Materials

Commercially available materials used in SLS come in powder form and include, but are not restricted to, polymers such as polyamides (PA), polystyrenes (PS), thermoplastic elastomers (TPE), and polyaryletherketone (PAEK). Due to its ideal sintering behavior as a semi-crystalline thermoplastic, polyamide is the most frequently used SLS material which results in parts with desirable mechanical properties. Polycarbonate (PC) is a material of great interest to SLS due to its high strength, thermal stability, and flame resistance; however, these amorphous SLS-processed polymers continue to result in components with reduced mechanical properties, dimensional accuracy, and are therefore restricted to applications where these are of low significance. Since the advancement of selective laser melting metal materials were not commonly used in SLS.

Powder Production

Powder particles are typically formed by cryogenic grinding in a ball mill at temperatures below the material’s glass transition temperature which can be achieved by grinding with added cryogenic materials such as dry ice (dry grinding) or liquid nitrogen and organic solvent combinations (wet grinding). The process can result in spherical or irregular shaped particles that are as small as five microns in diameter. Powder particle size distributions are usually Gaussian and range from 15 to 100 microns in diameter, but they can be modified to match different layer thicknesses in the SLS process. Chemical binder coatings can be applied to post-process powder surfaces; these coatings aid in the sintering process and are particularly useful for the formation of composite material parts such as aluminum particles coated with thermoset epoxy resin.

Sintering Mechanisms

The sintering mechanism in SLS occurs primarily in a liquid state when the powder particles form a micro-melt layer on the surface resulting in reduced viscosity and a concave radial bridge between the particles forming, due to the material’s reaction to lower surface energy, known as necking. In the case of coated powders, the laser’s purpose is to melt the surface coating that acts as a binder. Solid state sintering is also a contributing factor but has a decreased effect, and occurs at temperatures below the material’s melting point. The key driving force behind the cycle is again the material’s reaction to lower its free energy level, which results in the particlewide diffusion of molecules.