Selective Laser Sintering (SLS) is an advanced molding technology that shortens the design-to-manufacturing cycle, thereby reducing production costs and enhancing market competitiveness. In the SLS process, the heat emitted by a CO2 laser melts the powder material, which is then layered to form a three-dimensional object. Under computer control, the laser scans specific areas of the material layer-by-layer, based on two-dimensional data. As the laser beam scans the material, the powder fuses into solid layers while unscanned areas remain loose. After each scan, the platform is lowered, and a new layer of powder is spread. The laser then scans and bonds the new layer to the previous one, continuing this process until the entire part is formed.

The precision of SLS molding is influenced by several factors, including machine accuracy, CAD model slicing errors, scanning methods, powder particle size, ambient temperature, laser power, scanning speed, scan spacing, and layer thickness. Sintering process parameters significantly affect the accuracy and strength of the final part. Parameters such as laser power, scanning speed, scan spacing, sintering temperature, sintering time, and layer thickness all play crucial roles in the bonding between layers, shrinkage, warping, and even cracking during the process.

1. Laser Power

Increasing the laser power generally leads to an increase in size errors, particularly in the thickness direction. This occurs because, with a fixed laser wavelength, the laser spot size remains constant, but the heat emitted increases as the power rises. This additional heat causes a larger dimension error in the width and length directions, while the thickness direction experiences greater growth. When laser power increases, the intensity also rises, as more powder particles melt and fuse, increasing the density of the sintered part. At lower power levels, only the edges of the powder particles melt, creating voids between spherical particles, which results in a lower strength. However, when the power is increased to a certain level, the particles fully melt and fuse into a solid structure, increasing strength significantly.

On the flip side, excessively high laser power can intensify thermal shrinkage and lead to warping in the part. Therefore, an optimal combination of laser power and sintering parameters is necessary to achieve a balance between strength and dimensional accuracy.

2. Scanning Speed

As the scanning speed increases, the size error decreases in the negative direction, but the strength of the part also reduces. The impact of scanning speed on the dimensional accuracy and performance of the prototype is the opposite of the effect of laser power. When the scanning speed is increased, the energy density per unit area decreases, which is equivalent to lowering the laser power. While scanning speed does affect the efficiency of rapid prototyping, it must be selected based on the specific requirements of the part and the overall production process.

3. Scan Spacing

Scan spacing refers to the distance between two laser scanning lines. As scan spacing increases, the size error shifts towards negative values, while strength decreases. A smaller scan spacing results in a higher energy density per unit area, leading to better powder melting and higher strength. Conversely, when the spacing is larger, the overlap between adjacent laser beams decreases, which reduces the amount of powder sintered together, resulting in a decrease in strength. However, larger scan spacing can increase the efficiency of the molding process, making it an important factor to balance with the strength requirements in actual production.

4. Layer Thickness

Increasing the layer thickness tends to decrease the strength of the part, while the size error moves towards negative values. As layer thickness increases, the bond between layers becomes weaker, making it more prone to delamination or even failure in some cases. With thicker layers, more powder needs to be melted, which reduces the amount of heat transferred out of the part, causing the size error to shift towards the negative direction. The thickness of each layer also greatly impacts the efficiency of the forming process, and it should be chosen based on the shape and complexity of the part.

5. Preheating

Preheating is a critical step in the SLS process. Without preheating or with uneven preheating temperatures, the molding time may increase, resulting in lower part performance, poor quality, and poor precision. In extreme cases, the sintering process may not occur at all. Preheating the powder material helps to reduce thermal stress inside the part during sintering, preventing warping and deformation and improving molding accuracy. Proper preheating ensures a more uniform temperature distribution throughout the part, which contributes to better dimensional precision and overall part quality.

Conclusion

The selective laser sintering (SLS) process is a complex yet highly effective technology that relies on careful control of various process parameters. These parameters, such as laser power, scanning speed, scan spacing, layer thickness, and preheating, directly influence the dimensional accuracy, strength, and quality of the final part. By optimizing these parameters, manufacturers can achieve high-quality prototypes and production parts with the desired mechanical properties. Understanding the impact of these factors and balancing them appropriately is key to successful SLS-based manufacturing.