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In the lost foam casting, the foam coated with refractory coating is placed in a sand box, and the surrounding of the pattern is filled with dry sand. The microseismic and negative pressure are used to compact the liquid metal without casting the core, in the casting and A new casting process that continues to maintain a constant negative pressure during the solidification process to vaporize the foam and then replace it with metal to form a casting.

1. The process flow of lost foam casting is as follows:

1) Pre-expansion

Model production is the first process of the lost foam casting process. Complex castings, such as cylinder heads, require several foam models to be separately fabricated and then glued into a single overall model. Each block model requires a set of molds for production. In addition, a set of tires may be needed in the gluing operation to maintain the accurate positioning of each block. The molding process of the model is divided into two steps. The first step is to Polystyrene beads (EPS) are pre-expanded to a suitable density, typically by rapid steam heating, which is referred to as pre-expansion.

2) Model molding

The pre-expanded beads are first stabilized, then sent to the hopper of the molding machine, and fed through the feeding hole. After the mold cavity is filled with the pre-formed beads, steam is introduced to soften the beads. Expanding, squeezing all the voids and bonding them together completes the manufacturing process of the foam model, which is called autoclaving.

After molding, the model is cooled by a large flow of water in the water-cooling cavity of the mold, and then the mold is opened to take out the model. At this time, the model temperature is high and the strength is low, so care must be taken during demolding and storage to prevent deformation and damage. .

3) Model cluster combination

Before the model is used, it must be stored at the appropriate time to make it mature. The typical model storage period is up to 30 days. For the model formed by the uniquely designed mold, it only needs to be stored for 2 hours. After the model is matured, it can be divided. The block model is glued together. The mass-produced castings must be bonded to the automatic bonding machine using a hot-melt adhesive to ensure the bonding accuracy. Castings produced in small and medium-sized batches can be used, hand-bonded with cold glue, and the joints of the glued surfaces should be tightly sealed to reduce the possibility of casting defects.

4) Model cluster dip coating, drying

In order to produce more castings per box casting, sometimes many models are glued into clusters, the model clusters are immersed in refractory coatings, and then dried in air circulating ovens of about 30~60C (86-140F) for 2~3 After the hour, after drying, put the model cluster into the sand box, and fill the dry sand to make the vibration tight. The internal cavity of all the model clusters and the dry sand of the periphery must be tight and supported.

5) Pouring

After the model cluster is filled in the sandbox by the dry sand vibration, the vacuum is vacuumed to form a negative pressure to strengthen the compactness. The mold can be poured. After the molten metal is poured into the mold, the model gasification is replaced by metal to form a casting. In the lost foam casting process, the casting speed is more critical than conventional hollow casting. If the casting process is interrupted, the sand pattern may collapse and cause waste. Therefore, in order to reduce the difference between each casting, it is best to use an automatic pouring machine.

6) Falling sand cleaning

After the pouring, the vacuum is released for a while, the vacuum is released, the casting is solidified and cooled in the flask, and then the sand is dropped. Casting sand is quite simple, and the tilting sandbox castings fall out of the loose dry sand. The castings are then automatically separated, cleaned, inspected and placed in a casting box for transport. The dry sand can be reused after being treated and cooled by the sand treatment system, and other additional processes are rarely used, and the metal scrap can be remelted in production.

2. Advantages of the lost foam casting process

1) High precision of castings Lost-die casting is a new process with almost no allowance and precise molding. This process does not require mold taking, no parting surface, no sand core, so the casting has no flash, burr and draft angle, and The dimensional error caused by the core combination is reduced, the surface roughness of the casting can reach Ra3.2 to 12.5μm; the dimensional accuracy of the casting can reach CT7 to 9; the machining allowance is at most 1.5 to 2mm, which can greatly reduce the machining The cost can be reduced by 40% to 50% compared to conventional sand casting methods.

2) Flexible design: Provides sufficient freedom for the structural design of the casting. Highly complex castings can be cast by foam molding.

3) There is no sand core in traditional casting, so there is no uneven wall thickness of the casting due to inaccurate sand core size or inaccurate core position in traditional sand casting.

4) There is no chemical binder in the clean production sand. The foam is harmless to the environment at low temperature, and the recovery rate of the old sand is over 95%.

5) Reduce investment and production costs to reduce the weight of casting blanks, and the machining allowance is small.

Therefore, lost foam casting technology is in line with the general trend of casting development: it has broad development prospects.

