Preparation of powder layers in selective laser sintering process

Abstract

Selective Laser Sintering (SLS) is an additive manufacturing process by means of which three-dimensional objects can be manufactured using a laser beam to selectively sinter powder particles. Many are the advantages of SLS, among which are the capacity to build high resolution and very complex shaped objects by using a range of materials. Because of these qualities, SLS is used in advanced applications, e.g. scaffolds for tissue engineering. However, one drawback of SLS is the quality of the powder layer, which depends not only on the powder flow properties but also on the operating conditions. In particular, spreading of powder is a crucial step. Collecting information about the spreadability, that is the capacity of the powder to be spread, is relevant to optimize the distribution and the layering of powder. A new experimental setup able to simulate the powder spreading process and its DEM model were developed with the purpose of achieving this objective. The Johanson’s model was applied to the compaction of the powder layer to understand whether it is possible to increase the density of the layer. Based on the results of the model, a new configuration was designed and built. It combines the action of the blade and of the roller so as to spread and compact the layer, respectively. The Domo's Sinterline unfilled polyamide 6 3400 HT110 Natural was used as a test material. Layers of the powder were produced with a blade running at two different scrolling velocities, namely 3 mm s-1 and 3 cm s-1. An optical system was set up to observe the powder layer at the particle level. It consists of a microscope coupled with a camera, that can be positioned on the powder surface in defined locations. Quality of the powder surface was analysed in terms of characteristic length of roughness, calculated by applying the power spectrum of the wavelet transform to greyscale images of the deposited layers. Comparison between the tests at different blade velocities showed slight differences in the powder surface quality. The slight dependence of surface asperities from the scrolling velocity of the blade would suggest that there is not a direct relationship between characteristic length of roughness and dimension of aggregates, which depends on the Bond number. The effect of recycling the powder was also analysed suggesting that adequate preparation of the used powder and proper test conditions are necessary. A DEM model was created to simulate the spreading of powders with the future aim of understanding the mechanisms and the powder properties which are responsible for good or bad quality of the layer. The Hertz-Mindlin (no slip) with JKR cohesive model was used to describe interparticle interactions. Three different approaches were tried to calibrate the model parameters and the one based on the comparison between the experimental bulk density and porosity with those measured from the simulations turned out to be the right one. DEM model was validated by applying the wavelet power spectrum to greyscale images of the layers obtained with the simulations and by comparing the characteristic length of roughness of the experimental layers with the simulated ones. Therefore, the DEM model allows to quantitatively evaluate the quality of the powder layer, i.e. in terms of dimension of surface asperities, obtained through deposition in the same spreading conditions of the experiment. To the best of my knowledge, this procedure of analysis has been applied for the first time to experimental and simulated layers of additive manufacturing processes. Analysis of simulated powder layer can be used to choose the suitable spreading configuration for a certain powder or to properly engineer powders with desired particle properties to obtain good quality layers with a given spreading configuration. [edited by Author]