22.02.2021 by Dr. Natalie Rudolph, Doreen Rapp

Estimating Warpage of Selective Laser Sintering Parts Using Thermomechanical Analysis

The plastics used in Selective Laser SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. 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!

This is the case because the higher the thermal expansion coefficient, the larger is the effect of even the smallest temperature differences in the process. The thermal expansion or rather shrinkage during the process depends on the thermal shrinkage of the material and – in the case of semi-crystalline materials often used in SLS – the shrinkage due to crystallization. The amount of crystallization shrinkage depends on the polymer structure itself, but also the cooling temperatures. Looking at the SLS process, temperature gradients are inevitable and cooling rates are small.

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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).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 SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. 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 IsothermalTests at controlled and constant temperature are called isothermal.isothermal crystallization of SLS powders with Differential Scanning Calorimetry (DSC) as well as studying residual StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.stress and warpage in SLS.

Sources

[1] Schmid, M. (2018): Laser SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. 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 

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Estimating Warpage of Selective Laser Sintering Parts Using Thermomechanical Analysis 

The plastics used in Selective Laser SinteringSintering is a production process for forming a mechanically strong body out of a ceramic or metallic powder. 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 StressStress is defined as a level of force applied on a sample with a well-defined cross section. (Stress = force/area). Samples having a circular or rectangular cross section can be compressed or stretched. Elastic materials like rubber can be stretched up to 5 to 10 times their original length.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 (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.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.

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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 

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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.

Why the Effect of Anisotropic Fillers on Thermal Expansion is Process-Dependent 

Fillers are added to a polymer matrix to improve the mechanical performance of the finished product. The orientation of such fillers depends on the processing conditions. Learn how the overall content, shape and orientation of copper fibers influence the coefficient of thermal volume expansion.