3. Disadvantages and limitations of the lost foam casting process

The lost foam casting process and other casting processes have their shortcomings and limitations. Not all castings are suitable for production by the lost foam process, and specific analysis is required. The use of this process is mainly based on the following factors.

1) The batch size of castings is larger, and the economic benefits are more impressive.

2) The order of good and poor applicability of casting materials is roughly: gray cast iron – non-ferrous alloy – ordinary carbon steel – ductile iron – low carbon steel and alloy steel, because the foam is burned and decomposed during the casting process The effect of the substance on the alloy solution is different. For example, for steel castings with low carbon content, the use of lost foam casting may cause carbonation problems in the casting skin. Therefore, it is necessary to make necessary preparations before production to prevent the process experiment and debugging cycle from being too long.

3) The size of the casting mainly considers the scope of use of the corresponding equipment.

4) Casting structure The more complicated the structure of the casting, the better the economical and economic benefits of the lost foam casting process. For the case of a narrow internal cavity channel and interlayer, it is necessary to carry out the experiment before the lost foam process before it can be put into production. . For some simple conditions, the sand casting method can also produce high quality castings, and the production efficiency and casting cost are lower than that of the lost foaming process. In this case, the lost foam casting method is not necessarily used.

4. Development status of the lost foam casting process in China:

Through years of production practice, China’s enterprises using the lost foam technology for casting production have grown from less than ten in the initial stage to hundreds of current ones. There are many successful examples of lost foam casting production in China, and there are quite a few companies that have not achieved the expected results. Analysis of successful experience and failure lessons, the key to the development prospects of lost foam casting in China is the degree of awareness of this process, which lies in the optimal control of the production system of the process, including raw materials, coating technology, dry sand tight Real technology and optimized control of lost foam casting process technology.

Almost all the introductions about lost-foam casting coatings will narrowly describe the “coating permeability” of lost-foam casting coatings as “breathability”.

In production practice, we can understand that the paint after casting changes color. This is because the free carbon produced by the violent combustion of the foam model in the cavity passes through the coating, and the cavity is discharged and adhered to the surface of the coating. It can also be seen that when the cross-sectional area of ​​the pores through which the coating penetrates is large, or the absolute value of the vacuum is excessively high, or the temperature of the liquid metal is high, and the surface tension is small, the liquid metal permeates through the coating. In the sand gap, the unique sand-sand phenomenon of lost foam casting is formed: iron-clad sand.

The characteristic of the iron-clad sand unique to lost foam casting is that the coating is still intact, and the liquid metal penetrates the gap of the sand through the pores of the coating, not from the crack of the coating and enters the gap of the sand. The latter can be removed, and the cracks can be seen on the surface of the casting after the sand is removed. The former cannot be removed. I have had the experience. When I poured the large cold die bottom plate for the first time, in order to prevent the collapse of the box, the vacuum was pumped to 0.08Mpa, and the temperature of the molten iron was also high. As a result, the entire casting was poured into a hedgehog, and a serious iron occurred. Sand.

In summary, the lost foam casting coating not only enables the passage of gas, but also allows the solid free carbon and liquid metal to pass. We call it the permeability performance more apt and more accurate than the gas permeability performance!

Lost Foam Casting During the casting process, there are three physical states from bottom to top in the coating, the lowermost part is liquid metal, the uppermost part is unliquefied and gasified combustion foam, and the middle part is mixed with free carbon and flammable. The space of the gas is called the air gap. If the temperature is used to describe the three physical regions, the bottom-up is the high temperature zone, the medium temperature zone and the low temperature zone. With this method of differentiation, we can draw the following conclusions:

(1) The permeability of the coating is meaningless in the low temperature zone;

(2) The permeability of the medium temperature zone determines whether the coating can discharge the gas and free carbon generated during the disappearance of the foam;

(3) The permeability of the high temperature zone is only harmful, and there is no benefit. If the permeability of the coating cannot be closed in the high temperature zone, the liquid metal will overflow and cause “iron-clad sand”.

How is the permeability of the mid-temperature zone produced?

In the description of the formulation of the coating, we have mentioned that a certain amount of organic binder is added to the lost foam casting coating. During the drying process, the water is volatilized, and the water molecules leave fine, nano-scale pores in the volatilization process, forming the low-temperature (normal temperature) permeability of the coating. The coating is semi-permeable, like sugar. The block of wax paper can only pass gas molecules and cannot pass substances larger than water molecules.

