Precision casting. Baidu Encyclopedia.

Precision casting
Baidu Encyclopedia

I. Basic concept
It includes: investment casting, ceramic mold casting, metal mold casting, pressure casting, and lost foam casting.
Among them, investment casting is relatively commonly used. Also known as lost wax casting: select appropriate investment casting materials (such as paraffin) to make investment molds; repeat the steps of coating refractory coatings and sprinkling refractory sand on the investment molds to harden the shell and dry it; then melt the internal investment mold to obtain the cavity; roast the shell to obtain sufficient strength and burn off the remaining investment casting materials; pour the required metal materials; solidify and cool, remove the shell and clean the sand to obtain high-precision finished products. According to product needs, heat treatment, cold working, and surface treatment can be carried out.

II. Development
Precision casting is also called lost wax casting. Its products are precise and complex, close to the final shape of parts, and can be directly used without processing or with little processing. Therefore, investment casting is an advanced near-net-shape forming process.
In ancient China: Prince Wu's tripod, copper prohibition, copper lion, etc. are all masterpieces of investment casting. Since investment casting was used in industrial production in the 1940s, it has been developing at a relatively fast speed for half a century. Especially in European and American countries, it has developed rapidly. In addition to being used in the aviation and weapons sectors, investment casting is almost applied to all industrial sectors, especially in electronics, petroleum, chemical industry, energy, transportation, light industry, textiles, pharmaceuticals, medical devices, pumps and valves and other sectors. In recent years, China has also developed rapidly.
The development of investment casting technology enables investment casting to produce not only small castings but also larger castings. The largest investment casting has an outline dimension of nearly 2 meters, while the minimum wall thickness is less than 2 mm. At the same time, investment castings are becoming more precise. In addition to linear tolerances, parts can also achieve higher geometric tolerances. The surface roughness of investment castings is also getting smaller and can reach Ra0.4μm.

III. Classification

1. Ceramic mold casting
   A casting method that uses a ceramic slurry to make a mold and produce castings. The ceramic slurry is composed of ethyl silicate hydrolyzate and fine refractory sands with relatively pure texture and high thermal stability, such as fused quartz, zircon, corundum, etc. In order to make the ceramic slurry gel in a short time, calcium hydroxide or magnesium oxide is often added as a catalyst. Since the composition and appearance of the refractory materials used are similar to ceramics, it is called ceramic mold. There are two types of ceramic molds: ① The ceramic mold is entirely made of ceramic slurry. The production process is to first fix the pattern on the template, put on a sand box, then pour the adjusted ceramic slurry into the sand box. After gelation and hardening, demold and roast at high temperature to become a mold. ② Use a liner and pour ceramic slurry into the gap between the liner and the pattern to make a mold. The liner can be a sand mold or a metal mold. Pouring a ceramic shell layer with a liner can save a large amount of ceramic slurry and is widely used in production. The surface roughness of ceramic mold castings can reach Ra10～1.25 microns, and the dimensional accuracy is as high as 3～5 grades, which can achieve the purpose of little or no cutting. The service life of ceramic mold casting molds can be comparable to those made by mechanical processing methods, while the manufacturing cost is lower than that of molds made by mechanical processing methods.

2. Sand casting
   Among all castings production, 60-70% of castings are produced by sand casting, and about 70% of them are produced by clay sand casting. The main reason is that compared with other casting methods, sand casting has low cost, simple production process, and short production cycle. Therefore, castings such as automobile engine cylinder blocks, cylinder heads, and crankshafts are all produced by the clay green sand process. When the green sand cannot meet the requirements, consider using clay sand surface-dried sand molds, dry sand molds or other sand molds. The weight of castings produced by clay green sand casting can range from a few kilograms to dozens of kilograms, while castings produced by clay dry sand can weigh dozens of tons.
   Generally speaking, for medium and large castings, resin self-hardening sand molds and water glass sand molds can be used for cast steel parts to produce castings with accurate dimensions and smooth surfaces, but the cost is relatively high.
   Of course, in terms of casting accuracy, surface finish, material density, metallographic structure, mechanical properties, etc., castings produced by sand casting are often relatively poor. Therefore, when these performance requirements of castings are higher, other casting methods should be adopted, such as investment casting, die casting, low-pressure casting, etc.

