Chapter III 3.1 Understanding of Lost Foam Casting Coatings in Yellow Area


3.1.1 Introduction of lost foam coating

First of all, we briefly compare the ordinary sand casting coating with the lost foam coating, and then understand the role of the lost foam coating in the lost foam casting.

In the ordinary casting process, the paint is applied to the surface of the mold cavity. However, in lost foam casting, paint is applied to the foam mold. The main function of the lost foam casting coating is:

(1) Improve the strength and rigidity of the foam plastic model to prevent the pattern from being damaged or deformed during transportation and sand filling vibration. In the process of buried operation, no matter how much attention you pay, it is difficult to avoid the deformation effect of the impact force. Since we cannot prevent the occurrence of this phenomenon, we can only reduce the consequences of this phenomenon. Lost foam coatings can withstand the impact force more than other coatings at the same thickness and can protect the foam to the greatest extent, so that the degree of deformation is greatly reduced, even can not be seen. Figure 1 is the relationship curve between the force and deformation of the foam model with and without paint, and it can be seen from the figure that the paint can greatly reduce the deformation of the foam model after the force.

 

Fig.1 Relationship curve between stress and deformation of foam model with or without coating

Part 1-B has no paint; Part 2-A has no paint; Part 3-A has paint; Part 4-B has paint

    (2)During pouring, the coating separates the liquid metal from the mold to prevent the liquid metal from penetrating into the dry sand to ensure a smooth surface of the casting without sticking sand. At the same time, the dry sand is prevented from flowing into the gap between the metal liquid and the foam plastic model, causing the mold to collapse.

Some lost foam in the casting process because the back pressure between the sally gap is less than the liquid metal seepage pressure in the manufacturing process, which will cause the phenomenon of mechanical sand sticking. If a layer of lost foam coating is applied to the surface of the lost foam, this gap can be prevented, thus preventing the phenomenon of mechanical sand sticking. However, some need to be treated at high temperature during the production process. Once, the phenomenon of thermochemical sand sticking will occur when the lost vanisher cannot withstand high temperature, and the lost vanisher coating just has good fire resistance, thus preventing the phenomenon of thermochemical sand sticking.

(3) the coating layer can make the product of thermal decomposition of the foam plastic mold (a large amount of gas or liquid, etc.) smoothly discharged into the mold to prevent the casting from producing pores, carbon defects, etc.

When pouring aluminum alloy, the coating should also have good thermal insulation to prevent the temperature of aluminum liquid from falling too fast due to the heat absorption of plastic mold combustion, resulting in insufficient pouring and other defects.

3.1.2 Lost foam coating composition (material)

Lost foam coatings are generally composed of refractory materials, binders (solvents), surfactants, suspending agents, thixotropic agents, and other additives.

1. Refractory materials

Refractory material is the backbone of the coating, which determines the refractoriness, chemical stability and thermal insulation performance of the coating.

Commonly used refractory materials are corundum, zircon, quartz, bauxite, kaolin clinker, magnesia, diatomite and so on. Table 1 is the physical and chemical properties of several commonly used refractory materials.

Table 1 Physical and chemical properties of several common refractories

refractory materialrefractory material

Molecular formulaMolecular formula

Chemical propertieschemical performance

melting temperature() melting temperature

Density(g/cm3) density

coefficient of linear expansionlinear expansion coefficient (l/)×106

coefficient of thermal conductivityheat conductivity coefficient(W/m.)

