The sand molding process plays an important role in the casting production process, which directly affects the quality level, production cost, production efficiency and environmental pollution degree of the casting. The sand casting method can be divided into two categories: physical hardening modeling method and chemical modeling method.
The physical hardening modeling methods mainly include clay sand type, solid casting, V method modeling method, and freezing modeling method. Among them, clay sand type is divided into wet type, dry type and dry type. The solid shape and the V method are a binderless method (dry sand) and are formed by vacuum molding. The frozen molding method uses water as a binder.
The chemical modeling methods mainly include: water glass sand type, resin sand type and the like. They can be divided into three types: heat hardening, self hardening, and air hardening. Water glass is an inorganic binder and resin is an organic binder. When selecting a sand molding process, the following principles should be followed:
1. Should be able to guarantee the quality requirements of castings
At present, the widely used molding processes mainly include clay wet sand technology, CO2 blown hard water glass sand process, organic ester self-hard water glass sand process, and acid self-hardening furan resin sand process. In recent years, the hardening sand molding process of ester hardening phenolic resin has also been promoted and applied to a certain extent. The characteristics of these modeling processes, the impact on the quality of castings and the scope of application are as follows:
(1) Clay wet sand process
The advantages of the clay wet sand process are:
A. The raw materials used are cheap and the sources are abundant.
B. The shape is convenient, the sand type does not need to be dried, the casting production cycle is short, the efficiency is high, and it is easy to realize mass production.
C. The bentonite which has not been dehydrated in the recycled sand can be restored after mixing with water. The old sand has good recyclability and less investment in recycling equipment.
D. After a long-term application, a series of modeling equipment has been developed.
E. The dimensional accuracy of castings produced by general modeling is no less than that of chemical self-hardening sand. The high-precision modeling methods such as injection molding, air-punching and static pressure modeling can produce dimensional accuracy comparable to investment casting.
(2) CO2 blowing hardened water glass sand process
Ordinary CO2 blowing and hardening water glass sand process is the earliest rapid prototyping process in the field of water glass binder. Its main advantages are:
A. The device is simple, easy to operate and flexible to use.
B. the binder is non-toxic and tasteless, and the cost is low.
C. The sand type has high temperature repellent property and the shrinkage stress of the casting is small.
D. The binder system does not contain S, P, N, and there is no sulfur increase on the surface of the casting.
The CO2 blowing hardening water glass sand process has been widely used in the production of most steel castings at home and abroad, and is mainly used for the production of medium and small steel castings. However, the shortcomings of the CO2 blown hardened water glass sand process are also very obvious:
A. The sand type (core) has low strength and the water glass is added in a high amount.
B. large water content, easy to absorb moisture, poor hard penetration in winter.
C. The sand type (core) has poor collapsibility, the old sand is difficult to regenerate, and a large amount of old sand is discarded.
In the past, the problem of collapsibility and reuse of old sand was not well solved, which affected the expansion of water glass sand to a certain extent. In recent years, people have made breakthroughs in deepening the understanding of the basic composition of water glass and the essence of “aging” phenomenon and new hardening processes (such as vacuum-exchanged CO2 gas hardened water glass sand process), which are maintained in core sand. Under the condition of sufficient process strength, the high-quality natural silica sand with low content of mud can reduce the amount of water glass to 4.0%, so that the long-term existence of water-glass sand is poor, and the old sand can not be reused. Good solution. Water glass old sand recycling equipment has also matured, and water glass sand has developed a good momentum.
(3) Organic ester self-hardening water glass sand process
Organic ester self-hard water glass sand is widely used in cast steel and has certain applications in cast iron. The main advantages of this hardening process are:
A. The molding sand has a high strength, and the amount of water glass added can be as low as 2.5-3.5%.
B. Sand type (core) has good collapsibility, and the old sand dry regeneration rate is ≥80%.
C. The sand type has good thermoplasticity and low gas generation, which can overcome the defects such as cracks and pores which are prone to occur when the furan resin sand is used to produce steel castings. The quality and dimensional accuracy of the castings are comparable to those of the resin sand.
