Analysis on macro error sources of rapid investment casting

Because the sintered materials are mainly thermoplastic polymer materials and their composites, under the action of laser, the powder absorbs the laser energy, and then the temperature of the powder increases and flows through heat transfer. Through the diffusion and flow of the sintered powder materials, the sintering is realized. Then, with no continuous energy input, the temperature of powder material is reduced. In the sintering process, the powder particles undergo multiple phase transformations and material flow, so the sintering shrinkage occurs, including the shrinkage of melt and temperature. At the same time, due to the different shrinkage sequence and shrinkage, the warpage of the specimen will be caused.

In conclusion, it can be concluded that the size error of SLS prototype on z-axis is mainly affected by powder spheroidization, secondary sintering and z-axis surplus, while the X and Y directions are mainly affected by secondary sintering, and the sintering shrinkage acts on the whole SLS prototype.

The post-treatment process is due to the SLS prototype in the molding process, part of the particles can not fully melt, and the particles are bonded with each other, resulting in the generation of gaps, which affects the strength of SLS prototype. At the same time, in order to improve the surface quality, post-treatment is needed. However, the use of polyester post-treatment materials leads to a little volume shrinkage and warpage. In the post-treatment process, the volume of specimen changes due to the action of high temperature wax liquid, which leads to the size error of specimen.

The casting changes from liquid to solid, and the casting is accompanied by volume shrinkage, including the free shrinkage of the casting and the effect of actual obstruction. Especially for complex parts, in addition to the comprehensive consideration of the free shrinkage of the alloy, it is also necessary to analyze the actual obstruction of the casting in the actual pouring process. It is because of the above factors that the final metal casting has a certain size error relative to the wax mold size.

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