4.1 Polymer materials
Figure 13 shows a generic printer head, such as you might find in an office inkjet printer. As the print head moves across the paper, a signal from the microprocessor controller in the printer causes droplets of ink to be fired at the paper in appropriate places. The print head moves back and forth as the paper advances under the head.
Now imagine printing a document using an ink that deposits on the paper as a thick layer when it is dried. Once the first layer is finished and dried, the same paper could be put back into the printer and another layer added. This will result in the deposition of more ink on top. By repeating the process, and maybe altering the design of each layer a little from its predecessor, it should be possible to create a print with a built-up surface.
Some adaptation to the printer would obviously be needed to allow the layers to build-up under the print head. This is the principle for a basic 3D printing machine (Figure 14). However, not all AM is done like this.
Stereolithography (SLA) is an additive process in which an ultraviolet laser beam is used to set off a chemical reaction in a bath of a liquid ultraviolet-sensitive resin (a liquid polymer) that causes the polymer to solidify at one pinpoint position (Figure 15).
The laser beam traces out the shape of each layer of the object on the surface of a pool of resin above a moving platform. Once the first layer is formed, the platform moves down by the depth of one layer. The next layer is deposited directly on the previous layer. Repetition of the process results in a 3D object being built up. The entire geometry of the machine can also be inverted so that the developing part is raised out of the reservoir of liquid rather than lowered into it (Figure 16).
After building, the object will be rinsed in a solvent to remove excess resin. Curing can be carried out in a UV furnace.
Materials suitable for SLA are limited to specialised polymers, so this process can only be used for models or components that are not meant to have mechanical or structural purpose.
One other limitation is that the liquid resin cannot support isolated islands of solid material. So, a complex shape like the propeller shown in Figure 15 would need additional supports added to the design. The supports are often referred to as scaffolding, which would be removed after building.
Fused deposition modelling (FDM) is another process which involves the application of semisolid polymer beads, almost analogous to laying lengths of toothpaste on top of each other (Figure 17).
FDM is probably a technique that should be categorised as RP because it is limited in resolution and detailed finishing. Furthermore, because of the surface tension of semisolid polymer, wetting angles are relatively high, resulting in pores and defects. However, because the technique is relatively simple and does not rely on complex chemistry or lasers, the technique has found applications where mechanical performance isn’t critical. Children’s toys are a common application (Figure 18).