Efforts to reduce part bed thermal gradients during laser sintering processing

Abstract

Part bed surface thermal gradients (x-y plane) are usually present in laser sintering (LS) fabricators. The purpose of this study was to investigate various means to reduce these thermal gradients. Several experiments were conducted using a FLIRTM infrared camera to examine the thermal profile of the part bed during the LS operation. Experiments included thermal profile characterization of the part bed with different nitrogen shielding gas flow rates, assessment of the proper experimental settings, and a temperature profile record of the part bed from the warm-up to the cool-down stage. A series of experiments were conducted using the laser as a heat source to preheat part bed surface cold spots to decrease the thermal gradients, which effect was limited by the natural low thermal conductivity of nylon 12 powder and large heat convection. Moreover, manifolds were mounted below the piston to provide warm nitrogen down draft flow during the LS operation. Introduction Laser sintering is an additive manufacturing technology that uses a high power laser to fuse polymer powder into a mass that has a desired three-dimensional shape. The laser selectively scans and fuses powder material on the surface of the powder bed based on a previously generated CAD file. After one layer is scanned, the powder bed is lowered by one layer thickness, creating a new layer that is scanned. The process is repeated until the part is completed [1]. This process has the potential to be a valuable industrial tool [2]. Thermal processing during the LS operation is essential in improving the efficiency of the process and the accuracy of product quality and dimension. Polyamide 12, also known as Nylon 12, is a thermoplastic material that is widely used in laser sintering. It has high elongation, good abrasion resistance, good specific strength, and melts at a temperature around 180°C [1]. Thermal management and heat transfer analysis of polyamide 12 in the LS part bed are essential to better understand the effect of processing on the service part properties and performance. Temperature distribution of powder bed during LS has been analyzed in different materials including metal, polymer, etc. Some were focused on the surface temperature distribution, while some others were about the LS operation chamber. S. Kolossov et.al [3] developed a 3D thermal model of LS using finite element simulation, which incorporated the nonlinear behavior of the titanium powder. They found the temperature profiles during LS operation along both Xand Yaxes reached a maximum around 2500°C in the middle of the axes, and decreased to 500750°C on both sides. These results were in agreement with the infrared red camera experiments.