## 3 Quality Control of Silicon Carbide Printed Part Formation
### The Impact and Control of Key Printing Parameters on Green Part Formation Quality
#### 3.1.2 Uniform and Dense Powder Bed
In the study by [PERSON] et al. [98], the wiper profile was governed by the following equation:  
\[\left|\frac{y_{s}}{a_{s}}\right|^{n_{s}}+\left|\frac{z}{b_{s}}\right|^{n_{s}}=1 \tag{1}\]  
By adjusting these parameters (\(a_{s}\), \(b_{s}\), and \(n_{s}\)), various wiper profiles were obtained, as illustrated in Figure 11. \(D_{\text{sph}}\) represents the sphere diameter (for rods), with a value of \(10^{-4}\). The parameter \(a_{s}\) denotes the spreader profile thickness parameter, ranging from 10 to 25 or 100 \(D_{\text{sph}}\). The parameter \(b_{s}\) indicates the spreader profile height parameter, with a range of 10 to 20 or 50 \(D_{\text{sph}}\). The shape parameter \(n_{s}\), which determines the profile geometry, varies among 0.5, 1.0, 1.5, 2.0, and 5.0. Here, \(x\) and \(z\) correspond to the coordinates of the wiper along the spreading and building directions, respectively. Their simulation results revealed that the powder bed density reached its maximum value at \(n_{s}=5\) across different \(a_{s}\) and \(b_{s}\) combinations [98]. On the other hand, extensive studies have demonstrated that rollers, when employed as powder spreaders, can achieve higher powder bed densities compared to  
Figure 11: Different spreader profiles used for optimization [98]. wipers [99; 100; 101; 102; 103]. However, it should be noted that the rotation direction of rollers may affect powder bed surface integrity. Based on their rotational orientation, rollers can be classified into two types: forward-rotating rollers (as shown in Figure 12a) and counter-rotating rollers (as illustrated in Figure 12b). Forward-rotating rollers exerts significant shear force [105] and compressive force [106], allowing for higher compaction density when passing over the powder bed. However, the fine SiC powder used in the printing process has a high viscosity, and the large compressive force can cause the cohesive particles to adhere more readily to the rotating roller, forming powder clumps, which results in the formation of pits on the surface of the powder bed and leaves new \"craters\" in other areas of the powder layer, severely affecting the surface accuracy of the printed parts. To mitigate or prevent such surface defects caused by the substantial compaction force of forward-rotating rollers on the powder bed, reducing the powder quantity ahead of the forward-rotating rollers proves effective [106]. This can be implemented by incorporating a pre-spreading step using either a wiper or counter-rotating roller prior to final spreading with forward-rotating rollers [107; 102]. The pre-spreading and final spreading procedures can be executed either through separate traverses or within a single traverse operation. However, this approach inevitably compromises printing efficiency and increases process costs. In comparison, backward-rotating rollers are more beneficial for the movement of powder particles in the BJ 3D printing of SiC materials. [PERSON] et al. [108] conducted a simulation study on the powder spreading effects of backward-rotating rollers, which indicated that, during the passage of the backward-rotating roller over the powder bed, some particles are lifted by the rotating roller and enter the powder bed, subsequently circulating in the stacking area. This allows particles more time to rearrange, which reduces the occurrence of powder agglomeration and achieves a higher and more uniform powder bed density. Moreover, rollers with larger diameters and smoother surfaces can achieve higher powder bed densities [109; 110]. Therefore, the counter-rotating roller is most commonly used for powder spreading. Finally, when employing a roller for powder spreading, the influence of the roller's lateral speed must also be taken into consideration. Earlier research by [PERSON] et al. [111] concluded that, when the lateral speed does not exceed 12 mm/s, the packing density of the powder bed increases slightly with the increase in lateral speed. However, with  
Figure 12: (**a**) Forward-rotating roller. (**b**) Counter-rotating roller [104].