Selective laser sintering
Laser sintering is a generative layer construction process: the workpiece is built up layer by layer. As a result of the effect of the laser beams, any three-dimensional geometries can also be created with undercuts , e.g. B. Workpieces that cannot be manufactured using conventional mechanical or casting technology.
Because of the high level of machine effort and, in particular, the process times that depend on the volume generated (which can be in the range of hours, for large parts with high accuracy requirements, even days), the methods are particularly used for the production of prototypes and small numbers of complicated parts. The trend, however, is towards using the technology as a rapid manufacturing or rapid tooling process for the rapid generation of tools and functional components.
The basic requirement is that the geometric data of the product are available in three dimensions and processed as layer data. In the traditional production of casting molds, a casting model must first be made from the geometry data. a. the shrinkage of the cooling metal and other technical foundry requirements are taken into account. For laser sintering, on the other hand, numerous layers are generated from the available CAD data of the component (usually in STL format ) by so-called "slicing".
The powder is applied over the entire surface of a building platform with the aid of a doctor blade or roller in a thickness of 1 to 200 μm. The layers are gradually sintered or melted into the powder bed by controlling the laser beam in accordance with the layer contour of the component. The construction platform is now lowered slightly and a new layer is raised. The powder is made available by lifting a powder platform or as a supply in the doctor blade. The processing takes place layer by layer in the vertical direction, which makes it possible to create undercut contours. The energy supplied by the laser is absorbed by the powder and leads to a locally limited sintering of particles with a reduction in the total surface area.
In the case of the plastic powders used, it is customary not to produce them by grinding, but to polymerize them directly as spheres, as very high demands are made on the texture in the process, such as B. the flowability of the powder used.
A major advantage of the SLS is that support structures, as required in many other rapid prototyping processes , are not required. The component is always supported by the surrounding powder during its creation. At the end of the process, the remaining powder can then simply be tapped off and some of it can be reused for the next run. Complete reuse is currently not possible, especially with plastic powders, as these lose quality as a result of the process.
A special form of creating microstructures is the laser microsintering developed at the Laser Institute of the Mittweida University of Applied Sciences . A Q-switched laser with short pulses is used here. The process can take place both in a vacuum chamber, which also allows nanopowders to be processed, as well as under protective gas or, for special metals, under air. A special design feature is the globally patented ring doctor blade, which can be used to precisely apply extremely thin layers of powder. By using several squeegees, alternating and gradient layers can be created. The resolution of the method is in the µm range with regard to the realizable layer thicknesses and in similar areas with regard to the representable geometric details. For a short time it has also been possible to process ceramic powders in high quality. Ceramic tooth inlays were also generated with the process .
Related procedures and synonymous terms
The term laser sintering is interpreted inconsistently. In the academic environment, laser sintering is sometimes defined as a process that only partially melts the powder grains, in which virtually no liquid phase sintering process takes place. In fact, such processes had a certain breadth of application and market relevance in the late 1990s and early 2000s. Today they hardly play a role. In today's usual and established use, laser sintering stands for processes in which plastic or metal powder is completely melted in layers without the use of binders and a homogeneous material of high density is created after the solidification of the melt or processing of all layers. The term DMLS (Direct Metal Laser Sintering), which was originally introduced as a brand name, is also widely used for metal processes. Another synonymous name with a certain distribution is LaserCusing, also a brand name. The term selective laser melting (SLM) is used for processes that process metal powder in basically the same way without the addition of a binder. The metal powders are also completely melted, mostly with CW lasers .
Various methods are used to increase the build rate - the sintered volume per unit of time. Laser powers over 1 kW are used for this. With laser microsintering, a high-speed process is implemented by means of ultra-fast beam deflection, deflection speeds of 150 m / s being experimentally achieved. The electron beam sintering process is currently being developed . Here even higher outputs of up to 10 kW are used. This also enables high-strength steels to be processed quickly, especially tool steels.
- Manfred Schmid: Additive Manufacturing with Selective Laser Sintering (SLS) - Process and Material Overview , Springer, 2015.
- Laser sintering will establish itself as a manufacturing process - the electron beam is now also forming tool steel - Konradin Verlag ( Memento of the original from May 27, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.