**4. Fused deposition modeling**

FDM, also known as Fused Filament Fabrication (FFF), is a material extrusion process in which material is heated and then dispensed through a nozzle layer by layer. The process was invented and patented by Scott Trump in 1989, who then founded Stratasys Inc. The basic process is based on a robot arm that moves two nozzles where a filament of polymeric material is fused and then deposited layer by layer on a platform (**Figure 5**). The filament is supplied by an unrolled spool and

**Figure 5.** *FDM process.*

## *Additive Manufacturing of RF Waveguide Components DOI: http://dx.doi.org/10.5772/intechopen.104106*

pressed into the extrusion head. The material is then heated utilizing electrical resistance [1]. Heat is conducted to the liquefier chamber to obtain a liquid state. The material inside the chamber should be maintained in a molten state, with particular care on the temperature since some polymers degrade at high temperatures and could burn [2]. The filament is softened, the molten material gets off through a nozzle of reduced diameter and deposits on the building platform [1].

The nozzle diameter determines both the shape and size of the extruded filament and the minimum feature size that can be printed—the larger the nozzle diameter, the faster the process, but with lower precision. Material extrusion is controlled by the pressure difference between the chamber and the atmosphere. If the pressure is maintained constantly, the material flows at a constant rate with a constant cross section. The same happens if the nozzle speed is kept constant. Any change in the direction of the extrusion head must result in a change in the corresponding material flow rate. Otherwise, a different amount of material will be deposited in that region. Once the material is extruded, gravity and surface tension can cause a change in the shape, while the cooling and drying effects can change the size of the material deposited. This effect can be reduced by minimizing the differential temperature between the chamber and the atmosphere. Bonding with the previous layer and the adjacent region can be ensured by residual heat energy or solvents and wetting agents in the extruded filament [2].

FDM machines can be equipped with one to three nozzles. In this case, two nozzles deposit two different building materials, while the third one is filled with a soluble material for supporting structures. The former case is employed for a low-cost solution for large industrial machines. In this way, the support material is not removed manually but is dissolved in a chemical bath after the fabrication process.

Many thermoplastic materials are available for FDM and FFF processes; the most commonly used for RF applications are listed below [1].


more than ABS and PLA. For this reason, the use of a heated building volume is recommended.


FDM allows the manufacturing of multi-material parts. The use of a soluble support material offers good design flexibility. The main concern with this technology is that the nozzle dimensions limit the accuracy. The typical resolution of the process is about 100–200 μm. As in the case of SLA, a metal plating of the internal channels is mandatory for waveguide RF application.
