Analysis of Technical Characteristics and Improvement Direction of Precision Casting Shell.

Four shell-making processes widely used in China’s precision casting industry are analyzed and compared. From the quality comparison of precision castings, the water glass mold shell is poor, the composite mold shell, silica sol-low temperature wax mold shell is the second, and silica sol-medium temperature wax mold shell is the best. From the comparison of the cost of shell making, the water glass shell is the lowest, and the silica sol-medium temperature wax shell is the highest. Improvement measures were put forward for these four kinds of shell-making processes.

At present, the shell-making processes widely used in the production of precision castings in China are as follows :

A. water glass shell;

b. composite shell;

c. Silica sol shell (low-temperature wax);

D. Silica sol shell (medium temperature wax).

The first 3 schemes all use low-temperature wax (mold).

1. The process of water glass shell has a production history of nearly 50 years in China, and its factory points still account for more than 75% of China’s investment casting manufacturers. After half a century of unremitting efforts by colleagues in the precision casting industry, the application and research of water glass shell technology have reached a high level. Over the years, due to the improvement of the back layer shell refractory material and the popularization and application of new hardeners, the strength of the water glass shell has doubled. The surface quality, dimensional accuracy, and yield of castings have been greatly improved, and they still occupy a large market share and replace foreign sand castings for export in batches. Low cost, shortest production cycle, excellent shelling performance, and high air permeability are still advantages that any other shelling process can’t reach so far. However, the quality of castings, including surface roughness, number of defects, dimensional accuracy, yield, and repair rate, are all worse than those of the other three processes.

1.1 The main problems that exist

(1) The inherent disadvantage of a water glass binder is that the content of Na2O is high, and the high-temperature strength and creep resistance of the shell are far less than that of the silicon solvent shell (only 1/30-1/50 of it). In addition, the surface layer refractory uses quartz sand (powder) with low price and poor particle size gradation, and the hardener is still limited to the use of ammonia chloride, so high-quality precision castings cannot be obtained.

(2) The production conditions of the shell are poor, and there is a lack of strict control of the production process and parameters. Due to the strong corrosiveness of the hardener and the rudimentary dust removal equipment, few workshops have a production environment with constant temperature, constant humidity, and dust removal. Coating preparation, hardening, air-drying, dewaxing, and other processes that affect the quality of mold shells and castings are rarely strictly controlled according to the operating specifications stipulated by the industry. Such as regular detection of paint viscosity, smear weight, hardener concentration, pH value, etc. The temperature, humidity, wind speed, etc. of the air-drying part of the shell are not controlled, so the quality accident of batch scrapping often occurs in high, low temperatures or during the rainy season. In short, most factories remain in the stage of manual workshops, relying on craftsmanship rather than scientific quality management for production. This is one of the important reasons for the unstable quality of castings, high rejection rate, and high repair rate for water glass shells for decades.

1.2 Improvement direction

(1) Use high-purity silicon micro powder (vein quartz) to replace the commonly used low-grade quartz sand powder as the surface layer refractory material, and use the “double- peak ” type particle size gradation circular quartz powder to prepare the surface layer coating. It can not only improve the thermochemical stability of the surface layer shell but also obtain high powder-to-liquid ratio coatings. Our factory uses water glass ingredients with a modulus of 3.4 and a density of 1.28g/cm3, and the powder-to-liquid ratio can reach 1.4. See the technical requirements for silicon micro powder. After wet ball milling, single-tank precipitation, magnetic separation, and ion high-purity water treatment, it naturally forms a circular, bimodal particle size grading. This high-purity and low-impurity powder are more ideal than artificial grading. It has been practically applied in our company with good effect.

(2) Strengthen the on-site quality management of the shell-making process and operate according to industry standards. At the same time, the coating, sanding, and hardening site should be isolated from the drying of the shell. The latter controls temperature and humidity, while the former strengthens dust removal and anti-corrosion, which is beneficial to the stability of the shell quality and the improvement of the operating environment.

(3) Quartz-silica sol shell is used to replace the first and second layers of a quartz-water glass shell, and the water glass and ammonium chloride hardener of the surface layer and transition layer is completely eliminated. The calculation shows that the cost of castings only increases by 0.46 yuan/kg, and the production cycle of shell making is basically the same as that of water glass shells.

