IN SITU OBSERVATION OF SHAPE LOSS DURING POLYMER BURNOUT IN PM PROCESSING

Abstract

Shape loss or distortion of powder metal (PM) components can occur during polymer burnout. A study has been carried out to understand the evolution of shape loss during polymer burnout utilizing thin beams compacted from admixed powders of 316L stainless steel and 1 w/o ethylene vinyl acetate (EVA). The effect of particle shape on shape loss was studied by using gas-and water-atomized 316L stainless steel powders. The results showed that shape loss occurs primarily due to viscous creep during softening of the polymer. The shape loss was also found to depend on the degradation behavior of the polymer. Recovery of shape loss was observed on burnout in the case of polymers degraded by formation of double-bond product. The recovery of shape loss was found to be absent in injection molded samples in which the burnout of the polymers occurred without the formation of a double-bond product. The midpoint deflection of the beam under the influence of gravity was monitored in situ to quantify shape loss during polymer softening and burnout. Viscosity evolution during polymer burnout was estimated from the rate of midpoint deflection and was found to increase on heating from room temperature to 250 degrees C.