Flow and void formation in powder injection moulding feedstocks made with PEG/PMMA binders Part 2 – Slip band model

Abstract

Flow and deformation of powder injection moulding (PIM) feedstocks are considered to occur in similar ways to those observed in materials containing substantial amounts of clay and water, i.e. by a slip band mechanism. A slip band model for PIM feedstocks has been postulated, which allows flow behaviour and void formation to be explained qualitatively. For feedstocks, the slip bands are assumed to be layers of mobile liquid, which, for polyethyleneglycol/polymethylmethacrylate (PEG/PMMA) binders, are considered to be mainly PEG. It is proposed that some of the slip band liquid is drawn from between the randomly densely packed particles between slip bands to form voids. It is considered for given moulding conditions that increasing the apparent viscosity of a feedstock, by decreasing the volume fraction of binder in slip bands, reduces voidage. This can be achieved by increasing the nominal powder loading of a feedstock and/or increasing the effectiveness of the PMMA in holding the particles together more strongly.

Keywords:

• POWDER INJECTION MOULDING
• FLOW
• VOIDAGE
• SLIP BANDS