Casting of metal: process, methods, methods

Metal is the basis of all modern civilization. For a year, modern mankind extracts and processes such a quantity of iron alone that the whole world would have picked it for at least a couple of centuries before. And this need is fully justified, since the building alone takes an incredible amount of steel. It is not surprising that metal casting under such conditions is constantly being improved.

A bit of history

The most important feature of iron to take, frosting, "proposed" to him the form, the person noticed even in ancient times. Today, almost all scientists assume that the initial acquaintance of a man with a metal was due to meteorites. Meteoritic iron was fusible, it was easy to process, so the foundations of casting some of the emerging civilizations studied a long time ago.

In our country, metal casting has always been a respected and honorable business, people have always been treated with great respect for this craft. Widely known are "The Tsar Cannon" and "Tsar Bell", which are masterpieces of the foundry skills of Russian masters, even though one of them never called, and the second - did not shoot. Uralic foundrymen in the reign of Peter the Great gained special popularity as suppliers of reliable weapons for the army. However, this title is quite rightly worn even now. Before we consider the main types of metal casting, it is necessary to say a few words about the required characteristics of the raw materials.

What should be the metal for casting

The most important property of the metal, which is supposed to be used for casting, is its fluidity. The alloy in the molten form should flow as easily as possible from one crucible into the other, while filling its minute recesses. The higher the fluidity, the thinner the wall can be made with the finished product. With a metal that spreads badly, it is much more difficult. Under normal conditions, he manages to grasp much earlier than fill all the gaps of the form. It is with this complexity that industrialists come across when metal alloys are cast.

It is not surprising that it was cast iron that became the favorite material of foundry workers. And all because this alloy has excellent fluidity, because of what to work with it is relatively simple. Steel is not so fluid, and therefore to fully fill the form (to avoid cavities and voids) have to resort to a variety of tricks.

In the simplest case, when homemade metal casting is required, the raw materials are melted and poured into small portions into water: in particular, it is possible to produce a fishing weigher. But this method is relatively widely used even in the arms industry! From the top of the special tower, outwardly reminiscent of the cooling tower, molten metal begins to pour out as well. The height of the structure is such that an ideally formed droplet reaches the ground, already cooled. This is how the shot is produced in industrial quantities.

"Earth" method of casting

The most simple and ancient way is to cast metal into the ground. But the "simplicity" of it is a relatively conventional concept, since this work requires extremely painstaking preparation. What is meant by it?

First, a full-sized and maximally detailed model of the future casting is made in the model shop. And its size should be slightly larger than the product that should be produced, since the metal will settle on cooling. As a rule, the model is made detachable, from two halves.

As soon as this is done, a special molding mixture is prepared. If the future product should have internal cavities and voids, then it is necessary to prepare also rods, as well as additional molding composition. They should temporarily fill those areas that are "empty" in the finished part. If you are interested in metal casting at home, be sure to remember this circumstance, otherwise the already filled flask can simply burst with pressure, and the consequences of this can be the most sad.

What are the molding mixtures made from?

The basis is a variety of varieties of sand and clay, as well as binders. In their role can serve oils natural and synthetic, drying oil, resin, rosin, but at least tar.

Next comes the time of molders, whose task is to make molds. If it's easier to explain, it's done like this: a wooden box is taken, it puts a half of the form (it's detachable), and the spaces between the walls of the model and the form are clogged with a molding compound.

The same is done with the second half and fasten both parts with pins. It is important to note that two special cones are inserted into the part of the mold that, when pouring, will be at the top. One of them serves for pouring molten metal, the second - for the escape of expanding gases.

Completion of the preparatory phase

And now the time comes for almost the most crucial part of the operation. Opoki very carefully disconnected, trying not to allow damage to the integrity of the molding mixture. After that, two clear and detailed prints of the future detail remain in the ground. After that they are covered with special paint. This is done so that the molten metal does not come into direct contact with the ground of the molding mixture. This technology of casting metals should not be allowed, because otherwise the quality of the finished product can significantly deteriorate.

If this is necessary, at the same time, an additional sprue passage is cut, necessary for casting the melt. Opoki again folds and the most solidly connected. Once the molding mixture is slightly dry, you can start casting.

Start casting

First, in the cupola furnaces, that is, special furnaces, cast iron blanks are melted. If it is required to cast steel, the raw materials are melted in blast furnaces, open-hearth furnaces, inverter and other furnaces. To bring into the state of melt non-ferrous metals, use specialized melting devices.

