Method of wax mold size control in precision casting

Method of wax mold size control in precision casting

The size of precision castings is guaranteed by the size of the mold. Generally, in mold design, the first step is to calculate the shrinkage rate of the casting. The shrinkage rate of castings is affected by three factors: wax mold shrinkage, alloy shrinkage, and mold shell expansion. Sometimes, due to the small expansion of the mold shell, the expansion of the mold shell can be disregarded. Therefore, in terms of casting size, the main factors are the shrinkage of the wax mold and the shrinkage of the alloy. Therefore, the shrinkage control of wax molds is the first step in controlling the size of castings. Today, precision casting manufacturer Tianyao Metal will talk to you about the methods of wax mold size control.

The shrinkage rate of wax molds in precision casting is related to various factors that contribute to the formation of wax molds. For example, the treatment of wax, the ratio of new and old wax, the injection temperature of wax, the temperature of the mold, the injection pressure, and the cooling temperature, etc. Therefore, it is necessary to take into account the various factors that contribute to the formation of wax molds.

1、 Wax treatment requires testing the shrinkage rate of the wax mold at regular intervals to confirm whether the shrinkage of the wax mold meets the standards. All parameters must be fixed during the first piece production, only in this way can the shrinkage of the wax mold be controlled. In actual production, some units do not pay attention to these detailed issues, which leads to the inability to control the size of castings or poor consistency of castings within the tolerance range. Because most castings are not final products and require processing. Modern machining basically eliminates the need for marking lines and simply uses CNC machining on fixtures. The consistency requirement for castings is very high. Moreover, as a customer, there is no obligation to improve the mistakes made by the supplier. Therefore, as a casting manufacturer and casting engineer, it is necessary to pay attention to these control details. Only in this way can the size of the casting be controlled.

2、 The size control of wax molds generally includes two aspects: deformation and size. Sometimes these two aspects intersect, which means that one measure can control both deformation and size. In principle, deformation mainly refers to geometric tolerances, while size refers to dimensional tolerances.

There are several methods to control the deformation of precision castings:

1. By controlling injection pressure and injection time, holding pressure and holding time;

2. Control the molding method to ensure that the wax molds produced from the mother's womb do not deform. There are two methods for demolding, one is hand picking and the other is relay. Hand picking can easily cause deformation of wax parts, so it is recommended to use relays. To ensure that the wax piece does not deform, manual and compressed air or ejection mechanisms are used together. In addition, after removing the wax, it is necessary to carefully check the shape and size. If there is any deformation, it must be corrected first;

3. Based on the structure of wax parts, use cold wax blocks reasonably to reduce the adverse factors of local shrinkage and deformation of wax parts; You can also use methods such as compressed air cooling and setting cooling blocks at specific positions on the mold;

4. The cooling of wax parts is also an important factor in ensuring that wax parts do not deform. Generally, there is an ice water machine in the wax mold room, mainly used for cooling molds and wax parts. The water temperature is generally lower than room temperature. When cooling, the wax parts must be placed underwater or covered with a wax plate. The above is how to ensure that wax parts do not deform.

So if it deforms, measures need to be taken to correct it. Therefore, for products with high size requirements. There must be inspection tools and calibration tools. The inspection tool is used to check whether the wax piece is deformed, while the calibration tool is used for correction. Generally speaking, the size stability period of wax molds is 24 hours, which means that after 24 hours, the size of wax molds is basically stable. Because wax molds need to undergo shaping before assembly, checking and correcting them before assembly is a necessary process.

And size control is actually basically the same as deformation control methods. Sometimes, after the wax mold is pressed out, it is immediately placed in a fixture (called in Taiwan, a tool that prevents deformation and maintains size), and then cooled in cold water. In fact, deformation and size are two aspects of the same problem. For the sake of clarity, they are separately mentioned, but in reality, the two are done simultaneously.

Precision machining