The shape loss during polymer burnout was observed in situ for the first time in the field. The schematic of the equipment and experimental set up used for in situ observation is shown in Fig. 1.
The experiments were carried out on thin beams compacted from admixed powders of gas atomised 316L stainless steel and 1 wt.% ethylene vinyl acetate (EVA). The in situ images of shape loss during polymer burnout are shown in Fig. 2. The results showed shape loss to occur primarily due to viscous creep during the softening of the polymer. A recovery of shape loss was observed on burnout of EVA.
The in situ images of shape loss during polymer burnout of injection moulded samples made from gas atomised 316L stainless steel and a polymer mix of 60 wt. % paraffin wax, 32 wt. % polypropylene and 8 wt. % polyethylene is shown in Fig. 3. The shape loss in this case also occurred primarily during the softening of the polymers. The recovery of shape loss was found to be absent during polymer burnout.
The recovery in shape loss was attributed to the burnout characteristic of EVA. During the first stage burnout, EVA degrades to form acetic acid and polyethylene co-polyacetylene. The polyethylene co-polyacetylene forms with a carbon-carbon double bond. The phase change due to the formation of polyethylene co-polyacetylene was attributed to the recovery of distortion during the first stage of polymer burnout. The recovery of shape loss was found to be absent in case of injection moulded samples in which the burn out of the polymers occurred without the formation of double bond product.
The research study shows that the shape loss during burnout depends on the degradation behaviour of the polymer. The shape loss during polymer burnout can be minimised by selecting polymers that degrade by yielding carbon-carbon double bond products.
The research has been carried out with the support of National Science Foundation under the grant DMI-0200554: Protocol Development for Net Shape Powder Metal Part Production via Cold Compaction.
The complete details of the findings from the research study were recently published in the International Journal of Powder Metallurgy and can be found at the following reference: Ravi K Enneti, Seong Jin Park, Randall M German and Sundar V Atre, “In situ observation of shape loss during polymer burnout in powder metal processing” International Journal of Powder Metallurgy, Vol.47, Issue 3, pp.45-54, 2011.