Defect 1: casting shrinkage
The main reasons of precision parts processing are casting shrinkage caused by alloy solidification shrinkage, oxygen and nitrogen absorbed in air when alloy dissolves, and casting shrinkage caused by gas released during alloy solidification.
Solutions:
1) Place the gold ball.
2) Increase the diameter of the sprue or reduce the length of the sprue.
3) Increase the amount of metal.
4) The following methods are used to prevent the pitting of the structure towards the direction of the sprue.
a. A cooling channel is placed at the root of the channel.
b. In order to prevent the molten metal from hitting the cavity vertically, the sprue should be curved.
c. The sprue is placed obliquely.
Defect 2: the surface of casting is rough and not smooth
The chemical reaction between the rough and melted metal on the cavity surface of precision parts mainly reflects the following situations.
1) The particles of the embedding material are coarse and not fine after mixing.
2) After curing, the embedding material is directly put into Maofu furnace for roasting, and the moisture content is too much.
3) If the heating rate of baking is too fast, the expansion difference will be produced in different positions of the cavity, which makes the cavity surface spalling.
4) The highest baking temperature is too high or the baking time is too long, which makes the cavity surface too dry.
5) The melting temperature of the metal or the baking temperature of the casting ring is too high, which makes the metal react with the cavity, and the surface of the casting is burnt with the embedded material.
6) When the molten metal is cast into the mold due to insufficient baking, the embedded material will decompose and more gas will occur, resulting in pitting on the surface of the casting.
7) After the molten metal is cast in, the local temperature in the cavity is too high, and the surface of the casting is rough.
Solutions:
a. Don't over melt the metal.
b. The baking temperature of the mold should not be too high.
c. The calcination temperature of mold should not be too low (the calcination temperature of phosphate embedding material is 800-900 ℃).
d. To avoid the phenomenon that the structure faces the direction of the casting channel.
e. Coating liquid to prevent burning and sticking on wax mold.
Defect 3: Cracking of casting
There are two main reasons for precision parts processing. One is that the metal solidification is too fast, resulting in casting defects (seams); The second is the cracking caused by high temperature.
1) The casting joint caused by the rapid solidification of metal can be solved by controlling the casting time and solidification time. The related factors of casting time: the shape of wax mold. The amount of thickness cast. Casting pressure (casting machine). The permeability of the embedding material. The related factors of setting time: the shape of wax mold. The highest baking temperature of casting ring. The type of embedding material. The type of metal. Casting temperature.
2) The cracking caused by high temperature is related to the mechanical properties of metal and embedded materials. Cracking is easy to occur under the following conditions: high casting temperature is easy to cause cracking; The high strength of the embedded material is easy to crack; The nickel and cobalt alloys with low elongation are easy to crack.
Solutions:
Use low strength embedding material; The casting temperature of metal should be reduced as far as possible; Don't use the one with small ductility. Brittle alloy.
Defect 4: spherical protuberance defect
It is mainly caused by the residual air (bubble) remaining on the surface of the wax after mixing the embedded material【 Dongguan CNC processing plant, good content, worthy of attention]
1) The effect of vacuum embedding is better than that of vacuum embedding.
2) Spray surfactant (such as castmate of Rijin company) on the surface of wax before embedding
3) First, the embedding material is coated on the wax mold.
4) The method of pressure embedding was used to extrude bubbles.
5) When embedding, pay attention to the direction of wax mold, and there should be no depression below the joint between wax mold and casting channel.
6) To prevent the inclusion of bubbles during embedding. Casting ring and casting seat. The buffer paper should be closed; It is necessary to pour the embedding material along the inner wall of the casting ring (using the vibrating machine).
7) Do not vibrate after filling the casting ring.
Defect 5: Flash defect of casting
The main reason is that the casting ring cracks and the molten metal flows into the cracks in the cavity.
Solutions:
1) Change the embedding conditions: use high strength embedding materials. The strength of gypsum is lower than that of phosphate, so it should be used with caution. Try to use ring casting. When casting without ring, the ring is easy to crack, so it needs to be injected【 The content of Dongguan CNC processing plant is good, worthy of attention]
2) Baking conditions: do not bake directly after the embedding material is solidified (it should be roasted after several hours). The temperature should be raised slowly. Cast immediately after baking, do not bake the casting ring repeatedly.