Abstraction
Casting defects and prevention of disappearing mould castings are unavoidable phenomena in the casting process of some castings. In this paper, the causes of disappearing mould castings, prevention methods and countermeasures for castings and related equipment during the process of disappearing mould castings are mainly discussed from the perspective of technology.
Keywords：
Castings； Disappearing mould castings； Prevention
Introduction
Due to the limitation of equipment and sand moulds, the castings that cannot be produced obviously exist in the casting process. Thus, the phenomenon of disappearing mould castings is produced, which results in the quality of the castings not being up to the required standard and wasting of resources. The standard of the castings directly affects the cost and quality of the product, and the quality of the castings cannot be ignored. Disappearing mould castings usually occur in casting defects, resulting in an increase in production costs. In order to reduce the influence of disappearing mould castings, it is necessary to have a corresponding analysis of its causes and prevention based on the characteristics of the castings.
What are disappearing mould castings
Disappearing mould castings refer to the phenomenon that the structure of the upper pattern in the investment sand mould is lost due to the plasticity of the clay under the action of the molten metal pressure, thus the cavity of the pattern disappears in the sand mould. This defect is mainly caused by the shrinkage of the casting caused by the hot metal filling process, the large metal pressure in the high-rise sand mould cavity and the poor adhesion between the core sand and the sand mould.
Reasons for disappearing mould castings
1. Too thin sand mould.
The mould under the thin wall or large-area hollow core is prone to shrinkage due to the improper casting size of the core, resulting in the mould being too thin and being easily damaged by molten metal pressure. This happens particularly in the production of thin wall, low height and small core castings. In order to prevent disappearing mould castings, the casting size should be properly increased.
2. Pre-castings before pouring
When the thickness of the upper pattern is thin and the wall thickness is thin, most of the core sand will be washed away when the molten metal is poured. The molten metal pressure continues to pile up on the thin wall sand mould, thus squeezing out and destroying the upper pattern surface. In order to reduce the molten metal pressure, the thickness of the upper pattern of the sand mould can be increased, and the pouring should be done after the first pouring.
3. Inadequate compressive strength
The compressive strength of the sand mould is not up to a certain range of values, resulting in the sand mould becoming powerless when it is squeezed by the metal pressure and the pits or glue surface uncovered or destroyed, forming the disappearance of the upper mould surface. In order to improve the compressive strength of the sand mould, the sand mould can be strengthened by changing the properties of the sand mould material.
4. Poor sand-core adhesion
The strength of the core body and sand mould depend on the bonding of the adhesion between the core body and sand mould. The weak adhesion of the core-sand interface easily causes the core surface to deform and separate from the sand mould, resulting in the disappearance of the mould. In order to reduce the occurrence of this kind of phenomenon, the strength of the adhesive can be increased by changing the adhesive material and improving the adhesive process.
Prevention of disappearing mould castings
1. Increase sand strength
Adopting a higher strength clay and a stronger cement paste will help increase the strength of the sand and reduce the occurrence of disappearing mould castings.
2. Improve the strength of the sand-core adhesion interface
We can use new core types such as resin sand, coated sand or bonded sand, or use no binder to reduce the strength of the sand-core adhesion interface.
3. Reduce the number of pouring
The number of pouring should be adjusted properly according to the actual conditions of the core. If possible, multiple casts should be avoided.
4. Properly increase the height of the sand mould
When the height of the sand mould is higher than the allowable range, disappearing mould castings may occur. Therefore, when the expected strength of the sand mould is not reached, the height of the sand mould should not be too high.
5. Properly adjust the size of the core
The size of the core should be calculated according to the casting size, the high temperature shrinkage rate of the casting material, the thickness of the sand mould and other factors, so that it can guarantee the best casting effect and the least defective rate.
Conclusion
Disappearing mould castings are unavoidable in the casting process, but we can reduce or prevent the occurrence of disappearing mould castings by properly adjusting the sand setting process, sand strength, pouring number and other factors. Therefore, most of the occurrences of disappearing mould castings can be avoided by paying attention to the problems in the production process and formulating corresponding preventive measures.