In the pouring process, the liquid metal first transfers heat to the foam through convection and gas convection. The foam shrinks into a gel-like substance when it is heated, and is vacuum-drawn and adsorbed on the paint wall (Coanda effect), and then vaporized under high temperature. , a gas gap is formed. When the temperature of the gas gap reaches 300 400 C or more, the organic binder is denatured and coked, and the cross-network structure formed by the organic binder in the process of mixing the paint forms a network-like passage, and the coating is transparent. performance.

The permeability of the coating has two process parameters: (1) the size of the channel aperture cross-sectional area, and (2) the density of the pore size distribution.

The combination of the two indicators determines the permeability of the coating. Therefore, the adjustment of the permeability of the coating includes the adjustment of the aperture cross-sectional area and density.

The adjustment of the permeability aperture is achieved by the choice of organic binder. The thickness of the network structure formed by hydrolysis and stirring of the organic binder (relatively) determines the size of the permeability pore size.
The adjustment of the density of the through holes is regulated by the amount of organic binder added. The proportion of the added amount is high, and the number of through holes formed per unit area is large, and vice versa.

In specific applications, cast iron has good fluidity, low surface tension and strong penetrability. Therefore, the cross-sectional area of ​​the pores of the coating is required to be small to prevent the occurrence of iron-clad sand. Correspondingly, the fluidity of the cast steel is poor. Large, low penetration, the cross-sectional area of ​​the through hole can be larger. Of course, this adjustment also needs to match the pouring temperature and the degree of vacuum.

The ratio of the surface area to the weight of the casting is called the modulus. The ratio of the surface area to the weight of the thin-walled member is greater than the ratio of the thicker member. The throughput per unit area of ​​the coating is thinner than that of the thick-walled parts. Therefore, in the preparation of the coating, the amount of the organic binder added to the thin-walled parts can be reduced under the premise of ensuring the coating performance. the amount. In the formulation of the coating, some binders are added to adjust the permeability of the coating. For example, the BY binder in the formulation of our company’s coatings is the role. The jute fiber added in some professional formulas also adjusts the permeability of the coating.

 

 

 

The development and continuous improvement of the lost foam casting method is the evolution process from the foam model wet sand method (FM method) to the foam model thousand sand negative pressure method (FV method). This kind of evolution is not only the evolution of the modeling method, but the fundamental change of the process principle. The disappearance of the bubble model has undergone a fundamental shift.

The solid-cast foam model is violently burned away under open conditions.

Dry sand solid casting is mostly used for cast aluminum body, and the foam model disappears mainly in liquefaction mode.

The negative compaction type dry sand (foam mode) casting method, the foam model disappears in a gasification-based manner.

The three methods described above not only have different ways of disappearing the foam, but also the principles of the setting are different. Therefore, the requirements for the performance of the coatings are not the same.

The solid casting relies on the powdering agent to shape the sand and the open casting. The coating mainly acts as a partition between the molding sand and the liquid metal to prevent the sand from being difficult to remove.

The setting mechanism of the dry sand solid casting sand without vacuuming is relatively complicated. This method must be filled by the bottom of the model to the human metal liquid, and the liquid metal is filled by the static pressure, and the radiation heat conduction mode is adopted during the rising process of the liquid surface. The carried heat is transferred to the foam model, and the foam shrinks rapidly after being heated, liquefying and dripping on the surface of the liquid metal layer. The liquid foam dropped on the surface of the liquid metal is subjected to a higher temperature and rapidly vaporized to generate a high pressure gas. The sand in the air gap is the shape of the model cavity that is maintained by the high pressure of the gas.

The continuous convex form of the molten metal surface rises, and the liquid foam is instantly squeezed between the liquid metal and the coating. In order to avoid the occurrence of casting defects, this method requires the coating to have good wetting properties, absorb the liquefied material of the foam model, and remove the liquefied material of the foam model into the sand gap under the action of the liquid metal pressure.

Under the action of high temperature of liquid metal, the liquid foam continuously vaporizes into the peripheral sand gap during the discharge process. At this time, the vaporized foam is condensed into a liquid substance in the cold, so that the molding sand is bonded and shaped.

In the state of vacuuming, the pressure difference generated by the atmosphere makes the sand compact as hard as a stone. This paper discusses the problems related to the coating used in the lost foam casting method with foam model, dry sand and vacuum. .

There are three methods of using foam casting. Although the foam molds disappear, in China’s atmosphere, only the casting method with three types of foam, dry sand and vacuum should be called lost foam casting.

