Technological approaches in 3D printing

Under the term 3D printing, a lot of technologies have now “accumulated”. These processes differ in terms of the raw material used and the processing method. However, these technologies often rely on the same basic principles. Consequently, not every new process is necessarily a ‘revolution’, but mostly a further development or modification of already existing 3D printing technologies.

There are generally three approaches here: the most well-known are ColorJet 3D printing and laser sintering (SLS) or melting (SLM). The former works chemically (bonding), while the latter processes are thermal (laser-based).
The ColorJet process uses a chemical binder (glue) to shape polymer gypsum. The binder contains pigments, allowing objects to be directly printed in color. The advantage of bonding is that it is irrelevant which material the powder is made of. Thus, with a comparable technology, sand molds for metal casting (e.g., Voxeljet, ExOne), metals (ExOne), or other materials such as ceramics, glass, or wood can be used.

Recently, HP introduced another system related to powder-based 3D printing. In the Multi Jet Fusion process, thermoplastics are initially wetted with a heat-conducting liquid and then sintered using infrared light. The result is parts that resemble those from laser sintering, but with higher density and therefore stability.

In these processes, photosensitive resins are cured using a light source (UV light, laser). The most well-known methods are Stereolithography (SLA), Digital Light Processing (DLP), and the PolyJet process. In Stereolithography, a liquid resin is cured layer by layer using a (UV) laser. This process was in fact the very first rapid prototyping process, with developments starting in the 1970s. DLP systems operate on almost exactly the same principle, but instead of a laser, a projector is used. The advantage here is that much higher printing speeds can be achieved (a layer is completely cured by the projection), although individual pixels may still be visible depending on the resolution.
A new process (CLIP, a DLP-based process) is currently emerging in this technology field and promises to set new standards in terms of speed, as layers are completely eliminated and printing is continuous.

Polyjet systems (Stratasys) or Multijet Modeling (3D Systems) work slightly differently. The smallest droplets of resin are applied to a platform and immediately cured using a UV laser attached to the print head.

In these 3D printing systems, materials are extruded using an extruder and solidify through various means. By far the most important method is filament 3D printing (FDM or FFF), in which plastic (thermoplastics; often ABS, PLA, and polyamides) is melted in wire form and applied. Once applied, the plastic cools and solidifies. Newer methods use different materials for the plastics, such as ceramics or metals. In the case of the latter, Markforged and Desktop Metal have implemented new approaches. Metal powders are included in the filaments, which are sintered after printing.

Other systems are currently rather exotic and work, for example, with chemical curing (cement in house 3D printing, e.g., Apis-Cor) or even biologically (bio 3D printers, e.g., Organovo). These systems mostly work with print heads resembling syringes. With such systems, even food (e.g., Choc Edge) can now be printed.