corundumcorundum

Al2O3

Genderamphoteric

2000~2050

3.8~4.0

8.6

5.2~12.5

Zirconzirconite

ZrO2.. SiO2

weak acidslightly acidic

>1948

4.6~4.9

4.6

2.1

Quartzquartz

SiO2

Acidicacidic

1713

2.65

12.5

1.8

bauxite clinkercalcined bauxite

-

-

1800

3.1~3.5

5-8

-

kaolin clinkerkaolin clay clinker

-

-

1700~1790*

2.62~2.65

5.0

0.6-0.8

Talcum powder

Talcum powder

Mg3Si4O10(OH)2

-

800~1350

2.7

7-10

-

Graphitegraphite

C

-

>3000

2.25

2

13.7

Magnesium Oxidemagnesium oxide

MgO

Alkalinealkaline

>1800

3.6

14

2.9~5.6

DiatomitePebble guhr

SiO2

-

-

1.9~2.3

-

0.14

* for refractoriness

Different refractory materials should be used to make coatings when producing lost foam castings of different alloys. This is because different alloys have different requirements on the refractoriness of refractory materials, different requirements on the chemical stability of coatings, and different requirements on the thermal insulation of coatings. Usually the production of aluminum castings when the lost foam coating to use diatomite, talc and other refractory materials; iron castings commonly used quartz powder, bauxite, kaolin clinker, brown corundum powder and other refractory materials; steel castings commonly used corundum, zircon, magnesium oxide and other refractory materials.

In addition to the correct selection of refractory types, the particle size and distribution of refractory materials and the particle shape of refractory materials should be correctly selected. Because the particle size and distribution and particle shape will affect the permeability of the coating. It is reported that the refractory particles used for the lost foam coating are preferably round, and the particle size is coarse and concentrated.

2.Binder

As mentioned earlier, in order to ensure that the lost mold coating has both high strength and high air permeability, it is necessary to reasonably select organic and inorganic binders, and use a variety of binders to ensure that the coating performance meets the requirements.

(1)Inorganic binders: the most used inorganic binders are bentonite, water glass, silica sol, phosphate, etc. Here focuses on bentonite.

Foundry clay is divided into two categories: ordinary clay and bentonite. Coatings often use sodium bentonite as an inorganic binder, the effect is better.

Bentonite is mainly composed of montmorillonite minerals, chemical expression is.Al2O3.4SiO2.H2Wall to Wall2O, whereH2O is structural water,nH2O is the inter-layer water. Under the electron microscope, it is an irregular sheet or villous, lamellar tissue. The surface of clay particles is often negatively charged, which can adsorb cations, which in turn affect the properties of bentonite. adsorptionNa 1,Ca 2The ionic bentonite is called separatelyNabentonite andCaBentonite. Domestic sodium bentonite is less, calcium bentonite can be converted into sodium bentonite with the right amount of sodium carbonate treatment. The characteristics of two kinds of bentonite are shown in the table2.

Table2 Comparison of properties of sodium bentonite and calcium bentonite

Kindtypes

Chemical composition (%)chemical components

Amount of sodium ion adsorbed(mgequivalent/100g) With ion absorption amount of

PH

gum valuecolloid index

Expansion ratioexpansion times

adsorption rate() absorption rate

Blue absorption(%) blue absorption amount

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

Na2O

Na groupAt the base

64.31

19.35

2.18

1.44

2.90

0.87

2.40

35.4

9.5

100

29

360

>20

Ca groupCa base

69.06

17.51

1.16

1.54

1.97

0.69

0.11

0.1

6.9

45

10

150

10

(2) organic binder: foundry commonly used organic binder syrup, pulp, polyvinyl alcohol, polyvinyl acetate emulsion, polyvinyl butyral, carboxymethyl cellulose (CMC), starch, dextrin and so on. Only a few of them are presented here.

Polyvinyl alcohol (PVA) is an organic polymer compound and is a water-soluble resin. It is a non-dielectric surfactant with good film-forming properties and is not damaged by mold. It can be used as a casting binder. Different grades of polyvinyl alcohol, its performance is also different. Some are soluble in water, some can only swell. The brand of polyvinyl alcohol for casting binder isPVA17-88 (polymerization degree of 1700, alcoholysis degree of 88%), the appearance is white granular.

Starch is a carbohydrate with the chemical formula (C6H10O5) n, its molecular structure has two kinds of straight chain and branched chain. Amylopectin has a large molecular weight, up to 6 of amylose.Times. The higher the amylopectin content, the more viscous the starch. According to the types of raw materials, it can be divided into corn starch, potato starch, sweet potato starch, cassava starch, glutinous rice starch and so on. According to its granular structure, starch is dividedα,BetaTwo kinds.BetaStarch, I .e., native starch, is mostly round or polygonal with a compound grain and microcrystalline structure. Mix natural starch with water, pre-gelatinization treatment at a certain temperature and pressure, so that its volume expansion, molecular chain breakage, microcrystalline structure destruction, particles entangled with each other to form.αStarch. There are deposits in the grain departments all over the country.Powder supply.

