Discussion And Practice of Water Glass Casting Process Upgrading Scheme

Views: 16     Author: Site Editor     Publish Time: 2024-09-20      Origin: Site

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Discussion And Practice of Water Glass Casting Process Upgrading Scheme

Abstract: In view of the high pollution of traditional water glass investment casting, the process upgrading scheme was discussed, and the silica sol thin shell process was confirmed as the best scheme. The process was optimized and upgraded from the aspects of mold material, tree formation, shell making and wax recovery, which completely eliminated the pollution of acid fog and sodium salt, and also improved the quality of castings.


Investment casting is a near-net forming process, which is more suitable for the production of small and medium-sized complex investment casting. According to the type of coating binder, split investment casting can be divided into three main processes, such as water glass shell, silica sol shell and ethyl silicate shell, among which the first two are the mainstream processes widely used in domestic investment casting enterprises. Water glass adhesive investment casting (referred to as water glass investment casting) process has been applied in China for nearly 60 years, during which although a lot of improvements and innovations have been made in shell materials, hardeners and process control, the problem of high pollution has not been fundamentally solved.


1.The remaining magnesium chloride breaks down into highly corrosive hydrogen chloride gas.

a. The main molding material is mainly paraffin and stearic acid, which is easy to saponification reaction during use. Hydrochloric acid is usually used for recycling, and the pollution of hydrochloric acid fog is serious; The poor heat resistance and low strength of the mold lead to high dropping rate in the process of shell making and large discharge of waste mold shell.

b. The chemical hardening of the shell process produces a large amount of sodium salt and hydrogen chloride acid fog pollution, and the hardening of ammonium chloride also increases the ammonia pollution. Magnesium chloride instead of aluminum chloride or ammonium chloride as hardener, environmental protection and has been improved, the hardening process also produces sodium salt, residue in the roasting process

c. Type shells have high sodium oxide content and low high temperature strength, and increase the thickness to ensure the overall strength leads to high shell refractory consumption and roasting energy consumption.

d. When dewaxing, ammonium chloride or hydrochloric acid is used to supplement hardening, there is ammonia gas or hydrogen chloride acid fog pollution.

e. The current ammonium chloride hardening water glass shell process has been listed as a backward process by the state, hydrochloric acid is also prohibited in many places, with the increase of environmental pressure, the transformation or upgrade of the traditional water glass shell process is imperative.


2 Discussion on process upgrading scheme

In general, there are two ideas to solve the problem of high pollution in water glass precision casting process. The first is to upgrade the main process, that is, upgrade the mold and hardener, adopt high-strength mold and environmentally friendly hardener, cancel the hardening of chlorine salt in shell making, and cancel the use of ammonium chloride or salt acid in dewaxing and wax treatment, so as to eliminate the generation from the root. The second is to use a new environmentally friendly process to replace the existing water glass precision casting process. Based on domestic research, at the implementation level, there are mainly 4 mature process schemes in application, and the description and comparison of the schemes are shown in Table 1.

Scheme one is the ester hardening water glass shell process, that is, the sand layer is hardened with organic ester, which has the advantages of no major adjustment of the process layout and low cost of equipment modification. However, because the half layer is hardened with aluminum chloride, ammonium chloride or hydrochloric acid are still used for dewaxing and wax treatment, and the pollution problem of sodium salt and hydrogen chloride cannot be completely eliminated. The problem of low mold and shell strength has not been solved.

The second scheme is the coated sand shell type process, which belongs to the process replacement, and its biggest advantage is that the production cycle is short, the deficiency is only suitable for the production of some products, and there is also the problem of poor environmental protection. The use of inorganic binder instead of resin to solve environmental problems is the development direction of this process, and no technical breakthrough has been achieved.

The third plan is the silica sol thin shell process, which does not need to make major adjustments in the process layout, and the shell making cycle and cost are better than the water glass shell process, eliminating the pollution of sodium salt, hydrogen chloride gas and ammonia gas, and the casting quality is also improved.

The fourth is the mainstream silica sol process in the investment casting industry, which has good environmental protection, the dimensional accuracy of the castings can reach CT4~CT3, and the surface quality has been improved. The deficiency is that the shell making cycle is long and the shell making cost is high, and it is generally suitable for the production of products with high added value. With the industry mainstream silica sol process to replace the traditional water glass process, mold making, shell making and dewaxing need to re-invest equipment, coupled with the shell cycle and cost constraints, for water glass investment casting enterprises, it is not easy to fully accept the program.

By comparing the OCDE(quality, cost delivery time and environmental protection) indexes of the above four schemes, scheme 3 (silica sol thin shell process) is the best scheme to upgrade the sodium silicate casting process.


3 Application practice of silica sol thin shell process

Based on the needs of environmental protection and quality improvement, Dongfeng Investment Casting has been committed to the research of water glass process upgrading technology in recent years, optimizing and upgrading from the aspects of mold material, tree formation, shell making and wax recycling, and successfully developed the "silica sol thin shell process" with environmental protection, cost and quality advantages.


