# Review and New Aspects in Combining Multipoint Moulding and Additive Manufacturing
## 3 Possible Additive Manufacturing Methods
### Plastic-Based Processes  
In addition to additive manufacturing processes using metals, there is the very widespread group of processes based on plastics. These processes can be classified according to the shape of the starting material. They can be divided into powder bed processes, extrusion processes, resin bath processes and processes using plate- or film-like starting materials. In the field of powder bed processes, selective laser sintering is available in analogy to laser powder bed fusion with metals. However, the construction chamber is heated and maintained just below the melting point or the glass transition temperature of the plastic powder. This significantly reduces the energy to be introduced by the laser and prevents distortion. To suppress oxidation processes and the associated degradation of the material, the process is carried out in a nitrogen atmosphere. The layer thickness is about 0.1 mm [30]. Figure 10: Schematic representation of directed energy deposition (based on the work in [29]). The widely used fused layer modelling originates from the field of extrusion processes. This method is often mistakenly referred to as fused deposition modelling. Due to the comparatively simple design of the devices and the possible placement at the workplace, this procedure has become established for home users. Nevertheless, there are also devices for commercial use with an extended range of functions. In most cases, the solid thermoplastic raw material is pressed through a heated nozzle and liquefied. This liquid extrudate is placed next to each other on one level to produce the individual layer and in several layers on top of each other to produce the actual component. In most cases, a filament is used as the raw material, which is fed directly from a coil to the nozzle. Therefore, the more specific name fused filament fabrication (see Figure 11) is often used. Rarely pellet raw material is applied, which is liquefied in a screw extruder. In most cases, fused filament fabrication is used with a heated build plate to reduce warping, deformation and facilitate a better adhesion between the first layer and the build plate. Whereas other additive manufacturing technologies are very material specific there are many materials available for fused layer modelling [28]. In addition to the often-used plastics, also gel-like or liquid materials are used, for example, in medical applications. [PERSON] et al. [38] present such a procedure for direct skin reconstruction. Despite the relatively high degree of diffusion of the technology, many questions still need to be clarified scientifically. These are often questions concerning the optimisation of process parameters to improve production quality. [PERSON] et al. [39] optimise the main process parameters to enable and improve the production of lattice structures. They show a clear influence on the manufacturability of the components, on the one hand, and on the mechanical properties of the component on the other. Apart from that, [PERSON] et al. [40] address the problem of sharp directional changes in the extrusion path. These changes in direction lead to accumulation of material if the extrusion rate remains unchanged as the tangential speed of the die is greatly reduced relative to the build platform. Therefore, [PERSON] et al. [40] develop a control algorithm that leads to a more uniform material deposition rate and thus to more precise components by influencing the extrusion parameters. [PERSON] et al. [41], on the other hand, optimise process parameters for polylactide (PLA) components with the aim of achieving the greatest possible bending strength. Another major factor influencing the component strength of fused filament fabrication components is the connection between the individual extrusion strands within the layer or between the layers. Based on empirical data [PERSON] and [PERSON] [37] clearly show that the component strength along the extrusion strands is significantly higher than the strength across extrusion direction.