Dextrin is an intermediate product of hydrolysis (saccharification) of starch by acid and heat after adding water. The product formed after the reaction contains dextrin and reducing sugar, etc. Dextrin general gelatinization degree above 70%, reducing sugar in15% below. Dextrins are mostly light yellow powder, but also white dextrin, but its adhesion is poor. Dextrin is an organic water-soluble binder with high wet strength and dry strength. It can be almost completely dissolved in cold water, but it is easy to absorb moisture and agglomerate when exposed.

3.Carrier (solvent)

There are water-based coatings and alcohol-based coatings, but more water-based coatings are used in production. At home and abroad, the lost matrix commercial coatings are mostly directly formulated into water-based paste coatings; or when purchased as powder, water is added when used to prepare water-based coatings.

4. surface active agent

In order to improve the sagging of water-based lost-form coatings, it is necessary to add surfactants to the coatings. The molecule of a surfactant is generally composed of two different groups of atoms: one is a non-polar, lipophilic (hydrophobic) group; the other is a hydrophilic group. The two groups are located at both ends of a molecule, forming an asymmetric structure. Thus, the molecule of the surfactant is an amphiphilic molecule that is both hydrophilic and lipophilic. When the surfactant is added to the lost foam coating, its hydrophilic group is combined with the water in the water-based coating, and the lipophilic end is attracted by the foam mold, so that it is aligned on the surface of the foam mold, as if a bridge pulls the plastic mold and the water-based lost foam coating together.

Commonly used surfactants such as JFC, also known as penetrating agent EA, the full name of polyoxyethylene alkyl alcohol ether.

5. Suspending agent, thixotropic agent

In order to make the refractory powder in the coating can be fully mixed with the binder and can be in a suspended state, a suspending agent can be added. Bentonite is a binder and a good suspending agent. In addition, organic compounds such as CMC can be used as suspending agents for water-based coatings. CMC is divided into high, medium and low according to its viscosity. Among them, the medium viscosity CMC is mixed into a 2% aqueous solution, and its viscosity is in the range of 300 to 600 centipoise, which is convenient to use and low in cost. CMC can be adsorbed on the surface of powder particles due to charge effect or hydrogen bonding, and its bridging effect can improve the suspension of the coating. Because CMC is a kind of polymer anionic electrolyte and the surface charge of clay is the same, so it is used in combination with clay, especially with sodium bentonite.

In order to improve the thixotropy of the coating, a thickening thixotropic agent can also be added to the coating. Bentonite,CMCare thickening thixotropic agent. In addition, polyvinyl alcohol, sodium alginate, attapulgite, sesbania gum, SN (a variety of inorganic materials), activated rectorite, etc. are thickening thixotropic agent.

6.Other additions

Additives may be added in order to improve certain aspects of the properties of the coating. For example, in order to prevent the deterioration of organic matter in the vanishing mold coating, preservatives such as formalin can be added. Another example is the British company Foseco proposed to add a small amount of cryolite (NaAlF6), so that it forms NaF and AlF with high activity at high temperature3And so on, to absorb the carbon from the thermal decomposition of the foam plastic mold, so that it does not precipitate on the surface of the casting, so as to eliminate the wrinkled skin, carbon and other defects on the cast iron parts.

3.1.3 technical requirements for coating performance (thickness, strength, permeability, etc.)

In order to enable the coating to play the above-mentioned role, the lost foam coating should control the following properties: strength, air permeability, fire resistance, thermal insulation, resistance to rapid cooling, low moisture absorption, good cleaning, coating hanging, dripping, suspension and other properties.

The numerous properties can also be summarized into two types of performance: work performance and process performance.

