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

2022-08-02 14:34:02 370

Characteristics of silica sol

Silica sol is a kind of amorphous SiO2 small particles (nanometer) dispersed in water to form a stable colloidal solution. As an adhesive for precision casting shells, silica sol has the following advantages and disadvantages:

1.1 Advantages

Different from water glass and ethyl silicate, the manufacturing process of silica sol shell does not involve chemical changes, has no irritating products, and is an environmental friendly adhesive. 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 input of equipment.

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

22 Main performance indexes of silica sol

The main physicochemical parameters for measuring the quality of silica sol include: SiO2 mass fraction. PH foundry can select the appropriate type of silica sol according to the specific product structure characteristics and quality requirements.

2.1 SiO2 mass fraction

SiO2 content determines the high temperature strength of the shell.

Generally, the SiO2 mass fraction of silica sol used in precision casting is 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 Relationship between high temperature strength of shell and SiO2% of silica sol

2.2 PH value (Na2O% in colloid)

PH This value directly affects the stability of silica sol.

When the PH value of silica sol directly affects the gelling speed, when the PH value is 5~7, the charge on the particle surface decreases, and the particles are easy to collide, resulting in condensation. When the PH value drops to 2~4, the silica sol particles change from the original negative electricity to the main hall with good stability.

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

Fig. 2 Relationship between gelling time and PH value

2.3 Kinematic viscosity

Silica sol with low kinematic viscosity can usually be configured with slurry with high powder ratio, high shell density, high strength, improved slurry fluidity, and easy to obtain high level of surface quality.

2.4SiO2 colloid particle size

Colloid particle size is an important physical index of silica sol, which affects the use performance of silica sol from 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 other is the influence on the wet strength of the shell. The smaller the particle size is, the more the colloidal indirect contacts are, and the higher the shell strength is.

3 Selection of silica sol

The foundry shall select the appropriate silica sol model according to the product structure characteristics. The following principles can be referred to:

3. The factory with conditions shall consider the use of surface and back silica sol.

The silica sol with small particle size and high strength (high strength) SiO2 content is selected as the surface glue. Silica sol with large particle size and low strength is selected as the back glue.

3.2 When producing large precision castings in small batches, silica sol with large particle size can also be selected to give priority to ensuring the stability of the surface slurry.

3.3. When the product surface quality and dimensional accuracy are required to be high, the glue with low kinematic viscosity shall be given priority, and the surface reward material with high powder ratio can be configured to make the shell with high density and high temperature strength.

4. Main process nodes of shell making

4.1 Preparation of slurry

4.1.1 Powder liquid ratio

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

The foundry can configure slurry with different powder ratios according to the product structure characteristics and quality requirements.

4.1.2 Wetting agent and defoamer

The purpose of adding wetting agent is to improve the adhesion between wax pattern and slurry. JFC is generally selected

After the wetting agent is added to the slurry, the surface tension of the coating will drop, and it is easy to be drawn into 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 make the facial mask on the bubble surface stretch and thin, so as to break the bubble.

4.1.3 Slurry concentration control

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

The new slurry shall be measured immediately after curing, and the slurry in use shall be measured every shift, and the need to add ingredients shall be determined according to the measurement results.

When the concentration is greater than 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 surface layer slurry concentration is generally controlled to be 32~45s

4.1.4 Aging of slurry

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. Conditional factories should 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.

The fourth is to measure the pH value of the slurry every shift. The normal value should be 9~10. When it is lower than 9, it is easy to breed bacteria and aggravate the aging.

4.2 Key points of operation

4.2.1 Grouting

The surface coating is generally done manually, and the main steps are as follows:

First, clean it with compressed air, blow dry the etching module, and then immerse it in the surface slurry at an angle of 30 ° for slight rotation.

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

4.2.2 Sand hanging

The surface layer shall be sandblasted with a sandblasting machine, which shall rotate 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, and the excess slurry shall be returned to the slurry bucket for sand hanging operation.

After the surface layer is sanded, it is found that there is slurry on the surface of the module, and you can gently press with your fingers to remove the excess slurry.

The sand particles in the sand shower machine and floating sand bucket shall be screened regularly, and the mortar blocks shall be removed.

Attention shall be paid to sand hanging on the back layer, and small flow of compressed air shall be used to gently blow out the previous layer of floating sand before slurry sticking.

If the parts have blind holes or slender through-hole structures, they can be dredged with fine iron wires to remove the slurry and floating sand in the holes. In order to make the next layer of mucilage evenly covered.

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, the drying environment temperature of medium temperature wax silica sol shell is 20~24 ℃, and low temperature easily leads to wax mold shrinkage, which affects the dimensional accuracy of castings. High temperature (especially in summer) will lead to wax mold expansion and deformation, leading to coating cracking, seriously affecting the shell quality and yield.

4.3.2 Humidity

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 shell manufacturing time. It should be noted that the rapid dehydration of SiO2 colloid will lead to sharp shrinkage of the volume, and the coating is easy to crack when the strength has not been established. Generally, the surface humidity should not be too low.

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

4.3.3 Wind speed

In principle, the upper layer should be dry without wind, and the wind speed of the back layer: 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 the product structure characteristics and environment, the surface of parts with large plane and simple structure can be dried for 3~4h.

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


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