Essentials of Shell Making by Wax Silica Sol Process at Medium Temperature

Key points of shell making by medium temperature wax silica sol process.

abstract

Medium temperature wax silica sol shell is a widely used process in investment casting industry. How to produce high-quality ceramic shell quickly and efficiently is the key to the whole casting process. This article focuses on the manufacturing process and control nodes of the intermediate temperature wax silica sol shell, aiming to bring many basic proposals to practitioners.

Characteristics of silica sol.

Silica sol is a stable colloidal solution dispersed in water. As a kind of investment casting shell adhesive, silica sol has the following advantages and disadvantages:

1.1 Advantages

Different from water glass and ethyl silicate, silica sol shell is an environment-friendly adhesive that does not involve chemical reactions and irritating commodities. Easy to use, good slurry stability, simple equipment.

The shell has good high temperature resistance strength and high temperature creep resistance, and is suitable for precision casting of high melting point alloys such as cast steel and superalloy.

1.2 Disadvantages

Compared with water glass, the wettability of wax mold is poor, and surfactant needs to be added to improve the coating performance of slurry.

The drying rate of silica sol shell completely depends on temperature, humidity control and wind speed regulation, which requires high investment in equipment.

The drying speed of the shell is slow, the humidity is low, and the shell cycle is long.

Main performance indexes of silica sol.

The main physicochemical parameters of silica sol mass measurement include SiO2 mass fraction, PH value, kinematic viscosity and colloidal particle size. The foundry can select the appropriate type of silica sol according to the structural characteristics and quality requirements of specific commodities.

2.1 SiO2 mass fraction.

The content of SiO2 determines the high temperature strength of the shell.

The mass fraction of SiO2 in silica sol is generally 29~31%.

The higher the SiO2 content, the better the high temperature strength of the mold shell, and the higher the residual strength of the mold shell after pouring, which is not conducive to shell vibration.

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Fig. 1 High temperature strength of shell is related to SiO2% in silica sol.

2.2 PH value (colloidal Na2O%)

PH directly affects the stability of silica sol.

The change of pH value of silica sol will directly affect the condensation rate. When the PH value is 5~7, the surface charge of particles decreases, and particles are easy to collide and condense. When the PH value drops to 2~4, the silica sol particles change from the original negative electricity, and the silica sol has good stability.

The silica sol used in the investment casting industry is weakly alkaline, with a PH value of 9~10, which is stable.

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Fig. 2 Relation between gel time and PH value.

2.3 Kinematic viscosity.

Silica sol with low kinematic viscosity is usually equipped with slurry with high powder ratio, high shell density, high strength, high slurry fluidity and high surface quality.

2.4SiO2 colloid particle size.

It affects the properties of silica sol in two aspects:

First, the stability of silica sol is affected. The larger the particle size is, the better the stability of silica sol is;

The second is the influence on the humidity intensity of the shell. The smaller the particle size is, the more the colloidal indirect contacts are, and the higher the shell strength is.

Select silica sol.

The foundry shall select the appropriate silica sol model according to the structural characteristics of the commodity. The following principles can be referred to:

3.1 Factories with conditions shall consider the use of silica sol on the surface and back.

The surface glue adopts silica sol with small particle size and high strength (high SiO2 content). Silicone sol with large particle size and low strength is used as the backing.

Large particle size silica sol can also be used for small batch production of large precision castings to ensure the stability of the surface slurry.

3.3 When the surface quality and dimensional accuracy of the commodity are required to be high, the glue with low kinematic viscosity shall be given priority. The shell with high density and high temperature strength can be equipped with high powder ratio surface reward materials.

Main process node 4 shells.

Prepare 4.1 slurry.

4.1.1 Powder ratio.

The slurry with high powder ratio is helpful to obtain a dense shell. The surface quality of the casting is good, the wet strength of the shell is high, the high temperature strength is high, and the dimensional accuracy is high;

The foundry can provide slurry with different powder ratios according to the structural characteristics and quality requirements of the commodity.