Coating strength, air permeability, fire resistance, thermal insulation, resistance to rapid cooling and heat and other properties are its working performance. The most important performance is strength and air permeability. The lost foam coating requires high strength and high air permeability, which is different from other casting coatings.

The properties of coating, dripping and suspension belong to its process performance. Lost foam casting often uses water-based coatings, because water-based coatings and organic foam plastic mold is not wet, the need to improve the coating coating hanging, which is a major process performance of the lost foam coating. Another major process property is the dripping of the coating. After the pattern is coated, it is generally hung and dried, hoping that the coating can be leveled and not dripped as soon as possible, so as to ensure a uniform coating with a certain thickness and make full use of the coating without polluting the environment.

The following focuses on the four properties of coating strength, breathability, coating and dripping and their test methods.

1.Coating strength

The strength of the coating includes room temperature strength, high temperature strength and residual strength. The room temperature strength will ensure that the foam plastic model is not deformed or damaged during transportation and sand filling vibration. High temperature strength prevents coating damage during casting and forming of liquid metal, resulting in defects such as box collapse and sand sticking, and ensures castings with good surface quality. Residual strength is to determine the quality of the coating cleaning after pouring.

The coating strength mainly depends on the type and amount of coating binder. Room temperature strength is composed of organic binder and inorganic binder, the biggest influence is the organic binder. The high temperature strength depends on the inorganic binder. In addition, the type and particle size distribution of refractory materials also affect the strength of the coating.

There are many methods for measuring coating strength, and only two easy and simple methods are introduced here.

Method1: with a certain formula of sand100g, pound the standard sand sample, pay attention to empty out above the sand sample1.5mmto apply the coating, see Fig.2a). After the coating dries, the sand sample is carefully removed and the strength of the coating is measured. See Figure2b)。

 

The strength σ of the coating layer is calculated by the following formula:

(Formula1)

In the formula:G-Total load weight,kg;

F-Coating area,cm2

Figure2 Schematic diagram of coating strength test

Method2: Use thick (paste) paint to make rectangular coating samples, see figure3a)。

After drying, put it on the bracket of the bending strength tester (see figure3(b)), load from the middle of the sample until the sample breaks. Bending strength of coatingsSCan be expressed by the following formula.

(Formula2)

                        

In the formula:G-Total load weight,kg;

B- Specimen width,cm;

h-Specimen thickness,cm.

Figure3 Schematic Diagram for Determination of Bending Strength of Coating

The coating room temperature strength was measured as above. To determine the high temperature strength of the coating, the measuring device is placed in a high temperature furnace to determine the high temperature strength of the coating at a certain temperature. The residual strength can be measured after the sample is burned in a high temperature furnace and cooled to room temperature.

2. permeability of coating

The breathability of the coating is also divided into room temperature breathability and high temperature breathability. The high temperature permeability has a greater impact on the quality of castings.

There are many factors that affect the permeability of the coating, such as the particle size and distribution of refractory materials in the coating, the type and content of organic and inorganic binders, the type and content of additives, the mixing process, the thickness of the coating, the density of the coating or the solid content of the coating.

The coarser the particle size and the more concentrated the distribution of refractory materials in the coating, the higher the permeability of the coating, such as when using the same formulation and preparation process method, using the figure.4(a) and figure4b2) The air permeability of the coatings prepared by two kinds of refractory materials with particle size distribution is nearly doubled, and the coatings prepared by coarse refractory materials have high air permeability.

Figure4 Refractory material particle size distribution curve

Figure5 for the same binder, the relationship between the coating prepared by different refractory materials, refractory material particle size and coating permeability. It can be seen that the permeability of the coatings prepared by the two refractory materials increases with the change of the particle size of the refractory material.

Figure5 The relationship between the particle size of refractory materials and the permeability of coatings.

