The traditional surface layer shell-making process uses 100% zirconia powder, and the price of zirconia powder and zirconia sand is high due to the lack of zirconia resources in China.
The price of zircon powder and zircon sand is high due to the lack of zircon resources in China. Fused silica has abundant domestic resources and is cheap. Zirconia powder plus fused silica powder is prepared into a mixed slurry and the two kinds of powder with different densities are fully intermixed, the slurry is evenly mixed and the fluidity, coverage and dispersion of the slurry present good performance. Not only the surface roughness and dimensional accuracy of the castings are comparable with 100% zirconium powder slurry, but also it is convenient for sand cleaning and the cost of shell making is greatly reduced.
Australian zircon has excellent casting performance with good thermal conductivity, good thermal stability and high heat storage capacity.
Domestic fused silica, especially from Lianyungang, Jiangsu, has 99.9% silica content and a melting point of 1713°C. The coefficient of linear expansion of fused silica is the smallest among almost all refractory materials, and the thermal shock stability of fused silica is excellent, from 1100°C to 20°C.
The thermal shock stability of fused silica is so good that it can be dropped suddenly from 1100°C to 20°C in cold water without damage.
The high purity of fused quartz, the good stability of the slurry mixed with zirconia powder, the better compatibility of the shells made, the advantages of both
The advantages of both are that they prevent the shells from cracking and deformation during the dewaxing and baking process, and achieve low surface roughness values of the castings while
It also ensures the dimensional stability of the castings. In addition, the cost of shell production is greatly reduced, and it facilitates sand cleaning.
Preparation of mixed slurry
Amount of homogenizer added
33kg of silica sol and 2.5kg of homogenizer, stirring for 3~5min.
The density of zircon is 4.5g/cm³, while the density of fused silica is 2.2g/cm³, the difference between the two densities is more than double.
In order to overcome the layering tendency of the slurry, the two refractory powders can be integrated together, and the homogenizing agent of the slurry is a synthetic additive of several polymers.
The amount of powder added.
Then add 82kg of 325 mesh zirconium powder, then add 35kg of 270-325 mesh fused silica powder and stir for 24 hours. Mix for 24h.
Among the above powders, zirconia powder accounted for 70% and fused silica powder accounted for 30%. The mass to liquid ratio of the mixed slurry was 3.54:1.
The viscosity value of the mixed slurry was measured
After 24h mixing, the viscosity value was 41s (4# Jen’s flow cup).
It was found that under the same powder-liquid ratio, the viscosity of 100% zirconia powder slurry and 70% zirconia powder + 30% fused silica
The viscosity value of the mixed slurry is 1~2s higher than that of the homogenized slurry.
Determination of the quality of the mixed slurry smear
The smear was 40mm×40mm×2mm stainless steel sheet, and the smear was immersed in the slurry for 5S.
The smear is dipped into the slurry for 5S, and the smear is dripped for 1min, and the slurry dripped after 1min is retained, and the difference in quality before and after dipping is calculated.
The smear mass of the mixed slurry is 0.802g.
Chemical mechanism of homogenizer
Homogenizer is a combination of a variety of water-soluble polymeric organic compounds, easily soluble in water, resistant to Acid, alkali, salt, non-toxic, non-hazardous, non-polluting, non-flammable, non-explosive water-based viscous colloid.
In aqueous solution, the homogenizer colloid usually carries both positive and negative ions, while the surface of quartz powder colloid is usually Through electrostatic attraction, the homogenizer molecules act like a bridge to combine these two molecules to form a macromolecular group bond, which in turn makes the entire refractory slurry less prone to delamination and the refractory coating components relatively uniform and stable.
In order to improve the coating performance of the paste and eliminate the air bubbles in the paste, it is necessary to add penetrant (wetting agent) and defoamer in the top layer paste, however, there are cationic, anionic and non-ionic penetrants in the market, and there are many manufacturers, and there is no national standard for penetrant and defoamer. A small amount of increase, should be used and homogeneous agent chemical properties do not conflict with the penetrant and defoamer.
Coated products
Fittings.
Right angle elbow, material 304, 18 pieces/group, single piece mass 200g.
After the actual dipping operation, it was found that the flowability of the mixed slurry was better than that of 100% zirconia powder slurry, and the flow was smoother than that of 100% zirconia powder slurry, which was considered normal. The slurry was evenly distributed on the wax mold, the leveling performance was good, the slurry did not accumulate, and the sand layer was very even after skimming zirconia sand.
The drying time of the top layer is 8~10h, and the second layer and later is made of molai powder slurry sprinkled with molai sand. Dewaxing, roasting and fusing and pouring are carried out according to the normal process.
Fused quartz undergoes volume change when α square quartz is transformed into β square quartz, which can prompt micro-cracking and flaking, and the improvement of permeability and collapse of the mold shell is found when cleaning up.
Casting morphology.
The surface quality of the pipe fittings is required to be A grade, and A grade is the highest required level. The pipe fittings castings were identified by the QC department as not only having a low roughness value and good finish, but also having a fine surface, and the 70% zirconium quartz powder + 30% fused silica powder mixed slurry is better than the pipe fittings products made with 100% zirconium quartz powder slurry. After several batches of verification, the castings were dimensionally stable and the surface roughness remained excellent.
(iii) 50% each of zirconium quartz powder and fused silica powder. The zirconium quartz powder was reduced from 70% to 50% and the fused silica powder was increased from 30% to 50%. This configuration requires a high quality homogenizer to function, so we made further optimization and improvement of the homogenizer. The slurry preparation and related process data are as follows.
- Slurry preparation.
Silica sol 35kg+homogenizer 3.5kg, stir for 5min.
Add zirconium powder 59kg, add fused silica powder 59kg. stir for 24h.
- Slurry test.
Viscosity value: 39s, 4# Jen’s measuring cup.
The coating quality: 0.778g, which is normal because the proportion of fused silica powder increases, and the coating quality decreases slightly.
The powder-liquid ratio of the mixed slurry was 1:3.37.
The amount of homogenizing agent added is 2.2% of the slurry mass, i.e. 3.5/(35+58+59) × 100%.
- Shell products
Elbow flange, material 304, 16 pieces/group, single piece mass 250g.
Pump body, material 301, 1 piece/group, single piece mass 6.15kg.
- Shell-making process. Top layer.
Dip 50% zirconium quartz powder + 50% fused silica powder mix slurry, sprinkle zirconium quartz sand. Second to fifth layer, according to the conventional process.
- Stability of slurry
After 50% zirconium quartz powder + 50% fused silica powder mixed slurry is done with the top layer, continue to stir in the slurry barrel
The viscosity value of 41s was measured after suspending the production for three days, and the coating test was good.
Pouring results：
| Bend flange size accuracy comparison table （ Unit ：mm） | ||
|---|---|---|
| Location/Craftsmanship | 100% Zirconium quartz powder | 50% Zirconium quartz powder + 50% Fused silica powder |
| Outer diameter | 37.58 | 37.35 |
| Outer diameter | 37.29 | 37.20 |
| Outer diameter | 78.33 | 78.31 |
| Outer diameter | 47.65 | 47.65 |
| Outer diameter | 35.32 | 35.31 |
| Outer diameter | 34.98 | 34.99 |
| Inner diameter | 30.53 | 30.52 |
| Inner diameter | 30.23 | 30.22 |
| Inner diameter | 30.09 | 30.07 |
| Inner diameter | 29.86 | 29.87 |
| Inner diameter | 9.23 | 9.17 |
| Inner diameter | 9.22 | 9.14 |
| Center distance | 50.92 | 50.90 |
| Center distance | 69.37 | 69.18 |
