refractory coating for lost foam casting

Lost foam coatings are becoming increasingly important in the lost foam casting process. The lost foam casting process is closely matched with the coating, and the smoothness of the surface quality of the produced castings is greatly increased, which improves the qualification rate of the finished castings. It is an indispensable auxiliary material for high-quality castings with a smooth surface. If there is a problem with the performance, preparation and drying of the coating, the casting will be unstable in quality and the rejection rate will increase. In order to get a clearer picture of all the relevant aspects of lost foam casting coatings, we have a list for you to view.

1,The lost foam coating material component
2, The role of lost foam casting coating materials
3, Technical requirements for refractory coatings for lost foam casting
4, The surface coating method of lost foam casting
5, The characteristics of lost foam casting coating
6, Coating thickness

The lost foam coating is generally composed of a refractory coating aggregate, a binder, a carrier (solvent), a surfactant, a suspending agent, a thixotropic agent, and other additives. The various components are uniformly mixed together and act in a combination of coating application and metal liquid casting.

What parts of the lost foam coating material are included?

Lost foam coatings generally consist of refractory aggregates, binders, carriers (solvents), surfactants, suspending agents, thixotropic agents, and other additives. The various components are uniformly mixed together and act in a combination of coating application and metal liquid casting.

1. Commonly used refractory materials are corundum, zircon sand, silica sand, bauxite, kaolin clinker, magnesia, diatomaceous earth, kyanite and the like. When producing different lost foam castings, different refractory materials should be used to make the coating. Castings of different materials have different requirements for refractoriness, chemical stability and thermal insulation of the coating. Generally, when producing aluminum castings, the lost foam coatings should use diatomaceous earth, talc and other refractory materials; cast iron parts are commonly used in silica sand, bauxite and kaolin. Materials, powdered refractory materials such as brown fused alumina; cast steel parts are commonly used in corrugated refractories such as corundum, zircon sand, magnesia. In addition to properly selecting the type of refractory material, it is also necessary to correctly select the particle size and distribution of the refractory material, as well as the particle shape. Because the particle size and distribution and particle shape will affect the breathability of the coating. It has been reported that the particles of the refractory material for the lost foam coating are preferably round, and the particle size is coarse and concentrated.

2. Commonly used binders can be roughly classified into inorganic and organic types. In one coating, inorganic binders (high temperature binders) can be used simultaneously: hydrophilic type: clay, bentonite, water glass, silica sol, Phosphate, sulfate, polyaluminum chloride, water type: organic bentonite organic binder (low temperature binder): hydrophilic: syrup, pulp waste, dextrin, starch, polyvinyl alcohol (PVA), poly Vinyl acetate emulsion, water-soluble phenolic resin, natural gum. Water-repellent type: asphalt, coal tar, rosin, phenolic resin, dry vegetable oil, fat oil, ethyl silicate, polyvinyl butyral (PVB)

3. Carrier: The lost foam coating has a water-based coating and an alcohol-based coating, but more water-based coatings are used in the production.

4. Suspending agent: a substance added to prevent the precipitation of solid refractory material in the coating is called a suspending agent. The commonly used suspending agents for water-based coatings are: bentonite, attapulgite, sodium carboxymethylcellulose, polypropylene decylamine, alginic acid. Suspending agents commonly used in organic solvent coatings such as sodium include: organic bentonite, lithium bentonite, attapulgite, polyvinyl butyral, and the like.

Lost foam casting coating material

The molding material used for lost foam casting is generally ordinary dry quartz sand. When the foam plastic model is replaced by liquid metal, a large amount of gas generated by thermal decomposition can pass through the gap between the dry sand to discharge the mold. However, liquid metal easily penetrates into the gap between dry sand under the action of negative pressure to form mechanical sand. In order to avoid mechanical sanding, the surface of the lost mold must be coated with a layer of refractory paint. In the casting process, this layer of refractory coating is between the molten metal and the molding sand, which plays an important role in the vaporization of the lost foam pattern, the filling of the molten metal and the formation of the casting.

