In the experiment, the dimension deviation is used as the measurement index of the dimension accuracy of the specimen. The smaller the dimension deviation is, the higher the dimension accuracy is
η – dimension deviation;
A1 – measured dimension, mm;
A0 – theoretical dimension, mm.
The main changes of SLS prototype in this process are as follows: first, the residual floating powder on the surface of the specimen falls into the wax pool when heated; second, the wax liquid enters into the specimen through the gap after PS sintering, and the gap is filled to take out the wax immersed specimen and cool it to room temperature; third, the wax liquid does not enter into the specimen due to the volume change from liquid to solid The wax on the surface of the specimen changes the size of the specimen. Figure 1 (a) shows the three-dimensional deviation of SLS prototype minus the three-dimensional dimension after one wax immersion, and figure 1 (b) shows the three-dimensional deviation of SLS prototype minus one wax immersion after two wax immersion.
It can be seen from Figure 1 that the change trend of three-dimensional dimensional deviation of the specimen after one-time wax immersion is basically the same, showing a change trend of first decreasing and then increasing; after two-time wax immersion, the overall change of X and Y directions is small, and the dimensional deviation of Z direction decreases in a large range with the increase of wax temperature. After two-time wax immersion, the dimensional deviation of the specimen gradually decreases with the increase of wax temperature. The main reason is that the whole volume change and the size change caused by the adhesion of wax layer on the surface of the specimen are caused by the action of high temperature wax liquid in the first wax immersion, while the size change of the specimen is mainly caused by the adhesion of wax layer on the surface of the specimen in the second wax immersion.
It can be seen from Fig. 1 (a) that when the wax dipping temperature is 55 ℃, the wax temperature is lower, the viscosity of the wax liquid is higher, and the wax liquid enters into the SLS prototype less, but the thickness of the wax layer adhered on the surface of the specimen is larger. At this time, the size deviation of the specimen is determined by the thickness of the wax layer adhered on the surface and the increase of the volume of the wax dipping specimen, and the overall performance is larger; when the wax dipping temperature is 65 ℃, the wax temperature is higher, and the specimen size deviation is larger When the thickness of surface adhesion wax layer decreases, the wax liquid infiltrating into the specimen increases gradually, and the volume change caused by it also increases further. At this time, the decrease of surface adhesion wax layer thickness is greater than the increase of wax immersed specimen volume, and the overall size deviation is small; when the wax temperature is 75 ℃, the wax temperature further increases, the surface adhesion wax layer thickness of the specimen decreases continuously, but the wax infiltrating into the specimen increases The decrease of the thickness of the wax layer on the surface is less than the increase of the volume of the wax impregnated specimen, and the overall performance is the increase of the size deviation.
The wax molecules infiltrate into the specimen during the first wax soaking process, which improves the strength of the specimen, but the thickness of the wax layer on the surface is small and the surface quality is poor. Therefore, in order to meet the requirements of investment casting on the surface quality of wax pattern, it is necessary to conduct secondary wax impregnation on the specimen . As shown in Fig. 1 (b), the dimensional deviation diagram of the specimen after the second wax immersion is shown. When the wax temperature increases from 55 ℃ to 70 ℃, the z-direction variation rate is larger. When the wax temperature increases from 70 ℃ to 75 ℃, the three-dimensional variation trend tends to be the same. Because the XY plane is a laser firing plane, the surface has a smaller surface gap compared with other sides (XZ plane, YZ plane), and the infiltration of wax on the surface is reduced, resulting in a large amount of wax adhering to the plane, which makes the z-direction dimension deviation larger. With the further increase of wax temperature, the viscosity of wax decreases, especially at 75 ℃. When the wax temperature is further increased, the thinnest part of the specimen will soften to a certain extent. At the same time, under the action of high-temperature wax liquid, the wax layer will adhere to the surface of the wax mold after one-time wax soaking, which will melt when heated, resulting in the final surface quality of the wax mold being reduced. Therefore, the optimal temperature of the second wax soaking is 75 ℃.