Stainless steel is a low carbon steel that must have a minimum of 10% chromium. The addition of chromium to the steel causes the formation of a chromium-oxide film which gives stainless steel its unique properties such as its resistance to corrosion and its ability to heal itself when in the presence of oxygen.
For its good rust resistance and corrosion resistance, stainless steel has become an ideal choice to produce components with different shapes in investment casting method. Stainless steel investment casting, also knowns as stainless steel lost wax casting, is a metal forming method to produce stainless steel parts with complex structures, accurate dimensions and good surface finish. For centuries, stainless steel castings has been widely used in architecture, engineering, food machinery, medical machinery, marine industry, etc.
CFS Foundry, the top leading stainless steel investment casting manufacturer and exporter in China, aims to customize high quality stainless steel castings with reasonable prices. To help our customers to better understand the stainless steel investment casting process, here we would like to introduce the steps in detail.
Step 1: Creating a Mold
In ancient times, wax patterns were made from beeswax, often by hand and just one at a time. Today, stainless steel foundries creat a metal mold to inject wax patterns directly, which improves the production effeciency greatly. Normally, the mold is precise machined by CNC, whose cavity is the same as the desired shape. The material of mold selected is aluminum, and a pair of mold can creat wax patterns repeatedly. This mold is a permanent fixture for each part, and is only replaced if it is damaged or if the design changes.
Step 2. Wax Patters Forming & Assembly
Wax patterns, formed by injecting wax material into aluminum mold, has the same dimensions with final stainless steel casting blanks. The wax patterns forming process is in CFS Foundry, which can help to avoid human damage and improve production efficiency. When wax patterns are ready, they have to be districtly inspected and repaired if there is any defects. Bad wax patterns have to be rejected, and can not be used to continue stainless steel casting production.
After producing of wax patterns, they are affixed to wax bars to form the tree, sometimes referred to as a “cluster.” The patterns are attached where the gates are located, which is where the metal will enter the individual casting. The tree must be of adequate size to provide enough feed metal to help provide a sound casting during the metal-solidification process.
Step 3. Shell Building
The assembled trees are then consecutively dipped in refractory slurries (liquid mixtures of heat-resistant materials) and coated with sand to produce a ceramic shell around the wax.
The first investment coat is critical to produce a superior surface finish, one of the advantages of stainless steel investment casting process. The first coat of slurry is applied by submerging the wax tree into a well-mixed vat of zircon-based slurry, with a consistency similar to latex paint. The assembly is then covered with zircon sand, then left to dry. After drying, the assembly is dipped in additional slurries and coated with increasingly coarse fused-silica sands, which are utilized to build mold strength. Then a shell outside the wax assembly is built.
Step 4. Dewaxing
In order to pour metal into the mold, the wax inside the shell has to be removed. In this step, the shell are placed into an autoclave with the wax-exposed end facing downwards. The autoclave is closed, and steam is injected into the autoclave in a matter of seconds which pressurizes the vessel and exposes the molds to temperatures exceeding 300F. The steam heats the wax, and the pressure of the steam is required to offset the pressure of the heating, expanding wax inside of the molds.
As the wax heats, it melts rapidly and drains quickly out of the open end of the mold. Nearly 100% of the wax is captured and recycled for use again in the wax-injection process. After a short time, the molds are removed from the autoclave, and are allowed to cool and dry.
Step 5. Pouring
After dewaxing, the shell is hollow and ready for pouring. Before pouring, the stainless steel material has to be melted into liquid. In this step, it is critical to test the stainless steel to ensure the compositions are within its range. Then, under the workforce of gravity, pouring stainless steel liquid into the shell until it fills the cavity completely. Under room temperature, the metal inside the molds begins to solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mold, and the alloy used. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.
Step 6. Casting Cleaning
When the poured stainless steel get solidified, it is time to remove the shell and pouring gates to achieve net shape castings. The ceramic shell can be shaken away through vibration and blasting. Individual castings are cut away from the metal tree, and the gates are removed through grinding machine. The castings are then shot or sand blasted to remove surface mill scales or impurities, whose surface will be clean and smooth.
Step 7. Secondary Operations
When the final stainless steel castings are achieved, to meet the requirement of the finished drawing, the part may have to be heat treated, machined or polished. CFS Foundry can provide different types of secondary operations for our customers to provide finished stainless steel parts.
Investment casting with stainless steel is an excellent option for many applications. Its corrosion-resistant properties enable it to stand up to any severe environments. It is also easily cleanable, making it ideal for uses where sterile conditions are necessary. CFS Foundry can cast and machine different types of stainless steel alloys, including 304(L), 316(L), 17-4 ph, 2205, etc. Welcome your inquiry on all the custom stainless steel casting projects!