(1) Prevent mechanical sanding and thermochemical sanding of castings.
The essence of mechanical grit is the filling and infiltration of the pores between the sand by the molten metal. In ordinary sand casting, the infiltration pressure of the molten metal at a certain point of the mold is the pressure of the molten metal. When the infiltration pressure exceeds the gas back pressure in the interstitial space and the additional pressure caused by the surface tension of the molten metal, the molten metal can penetrate into the grit void to form a mechanical grit.
Under normal circumstances, the infiltration pressure of molten metal in lost foam casting is greater than that of sand casting, so the surface of the casting has more serious acupuncture and sand sticking. Applying a dense coating of a certain thickness on the outer surface of the foam mold can prevent the penetration of the molten metal, thereby preventing the casting from being needled and mechanically stuck.

Lost-molded castings currently widely used such as iron pellets, quartz sand, etc., have low melting temperatures due to low refractoriness. When poured into a high-temperature metal liquid, it is easily melted by the molten metal to form defects such as nodules, resulting in thermochemical grit. Therefore, it is helpful to prevent hot chemical sanding by separating a high-resistance coating between the molten metal and the molding.

(2) Improve the rigidity of the foam plastic model to prevent deformation during the buried type.
This is the unique role of the refractory coating in lost foam casting. When the foam plastic mold is in the buried operation, it is inevitably subjected to a certain amount of impact force, and thus the model may cause compression deformation and bending deformation, thereby affecting the final dimensional accuracy of the casting. The surface of the model is coated with a layer of refractory paint. After drying, it will form a hard shell layer with a certain strength tightly coated on the outside, which will increase its strength more than double before the unpainted coating, and the stiffness of the model will also be corresponding. Enhanced, so you can minimize the amount of deformation. For example, a 2 mm thick coating can reduce the compression deformation of the foam model by 2/3 under the same stress conditions, and reduce the bending deformation by more than 9/10.

(3) It contributes to the rapid discharge of the thermal decomposition gas of the lost mode.
If the gas generated by the thermal decomposition of the foam model cannot be discharged in time, defects such as incomplete casting, pores, slag inclusions and carbon black may occur. The surface refractory coating with a certain gas permeability allows the gas to be discharged out of the mold in time under the action of a negative pressure.

Technical requirements for refractory coatings for lost foam casting

The refractory coating for lost foam casting is finally applied to the surface of the foam model. Different types of foams and different casting materials have different requirements for coating properties. In addition, the foam is not gas permeable, the heat distortion temperature is low (about 70 °C), the affinity for water is small, and the gas generation after gasification is large. These characteristics determine that the refractory coating applied to the surface should have the following technical requirements:

(1) The coating has a certain gas permeability
From the point of view of preventing the sand from sticking of the casting, the appearance of the foam model must be coated with the coating, and the coating should be as dense as possible; but the dense coating is not conducive to the escape of the gas generated by the gasification of the gasification mold. Therefore, the coating is required. It has a certain gas permeability, so that the gas generated by the gasification of the model easily passes through the coating, and is evacuated by the vacuum pump through the gap between the samples.
In order to obtain good gas permeability of the coating, the following technical measures can be taken:

A: Properly matching different types and sizes of refractory aggregates. The type and particle size of refractory aggregates have a great influence on the permeability of coatings. Generally, when a certain type of aggregate is used alone, the permeability of the coating is very poor. Different refractory aggregates have better gas permeability when used together. For example, bauxite can be combined with an appropriate amount of quartz powder to obtain good gas permeability. At the same time, the particle shape of the aggregate also has a great influence on the gas permeability of the coating. Spherical particles (quartz, corundum) form exposed micropores, while flake particles (talc, mica) form concealed micropores. The packing of the cell-like structure (perlite) and the fibrous structure (asbestos, vermiculite) form a disordered structure, which constitutes a loose porous structure and has high gas permeability. Properly matching different types of aggregates can adjust the breathability of the paint within a certain range.

B: The organic binder is selected to decompose at high temperature to form some voids, thereby increasing the high-temperature permeability of the coating, and the coating prepared by the high-temperature binder such as silica sol or water glass is poor in gas permeability.

