The surface crust refractories of the all-silica sol crust process have always used expensive zircon powder and zircon sand. According to statistics, the cost of shell-making materials accounts for about 30% of the cost of precision casting, refractory materials account for about 80% of the cost of shell-making materials, and the cost of surface layer materials accounts for 60% of the cost of the entire shell-making materials. Therefore, it is necessary to innovate the traditional process, improve the application of surface material and surface encrustation process, and reduce the production cost of surface encrustation.
Kaolinite belongs to the aluminum-silicon series refractory material. It is an aluminosilicate mainly composed of alumina and silica. The phase composition is mullite, glass phase, and quartz process. The characteristics of aluminum-silicon series materials are high refractoriness, smaller linear expansion coefficient than silica sand and corundum, good high temperature chemical stability and high thermal stability. Therefore, the clay-based refractory powder was applied to the back layer slurry of the water glass process in the 1970s, and was hardened with crystalline aluminum chloride, which was the so-called high-strength shell at that time. As for the back layer slurry and sand for the back layer of the silica sol process, the application of mullite powder and mullite sand has become a classic process.
Mullite powder is used as surface layer powder.
Add mullite powder to zircon powder. According to theoretical composition analysis, 1/3 of zirconium silicate is silica, 50% of aluminum-silicon materials are silica, and zirconium powder is mixed with mullite powder. , forming a triad mixture ZrO 2—Al 2 O ₃-SiO 2 , with good compatibility, and analyzing the crystal phase structure, it is theoretically feasible.
The surface layer mixing process is as follows: the mass ratio of powder to liquid is 3.4~3.5:1, zircon powder: 320 mesh mullite powder (clinker) = 1:1 viscosity value (Zhan’s 4#) is (34±1)s . Add 30mL wetting agent to 10kg surface layer silica sol, add 22mL defoamer to 10kg surface layer silica sol, sprinkle 90~120 mesh zircon sand on the surface layer, and dry the surface layer for 7~8h. The ambient temperature is 22~25°C and the temperature is 22%~25%. The second layer and later use mullite slurry and samolite sand, and the process parameters still follow the all-silica sol crusting process. The dewaxing and roasting process is also the same as the all-silica gel crusting process.
Production practice shows that this mixed slurry has good coating and hanging properties, is easy to operate, and helps to improve the strength and refractoriness of the surface layer. The surface roughness of the casting is close to that of the all-zircon powder slurry, with high dimensional accuracy, non-sticky sand, and the same collapsibility as the all-zircon powder slurry. The mixed surface layer slurry is suitable for larger castings with thicker walls, such as pumps and valve castings. Most of them use low-temperature molding materials, and the surface roughness and dimensional accuracy of cast 300 series austenitic stainless steel castings are very high, and cast carbon steel and low alloy steel are more ideal. With the surface roughness without shot blasting after shelling, the cost of shell making is 60%~70% lower than that of all-coal powder slurry.
Mollite is used as sanding material for the surface layer.
The two refractory materials, zircon and kaolinite, have good compatibility and composite properties, and mullite sand can be used instead of Haoying sand for the surface sand. The mixing process of the surface layer is as follows: the powder-liquid ratio (325 mesh flour) is 1:3.3~3.4, the viscosity value (Zhan’s 4#) is 42~45s, the wetting agent is 10kg surface layer silica sol plus 28mL; defoaming The agent is 10kg surface layer of silica sol plus 20mL, the surface layer is sprinkled with 100 mesh sand, and the surface layer is dried for 8~9h. The ambient temperature is 2225C, and the humidity is 22%~25%. The second and subsequent layers use mullite slurry and samolite sand, and the specific process parameters are the same as the full silica sol crusting process.
Production practice shows that the powder-to-liquid ratio and viscosity of the surface layer slurry should be appropriately increased accordingly, and the bonding force between the sand and the slurry layer is good. If you use a floating sand machine to sprinkle sand, the effect will be better and the drying time will be slightly longer. The surface roughness and dimensional accuracy of the casting are high, the quality is stable, the sand is not sticky, and the collapsibility is the same as that of zircon powder and zircon sand.
This process is suitable for castings with not very thick walls such as mechanical parts and plumbing pipe fittings, and can be used for wax molds of low and medium temperature molding materials. Casting 300 series austenitic stainless steel and precipitated stainless steel, the surface quality of the casting is very good. If carbon steel and low alloy steel are cast, the surface quality of the casting will reach the same level as that of the sand casting. The cost of shell making is 80% lower than that of sacoin sand.