Experimental studies have shown that the increase of inorganic binder content in the coating decreases the permeability, while some organic binders increase within a certain range, can make the coating room temperature permeability slightly increased, so that high temperature permeability has a greater improvement. It is also reported that the type of binder has different effects on the permeability of the coating, as shown in the figure.6. Figure6It is used with two binders (A,B) and the same kind of refractory (200mesh alumina) the breathability of the paint. With the binderAThe increase in the permeability of the coating is increased due to microcracks in the coating (see Figure6a)). BinderBIt is easy to disperse and can be uniformly coated on the surface of refractory materials, thus with the binder.BThe voids in the coating decrease and the air permeability decreases.

In addition, the density of the coating, the solid content and the number of coating will affect the permeability of the coating. Generally speaking, the density of the coating increases with the increase of the solid content in the coating, while the thickness of the coating increases with the increase of the density of the coating and the number of coating, so the permeability of the coating decreases with the increase of the thickness of the coating, as shown in the figure.7 and Figure8.

Figure6 Relationship between binder content and coating permeability

  

Figure7 The relationship between coating density and coating permeability.

From Figure7 Visible, when the coating density is1.411.56g/cm3Between when, onThe coating thickness of primary paint is0.100.23mm, up2 The coating thickness increases0.250.60mm, up3 The thickness of the coating obtained from the secondary coating is up0.330.84mm. Figure8It is shown that when the coating is applied for the same number of times, the permeability of the coating decreases with the increase of the coating thickness, but for the same coating thickness, the permeability decreases with the increase of the coating number.

Figure8 Relationship between coating thickness and coating permeability

So far, there is no standard method for measuring the permeability of coatings at home and abroad. Many units and materials have put forward many methods for determining the permeability of coatings, and the samples, instruments and principles used in these methods are also different. From the test principle can be divided into the following three categories:

(1) The measurement was carried out according to the principle of air permeability.

     

              (Type 3)

Among themV-the volume of air passing through the specimen;

H-height of sample;

F-Sample area;

P-pressure before the gas passes through the sample;

t-time for a certain gas to pass through the sample.

There are many methods for determining the permeability of coatings in this category. For example, the permeability of the coating is measured directly by the sand permeability meter. or transform the above, measure a certain pressureP gas passes through the coated specimen when the flow rate by the gasV/t, calculatedKValue. Or measure the time required for the pressure to drop from one value to another as the gas passes through the coating, etc. This broad category of methods is used more.

(2) Gradually increase the gas pressure through the coating sample until the coating sample breaks. The gas pressure is used to express the gas permeability of the coating.

 (3)The pore size of the coating is measured to indicate the gas permeability of the coating, and the pore size and distribution of the coating can be measured using an optical microscope or the like.

 Three types of air permeability determination methods, due to different test principles, the data are not the same. Here only by principle (1) A method of determination, which is to make the coating into a circular sheet sample, placed in the figure.9a) in the sample holder, use the figure9b) to determine its permeability.

Figure9 Schematic diagram of the coating air permeability tester.

   The specific operation steps are as follows: open the oil-free air compressor switch, provide compressed air at two atmospheres, adjust the pressure reducing valve to stabilize the pressure value of the pressure gauge at a certain value, open the two-way valve, and let the air flow to the coating sheet on the sample seat through the flow meter. Read the pressure value and flowmeter value on the pressure gauge, and substitute the following formula to calculate the air permeability of the coating.K:

    (Formula4)

In the formula:θ/t-gas flow,cm3/min;

Δ -- Coating thickness,cm;

F-air permeable area of coating layer,cm2;

P-air pressure acting on the coating layer,g/cm2.

High temperature permeability can be the coating at high temperature after burning, cooling to room temperature and then measured.

3.Coating Sagging

The coating hanging of the coating can be measured by a plate-shaped foam plastic mold sheet. Holding one end of the foam plastic mold sheet or glass sheet by hand, immersing it in the coating. After taking it out, observe with naked eyes whether the coating has been hung on all parts and whether the thickness is uniform.

