Study of Recycled and Virgin Compounded Metal Injection Moulded Feedstock for Stainless Steel 630

Fine rounded powders preferable for metal injection moulding (MIM) are expensive. This forces MIM makers to recycle green scraps, for example, the runner system and defected green parts. This is particularly necessary for injection moulded small parts where parts are only a small portion of the injection short size. There is very little published data, although recycling feedstock has been practise throughout the industry. This work aims at investigating the effects of recycled stainless steel 630 feedstock content on the density, mechanical properties, dimensional changes and microstructure. Five batches of compounded virgin and recycled feedstock were studies from 0% to 100% recycled feedstock with the increment of 25%. Homogenously compounded feedstock was injected using the same injection condition. Subsequently, green parts were debinded and sintered at 1325°C for 2 hours in argon atmosphere. The results suggest that the green density increases linearly with increasing percentage of recycled feedstock because the polymeric binder was broken down during previous process. However, the sintered density remains nominally constant. As a result, the mechanical properties and microstructure of sintered parts are independent of recycled feedstock content. However, the volumetric and linear shrinkage decreases linearly with the increase in percentage of recycled feedstock. The difference in shrinkage is vital to dimensional control during commercial production. For example, only 4.5% of recycled feedstock can be added to virgin feedstock if a tolerance of ±0.3 mm is required for a 25 mm MIM part.