4.1.2 Wetting agent and defoamer.

JFC is usually combined with slurry, and JFC is usually used to add wetting agent.

After the wetting agent is added to the slurry, the surface tension of the coating is reduced, and it is easy to get involved in the air during the mixing process, forming a large number of bubbles. At this time, it is necessary to add defoamers such as methyl silicone oil to thin the bubble facial mask and break the bubble.

4.1.3 Slurry concentration control.

Use a viscometer (4 # Jens cup) to measure the slurry concentration and make records.

After the new slurry is cured, the slurry measured in each shift shall be measured immediately, and the need for adding ingredients shall be determined according to the measurement results.

When the concentration reaches the control standard, silica sol or distilled water can be added for dilution (silica sol is recommended for the surface and distilled water for the back).

The slurry concentration is generally controlled within 32~45s.

4.1.4 Slurry aging.

Slurry aging refers to that the colloids in the silica sol merge with each other, lose stability and produce agglomeration. To avoid aging, the following points should be done.

First of all, the prepared slurry should be used up as soon as possible, especially the surface slurry. Many factories use small particle size glue, which can age faster;

The second is to control the slurry temperature to avoid contact with high and low temperatures. The factory with conditions shall use ice water to cool the slurry barrel to better control the slurry temperature.

Third, add appropriate amount of wetting agent and defoamer according to the proportion recommended by the silica sol manufacturer, and do not overdo them.

Fourthly, the normal PH value of materials in each shift is 9~10. When the PH value is less than 9, bacteria are easy to breed and the aging is aggravated.

4.2 Key points of operation.

4.2.1 Paste.

The main steps are as follows:

First, clean the etching module with compressed air, and then soak it in the surface slurry with an angle of 30 ° and rotate it slightly.

Second, if there are words, grooves, corners, small holes and blind holes on the wax mold, blow off the bubbles with compressed air. Use a brush if necessary.

4.2.2 Sand hanging.

The sandblasting surface of the sandblasting machine rotates continuously to form a complete and uniform coating on the module. After the slurry is taken out, the module shall be inverted for 2~3s to make the excess slurry flow back to the slurry bucket, and then hang sand.

After the surface is sanded, it is found that the module surface is gently pressed with fingers to remove the excess slurry.

Regularly screen, clean the sand particles in the sand shower machine and floating sand bucket, and remove the mortar blocks.

A layer of floating sand must be gently blown out with small flow of compressed air.

If the part has a blind hole or a fine hole structure, it can be dredged with fine iron wire to remove the slurry and floating sand in the hole. Evenly cover the mucilage.

4.3 Shell drying factors.

The drying process of silica sol shell is actually the dehydration process of silica sol, which is related to the following four main factors.

4.3.1 Temperature.

Generally speaking, the drying environment temperature of medium temperature wax silica sol shell is 20~24 ℃, which is easy to cause wax pattern shrinkage and affect the size and accuracy of castings. Too high temperature (especially in summer) will cause the wax pattern to expand and deform, leading to coating cracking, which will seriously affect the quality and output of the shell.

Humidity 4.3.2.

The shell drying process is actually a process in which water diffuses from the shell to the surrounding environment. Low ambient humidity will accelerate the diffusion of water and shorten the production time of the shell. It should be noted that the rapid dehydration of SiO2 colloid will lead to rapid volume shrinkage, and the coating strength is easy to crack before establishment, so the general surface drying humidity should not be too low.

Generally, the surface drying humidity is 60~70%, and the back drying humidity is 30~60%.

4.3.3 Wind speed.

The upper layer shall be dry without wind, and the leeward speed shall be 6~8m/s.

Factories where conditions permit shall adopt 360 degree rotating adjustable air tower to ensure that the part slot and hole structure can blow air and promote uniform drying.

4.3.4 Drying time.

According to different commodity structure characteristics and environment, the surface of parts with simple structure can be dried for 3~4h.

Generally, 4~6h for the surface and more than 8h for the back.