Advantages and disadvantages of precision casting shell making process

Date：[2023/10/20]

The advantages and disadvantages of the main shell making processes in domestic precision casting production are as follows:
1. At present, there are three widely used shell making processes in the production of precision castings in China: A. Water glass shell; B. Composite shell; C. Silicone sol shell; The process of D water glass shell has a production history of nearly 50 years in China, and its factory number still accounts for a considerable proportion among precision casting manufacturers in China. Over the years, with the improvement of refractory materials for the back layer shell and the promotion and application of new hardening agents, the strength of water glass shell has increased exponentially, and the surface quality, dimensional accuracy, and yield of castings have greatly improved. The low cost, shortest production cycle, excellent shelling performance, and high breathability are still unmatched by any other shelling process to this day.

1.1 Main problems
(1) The inherent disadvantage of water glass binder is its high Na2O content, and the high-temperature strength and creep resistance of the shell are far inferior to those of silicon solvent shell. In addition, the refractory material for the surface layer uses quartz sand (powder) with low quality and poor particle size distribution, so it is necessary to not obtain high-quality precision castings.
(2) The production conditions of the shell are poor, and there is a lack of strict production process and parameter control. Due to the strong corrosiveness of hardeners, dust removal equipment is rudimentary, and few workshops have production environments with constant temperature, humidity, and dust removal. The coating preparation, hardening, air drying, dewaxing and other processes that affect the quality of the shell and casting are rarely strictly controlled according to industry regulations. The temperature, humidity, wind speed, etc. at the air dried area of the shell are not controlled, so quality accidents such as batch scrapping often occur during high, low temperature or rainy seasons. In short, most factories remain in the manual workshop stage, relying on craftsmanship rather than scientific quality management for production. This is one of the important reasons for the unstable casting quality, high scrap rate, and repair rate of water glass shell for decades.
2. At present, many factories are using zircon powder and mullite powder for the first and second layers of composite shells, as well as silica sol shells. The back layer still needs to use the original water glass shell process. It is an improvement plan that combines the excellent surface quality of silica sol shell with the advantages of low cost and short cycle of water glass. Compared with water glass shell, its casting surface quality has been greatly improved, with reduced surface roughness, reduced surface defects, and reduced repair rate. The production cycle is similar to that of water glass shell.

2.1 Main problems
(1) Due to the retention of water glass adhesive in the back layer, the overall high-temperature strength and creep resistance of the shell are lower than those of the silica sol shell, and the dimensional accuracy and positional tolerance of the poured castings are not as good as those of the silica sol shell.
(2) The breathability of the shell is not as good as that of the water glass shell or the silica sol shell, and the high-temperature strength of the shell is not as good as that of the silica sol shell, which is easy to cause waste.
(3) The quality stability of composite shell castings is better than that of water glass, but it is far inferior to silica sol shell castings.
(4) Due to the use of expensive zircon powder as the surface layer, the cost of the composite shell is 4.5 times that of the water glass shell. If the back layer is made of mullite sand powder, the cost of the shell is almost the same as that of the silica sol shell, and the advantage of low cost is not obvious.
(5) Composite shells cannot use medium temperature wax. Medium temperature wax cannot be dewaxed using hot water. When dewaxing in a high-pressure kettle, due to high temperature and pressure, the medium temperature wax liquid will undergo intense saponification reaction with water glass and residual hardener in the back layer, and cannot be reused without recycling treatment.
3. In the 1960s, silica sol shell was used as a ceramic shell binder in the casting field, and immediately replaced the previously widely used water glass and ethyl silicate hydrolysate with its unparalleled advantages, which were complicated, polluting, and costly in preparation processes. Practice has proven that using silica sol solution as a ceramic shell adhesive has the following advantages: simple preparation process, easy storage of coatings, continued use of the slurry after adjusting its composition, reduced material consumption, stable coating process performance, easy operation, elimination of ammonia drying when using silica ester hydrolysis solution, improved working environment, use of controllable humidity hot air drying, high shell strength, and use of water-based silica sol, Good safety. It is said that after switching from ethyl silicate to silica sol, energy consumption decreased by 20-30%, the shell qualification rate increased by 15-20%, and the working environment greatly improved.
3.1 Silicone sol (low-temperature wax) shell low-temperature wax molding is easy and the equipment is simple. The surface roughness of the wax mold is not significantly different, and the quality of the process is stable compared to the composite shell, especially with high dimensional accuracy of the casting. Due to the absence of water glass, the shell has good high-temperature performance, high permeability after roasting, and strong creep resistance. It can be used for thin-walled parts, small and medium-sized parts with complex structures, and can also produce oversized parts weighing 50-100kg.

3.1.1 Existing problems
(1) Due to the use of low-temperature wax, most of the mold shells are dewaxed in water, and it is inevitable that saponification residues enter the mold shell, which can easily cause surface inclusions on the casting and result in a slightly higher repair rate.
(2) The long production cycle of shell making is its biggest drawback and deficiency, especially when producing large parts with deep holes and grooves, each layer is usually dried for 24-48 hours.
(3) The cost of silica sol shell (low-temperature wax) shell is 5 times higher than that of water glass shell and 17% higher than that of composite shell. The cost of castings is correspondingly high.
3.2 Silica sol (medium temperature wax) shell is a commonly used precision casting production process internationally, which has the highest casting quality and the lowest repair rate, especially suitable for small and medium-sized parts with high surface roughness requirements and high dimensional accuracy.
3.2.1 Existing problems
(1) The cost is high, and the production cost of its shell is 8 times that of water glass shell. It is also 25% higher than the low-temperature wax silica sol shell.
(2) The production cycle is the same as that of low-temperature wax silica gel sol shell, but much longer than that of water glass and composite shell.
(3) Medium temperature liquid wax and high-pressure wax injection are often used in the production of large parts. Thick wall wax molds are prone to shrinkage and the dimensional accuracy of castings is not very high. The requirements for dimensional accuracy and surface roughness of large and medium-sized parts are not as high as those of small parts, so silica sol (medium temperature wax) shells are less commonly used for large and medium-sized parts.