Effect of process parameters on precision of SLS rapid casting prototype

In the selective laser sintering process, the process parameters have a great influence on the sintering quality. The main influencing process parameters are preheating temperature, laser power, powder layer thickness, scanning speed and scanning distance. The specific factors are analyzed as follows.

(1) The influence of preheating temperature on SLS prototype. When the laser acts on the sintered materials, especially for the polymer materials (PS / PA, etc.), the thermal deformation is large. When there is a significant temperature difference between the sintered area and the non sintered area, the SLS prototype will produce internal stress, which will lead to warping deformation. The setting of preheating temperature is mainly determined by the sintering temperature window of the sintering material. The wider the sintering temperature window is, the easier the forming process is to be controlled, and the smaller the warpage is.

(2) The influence of powder layer thickness on SLS prototype. Powder layer thickness refers to the distance that the forming cylinder descends each time during SLS forming. It determines the thickness of each powder spread on the molding surface. According to the principle of SLS, the layer thickness determines the powder layer thickness, and then determines the principle error. The smaller the thickness of powder layer is, the smaller the corresponding principle error is, the less obvious the step effect is and the higher the corresponding surface quality is, especially for the complex surface model. However, in the actual processing process, the powder layer thickness is generally within a certain range due to the influence of equipment powder laying accuracy, powder average particle size and slicing software accuracy. Through preliminary experiments, the best powder layer thickness of PS material is 0.14mm. The parameters are shown in Figure 1.

(3) The influence of laser power on SLS prototype. In this experiment, CO2 laser is used as the rapid prototyping equipment, the maximum power is 60W, and the laser mode is single mode. In the center of the laser spot, the laser intensity is the highest, and the corresponding energy is also the most. Far away from the center of the laser spot, the laser intensity decreases, and the energy of the corresponding powder decreases. Similarly, the closer to the center of the spot, the greater the sintering depth of the powder bed, and the farther away from the center, the lower the sintering depth of the powder bed, as shown in Figure 2. Therefore, when the laser power changes, the corresponding energy acting on the powder bed and surrounding will change, and the sintering depth will also change. At the same time, the sintered layer under the action of laser will act on the unfired material layer through heat conduction to achieve the temperature rise.

(4) The influence of scanning speed on SLS prototype. Scanning speed refers to the moving speed of laser spot on the powder bed in selective laser sintering process, which is mainly determined by laser step length, step period and scanning radius. The change of scanning speed will directly affect the laser energy density on the powder bed, and then affect the laser sintering depth. When the laser power is constant and the scanning speed is increased, the laser energy received by the powder bed in the corresponding unit time will be decreased, and the corresponding sintering depth will be decreased; on the contrary, when the scanning speed is decreased, the corresponding sintering depth will be increased. When the sintering depth increases, the powder at the bottom of the molded part is fused and bonded to the molded part, resulting in the Z dimension of the molded part larger than the theoretical dimension, and the dimensional accuracy of the molded part decreases. The best scanning speed of PS is 3000mm / s.

(5) The influence of scanning distance on SLS prototype. The scanning distance refers to the distance between the centers of two adjacent parallel laser beams. The scanning distance is a parameter related to laser spot diameter and laser energy density. If the scanning distance is too large, the sintering area between the adjacent laser scanning paths can not be fully sintered, which makes the sintering area between the adjacent laser paths not dense enough and reduces the intensity. If the scanning distance is too small, the adjacent two laser beams overlap, and the powder is heated repeatedly, which may even cause the powder spatter, resulting in overheating of the molded parts and warping deformation, affecting the molding accuracy of the molded parts. Ideally, the scanning distance should be equal to the laser spot diameter, so that the powder bed can be fully sintered. However, in the actual sintering process, due to the effect of laser heat radiation and heat transfer, the actual laser sintering area will be larger than the spot diameter, so the scanning spacing is generally selected to be slightly larger than the spot diameter. Therefore, the scanning spacing of 0.31mm is selected for the experiment.

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