Investment casting process is to make a model with wax, wrap a layer of clay and other refractories on its surface, heat to make wax melt and flow out, so as to obtain the empty shell formed by refractories, and then melt the metal and pour it into the empty shell, after the metal is cooled, the refractories are crushed to obtain the metal mold. This process of metal processing is called precision casting, also known as investment casting or wax loss casting.
Advantages and disadvantages of investment casting process
Investment casting is also called investment casting. Compared with other casting methods and parts forming methods, investment casting has the following advantages:
1. The casting has high dimensional accuracy and fine surface roughness. The dimensional accuracy of the casting can reach 4-6 levels, and the surface roughness can reach 0.4-3.2 μ m, which can greatly reduce the machining allowance of the casting, realize the manufacturing without allowance and reduce the production cost.
2. For castings with complex shape and difficult to be processed by other methods, the contour size of the castings is as small as several millimeters and as large as thousands of millimeters, the thinnest wall thickness is 0.5mm, and the smallest hole diameter is less than 1.0mm.
3. Alloy materials are not limited: such as carbon steel, stainless steel, alloy steel, copper alloy, aluminum alloy, high temperature alloy, titanium alloy and precious metal and other materials can be produced by precision casting, especially for alloy materials that are difficult to forge, weld and cut.
4. High production flexibility and adaptability. It can be used not only in mass production, but also in small batch or even single piece production.
To sum up, precision casting has the advantages of small investment scale, large production capacity, low production cost, simple process of complex products and quick investment effect. Thus, it is in a favorable position in the competition with other processes and production methods.
But in the process of wax mold making, the following defects are easy to appear
1. Wax mould is deformed. After the wax mould is taken out of the mould, in addition to the reduction of the size, sometimes it will be artificially deformed due to the incorrect technique when it is taken out. As the wax mould is common to be bent and deformed when it is cooled, the wax mould just taken out of the mould should be placed carefully, usually with a large plane as the datum plane, or the wax material may be too soft, Unreasonable pressure design and other factors.
2. The main reason for wax mold filling dissatisfaction is that the temperature of wax material is too low, the injection speed is slow, and the molding temperature is low, which causes wax material to cool quickly in the flow process, which is shown in the part of corner and edge or the thin-wall part of wax mold filling dissatisfaction, and the corner appears at the corner, which is very similar to the lack of pouring of metal castings.
3. There are wrinkles on the surface of wax mold. Because the temperature of wax material is too low and the speed of injection is too low, the movement of wax material and the pressing temperature are not matched properly; or because the inner surface of pressing mold is damaged or unclean; the place of chill metal block is not proper, etc., the movement traces are left on the surface of wax mold. There are deep lines, similar to the cold insulation defects of metal castings; there are also joints around the core and holes, which are slightly sunken. In fact, the joint of two streams of wax is not well fused, which is the result of insufficient temperature and pressure of wax.
5. The surface depression of wax mold is mainly caused by insufficient injection pressure and time, high wax temperature and insufficient cooling time, sometimes too much release agent, large area of surface depression, difficult to repair and easy to cause waste products.
6. The wax mould has a draping seam, which is the most common defect, that is, there are very thin wax flakes escaping at the joint of the pressing mould, the joint of the pressing mould combination block, the joint of the core and the core seat, etc. The main reasons are that the precision of the die is not enough, the parting surface or the joint surface of the internal part of the die is damaged or attached with unclean objects, or the force of the die is not enough, and the injection pressure is too high. Or the temperature of wax is too high. The hairy wings must be completely removed before the wax mold can be used.
7. The adhesion between wax mold and pressing mold is caused by not using release agent, too high temperature of wax and pressing mold, or the inner surface of pressing mold is not clean enough.
8. The surface of wax mold is rough, because the injection pressure is too low, or the contact density between wax material and the inner surface of the mold is not enough, serious wrinkles will occur.
9. There are bubbles in the wax mold, one is the surface bubbles visible to the naked eye, the other is the internal bubbles of the wax mold, which are usually large and cannot be seen by the naked eye, but they can be found through the local bulge of the wax mold, which is caused by the expansion of the gas in the wax mold.
Key points of investment casting process
1. Do not spray too much parting agent into the mold cavity.
2. Do not change the circulation parameters of pressing and melting (wax) mould.
3. The wax molds shall be placed in the storage tray and isolated from each other to avoid damage. Clamps can be used when necessary to avoid deformation of wax mould.
4. Pay attention not to damage the profile during the correction.
Application scope of investment casting process
Precision casting is used in almost all industrial sectors, especially in electronics, petroleum, chemical industry, energy, transportation, light industry, textile, pharmaceutical, medical equipment, pumps and valves.
