Five common defects of casting molds and their solutions

2021-03-23 15:20:07 admin

Five common defects of casting molds and their solutions


Defect 1: Casting shrinkage cavity


The main reasons are that the alloy solidification shrinkage produces casting shrinkage cavities and the alloy absorbs a large amount of oxygen and nitrogen in the air when the alloy is dissolved, and the gas is released when the alloy solidifies to cause casting shrinkage cavities.


The solution:


1) Place the gold ball.


2) Increase the diameter of the sprue or shorten the length of the sprue.


3) Increase the amount of metal.


4) Use the following methods to prevent the organization from sinking in the direction of the sprue.


a. Place a cooling channel at the root of the sprue.


b. In order to prevent the molten metal from hitting the cavity vertically, the sprue should be curved.


c. Place the sprue diagonally.


Defect 2: The surface of the casting is rough and not clean


The rough surface of the cavity and the molten metal have a chemical reaction with the surface of the cavity, which mainly reflects the following situations.


1) The embedding material has coarse particles and is not delicate after mixing.


2) After curing, the embedding material is directly put into the Maofu furnace for roasting, and the moisture is too much.


3) The heating rate of the roasting is too fast, and different positions in the cavity produce differential expansion, which causes the inner surface of the cavity to peel off.


4) The maximum firing temperature is too high or the firing time is too long, so that the inner surface of the cavity is too dry.


5) The melting temperature of the metal or the baking temperature of the casting ring is too high, causing the metal to react with the cavity, and the surface of the casting is burnt with the embedded material.


6) Insufficient firing of the casting mold. When the molten metal is cast, it will cause the decomposition of the investment material, generate more gas, and cause pitting on the surface of the casting.


7) After the molten metal is cast, the temperature in the cavity is too high, and the surface of the casting is locally rough.


The solution:


a. Do not melt the metal excessively.


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 embedded material is 800 degrees-900 degrees).


d. Avoid the phenomenon that the organization faces the direction of the sprue sag.


e. Coat the wax to prevent burning liquid.


Defect 3: Cracking of the casting


There are two main reasons, one is usually because the metal at the place solidifies too fast, resulting in casting defects (joints); the other is cracks caused by high temperature.


1) For metal solidification too fast, the resulting casting joints can be solved by controlling the casting time and solidification time. Factors related to casting time: wax shape. The amount of thickness cast. Casting pressure (casting machine). The permeability of the embedding material. Factors related to setting time: wax shape. The highest firing temperature of the casting ring. Type of investment material. The type of metal. The temperature of the casting.


2) Cracks caused by high temperature are related to the mechanical properties of metals and investment materials. Cracks are prone to occur in the following situations: high casting temperature is prone to cracks; high-strength investment materials are prone to cracks; nickel alloys and cobalt alloys with low elongation are prone to cracks.


The solution:


Use low-strength investment materials; reduce the casting temperature of the metal as much as possible; do not use low ductility. Brittle alloy.


Defect 4: Spherical protrusion defect


It is mainly caused by the residual air (bubbles) remaining on the surface of the wax after the embedding material is blended.


1) Vacuum embedding for real air conditioners and embedding materials has a better effect.


2) Spray a surfactant on the surface of the wax before embedding (e.g. castmate from Nissin)


3) First coat the embedding material on the wax mold.


4) Adopt pressure embedding method to squeeze out air bubbles.


5) Pay attention to the direction of the wax mold when embedding, and there should be no depression under the joint between the wax mold and the sprue.


6) Prevent air bubbles from being mixed during embedding. Casting ring and casting seat. The buffer paper must be tightly closed; the embedding material must be poured along the inner wall of the casting ring (using an oscillator).


7) Do not oscillate after filling the casting ring.


Defect 5: Flash defects of castings


Mainly due to cracks in the casting ring, the molten metal flows into the cracks in the cavity.


The solution:


1) Change the embedding conditions: use higher-strength embedding materials. The strength of gypsum-type embedding materials is lower than that of phosphate-type embedding materials, so be careful when using it. Try to use ring casting. When casting without ring, the casting ring is prone to cracks, so it needs to be noted.


2) Roasting conditions: do not directly fire after the embedding material has solidified (should be fired after several hours). The temperature should be increased slowly. Cast immediately after firing, do not repeat firing the cast ring.