IV. Technical requirements

1. Adaptation of methods
   For example, in sand casting, factories with large-scale production should create conditions to adopt advanced modeling and core-making methods. The old-fashioned jolt-squeeze or jolt-vibratory molding machine production lines have insufficient productivity, high labor intensity for workers, and high noise, and are not suitable for large-scale production and should be gradually transformed. For small castings, a horizontal parting or vertical parting boxless high-pressure molding machine production line or full-mold molding can be used. The production efficiency is high and the floor space is small. For medium-sized castings, various boxed high-pressure molding machine production lines and air-impact molding lines can be selected to meet the requirements of fast and high-precision molding production lines. For core-making methods, cold box, hot box, shell core and other high-efficiency core-making methods can be selected.
   For medium-batch production of large castings, resin self-hardening sand molding and core-making can be considered.
   For single-piece and small-batch production of heavy castings, manual modeling is still an important method. Manual modeling can adapt to various complex requirements and is relatively flexible without requiring a lot of process equipment. Water glass sand molds, VRH water glass sand molds, organic ester water glass self-hardening sand molds, clay dry molds, resin self-hardening sand molds and cement sand molds can be used. For single-piece production of heavy castings, the pit molding method has low cost and quick commissioning. For batch production or long-term production of stereotyped products, multi-box molding and split-box molding methods are more suitable. Although the initial investment in molds and sand boxes is high, it can be compensated by saving modeling working hours and improving product quality.
   Casting methods such as low-pressure casting, die casting, and centrifugal casting are only suitable for batch production due to the expensive prices of equipment and molds.

2. Adaptation of conditions and methods
   For example, for producing large machine tool beds and other castings, generally core assembly molding method is used without making patterns and sand boxes, and cores are assembled in pits. While in other factories, sand box molding method is used and patterns are made. Different enterprises have different production conditions (including equipment, site, staff quality, etc.), production habits, and accumulated experience. Appropriate products and inappropriate (or impossible) products should be considered according to these conditions.

3. Precision requirements and costs
   Casting methods obtain different casting precisions, initial investments and productivities are also inconsistent, and the final economic benefits are also different. Therefore, to achieve more, faster, better, and more economical results, all aspects should be taken into account. Preliminary cost estimates should be made for the selected casting method to determine the casting method with high economic benefits and that can ensure casting requirements.
   Although China's foundry industry is in a relatively difficult period of development, in the long run, there is still certain hope for the development of China's precision casting industry. Market demand has gradually begun to pick up. Coupled with the strong strength of China's foundry industry development, it is believed that China's foundry industry will definitely achieve gratifying results.
   Experts say that to fundamentally improve the level of casting technology, the following four points must be achieved: First, develop simulation technology, improve prediction accuracy, strengthen process control, and increase the yield. The regular problems are not mastered very well, which affects the yield in mass production. Second, combine production, education, and research. In addition to enhancing innovation consciousness and improving R & D capabilities, the independent innovation of enterprises also needs to attach importance to and strengthen the combination of "production, education, and research" with precision casting enterprises as the main body. Third, attach importance to material research and development. Materials are the foundation of industry, and there is still a lot of work to be done at present. In the aerospace field, the research and smelting technology of alloy materials, especially superalloys and some other new materials, still need to be improved, and the gap with foreign countries is still relatively large. Fourth, focus on improving equipment technology. In terms of process, it is mainly equipment problems. Many key equipments, such as some directional solidification equipments, mainly rely on imports. Therefore, the research and development of precision casting equipment is still the focus.

V. Industry development
In terms of technology, quality, and price, China's foundry industry has a great advantage over Southeast Asian and Middle Eastern countries. The export prospects are very promising. The entry of foreign-funded enterprises into China and the re-export and export of their castings will also become an important part of China's casting exports. Due to the much lower foundry production cost in China, and with the gradual improvement and quality improvement of foundry technology, not only can the import of castings be reduced, but it is also possible to gradually open up export markets and export more casting products to foreign countries. During the "12th Five-Year Plan" period, casting imports will continue to maintain a continuous growth trend in the short term, and casting exports will have great development.

[1]
It is reported that in the wave of economic globalization, the international division of labor in the industrial development process is gradually taking shape. Due to cost pressure, foreign businessmen purchase a large number of castings in China and even set up precision casting production bases in China. It can be predicted that in the future, China will still undertake the production and manufacturing tasks of international non-ferrous metal castings and products for a period of time. On the other hand, due to the unremitting efforts of foundry colleagues, the production level and ability of casting molds in China have been greatly improved. The gap between casting quality and advanced industrial countries has gradually narrowed. The low price has led to a sharp increase in foreign purchases. The huge demand in overseas casting markets has greatly promoted the prosperity of China's foundry industry.

