Small and medium-sized solid mold casting companies use outsourcing coatings to solve the problem of coating sources. These companies use a small amount of paint, and after purchasing a certain amount of paint at one time, they cannot use it up in a short period of time. Quality problems such as paint deterioration and sinking often occur, causing the paint to be scrapped. Once there are quality problems with the paint purchased by some enterprises with inconvenient transportation, they cannot be returned and the paint will be scrapped in batches, causing large economic losses. Therefore, these companies urgently need to make their own paints and master certain paint performance adjustment knowledge, so that they can solve some paint quality problems and take on-site measures to save the problematic paints. Enterprises that make their own paint need to understand the paint formula, mixing methods, testing methods and the changing rules of paint performance. The conditions of these enterprises are relatively simple and require formulation personnel to be able to use their own conditions and characteristics to select equipment and materials. There are many choices for paint formulas and mixing equipment. Master the paint preparation rules and casting process requirements to prepare paint that meets the usage requirements. The formulated coating meets the needs of production, reduces costs for enterprises, and improves product quality. This article summarizes the relevant experience as follows, for reference only by relevant small and medium-sized enterprises.

1. Introduction to the raw materials used in coatings

To determine the formula of the coating, you need to determine the refractory composition of the coating, the types of suspending agent, defoaming agent, and carrier solvent, and master the physical and chemical properties of the relevant materials. properties and microstructure.

1. 1 Refractory materials

Refractory materials should be selected mainly based on the type of casting alloy. Refractory materials for cast iron coatings often use flake graphite and ordinary graphite, quartz powder, bauxite, talc powder, kyanite powder, etc. X-ray fluorescence spectrometry and diffraction analysis were conducted on several foreign refractory materials for cast iron coatings and found that Their refractory materials often include wollastonite, mullite, mica, corundum, spodumene, etc. Various particle types in the aggregate are also used together, specifically to improve the permeability and strength of the coating. The particle types include flaky , fibrous and granular. Magnesia powder, zircon powder, high aluminum powder, brown corundum powder and other refractory powders are often used as coatings for steel castings. Fused magnesia powder and forsterite powder are commonly used for high manganese steel. These materials can resist alkali corrosion of high manganese steel. The particle size is generally between 320 mesh and 200 mesh, which should also be used in combination. This kind of coating must be noted that when the SiO2 (quartz) content in forsterite powder is ≥ 40%, it often affects the function of the coating, because SiO2 and MnO will react chemically and cause sand to stick.

Wollastonite CaO*SiO2: It is a metasilicate, belonging to the triclinic crystal system. It is divided into two types: low-temperature type and high-temperature type. The low-temperature type transforms into the high-temperature type at 1125°C. Wollastonite has acicular and fibrous crystal morphology (length/diameter ratio ≥ 22:1) and good low heat resistance (heat resistance ≥ 1500°C) and sinterability. It can increase the strength and suspension of coatings in coatings. and high temperature breathability. Generally, high-temperature materials with SiO2%≥50 and settlement degree <70 should be used. It has very large reserves in Jilin and Liaoning areas and can be widely used in cast iron coatings.

Mullite 3AI2O*2SiO2: orthorhombic crystal system, melting point 1810°C, polygonal grain type. Chemically stable. The linear expansion coefficient is small (20~1000℃, 5.3×10-6/℃), and the resistance to cold and heat shock is good. The commercial powder can be made of coal gangue powder that has been sintered at high temperature, which can ensure the high temperature stability of the coating. This material is abundantly available in China, has a low price, and can be used in cast iron and ordinary cast steel.

Mica KAI2 (AISi3O20) (OH*F)2: a lamellar silicate with a density of 2.6 to 2.86, low thermal conductivity (average 0.67W/m.K), and good thermal insulation performance . Its scales are elastic, the crystal lattice is stable, and the thermochemical stability is good. This material has a low melting point (1270~1330℃) and is prone to sand sticking when used in cast iron coatings. Due to its flaky shape, it can provide the coating with anti-falling properties and flow parallelism, making the coating have good toughness and cracking resistance, and can be used in coatings for aluminum castings.

Kyanite AL2[SiO4]O: According to the morphological characteristics of kyanite, kyanite ore is divided into three types of metamorphosis: (1) needle-like and fibrous aggregates (fibrous needle-like ore); ⑵ False kyanite aggregates rich in hollow crystals; ⑶ Kyanite nodules (nodule-type ores).

