Creating Molds for Investment Casting
The investment casting process produces parts with fine features, thin walls and excellent surface finish. This cuts down on machining operations, and reduces the amount of metal needed, lowering part mass and saving time and cost.
However, these advantages are countered by the complexity of making the molds. This process has more steps than mold-making in other casting methods. The lead time to get sample parts can be longer and tooling costs may be higher. For engineers and buyers trying to determine if investment casting is the right process for their parts and understanding mold-making in investment casting will help.
The “Lost Wax” Process
The principle behind investment casting is that a pattern of the part to be cast is produced in wax. This is coated in a hard shell and the wax is melted out to leave a cavity for the molten metal. This means, unlike sand casting where the pattern is reused many times, a new pattern is made for each part to be cast. It’s also the origin of the expression “lost wax casting”.
The steps in the mold-making process are:
- Mold cores
- Produce wax pattern
- Assemble wax patterns to a sprue
- Coat the tree and melt out wax
Here’s a closer look at each step.
Cores are used to create empty spaces inside the cast part. These are molded from may be soluble wax or ceramic material and placed into the mold for the wax pattern so the wax forms around them. Soluble wax cores are dissolved out of the wax pattern while ceramic cores remain in place for metal pouring.
Produce Wax Pattern
This is a replica of the part to be cast, only made in wax and with dimensions adjusted to allow for shrinkage of both wax and the molten metal. Various types of wax are used, depending on the properties required.
Wax patterns are produced in aluminum molds by an injection molding process. As the molds don’t need high strength or durability they are less time-consuming to manufacture than those used for plastic injection molding.
Assemble Wax Patterns to a Sprue
The cavity formed by melting out the wax needs a channel to guide the metal in. This is created by attaching a wax sprue to the pattern.
To maximize productivity when patterns are smaller, (measuring in inches and weighing less than a few pounds,) multiple patterns are attached to a central sprue in an arrangement that looks like a tree.
Coat the Tree and Melt Out Wax
Once assembled, the wax tree is coated with a ceramic slurry. This may be done by either dipping or spraying but in both methods the coating is built up as a series of layers. (Applying a coating this way was once called “investing”, which is how this casting process gets its name.)
The desired coating thickness is a compromise between being thick enough to take the weight of the metal, but not so thick that the metal cools excessively slowly. The first layer of slurry determines the surface finish and level of detail reproduced on the surface of the cast part. It is usually a low viscosity mix composed of particles measuring less than 0.0008” (0.020mm).
Once the required layers have been applied, the coating is dried to form a hard shell. This is placed upside down in an oven and the wax melted out. At this point the resulting ceramic shell is ready for preheating and metal pouring.
After the metal has solidified the shell is broken apart to release the parts inside.
Investment Cast When the Savings Justify Mold Production Time and Cost
Investment casting minimizes the amount of metal needed to produce each piece and reduces final machining requirements. This makes it especially attractive for casting expensive and hard-to-machine metals. Stainless steel and superalloys are excellent applications.
With less costly or difficult metals the complexity of mold-making may still be justified by the savings in material and machining, but this is design-dependent. Contact us to discuss your applications with an investment casting specialist.