How Does Selective Laser Sintering (SLS) Work?

Powder Bed Fusion (PBF), often called Selective Laser Sintering (SLS), is one of the most used Additive Manufacturing technologies to produce structural plastic parts. In this article, we explain the process principle and materials used in the SLS process.

Powder Bed Fusion (PBF), often called Selective Laser Sintering (SLS), is one of the most used Additive Manufacturing technologies to produce structural plastic parts. It does not require molds or support structures. Furthermore, it can produce complex geometries, internal structures and thin walls with mechanical properties comparable to injection-molded parts. This shortens the development cycle and makes it an alternative for many work pieces and even whole assemblies.

Figure 1: Schematic of an SLS machine

The SLS process principle

In the SLS process, a thin layer of powder is applied on the build platform and heated to just below the melting temperature of the material, which is often referred to as the build temperature (heaters not shown in the schematic). Next, a laser traces the cross-section of the part geometry of the first layer, providing enough energy to locally melt the material. Without any shear forces, the melt needs to have a low viscosity and surface tension to coalesce and form a uniform melt pool. The surrounding powder stays solid and keeps the shape of the molten geometry. Therefore, no support structures are needed. This can be seen by the three N-shaped built parts in the powder bed. Now the build platform is lowered by one layer height making room for the next layer. A sweeper or recoater roller moves across the surface, picks up excess material from the reservoir and deposits new and colder powder on top of the build platform to create the next layer. Again, the powder is heated to keep it at the build temperature. This is important to hinder crystallization. The whole build envelope is kept in a nitrogen atmosphere to reduce effects of aging. These process steps of powder coating and laser melting are repeated over and over until the whole part is built. Only then is the build envelope cooled down, which initiates the crystallization and thus solidification process of the part. After the part and surrounding powder is cooled completely, the part is unpacked.

Materials used in the SLS process

The first material used in this process was PA12, because of its good mechanical performance and the ability to generate powders by precipitation. This yields powder with close to perfect spherical shape, which is necessary to create a uniform layer during coating. It still makes up 90-95% of all materials used in SLS today. However, in recent years, more and more materials have been qualified for the process including high-performance materials such as PEEK, elastomeric materials such as TPUs and even commodity materials such as PP. Most of them are produced by cryogenic grinding and show more or less pronounced deviations from the circular shape [1].

More basics about Selective Laser Sintering and other Additive Manufacturing technologies are available on our YouTube channel!

Learn more about Polymer Powder Bed Fusion here!

Thermal analysis and rheology supporting successful SLS processes

Research and development focusing on SLS processes are targeted when investigating new materials for SLS. The aim is to determine their suitability for SLS, define the process window, analyze the formation of the pool melt and understand how fillers change the properties of powder and the finished parts. In the following blog posts, we will shed light on different analysis methods using thermal analysis and rheology instruments to characterize key parameters, including the determination of the process window and isothermal crystallization of SLS powders with Differential Scanning Calorimetry (DSC) as well as studying residual stress and warpage in SLS.

Sources

[1] Schmid, M. (2018): Laser Sintering with Plastics – Technology, Processes and Materials, Carl Hanser Verlag, Munich.

How to Determine the Process Window for SLS Powders Using DSC

In order to characterize a polymer powder for its suitability for SLS and to determine the possible process window, Differential Scanning Calorimetry (DSC) is used. Learn how to set up and interpret the measurements!

How to Study the Isothermal Crystallization Behavior of SLS Powder Using DSC

In a previous article, the process window in the Selective Laser Sintering process with polyamide 12 powder was determined with dynamic measurements. In this article, we explain how isothermal measurements can be used for more advanced studies.

Wilo: Better Performance with Fiber-Reinforced 3D Printed Components

Wilo SE is a worldwide manufacturer of pumps and pump systems for building services, the entire water management chain and industry. It comes as no surprise that Wilo is working with cutting-edge technologies such as Additive Manufacturing. Learn how they use the NETZSCH DSC 214 Polyma to understand the thermal behavior of new material choices.

Estimating Warpage of Selective Laser Sintering Parts Using Thermomechanical Analysis

The plastics used in Selective Laser Sintering (SLS) have a higher thermal expansion when compared with other materials. Therefore, it is important to know how the dimensions of an SLS part change at different temperatures during the build and during use. The higher the thermal expansion coefficient, the more prone are the parts to warpage or curling and the build-up of residual stresses. Learn more!

Estimating Residual Stresses in SLS Parts Using DMA

Selective Laser Sintering (SLS) is one of the most used Additive Manufacturing technologies to produce structural plastic parts. When operated at elevated temperature, any residual stresses could be detrimental for the part performance. In order to better understand residual stresses, knowledge of a material’s modulus is needed. Learn more about residual stress and how to measure the material property using a thermal analysis method.

Measuring Specific Heat Capacity to Simulate SLS Processes

Significant efforts have been made to model and simulate the Selective Laser Sintering process as information about the temperature field in lower layers is difficult to measure. Learn how specific heat capacity can help!

How Fillers Affect the Crystallization Behavior of SLS Powders

Due to the still limited number of available materials for the Selective Laser Sintering process, there is a constant demand for materials with different properties. The addition of any filler to SLS powder typically has an effect on the processing behavior. Today, we investigate the crystallization behavior of PA12 powder filled with copper spheres and flakes.

How Specific Heat Capacity of Filled Powders Affects SLS Processing Parameters

The modification of Selective Laser Sintering (SLS) powders with fillers is a good way to modify the properties of the produced parts without the necessity for new powder materials. Learn how to assess the effect of copper fillers on the processing behavior.

How to Prepare SLS Parts for Thermal Analysis Measurements: LFA

The build orientation of samples has an effect on the mechanical properties of Selective Laser Sintering (SLS) parts. Therefore, thermophysical properties need to be assessed in different directions. Learn how to prepare filled samples for laser flash analysis!

How Thermal Diffusivity Affects the Build Temperature in the SLS Process

Conductive fillers in polymer powder, like copper spheres and flakes, influence Additive Manufacturing processes. Learn how laser flash analysis allows determination of process setting to print highest quality parts.

How Fillers Increase Isotropic or Anisotropic Behavior of SLS Parts through Their Alignment

In general, the addition of fillers leads to an increase in mechanical performance. To understand how the stiffness or modulus change as a function of the filler geometry and filler content, Dynamic Mechanical Analysis (DMA) can be used. Learn more in our article. 

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