## 5 Quality Assurance: Raw Materials and Final Printed Parts
### Quality Control of Final Printed Parts
#### 5.2.1 Issues That Occurred during the Printing Process  
The following quality issues were observed and will be discussed further in the following paragraphs: insufficient adhesion, stringing, within-stack lack of separation, and breaking of parts.  
Insufficient adhesion is as result of a cold build platform, a cold preceding layer, or insufficient calibration of the Z-axis. Consequently, the filament does not attach to the surface of the bed or the previous layer, instead, it detaches from the whole part or the surface due to insufficient bonding between layers. Figure 5 shows an example of mid-print failure due to insufficient layer adhesion. Detachment of the first layer can cause the part to be stuck to the heated nozzle, which can result in material build-up at the hot end and in turn fatal damage to the heaters, thermistors, or even the whole printer. Therefore, close monitoring of the first layer is crucial for a successful print.  
Stringing is a more common issue with PETG filaments. Due to overheating of the nozzle, small stringy filaments are attached to the actual parts, see Figure 6. These small stringy filaments are particular  
Figure 5: Insufficient adhesion between layers on a part made from polylactic acid (PLA).  
visible in areas with rapid printhead movements, where the material is stretched out as the retraction of the filament is not sufficient. A solution for this issue is to either print with a lower printing temperature or to tweak the retraction settings. The optimized settings for avoiding major stringing issues with PETG are given in Table 1.  
Another major issue was observed during the printing of stacks of the face shields. During the stack printing approach, multiple face shields would unintentionally attach to each other, preventing later separation of the individual parts (Figure 6). The printing of stacked parts is especially useful, as it maximizes the daily output of the headbands (one hour printing time per headband) and allows for a continuous printing process overnight. However, some prints showed insufficient separation between the individual headbands. This was due to imperfect bridging parameters in the gaps between two parts. After optimizing the printing parameters by increasing the bridging speed and lowering the bridging temperature these issues could be solved, and stacked prints were possible.  
Breaking of parts during the removal process was another common issue. This issue occurred especially in the initial stages of ramping up the production and was even exacerbated due to issues of increased adhesion to the bed or to other parts (such as the aforementioned bonding in stacks). Establishing a standardized practice for part removal solved this problem, see Figure 7.  
The issues mentioned in the sections above were mainly specific to the printing of headbands during the production period. However, as in any production environment, general 3D printing issues were experienced occasionally, such as the clogging of the nozzles or warping of printed parts.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Nozzle** & **Print Speed** & **Filament** & **Retraction** & **Fan Speed (\%)** \\
**Temperature (\({}^{\circ}\)C)** & **Perimeters (mm/s)** & **Retraction (mm)** & **Speed (mm/s)** & **Fan Speed (\%)** \\ \hline
255 & 50 & 0.8 & 35 & 50 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Optimized setting for 3D printing of polyethylene terephthalate glycol (PETG).  
Figure 6: Image of 8 head bands that were bonded together and the surface shows stringing of the PETG filament.