A. Causes and countermeasures of casting shrinkage
The main reasons are that the alloy solidifies and shrinks to produce casting shrinkage, and when the alloy dissolves, it absorbs a large amount of oxygen and nitrogen in the air. When the alloy solidifies, gas is released to cause casting shrinkage.
The solution:
- Place the gold storage ball.
- Thicken the diameter of the sprue or shorten the length of the sprue.
- Increase the amount of metal.
- Use the following methods to prevent the structure from denting in the direction of the sprue. a. Place a cooling channel at the root of the sprue. b. To prevent the molten metal from hitting the cavity vertically, the sprue should be curved. c. Oblique Place the sprue.
B. Reasons for the rough surface of the casting
The rough and molten metal on the surface of the cavity has a chemical reaction with the surface of the cavity, which mainly reflects the following conditions.
- The particles of the embedding material are coarse and not fine after stirring.
- After the embedding material is cured, it is directly put into the Maofu furnace for roasting, and the moisture is too much.
- The heating rate of the co-firing is too fast, and the different positions in the cavity will have an expansion difference, causing the inner surface of the cavity to peel off.
- The maximum temperature of roasting is too high or the roasting time is too long, which makes the inner surface of the cavity too dry, etc.
- The melting temperature of the metal or the roasting temperature of the casting ring is too high, which causes the metal to react with the cavity, and the surface of the casting burns the embedding material.
- Insufficient roasting of the casting mold, when the molten metal is cast, it will cause the decomposition of the embedding material, generate more gas, and produce pitting on the surface of the casting.
- After the molten metal is cast, the local temperature in the cavity is too high, and the surface of the casting is partially rough.
The solution:
a. Do not over-melt the metal.
b. The baking temperature of the mold should not be too high.
c. The baking temperature of the mold should not be too low (the baking temperature of the phosphate embedding material is 800-900 degrees)
d. Avoid the phenomenon that the organization faces in the direction of the sprue and the phenomenon of depression.
e. Apply a liquid to prevent sticking on the wax-up.
C. Reasons for cracks in castings
There are two main reasons. One is that the metal at the place solidifies too fast, resulting in casting defects (joints); the other is cracks caused by high temperature.
- For the metal solidification too fast, the resulting casting joint can be solved by controlling the casting time and solidification time. Factors related to the casting time: the shape of the wax pattern. The number of thicknesses cast. Casting pressure (casting machine). Air permeability of the embedding material. Factors related to setting time: Wax shape. The maximum firing temperature of the casting ring. Type of embedding material. Type of metal. Casting temperature.
- Cracks caused by high temperatures are related to the mechanical properties of metals and embedding materials. Cracks are likely to occur in the following situations: cracks are likely to occur due to high casting temperature; embedding materials with high strength are prone to cracks; elongation is small The nickel solder alloy and cobalt solder alloy is prone to cracking.
The solution:
Use low-strength embedding materials; try to reduce the casting temperature of the metal; do not use less ductile and brittle alloys.
D. Causes and countermeasures of spherical protrusions
It is mainly caused by the residual air (bubbles) remaining on the surface of the wax-up after the mixing of the embedding material.
The solution:
- For vacuum conditioning and embedding materials, the effect of vacuum embedding is better.
- Before embedding, spray surfactant on the surface of the wax type (such as the castmate of Rijin Company)
- First coat the embedding material on the wax-up.
- Use the method of pressure embedding to squeeze out the air bubbles.
- Pay attention to the direction of the wax pattern when embedding, and there should be no depression below the connection between the wax pattern and the sprue.
- Prevent air bubbles from being mixed in during embedding. The casting ring, the casting seat, and the buffer paper should be tightly sealed; the embedding material should be poured along the inner wall of the casting ring (using a shaker).
- Do not vibrate after filling the casting ring.
E. How is the flash of the casting formed?
Mainly due to the cracking of the casting ring, the molten metal flows into the cracks of the cavity.
The solution:
- Change the embedding conditions: use the embedding material with higher strength. The strength of the gypsum type embedding material is lower than that of the phosphate type embedding material, so it should be used with caution. Use ring casting as much as possible. The ring is prone to cracks, so be careful.
- Roasting conditions: Do not roast directly after the embedding material is solidified (should be roasted after a few hours). It should be heated slowly. Cast immediately after roasting, and do not repeatedly roast the cast ring.
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