In the previous article, we presented the basic advantages of 3D printing (additive manufacturing) and emphasized that each component is manufactured layer by layer by solidifying the material. In this article, we will take a closer look at different additive manufacturing processes for polymers.
There are various ways of classifying the many different processes. For our purposes, we separate them according to the aggregate state of the raw material. We will focus on the best-known and most common processes and therefore we will focus on the processes for manufacturing components from liquid and solid polymers.
Stereolithography (SLA) and Digital Light Processing (DLP) are two of the most common additive manufacturing processes that produce components from liquid polymers (resins). Both processes polymerize synthetic resins with the help of light. The SLA process uses a LASER as the power source, which is controlled with the aid of deflection mirrors. The DLP process, on the other hand, uses a projector or display to apply heat (energy) to cure the resin.
Figure 1: A component is manufactured using the SLA and DLP processes.
First, liquid resin is filled into a container that is transparent from the bottom. The building platform onto which the future component is produced is moved down into the resin so that the gap between the transparent bottom of the container and the building platform is equivalent to the thickness of the first layer. In figure 1 the top of the building platform is submerged into resin.
The resin in the gap is then cured by the power source (LASER, or projector) on the building platform through the transparent bottom. Since the power source is located under the container, the layers are built up from below. This means that in order to produce further layers, the Z-axis must move upwards after curing to be able to cure the next layer. This process is repeated until the component has been manufactured. Since the component is manufactured from below and the Z-axis moves upwards layer by layer, both the component and the necessary support structures are "turned upside down" and hang from the top, i.e. SLA and DLP manufacture upside down!
|Advantages of SLA and DLP||Disadvantages of SLA and DLP
|Low layer height -> high level of detail||Support structure necessary|
|High accuracy||Low construction volume|
|Smooth surface even without finishing||Cleaning of the component necessary|
|Use of synthetic resins with different mechanical properties||No reinforcement with fibers possible|
When manufacturing from the solid phase there are three ways to apply the polymer, as shown in Figure 2.
Figure 2: Categorizing the manufacturing processes from the solid raw material state 
Powder bed processes are some of the possible ways used to manufacture components from solid-state material. In these processes (SLS and 3DP), polymer powder is applied in thin layers to the building platform and sintered by an energy source (usually light) at the designated areas. Unlike the SLA and DLP processes, in the powder-based processes, the building platform (printer bed) is moved down for each new layer in order to apply new powder for the respective layer on top of the existing material.
The only difference between the SLS and 3DP processes is that in the SLS process the powder is sintered directly by a LASER, and in the 3DP process a binder is used.
|Advantages of powder bed process (SLS und 3DP)||Disadvantages of powder bed process (SLS und 3DP)
|Low layer height, high level of detail||Rough surface|
|High accuracy||Low construction volume|
|No support structure necessary||Cleaning of the component necessary|
|Use of different thermoplastics with different mechanical properties||No reinforcement by fibers possible
Another possibility for producing 3D printed components from solid polymers is the layer-by-layer bonding of polymer films (see figure 2 “Applying laminates”) that have already been cut to size. However, this process is little used in industry, so it will not be discussed here further.
The last option is to melt the solid polymer within a nozzle or print head and apply it sequentially to the building platform. The solid polymer is usually fed into the nozzle or print head as a filament (called FFF or FDM) or pellet (FGF). To be able to create the required layers, either the nozzle or print head must be moved upwards or the building platform alternatively moved downwards.
|Advantages of FFF/FDM and FGF||Disadvantages of FFF/FDM and FGF
|No cleaning of the component necessary||High layer height, low level of detail|
|Large construction volume||Rough surface|
|Use of different thermoplastics with different mechanical properties||Support structure necessary|
|Reinforcement by fibers possible|
By now, you should have some understanding of the most common ways of manufacturing components using polymer materials. We will showcase some advanced processes utilizing robots and 3D printing nozzles in our following articles. Our next article will be about the often-mentioned layers, how they are created by slicing, and how they are parameterized.
Until then, stay safe and stay tuned.
Title Image: Credits go to the Technical University of Munich.
 Berger Uwe, Hartmann Andreas und Schmid Dietmar, Additive Fertigungsverfahren RAPIDPROTOTYPING • RAPID TOOLING • RAPID MANUFACTURING [Buch] Verlag Europa-Lehrmittel, 2013.