Investment casting is a metal casting process that pours molten metal into a ceramic mold that is the shape of the desired product. This type of metal casting process is used to make a wide variety of products ranging from jewelry to airplane components. There are different procedures involved with this process depending on which type of medal is being used and what it is being used for.
Alloys
Aside from jewelry the most common metals used in the investment casting process are steel and aluminum. There are also some nickel and cobalt steel alloys that are vacuum cast and desirable for their high temperature properties. They are used for the turbine components on jet engines. Aluminum alloys are used primarily on aircraft because they are lightweight. Aluminum is poured at around 1300º F and can be mixed with small amounts of other metals to enhance the flow of the metal during pouring. Steel is also a very popular metal used in many applications. The 15-5 and 17-4 alloy steels are common among investment casting foundries. These alloys pour at temperatures close to 3000º F.
Molds
The wax mold starts out with a drawing or a CAD file of the part to be made along with specifications. It is looked at by a group of manufacturing engineers, quality assurance personnel, operations manager, and others depending on the size and importance of the job. All of the standards are looked at and planning is initiated at this stage. The mold is made out of steel or aluminum in a CNC shop using the CAD file. The mold is designed to receive an injection of molten wax, which is cooled and then disassembled to leave the wax part left in pristine condition with consistent accurate measurements. It also has a shrinkage rate applied to it depending on the metal and expertise of the engineer. The pattern is then attached to a gating system that is designed to produce the highest yield for the foundry. If the parts are small then it is typical to see several parts on one gating system. The wax is then dipped in liquid slurry and zirconium or other type of sand and dried repeatedly over a few days.
Process
After the sand shell is dried it is tipped upside down and placed in an autoclave, which melts out the wax without damaging the shell. The mold is sent to the foundry where it gets prepped and placed in the oven to pre-heat the mold sometimes at over 1500º F. After the mold is filled with metal it is left for hours to cool before the sand mold is blasted off with high-pressure water. The gates are cut off and the parts get tested for internal flaws such as cracks or shrinkage cavities. X-rays, fluorescent liquid penetrant exam, and tensile strength tests are required for certifications based on the grade of the metal. If the parts are good they are checked dimensionally and reworked until they conform.
Investment castings can be a good choice of manufacturing for many reasons, but there are also some things to watch out for. The process of making molds and getting good castings is not quick. Typical development time is over 2 months and could stretch into years if the part is complex, high quality, and with tight dimensional tolerances. This is one of the most efficient processes for medium complex parts after development is complete so it is often worth the wait.
