Lost Wax Casting

1. Introduction
Lost wax casting is to use medium temperature wax to make the wax mold of the required parts, then wrap the wax mold with refractory material, dry, harden, lose wax and then fire into a ceramic mold shell, and pour the molten metal into the mold shell to cool the product. Casting method.
It is not only suitable for precision casting of various types and alloys, but also produces castings with higher dimensional accuracy and surface quality than other precision casting methods, and even other precision casting methods are difficult to cast, high temperature resistant, and difficult to process. All castings can be made by investment precision casting.
The so-called investment casting process is simply to use fusible materials (such as wax or plastic) to make a fusible model (referred to as investment or model), coat it with several layers of special refractory coatings, and dry and After being hardened to form an integral shell, use steam or hot water to melt the model from the shell, then place the shell in the sand box, fill with dry sand around it, and finally put the mold in the roasting furnace. High-temperature firing (if a high-strength mold is used, the mold shell after demolding can be directly fired without modeling). After the mold or mold is fired, molten metal is poured into it to obtain a casting.
The dimensional accuracy of investment castings is relatively high, generally up to CT4-6 (CT10~13 for sand precision casting and CT5~7 for die casting). Of course, due to the complex process of investment casting, there are many factors that affect the dimensional accuracy of castings. For example, the shrinkage of the mold material, the deformation of the investment mold, the linear change of the shell during heating and cooling, the shrinkage of the alloy, and the deformation of the casting during the solidification process, so the dimensional accuracy of ordinary investment castings is relatively high, However, its consistency still needs to be improved (the size consistency of castings with medium and high temperature waxes should be improved a lot).
When pressing the investment mold, a pressure type with a high surface finish of the cavity is used, so the surface finish of the investment mold is also relatively high. In addition, the shell is made of a refractory coating made of a high-temperature-resistant special adhesive and refractory material and is coated on the investment mold, and the inner surface of the cavity directly in contact with the molten metal has a high smoothness. Therefore, the surface finish of investment castings is higher than that of general precision castings, generally up to Ra.1.6~3.2μm.

Process Flow

1. Model making. An artist or mold maker uses wax, clay, or other materials to create the original model. Wax and oil-based clays are more commonly used because these materials are relatively soft.
2. Reverse mold. Use the original model to mold. Generally, there is a rigid outer mold plus a softer inner mold, and the inner mold is the exact negative shape of the original model. The inner mold is usually made of latex, urethane rubber or silicone and supported by the outer mold. The outer mold can be made of plaster, but it can also be made of fiberglass or other materials. Most molds are made of at least two pieces, and during construction, some spacers are placed between the parts so that the mold can be accurately put back together. If there are long and thin parts that extend out, they are often cut from the original model and molded separately. Sometimes many molds are needed to reconstruct the original model, especially for large models.
3. Cast wax. Once the mold is complete, pour the molten wax into it and shake it to form a uniform coating on the inner surface, usually about 1/8 inch (3 mm) thick. Another method is to fill the entire mold with molten wax and let it cool until the desired thickness is set on the surface of the mold. Then pour out the rest of the wax again, invert the mold to cool and harden the wax layer. Using this method is more difficult to control the overall thickness of the wax layer.
4. Remove wax. Remove the hollow wax version of the original model from the mold. The mold can be reused to make multiple copies, which is only limited by the durability of the mold.
5. Polishing. Polish each hollow wax replica and wipe the parting line with a heated metal tool. The surface of the wax model should not have any defects, it looks like the finished product. The individually molded wax molds can now be heated and combined together.
6. Open the sprue. The wax replica is made into a wax tree with a tree-like structure, which will eventually provide a path for the molten casting material to flow and allow air to escape. There is usually a wax “cup” from the top, and the wax mold is connected to various points by wax pillars.
7. Sizing. The wax tree is immersed in silica slurry, and then immersed in sandy plaster, or crystalline silica with a certain grain size. The combination of slurry and gravel is called a ceramic mold. Dry and repeat the process until at least half an inch of coating covers the entire part. The larger the object, the thicker the shell needs to be.
8. Burn out. The ceramic shell coating is placed in the kiln, and the heat hardens the silica coating into a shell, melting the wax and expelling it. The melted wax can be recycled and reused, but it is generally simply burned. Now, the shape of the original object is the inside of the hardened ceramic shell, which is the negative shape previously occupied by wax. The feed tube is now also hollow.
9. Test. Let the ceramic shell cool, and then test to see if water will flow freely through the feeder and exhaust pipe. Cracks or leaks can be repaired with thick refractory paste. To test the thickness, holes can be punched in the shell and then patched.
10. Pouring. Reheat the shell in the kiln to reinforce the patch and remove all moisture, and then put the mouth up into a bucket full of sand. The metal is melted in the crucible in the furnace and then carefully poured into the shell. The enclosure must be very hot, otherwise the temperature difference will destroy it. The filled shell is then cooled.
11. Demoulding. The shell is knocked off, removing rough castings. The metal gate is also cut off and recycled for the next pouring.
12. Metal polishing. Finally, the metal castings are ground and polished


The biggest advantage of investment casting is that because investment castings have high dimensional accuracy and surface finish, it can reduce machining work, just leave a little machining allowance on the parts with higher requirements, even some castings Only leave the grinding and polishing allowance, it can be used without machining. It can be seen that the use of investment casting method can save a lot of machine tool equipment and processing man-hours, and greatly save metal raw materials.
Another advantage of the investment casting method is that it can precision cast complex castings of various alloys, especially high-temperature alloy castings. For example, the blade of a jet engine, its streamlined outline and cooling cavity, can hardly be formed by machining technology. Production with investment precision casting technology can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.


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