The kyanite mineral is calcined at high temperatures (1100~1650℃) and transformed into mullite and molten free silica (cristobalite), while producing varying degrees of volume expansion. The transformation reaction formula is:

3(AI2SiO5) 3AI2O3*2SiO2+SiO2 (above 1300℃)

The casting coating should use calcined fiber needle-shaped products, and its refractoriness Higher than wollastonite and quartz powder, similar to zircon powder, due to its fibrous needle-like form, it can improve the strength, air permeability and fire resistance of the coating. At present, this product is also produced in Jilin Panshi and other places, and can be used in coatings for cast iron, ordinary cast steel, etc. The performance of other materials will not be detailed in this article, and relevant information should be carefully searched when selecting.

1.2 Carrier liquid

Water or ethanol (methanol) can be used as the carrier liquid. The cost of water-based paint is lower than that of alcohol-based paint, and the suspension is easy to control, but it requires a set of equipment for drying and removing water vapor.

Alcohol-based paint is easy to dry and is often used by many small businesses. However, it has the following problems: it generates a lot of gas during pouring, is high in cost, has a strong paint smell, affects the health of workers, and is not safe. It is generally recommended to use water-based coating technology, especially for some castings with simple shapes, such as manganese steel plates. The application of methanol can improve drying quality and can be used in conjunction with ethanol.

1.3 Binders

According to different carrier liquids, binders are divided into two types: water-based and alcohol-based. The general requirements for binders are: easily soluble and dispersible in water or ethanol; and have high dry strength, especially high temperature strength. Syrup, paper pulp, phenolic resin, polyvinyl alcohol (PVA), polyvinyl acetate emulsion (white latex), polyvinyl butyral (PVB), carboxymethyl cellulose (CMC), as well as clay and phosphoric acid with good high temperature properties Aluminum dihydride, sulfate, rosin, water glass and silica sol can all be used as binders in coatings. Phenolic resin has both high normal temperature strength and high high temperature strength, and is widely used in coatings. Water-based coatings can directly use alkaline liquid phenolic resin, while alcohol-based coatings use thermosetting phenolic resin, which is first soaked in alcohol, dissolved and then added to the coating. It works better when used in conjunction with PVB. When polyvinyl alcohol (PVA) and aluminum dihydrogen phosphate are used together, the effect is better and the cost is low. Some commercial paints are reversible in hardening, that is, the hardened paint dropped to the ground can still be mixed and used repeatedly. Some water-soluble resins, such as PAM, have this property.

1. 4 Suspending agent

Commonly used suspending agents include lithium-based, sodium-based or organic bentonite, attapulgite, CMC, etc. Lithium-based bentonite not only has good suspension properties, but also has good painting performance. It is often used in alcohol-based paint baths. However, paints using lithium-based bentonite are prone to "sinking to the bottom". It is best to prepare the paint freshly. Currently in use. Soda soil and CMC can be used in water-based paints, and the effect is better. Organic soil has the best effect, but the price is too high. It is generally added in small amounts in alcohol-based paints with higher requirements. When choosing a bentonite suspending agent, on the premise of ensuring the suspension, the less the added amount, the better. If the added amount is too large, the coating will easily crack.

1.5 Other additives

Real type (or lost foam) coatings also need to add the following additives: ⑴ Surfactants are used to improve the lubrication of the coating on the plastic surface. Wet, commonly used non-ionic surfactants JCF (fatty alcohol polyvinyl ether), OP-10 (condensate of alkyl phenol and ethylene oxide), NNO (sodium naphthalene sulfonate formaldehyde condensate), it It has both dispersing and water reducing effects. Surfactants tend to produce bubbles, so add as little or no surfactant to the paint as possible. ⑵ Defoaming agent During the high-speed mixing process, the paint may also be involved in air to generate bubbles, so a small amount of defoaming agent must be added. Commonly used ones include n-octanol, n-butanol, SPA-202 aliphatic mineral oil, and SAF (polymethylsiloxane emulsion). ⑶Preservatives Commonly used preservatives include pentachlorophenol, pentachlorophenol, sodium benzoate, formaldehyde, etc. ⑷Carbon adsorbent Cryolite (Na3AIF6) forms active NaF, AIF3, etc. at high temperatures, which adsorbs the carbon decomposed from the sample so that it does not precipitate on the surface of the casting, thereby preventing the generation of carbon on the surface of the casting. In addition, iron oxide powder (Fe2O3) is also incorporated into the paint to improve the paint's self-peeling ability and resistance to nitrogen pores. The surface of the parts is smooth and clean, and the dimensional accuracy is improved to a certain extent.

