Compensation method for dimension error of typical complex parts in rapid investment casting

The size correction of the typical complex diffuser was completed by using magic software. Through the analysis of the size deviation of the diffuser impeller, the existing size error compensation coefficient is applied to compensate the size of the part, and the size compensation is carried out again until the qualified model is obtained. Finally, the final size correction is completed combined with the machining allowance of each direction. Specifically, the STL format file of the diffuser impeller is imported into magics, and triangular patches are marked between the two vanes on the side of the diffuser impeller, the outer side of the vanes and the upper and lower planes, as shown in Figure 1. And select the “offset” function to compensate the corresponding size.

(a) Between two blades of measuring surface (b) Outside of side blade

Selective laser sintering (SLS) technology was applied to burn the impeller of the diffuser after size correction. After powder cleaning and wax soaking, the point cloud data of the diffuser was detected again. The laser three-dimensional scanning method was used to detect the point cloud data of the part again. The measured point cloud data was compared with the size corrected model in three-dimensional dimension, and finally the diffusion coefficient was obtained The results of the size deviation are shown in Figure 2 (a).

(a) Diffuser size comparison results (b) Mechanical cutting allowance addition

It is found from figure (a) that it is not difficult to find that most of the size deviation of the parts with size compensation is within ± 0.5mm by correcting the diffuser impeller dimensions in all directions. Compared with the parts without size compensation, the dimensional accuracy of the diffuser wax mold has been significantly improved. Therefore, on the basis of the qualified diffuser, the mechanical cutting allowance of the upper and lower planes and the inner cavity is added, as shown in figure (b).

According to the size transfer law of the rapid prototyping process and the size deviation results of the diffuser, the section size of the diffuser is shown in Figure 3, and the thinnest section size of the diffuser is 12.5488mm and the thickest is 41.411mm. According to the test piece CAD According to the size error transfer law from the model to the metal casting, there is a certain volume shrinkage in the alloy solidification process, and the volume shrinkage has a certain relationship with the size of the wax mold. Combined with the scanning comparison deviation results of the diffuser impeller after a wax immersion, and according to the requirements of the machining allowance of the casting, the final part size correction and the size correction of the diffuser impeller are carried out The addition of machining allowance.

The upper and lower parts of the part are welded with gating system, and the corresponding machining needs to be carried out after the removal in the later stage, so a certain machining allowance needs to be left for the upper and lower parts; the corresponding machining allowance also needs to be added for the convenient assembly of the inner cavity. The corresponding machining allowance is determined according to the formula.


R1 – one side dimension of casting blank, unit: mm;

R2 – casting blank cavity size, unit: mm;

F – final machining dimension, unit: mm;

RMA – required machining allowance, mm;

CT – casting tolerance.

According to the above formula, the machining allowance of one side is RMA + CT / 2 = 1.4 + 0.56/2 = 1.68, so it is necessary to add 1.68mm machining allowance to the upper and lower planes respectively after the modification of the diffuser CAD model. The machining allowance of inner cavity is 2rma + CT / 2 = 2 × 1.4 + 0.56/2 = 3.08mm. Combined with the compensation of dimensional error and the addition of machining allowance, a qualified SLS prototype was finally fired.