VI. Industry requirements
[2] Precision casting is the leader of the world's foundry industry. As a representative of the foundry industry in high-tech fields, precision casting must play its exemplary and leading roles. However, the prerequisite for precision casting to play these two roles is to strengthen itself. Therefore, China's precision casting should be given priority for development. Development requires established guidelines and development goals. The development goals of the precision casting industry are the following aspects:
First, complete the transformation from a labor and resource-intensive growth mode to a technology and capital-intensive one, and from an extensive and polluting type to a green and intensive type. The quality, variety, and quantity of products are comparable to the world's advanced level. The proportion of high-quality materials increases rapidly. The production efficiency and economic benefits of the entire precision casting factory multiply. Environmental pollution is effectively controlled, and a Chinese foundry material industry system coordinated with the environment is initially established.
Second, form some research and development and education and training bases with reasonable structures and high overall levels that combine production, education, and research. Continuously send professionals to enterprises and continuously develop new varieties of high-quality and high-grade casting materials to meet the growing needs for casting materials. At the same time, gradually increase the intensity of technology exports.
Third, complete the adjustment of industrial structure, eliminate a batch of small precision casting factories with low technical levels, poor product quality, serious pollution, and poor economic benefits, and form relatively concentrated "small giants" and "star enterprises" to achieve "specialized and large-scale" production. In particular, one or two backbone enterprises that can compete with world-renowned enterprises should emerge. They have high technical levels, excellent process equipment, abundant funds, and advanced management levels. They can continuously provide high-end and high-quality casting materials to the market and have a high market share.
If the precision casting industry can achieve the above development goals and strengthen itself, then the precision casting industry will surely play its vanguard and exemplary role in the foundry industry and drive the development of the foundry industry.

VII. Industry reform
To promote the development of the foundry industry and keep the scale of precision casting enterprises within a reasonable range. The "12th Five-Year Development Plan for China's Precision Casting Industry" proposes that by 2015, the number of precision casting enterprises will be reduced from 30,000 to about 20,000, and by 2020, it will be reduced to about 10,000. At the same time, significantly reduce foundry energy consumption, and 30% of enterprises will reach the level of developed industrialized countries; the waste and exhaust gas emissions of foundry enterprises will meet national or local standards; on the basis of the "11th Five-Year Plan", achieve energy conservation and emission reduction goals of reducing energy consumption by 10% and reducing emissions by 15%, implement the "access conditions for China's precision casting industry", and close down enterprises that waste resources, pollute the environment, and do not meet safety production conditions. [3]
In addition to having many factories, many employees, and large output, compared with developed countries, China's precision casting foundry technology still has gaps in terms of quality, efficiency, energy and material consumption, labor conditions, and environmental protection. It is reported that currently, the development of precision casting technology in industrial developed countries in the world can be roughly summarized into four goals. China's precision casting industry should also move forward in these four aspects: 1. Improve casting quality and reliability and produce high-quality near-net-shape castings; 2. Shorten delivery time; 3. Protect the environment and reduce or even eliminate pollution; 4. Reduce production costs. China's precision casting cannot achieve these goals yet. To make China's precision casting industry reach the world's advanced level, it must start from China's actual situation to promote the development of precision casting industry as soon as possible.

VIII. China's precision casting industry will achieve intensive development in the near future
In recent years, with the increasing demand for precision casting industrial products in the domestic market, relevant enterprises in China have also begun to get rid of the constraints of foreign products and embark on the road of transformation towards independent development to provide market competitiveness for products. At present, independent research and development has become the primary goal of domestic foundry enterprises. Luo Baihui, secretary-general of the International Mold & Die Association, said that only by completely getting rid of dependence on technology can we be fundamentally independent. In order to get rid of this long-term dependent situation, some domestic foundry enterprises have begun to obtain the market through independent development. Recently, the non-ferrous metal casting factory in Weitang Town, Xiangcheng District, Suzhou City has taken the road of independently developing and processing molds and embarked on a transformation path of precision casting.
It is reported that this precision casting enterprise currently has 118 employees and produces more than 800 kinds of non-ferrous metal parts. In the past two years, although the non-ferrous metal casting business of other enterprises that mainly exports has fluctuated greatly under the influence of the financial crisis. However, the non-ferrous metal casting factory in Xiangcheng District is not "anxious" about this at all. Instead, it believes that this is a good time to adjust the product structure. In order to fully meet customer needs, it has started the road of independently developing and processing molds. It has increased market development efforts and achieved intensive development in line with the international market. This will greatly promote the continuous development of the domestic precision casting industry. [4]