Investment casting: the best way to cast stainless steel

Stainless steel - we see it everywhere now, from our glasses to our smartphones to complex machinery. Stainless steel is everywhere. The reason is its corrosion resistance.

The investment casting process, also known as precision casting or lost-wax casting, involves the forming of a ceramic mold from a wax pattern. Wax patterns are made in the exact shape of the item to be cast. Refractory ceramic is used to coat this pattern. Once the hardened ceramic material has been turned upside down, the wax melts and drains out. Hardened ceramic shells become disposable investment molds. The molten metal is poured into the mold and allowed to cool. The metal casting is then removed from the spent mold. The process of "investing" (enclosing) a pattern with refractory materials is called investment casting. The advantages of investment casting over other molding methods include fine details and excellent surface finishes as-cast. Castings with thin walls and complex internal passageways are also possible. Investment casting does not require a draft, as does sand casting. The process quality can produce net shape or near net shape castings, resulting in significant material, labor, and machining cost savings for the customer. 

What type of stainless steel is best for casting?

Stainless steel has its own classification and grading. Stainless steel contains iron, carbon, chromium, and other materials.

Chromium is the primary element that makes stainless steel corrosion resistant and is contained in no more than 10.5% of each unit. Depending on the availability and percentage of other materials, stainless steel can be classified into different grades.

For casting purposes, we would say that grades 304 and 316 stainless steels are the best. They are both above the 300 level and both contain nickel.

Therefore, make sure the stainless steel foundry you hire has 304 and 316 stainless steel available.

Stainless Steel Investment Casting Procedures

1. Prepare the pattern with wax

At the beginning, you will need to prepare a replica of the final product in wax. You can use other materials, such as foam or plastic. But wax seems to be the best choice because it melts easily, which will pay off in the later casting stages.

2. Make a tree with wax patterns

If you are casting a part multiple times, then you need to make the shape out of wax (or whatever you chose in the previous step). Then put all these wax parts together to form a tree structure that can be dipped together.

3. Dip the wax tree

Now you need to dip the tree structure into a substance to create the mold. This substance can be a ceramic paste or a white industrial latex, also known as a silicon dissolving paste.

The reason why you need to use these materials to prepare the mold is that the mold needs to be removed later to remove the steel. In addition, these materials can be reused, thus reducing the cost slightly.

When dipping the wax structure in the mold, you may need to dip it several times. This will ensure that there is no leakage and that the mold will be thick. The thicker the mold, the stronger it will be to handle hot molten stainless steel.

4. Melt the wax

After dipping the shape into the substance several times and reaching the desired thickness, place the mold in the oven to heat it up.

Heating the mold will accomplish two things. First, it will melt the wax or the plastic inside, making the mold hollow. Second, the structure of the mold will become stronger and less fragile.

If you have used wax, the temperature of this oven should be between 900C and 1800C. This is enough to melt all the molds. The melted wax will flow through the sprues. The sprue is also the gate into which you need to pour the melted stainless steel.

5. Prepare the stainless steel

As mentioned earlier, you should choose stainless steel 304 or 316 for casting. It is best to pour into the mold while the mold itself is hot. To do this, you should start heating the steel and preparing the mold at the same time.

6. Pour the heated alloy

When the mold is ready and the steel is melted, pour the steel into the mold. But before that, heat the mold to 550 ⁰C - 1100 ⁰C. Pour the melted alloy when the mold is close to that temperature.

Keep the mold heated for two reasons. One, the molten alloy will be able to flow smoothly through the mold. Secondly, the smooth flow will allow the alloy to fit into thin spaces, which will ensure a perfect dimensional fit.

7. Solidification and mold breaking

After pouring the steel, now allow the steel to cool and solidify. It does not take much time.

Once fully solidified, break the mold. Stainless steel foundries have different ways of breaking the mold. Do not stress this part. Besides, the steel inside will be stronger than the old one. So, hitting it with a hammer will not cause too much damage.

8. Cut the component from the tree

Now, all you need to do is to cut the component from the tree. You may need to polish the assembly before using it. But the casting process is complete.

Investment casting is the best way to cast stainless steel

The investment casting method is one of the easiest ways to cast stainless steel. You can use this process to create any type of shape, but it is best to keep the mass below 75 lbs. Otherwise, the mold may be damaged. The investment casting method can produce complex parts with excellent surface finishes. Because the ceramic shell detaches from the part as it cools, the investment casting does not require a taper to remove the part from the mold. With this production feature, castings can be designed with 90 degree angles, with no shrinkage allowance and no additional machining required. Investment casting produces parts with excellent dimensional accuracy; mesh parts can often be produced, and the finished shape usually requires no secondary machining.