4.dripping property of paint

The dripping of paint can be comprehensively evaluated by smear weight, dripping amount and dripping time. A quick balance, stopwatch, smear and stand are required for testing. First weigh out the smear with a quick balance (40'40'20mm stainless steel sheet) and bracket weight (G1). Dip the smear into the paint to be tested, take it out and hang it on a bracket, under which a film is placed to receive the paint dripping from the smear. Use a stopwatch to record the time the paint drips,2minWeighing out the weight of the smear plus paint and bracket (G2), dripping paint (G3). Then:

Smear weight is the weight of the coating attached to the smear,G=G2-G1;

The dripping amount is the weight of the coating dripped into the film after the smear is taken out, which isG3;

The dripping time is the time taken from the smear to the time when the paint does not drip.t。

The smaller the dripping amount and the shorter the dripping time, the better the dripping property of the coating.

The dripping property of the coating is related to the rheology of the coating.-Thixotropy is directly related, and if the coating has good thixotropy, it will not drip. The thixotropy of the coating is related to the composition of the coating, especially bentonite, thickening thixotropic agent, and whether the coating is in a stirring state when the coating is hung.

3.1.4 Types and selection principles of lost-form coatings

The types of vanish coating are: gray iron, high chromium gray iron, ductile iron, manganese steel, carbon steel, cast aluminum, cast copper, stainless steel.

The selection principle is mainly based on the different properties of different materials such as refractoriness (high temperature strength), permeability and peeling. Even if the same material, the wall thickness, weight and even the quality requirement level of the casting are different, the selected coating types will be different. The following examples are given in turn:

Examples1. Different types of coatings selected due to different materials

The maximum required temperature for the casting of gray cast iron lost foam casting is1480-1500C, and the pouring temperature required for the lost foam to make carbon steel parts is about 1600 ℃. Therefore, the two materials have different requirements for the high temperature strength of the coating, so the general carbon steel lost foam coating has higher refractoriness than gray iron. Of course, the main aggregate of the coating will be different.

Examples2. Castings of the same material have different types of coatings selected because of different shapes and sizes of castings.

The temperature required for the pouring of the lost vanity pattern will be determined by the thickness of the wall of the casting. The thinner the wall thickness is, the higher the pouring temperature should be. For example, the gearbox of the tractor, the thinnest part6mm, the required pouring temperature is about 1460 ℃-1480 ℃. However, the same gray iron machine tool bed castings, due to the large overall wall thickness, the pouring temperature is generally selected at 1400 ℃. Although it is the same material, the pouring temperature corresponding to different shapes and sizes of castings is different, which requires the fire-resistant temperature of the coating to be different, so the corresponding coating will be adjusted accordingly.

Examples3, the same material castings due to different weight coating selection is also different

The lost mode is replaced by molten iron.The process of the EPS model, so the volume and weight of the EPS model required for castings of different weights are also different. Different EPS models in the process of gasification gas will be different, so the requirements of the air permeability of the lost foam coating is not the same.

3.1.5 Field Control of Coating

The performance of the coating control is more, as described earlier. But in the production site can mainly control the density of the coating, coating weight and coating.The pH value.

1.coating density

The density of the coating can be measured by a density meter (specific gravity meter) or by a volumetric method.

The density of the coating reflects its thin and thick state. Under certain coating methods, it also reflects the thickness of the coating on the foam mold, that is, the coating thickness of the coating. Therefore, this is an important performance of the coating and should be controlled.

Many casting coatings are often measured in the field of its flow cup viscosity, with the flow cup viscosity to control its thin thick degree. However, the lost foam water-based paint is a pseudoplastic fluid with high thixotropy and yield value, and its flow cup viscosity is difficult to measure. Therefore, the density of the coating is used to control its thinness at the production site.

2.Coating weight

The weight of white mold (foam plastic mold) can be measured at the production site (G1), after the coating is applied, the total weight (G2) of the white mold and the coating is measured after the coating is dried, and the G2-G1 is the coating weight g. Knowing G when producing a certain product, the thickness of the coating can be estimated. You can also take the coating layer directly on the dried model and measure the thickness with a vernier caliper.

The coating weight can also be determined by the method described previously herein.

  3.PaintpHValue

Bentonite is used as the inorganic binder for the water-based coating of the lost pattern, and various organic binders are added at the same time. of paintThe pH value reflects the stability and performance of the coating and should be controlled.

of paintpH can be used.pHTest paper to be measured.

