Question:
I am a chinese student and have two questions for Dr. Satya Banerjee:
1.How can you improve the relative density after sintering in pure copper powder metal injection moulding process?
2.I would like to know the d50 of the metal powder for MIM.
Your guidance will be appreciated.

Answer:
In answer to your first question: There are two issues here; the purity of copper and the sintering process.
Even if you start with the purest copper powder, the powder will oxidize when exposed to air during feedstock formulation. So you need to reduce the oxides during the sintering cycle. Since the copper oxides are easily reduced by a number of atmospheres, this is not a difficult situation.
The extent of sintering achieved as measured by the density attained is a function of temperature and time, when other factors are constant. The maximum density attained is always lower than the theoretical density and once this is attained, further time results only in grain growth.
Post-sintering densification may be achieved by HIPing, assuming that the majority of the pores are closed.

In answer to your second question: Typical d50 used for metal powder injection moulding is 5 to 8 μm for carbonyl iron type powders and 20 to 25 μm for atomized powders. I am sure larger particle sizes may be used for making parts if the property requirements for the parts permit the use of lower densities.