A Model System for Powder Injection Moulding – Physical Properties and Mould Filling Analysis

Licentiatavhandling, 2009

ABSTRACT Powder injection moulding (PIM) is an important and accepted industrial technique for producing net-shaped components which combines the advantages of injection moulding and powder metallurgy. The properties of the feedstock, a mixture of powder and binder system (often based on a thermoplastic polymer), strongly influence the process. An accurate characterisation of the feedstock properties are then of great significance in many situations, e.g. when simulating moulding of PIM-components. Here it is important to perform the measurements at conditions similar to those experienced during the injection moulding. In the present work, a model system, consisting of steel powder, poly(ethylene glycol) and wax, is used in order to illustrate how the rheological properties as well as thermal properties, such as the conductivity and the specific heat, of the system can be related to the corresponding properties of the polymeric binder system. In a similar way, the pvT (pressure-volume-temperature)-behaviour of the model system is analysed and discussed. With the model system described above and a medium-pressure injection-moulding machine, the mould filling phase was studied in situ with a special mould equipped with a sight glass using a high speed camera. Effects of features such as the mould temperature, the melt temperature, the surface roughness of the mould cavity, the gate dimensions, the mould design and the flow rate on the filling behaviour were demonstrated. In particular it was noted that a rougher mould surface facilitated to some extent the filling of the mould. The mould temperature, the nominal melt temperature and the flow rate affected the position of weld lines via the cooling of the feedstock and the corresponding change of the melt viscosity. The experimentally determined weld-line positions were compared with predictions from numerical simulations performed with a commercial software. The physical properties of the model feedstock, as measured in the first part of this study, were used as input data for the simulation. The agreement between the experiments and the predictions was in most cases quite fair or good. The in situ observation of the mould filling also provided the possibility to study how the gate dimensions and the flow rate influenced the jetting behaviour. In summary, this work combines feedstock characterization, in situ visualization of practical mould filling experiments and computer simulations in order to get a better understanding of the properties courses of events influencing the mould filling Key words: powder injection moulding, rheology, thermal properties, mould filling, processing conditions, jetting, computer simulations