Research and Practice on Rapid Shell Making Process of Investment Casting Silica Sol

Views: 8     Author: Site Editor     Publish Time: 2024-08-30      Origin: Site

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Research and Practice on Rapid Shell Making Process of Investment Casting Silica Sol

Abstract: This article introduces the transformation and upgrading of investment casting technology for water glass, achieving rapid shell making and batch application of investment casting silica sol. This process has been optimized and upgraded from the selection of mold and shell materials, addition of fast drying agents, online rapid shell making, steam dewaxing, and wax material recovery processing, completely eliminating the use of pollutants such as water glass, hydrochloric acid, and chloride salts, and achieving zero emissions of acid mist and sodium salts.

The investment casting water glass precision casting process has a history of nearly 60 years of industrial application in China, with an annual production of approximately 1.5 million tons of precision castings, making significant contributions to the country's economic development. Although significant improvements and innovations have been made in mold materials, shell making materials, hardening agents, and shell making processes during this period, the pollution issues of acid mist, chloride salts, and other pollutants have not been fundamentally resolved.

Discussion on Process Upgrade Plan

The traditional water glass precision casting process mainly uses paraffin stearic acid, which is prone to saponification reaction during use. The recovery is usually treated with hydrochloric acid, which causes serious pollution from hydrochloric acid mist; The shell making process uses chloride salt chemical hardening to produce a large amount of sodium salt and hydrogen chloride acid mist pollution. During dewaxing, ammonium chloride or hydrochloric acid is used to supplement hardening, and chloride salt and hydrogen chloride products will be produced during the calcination process.

The current ammonium chloride hardened water glass shell process has been listed as a backward process by the country, and hydrochloric acid is also prohibited from use in many places. With the increasing environmental pressure, the transformation and upgrading of traditional water glass precision casting process is imperative. To address the key issues of pollution, cost, cycle, and quality in traditional water glass precision casting technology, the overall strategy for the transformation and upgrading of Dongfeng precision casting technology is as follows:

(1) Using the original mold material (paraffin stearic acid), the binder is silica sol, and the shell is made without chloride hardening. The dewaxing and wax treatment eliminate the use of ammonium chloride or hydrochloric acid, eliminating the generation of pollutants from the root;

(2) The shell material is made of quartz sand powder;

(3) Independently developed fast drying and strengthening additives to achieve fast drying of shells and thin-walled high-strength shell forming;

(4) Effective control of "temperature+humidity+wind speed" in the shell making environment;

(5) Low temperature mold material micro pressure steam dewaxing.

On this basis, there is no need for major adjustments to the original process layout, and the shell making cycle and comprehensive cost are not higher than traditional water glass shell making processes, completely eliminating sodium salt, hydrogen chloride gas, and ammonia pollution. At the same time, the quality of castings has also been improved, as shown in Table 1. Through comprehensive comparison of quality, cost, delivery time, and environmental indicators, it can be concluded that Dongfeng Precision Casting's low-cost thin shell silica sol quick drying process is the best solution for the transformation and upgrading of traditional water glass precision casting technology.

Research and application of rapid shell making process using Dongfeng precision casting silica sol

At present, 80% of investment casting enterprises in China use traditional water glass investment casting process or silica sol water glass composite investment casting process, mainly producing components for automobiles, ships, engineering machinery, rail transit, machine tools, etc. The traditional water glass precision casting process requires a chloride salt hardened shell, which causes serious environmental pollution. The all silica sol process for medium temperature mold materials has a shell making cycle and cost 2-3 times that of traditional water glass precision casting process, which is not suitable for upgrading and mass production of traditional water glass precision casting process.

2.1 Requirements for rapid shell making process using silica sol

At present, the rapid shell making technology of silica sol in the precision casting industry mainly involves adding organic polymer aqueous solutions such as fast drying and strengthening to coatings, accelerating the gelation process by increasing the gelation core, and achieving fast drying through physical binding and strengthening effects (Table 2). At present, there are investment casting specific fast drying silica sol and NP fast drying additives on the market [4]. Dongfeng Precision Casting has independently developed a fast drying agent suitable for medium to large-sized and mass production modes, which can achieve rapid shell making of silica sol online in a catenary system. Tables 3 and 4 respectively show the environmental parameters for rapid shell drying and the selection of refractory materials for rapid shell making by Dongfeng Precision Casting. The number of reinforcement layers is generally ≥ 2, depending on the size of the product; The drying time and environmental parameters for sealing slurry do not require specific requirements and can be carried out simultaneously with module storage. If it is steam dewaxing, there may be no sealing layer.