2. Composite shells In order to overcome the shortcomings of the above water glass shells, many factories currently use zircon and mullite-silica sol shells for the first and second layers. The back layer still adopts the original water glass shell process. It is an improved solution combining the excellent surface quality of silica gel shell with the advantages of low cost and short cycle time of water glass. Compared with the water glass shell, the surface quality of the casting has been greatly improved, the surface roughness is reduced, the surface defects are reduced, and the repair rate is reduced. It can be applied to high alloy steel such as stainless steel and heat-resistant steel. The production cycle is much shorter than that of the low-temperature wax-silica sol mold and is similar to the water glass mold.

2.1 The main problems that exist

(1) Since the back layer retains the water glass binder, the overall high-temperature strength and creeps resistance of the shell are lower than those of the silica sol shell. Its calcination temperature is limited to below 950℃. After 900 ℃, the deformation of the shell increased by 30%. The calcination temperature of the silica sol shell can reach 1000-1200 °C, and the shell does not deform before 1000 °C. Therefore, the dimensional accuracy (including shape and position tolerance) of castings cast with composite shells is not comparable to that of silica sol shells. When casting large (10kg or more) castings, it is often necessary to increase the number of silica sol shell layers (generally at least 2 layers) in order to obtain high-temperature strength and prevent casting deformation.

(2) Since the first two layers of the mold shell are the main factors affecting the air permeability of the mold shell after the water glass mold shell is changed to silica sol, the overall air permeability of the mold shell is greatly reduced. When the baking temperature is low and the holding time is not long enough, It often causes defects such as pores, insufficient pouring, and cold insulation in the casting, so the composite shell is difficult to apply to thin-walled (δ≤3mm) parts, small parts and extra-small parts (less than 50g). And because the high-temperature strength of the shell is not as good as that of the silica sol shell, it is more likely to cause the above-mentioned waste products. In short, the air permeability of the composite shell is not as good as that of the water glass shell and the silica sol shell.

(3) The quality stability of composite shell castings is worse than that of water glass but far less than that of silica sol shells. The back layer still retains the water glass binder. In order to reduce the cost, refractory materials with low prices and unstable quality, such as clay and granular sand, are still used, and the shell-making process control is the same as that of the water glass shell, resulting in poor quality stability of castings. Especially for large parts above 10kg and small parts below 1kg, the scrap rate and repair rate are higher than those of silica sol shells.

(4) Due to the use of expensive zircon as the surface layer, the cost of the composite shell is 4.5 times that of the water glass shell. If the back layer uses mullite sand powder, the cost of the shell is the same as that of the silica sol shell. The cost is almost the same, and the cost per kg of casting is only 1 yuan. The advantage of its low cost is not obvious.

(5) The compound shell cannot use medium-temperature wax. Medium-temperature wax cannot be dewaxed with hot water. During dewaxing in the autoclave, due to the high temperature and high pressure (170°C, 0.7MPa), the medium-temperature wax liquid will have a violent saponification reaction (white foamy saponification) with the water glass and residual hardener in the back layer. Processing cannot be reused. For the silica sol shell, low and medium-temperature waxes can be used without this drawback. In summary, the composite mold is an improvement of the water glass mold, which is superior to the former in terms of casting surface quality, yield, and repair rate, but there are still essential differences with the silica sol mold. In addition to the shorter production cycle and lower shell-making cost, the quality and stability of castings are not as good as those of silica sol shells.

2.2 Improvement direction

(1) Quartz is used instead of zircon sand for surface shell refractories. The surface quality of castings does not completely depend on the surface shell refractory material but is closely related to the binder, and also related to the wax material (surface roughness of the wax mold, saponification residue, etc.). Compound shells can only use low-temperature wax, and most of them are used in precision castings with medium surface roughness (Ra=6.3-12.5) and low dimensional accuracy (CT4-CT6). The practice has proved that the quartz-silica sol surface layer is used instead of zirconium The quartz-silica sol is completely feasible. This measure reduces the cost of each t casting shell from the original 4150-4830 yuan to 1360 yuan, which is only 460 yuan more than that of the water glass shell.

(2) Strengthen the quality management and environmental improvement of the shell-making process, especially the shell-making of the back layer (see Sections 1 and 2 of this paper for details).

