## 3 Results and Discussion  
### Fabrication Strategy by Combined Binder Jetting 3D Printing and Casting  
Conventional manufacturing methods are limited mainly by the total production volume. For this reason, 3D printing is a valuable alternative when few copies of a given part are needed. In our case, the aim was to replace a very valuable old impeller used in the centrifugal pump of a polyol plant and compare the performances of the original and the 3D-printed copy. The sequential workflow is depicted in Figure 4 and involved the following steps:  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Recipe**} & \multicolumn{2}{c}{**Final Product Properties**} & \multicolumn{2}{c}{**Duration of Tests**} \\ \cline{2-5}  & **Concentration (\% _w/v_)** & **Viscosity (cSt)** & **KSB Impeller** & **BJ Impeller** \\ \hline
**1** & 80 & 500 & January 2020–October 2020 & November 2020–April 2021 \\ \hline
**2** & 50 & 180 & August 2019–October 2020 & November 2020–April 2021 \\ \hline
**3** & 20 & 30 & May 2019–October 2020 & November 2020–April 2021 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Product recipe compositions and test durations.  
Figure 3: Work process flowchart at the polyol/polyglycol plant. The impeller was mounted in pump P. T: reaction tank, D: distillation column, V-A and V-B: fail-open valves.  
* analysis of the original part (KSB impeller);
* part digitizing, CAD design, and 3D printing of the plastic model;
* sand mould design and manufacturing;
* casting using molten stainless steel and finishing (BJ impeller);
* performance tests in a chemical plant and comparison with KSB impeller.  
Since there were no original drawings of the KSB impeller available, the part was reverse engineered (Figure 5) using an industrial metrology 3D scanner bearing an optical automatic arm. This is a useful strategy when a precise digital model of a mechanical component needs to be reproduced [21]. After scanning, the small complex components were measured with an optical 3D coordinate measuring machine at up to 0.5 mm resolution to obtain the point cloud geometry. These points were then used to extrapolate the shape of the impeller, and finally, a parametric CAD model was made. Optical 3D coordinate measuring machines are replacing tactile measuring systems and gages because they capture more detailed and more easily interpretable information on an object in significantly shorter measuring times. Whereas old mechanical measuring systems captured data in a point-based or linear manner, optical measuring systems return full-field data about deviations between the actual 3D coordinates and the CAD data [41]. The point cloud geometry was revised and corrected before generating the STL file that the 3D printer understood and fabricating a \"communication model\" on PA12 plastic.  
The 3D sand mould was fabricated using binder jetting on rapid prototyping silica. It was designed in separate pieces that assembled together to contain two impellers (Figure 6a,b) and had an excess dimension of 0.5 mm to account for material contraction after cooling. There were superior, intermediate, and bottom pieces that assembled like a puzzle. The fluid (liquid metal) entered through the superior part (cone shape inlet in Figure 6c) to fill internally, from down to top, the 3D features that generated the structure of the two impellers. Figure 6d shows photographs of the moulds after 3D printing, and Figure 7 shows photographs of the resulting casted impellers after mould separation. Excess casted material was then removed, and the parts were machine finished (Figure 8) and tested for dynamic balancing according to ISO 1940.