What is Investment Casting Process?
Investment casting (also known as lost wax investment casting process) is a process in which a wax pattern is coated with a refractory ceramic material.
As the refractory material gets hardened, it’s internal geometry takes the shape of the wax pattern.
The wax is then melted out and the molten metal is poured into the mold cavity.
This molten metal solidifies and the outer ceramic material is broken down to take the casting out.
This manufacturing process is also known as Lost Wax Investment Casting.
By using investment casting, parts like dental fixtures, cams, turbine blades, gears, jewelry, as well as other components with complicated shapes can be manufactured.
This was just a simple explanation of investment casting process, now let’s proceed towards the detailed step by step procedure involved in investment casting process (or Lost wax investment casting process).
Investment Casting Process (9 Steps)
The following are the steps involved in investment casting process.
Step 1: Manufacturing a pattern
First of all the wax pattern of desired shape is manufactured.
This pattern may be made from plastic, but generally it is made with wax so that it can be easily melted out and can be reused.
In investment casting process, the pattern is destroyed for removing the casting. So for every casting, a new pattern is needed.
Just like the metal pattern used in shell molding process, the wax pattern is used in investment casting process for mold preparation.
The size of the master die used to prepare the wax pattern is calculated very carefully.
The pattern allowances like shrinkage of wax, shrinkage of ceramic material, shrinkage of metal, etc should be considered for manufacturing the pattern.
Trial and error should be done before finalizing the right dimensions. Hence these molds are expensive.
Since the mold does not need to be opened, various complex shaped castings can also be manufactured by using an investment casting process.
Step 2: Preparing a wax pattern tree
You can see in the diagram below that several wax patterns can be combined for a single casting.
Sometimes many wax patterns are connected to make many castings in a single process.
This can be achieved by attaching the wax pattern to the wax bar, and the wax bar serves as a central sprue.
As shown in the picture, the ceramic pouring cup is attached at the top of the bar.
The complete tree-like arrangement is shown in the picture above.
Step 3: Applying ceramic slurry over a pattern
The entire pattern is dipped in the slurry which is made up of silica, water and binders. (See above image).
Because of this, the ceramic layer is formed on the surface of the pattern.
In order to increase the thickness of ceramic layer on the pattern, it is repeatedly dipped in the slurry.
In some cases, instead of dipping the pattern in slurry, the pattern is kept in a flask and the slurry is poured over it.
Step 4: Drying the slurry on a pattern
Once the desired thickness of slurry is achieved on a pattern, it is allowed to dry in the air so that it becomes hard.
Step 5: Wax melted out of mold
Now in the next step, the hardened ceramic mold is turned upside down and it is heated at around 90 °C to 175 °C.
Because of this, the wax is melted out and the mold cavity is formed inside the ceramic mold.
Step 6: Heating of mold before pouring
This ceramic mold is then heated at temperatures around 550 °C to 1100 °C.
Because of this the residual wax or other contaminants dries out from the mold and this ultimately gives strength to the ceramic mold.
Step 7: Pouring the molten metal
Now, the casting is poured into the mold cavity when the mold is still hot.
By pouring the molten metal in the hot mold, allows the molten metal to flow easily and it also fills up the finer details inside the mold, which gives good dimensional accuracy.
Step 8: Solidifying the molten metal
After this process, the molten metal is allowed to solidify.
Step 9: Breaking of ceramic mold to get final casting
Now, in the final step, the ceramic mold is broken out to get the casting part.
If the casting components are in tree-like arrangement, then they are cut out into separate parts.
Now let’s discuss the advantages and disadvantage (pros and cons) of investment casting process.
Advantages of investment casting process
The advantages of investment casting process are mentioned below.
- By using investment casting process, very thin sections can be casted.
- Higher dimensional accuracy can be achieved using investment casting process.
- Almost every metal can be casted by using investment casting process.
Disadvantages of investment casting process
The disadvantages of investment casting process are mentioned below.
- Investment casting process is expensive process as it requires new mold everytime the new casting is made.
- Investment casting is relatively a complicated process.
- More labor is required for the manufacturing of pattern and mold.
Applications of investment casting process
The applications of investment casting process are mentioned below.
- Investment casting is used in aerospace industries for manufacturing blades or cooling systems which have complex shapes.
- Previously, investment casting was used for manufacturing artifacts, surgical instruments as well as jewellery.
- This process has been used in manufacturing aircraft door frames, aluminum castings (upto 30 kg), and steel castings (upto 300 kg).