(3) The back layer should use refractory materials with stable quality, excellent high-temperature performance, and relatively low cost, and at the same time, it should match the expansion rate of the surface layer shell refractory materials.

The following 2 commonly used backing materials are recommended.

①Refractory clay -quartz powder coating (50% each), sprinkled with granular sand (made by crushing and sifting refractory brick waste), its advantages are the wide source, low price, and its high-temperature strength and creep resistance of the shell are higher than Mullite, bauxite. The price is only 1/2-1/3 of bauxite. It is suitable for composite shells with zircon or quartz as the surface layer.

②Refractory clay -granular powder coating (volume ratio is 3:7), sprinkle with granular sand. This solution is only suitable for zircon composite shells. Some factories use mullite sand powder or bauxite for the back layer of the composite shell. Its coating performance is relatively stable, the shell is thin and easy to bake, but the cost is too high and the high-temperature performance of the shell is not as good as the previous two kinds of shells. Bauxite has poor shelling performance. As for the broken materials such as waste ceramic utensils, sulfur glass tiles, floor tiles, etc., the price is low, but they have not been fired at high temperature, the composition is complex, and the shell has a high tendency to crack at high temperature, and the refractoriness is low. After pouring (especially thick and large pieces) shelling is difficult and should not be used.

3. The process of silica sol (low-temperature wax) shell conforms to the national conditions. It has greater adaptability and superiority (compared with medium temperature wax) when casting more than 1kg, especially large and medium castings above 5kg. Generally speaking, the quality requirements of medium and large castings, especially the requirements of surface roughness, dimensional accuracy, and geometric tolerance are not too high, so it is not necessary to use high melting points and medium temperature wax. Medium-temperature wax requires high pressure (greater than 6-7MPa) or liquid wax injection into the wax mold, and the equipment investment is large. Medium temperature wax is thick and large wax mold is prone to shrinkage, deformation, and high cost. The low-temperature wax is easy to form and the equipment is simple, and the surface roughness of the wax mold is not much different. This process is more stable in quality than the composite mold, especially the casting has high dimensional accuracy, because it has no water glass, and the mold has good high-temperature performance. It can be used for thin-walled parts, small and medium-sized parts with complex structures, and can produce extra-large parts weighing 50-100kg, such as water pumps, impellers, guide casings, pump bodies, ball valve bodies, valve plates, etc. For thin-walled, small, medium, or large parts, fork shells or lift shells can be used for direct casting in front of the furnace, and a high yield can be obtained.

3.1 There are problems

(1) Due to the use of low-temperature wax, most of the mold shells are dewaxed in water, and it is inevitable that saponified residues will enter the mold shell (especially when the composite mold shell and water glass mold shell are dewaxed at the same time). Slightly higher, which is one of its drawbacks.

(2) The long production cycle of shell making is its biggest shortcoming and deficiency, especially in the production of large pieces with deep holes and deep grooves, each layer usually takes 24-48 hours to dry. Taking a 50kg dual-channel impeller as an example, it usually takes 10-15d to make the shell, and there is a slight dead angle that is not dry. When dewaxing with water, it will cause the silica sol to redissolve and the shell to crack.

(3) The cost of the silica sol mold (low-temperature wax) mold is 5 times higher than that of the water glass mold (the cost of making a shell per t of castings is 5,000 yuan), which is 17% higher than that of the composite mold. Casting costs are correspondingly higher.

 3.2 Improvement direction

The following measures should be taken to prevent the occurrence of saponification inclusion defects in castings due to incomplete recovery of low-temperature wax and water dewaxing from sharing the same hot water tank with the composite mold or water glass mold.

①The use of steam dewaxing (steam pressure 0.2-0.4MPa, temperature 120-130℃) instead of water dewaxing can not only prevent the inclusion of saponified substances but also prevent cracks in the mold shell, which is more guarantee the quality of castings.

②If hot water is used for dewaxing, industrial hydrochloric acid with a volume fraction of 1%-3% should be added to the water, and after dewaxing, each group of shells should be washed with hot water containing hydrochloric acid to reduce the residue of saponification. Try not to share the same tank of water for dewaxing with water glass shells and composite shells. You can also replace the water and dewax separately to reduce saponification into the shell.