2 The role and performance requirements of coatings

Coatings can fill the surface pores of the mold and core and inhibit the thermal interaction (mechanical penetration and chemical erosion) between the sand mold and the metal liquid at high temperatures. This prevents mechanical sand from sticking, makes the surface of the casting smooth, and improves dimensional accuracy to a certain extent. Coatings can also improve the surface strength of molds and cores and prevent blisters, burrs and other defects caused by the erosion of liquid metal. The isolation effect of the coating layer can also prevent a large amount of gas generated by the heating of the mold and core from invading the casting and causing pore defects. Coatings with low thermal expansion can also reduce sand inclusion defects in castings. In addition, by adding thermal insulation material to the coating, the transfer and movement of heat flow can be adjusted to control the solidification and crystallization process of the alloy, thereby eliminating shrinkage defects. Adding certain special additives to the coating can also achieve local incubation or surface alloying to improve the structure. These functions of coatings have also attracted people's attention recently. There are many requirements for the performance of coatings:

2.1 Suspension stability, the coating should not precipitate, layer, or agglomerate within a certain period of time and maintain uniform density. The suspension property mainly depends on the quality and amount of the added suspending agent, as well as the particle size and specific gravity of the refractory filler.

2.2 Permeability, which represents the ability of the coating to penetrate into the surface of the mold or core to a certain depth after application. Permeability is not always better. Excessive permeability will cause unnecessary waste or make it difficult to maintain a certain thickness. If the permeability is too small, the adhesion of the paint on the mold or core will be small, and it will easily peel and break during drying and pouring. The penetration depth depends on the particle size of the refractory material, the specific gravity and viscosity of the coating, the surface characteristics and internal structural characteristics of the coating suspension system, and the wettability of the coating with the mold or core.

2.3 Thixotropy (thixotropy) In physical chemistry, the thixotropy phenomenon is regarded as a reversible isothermal transformation process of sol to soft glue.

Thixotropy includes two aspects of shear thinning viscosity time-dependence: one is that it can thin under the action of shear force, and will return to its original state after the shear force is removed; the other is that the above-mentioned thinning or thickening has both A time process. In layman's terms, it means "it becomes thinner when stirred and thicker when it rests". A good paint should have a certain degree of thixotropy and be "thick but not sticky, slippery but not dripping" when used, making it easy to brush, dip, and spray, and the paint will neither accumulate nor run off. Thixotropy is one of the main technological properties of coatings used in modern castings. To a certain extent, it is a comprehensive index that reflects the quality of many technological properties of coatings (such as suspension, brushability, etc.). Thixotropy depends on the internal structural characteristics of the paint suspension system, which can be judged by the relationship between the apparent viscosity of the paint and time at a certain shear rate.

2.4 Coatability, which indicates the difficulty of coating evenly covering the surface of the mold or core with a certain thickness. It should be applied without buildup, runoff, or damage to the surface of the mold or core. At present, there is no direct method to measure the coating property. Generally, it can be comprehensively measured by indicators such as the viscosity, specific gravity, thixotropy, coating thickness or coating weight of the coating.

2.5 Surface strength, the coating must have sufficient surface strength to prevent damage to the coating during handling and packing.

2.6 High temperature crack resistance. The coating should not crack during drying and pouring to prevent burrs or veins in the casting.

2.7 Resistance to sand sticking. The ability to prevent sand sticking is one of the main properties of coatings. It mainly depends on the composition of the coating, especially the properties of the refractory material and the atmosphere during pouring. The thickness, strength, and crack resistance of the coating also have an impact on the resistance to sand sticking.

Long shelf life, no pollution, low cost, wide adaptability, etc. In addition to the above-mentioned properties, coatings are also required to produce less gas, have a long shelf life, be pollution-free, be low-cost, and have wide adaptability. It is actually difficult for coatings to fully meet the above requirements. For different types of sand molds and different castings (or even different parts of the same casting), appropriate coatings should be selected and used based on the principle of low cost and high quality.