C: Adding an appropriate amount of oxidant foam plastic mold will deposit some carbon on the surface of the coating when it is not completely decomposed at high temperature, blocking the exhaust passage, thereby reducing the high temperature permeability of the coating. Adding some oxidant can accelerate the decomposition of the foam mold, thereby preventing The deposition of carbon increases the breathability of the coating.

(2) has good wettability and adhesion
Styrofoam is a non-polar material with low surface tension and is not easily wetted and penetrated by water-based coatings. Therefore, a small amount of surfactant should be added to the coating and a suitable binder should be selected. The adhesive used is required to have good adhesion to the foam mold, so that a certain thickness of the refractory coating can be obtained on the outer surface of the mold after application. If the adhesion of the coating is poor, the thickness of the coating may be insufficient or uneven, and even the coating may be discontinuous, and it is difficult to obtain a casting of good quality.

(3) good high temperature strength
The refractory coating has high refractoriness and low thermal expansion coefficient. Otherwise, due to the scouring action of the molten metal during the casting process, it is easy to scouring the unsatisfactory coating and winding it into the casting, so that the product produces sand and slag. defect.

(4) Fast drying speed
The coating dries at low temperatures (typically below 60 ° C) at a fast rate, does not crack, and forms a strong refractory coating. If the drying speed is too slow, it will not only affect the production efficiency, but also may peel off from the surface of the gasification mold because the coating cannot be dried for a long time and solidified.

(5) The amount of gas generated should be small
Once the coating layer has been dried, no other gases may be produced when it is in contact with the high temperature metal liquid during the casting process. Otherwise, the coating layer is liable to collapse and fall off, and the generated gas also affects the smoothness of the casting process.

(6) Good coating properties
The coating of the lost foam coating is relatively thick, and the coating is required to have a high yield value and plastic deformation ability. The flowability of the coating should be strictly controlled. For the coating of the thermosetting resin as a binder, a small amount of a thixotropic agent may be added thereto. Improve the coating properties of the coating.

(7) Good sintering and peelability
That is, after the casting, a coating shell which is easy to be automatically peeled off is formed, thereby obtaining a smooth surface casting, and the smoothness of the surface quality of the produced casting is greatly increased, and the yield of the finished product of the casting is improved.

Surface coating method for lost foam casting

The lost foam casting coating is applied to the surface of the lost mold pattern, while the traditional sand casting coating is applied to the surface of the prepared mold. The coating methods of the two are very different, and the technical requirements are Not the same. The application of the lost foam coating can be combined by various methods such as brushing, dipping, pouring, spraying or dipping. In general, the surface roughness and dimensional accuracy of lost foam castings depend on the quality of the lost mold body, and the thickness of the coating does not affect the size of the casting. The rough surface coating layer not only does not affect the surface roughness of the casting, but is advantageous for improving the gas permeability.

The brushing method is done manually by brush. This method does not require certain performance requirements for coatings to be as good as spray or dip coating, but requires a higher level of operator skill and experience.
When brushing, the thickness uniformity of the coating is not as good as that of spraying or dip coating, but the effect is satisfactory. For some vanishing molds with complex structural shapes, it is best to choose a brush. Because some “dead spots” are often not sprayed when spraying, dip coatings often make it difficult to discharge accumulated coatings in certain grooves.

The main disadvantages of brushing are the high labor intensity and low production efficiency. Therefore, it is only suitable for the repairing brushing operation or single-piece production that is bulky and cannot be dip-coated or spray-coated.

The spraying method is to spray the paint on the lost foam pattern with a spray gun. The efficiency of spraying is higher than that of brushing and the labor intensity is much lower.

Due to the large specific gravity of the refractory coating and the high viscosity, the coating must be sprayed in a mist form. In addition, the coating is applied to a foam mold that is easily damaged, and the impact of the coating on the foam model is required. Be as small as possible.

