## 3 Results and Discussion
If holes are visible in the layers, there may be more defects, such as an incorrect (or even inconsistent) diameter of the used filament, inappropriately selected operating parameters for layer height and filament dosing. Conversely, when a certain percentage of excess filament is dosed, material may accumulate in the nozzle, resulting in a visually distinct wider layer, often repeated periodically [48]. With seemingly beautiful surfaces, more accurate measurements are needed to determine the surface quality. New FDM 3D printed parts can be used as final components in specific cases for example in machines with short life such as missiles, emergency service solutions (temporary replacements), alternative mechanisms with less efficiency, or in prototyping [49]. It is also necessary to know the surface quality of raw parts and possible treatment options. When using a production machine containing secondary machin  
Figure 10.7 Tensile test load-extension curves for material printed at 0\({}^{\circ}\) print orientation. Figure 10.8 Internal structure of the printout. to machine each layer after extrusion to achieve a higher surface quality, even in internal structures [50]. For example, extruded bearings with metal additives on the rolling parts can be more abrasion-resistant. When machining the surface with a milling cutter immediately after the formation of the layer and repeating it in the whole 3D printing process it is possible to achieve higher quality surfaces for their smoother operation. Presented results show possibilities of material finish for future use of parts with high-quality surfaces. When measuring the surface roughness at the edge of test samples, the surface quality is significantly affected by the filament used. In addition to additives in the form of beads of different sizes and the adjustment of the height of the working layer and the flow of the filament itself, it is necessary to pay attention to the correct storage of the filament. Certain plastics can absorb atmospheric moisture very quickly, which results in uneven printing and thus weakening the product walls. Table 4 offers surface roughness measurement values of different layer heights for the materials used. The pure PLA filament without additives shows the best parameters. In others, the presence of additives greatly roughens the samples' outer surface. In the next step, the samples were subjected to wet grinding for 10 min on 2000 grit sandpaper. Figure 19 offers a close-up view of the wall of the test specimens before and after grinding. Images were captured under an inverted microscope Nikon Eclipse MA100. The grinding achieved the sample surface became smooth. Due to the high concentration of copper in the PLA plastic, it was possible to process this material very well. The best values for the surface of 0.2 mm were: Ra = 0.274 [\(\mathrm{\SIUnitSymbolMicro m}\)], Rq = 0.352 [\(\mathrm{\SIUnitSymbolMicro m}\)], Rz = 2.338 [\(\mathrm{\SIUnitSymbolMicro m}\)]. Figure 20 offers a view of the sidewall surface measured before grinding at a layer height of 0.1 mm. This layer offered excellent surface properties for all materials without visible external defects right after 3D printing. The measurement has made it obvious that, despite constant settings, the fiber in each layer flows out inconsistently, an overpressure builds up in the nozzle during certain working transitions, which is manifested in certain layers by a certain increase in the percentage of material flow. This shortcoming can be partially compensated for, but not completely suppressed in the production process on FDM devices. For basic applications of the model, this is only a visual error. If the production process is to be designed to generate functional assemblies, inequalities must be eliminated as much as possible. In the production of individual parts only tenths of a millimeter or less apart without the use of a separating material, surface irregularities result in the parts being connected into one non-functional unit.