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Selective laser sintering
Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material (typically nylon/polyamide ), aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to Selective Laser Melting (SLM); the two are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered, allowing different properties (crystal structure, porosity, and so on). SLS (as well as the other mentioned AM techniques) is a relatively new technology that so far has mainly been used for rapid prototyping and for low-volume production of component parts. Production roles are expanding as the commercialization of AM technology improves.

Materials
Commercially-available materials used in SLS come in powder form and include, but are not limited to, polymers such as 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, such amorphous polymers processed by SLS tend to result in parts with diminished mechanical properties, dimensional accuracy and thus are limited to applications where these are of low importance. Metal materials are not commonly used in SLS since the development of selective laser melting.

Powder Production
Powder particles are typically produced by cryogenic grinding in a ball mill at temperatures well below the glass transition temperature of the material, which can be reached by running the grinding process with added cryogenic materials such as dry ice (dry grinding), or mixtures of liquid nitrogen and organic solvents (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. Chemical binder coatings can be applied to the powder surfaces post-process ; these coatings aid in the sintering process and are especially helpful to form composite material parts such as with alumina particles coated with thermoset epoxy resin.

Sintering Mechanisms
Sintering in SLS primarily occurs in the liquid state when the powder particles forms a micro-melt layer at the surface, resulting in a reduction in viscosity and the formation of a concave radial bridge between particles, known as necking, due to the material's response to lower its surface energy. In the case of coated powders, the purpose of the laser is to melt the surface coating which will act as a binder. Solid state sintering is also a contributing factor, albeit with a much reduced influence, and occurs at temperatures below the melting temperature of the material. The principal driving force behind the process is again the material's response to lower its free energy state resulting in diffusion of molecules across particles.

Applications
SLS technology is in wide use at many industries around the world due to its ability to easily make complex and geometries with little to no added manufacturing effort. 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. Because the process requires the use of a laser and other expensive, bulky equipment, it is not suited for personal or residential use; however, it has found applications in art [EOS artist citation with images].

Advantages

 * The sintered powder bed is fully self-supporting, allowing for:
 * high overhanging angles (0 to 45 degrees from the horizontal plane)
 * complex geometries embedded deep into parts, such as conformal cooling channels
 * batch production of multiple parts produced in 3D arrays, a process called nesting


 * Parts possess high strength and stiffness


 * Good chemical resistance


 * Various finishing possibilities (e.g., metallization, stove enameling, vibratory grinding, tub coloring, bonding, powder, coating, flocking)


 * Bio compatible according to EN ISO 10993-1 and USP/level VI/121 °C


 * Complex parts with interior components can be built without trapping the material inside and altering the surface from support removal.


 * Fastest additive manufacturing process for printing functional, durable, prototypes or end user parts


 * Wide variety of materials with characteristics of strength, durability, and functionality


 * Due to the reliable mechanical properties, parts can often substitute typical injection molding plastics

Disadvantages

 * parts have porous surfaces; these can be sealed by several different post-processing methods such as cyanoacrylate coatings, or by hot isostatic pressing.