The spraying method has the following problems:
1The technical requirements of the coating are high. When the concentration of the coating is too large, it is often impossible to spray or spray unevenly; the concentration is too small, and it is easy to turbulence or sticking. In addition, the large particles in the coating are easy to block the nozzle. , interrupt production;
2 The lost mode is not easy to be clamped, and it often causes deformation or damage of the lost mold during the spraying process;
3 waste paint: a considerable part of the spray paint sprayed from the spray gun can not fall onto the lost mold pattern, not only waste paint and pollute the environment;
4 quality is not easy to control: some dead angles can not be sprayed, and the first layer of paint is easily destroyed during the second spraying.

Dip coating
Dip coating has the characteristics of high production efficiency, saving paint, uniform coating and low requirements on coatings, easy operation and no special equipment. Since the density of the foam plastic pattern is small (several times the density of the paint) and the strength is not high, it should be noted that the excessive density of the paint is prevented, so that the lost mold pattern is difficult to be immersed and deformed.

The coating thickness of the dip coating can be controlled by the coating concentration, the number of dip coatings and the controlled dip coating operation. Generally, the dip coating operation is used twice: the first time, dilute the thin coating with a small concentration to obtain a uniform thin Coating to improve the coating performance of the two-pass operation; the second pass, dip coating to achieve uniform coating thickness by adjusting the coating concentration. If the concentration of the coating does not change and the thickness of the coating is required to be controlled, it can only be dip coated. The amount of post-jitter disappearance mode is adjusted. It is generally not difficult for a skilled painter to do this.

Precautions for dip coating operation:
(1) Before the dip coating operation, the paint must be properly stirred, preferably in a slow agitation process during the dip coating process to prevent entrapment of air;
(2) When dip coating, the direction and part of the disappearing mold should be selected to prevent the deformation of the pattern;
(3) The coating should be uniform and there should be no whitening on the coating;
(4) The lost mold pattern after dip coating shall be taken out from the container, transported, and placed to prevent deformation.
(5) After the dip-molding pattern is dip-coated, it should be shaken in time to make the coating uniform and remove excess paint.

In order to overcome the deformation of the lost mold pattern caused by the buoyancy of the dip coating method, the suitable mold can be used to fix the lost mold pattern and then immersed in the paint container. In mass production, a special mechanized dip coating device can be used. The lost mold pattern is first fixed with a clamp, then the paint is pressed from the bottom, the mold is removed after the mold is removed, and the paint is automatically dropped to complete the dip coating process.

The dip coating method is suitable for the production of small castings, and the larger size of the lost mold pattern can also be used as a multi-stage dip coating as long as the volume of the paint bucket is allowed.

Lost Foam Casting Coating Features:

The lost foam casting coating has the functions of preventing mechanical sanding and thermochemical grit sanding of the casting, improving the rigidity of the foam plastic model, preventing deformation during embedding, and facilitating the rapid discharge of the thermal decomposition gas of the lost mold pattern, and then the lost foam casting coating Why are there so many functions gathered? Below we will analyze them from six aspects.

Lost foam casting coating has these six characteristics

(1) High strength and rigidity to prevent deformation and dry sand damage during molding; (2) High fire resistance to prevent sanding on the surface of castings during casting;
(3) Excellent gas permeability, which can quickly export the product of the gasification of the sample;
(4) Strong adhesion to prevent cracking or even falling off of the coating during production;
(5) Good coating property, it can be conveniently coated with a certain thickness of coating when used;

(6) Good sintering and peeling property, that is, a coating shell which is easy to be automatically peeled off after casting, thereby obtaining a smooth surface casting .

Coating thickness

The thickness of the coating of the lost foam coating affects its gas permeability and fire resistance. In terms of fire resistance, the company hopes that the coating of the coating is as thick as possible because it is a very effective process for solving the problem of sand sticking of castings. However, if the coating is too thick, it will affect the breathability of the coating, and the coating work is also very troublesome; if it is considered from the breathability of the coating, it is better to make the coating as thin as possible, but the coating is too Thinness can cause needle-punched defects or even sand-bonding defects on the surface of the casting, which affects the surface quality of the casting.

Casting weight (kg) coating thickness (mm)
<100 0.5~1 aluminum, copper casting, small iron casting 100~500 1.0~1.5 medium castings, and steel castings>500 1.5~2.0 Large cast iron and steel castings

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