③ Recycled wax can be treated with acidified water with a volume fraction of 3%-5% hydrochloric acid, and the boiling and precipitation time should be long enough. In winter, the temperature of the hardening water is low, the residual amount of Na2O in the water glass and the composite shell is high, and the saponification of the wax is also serious. More hydrochloric acid should be added to treat the recovered wax to reduce saponification. After the wax treatment, it is also important to add stearic acid in time.

(2) In order to shorten the production cycle of shell making, “quick-drying silica sol” can be used to make shells. This process has become more and more mature, and the drying time of each layer of the shell can be shortened by more than 1/2. Each layer of small pieces (except the last layer) only takes 3 hours to dry, and the shell-making time is shortened from 63 hours to 24 hours. Medium and large pieces are also 50% shorter than general silica sol. However, its market price is only increased by 20-30%, which can be fully compensated by the reduction of space and power consumption and the improvement of productivity. The popularization and application of quick-drying silica sol is the only way the reform the silica sol shell-making process, and its application will be gradually expanded.

(3) In order to reduce the cost of the silica sol shell, the most effective method is to use quartz instead of zircon as the surface layer shell refractory. At present, zircon refractory materials account for 60% of the total cost of silica sol shell making. After switching to quartz, the cost of shell making per ton of castings is reduced from 5,000 yuan to 2,210 yuan, a decrease of 55.8%. For medium and large pieces, fused silica sand (powder) can be used to replace zircon sand (powder), which has been gradually popularized and applied.

4. Silica sol (medium temperature wax) shell This is an international common precision casting production process. It has the highest casting quality and the lowest repair rate, especially suitable for high surface roughness requirements (Ra0.8-3.2), size Small and medium-sized pieces, and extra-small pieces (2-1000g) with high precision (CT3-CT5 grade). However, due to equipment and cost constraints, it is less used in medium and large pieces (5-100kg).

4.1 There are problems

(1) The cost is high, and the production cost of the shell is 8 times that of the water glass shell. It is also 25% higher than the low-temperature wax-silica sol shell. The main reason is that the cost of shell and wax mold materials is high, the equipment power consumption is also much larger, and the equipment investment is also large.

(2) The production cycle is the same as that of the low-temperature wax-silica sol shell, much longer than that of water glass and composite shells.

(3) For the production of medium and large parts of 5-50kg, medium temperature liquid wax (65-70°C) and high pressure (4.0-7.0MPa) are often used for wax injection. Thick-walled wax molds are easy to shrink and concave, and the dimensional accuracy of castings is not too high. The requirements for dimensional accuracy and surface roughness of medium and large parts are not as high as those of small parts, so silica sol (medium temperature wax) shells are less used for medium and large parts.

4.2 Improvement direction

(1) In order to reduce costs and ensure quality, after solving the problem of poor wettability of quartz to medium temperature wax, it is undoubtedly a direction to use quartz stone or fused silica instead of zircon. The thermal expansion coefficient of fused silica is only 5×10-7/℃, and its price is only 1/6 of that of zircon. In foreign countries, fused silica has gradually expanded its application range.

(2) The use of quick-drying silica sol to shorten the shell-making cycle is the direction of the joint efforts of Chinese and foreign counterparts.

(3) The development of domestic medium-temperature wax or improved paraffin-stearic acid low-temperature wax is an important task in my country’s precision casting industry. How to solve the problem of domestic medium-temperature wax or improved low-temperature wax recycling and processing, so that it can keep the wax material performance unchanged for a long time in production is the key to popularization and application.

5. Conclusion

(1) Various shell processes have their different applicable objects, and the selection is based on: quality requirements, price, and delivery time of castings. Comprehensive consideration and the correct selection of the most economical and reasonable shell-making process is the basis for ensuring the production of high-quality and low-cost castings.

(2) Although the water glass shell has many advantages, the inherent shortcomings of the binder make it difficult to improve the quality of the casting, and the quality stability is also poor. In the future, it will be gradually replaced by composite shells, especially low-cost quartz-silica sol composite shells.

(3) Silica sol is an ideal binder, and its high-quality shell, stable casting quality, and low repair rate are the future development direction. The application of quartz stone and fused silica refractories in surface shells and the promotion of quick-drying silica sol greatly reduce and shorten the production cost and shell-making cycle, and overcome these two deficiencies. Or medium temperature wax) will be widely used in my country’s precision casting industry, after all, high casting quality is the most important indicator.

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