Analysis of the characteristics of four shell making processes in precision casting and discussion on the improvement direction
The characteristics of four shell making processes widely used in domestic investment casting 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. The improvement methods for these four shell making processes are proposed.
At present, there are four shell making processes widely used in domestic production of precision castings:
A. Water glass shell;
B. Composite shell;
C. Silica sol shell (low-temperature wax);
D. Silica sol shell (medium temperature wax). The first three plans use low-temperature wax (mold).
Our company has 4 kinds of processes to fully satisfy the different demands of the market for the quality and price of investment castings, and increase the competitiveness and adaptability of the market.
3.1 Existing problems
(1) Due to the selection of environment-friendly low-temperature wax, most mold shells are dewaxed in water, and it is inevitable that saponification residues enter the mold shell (especially when the composite mold shell and sodium silicate mold shell are dewaxed together), which is easy to produce casting surface blending, and the repair rate is slightly high, which is one of its disadvantages.
(2) Long production cycle of shell making is its biggest disadvantage and lack (see Table 1), especially in the production of large pieces with deep holes and grooves, each layer is usually 24-48h monotonous. Take 50kg double channel impeller as an example, usually 10-15d
During shell making, there is a little dry corner, which will form silica sol re dissolution and shell crack during water dewaxing.
(3) The cost of silica sol shell (low-temperature wax) shell is 5 times higher than that of sodium silicate shell (the cost of shell making per t casting is 5000 yuan), and 17% higher than that of composite shell. The casting cost is relatively high.
3.2 Improvement direction
(1) The following methods shall be adopted to avoid casting saponification due to incomplete recovery of low-temperature wax and sharing the same hot water tank with composite mold shell or water glass mold shell when dewaxing with water.
① Steam dewaxing (steam pressure 0.2-0.4MPa, temperature 120-130 ℃) is selected to replace water dewaxing, which can not only avoid the mixing of saponifiers and avoid cracks in the mold shell, but also ensure the stability of casting quality.
② If hot water is selected for dewaxing, industrial hydrochloric acid with a volume fraction of 1% - 3% shall be added to the water. After dewaxing, each group of shell shall be washed with hot water containing hydrochloric acid to reduce the residue of saponification. Try not to use the same tank of water for dewaxing with water glass shell and composite shell, or replace the water solution and dewaxing alone to reduce the amount of saponified matter entering the shell.
③ The recovered wax can be treated with acidified water with the volume fraction of hydrochloric acid of 3% - 5%, and the entertainment and accumulation time should be long. In winter, the temperature of hardening water is low, the residue of Na2O in water glass and composite shell is high, and the saponification of wax is severe. Therefore, more hydrochloric acid should be added to recover wax to reduce saponification. It is also important to add stearic acid in time after wax treatment.
(2) In order to shorten the production cycle of shell making, "quick drying silica sol" can be selected for shell making. This process has become increasingly old, and the monotonic time of each layer of shell can be shortened by more than 1/2. Each layer of small parts (except the final layer) is monotonous only for 3h, and the shell making time is shortened from 63h to 24h. Medium and large pieces are also 50% shorter than ordinary silica sol. However, its market price is only 20-30%, which can be completely compensated by the reduction of place, power consumption and production rate. The promotion and application of quick drying silica sol is the only way to innovate the silica sol shell making process, and will gradually expand the application.
(3) In order to reduce the cost of silica sol shell mold, the most useful method is to choose quartz instead of zircon as the surface layer shell refractory. At present, zircon refractories account for 60% of the total shell making cost of silica sol. After the use of quartz, the shell making cost per t of castings is reduced from 5000 yuan to 2210 yuan, down 55.8%. Fused quartz sand (powder) can be used to replace zircon sand (powder) for medium and large pieces, which has been gradually popularized.
4. Silica sol (medium temperature wax) shell
This is an internationally common production process for investment castings. It has the highest casting quality and the lowest repair rate. It is especially suitable for small and medium-sized pieces and special small pieces (2-1000g) with high surface roughness requirements (Ra0.8-3.2) and high specification accuracy (CT3-CT5). However, due to equipment and cost bundling, it is rarely used for medium and large pieces (5-100kg).
4.1 Existing problems
(1) The cost is high, and its shell production cost is 8 times that of water glass shell. It is also 25% higher than low-temperature wax silica sol shell. The main reason is that the cost of shell making and wax model materials is high, and the power consumption of equipment is much higher, and the investment in equipment is also large.
(2) The production cycle is the same as that of low-temperature wax silica sol shell, which is much longer than that of sodium silicate and composite shell (Table 1).
(3) Medium temperature liquid wax (65-70 ℃) and high pressure (4.0-7.0MPa) wax injection are often used for medium and large pieces with a capacity of 5-50kg. Thick wall wax molds tend to shrink, and the specification accuracy of castings is not too high. Medium and large pieces also have less requirements on specification accuracy and surface roughness than small pieces. Therefore, silica sol (medium temperature wax) shells are less used for medium and large pieces.
4.2 Improvement direction
(1) In order to reduce the cost and ensure the quality, after dealing with the problem of poor wettability of quartz to medium temperature wax, it is undoubtedly a direction to choose quartz or fused quartz to replace zircon. The thermal expansion coefficient of fused quartz is only 5 × 10-7/℃, and its price is only 1/6 of zircon. In foreign countries, fused quartz has been gradually expanded in application planning.
(2) Using fast drying silica sol to shorten the shell making cycle is the direction of joint efforts of domestic and foreign peers (see the above).
(3) It is an important task for China's investment casting industry to develop domestic medium temperature wax or improve white wax stearic acid low temperature wax. How to deal with the problem of recovery and treatment of domestic medium temperature wax or improved low temperature wax, so as to keep the wax function unchanged for a long time in production is the key to whether it can be popularized.
5. Concluding remarks
(1) Various shell molding processes have different applicable policies. The selection is based on the quality requirements, price and delivery date of castings. In summary, 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 shortcomings of the binder make it difficult to travel the quality of the casting, and the quality stability is 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 ambitious binder. Its high shell quality, stable casting quality and low repair rate are the future development direction. 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 shortages. Silica sol shell mold (low temperature wax or medium temperature wax) will be widely used in China's precision casting industry. After all, high casting quality is the most important goal.
Qingdao Xinnuoke Holdings Group limited company.