3.1.6 Coating method and coating thickness

The method of coating generally has four kinds of brush, dip, shower and spray. A single piece of small batch production of medium and large patterns using brush coating method. The dip coating method and the flow coating method are selected for small patterns with large batches and complex shapes. For thin-walled, easily deformed or easily damaged models can not be dip-coated, must be sprayed. Several methods can also be used in combination in actual production.

Dip coating method has high production efficiency and uniform coating, but due to the light weight of foam plastic mold and large buoyancy during dip coating, attention should be paid to prevent deformation of the model. In order to overcome buoyancy, a fixture can be used to fix the foam plastic mold in the fixture, and then the paint is pressed in from the bottom. After the whole model is submerged from bottom to top, the pressure is withdrawn to make the paint fall automatically.

Dip-dip combination can solve the problem of model buoyancy, but the efficiency and uniformity of coating are not as good as dip-coating method. The coating should be uniform, the thickness is determined by the size of the piece, the wall thickness30mm weight20kgThe general coating of the following parts is not more1.5mmLarge pieces can be thicker.

The thickness of the coating shall be determined according to the alloy type, structure shape and size of the casting. If the coating is too thin, the strengthening effect on the model is small; if it is too thick, the permeability will decrease, and the coating is easy to crack and peel off, increasing the drying time.

For small parts and thin-walled parts, try to use2 times coating, coating thickness approx.0.8~1.2mm, thick iron castings and steel castings, can be used2times, even3The coating thickness is1.5~2mm.

3.1.7 Dipper Precautions

 1.Continuous stirring and temperature control

The water-based coatings for lost foam casting are all thixotropic coatings, that is, the viscosity of the coating decreases during the stirring process, and the viscosity increases after the stirring is stopped. Figure10 is the change in the degree of paint due to thixotropy after stopping stirring.

In order to obtain a uniform coating thickness, the coating should be applied with stirring. This can also be seen from the graph.11, the same model of the specimen (inner diameter3in, outside diameter3.5in, long4inThe tube) is applied with and without stirring, and the weight change of the coating after drying. The coating weight change is small under continuous and slow stirring, that is, the coating thickness is uniform. The weight of the coating applied in the unstirred state varies greatly and the coating is not uniform.

Figure10 The effect of thixotropy on the baumensity of coatings after stopping stirring.

Figure11 The relationship between the uniformity of the coating and the stirring condition after the drying of the model.

2. slow mixing, to prevent the volume of gas

Mixing paint speed should not be too slow, otherwise the paint will precipitate; but the speed can not be too fast, otherwise the paint is easy to roll gas.Figure12For the shape of blade type mixing drum equipment, figure13ForL type mixing barrel, can achieve slow mixing. Figure14is a schematic diagram of a paint agitation tank.

Figure12Equipment drawing of blade type paint mixing barrel

Figure13 LSchematic Diagram of Mixing Barrel for Shaped Coating

1-LType mixing blade (fixed)2-Rotating paint bucket

   3.Dip coating direction to prevent deformation

When the paint is dipped, the direction and position of the model immersed in the paint should be reasonably selected to prevent the model from deforming.

   4.Uniform coating, not exposed

The paint applied to the model shall be comprehensive and uniform, and there shall be no exposed parts of the model without paint.

5. Stable operation to prevent deformation

    

Figure14 Schematic diagram of paint tank structure

1-Paint tank2-Slide3-Paint4-Mixing blade5-Mobile trolley6-Reducer7-Motor

Model removal from the paint, transport, placement should be considered to prevent model deformation.

3.1.8 Drying specifications and precautions

Model coating drying is subjectEPS model softening temperature limit, often using low temperature or room temperature drying.

In mass production, often40~60℃, dry after each application812h. The drying process should pay attention to the following matters:

1) Pay attention to the reasonable placement and support of the model during the drying process to prevent the model from deforming.

2) It must be dried thoroughly.

3) The dried model should be placed in a place with low humidity to prevent moisture absorption.

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