3. Preparation and coating

3.1 Water-based solid coating formula

First of all, the preparation method of coating should be determined according to the type of casting alloy and the size of the casting. Performance, such as fire resistance, coating strength, coating storage time, etc., then select refractory powder, carrier liquid, binder, suspending agent and additives, adjust and determine the formula after testing. Table 1, Table 2 and Table 3 list several formulations prepared.

3.1 Table 1 Water-based solid coating formula for cast iron

Coating

Components

Alkaline phenol

Aldehyde resin

CMC

Sodium soil

Water

Oxidation

Iron powder

Spodumene

Mullite

WT/%

60

3

30

600

5

100

400

500

3.1 Table 2 Formula of solid alcohol-based coating for high manganese steel

Coating

Components

Phenolic

Resin

PVB

Lithium earth

Alcohol

Oxidation

Iron powder

Zirconium Powder

Electrofusion

Magnesia powder

n-Octanol

WT/%

30

6

30

500

5

170

830

3.1 Table 3 Alcohol-based solid coating formula for cast iron

Coating

Components

Phenolic

PVB

Lithium earth

Alcohol

Oxidation

Iron powder

Graphite

Scale

Graphite

WT/%

25

8

25

550

5

150

800

50

3.2 Preparation of coating

During preparation, it is generally dispersed in a disperser and then ground by grinding equipment. For enterprises with an annual consumption of less than 100 tons, a 2 to 3 kilowatt disperser is sufficient. The impeller has two types: blades and discs. The blade-type stirring force is large and can turn the material up and down, but a slightly higher speed will cause the material to splash. The disc-type blades are staggered up and down, which has a strong shearing effect on the material. It can rotate smoothly at high speed and has a better dispersion effect. Good, the productivity is also high, and the equipment can be homemade. The linear speed of the impeller can be selected to be greater than 200cm/min. For enterprises with low output requirements, a small ball mill can also be used. The diameter of the barrel is about 0.5-0.8mm and the rotation speed is about 100 rpm. The coating is ground in a ball mill. In addition to good dispersion effect, it can also break the aggregate, activate the coating, and improve the quality of the coating.

The ball mill is more effective than the colloid mill. The coating ground by the ball mill not only has high coating strength, but also has better process properties such as leveling and brushability. The ball milling time is more than 4 hours.

The mixing procedure for water-based coatings is generally to place bentonite, CMC and water in a disperser and stir into a slurry, then add refractory materials for continuous stirring, and then add additives such as binders and surfactants in sequence. . If bentonite and CMC are soaked in advance, the effect will be better. If alcohol-based paint uses lithium bentonite as the suspending agent, it should be pre-expanded with water for more than 24 hours. Phenolic resin and PVB should also be soaked with alcohol first, and then added to the disperser for mixing. We have also mixed and crushed the soaked phenolic resin and lithium bentonite into a premix for long-term storage. When used, it is added to other mixtures in proportion to form a coating. This law is great for small businesses.

3. 3 Coating application

Select the coating method according to the production batch of castings. For single-piece and small-batch production, brushing should be used. Flow coating can be used for larger castings. The viscosity of the flow-coated paint should be smaller and have better coating and hanging performance; dip coating and flow coating are used for batches and complex shapes, and spray coating is used for thin-walled and easy-to-deform shapes. Several methods can be used in combination. The paint must evenly cover the surface of the pattern, with no gaps, excessive run-through, or air bubbles. The coating thickness is generally 0.5-2.5mm, which can be determined according to the shape, thickness, complexity, alloy type, gate static head height and other factors of the casting.

Generally, it needs to be applied 1-3 times. Each coating needs to be dried. If cracks occur in the dried coating, the specific gravity of the coating and the amount of bentonite should be reduced in time, and the amount of binder should be increased appropriately. quantity. Coating drying is generally achieved in an oven by circulating hot air (below 60°C). Drying time is 3-10h. Outdoor drying, infrared or microwave drying can also be used.

4. Quality and performance testing

The testing of coating performance is a necessary and important condition to ensure the quality of coatings. The physical properties of coatings, such as specific gravity, viscosity and suspension, can be tested in the laboratory Check with conventional instruments. It is best to use the weighing method to test the specific gravity, because the viscosity of the solid paint is high, and the hydrometer and Baume meter are not easy to float freely, which affects the test results. The detection of specific gravity is very important, as it directly affects the thickness of the coating and can control the amount of solvent added. The coating thickness can be measured using the weighing method and special calipers. There are many methods for measuring the air permeability of coatings. A standard sample of bentonite green sand with a fixed formula is used in the laboratory, coated with paint on one end and then dried, and measured on an ordinary air permeability meter. At the production site, the air permeability of the coating can be indirectly estimated by measuring the pouring time.

