Supercritical Extraction of Binders for Metal Injection Moulded Components
A common bottleneck of the powder injection moulding technique is the inevitable debinding step. The debinding process is very time-consuming because of the risk of deformations and flaws, especially for thick-walled parts. A newly implemented debinding technique that has proven to be efficient for certain organic binder systems is supercritical CO2 extraction.
CO2 is a supercritical fluid when the pressure is above 73.8 bar and the temperature is above 31.0° C. The feature of a supercritical fluid is that the viscosity is like gas, the density is like liquid and the diffusion coefficient lies between those of gas and liquid. This implies that the fluid diffuses into an injection moulded component more like a gas and dissolves the binder more like a liquid. The dissolved binder will then be transported out from the component by diffusion. The non-polar CO2 will primarily dissolve small and medium-sized, non-polar hydrocarbons like paraffin and, to a limited extent, stearic acid and beeswax, but not high-molecule polymers. Another characteristic of supercritical CO2 is that it has no surface tension, which means that no capillary forces will appear during extraction.
In this particular project, a medium pressure injection moulding machine from Goceram AB was used to produce parts for the supercritical CO2 extraction experiments. Feedstock containing 65 vol% stainless 316L steel, mixed with a binder system of paraffin, stearic acid and polymer was injection moulded in various geometries for subsequent extraction in supercritical CO2. After a complete extraction cycle at 200 bar and 60° C for six hours, approximately 90 % of the paraffin was removed. Residual paraffin, stearic acid and polymer were thermally burned out in a couple of hours. Despite the rapid burnout, no powder bed was needed either to support the parts or for any wicking process.
Summarised, the advantages of supercritical CO2 extraction are reduced debinding times, dispensable powder embedment, improved surface finish, production of more thick-walled parts and no rearrangement of particles during the process. In addition, all extracted paraffin can be reused without any refinement, which makes the process environmentally friendly.