11 Steps of The Investment Casting Process

A fire-resistant ceramic material is placed over the surface of the wax pattern to form a shell, and once it hardens, the inside of the shell forms the shape of the desired casting. The wax is then thawed and drained out from the shell, and the cavity is going to be filled with molten metal. After the metal solidifies in the shell, it forms the castings. So this investment casting technique is also known as lost-wax casting.

And the definition of the Investment Casting process has been indicated on Wikipedia. We can check this link to review it.

Investment casting has a history of more than 5,550 years, dating back to ancient times in Egypt and China. In the industrial production field, parts made by this method include teething, gears, cams, ratchets, jewels, turbine blades, mechanical parts, and other parts with complex shapes. Click here to have a look at the products Sinoway has manufactured for our clients with investment casting(lost-wax casting).

Now let’s go through each step one by one.

The first step of the investment casting process is the preparation of wax patterns. Wax is easily melted and reusable, and regardless of how many cast parts a given number of wax patterns must be made. The molds used to make wax patterns are usually machined from metal. These molds are generally expensive because of their high accuracy, and the shrinkage of the metal casting and wax and the ceramic material on the surface of the wax mold must be carefully accounted for.

After the mold is completed, we can start preparing for the next step: wax injection. Before injecting the wax, we need to apply a release agent on the inner cavity surface of the mold so that the wax patterns can come out of the mold smoothly. Then we have to assemble the molds and put them on the wax injection equipment for the injection process. After injection, we can open the molds, take out the wax patterns, and put them into cold water for complete cooling.

Generally, once the wax pattern is injected, we need to adhere multiple patterns to a single wax rod in the middle. The wax rod acts as a central sprue, connecting with the casting gate, which is like a tree, and the wax patterns attached to it are like the branches. Therefore, this structure is referred to as a wax tree or a wax cluster. This way, all wax patterns are connected to the main sprue, and multiple castings can be made in a single pour.

The materials used to make the shells consist mainly of refractory materials and binders. At present, the refractory materials used in investment casting are primarily quartz, corundum, and aluminosilicate refractory materials. The most common binder is colloidal silicate solution (silicate sol), such as water glass and silica sol. Water glass shells are easier to deform and crack. And the dimensional accuracy and surface roughness of the castings are relatively poor compared with the silica sol shells. The stability of silica sol is better, it can stay for a long time, and no special hardener is needed for shell making. But the wetting ability of silica sol on the wax patterns is not good, and the hardening of the shells is a drying process that takes longer.

To improve the ability of the coating to wet the wax, it is necessary to remove the oil from the wax surface in advance. Moreover, the coating should be mixed well before applying it to the wax tree. And the viscosity and gravity of the coating should be optimized so that the coating can fill and wet the wax pattern well. While applying the coating, dip the wax tree into the coating and shake it up and down so that the coating can wet the wax patterns well and cover the wax surface evenly. After the wax tree is coated with refractory paint, it is sprinkled with refractory powder material, then dried and hardened, repeated many times until the refractory coating reaches the required thickness to form a multi-layer shell on the wax cluster.

After coating each layer of the shell, we have to dry and harden it so that the binder in the coating transforms from sol to jelly and gel, which joins the refractory materials together and finally forms the shell to satisfy the casting requirements.

After the shell is completely hardened, the wax patterns need to be melted away from the shell, which is called dewaxing. Dewaxing is mainly achieved by either hot water or steam with the same pressure, depending on the heating method.

If a sand filling is needed, before baking the shell, it will be buried in the sand in the box and then put into the furnace for baking.

If the shell has high-temperature resistance and does not need sand filling, the dewaxed shell can be sent directly into the furnace for baking. During baking, the furnace temperature is gradually increased, and the shell is heated to 550-1000℃ and kept in the heat for a period of time before casting.

After the shell is taken out of the baking furnace, it is immediately poured at a high temperature. The molten metal cools slowly in the shell over time, and it can fill the cavities with high fluidity. Therefore, the casting can replicate the cavity’s shape well, improving the casting’s accuracy.

After casting, we need to remove the shell to obtain the castings. Factories usually use a vibratory process to remove the shell and blast to clear the refractory material adhering to the castings.

Once the casting cluster is cleaned up, we need to cut each casting off. The casting ingates and risers are ground off with the abrasive belt grinding machine to form the individual castings.

We should check the workpiece for casting defects when the castings are ground. Some defects are unacceptable and cannot be repaired. And these castings can be returned to the furnace as scrap for the next casting. Other casting defects are acceptable after being fixed, such as welding.

After the above 11 steps, the castings are basically formed. Depending on different materials, some castings need to be heat treated. And for the castings with higher precision requirements, post-machining is required. We will discuss this in the next article.