2.2 Development of rapid shell making and drying agent for silica sol

In the field of investment casting shell coating technology, fast drying agents will directly affect the quality, production cycle, and cost of the shell and castings. In order to seek a suitable quick drying agent, Dongfeng Precision Casting has conducted extensive research and experiments, and found that most of them have developed from alcohol based ethyl silicate binders to water-based silica sol binders, greatly reducing environmental pollution. However, the fast drying agents in existing technology are still unable to meet the production needs and have many shortcomings. For example, the fast drying agents in existing technology have disadvantages such as slow drying speed, low stability, and high casting scrap rate, which seriously affect the production cycle and increase production costs, and need further improvement. In addition, silica sol contains 60% free water, and the addition of quick drying agents will inevitably greatly shorten its shelf life. Experiments have found that if fast drying agents are insoluble in water and mixed with refractory materials before being directly added to silica sol coatings, the coatings will gel and "age" in a short period of time. Therefore, the method of adding fast drying agents will directly affect the service life and cost of silica sol (coatings). Therefore, "modification" treatment must be carried out to extend its service life to more than six months, resulting in a sharp increase in costs. This is one of the main reasons why fast drying silica sol (agents) have not been widely used for many years.

The technical problem that Dongfeng Precision Casting aims to solve is to provide a fast drying agent for efficient precision casting shell coating that is convenient to use, anti-aging, and has a fast drying speed, in response to the shortcomings of the existing technology mentioned above. One of the technical solutions is composed of a flocculant with a mass fraction of 0.05% to 2.00% and a solvent of 98% to 99.95%. The flocculant is one or a combination of polyacrylamide and sodium polyacrylate, and the solvent is distilled water or deionized water.

Research on fast drying mechanism: By adding different ratios of fast drying agents to the investment casting silica sol shell coating, the coagulation, adhesion, thickening, drag reduction, dispersibility, stability and other characteristics of the coating are further studied, aiming to improve the colloidal stability of the coating, improve the gas permeability and humidity of the shell

Strength, high temperature strength, and residual strength.

Flocculation: It can cause the colloidal particles of silica sol to agglomerate into large particles, while displacing the free water (about 60% of the total water) into the gel clusters

On the surface, it is easy to evaporate during air drying, achieving "rapid drying".

Adhesive properties: The quick drying agent aqueous solution itself is a "soft" colloid with excellent elasticity. It can absorb and relieve stress (with yielding properties) when silica sol (a "rigid" colloid) loses water and solidifies, and has a physical "binding" effect. It can increase the wet strength of the shell by up to 30% and prevent cracking of the shell.

Thickening: It also has the function of improving the coating and coverage of silica sol, and increasing the coating thickness. For example, the silica sol coating on the surface layer of quartz (refined quartz powder or fused silica powder) has poor wettability with the wax film surface, which will improve the coating performance of the surface slurry.

Resistance reduction: It can increase the fluidity of fluids (coatings) by 50% to 80%. Beneficial for the stability and improvement of the quality of the shell (casting), it prevents the occurrence of "steel infiltration" defects in the casting due to coating accumulation and shell baking process.

Dispersion: It can improve the dispersion of refractory materials in coatings and increase the content of fine powder (d<5-10 μ m). The coating's adhesion, coating thickness increase, and density increase.

Stability: Quick drying agents can improve the stability of silica sol coatings, generally increasing from 3 days to 7 days. It is a type of "chelating agent" that chelates with metal ions (Na, Ca, Fe, etc.) in water or refractory powder, also known as a "chelating agent", metal blocking agent, or "water softening agent". It can reduce the harmful effects of metal oxides on coatings, improve the "stability" of coatings, and extend their service life.

Porosity: After high-temperature calcination, the organic high molecular weight polymer (NP) in the investment casting shell is completely burned without leaving any residue or ash. After gasification and decomposition, a large number of micropores are left, which can improve the permeability of the shell by 20% and the shell stripping performance by 10%, reduce the scrap rate of castings, and improve the efficiency of post-processing production.

Three experiments were conducted to compare the effects of different ratios of quick drying agents, as shown in Table 5.

After the above experiments, it was found that Experiment 1 has good drying performance. The present invention provides a high-efficiency precision casting shell coating special fast drying agent, which has the advantages of fast drying, improving the wet strength of the shell, preventing cracking, improving breathability and shell shedding compared to traditional fast drying agents. It is also very convenient to use, effectively improving work efficiency, shortening production cycle, reducing the consumption of shell sand powder materials, and lowering manufacturing costs. It has good economic value. However, the biggest disadvantage of organic polymers is that their water-soluble liquids have a short shelf life, which can degrade and cause flocculation failure after expiration, especially under shear stress, making them more prone to "aging". Due to different timing of addition, the effect can vary greatly. During use, it is necessary to strictly follow the technical requirements of the quick drying agent to avoid losses.


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