Elsevier

Additive Manufacturing

Volume 12, Part B, October 2016, Pages 189-196
Additive Manufacturing

Full length article
Modelling of temperatures and heat flow within laser sintered part cakes

https://doi.org/10.1016/j.addma.2016.06.002Get rights and content

Abstract

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 flow within laser sintered part cakes. Therefore, a thermal Finite Element (FE) model of a part cake is developed based on experimental in-process temperature measurements. Thermal boundary conditions and properties of the used bulk polymer powder are analyzed and relevant parameters are identified. The model is validated and optimized considering different job heights and ambient conditions during the cooling phase. It is finally possible to predict position-dependent temperature histories as a function of significant 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.

Introduction

Polymer laser sintering is one of the most used industrial Additive Manufacturing (AM) technology for the processing of thermoplastic polymers, especially polyamide 12. Thin layers of powder are recoated onto the build area, the part bed is heated up close to the melt temperature of the material and a laser melts the material selectively to the desired slice geometry of the part(s). This process repeats until the build finishes. Parts can be placed throughout the whole build volume and are supported by the unsintered powder. The upper process chamber as well as the removal area containing the build frame are heated up to a defined temperature. The process is illustrated in Fig. 1.

Since the polymeric material in bulk powder form has a very low thermal conductivity, it takes a very long time for the so-called part cake (powder + contained parts) to cool down after the build finishes. In addition, temperatures within the part cake are position dependent and change inhomogeneously throughout the build and cooling process. This effect is expected to be an important aspect regarding the reproducibility of the process and part quality [1]. In literature, temperature inhomogeneities on the part bed surface have been analyzed and correlated with part quality several times [2], [3], [4], [5]. However, there is not much knowledge about the inner part cake temperature distribution and history. Material solidification and crystallization occurs after the laser exposure within the part cake. These effects have been investigated using thermo-mechanical models of the laser energy input and the crystallization phase [6], [7], [8], but a validation of results is difficult due to lack of information about part and powder temperatures below the part cake surface. Nevertheless, experimental correlations of part quality, exposure strategy and position within the build area are possible as a result of individual temperature histories and consecutive shrinkage effects [9], [10]. In addition, thermal powder ageing effects and thereby the recyclability of the material is highly dependent on the temperature history [1].

In a previous work, the authors implemented a temperature measurement system into an EOSINT P395 laser sintering system from EOS Electro Optical Systems, Germany to analyze the inner part cake temperatures throughout the whole manufacturing process [1]. To transfer these results into machine and process design, a consecutive FE model of the heat flow within laser sintered part cakes has been developed and is presented in this work. The overall aim is to predict inner powder temperatures and to understand decisive thermal processes during cooling. Later, these results can be used to correlate individual temperature histories with part properties and finally develop strategies for a more homogeneous, reproducible and economic build process.

Section snippets

Materials and methods

The simulation of 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 initial temperature distribution when the cooling phase starts on the

Thermal finite element method

The finite element model is a transient thermal model governed by the transient conduction energy equation, derived from the energy balance for a finite volume element. The heat flux across a surface due to conduction in a Cartesian coordinate system is given by: [16]q=kT=k(Txiˆ+Tyjˆ+Tzkˆ)

The energy equation governing the heat flux and temperatures within the part cake is given by the following equation, whereas the heat generation here can be neglected (q˙=0): [16](kTx)x+(kTy)

Experimental

The results of the full process temperature history are shown in Fig. 6 for the sensor tube located in the center of the build area. Each profile corresponds to the given z height within the part cake. The process can be divided into four phases: The pre-heating phase (I), the build phase (II) and the cooling phase within (III) and outside of the machine (IV). During the build process, the thermocouples penetrate into the part cake one after another; during cooling, the positions are fixed. It

Discussion

The model developed shows a good applicability for a cooling simulation of laser sintered part cakes and is suitable for the development of optimized cooling strategies for individual job layouts. It is possible to analyze the relative influence of ambient conditions without the need of further experiments. More challenging is an analysis of the interaction between powder bed density and thermal conductivity of the bulk material, the occurrence of cracks and the influence of parts (=molten

Conclusions

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 + convection) and the thermal conductivity of the part cake were determined in a parameter study. A parameter set has been

Acknowledgement

The authors want to thank all industry partners of the DMRC as well as the federal state of North Rhine-Westphalia and the University of Paderborn for the financial and operational support within the project “AMP2: Advanced Additive Material and Part Properties—Reduced Refresh Rates & Cooling Process Regarding LS”. Special thanks to Lavish Ordia from the Indian Institute of Technology (IIT), Mumbai, for his support in the model development.

References (16)

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