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Process of Investment Casting

Ⅰ. Basic definition of investment casting

Investment casting is usually a method of coating several layers of refractory materials on the surface of the wax mold. After it is hardened and dried, the wax mold in it is melted to form a mold shell, which is then fired and then poured to obtain a casting. It is also called investment casting because the obtained casting has high dimensional accuracy and surface finish.

Investment precision casting has a history of more than 5,550 years, which can be traced back to the ancient times of ancient Egypt and China.

In industrial manufacturing, parts manufactured by this method include: gears, cams, ratchets, turbine blades, mechanical parts and other parts with complex geometries.

Ⅱ. Process of investment casting

1. To make a wax mold, that is, press the molten wax material into the mold, and take it out after condensation to get the wax mold. The material of the mold can be resin, but wax is usually used because wax melts easily and can be reused.

The dimensional accuracy of the mold is relatively high, which requires careful calculation and scrutiny. The shrinkage of the metal casting parts and wax and the refractory material on the surface of the wax mold must be considered. It takes multiple measurements to reach the required size range.

2. Group tree: Combine the finished product wax mold and the wax mold that forms the pouring riser system, connect multiple wax molds, and complete multiple castings through one casting. The wax rod is used as the central main runner, all wax molds are connected with the main runner, and the starting end of the wax rod is connected to the main gate. This structure is called a group tree, where the main runner is like the trunk of a tree and the multiple wax molds it connects are like the branches of the tree.

3. Making the shell: The surface of the wax model is repeatedly painted, sanded, dried and hardened.

4. Dewaxing: The hardened mold shell is heated to a temperature of 90°C-175°C, so that the wax melts and flows out, leaving a cavity that will be used to cast the desired metal casting.

5. Firing the shell: Heating the dewaxed shell to 800°C-1100°C, which further strengthens the mold, eliminates residual wax and impurities, and removes moisture from the mold material.

6. Pouring: The casting melt is injected when the shell is not cooled, so that the melt has better fluidity in the mold cavity, thereby filling every detail in the mold cavity.

Generally, injecting molten metal into the heated mold cavity will result in higher precision castings, because the mold and metal casting parts will eventually shrink simultaneously during cooling, thereby reducing the impact of shrinkage on dimensional accuracy.

7. After the casting melt is poured into the mold, the starting time of the casting solidification process can be set.

8. Cleaning of castings, removal of mold shells, cutting of pouring risers, removal of oxide scale, etc.

Ⅲ. Characteristics of investment casting

The biggest advantage of investment casting is that investment castings have high dimensional accuracy and surface finish. Therefore, the machining work can be reduced, and only a little machining allowance can be left on the parts with higher requirements. Even some castings only have grinding and polishing allowances, and can be used without machining. It can be seen that the use of investment casting method can save a lot of machine tool equipment and processing time, and greatly save metal raw materials.

Another advantage of the investment casting method is that it can cast complex castings of various alloys, especially high temperature alloy castings. For example, the blade of a jet engine has a streamlined outline and an inner cavity for cooling, which is almost impossible to form by machining processes. The investment casting process can not only achieve mass production and ensure the consistency of castings, but also avoid the stress concentration of residual knife lines after machining.

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