All, you can start casting. If the form is one, then the melt is poured into the bucket, individually. In other cases, as a rule, a conveyer is organized: either the belt with the blanks goes under the bucket, or the bucket moves over the rows of the flasks. Here everything depends solely on the organization of production. When the time comes and the metal cools, it is taken out of the mold. In principle, this method is ideal in cases where metal casting is required at home (for forges, for example). Something more perfect in such conditions is still impossible to achieve.

With sandblasting or grinding machines, scales and adherent molding mixture are removed from the finished product. Incidentally, this method was actively used in the production of tanks during the Great Patriotic War. This is how the cast towers were made, and the simplicity and manufacturability of this process made it possible to produce a huge number of combat vehicles that the front needed so much. What other types of metal casting exist?

Casting into chill mold

But nowadays they use much more advanced and technological ways of producing cast products. For example, metal casting in chill mold. In principle, this method is in many respects similar to the one described above, since in this case also the molds are used. Only at the same time they are metal, which greatly simplifies the process of large-scale production.

So, in two halves insert the cones and rods (for pouring metal and forming voids), and then firmly fasten them to each other. All you can get to work. The peculiarity of this method is that here the molten metal is extremely rapidly solidified, there is the possibility of forced cooling of the molds, and therefore the process of release is much faster. With the help of a chill mold alone, you can get hundreds or even thousands of castings without spending a lot of time on individual preparation of molds and molding mixtures.

Some disadvantages of the method

The drawback of this casting method is the fact that only those kinds of metals that are distinguished by increased fluidity in the molten form are suitable for it. For example, steel is only suitable for casting under pressure (about it below), since this material does not possess good flow properties at all. Under the action of compressed air, even the most "viscous" grades have become much better pressed the required shape. The bad thing is that the ordinary chill mold of such extreme conditions of production simply can not stand and fall apart. That is why we have to use a special method of production, which we will discuss below.

Injection molding

How is casting - under pressure - metals? We have already considered some aspects above, but it is still necessary to disclose this issue in more detail. Everything is simple enough. First, a mold is needed from quality steel grades, which can be a multistage, complex internal shape. Secondly, it is necessary to inject equipment, capable of issuing from seven to seven hundred MP.

The main advantage of this method of smelting is high productivity. What else does injection molding provide? Metals in this case are much less, and the surface quality of the finished product is very good. The latter circumstance implies the rejection of a complicated and rather dreary procedure for cleaning and polishing. From what materials with this method of production is it preferable to produce finished products and parts?

Most often used alloys based on aluminum, zinc, copper and tin-lead (casting of non-ferrous metals). The melting point in them is relatively small, and therefore very high processability of the whole process is achieved. In addition, this raw material has a relatively small sediment upon cooling. This means that it is possible to produce parts with very small tolerances, which is extremely important in the production of modern technology.

The complexity of this method is that when separating finished products from molds, they can be damaged. In addition, this method is suitable only for the manufacture of parts with a relatively small wall thickness. The fact is that a thick layer of metal will be extremely unevenly frozen, which will predetermine the formation of shells and caverns.

Varieties of installations for "crushing" casting

All machines that are used in this method of casting metal products are divided into two large groups: with a hot and cold casting chamber. The "hot" variety can most often be used only for zinc-based alloys. In this case, the casting chamber itself is immersed in a hot metal. Under the pressure of air or a special piston, it flows into the casting cavity.

As a rule, a strong injection force is not required, pressure is sufficient up to 35-70 MPa. So the molds for metal casting in this case can be much simpler and cheaper, which most favorably affects the final cost of the product. In cold casting forms, molten metal has to be "pounded" deep into the casting chamber under especially high pressure. In this case, it can reach 700 MPa.

Where are injection molded parts used?

They are everywhere. In telephones, computers, cameras and washing machines, everywhere there are parts obtained exactly by this method. Especially widely it is used by mechanical engineering, including directly connected with aviation and even space branch. The weight of the cast parts can range from several grams to 50 kilograms (and even higher). Can some other "processing" of metals be cast? Yes, there are still many ways.

