Several Measures to Prevent Deformation of Precision Casting Wax Patterns

Several Measures to Prevent Deformation of Precision Casting Wax Patterns

Due to the smaller shrinkage, faster solidification, and higher productivity of paste based molds compared to liquid based molds, paste based molds are commonly used in investment casting production. In the production process of pressure molding, wax mold, and casting, the dimensional stability and degree of deformation of the wax mold affect the quality status of the casting.

For deformable parts, effective measures such as placing obstacles in the wax mold to restrict shrinkage deformation, sticking the sprue after opening the sprue root, and cooling in water after pressing the wax should be taken from the first process - wax injection, to keep the wax mold in a good quality state.

1、 Placing obstacles in wax molds to restrict shrinkage deformation

The right page structure of the precision casting requires two holes with a diameter of 10.5mm to be coaxial in order to ensure good rotation of the door. The customer requires a specific pin shaft inspection tool to be used for full inspection, and one end of the specific pin shaft should freely fall through the two holes without obstacles. The original production process was: after pressing the wax, the mold was placed in water and cooled for 30 minutes before removing the wax mold. After repairing the wax mold, the tree was assembled. The casting produced by pouring is pierced through two holes with a specific pin shaft, which is 10.5mm in diameter. Due to the different axes of the two holes, the pin shaft cannot penetrate and requires 100% correction. Moreover, there is no regular deformation, which increases the workload of correction and seriously delays the production progress.

In response to this situation, the process was improved. After calculation, 10 pin shafts with a diameter of 10.80-0.1mm were made. After pressing the right page wax mold, the pin shaft was inserted into the wax mold at the hole with a diameter of 10.5mm, and then placed in water. After cooling for 30 minutes, it was taken out. Two methods were used for these wax molds: one was to directly assemble the tree after repairing the wax mold, and the other was to stick a tie bar on it. After the casting was produced, the pin shaft was used for inspection. Only one piece had slightly skewed two holes, while the rest fully met the requirements. The skewed right page of this one piece can be slightly corrected to meet the requirements. After classification, it was found that the first method without adhesive reinforcement already meets production requirements and can be put into mass production.

The 4 claws of the 5 × 5BT valve sleeve precision casting valve sleeve have strict requirements, and the outer circle of the claws needs to be processed to ensure sufficient processing quantity. Exposed black skin is not allowed. For this reason, there are sample inspection claws with changes in size on wax molds and castings

The original process was to press the wax and cool it in water for 40 minutes before removing the wax mold. After inspection, the four claws on the wax mold are generally deformed inward, with each claw still swinging left and right, and the deformation is irregular. The wax shooter can only repair 7 wax molds that meet the sample in one hour. To solve the deformation problem, a metal support ring was specially made through calculation, as shown by the double dotted line. After the valve sleeve wax mold is ejected, the metal support ring is inserted and cooled in cold water for 40 minutes before being removed. After using metal support rings and correcting wax molds according to process requirements, there is almost no need to repair wax molds, greatly improving the efficiency of wax mold repair work. The improved process has been followed in mass production and good results have been achieved.

2、 Measures for sticking the sprue after using the root of the sprue

DNl50 flange structure, with symmetrical structure and uniform wall thickness. If a sprue is directly opened on it, due to the thickness of the sprue, there is a wall thickness difference with the wax mold body, which not only makes it difficult to remove the wax mold from the mold, but also causes deformation due to improper mold removal methods; It may also cause shrinkage deformation during the cooling process of the wax mold due to uneven wall thickness. Therefore, considering the above, the DNl50 flange plate adopts the method of pre setting the sprue root (height of 1.0-1.5mm) and then sticking the sprue to avoid uneven wall thickness caused by large sprue thickness on the wax mold, resulting in asynchronous shrinkage deformation.

3、 Wax mold cooling method

After waxing, attention should also be paid to the choice of cooling method, usually cooling in air or in cold water. Cooling in cold water can force the wax mold to shape, while some three or two ends are flange plates, such as DN40 flange gate valve bodies, which are cooled in a water tank under normal production conditions, resulting in a significant difference in the distance L1 and L2 between the two flange plates on the wax mold, which can reach 2.0-2.5mm.

After analysis, apart from the characteristics of the structure itself, the wax model is in a floating state in the water tank, with a portion of the wax model in water and a portion in air. Due to the different cooling conditions of the flanges above and below the water surface, the resulting shrinkage varies, resulting in significant deformation of the flanges. To prevent this phenomenon, efforts should be made to completely immerse the wax model in water. Making the cooling conditions of each part of the same wax mold consistent and taking it out after the size is stable can greatly reduce the occurrence of such problems.

Precision machining