Low pressure metal powder injection molding is a forming technique of complex parts. Metal powder is mixed with low viscosity polymers (eg. Wax) to form a homogeneous mixture at a temperature above the polymer melting point temperature. To facilitate injection into the mold cavity, the composition of the mixture is adjusted to decrease the viscosity. On the one hand, the low viscosity feedstocks have good moldability. On the other hand, the phenomenon of segregation rapidly manifest with low viscosity feedstocks.
The commercial machines are equipped with an injection channel and a valve which connect the mixing container and the mold cavity. The feedstock remains stationary in these components between two injection sequences, which make it suitable to segregation. Several patents attempt to solve this problem by using pumps and recirculating channels. These components are difficult to clean because of their complexity.
A low pressure injection machine has been designed for the study of moldability of very low viscosity feedstocks (0.1 Pa·s) which takes into account the phenomenon of segregation and cleaning constraints. An injection piston withdraws a desired volume of a container. Next, a lateral movement shear the feedstock at the intersection between the container and the cylinder and plug the outlet of the container. The cylinder is then released and can receive the mold. Following the injection, the piston returns to the container position and plug the outlet of the container. The residual mixture is returned to the container, mixed and deaerated again.
The device has been validated by injection tests with a mix of stainless steel powder and low viscosity binders. Injection tests have shown that the feedstock containing stearic acid has covers the greatest distance in the spiral shape mold, followed by the feedstock containing stearic acid and ethylene vinyl acetate and finally the feedstock containing only the wax paraffin. Rheological tests were correlated with results for viscosity associated with low shear rates. A segregation test showed that waiting time less than one minute should be respected before measuring a significant segregation. In addition, the operation of the vacuum system has been validated by radiographic analysis. A vacuum under 30 min mixing time is sufficient to completely eliminate the presence of air bubbles in the injected part.
G. Lamarre, S. (Author),
Demers (Supervisor) &
Chatelain (Co-supervisor),
12 Dec 2016Student thesis: Master's thesis › Master in Engineering: Mechanical Engineering