Compensation of Non-Linear Shrinkage of Polymer Materials in Selective Laser Sintering

Inaccuracies in the selective laser sintering (SLS) process using polymer materials are typically caused by inhomogeneous shrinkage due to inhomogeneous temperature distribution in the powder bed of the SLS machine. These shrinking effects lead to stress in the sintered parts, causing the part to distort. The inhomogeneous shrinkage of benchmark parts has been compensated empirically in a former work. The results cannot been transferred to all geometries, because each geometry requires a specific temperature for laser sintering and, thus, has its own related shrinkage distribution. In a new theoretical approach, shrinkage behavior is to be integrated in a thermal simulation of the SLS process and the thermal shrinkage calculated prior to the building process. In the following, experimental data of the temperatureand pressuredependent shrinkage of laser-sintered powder samples is presented. Possible theories for a physical model of thermal shrinkage are discussed. In particular, these models have to consider granular characteristics such as internal friction, particle sliding, and powder compaction. Introduction Today rapid prototyping, rapid tooling and rapid manufacturing processes are counted as established means of accelerating the product development cycle in the automobile and aeronautics industries. Thus we see more and more prototype parts, which in the past were traditionally produced using high-cost, time-consuming milling and joining techniques, being replaced by investment cast parts. And the master forms deployed in investment casting are now typically plastic forms produced by means of selective laser sintering (SLS). In addition SLS processes are used to generate plastic functional prototypes for both the automobile and aeronautics industries. Yet the above applications alone are not what makes SLS techniques so interesting for industry: the growing demand for customized automotive products and the resulting small lots needed for low-volume production has caused a commensurate demand for the fabrication of volume-produced parts using generative processes. As a key manufacturer of premium cars, DaimlerChrysler is driving research in these direct-to-product technologies to accelerate product development and reduce costs. Thus, an essential part of our work focuses on the very strict requirements for accuracy in the SLS process to achieve the desired high quality of the parts produced.