Investment Moulding: The Reliable Casting Method for Precise Metal Components

What Is Investment Moulding?

Investment moulding, also known as investment casting or lost wax casting, is a process that is used for making metal components, parts of a machine. Investment moulding is one of the casting methods that is relatively cost-efficient and versatile, but more time-consuming than others. The moulding or casting of metals should follow strict requirements. For example, the moulding process should be heat resistant as the products are made with molten metals.

In addition, the precision of the casts that are made by these processes is emphasized as well. It may cost a lot and take much effort when various machining works are required. However, with investment moulding, the process of making casts can be free from excessive machining works, except for the surface smoothing in the last step.
 

How Does Investment Moulding Work?

The processes of investment moulding are completed with the forming of the wax molds and the casting of the metals. In other words, before casting the metals, the wax molds should be made first, then the metals are casted in the wax molds afterwards.

For the forming of wax molds, it can be divided into four steps, including wax pattern casting, wax patterns assembling, shell configuration, and wax removal. As for the casting of the metals, it can be divided into metal casting, cutting, and customization.
 

Wax Pattern Casting

The first step of wax formation is to pour the wax into a metal injection die, which is usually the aluminum injection die. The cooled wax patterns should be larger than the final metal products since the metals would expand when they are heated and shrink after they are cooled off.
 

Wax Patterns Assembling

Once the wax patterns are cooled off, they are dipped into a hot iron, in order to allow the wax patterns to attach to each other, and create the so-called metal delivery system. The metal delivery system provides cavities for the molten metals to be poured later, so this is one of the important steps in the precision casting of metals, which is the key to reduce the requirements of the machining of the final products.
 

Shell Configuration

After that, the wax pattern assembly, or the metal delivery system, would be dipped into a ceramic solution that is called the slurry, and this step is performed manually so as to prevent the imperfection of the configuration by the machine. Then the assembly dipped with slurry would be strengthened by costing with the sand stucco, and leaving it to dry off. The dipping process in the slurry and the coating process with the sand stucco would be repeated until the desired thickness of the wax mold is achieved, which is about three tenths of an inch thick and should take about five days.
 

Wax Removal

After the thickened and dried wax mold is formed, it would be placed into the autoclave, which is a hot steam chamber, for five to ten minutes, in order to melt the wax out of the shell or the ceramic mold. Once the wax is removed and the shell is completely dried off, the processes of forming the wax molds are finished.
 

Metal Casting

Before the molten metal is poured into the shell, the shell would be heated into an oven for about two to three hours, so as to strengthen the shell and stabilize the properties of the shell for reducing the interaction between the molten metal and the shell later.

Once the metal becomes molten and reaches the temperature of 2200 degrees Fahrenheit, the molten metal would be poured into the chamber of the shell, then leave it to cool and harden. For the aluminum, it takes about two hours to cool, while the steel takes about four to five hours. After that, the ceramic shell would break off and the final metal part remained for the next step, and this process takes about five to ten minutes.
 

Cutting and Customization

Finally, the metal delivery system would be cut and grind for smoothing the finishes of the final products, then these products can be sold.
 

Investment Moulding vs. Sand Casting

Investment moulding or investment casting is one of the counterparts to sand casting, which is another casting method but replaces the wax into the sand. While there are some variations between them, including the process intricacy, the required molds, and the machining requirement.
 

Process Intricacy

Comparing the investment moulding and the sand casting, the processes of the former are more intricate than the latter, as the former is dedicated to making the integration of the mold, in order to accommodate the required final products. While the products that are made of sand casting are not as versatile as that of investment moulding, so the processes that sand casting requires are relatively fewer and simpler than investment moulding.
 

Required Molds

As the above paragraph has mentioned, the investment moulding takes more processes to make the mold as well as the final product, as the one and only mold for the final product can cover the entire configuration of the final product. In contrast, the formation of the mold in sand casting is simpler and takes fewer steps. To make the required configuration, it required two half molds to make one product in sand casting, which is more than the investment moulding.
 

Machining Work Requirement

Since the intricacy of the investment moulding is relatively higher, the integration and perfection of the mold are greater, the machining works are less required accordingly, whereas the molds that the sand casting makes are rougher. Therefore, more machining works are required later in order to achieve the geometries of the customized final products.
 

Investment Moulding vs. Die-casting

There is another method in the making of molds or dies, which is called die-casting. It is a method that transfers the electric energy into thermal energy and creates sparks from the electrodes in order to make the cavities of the molds or dies. The die-casting processes are performed by a machine called the EDM machine, and the electrodes that send the electric energy as the form of heat are made of gray cast iron, which results in the requirement of electronically conductive workpieces. This leads to the limitation of the workpieces that the EDM machine can be applied for. In contrast, the versatility of the ceramic molds that are produced by the investment moulding is much greater than the EDM machine.

However, when it comes to the efficiency of the works, the die-casting surpasses the investment moulding, as it is free from the intricate processes that the investment moulding requires. Therefore, owing to the versatility of the casting methods and the efficiency of the works, making a choice between these two may depend on the manufacturers’ personal preferences and considerations.