D. The production cost is the lowest and the working conditions are good in all self-hardening sand processes.
The hardening process still has the following disadvantages: the core sand hardening speed is slow, the brittleness is large, and the fluidity is poor.
(4) Acid self-hardening furan resin sand process
Acid self-hardening furan resin sand and organic ester self-hardening water glass sand are typical representatives of organic binder chemical self-hardening sand and inorganic binder chemical self-hardening sand. The common features of chemical self-hardening sand process are:
A. After the sand type hardens, the mold sand does not need high wet strength, the sand has good fluidity, the sand type has high strength, the deformation is small, the tooling is simplified, the shape is simple, the sand type does not need to be dried, and the dimensional accuracy of the casting is significantly improved, which can reach CT8~ Level 10, casting defects are also less.
B. Chemically hardened sand generally uses a liquid self-hardening binder. Therefore, it requires high quality of the original sand to minimize the amount of binder added.
C. Since the hardening of the binder is an irreversible chemical reaction, it cannot be re-used as simply as clay sand. When used in large quantities, a relatively complete old sand regeneration system must be used.
D, the model structure and surface quality requirements are higher, in order to demould.
The self-hardening sand process is mainly suitable for small batch or batch production of large-scale castings, and there is no competition or substitution relationship with the clay wet sand technology.
The acid self-hardening furan resin sand process is a self-hardening sand process which is widely used in cast iron, and its outstanding advantage is that the sand type (core) has good collapsibility and the old sand regeneration has high recycling rate.
The furan resin used in the foundry production needs to be modified with urea-formaldehyde and applied to the non-ferrous alloy, gray cast iron, ductile iron and cast steel according to the nitrogen content of the modified resin.
The disadvantages of the acid self-hardening furan resin process are:
A. The N, S, P and other gases generated after the pyrolysis of the resin binder and the curing agent may cause severe osmosis of the surface of the ductile iron casting and the steel casting, causing defects such as pores and cracks.
B. Sand type has high thermal expansion rate, large thermal stress, poor high temperature retreat, large shrinkage stress of castings, and cracks and burrs in castings.
C. The resin binder is expensive, and the furan ring produced after decomposition is very harmful to human health.
(5) ester hardening phenolic resin self-hardening sand
The ester hardening phenolic resin self-hardening sand process was developed by Bolton Company of the United Kingdom and is called a-set process. It was patented in 1981 and has been widely used in Europe in 1984. It was first used in cast steel production and has been expanded to Cast iron and non-ferrous alloy castings.
The phenolic resin has a strong basicity and a pH of from 1 to 13.5. The resin contains organic solvent, low flash point, flammable, and soluble in water. The shelf life is short. It can be stored for 6 months at 20 °C, 2-3 months at 30 °C, and only 1- at 40 °C. 2 months.
The hardener of such self-hardening sand is an organic ester, which can be selected according to the requirements of the curing speed. The amount of hardener is about 20~30% (mass fraction) of the resin, and the amount of phenolic resin added is 1.5~2.5% of the original sand. The sand mixing process is the same as the acid self-hardening furan resin. The sand temperature is usually controlled at 20~30%, the type (core) sand can be used for 5~30min, and the demolding time is 15~60min.
The main features of ester hardening phenolic resin self-hardening sand are:
A. Only partial reaction occurs under the action of hardener. The mold or core has a certain thermoplasticity after hardening. After pouring the metal, there is a short process of complete hardening due to heat. This is also the difference from the acid self-hardening furan resin sand. Therefore, the mold (core) made by this process is not very strong after hardening, and the compressive strength is only 2 to 4 MPa, but the dimensional stability and thermal stability of the mold are further hardened due to the initial stage of casting. All are good, the castings produced have high dimensional accuracy and good surface quality.
B. Because it does not contain N, P, S, it is especially suitable for the production of steel castings and ductile iron castings.
C, there will be no burr defects. Other self-hardening resin molds exhibit cracks at the mold/metal interface during casting and solidification. While the ester-hardened phenolic resin self-hardening sand has a short-term thermoplastic stage in the surface layer during the casting and solidification process to avoid cracking, a smooth casting with no burr defects can be obtained.
D. Alkaline phenolic resin has wide adaptability to raw sand, and is not only suitable for silica sand, but also suitable for special sand such as magnesia, forsterite sand and chromite ore with high acid value.
