Precision casting is the development direction of modern industrial forging, no cutting, high surface finish, high precision and other complex parts and assemblies that are difficult to process
The manufacturing technology has created a broad application prospect;The military and aerospace products have high requirements on the surface quality and dimensions of the casting parts, and ethyl silicate hydrolyzes
Liquid because of greater pollution, preparation process is complex, high cost reasons, and silica sol is prepared from diluted sodium silicate solution by a specific process
A colloidal dispersion system with discontinuous amorphous silica particles as the dispersing phase and water as the dispersing medium is alkalized and stable, in which SiO2 particles are shown
After the surface is modified by alkalization, acidification and silicon dissolution, it can be used as an adhesive by dissolving with organic polymers.
At present, silica sol adhesive is used to manufacture ceramic shell. The process has many advantages: the preparation process is simple, the coating is easy to store, and the coating has been dipped and coated
The slurry can be continued to be used after adjusting the composition of the slurry, reducing the material consumption, the dip coating process performance is stable, easy to operate, and the use of ethyl silicate hydrolysate is canceled
Ammonia dry, the working environment is improved, the use of humidity controlled hot air drying, high shell type strength, ensure the quality of ceramic shell type and precision casting parts,
At the same time, water-based silicon solvent is used to ensure good safety.Its disadvantages: the wettability is not as good as ethyl silicate, the paint flow is poor, the bubble should not be eliminated;with
Water evaporates faster than alcohol, and is greatly affected by the humidity and temperature of the operating environment, resulting in a relatively long production cycle.
1, The characteristics of silica sol silicon
Sol is milky white transparent colloid.Like liquid sodium silicate and ethyl silicate hydrolysate, it is bonded by silicate colloid in the manufacture of ceramic shell
The basic component is amorphous SiO2. The particles have a large specific surface area and a negative charge. The hydrated silicon alcohol layer provides wet strength during the drying process.
During shell roasting, most of the free and structural moisture evaporates, and the maximum strength can be obtained to withstand the impact of the casting molten alloy.
One of the important properties of silica sol is its stability.The stability is related to the PH value, and the pH range is 8.5 ~ 10.Stable at PH 2 ~ 4,
The most unstable PH is around 7.The dielectric in the silica sol affects the charge distribution of the double-layer structure and reduces the stability of the silica sol.Preparation of silica sol formula
There are many methods, such as acidification method, dialysis method, ion exchange method, gel gel solution method, hydrolysis method, etc.
2, The choice of silica sol
The performance indicators of silica sol include SiO2 content, density, content of stabilizer, PH value, SiO2 particle size, etc., special silica sol needs surface modification
Sex or additive active agent.
3, The preparation of silica sol
For high temperature alloy, steel, titanium alloy, aluminum alloy and copper alloy castings, due to the casting temperature of the alloy to different ceramic shell lithofacies composition
It’s also different.But the selection of refractory powder must be able to be bonded by silica sol, and the preparation of paint at room temperature will not cause slurry gelation.Often makes
With particle size of 0.075mm or so of alumina, zirconia, mullite, aluminum ore and quartz glass powder, in order to improve the coating dip-coating, often also
To add suspending agent, mineralizing agent, penetrant, leveling agent and other auxiliary materials.Diluents are often deionized water or alcohol.The addition of powder must be edge addition
Stir into the edge to prevent excessive water absorption of local powder resulting in silica gel.After the formulation is selected, the properties of the coating are determined by its viscosity and density.As the
The properties of coatings are bound to change when they are impregnated.Water evaporation is reduced and viscosity is increased, so a little silicon sol and deionized water need to be added according to the calculated amount.
To ensure the original coating SiO2 content and viscosity.
4, Silica sol coating process of drying
The drying condition in the coating process is an important factor to ensure the quality. Shell drying is the transfer of moisture from the inside of the shell through capillary action to the coating
The surface and then it evaporates outward.Therefore by the air relative humidity, ambient temperature, airflow speed, drying time, shell material and a series of reasons
The influence of element.
In the drying process, the moisture from the shell surface evaporation and absorption of heat, which will make the wax mold temperature drop, size reduction, easy to produce surface defects.
The drying process should be as fast as possible to prevent the effects of changes in temperature on the inner surface of the shell.The ideal drying system should be able to take advantage of the temperature drop during the drying process
A large amount of moving air is required to balance the wet bulb temperature with the room temperature in the dipping chamber.
Weighing method and measuring resistance method can be used to monitor the drying of each layer during the drying process of the shell, until the shell is fully dried and then coated with the next layer.
After dewaxing and roasting, the mold shell becomes the shell shape of castable liquid metal. The performance of each batch of mold shell can be checked with the furnace sample.Made of different silica sol
The strength and deformation resistance of the ceramic shell can be seen from the data analysis of the strength level and the high temperature self-weight deformation performance of the domestic silica sol shell
It is close to the advanced type of silicon sol abroad.
Post time: Mar-22-2021