Lost foam casting currently has three processes

Since the entry into China in the early 1980s, lost foam casting has experienced a long period of chaos. It has been circulating in the industry – the words: seemingly simple lost foam casting, a look If you do it, it will be scrapped!

As an independent technical method, lost foam casting should have its own theoretical basis. It is precisely because we neglect the study of the unique theory of lost foam casting. For a long time, we borrowed the theory of traditional casting to explain the lost foam casting. We have entered a misunderstanding with the actual ambiguity.

When we asked ourselves what topics should be studied for lost foam casting, we discovered that the name of the lost mold is very appropriate! Very good! Three words set the theme!

Lost foam casting is to study: the mold disappears!

To put it simply, we need to study three problems in the disappearance of the mode: the way the mode disappears, the time the die disappears, and the amount of die disappearance.

(1) The disappearance mode of the lost foam casting mold

The lost foam casting mold is a compound mainly composed of carbon and hydrogen, and it disappears in two ways.

Gasification disappears;

The burning disappeared.

In the process of casting in the lost foam casting, if the liquid metal can close the sprue and not let the air enter the cavity, the foam of the model will be cracked into small molecules by the macromolecule under the condition of high temperature and no oxygen, and the solid state will change into the gaseous state. The coating is vacuumed and discharged.

If air is drawn into the cavity during the casting process, the foam will burn violently. This common sense is shared by everyone. Foam burning produces a lot of free carbon and carbon bundles. This is because the concentration of oxygen in the air is insufficient, and the active hydrogen atoms combine with oxygen to free the carbon. When free carbon and carbon bundles are dissolved in liquid metal, diffuse carbonization occurs, and carbon segregates bright carbon, which changes the material quality, quality and processing properties of the casting.

The carbon concentration of gray iron and ductile iron is higher, the carbon adsorption tendency is not obvious, the carbon concentration of carbon steel is very low, the greedy absorption of carbon, the free carbon produced by foam combustion becomes a deadly killer of steel castings. Therefore, in the early days, few people were able to make casting molds for lost molds, especially small dry cast steel parts.

We know the principle of free carbon generation. We also know the danger of free carbon. We also know that the occurrence of free carbon is inevitable. As long as there is a way to make the free carbon produced by combustion out of the cavity, it is insoluble in liquid metal. It can prevent the occurrence of carbonization defects. Therefore, we tried to cast directly from the riser. The spacious riser channel allows the gas and free carbon produced by the combustion of the foam to be fully discharged, solving the serious carbonation problem of the casting. The open casting method is to solve the problem of carbon-discharging. Therefore, we call it the carbon-discharging method of lost-foam casting, which is mostly used for thick and cast steel parts that require Zengkou feeding. The closed casting method corresponding to this is to create a condition of high temperature and no oxygen, and to make the foam gasification disappear, so we call it the gasification method of lost foam casting.

(2) The disappearance time of the lost foam casting mold

a. The disappearance of the lost foam casting mold is actually the replacement of the liquid metal with the foam type. Since it is a replacement of two substances, there are three manifestations in the time relationship between one in and one out;

b. The filling speed of the liquid metal is faster than the disappearing speed of the foam model;

c. The filling speed of the liquid metal is synchronized with the disappearing speed of the foam model. If the filling speed of the liquid metal is faster than the disappearance of the foam model, the liquid metal will be buried in the foam before the foam cutting bottom disappears and fills the cavity. The gel, which causes the gas to continue to vaporize, cannot escape from the cavity and solidify in the casting, creating pore defects, which we do not want to happen. We hope that the rate of disappearance of the foam (whether gasification or combustion) will disappear with the filling of the liquid metal or before the filling of the cavity with the liquid metal. Thus, the disappearance of the foam type has no effect on the filling of the liquid metal. Therefore, when we summarize the design principles of the lost foam casting system, we propose to control the cross-sectional area of ​​the sprue, balance the feed rate of the liquid metal and the gasification speed of the foam. The best way is to burn the foam model first or first.

(3) Loss of lost foam casting mold

In the lost foam casting process, we hope that the foam will disappear completely without leaving any traces. The special defects such as pore defects, carbonization defects, and wrinkle defects in lost foam casting are traces of the mold lost during the disappearance process. We are known as: gasification defects.

The measures to overcome the gasification defects are – in general terms, seven words: one low, two high, three breathable. Namely: use a lower density foam model, higher temperature molten iron and a coating with good permeability.

