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 direct metal laser sintering (DMLS); 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.

Laser sintering is a generative layering process: the work piece is built up layer by layer. By the action of the laser beams so any three-dimensional geometries can be generated with undercuts, Work pieces that can not be produced in conventional mechanical or casting production.

Because of the high mechanical complexity and in particular the process time dependent on the generated volume (which can be in the range of hours, and in large parts with high accuracy requirements of days), the methods are used especially for the production of prototypes and small numbers of complicated parts. The trend, however, is to use the technology as a rapid-manufacturing or rapid tooling method for the rapid production of tools and functional components.

The basic prerequisite is that the geometric data of the product is available in three dimensions and processed as layer data. In the traditional production of casting molds, a casting model must first be produced from the geometric data, which u. a. the dwindling of the cooling metal and other casting-technical requirements are taken into account. For laser sintering, on the other hand, numerous layers are generated from the existing CAD data of the component (usually in STL format ) by so-called “slicing”.

Usually a laser uses a CO 2 laser, a Nd: YAG laser or a fiber laser. The powdery material is a plastic, a plastic-coated molding sand, a metal or a ceramic powder.

The powder is applied to a building platform with the aid of a doctor blade or roller over its entire surface in a thickness of 1 to 200 microns. The layers are successively sintered or melted into the powder bed by triggering the laser beam in accordance with the layer contour of the component. The build platform is now slightly lowered and a new layer raised. The powder is provided by lifting a powder platform or as a stock in the squeegee. The processing is done layer by layer in vertical direction, thus it is possible to create also undercut contours. The energy supplied by the laser is absorbed by the powder and results in localized sintering of particles with reduction of the total surface.

In the case of the plastic powders used, it is customary not to produce them by grinding, but to polymerize directly as beads, since in the process very high demands on the nature of such. As the flowability of the powder used are provided.

A major advantage of the SLS is that it eliminates the support structures required by many other rapid prototyping methods. The component is always supported during its formation by the surrounding powder. At the end of the process, the remaining powder can then be simply knocked off and partially reused for the next run. Full reuse is currently not possible, especially with plastic powders, as they lose quality through the process.

A special form for the production of microstructures is the laser micro-sintering developed at the Laser Institute of the University of Applied Sciences Mittweida. This is a Q-switchedLaser used with short pulses. The process can take place both in a vacuum chamber, through which nanopowders can also be processed, and under protective gas or, in the case of special metals, under air. A constructional feature is the worldwide patented ring doctor, with the help of which even extremely thin layers of powder can be wound up precisely. By using multiple squeegees alternating and gradient layers can be generated. The resolution of the method is in the micron range with respect to the realizable layer thicknesses and in similar areas with respect to the reproducible geometry details. For a short time, the processing of ceramic powders in high quality is possible. Thus, ceramic dental inlays were also generated with the method.

Selective laser sintering (SLS) was developed and patented by Dr. Carl Deckard and academic adviser, Dr. Joe Beaman at the University of Texas at Austin in the mid-1980s, under sponsorship of DARPA. Deckard and Beaman were involved in the resulting start up company DTM, established to design and build the SLS machines. In 2001, 3D Systems, the biggest competitor to DTM and SLS tec hnology, acquired DTM. The most recent patent regarding Deckard’s SLS technology was issued 28 January 1997 and expired 28 Jan 2014.

A similar process was patented without being commercialized by R. F. Housholder in 1979.

As SLS requires the use of high-powered lasers it is often too expensive, not to mention possibly too dangerous, to use in the home. The expense and potential danger of SLS printing means that the home market for SLS printing is not as large as the market for other additive manufacturing technologies, such as Fused Deposition Modeling (FDM).

The SLS prototypes are made from powdered materials that are selectively sintered (heated and fused) by a high power laser.

The machine consists of a construction chamber on a manufacturing piston, surrounded on the left and right by two pistons supplying the powder, a powerful laser, and a roller to spread the powder. The chamber should be kept at a constant temperature to prevent deformation.

The process begins with a 3D CAD file that is cut into 2D sections. The manufacturing piston is raised to the maximum while the pistons supplying the powder are at their lowest point. The roll spreads the powder in a uniform layer over the entire chamber. The laser then traces the 2D section on the surface of the powder, thus sintering it. The manufacturing piston goes down the thickness of a stratum while one of the pistons of powder supply rises (they alternate: one time out of two that of left). A new layer of powder is spread over the entire surface by the roll, and the process is repeated until the piece is finished.

The workpiece must then be removed carefully from the machine and cleaned of the unsintered powder surrounding it.

There are other machines where the powder does not come from below thanks to pistons, but from the top. This method saves time because it is not necessary to stop the manufacture of parts to replenish the powder machine.

If the piece is intended for lost wax casting, it must then be infiltrated with wax to make it less fragile. After drying, it is placed on a molding tree around which ceramic is poured. When the latter is hard, the mold is placed in an oven, the wax melts and the desired mold is obtained. It remains to sink a molten metal, let it cool, break the mold, recover the piece, cut the tree and treat the surface. The finished piece is there.

Various methods are used to increase the build rate – the sintered volume per time unit. For this purpose, laser powers over 1 kW are used. In laser microsinelling, a high-speed process is realized by ultra-fast beam deflection, with deflection speeds of 150 m / s being reached experimentally. In development, the process is electron beam sintering. Here, even higher powers of up to 10 kW are used. This also enables the rapid processing of high-strength steels, especially tool steels.

An additive manufacturing layer technology, SLS involves the use of a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired three-dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Because finished part density depends on peak laser power, rather than laser duration, a SLS machine typically uses a pulsed laser. The SLS machine preheats the bulk powder material in the powder bed somewhat below its melting point, to make it easier for the laser to raise the temperature of the selected regions the rest of the way to the melting point.

In contrast with some other additive manufacturing processes, such as stereolithography (SLA) and fused deposition modeling (FDM), which most often require special support structures to fabricate overhanging designs, SLS does not need a separate feeder for support material because the part being constructed is surrounded by unsintered powder at all times, this allows for the construction of previously impossible geometries. Also, since the machine’s chamber is always filled with powder material the fabrication of multiple parts has a far lower impact on the overall difficulty and price of the design because through a technique known as ‘Nesting’ multiple parts can be positioned to fit within the boundaries of the machine. One design aspect which should be observed however is that with SLS it is ‘impossible’ to fabricate a hollow but fully enclosed element. This is because the unsintered powder within the element can’t be drained.

Since patents have started to expire, affordable home printers have become possible, but the heating process is still an obstacle, with a power consumption of up to 5 kW and temperatures having to be controlled within 2 °C for the three stages of preheating, melting and storing before removal.

Materials and applications
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.

Advantages vs. disadvantages

A distinct advantage of the SLS process is that because it is fully self-supporting, it allows for parts to be built within other parts in a process called nesting – with highly complex geometry that simply could not be constructed any other way.

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, channels, 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.

Vast variety of materials and characteristics of Strength, durability, and functionality, SLS offers Nylon based materials as a solution depending on the application.

Due to the excellent mechanical properties the material is often used to substitute typical injection molding plastics.

SLS printed parts have a porous surface. This can be sealed by applying a coating such as cyanoacrylate.

Source from Wikipedia