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Expand/Improve Article on Selective Laser Sintering (SLS) to Add Polymers

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Materials and Applications (My edited draft)

Commercially-available materials used in SLS come in powder form and include polyamides (PA), polystyrenes (PS), thermoplastic elastomers (TPE), and polyaryletherketones (PAEK). Polyamides are the most commonly-used SLS materials due to their ideal sintering behavior as a semi-crystalline thermoplastic, resulting in parts with desirable mechanical properties. Polycarbonate (PC) is a material of high interest for SLS due to its high toughness, thermal stability, and flame resistance; however, amorphous polymers processed by SLS tend to result in parts with diminished mechanical properties and dimensional accuracy and thus are limited to applications where these are of low importance. SLS of metal powders was first invented at The University of Texas At Austin in the late 80's and early 90's, and has since evolved into laser powder bed fusion.

[Image of polymer powder at macro scale and also in SEM]

Powder particles are typically produced by cryogenic grinding in a ball mill at temperatures well below the glass transition temperature of the material; desired temperatures can be reached with the use of dry ice, known as dry grinding, or with a mixture of liquid nitrogen and organic solvents, known as wet grinding. The process can result in spherical or irregular shaped particles as low as five microns in diameter. Powder particle size distributions are typically gaussian and range from 15 to 100 microns in diameter, although this can be customized to suit different layer thicknesses in the SLS process.

Diagram showing formation of neck in two sintered powder particles. Original shapes are shown in red.

Solid state sintering of powders occurs at temperatures below the melting temperature of the material. The principal driving force behind the process is the material's response to lower its free energy state resulting in diffusion of molecules across particles and eventually the formation of a concave radial bridge between particles, also known as necking. Liquid state sintering also displays necking between particles, and results from melting of the surface of the powder particle. The melted surface can consist of either the same material as the core of the particle or a deposited chemical binder coating. Binder coatings can be used to form composite material parts in the SLS process, such as alumina particles coated with thermoset epoxy resin.

Applications (My edited draft)

SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data. Its most common application is in prototype parts early in the design cycle such as for investment casting patterns, automotive hardware, and wind tunnel models. SLS is also increasingly being used in limited-run manufacturing to produce end-use parts for aerospace, military, medical, and electronics hardware. On a shop floor, SLS can be used for rapid manufacturing of tooling, jigs, and fixtures. One less expected and growing application of SLS is its use in art.

Materials and applications (original verbatim copy)

Some SLS machines use single-component powder, such as direct metal laser sintering. Powders are commonly produced by ball milling. However, most SLS machines use two-component powders, typically either coated powder or a powder mixture. In single-component powders, the laser melts only the outer surface of the particles (surface melting), fusing the solid non-melted cores to each other and to the previous layer.

Compared with other methods of additive manufacturing, SLS can produce parts from a relatively wide range of commercially available powder materials. These include polymers such as nylon (neat, glass-filled, or with other fillers) or polystyrene, metals including steel, titanium, alloy mixtures, and composites and green sand. The physical process can be full melting, partial melting, or liquid-phase sintering. Depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large numbers of parts can be packed within the powder bed, allowing very high productivity.

SLS technology is in wide use around the world due to its ability to easily make very complex geometries directly from digital CAD data. While it began as a way to build prototype parts early in the design cycle, it is increasingly being used in limited-run manufacturing to produce end-use parts. One less expected and rapidly growing application of SLS is its use in art.

Because SLS can produce parts made from a wide variety of materials (plastics, glass, ceramics, or metals), it is quickly becoming a popular process for creating prototypes, and even final products. SLS has been increasingly utilized in industry in situations where small quantities of high quality parts are needed, such as in the aerospace industry, where SLS is being used more often to create prototypes for aircraft. Aircraft are often built in small quantities and stay in service for decades, so producing physical molds for parts becomes non cost effective, so SLS has become an excellent solution.