The coating strength is measured using the SVQ paint coating strength tester. Before the test, apply paint layer by layer into the paint groove of the base and dry it. After polishing, ensure that the coating thickness is 1.2mm. During the test, start the air compressor to pressurize the cavity until the coating ruptures, and read the maximum pressure from the pressure gauge. This pressure value is regarded as the coating strength value of the coating. The coating properties in Table 2 and Table 3 were measured, and the results are shown in Table 4 and Table 5.

Table 4 Properties of cast iron water-based solid coatings

Coating name

(24h)

Suspension

g/cm-3

Density

Breathability

Coating strength

MPa

High temperature emergency Thermal

Crack resistance

Gas generation

Ml/g

Homemade 1

98

1.90

17

0.15

Grade 1

21

Table 5 Real alcohols Base coating properties

Coating name

Suspension

(2h)

Density

g/cm- 3

Breathability

Coating strength

MPa

High temperature and rapid heating

Crack resistance

p>

Gas output

Ml/g

Homemade 2

Homemade 3

95

98

1.58

1.25

75

80

0.075

1.0080< /p>

Level 1

Level 1

22.5

20.0

5. Production application

A factory produces ball iron head castings and needs a batch of high-quality real-shape coatings. It uses imported Ashland lost foam coatings, which has good results, but the price is too high. The quality of domestic commercial paint is not very stable. If you rely solely on imported paint, it will inevitably increase a lot of costs. Through analysis, we believe that if most domestic materials are used, the cost of powder materials can be reduced to less than 4,000 yuan. The economic benefits are very obvious, and the performance can be adjusted at any time as needed.

Prepare a coating using 270-mesh high-aluminum powder as refractory material based on the data, and use white latex as the binder. After the round test, it was found that the paint had poor air permeability, the paint was prone to blistering, and there were many casting waste products. There were almost no finished products in the first two times.

Using 200-mesh mullite and spodumene as aggregates and water-soluble phenolic resin as binder, the test results are good. The casts produced are basically poured, and the castings have serious sand sticking. Immediately, 320 mesh zirconium powder was added to the formula and tested, and the effect was very obvious. After the casting is cast, the coating and the casting are very easy to peel off, completely solving the problem of sand sticking, and the yield of the casting is high, reaching the level of Ash paint cast in the same batch. The formula is shown in Table 1.

A factory produces high-manganese steel castings for ball mill jaw plates using the real-shape method. In the past, coatings relied on purchased commercial coatings, and coating waste such as coating cracking and sand sticking often occurred. In order to solve the coating problem by myself, I have tried various formulas without success, especially the selection of binders and aggregates, which is a big problem. For example, if a formula such as PVA plus sulfate binder + high aluminum powder is used, the normal temperature strength of the coating is acceptable, but the high temperature strength is low, and the castings are seriously stuck to sand. After testing, the formula in Table 2 was adopted, and a large number of castings were successfully produced using the ball mill and other current equipment of the factory. The formula of graphite coating for cast iron material is shown in Table 3.

6. Conclusion

6.1 Small and medium-sized solid mold foundries use various domestic materials to make their own coatings, which can reduce material costs and improve product quality. It can also enhance flexibility and improve market competitiveness. . The correct selection of paint formula, raw and auxiliary materials is the key to the success or failure of self-prepared paint. In addition to technical factors, the choice of formula should also consider the cost of materials. Priority can be given to using materials from local and nearby areas.

6.2 From an international perspective, the variety of commercial coatings will increase day by day, and the process performance and coating methods of coatings will continue to improve and innovate. Due to the high cost of oven drying, the use of water-based coatings is not so economical. The use of low "foam" carrier coatings, dry coatings, electrostatic bonding coatings, light radiation curing-resistant coatings, etc. will become the development trend of coatings. In short, the application efficiency of future coatings will be higher and the effect will be better. Due to strict environmental protection laws, it is expected that flammable coatings will no longer be used in some countries within a few years. Judging from the domestic situation, new results will be achieved in the basic theoretical research on coatings and the development and application of new thixotropic coatings, and the application of light-colored and self-colored coatings will gradually expand. Professional fixed-point production, commercialization and high-quality coatings will also be gradually realized in the short term.