In this article we outline three process and theoretical foundations for the development of lost foam casting to date. The three methods are:

(1) “sealed pouring” of lost foam casting gasification method;

(2) Lost-die casting carbon discharge method “open pouring”;

(3) The lost shell casting method of empty shells is “first burned and then poured”.

The purpose of introducing the three methods is to show that the development of lost foam casting has not been a single method, and the requirements for coating performance are different.

With the continuous development of the foundry industry, the most popular casting process used by Chinese foundry manufacturers in 2017 is lost foam and sand casting. Generally, manufacturers of single large-volume precision castings generally adopt the lost foam casting method, which mainly undertakes outsourcing and zero-lived foundry manufacturers. Most lost foam and sand casting are in use. Let’s summarize the advantages of the lost foam and sand casting.

According to the comparison between sand casting and lost foam production process, the lost foam has eight advantages over sand casting: wide production range and high precision standard. Suitable casting castings can reduce running cost, short cleaning time, suitable for mechanized production, and workers can find Easy to organize production quickly, high pouring efficiency and high customer satisfaction!

Since the molding materials used for sand casting are cheap and easy to obtain, the castings are easy to manufacture, and can be adapted to single-piece production, batch production and mass production of castings. For a long time, it has been the basic process in casting production. Sand casting is used to produce “a variety of complex geometric shapes of metal parts. These parts can vary greatly in size and weight, from a few grams to a few tons. Some smaller sand castings include component gears, pulleys, crankshafts, Connecting rods. Larger applications include large equipment and heavy machinery base housings. Sand casting is also suitable for the production of automotive components such as engine blocks, engine cylinder heads, and gearboxes.

The lost foam casting is a model made of foam material. After painting the special coating, it is buried in the negative pressure sand box with dry sand. After the dry sand is compacted by vibration, the metal liquid is poured under negative pressure. Thus, a casting having no flash burr consistent with the foam model is obtained. The foam model vaporization disappeared during the casting process, so the image was called lost foam casting.

Lost Foam Casting (also known as solid casting) is a combination of a paraffin or foam model similar in shape to the shape of the casting into a model cluster. After brushing the refractory coating and drying it, it is buried in the dry quartz sand for vibration molding, under negative pressure. Under the pouring, the model is vaporized, the liquid metal occupies the position of the model, and a new casting method for forming a casting after solidification and cooling is formed.

The eight main advantages of lost foam and sand casting are detailed:

1, Wide production range

Lost foam casting does not require parting and lower cores, so it is especially suitable for box-type, shell-like castings, and bobbin-type castings with complex geometric shapes and difficult traditional casting.

2, high precision standard

High dimensional accuracy can be achieved and thin walls (ie 3mm) can be cast. Because of the lack of modeling, the lost foam casting directly adopts the “fusion” method, so the shape of the casting is consistent with the mold. It is not necessary to consider the factors in the sand casting shape, the casting I casting wrong box, the collapse of the box and other artificial castings.

3, suitable castings can reduce operating costs

Compared to conventional sand forming methods, complex castings, especially those requiring high dimensional accuracy and having a thin cross section, can be produced at very high cost.

4, short cleaning time

Lost foam casting has a high dimensional accuracy, and there are no parting lines or vent needles and cores, so burrs and machining can be minimized. Cleaning hours can be reduced by more than 80%.

5, suitable for mechanized production

Lost foam casting is not only suitable for large-volume castings, mechanized operation, but also for manual splicing models of small batches of products.

6, people are looking for easy to organize production quickly

Lost foam casting eliminates the molding process and eliminates the need for skilled stylists. After a short training period, you can become a skilled worker.

7, high pouring efficiency

Lost foam casting is suitable for group casting, and dry sand is easy to remove sand. In some materials, castings can also be used for residual heat treatment according to the application. Multiple castings can be combined in one mold to increase casting efficiency.

8, high customer satisfaction

Lost foam casting is not only suitable for small and medium-sized parts, but also suitable for large-scale castings, such as: machine bed, large-diameter pipe fittings, large-scale cold-punching modules, large-scale mining equipment parts, etc., because the model production cycle is short, the cost is low, and the production cycle is short. Therefore, it is especially well received.

Of course, in some cases, sand casting is still required for casting production, and specific problems should be analyzed. With the advancement of society, casting production is becoming more and more “scientific”, so as a founder, it is necessary to follow the pace of the times and continuously learn and improve the casting production technology.

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.