Residual Stress Control Issues for Thermal Deposition of Polymers in SFF Processes 209
Controlling residual stress-induced warping and other tolerance losses is important for accurately creating parts by solid freeform fabrication (SFF). In this paper, results are presented from warping experiments on plate-shaped acrylonitrile butadiene styrene (ABS) specimensmcreated by an extrusion process used in Shape Deposition Manufacturing (SDM). Experimental results are compared to predictions from both one- and two-dimensional types of residual stress models. In addition to SDM, methods and results from this study are applicable to a number of other solid freeform fabrication processes involving extrusion of polymers or polymer slurries. Results from polymer extrusion are compared with those from existing work on thermal deposition of metals. Unlike metals, polymer deposition shows essentially no stress reduction due to preheating by the deposition process. Due to a greater number of deposited rows, directionality of warping is also greater than in metals. Polymer deposition experiments show that a preheat temperature near the glass transition temperature is needed for essentially no warping. Comparison of predicted and measured curvatures show that a simple 1-D thermomechanical model does not predict warping magnitudes well, but does provide insight into trends in warping as a function of preheat temperature. The effects of successive material deposition are substantial in this process and a 2-D model that includes the effects of successively deposited rows can provide much more accurate curvature predictions.