3.1 Mold optimization

The performance control of moulds is the key link to ensure the quality of castings and the emission of pollutants. The main purpose of mold optimization is to increase the strength and reduce the modulus drop rate, while improving the heat resistance and reducing the deformation of the wax mold. Under the premise that the molding equipment does not make large adjustments, the mold material optimization mainly has two schemes: the first scheme is the use of new low molecular mold material, which is suitable for new casting workshops or factories; The second plan is to strengthen the existing paraffin-stearic acid molding material to improve its strength and heat resistance.

3.1.1 Application of new low molecular weight moulds

Using the technical solution of "paraffin + low molecular weight polyethylene + third component (compound strengthening agent)", supplemented by three levels of boiling, two levels of standing and a recovery process of strengthening, a new type of low molecular weight mold was successfully developed, which was applied in batch in the new factory in Anhui Province in October 2013. The bending strength of the material can reach 3.5MPa stably, the strength of repeated use has no attenuation, the mold material has no "slagging", and the molding efficiency is the same as that of paraffin stearic acid. In view of the sharp decline in recovery performance and great impact on strength after mixing with stearic acid, it is very risky to switch to low molecular moulds in workshops that originally used paraffin-stearic acid moulds.

3.1.2 Paraffin-stearic acid molding material strengthening

Low molecular molding materials mixed with stearic acid will lead to a sharp deterioration in the performance of wax materials. In view of the difficulty in completely removing stearic acid from wax presses, wax storage barrels, recovery barrels and wax delivery pipelines, there is a greater quality risk in the workshop that originally used paraffin-stearic acid molding materials to switch to low molecular molding materials. The priority is to strengthen and upgrade the original paraffin-stearic acid molding materials to greatly improve its comprehensive performance .

There are many strengthening schemes for paraffin wax and stearic acid mould material. The relatively simple scheme is to replace the usual 58 or 60 with a high number of paraffin wax, such as No. 66 or No. 70. The bending strength of the mould material is increased by about 10% and the heat resistance is increased by 2~3℃. The composite strengthening scheme is adopted, that is, the special fortifying agent made by mixing a variety of materials such as microcrystalline wax is added to the molding material in proportion. Under the premise of keeping the shrinkage rate unchanged, the flexure strength of the composite reinforced mold can be increased by 20% to more than 3.2MPa, and the heat resistance can be increased by 3~4℃. The special fortifying agent itself has good strength and can refine the paraffin crystal at the same time, so the strength stability is better


3.2 Wax module tree process upgrade

Investment casting enterprises using wax-based mold materials have long been using heated welding knives for wax mold tree, and the paraffin smoke concentration reached 6mgm 'when the tree was formed, which was 3 times the upper limit of the national standard. In order to solve the problem of excessive paraffin smoke and dust, the technology of bonding wax group tree based on wax base mould material was developed and applied in batch. For the wax base mold, the key point of the process is to choose the right bonding wax and the choice of bonding wax

You can refer to the following three aspects. a. has no effect on the performance (such as strength, shrinkage, etc.) of the existing mold material;


3.3 Shell making process upgrade

All water glass shell or silica sol-water glass composite shell process has the advantages of short shell cycle and low cost, but its most fatal disadvantage is poor environmental protection, and there are pollutants such as sodium salt and hydrogen chloride gas emission problems. The silica sol shell adopts the drying and hardening method to promote the silica sol gel, which overcomes the pollution problem caused by the chemical hardening of the sodium silicate shell. At the same time, due to the low sodium oxide content, the high temperature strength of the shell, the thickness of the shell is only 30%-50% of the water glass shell, and the consumption of refractory materials is less. The all-silica sol shell process used in the investment casting industry, its shell production cycle and cost are usually 2 to 3 times that of silica sol-water glass composite shell, and the insufficiency of these two aspects restricts the full application of this process.

How to optimize and upgrade the water glass shell process has been carried out for 5 years, and the final upgrade plan is to develop a low-cost and short-cycle all-silica sol shell process instead of the original silica sol-water glass composite shell process. The premise of the project is as follows.

a. The shell line cannot be extended, and the online shell making time remains unchanged:

b. The comprehensive cost of shell making cannot be increased:

c. Shell quality (surface roughness and strength, etc.) meet the requirements. The research group took the silica-sol shell strengthening as the cutting point and carried out composite strengthening of the coating through polymers, fibers and inorganic strengthening agents. Figure 2 and Figure 3 show the sectional comparison of the silica-sol coating before and after composite strengthening. After strengthening, the connection between the coated sand powder is more uniform, and the comprehensive performance of the silica-sol shell is also improved. At the same time of coating strengthening, the best combination of "temperature + humidity + wind speed" can realize the rapid drying of the coating, and the drying time of the back layer of silica sol coating is controlled within 1h. Environmental protection is poor, there are pollutants such as sodium salt and hydrogen chloride gas emission problems. The silica sol shell adopts the drying and hardening method to promote the silica sol gel, which overcomes the pollution problem caused by the chemical hardening of the sodium silicate shell. At the same time, due to the low sodium oxide content, the high temperature strength of the shell, the thickness of the shell is only 30%-50% of the water glass shell, and the consumption of refractory materials is less. The all-silica sol shell process used in the investment casting industry, its shell production cycle and cost are usually 2 to 3 times that of silica sol-water glass composite shell, and the insufficiency of these two aspects restricts the full application of this process.