Fused silica is used as the surface material.
In recent years, in foreign advanced industrial countries, the refractory materials used for shell making have undergone significant changes. The use of malocite is seldom, and fused silica has increased sharply, which is different from the domestic situation. The thermal expansion coefficient of fused silica is relatively the smallest among all refractory materials, the cracking and deformation tendency of the shell during dewaxing and roasting is the smallest, and the high temperature creep resistance of the shell is the best, making the casting dimension stable, so the fused silica is introduced Human surface materials have theoretical and practical basis.
Zircon powder-fused silica powder mixed slurry-sprinkling zircon sand process
The surface layer mixing process is as follows: zircon powder – fused silica powder = 85:15, the powder-to-liquid ratio is 1:3:2, the viscosity value (Zhan’s 4#) is 42~45s, and the wetting agent is 10kg surface layer silicon Add 30mL of colloidal sol, add 22mL of defoamer to 10kg surface layer of silica sol, and sprinkle 100~120 mesh zircon sand on the surface layer. The drying time of the surface layer is 6~7h, the ambient temperature is 22~25℃, and the humidity is 22%~25%. The second layer and subsequent layers are the same as the all-silica sol crusting process, and the specific process parameters are the same as the all-silica sol crusting process.
A long period of production and application shows that this mixed slurry has good adhesion to zircon sand, the slurry flows evenly, and the operability of slurry dipping and sanding is better than that of all zircon powder slurry.
Using medium temperature wax, the surface roughness of the casting is R a6.3~3.2. Dimensional accuracy CT 3~5, see table.
The mass fraction of fused silica powder added is generally 15%, if it exceeds 15%, burrs will appear on the surface of the casting. The fused silica used by the author is produced domestically, and there are certain differences in the purity of silicon dioxide and the process of transforming into amorphous form after melting and cooling compared with foreign products. . Therefore, the powder-to-liquid ratio of the slurry should not be too high, the fluidity is similar to that of all-zircon powder, the drying time can be shortened properly, and the collapsibility is better than that of all-zircon powder slurry. This process is suitable for the production of auto parts and precision mechanical parts, and the cost of shell making on the surface layer is about 25% lower than that of full zircon powder.
| Casting size /mm zircon powder paste, zircon sand | Special powder + fused silica slurry, special sand | |
|---|---|---|
| the inside diameter of 13.50士0.06 | 13.46 | 13.53 |
| outer diameter 20.80士0.05 | 20.82 | 20.80 |
| inner width 6.50 | 6.48 | 6.54 |
| wall thickness 182士0.05 | 1.84 | 1.86 |
| Groove width 4 | 4.04 | 4.04 |
| high 46 | 46.1 | 46.09 |
Zircon powder – fused silica powder slurry – sprinkling fused silica sand process.
For the process parameters such as slurry preparation and drying time, please refer to the above process. The surface layer is sprinkled with 100-120 mesh fused silica sand, and the second layer and subsequent layers are the same as the full silica sol encrusting process parameters. The production practice shows that the powder-to-liquid ratio of the slurry should not be too high, the fluidity is similar to that of full zircon powder, the shell-making operability is good, the drying time can be shorter than the drying time of the above-mentioned process, the dimensional accuracy is good, and it does not stick to sand. , the quality is stable, and the collapsibility is better than that of cobalt powder and diamond sand. The surface quality of castings is shown in Figure 2-27. Dimensional accuracy reaches the level of the process described in Table 2-11.
This process is suitable for wax molds of low and medium temperature mold materials for auto parts and general mechanical parts. It is recommended to cast carbon steel and low alloy steel products. The cost of shell making is about 40% lower than that of zircon powder zircon sand.
Fused corundum refractory material.
Fused corundum (commonly known as white corundum) is industrial alumina, which is melted at high temperature, cooled and crystallized into alumina, which is the most stable variant of alumina used in investment casting. Fused corundum has high melting point, high density, good thermal conductivity, small thermal expansion coefficient, compact structure and good chemical stability. Therefore, before the silica sol-zircon powder surface layer shell making process was introduced, the ethyl silicate hydrolyzed solution-corundum powder and sand shell making process had been widely used in military production and aviation industry.
