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 up for at least a couple of centuries before. And this need is completely justified, since for an only construction takes an incredible amount of steel. It is not surprising that metal casting under such conditions is constantly being improved.
A bit of history
In our country, metal casting from time immemorial has beena matter of respect and honor, people always treated this craft with great respect. Widely known are the "Tsar Cannon" and "The Tsar Bell", which are masterpieces of foundry craftsmanship 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 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 metal, whichit is supposed to be used for casting, - 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 flows 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 no wonder that it was cast iron that became a favoritematerial foundry. 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), one has to resort to a variety of tricks.
"Earth" method of casting
The simplest and most ancient way ismetal casting 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 in the model shop is a full-sized andthe most detailed model of the future casting. And its size should be somewhat larger than the product that should be produced, as the metal will settle on cooling. As a rule, the model is made detachable, from two halves.
As soon as this is done, a specialmolding mixture. 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 must temporarily fill those areas that are "empty" in the finished part. If you are interested in metal casting at home, be sure to keep this circumstance in mind, otherwise the already filled flask may simply burst with pressure, and the consequences of this may be the saddest.
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, and at least tar.
Next comes the time of the moulders, the taskwhich includes the manufacture of molds. If it's easier to explain, then it's done like this: a wooden box is taken, it puts a half of the form (it's detachable), and the gaps between the walls of the model and the form are clogged with a molding compound.
The same is done with the second half andfasten both parts with pins. It is important to note that two special cones are inserted into that part of the mold, which, when pouring, appears at the top. One of them serves to fill the molten metal, the second - to exit the expanding gases.
Completion of the preparatory phase
And now it is time for almostresponsible part of the operation. Opoki very carefully separated, 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 timean additional gating pass is cut, necessary for casting the melt. Opoki re-fold and maximally connect. Once the molding mixture is slightly dry, you can start casting.
Start casting
All, you can start casting. If the form is one, then the melt is poured into the ladle, 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.
Sandblasting or grinding machines withThe finished product removes scales and adherent molding mixture. 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 in chill mold
But nowadays they use much more advanced andtechnological methods of production of cast products. For example, metal casting in chill mold. In principle, this method in many respects resembles 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 cones and rods(for pouring metal and creating voids), and then firmly fasten them to each other. Everything, you can get down 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 the 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 disadvantage of this method of casting isthe circumstance that only those kinds of metals which are distinguished by increased fluidity in the molten form are suitable for it. For example, for steel, only casting under pressure (about it below) is suitable, 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 on the desired shape. The bad thing is that the ordinary chill mold of such extreme conditions of production simply will not survive 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 multi-step, complex internal shape. Secondly, it is necessary to inject equipment capable of issuing from seven to seven hundred MP.
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 entire 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 separatingfinished products from molds may damage them. 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 with this methodcasting of 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 atThis is not required, enough pressure 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, the molten metal has to be "pounded" deep into the casting chamber under especially high pressure. At the same time, it can reach 700 MPa.
Where are injection molded parts used?
They are everywhere. In phones, computers, cameras and washing machines, everywhere there are parts obtained exactly by this method. Especially widely it is used by machine building, including directly related to the aviation and even space industry. The weight of the cast parts can range from several grams to 50 kilograms (and even higher). Can any other "processing" of metals be cast? Yes, there are still many ways.
Injection molding
Casting into shell molds
If the casting is relatively simple, and"Space" strength from the finished product is not required, the 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 dismountable metal models are taken,consisting of two halves, and placed on the surface, heated to about 300 degrees on a Celsius scale. 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 cool down slightly, the metalChutes 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 form,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 uniformly 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,which with full right can be called modern? 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, whichthe whole process has never touched the air, enters the crystallization chamber, which "in combination" is also a mold. This method is used for relatively simple and mass castings, for the manufacture of which many conditions do not need to be observed.
Vacuum fill
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 the introduction of air into their cavity.
After that, the ready melt can beDistributed according to the forms from which the air was previously evacuated by the pump. 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
For cast quality and simplicity, this methodis 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: So, in this case, the primary model is not made from wax or plasticine, but from a widely spread foam. Since this material has its own specifics, the binder sand mix 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 some hardthere is no restriction on the size of the parts. Thus, by this method of casting, even components for ship engines can be made, 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 whole molding mixture is pure quartz sandwithout 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.
