Modelling of temperatures and heat flow within laser sintered part cake

Objectives

Temperature effects in the polymer laser sintering process are an important aspect regarding the process reproducibility and part quality. Depending on the job layout and position within the part cake, individual temperature histories occur during the process. Temperature history dependent effects are for example part warpage, the crystallization rate and powder ageing effects. This work focuses on temperatures and heat fl ow within laser sintered part cakes.

Procedure

Therefore, a thermal Finite Element (FE) model of a part cake is developed based on experimental temperature in
situ measurements (Figure 1). Determining of the heat flow within laser sintered part cakes requires experimental information about the three-dimensional temperature distribution and history within the powder as a reference for the model development. Since the size of the part cake increases continuously during the build phase, here only the cooling phase is selected for the model development. Experimental temperature measurements are used to specify the temperature distribution and determine the starting of the cooling phase on the one hand and to validate and check the accuracy of the model on the other hand. Thermal boundary conditions and properties of the used bulk polymer powder are analyzed and relevant parameters are identifi ed. The FE model is validated and optimized considering different job heights and ambient conditions during the cooling phase.

Achievements

A model to simulate the temperature history and heat flow within laser sintered part cakes during the cooling phase has been set up. Thermal boundary conditions of a polymer laser sintering system were analyzed. Modelled data has been compared to experimental data obtained with 48 thermocouples inside the part cake. The outer heat transfer coefficient (thermal powder contact and convection) and the thermal conductivity of the part cake were determined in a parameter study. A parameter set has been validated with an accuracy of about 6 K for all sensor positions during the whole cooling process. To improve the model, possible disturbance variables were fi gured out. A consideration of time and location dependent heat transfer coeffi cients lead to an improved model with an accuracy of 3 K. Further aspects are for example cracks within the part cake or the influence of the powder bed density on its thermal conductivity. It is finally possible to predict position-dependent temperature histories as a function of signifi cant job parameters. The model allows a transfer of the results for varied boundary conditions during cooling. In combination with an implementation of built parts, this model will be an important tool for the development of optimized process controls and cooling strategies.