Almost all the introductions about lost-foam casting coatings will narrowly describe the “coating permeability” of lost-foam casting coatings as “breathability”.
In production practice, we can understand that the paint after casting changes color. This is because the free carbon produced by the violent combustion of the foam model in the cavity passes through the coating, and the cavity is discharged and adhered to the surface of the coating. It can also be seen that when the cross-sectional area of the pores through which the coating penetrates is large, or the absolute value of the vacuum is excessively high, or the temperature of the liquid metal is high, and the surface tension is small, the liquid metal permeates through the coating. In the sand gap, the unique sand-sand phenomenon of lost foam casting is formed: iron-clad sand.
The characteristic of the iron-clad sand unique to lost foam casting is that the coating is still intact, and the liquid metal penetrates the gap of the sand through the pores of the coating, not from the crack of the coating and enters the gap of the sand. The latter can be removed, and the cracks can be seen on the surface of the casting after the sand is removed. The former cannot be removed. I have had the experience. When I poured the large cold die bottom plate for the first time, in order to prevent the collapse of the box, the vacuum was pumped to 0.08Mpa, and the temperature of the molten iron was also high. As a result, the entire casting was poured into a hedgehog, and a serious iron occurred. Sand.
In summary, the lost foam casting coating not only enables the passage of gas, but also allows the solid free carbon and liquid metal to pass. We call it the permeability performance more apt and more accurate than the gas permeability performance!
Lost Foam Casting During the casting process, there are three physical states from bottom to top in the coating, the lowermost part is liquid metal, the uppermost part is unliquefied and gasified combustion foam, and the middle part is mixed with free carbon and flammable. The space of the gas is called the air gap. If the temperature is used to describe the three physical regions, the bottom-up is the high temperature zone, the medium temperature zone and the low temperature zone. With this method of differentiation, we can draw the following conclusions:
(1) The permeability of the coating is meaningless in the low temperature zone;
(2) The permeability of the medium temperature zone determines whether the coating can discharge the gas and free carbon generated during the disappearance of the foam;
(3) The permeability of the high temperature zone is only harmful, and there is no benefit. If the permeability of the coating cannot be closed in the high temperature zone, the liquid metal will overflow and cause “iron-clad sand”.
How is the permeability of the mid-temperature zone produced?
In the description of the formulation of the coating, we have mentioned that a certain amount of organic binder is added to the lost foam casting coating. During the drying process, the water is volatilized, and the water molecules leave fine, nano-scale pores in the volatilization process, forming the low-temperature (normal temperature) permeability of the coating. The coating is semi-permeable, like sugar. The block of wax paper can only pass gas molecules and cannot pass substances larger than water molecules.
In the pouring process, the liquid metal first transfers heat to the foam through convection and gas convection. The foam shrinks into a gel-like substance when it is heated, and is vacuum-drawn and adsorbed on the paint wall (Coanda effect), and then vaporized under high temperature. , a gas gap is formed. When the temperature of the gas gap reaches 300 400 C or more, the organic binder is denatured and coked, and the cross-network structure formed by the organic binder in the process of mixing the paint forms a network-like passage, and the coating is transparent. performance.
The permeability of the coating has two process parameters: (1) the size of the channel aperture cross-sectional area, and (2) the density of the pore size distribution.
The combination of the two indicators determines the permeability of the coating. Therefore, the adjustment of the permeability of the coating includes the adjustment of the aperture cross-sectional area and density.
The adjustment of the permeability aperture is achieved by the choice of organic binder. The thickness of the network structure formed by hydrolysis and stirring of the organic binder (relatively) determines the size of the permeability pore size.
The adjustment of the density of the through holes is regulated by the amount of organic binder added. The proportion of the added amount is high, and the number of through holes formed per unit area is large, and vice versa.
In specific applications, cast iron has good fluidity, low surface tension and strong penetrability. Therefore, the cross-sectional area of the pores of the coating is required to be small to prevent the occurrence of iron-clad sand. Correspondingly, the fluidity of the cast steel is poor. Large, low penetration, the cross-sectional area of the through hole can be larger. Of course, this adjustment also needs to match the pouring temperature and the degree of vacuum.
The ratio of the surface area to the weight of the casting is called the modulus. The ratio of the surface area to the weight of the thin-walled member is greater than the ratio of the thicker member. The throughput per unit area of the coating is thinner than that of the thick-walled parts. Therefore, in the preparation of the coating, the amount of the organic binder added to the thin-walled parts can be reduced under the premise of ensuring the coating performance. the amount. In the formulation of the coating, some binders are added to adjust the permeability of the coating. For example, the BY binder in the formulation of our company’s coatings is the role. The jute fiber added in some professional formulas also adjusts the permeability of the coating.