The process of ethyl silicate hydrolyzate-corundum powder and sand encrustation is as follows. Ethyl silicate hydrolyzate: white corundum powder 20W =1:2:5, the viscosity of the first layer of paint is 22~30s (∮6mm, 100mL), sprinkle 70 mesh white corundum sand. The viscosity of the second layer of paint is 8~10s, sprinkle 46 mesh white corundum sand. The viscosity of the third layer of paint is 7~9s, sprinkle with 36 mesh brown corundum. The viscosity of the fourth layer of paint is 10~15s, sprinkle 20 mesh brown corundum sand. The viscosity of the fifth layer of paint is 10~15s, sprinkle with 16 mesh brown corundum. The sixth layer is only dipped in slurry and not sanded. Ambient temperature 18~27C, humidity 60%~80%. Coating drying process: 1.5~2h for self, 1.5~2h for air drying, 0.5h for ammonia drying. Chlorine gas flow rate: 10~15L/min for the surface layer, 10~20L/min for the reinforcement layer.
Control the quality of surface material.
Zircon sand is a high-quality shell refractory material for surface layer. Due to the different origins and grades of zircon sand, the mass fractions of zirconium dioxide and ferric oxide contained in it are significantly different. If the content of zirconium dioxide is low and the content of impurities such as ferric oxide is high, the decomposition temperature of the high-temperature shell will drop to 900°C, and amorphous silica will be precipitated, causing “black spots” and “spots” on the surface of the casting. Pitting”. In order to find out what the “pockmarks” and “black spots” are, and to explore the causes of the “pockmarks” and “black spots”, the author uses an imported energy spectrometer to detect the micro-components of the surface defects (black spots) , see Figure 2-28 for the element absorption peak diagram. The analysis report is shown in Table 2-8. It shows that the mass fraction of silicon dioxide is as high as 18.90%. The quality control department inspected the iron content of zircon powder and zircon sand recently used, and the ferric oxide reached 0.82%. Exceeding the standard of ferric oxide is the main cause of “pockmarks” and “black spots”.
| Element content % | |||
|---|---|---|---|
| OK | 30.08 | CrK | 22.35 |
| AIK | 0.57 | MnK | 20.12 |
| SiK | 18.9 | FeK | 4.1 |
| SK | 0.28 | NbK | 3.29 |
| CaK | 0.31 | total | 100 |
The manufacturer has very strict requirements on the chemical composition of the mixed sand used for investment casting. The silicon dioxide is listed as an impurity with a limit of 33.02% and the limit of ferric oxide is 0.03%. It can be seen that for natural mineral materials such as British sand, it must go through the surface layer mixing process of the subsequent silica sol-white corundum powder slurry-sprinkling white corundum sand process: the powder-liquid ratio is 1:3.43.45 (white corundum) deep processing, strict Only by removing impurities can good casting quality be guaranteed. The viscosity value (Zhan’s 4#) is 40~45s, the wetting agent is 10kg surface layer silica sol plus 27mL, and the defoaming agent is 10kg surface layer silica sol plus 19mL. Sand on the surface: 100 mesh white corundum sand for small pieces, 70 mesh white corundum sand for large pieces. The drying time of the surface layer is 8~10h; the ambient temperature is 22~25C, and the humidity is 22%~25%. The third layer and subsequent water glass encrustation process. The production results show that the density of fused corundum is relatively high (3.99~4.0g/cm), the powder-to-liquid ratio of the slurry can be prepared relatively high, the fluidity of the slurry is almost the same as that of British powder, and the viscosity of slurry and sand can be reduced. Operability is good, drying time can be shortened properly, quality is stable, surface roughness and dimensional accuracy are good. The disadvantage is that there is slight sticky sand in the deep hole and slot of the casting, and the collapsibility of this part is also poor. The process adopts low-temperature mold material, which is very suitable for making various valves and large valve bodies. The casting material is 300 series austenitic stainless steel. Figure 2-29 shows the surface condition of the 316 valve body after rough polishing. This process is a silica sol-water glass composite process, so the shell manufacturing cost is the lowest.
Application of fine casting special powder (sand) on the surface layer
Silica sol-special powder slurry for precision casting-sprinkle special sand for precision casting.