2. Should be compatible with the production batch
In mass production, priority should be given to mechanized, automated clay wet sand molding production lines and resin sand core production lines. For small castings of clay wet sand casting, it is possible to adopt a boxless high-pressure molding production line with horizontal splitting or vertical splitting. The production efficiency is high and the floor space is small. For medium parts (greater than 10kg), various high-pressure boxes are available. Modeling production line, pneumatic molding line to meet the requirements of fast, high-precision modeling production line. The old-fashioned shock-type or shock-pressure molding machine has low production line productivity, high labor intensity, high noise, and is not suitable for mass production. It should be phased out.
In medium-volume production, it is possible to consider the application of resin self-hardening sand, CO2 blowing hardened water glass sand, vacuum displacement blown hardened water glass sand molding and core making.
Manual styling is still an important method when producing single-piece small batches. Manual modeling can adapt to a variety of complex requirements, more flexible, does not require a lot of process equipment, can be applied to resin self-hardening sand type, CO2 blowing hardened water glass sand type, vacuum replacement blowing hardened water glass sand type, organic ester water glass self-hardening sand type , clay dry type and cement sand type. For heavy-duty castings produced in one piece, the pit-forming method is low in cost and quick in production.
The multi-box shape and the box-boxing method are suitable for mass production or long-term production of styling products. Although the initial investment of molds and sand boxes is high, it can be compensated for from saving man-hours and improving product quality.
3. Should adapt to the company’s own conditions
The production conditions (including equipment, site, staff quality, etc.), production habits, and accumulated experience of different enterprises are different. It is necessary to consider what modeling method is suitable according to these conditions. The best is applicable. At present, various technologies are competing for development. Each technology has its own advantages, and it also has certain limitations and scope of application. Advanced, high-tech processes are not necessarily applicable. According to the company’s own conditions, it is necessary to make practical choices by selecting technically applicable and economically reasonable processes and focusing on the combination of technology and economy.
For example, when producing castings such as large machine bed, the core forming method can be used, and the mold and the sand box are not made, and the core is assembled in the pit; while the other factory adopts the sand box modeling method to make the appearance.
4. To balance the quality and cost of castings
The quality of the castings obtained by various casting processes is different, the initial investment and production efficiency are also inconsistent, and the final economic benefits are also different. Therefore, to be more, faster, better, and more economical, we should take into account all aspects. Cost estimates should be made for the chosen casting method to ensure both economic efficiency and casting quality.
5. Pay attention to the environmental protection characteristics of the modeling process
Foundry production has always been known for its serious environmental pollution. Its pollution form is mainly caused by air pollution and waste slag pollution caused by steel melting and modeling materials. Among them, air pollution and waste residue pollution caused by modeling materials are the most serious.
According to statistics, it takes about 1t of new sand to produce 1t castings, and about 1t of old sand is discarded. At present, China produces about 28 million tons of castings a year, and the annual amount of old sand discharged is about 28 million tons. This not only accounts for a large amount of natural resources, but also causes serious environmental pollution. In order to reduce the amount of old sand emissions, it is necessary to use the casting process with high old sand recycling rate, clay wet sand and resin sand.
However, the clay wet sand has large dust pollution and black pollution of coal powder. The harmful gas generated by the combustion and decomposition of coal powder during the pouring process also causes serious air pollution; while the air in the resin sand production site is free of many organic substances. Exhaust gas (SO2, formaldehyde, benzene, methyl, etc.) will produce a lot of harmful gases after pouring, which is very harmful to human health. The water glass sand is composed of silica sand, inorganic water glass binder, etc., and uses CO2 gas or organic ester (such as ethylene glycol diacetate) as a curing agent, and the production environment is friendly, and no harmful gas is generated. Compared with clay sand and resin sand, the water glass sand process is the most suitable core making process for green clean casting production.
6. In conclusion
There are many kinds of products to be cast, and the process of modeling is also ever-changing. We must have a scientific attitude when choosing the production process of our products. We must proceed from the actual situation and make comprehensive considerations according to the material, structural characteristics and quality requirements of the castings.