Conventional casting sand is shaped by an adhesive to form a cavity. After contacting the high-temperature metal liquid, the binder and other auxiliary materials will instantaneously generate high-pressure gas, fill the sand gap and form a gas film between the liquid metal and the cavity wall to block the liquid metal from entering the molding sand gap. In the microscopic environment, this moment liquid metal can not touch the molding sand, only contact with the gas film, called liquid gas infiltration. Therefore, in the conventional casting, the surface of the molding sand may be rough, and the surface of the casting is very smooth, which is because of the existence of the gas film.
Vacuum is drawn during the casting of the lost foam casting. Although the foam model is vaporized, the organic binder in the coating can generate a large amount of gas by thermal carbonization and vaporization of the crystal water in the molding sand, and the sand gap cannot be formed due to the suction by the negative pressure. The high pressure of the gas and the gas film between the liquid metal and the coating, the liquid metal is in direct contact with the coating, which is called liquid solid infiltration. Therefore, lost foam casting can clone the fine structure of the surface of the foam model. The poorly-contaminated structure of the turtle’s back and the traces of the gas plug can be clearly cast out to form the surface features of the lost-foam casting castings that are distinct from the traditional casting. Therefore, some people joked that the lost mold casting is different from people. Sometimes human ugly mothers can produce beautiful children. In order to improve the surface quality of castings, it is necessary to improve the precision of the molds. First, make the ideal foam model, inferior mold. Can’t make beautiful castings.
Lost foam casting can not form the gas high pressure and gas film of the sand gap. If there is no coating shielding, the liquid metal will infiltrate the sand gap under the vacuum of the negative pressure, and the sand is wrapped to produce serious iron-clad sand, which cannot form a foam phase. Consistent fine castings. At the same time, if there is no shielding of the coating, the pressure difference between the cavity and the sand gap cannot be formed, and the dry sand will be smashed. Therefore, the main function of the lost foam casting coating is: the shielding sand forms a cavity.
We all know that foam models (especially thin-walled shell-like models) are not inherently strong and susceptible to damage and deformation. After brushing the paint and drying it, the foam model is like wearing a layer of armor, and the strength is much improved, and it is usually able to effectively overcome the shortcomings of deformation due to insufficient strength. Therefore, the auxiliary function of the lost foam casting coating is to improve the strength of the foam model, protect the model, prevent damage and deformation, and improve the operability of lost foam casting.
In order to obtain a good coating, the lost foam casting coating must have the following properties (working properties) during the coating process:
(1) suspension properties of the coating;
(2) Thixotropic properties of the coating;
(3) the flow properties of the coating;
(4) Adhesion properties of the coating;
(5) The strength of the paint (low temperature strength or dry strength).
During the casting process, the lost foam casting coating is subjected to strong scouring of high-temperature molten metal, and the foam model material is violently vaporized and burned to generate a large amount of combustible gas and free carbon and carbon bundle. The coating at high temperature not only maintains good strength, but also purifies the cavity to discharge the foamed gasification product, and does not allow the liquid metal to pass through the coating, thereby obtaining high-quality castings without inherent defects. Under the process conditions, the coating must have the following properties (process performance):
(1) good high temperature strength;
(2) Proper permeability;
(3) a small coefficient of thermal expansion;
(4) cold insulation performance;
(5) Alloying properties.
At the end of casting, during the casting process, the coating must also have: (1) self-exfoliation (cracking) performance; (2) easy shot blasting characteristics.
The unique properties of lost foam casting coatings come from the right formula, high quality materials and reasonable production. The three links are indispensable!
Formulation of lost foam casting paint
Lost-molded water-based coatings are low-cost, safe and reliable, stable in performance, convenient in transportation, clean and less polluted, easy to manufacture and coating, and widely used. They are the first choice for lost foam casting. This article discusses the lost foam casting water-based coatings.
The lost foam casting water-based coating consists of two parts: refractory aggregate and auxiliary material. Among them, aggregates account for 90-92%, and materials account for 8-10%. In addition to commercial coatings, defoamers and preservatives are added to the applicator. There are only three main ingredients:
(1) binder; (2) suspending agent; (3) wetting agent.
The classification or division of the three agents of adhesion, suspension and wetting is actually the subjective behavior of scholars. In the material that has been objectively surrounded by the mold-casting coating material, the individual’s role is not single. For example: bentonite and carboxylic acid based cellulose (CMC), usually used as binders, but in lost foam casting water-based coatings, because they can not overcome the hydrophobicity of the smooth surface of the polystyrene foam model, use them to make sticky The coating, the coating cannot be applied to the surface of the foam model. However, the fine particles of bentonite swell in water to form a hydrated film, which sticks to the macromolecular chain of CMC, and laps to form a body network structure, which prevents the refractory aggregate particles from sinking and sinking, thereby improving the suspension performance of the floating material. Therefore, these two substances having a bonding function are used as a suspending agent.
The substance as a binder must first have an affinity for a smooth surface of the foam model while having good low temperature strength and high temperature strength. Regardless of domestic coatings or foreign coatings, the preferred binder is: polyvinyl acetate emulsion (commonly known as white latex). White latex can significantly increase the viscosity of the coating while reducing the suspension of the coating. However, the white latex has a slight corrosive effect on the surface of the foam model, which improves the wettability of the coating, thereby improving the adhesion property of the coating, that is, improving the coating performance of the coating.
This is not comparable to any other binder. According to this feature, the amount of milk white glue added is determined according to the roughness of the surface of the foam model. Light-like foam model, the amount of white latex in the coating can be as high as 56%, the resistance wire cuts the spliced foam type, and the amount of white latex in the coating can be as low as 1% or less.
The choice of the lost foam coating binder should not only consider the low temperature strength, but also the high temperature strength of the coating. White latex has good low temperature strength, but coking will occur when it is 300C to 400C, and the binder will be lost. Therefore, when white latex is used as the binder, it is necessary to supplement the high temperature binder.
Among the many high-temperature binders, phosphates, especially sodium tripolyphosphate, not only have good high-temperature properties, but also a kind of wetting agent, which can reduce the surface tension of the coating and improve the coating properties and flow properties. The high-temperature strength of sodium tripolyphosphate is manifested in the promotion of low-temperature sintering ceramization of refractory aggregates, forming a ceramic shell that is highly resistant to high-temperature metal liquid scour.
Sodium tripolyphosphate can cause the refractory aggregate to aggregate and precipitate, and destroy the suspension property of the coating, and the ratio should be less and less.
The various excipients in the coating have interactions, some are positive interactions, the appropriate ratio can play a synergistic effect, and some are negative interactions, and the ratio imbalance will act as an antagonistic offset. Therefore, the effect of the coating formulation is not the sum of the independent effects and effects of various auxiliary materials. The reasonable total effect of the formula is greater than the sum of the independent effects, and the total effect of the unreasonable formula is less than the sum of the independent effects.
