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Fused Deposition Modeling is a 3D printing process in which molten plastic, in rare cases also liquefied metals, is applied layer by layer to a work platform. Layer by layer, the FDM creates a 3D print model. The basis for the FDM process is a digital 3D model which is broken down into a large number of individual layers (slices) by special CAD programs. These layers are then transferred to the working platform by the 3D printer.

The advantages of the Fused Deposition Modeling are above all in the economical, fast execution of the 3D printing. This enables short delivery times and economical printing. The printed components are also extremely dimensionally stable. A disadvantage is the low accuracy of the printed parts and the visibility of the individual print layers. With certain materials, especially ABS, thin parts can bend - the so-called "warp effect". Also in some cases, a technical peculiarity of FDM printing which can be seen as disadvantageous is that volumetric bodies are not printed solid but always with a filling structure.

All thermoplastics can be used as materials for FDM printing. ABS, polyamides, PEEK or PA6 are frequently used. Polycarbonates, PET or polypropylene are also used for FDM printing. FDM prints made of metals are still being tested. However, this has not yet been implemented on an industrial scale.

Stereolithography is the longest used 3D printing process to date. Stereolithography (SLA) was invented as early as 1983 - and is still used industrially in almost unchanged form. In stereolithography, light-curing properties of photopolymers are used to print components in 3D. The photopolymers, for example epoxy or synthetic resins, are in a liquid plastic bath. This consists of the basic monomers of the plastic. A laser is used to harden the plastics at the points specified by a CAD model. Once a layer has hardened, the working platform of the 3D printer lowers a few millimeters. A wiper distributes a new layer of liquid plastic and the laser is used again. Layer by layer, the desired object is built up.

For technical reasons, only photosensitive plastic resins (epoxy, acylate or elastomer) are used as the material for stereolithography. SLA is used for the production of filigree models, design models and prototypes, but also for the production of functional components or master models for molding. As a disadvantage, the brittleness of the objects created must be mentioned, which limits the areas of application. Since extensive reworking, such as curing the objects in a UV cabinet or manually removing support structures during SLA printing, cannot be avoided, the process is comparatively cost-intensive.

Solid components with high accuracy, high resolution and in the shortest possible time: Multi Jet Fusion is undoubtedly one of the most powerful 3D printing technologies available today. In this process, a heat-conducting liquid, the so-called "fusing agent", is applied to a layer of the material powder (usually polyamide 12). An infrared heat source is used directly afterwards. The fusing agent ensures that the area wetted by it is heated more than the rest of the material powder. This causes the material to melt together. In order to achieve sharp edges and precise contours, another active ingredient, the "detailing agent", is used. This agent is applied precisely around the areas on which the fusing agent is used. This results in a clear temperature difference - and the desired sharp edges and precise contours are created.

The advantages of the Multi Jet Fusion process lie in the almost 100 percent density of the printed components and the high printing speed. The printing speed at MJF is about 10 times higher than at other comparable processes. Due to the small droplets used in the Multi Jet Fusion for 3D printing, the resolution (1,200 dpi) exceeds any other additive manufacturing method. A disadvantage is the slightly rough surface of the printed objects and the limitation to only one material (PA 12). If colors other than gray or black are desired for the printed objects, there will be an extra cost for a coating.

The PolyJet method, also known as MultiJet Modeling, is one of the most widely used methods in rapid prototyping. For 3D printing, the PolyJet process uses a print head to apply a layered photopolymer distributed in tiny droplets to a building platform, where it is immediately cured by UV light. In PolyJet 3D printing, the finest layer thicknesses of only 16 - 32 µm are possible, which gives the models an extremely high level of detail accuracy. Smooth surfaces similar to those from injection molding are also produced. It is also no problem to combine several different materials for one workpiece with PolyJet printing. For example, solid and rubber-like materials can be combined or different materials can be mixed during 3D printing to create completely new workpiece properties. In addition to these advantages, it should also be mentioned that PolyJet 3D printing involves long printing times, which makes production quite cost-intensive. Supporting structures are also required in certain places for printing. As a result, the printed workpieces often have to be reworked manually, which also pushes up the price.

In selective laser sintering (SLS), a powdery starting material - usually polyamide or elastomer - is applied to the entire surface of the 3D printer's work platform. The powder is then sintered by a laser beam according to the specified contour, i.e. heated to just below the melting point. This causes the material to bond along the desired contour. Once a layer is finished, the building platform is lowered by the thickness of the layer. A new powder layer is applied and the process begins anew. Layer by layer and from bottom to top, the desired object is created in the 3D printer. Since overhanging structures are stabilized in the powder bed, the attachment (and subsequent removal) of support structures is not necessary. The components produced are mechanically resilient, thermostable and lightweight. SLS can also be used to print interlocked and thus movable objects. The comparatively rough surface and the long pressure cycles can be mentioned as disadvantages of the SLS process. The achievable tolerances are also higher than with other processes such as stereolithography or the PolyJet process. As already mentioned, the main material used for SLS is polyamide. Thermoplastic polyurethanes (TPU) or polyether ketones (PEK) are also used less frequently in selective laser sintering. If PA12 (nylon) is used for SLS, it can be enriched with additions to achieve specific material properties. Supplementary material is, for example, aluminium or carbon fibre.

Selective laser melting (SLM) is one of the most versatile 3D printing processes and a cost-effective alternative to welding, milling or casting. In selective laser melting, metal in powder form is applied to a building platform and then melted along the desired contour using a laser beam. SLM also works in layers, which means that the building platform is lowered after a layer is created, fresh powder is applied and then remelted. Required support structures are automatically installed parallel to the actual printing.

The advantage of the SLM is clearly its freedom of design. Since the components are built up layer by layer in the desired form, even extremely complex geometries and moving parts can be generated from materials that are difficult to machine. Selective laser melting is at the same time extremely economical due to minimal amounts of waste. The materials used are aluminum, stainless steel, titanium or cobalt-chromium alloys. Depending on the manufacturer or manufacturing company, selective laser melting is also referred to as DMLS (Direct Metal Laser Sintering), LMF (Laser Metal Fusion) or Additive Layer Manufacturing. However, the different terms all refer to the same process.

In addition to the methods commonly used in industry, there are other 3D printing possibilities. One example is 3D printing, in which gypsum-like powder is bonded layer by layer using a binder. Or Digital Light Processing (DLP), which roughly resembles stereolithography. However, a projector or LCD display is used instead of a laser to cure the base material. Electron beam melting (EBM) is also used in places for 3D printing. EBM always requires a conductive material in powder form. The material is "bombarded" with energy from electrons in the 3D printer and thus melted. 
Which 3D printing process is ultimately the most economical, fastest and optimal for the respective application cannot be answered in general. Each process has its own specific advantages and disadvantages. Therefore, for each 3D print, it is necessary to weigh up which process should be selected as being most suitable.