Injection molding

As in the very first case we have considered, mankind has known from the earliest times about the method of pouring molten metal into a pre-prepared model made of paraffin or wax. It is simply put into the flask and the gaps are filled with the molding mixture. The melt dissolves the wax and ideally fills the entire volume of the primary billet. This method is good because the model does not need to be extracted from the flask. In addition, it is possible to obtain details of simply perfect quality, this process of casting metals is relatively easy to automate.

Casting into shell molds

If the casting is relatively simple, and no "space" strength is required from the finished product, a method of casting into shell molds can be used. They have been made from time immemorial, and as a basis, fine quartz sand and resin are used. Today, naturally, various synthetic compounds are used as the latter.

Then take the dismountable metal models consisting of two halves, and put on the surface, heated to about 300 degrees on a scale of Celsius. Then, the molding mixture (sand and dry resin) is poured in there so that it completely covers the surface of the metal models. Under the influence of heat, the resin melts, and in the thickness of the sand a rather strong "flask" appears.

As soon as all this is slightly cooled, the metal ingots can be removed, and the sand sent to the "roasting" in the oven. After this, they are formed in a sufficiently strong shape: by joining two halves of them, it is possible to pour molten metal into them. What other methods exist for casting metals?

Centrifugal Casting

In this case, the melt is poured into a special shape, which rotates at a very high speed in a horizontal or vertical projection. As a result of the action of powerful equally applied centrifugal forces, the metal evenly flows into all the gaps of the mold, thereby achieving high quality of the finished product. This method of casting is ideal for the production of various types of pipes. It allows to form a much more uniform wall thickness, which is extremely difficult to achieve using "static" methods.

Electroslag casting

Are there any ways of casting metals that can be called modern ones with full right? Electroslag casting. In this case, the liquid metal is first obtained by acting on the previously prepared raw material by powerful electric arc discharges. Can be used and an iron-and-mortar method, when iron melts from the heat accumulated by the slag. But on the last, powerful discharges act.

After that, the liquid metal, which has never touched the air throughout the process, enters the crystallization chamber, which "in combination" is also a mold. This method is used for relatively simple and massive castings, for the manufacture of which many conditions do not need to be observed.

Vacuum fill

Applicable only in the case of "elite" materials, such as gold, titanium, high-grade steel. At the same time, the metal melts under vacuum conditions, and then quickly (under the same conditions) is distributed according to the shapes. The method is good in that it practically eliminates the formation of air cavities and cavities in the product, since the amount of gases present there is minimal. It is important to remember that the weight of castings in this case can not exceed hundreds or two kilograms.

Is it possible to receive details of a larger weight?

Yes, such a technology exists. But it can be used only in those cases when simultaneously processed from one hundred tons of steel and more. First, the metal is melted under vacuum conditions, and then it is poured into special molding buckets, which are also protected from entering their air cavity.

After that, the finished melt can be distributed into shapes, from which the air was also pumped out beforehand. Steel, obtained as a result of such a process, is quite expensive. It is used for forging, and also for some types of all the same casting, when it is required to obtain blanks and details of the highest quality.

Casting on gasifiable (burnt) models

On the quality of casting and simplicity, this method is one of the most profitable, and therefore in modern industry it is used more and more widely. Especially popular is the casting of metal, the production of which increases year by year, is used in China and the US, as the industrial bases of these two countries are the most demanded in quality steel. The advantage of this method is that it allows casting without any restrictions on weight and dimensions.

In many respects, this method is similar to the ones described above: thus, in this case the primary model is not made of wax or plasticine, but of a widely spread foam. Since this material has its own specifics, the sandy binder is filled into a flask under a pressure of about 50 kPa. Most often this method is used in cases where it is necessary to make parts with a mass of 100 grams to two tons.

However, we have already said that there are no strict limitations on the size of the parts. Thus, by means of this method of casting, even components for ship engines can be produced, which never "modest" in size did not differ. For each ton of metallic raw materials, the following quantity of additional materials is consumed:

• Sand quartz fine - 50 kg.
• Special non-stick coating - 25 kg.
• Granulated expanded polystyrene - 6 kg.
• Dense polyethylene film - 10 square meters. M.

The entire molding mixture is pure quartz sand without any additional additives and additives. It can be used approximately by 95-97% used repeatedly, which significantly increases the economy and reduces the cost of the process.

Thus, the casting of metals (the physics of the process was partially examined by us) is a "multifaceted" phenomenon, as today there are a lot of new techniques. At the same time, modern industry uses methods that were in use several thousand years ago, adapting them to the current realities.