How to optimize and upgrade the water glass shell process has been carried out for 5 years, and the final upgrade plan is to develop a low-cost and short-cycle all-silica sol shell process instead of the original silica sol-water glass composite shell process. The premise of the project is as follows.

a. The shell line cannot be extended, and the online shell making time remains unchanged:

b. The comprehensive cost of shell making cannot be increased:

c. Shell quality (surface roughness and strength, etc.) meet the requirements. The research group took the silica-sol shell strengthening as the cutting point and carried out composite strengthening of the coating through polymers, fibers and inorganic strengthening agents. Figure 2 and Figure 3 show the sectional comparison of the silica-sol coating before and after composite strengthening. After strengthening, the connection between the coated sand powder is more uniform, and the comprehensive performance of the silica-sol shell is also improved. At the same time of coating strengthening, the best combination of "temperature + humidity + wind speed" can realize the rapid drying of the coating, and the drying time of the back layer of silica sol coating is controlled within 1h.

Environmental protection is poor, there are pollutants such as sodium salt and hydrogen chloride gas emission problems. The silica sol shell adopts the drying and hardening method to promote the silica sol gel, which overcomes the pollution problem caused by the chemical hardening of the sodium silicate shell. At the same time, due to the low sodium oxide content, the high temperature strength of the shell, the thickness of the shell is only 30%-50% of the water glass shell, and the consumption of refractory materials is less. The all-silica sol shell process used in the investment casting industry, its shell production cycle and cost are usually 2 to 3 times that of silica sol-water glass composite shell, and the insufficiency of these two aspects restricts the full application of this process.

How to optimize and upgrade the water glass shell process has been carried out for 5 years, and the final upgrade plan is to develop a low-cost and short-cycle all-silica sol shell process instead of the original silica sol-water glass composite shell process. The premise of the project is as follows.

a. The shell line cannot be extended, and the online shell making time remains unchanged:

b. The comprehensive cost of shell making cannot be increased:

c. Shell quality (surface roughness and strength, etc.) meet the requirements. The research group took the silica-sol shell strengthening as the cutting point and carried out composite strengthening of the coating through polymers, fibers and inorganic strengthening agents. Figure 2 and Figure 3 show the sectional comparison of the silica-sol coating before and after composite strengthening. After strengthening, the connection between the coated sand powder is more uniform, and the comprehensive performance of the silica-sol shell is also improved. At the same time of coating strengthening, the best combination of "temperature + humidity + wind speed" can realize the rapid drying of the coating, and the drying time of the back layer of silica sol coating is controlled within 1h.

In February 2016, the research group comprehensively upgraded the process of silica sol-sodium silicate composite shell to a new all-silica sol process, which solved the pollution problem caused by chemical hardening of sodium silicate shell, and also improved the shell making cycle and cost. Figure 4 is before the upgrading of the shell making process


3.4 Dewaxing and wax Recovery process After upgrading the shell process to a new all-silica sol shell, dewaxing can be dewaxed by pure hot water (without supplementary hardener), or by low-pressure steam, and no ammonium chloride or hydrochloric acid is used in the dewaxing process. The new low molecular molding material is adopted, the molding material has no saponification phenomenon, the wax treatment does not need to add hydrochloric acid, there is no problem of hydrogen chloride fog pollution; Based on the environmental pressure, it is no longer feasible to continue to use the original hydrochloric acid treatment process for wax recovery by using the strengthened paraffin-stearic acid molding material, and the wax treatment process needs to be optimized.


The recovery process of paraffin-stearic acid molding material mainly includes acid treatment, electrolysis and active clay treatment. Acid treatment is usually treated with sulfuric acid or hydrochloric acid, and sulfuric acid is also used in electrolysis, so the two current methods are not environmentally friendly. Dongfeng precision Casting has tried to use active clay to treat wax, and the final trial conclusion is that the wax loss of this process is 5-10 times that of acid treatment, which is not suitable for industrial application. After several rounds of test verification, Dongfeng Precision Casting Wudangshan plant finally used environmentally friendly non-volatile organic acid instead of hydrochloric acid for wax treatment, and the recovered wax material has qualified strength and ash index.


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