefore, a better understanding of the wear mechanisms and the main parameters affecting mould wear is needed. In this research, three major failure mechanisms were studied: abrasion and erosion (due to fibre-reinforcements in the plastics), and corrosion (due to gas release from the polymers). Testing protocols were developed for each failure mechanism. Abrasion was simulated with a block-on-plate tribological test, erosion was evaluated in an air-jet erosion tester combined with a gravel-o-meter test, and corrosion was simulated using electrochemical techniques. Three different thermoplastic materials and several mould surface solutions (i.e., mould steel and four alternative surface treatments) were tested. Furthermore, this paper presents an innovative approach to simulate the behaviour of mould and plastic materials at the laboratory scale, in order to minimise the cost associated to mould failures, through mould lifetime prediction. The prediction, with some limitations has been made based on the model generated through laboratory tests, by using a multiparameter design of experiments, where different working conditions (such as pressure or speed) and plastic and mould materials were evaluated. A single best solution was not found to resist all failure mechanisms, but useful recommendations for potential solutions for each failure mechanism were proposed. The TiN coating applied by Physical Vapour Deposition represents a good compromise between wear (abrasive and erosion) and corrosion resistance, especially for fibre reinforced plastics. Ni-PTFE coating is proposed to reduce the friction and improve the corrosion resistance of the mould materials when high wear resistance is not needed, as in the case of non-reinforced plastics.