The characteristics of four shell-making processes widely used in the investment casting industry are analyzed and compared. From the quality comparison of investment castings, the water glass shell is poor, the composite shell, silica sol low-temperature wax shell take the second place, and the silica sol medium-temperature wax shell is the best. Compared with shell-making cost, water glass shell is the lowest, and silica sol wax shell is the highest.
A. Water glass shell;
B. Composite shell;
C. Silica sol shell (low-temperature wax);
D. Silica sol shell (medium temperature wax). The first three schemes all use low-temperature wax (mold).
1. Water glass shell
Over the years, the strength of the sodium silicate shell has doubled due to the improvement of the refractory for the back shell and the popularization and application of new hardeners. The surface quality, dimensional accuracy, and yield of castings have been greatly improved, and still occupy a large market share.
Low cost, the shortest production cycle, excellent shelling performance, and high permeability are still the advantages of any other shell molding process. However, the quality of castings, including surface roughness, number of defects, dimensional accuracy, yield, and repair rate, is worse than the other three processes.
2. Composite shell
To overcome the shortcomings of the above-water glass shell, many factories now use zircon and mullite silica sol shells for the first and second layers. The original sodium silicate shell process is still used for the back layer. It is an improved scheme combining the excellent surface quality of silica gel shell with the advantages of low cost and a short cycle of sodium silicate. Compared with sodium silicate shell, the surface quality of the casting is greatly improved, the surface roughness is reduced, the surface defects are reduced, and the repair rate is reduced. It can be applied to stainless steel, heat-resistant steel, and other high alloy steels. The production cycle is much shorter than that of low-temperature wax silica sol shells and similar to that of sodium silicate shells.
However, there are also some problems. The dimensional accuracy (including shape and location tolerance) of castings cast with composite mold shell is not comparable to that of silica sol mold shell. It is often necessary to increase the number of silica sol shell layers (generally at least 2 layers) to obtain high-temperature strength and prevent casting deformation when pouring large castings (above 10kg). It is difficult to apply a composite shell to a thin wall（ δ ≤ 3mm), small pieces, and extra small pieces (less than 50g). And because the high-temperature strength of the mold shell is lower than that of the silica sol mold shell, it is more likely to cause casting defects such as porosity, insufficient pouring, and cold shut. In a word, the permeability of a composite shell is not as good as that of a water glass shell and silica sol shell.
To sum up, the composite mold shell is an improvement of the water glass mold shell. It is superior to the former in terms of casting surface quality, yield, and repair rate, but it is still fundamentally different from the silica sol mold shell. In addition to the short production cycle and lower shell-making cost, its casting quality and stability are inferior to that of silica sol shell.
3. Silica sol (low-temperature wax) shell
This process has greater adaptability and superiority (compared with medium-temperature wax) when casting medium and large castings above 1kg, especially above 5kg.
Generally speaking, the quality requirements of medium and large castings, especially the requirements for surface roughness, dimensional accuracy, and geometric tolerance, are not too high, so it is unnecessary to use high melting points and medium-temperature wax. Medium-temperature wax requires high pressure (more than 6-7MPa) or liquid wax injection wax mold, and the equipment investment is large. Medium temperature wax thick large wax mold is easy to shrink, and deform and has a high cost. Low-temperature wax is easy to form and the equipment is simple, while the surface roughness of wax molds is similar.
This process is more stable than the composite mold shell in quality, especially since the dimensional accuracy of the casting is high. Because it has no sodium silicate, the mold shell has good high-temperature performance. After baking at 1000-1200 ℃, the mold shell has high permeability and strong creep resistance. It is not only suitable for thin-walled pieces, small and medium-sized pieces with complex structures but also can produce oversized pieces weighing up to 50-100kg, such as pumps, impellers, guide shells, pump bodies, ball valve bodies, valve plates, etc. For thin-walled small and medium-sized pieces or large pieces, a fork shell or shell lifting can be used for direct pouring in front of the furnace, which can achieve a high yield.
However, due to the use of low-temperature wax, most mold shells are dewaxed in water, and saponification residues inevitably enter the mold shell (especially when the composite mold shell and sodium silicate mold shell are dewaxed at the same time), which is easy to produce inclusions on the surface of the casting, and the repair rate is slightly higher, which is one of its shortcomings. Secondly, the long production cycle of shell making is its biggest disadvantage and deficiency, and there is a little dry corner in shell making, which will cause silica sol re-dissolution and shell crack during water dewaxing. The cost of silica sol shell mold (low-temperature wax) is higher than that of sodium silicate shell mold and composite shell mold, and the casting cost is correspondingly higher.
4. Silica sol (medium temperature wax) shell
This is an internationally common production process for precision castings. It has the highest casting quality and the lowest repair rate. It is especially suitable for small and medium-sized pieces (2-1000g) with high surface roughness requirements (Ra0.8-3.2) and high dimensional accuracy (CT3-CT5). However, due to equipment and cost constraints, it is rarely used for medium and large pieces (5-100kg).
However, the production cost is high, and the production cycle of warm wax silica sol shell is the same, which is much longer than water glass and composite shell, so it is not suitable for the production of 5-50kg medium and large pieces.
5. Concluding remarks
(1) Various shell molding processes have different application objects, and the selection is based on the quality requirements, price, and delivery date of castings. In comprehensive consideration, the correct selection of the most economical and reasonable shell-making process plan is the basis for ensuring the production of high-quality and low-cost castings.
(2) Although the sodium silicate shell has many advantages, the inherent disadvantages of the binder itself make it difficult to improve the quality of the casting and the quality stability is poor. In the future, it will be gradually replaced by the composite shell, especially the low-cost quartz silica sol composite shell.
(3) Silica sol is an ideal binder. Its shell quality is high, the casting quality is stable, and the repair rate is low. It is the development direction for the future. The application of quartz stone and fused quartz refractories in the surface layer shell mold and the promotion of quick-drying silica sol have greatly reduced and shortened its production cost and shell-making cycle, overcoming these two shortcomings. Silica sol shell mold (low-temperature wax or medium-temperature wax) will be widely used in the precision casting industry, and the highest casting quality is the most important indicator.
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