The raw material of precision casting special powder (sand) is fused corundum. The fused corundum is further processed, and the quality is improved by multi-stage gravity separation by flotation method. Iron is removed by strong magnetic separation method, and phosphorus, potassium and sodium are removed by pickling method. , calcium, magnesium, silicon, so as to increase the density of the particles, improve the shape of the particles, and add refractory materials such as promoting collapsibility. The production of special powder requires scientific particle size distribution and strict control of dust and moisture in the sand. Surface layer mixing process: powder-to-liquid ratio is 3.03.2:1 (special powder for precision casting), viscosity value is 3641s (E6100mL), wetting agent is 10kg surface layer silica sol plus 33mL, defoamer is 10kg surface layer Silica sol plus 25mL. Surface layer sanding: special sand for small pieces with 100-120 mesh, and special sand for large pieces with 80-100 mesh. The drying time of the surface layer is 8~10h. Ambient temperature 22~25C, humidity 22%~25%. The second and subsequent layers use mullite slurry and samolite sand, and the specific process parameters are the same as the all-silica sol crusting process. Mass production shows that the surface layer refractory material for precision casting has a high density (3.99g/cms), and the powder-to-liquid ratio of the material should not be very high. 1. The operability of sanding is good, the drying time of the surface layer is loose, the mold material of medium temperature is used, the dimensional accuracy and roughness are very good, the deep holes and slots of the casting are not sticky to sand, and the collapsibility is good. This process can be applied to both large and small castings. Wax molds of low and medium temperature molding materials can be used. Casting 300 series austenitic stainless cloth and hot steel, high-temperature alloys. Compared with sand, it is almost the same, and “black spots cannot be spotted” no longer appear. The shell making cost of this process is about 30% lower than that of zircon powder and zircon sand.
Zircon powder slurry-sprinkle special sand for precision casting.
The surface layer is made of silica sol colloidal powder (powder-liquid ratio 1:3.4, viscosity value 3~45s), sprinkled with 100~120 mesh precision casting sand, the second layer and subsequent layers are the same as the full cement shell process, and the surface is rough and precise And the dimensional accuracy is equivalent to the level of zircon powder zircon sand.
| Drawing size/mm | Casting size/mm | |
|---|---|---|
| Zircon powder slurry, sprinkle zircon powder | Special powder slurry, sprinkle special sand | |
| Middle hole8士0.05 | 8.08 | 8.06 |
| lefleft and right holes9.6士0.10 | 9.66 | 9.65 |
| Outer block 38.1 | 38.12 | 38.15 |
| Inside block 14 | 142 | 13.96 |
| width45士0.06 | 45.03 | 45.01 |
| length62.5士0.05 | 62.54 | 62.57 |
Special powder for precision casting fused silica slurry-sprinkle zircon sand.
Add 10% fused silica powder (domestic) to the special powder for precision casting to make the surface layer slurry, sprinkle zircon sand, the second layer and later, use the same all-silica sol crusting process, and the casting roughness and size cast out The precision is comparable to that of zircon powder zircon sand coating, and the collapse of the shell is very good. In addition, the amount of fused silica powder added should not exceed 10%, and the dimensional accuracy is shown in the table. The production cost of this process is about 40% lower than that of zircon powder zircon sand coating.
Hole diameter12 | 11.97 | 12 |
Outer circle diameter110士0.06 | 110.07 | 110.85 |
Inner circle diameter43.10 | 43.14 | 43.14 |
high28.20 | 28.18 | 28.23 |
Center distance85士004 | 84.97 | 85.02 |
Zircon powder-special powder fused silica slurry-sprinkle zircon sand.
The “three-in-one” slurry is applied to investment casting production, that is, 10% zircon powder + 80% fine casting special powder + 10% fused silica powder (domestic) as the surface layer slurry, sprinkled with zircon sand, the first The second layer and beyond are the same as the all-silica sol crusting process. This process is suitable for making small and medium-sized pieces. Low-temperature mold materials are used. The surface roughness and dimensional accuracy of castings are very good. The cost of shell making on the surface layer is 25% lower than that of fine casting special sand sprinkled with Haoying powder slurry.
Zircon powder – special powder – fused silica slurry – special sand for precision casting.
The “three-in-one” slurry is applied to investment casting production, that is, 10% zircon powder + 90% special powder for precision casting + 10% fused silica powder (domestic) as the surface layer slurry, sprinkle special sand for precision casting, The second layer and beyond are the same as the all-silica sol crusting process. This process is suitable for castings weighing 15-20kg. Low-temperature molding materials are used. The surface quality and dimensional accuracy of castings meet customer requirements. The shell-making cost of the surface layer is 20% lower than that of using zircon powder slurry and sprinkling special sand for precision casting.
Experience and experience
In investment casting production, according to the grade, cost performance, size, structure characteristics, and material type of the parts, on the premise of meeting the product quality requirements, the encrustation process is diversified and the implementation is reasonable The combination of materials, the targeted selection and use of powder and sand for the surface layer not only meet the performance and quality requirements of the shell, but also facilitate sand cleaning and shelling, which greatly reduces the cost of shell making.
