Metal is the foundation of all modern civilization. For a year, modern mankind produces and processes such an amount of iron alone that before the whole world would pick it for at least a couple of centuries. And this need is quite justified, since an incredible amount of steel is spent on construction alone. It is not surprising that casting of metal under such conditions is constantly being improved.
A bit of history
The most important feature of iron to take, solidifying, the "proposed" form, a man noticed in ancient times. Today, almost all scientists assume that the initial acquaintance of a person with metal took place thanks to meteorites. Meteorite iron was fusible, it was easy to process, so some nascent civilizations studied the foundations of casting a very long time ago.
From time immemorial, metal casting in our country has been a respected and honorable affair; people have always treated this craft with great respect. The Tsar Cannon and Tsar Bell are widely known as masterpieces of the foundry skills of Russian craftsmen, even if one of them never called, and the second did not shoot. Ural casters in the reign of Peter the Great gained particular fame as suppliers of reliable weapons for the army. However, they rightfully carry this title right now. Before we consider the main types of metal casting, it is necessary to say a few words about the required characteristics of 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 molten form should flow as easily as possible from one crucible to another, while filling its smallest recesses. The higher the fluidity, the thinner the walls can be made on the finished product. With metal, which spreads poorly, it is much more difficult. Under ordinary conditions, he manages to grasp much earlier than he fills all the gaps in the form. It is precisely this complexity that industrialists are faced with when casting metal alloys.
It is not surprising that it was cast iron that became the favorite material of casters. And all because this alloy has excellent fluidity, which is why working with it is relatively simple. Steel is far from so fluid, and therefore, to completely fill the form (so that there are no caverns and voids), one has to resort to a variety of tricks.
In the simplest case, when home casting of metal is required, the raw materials are melted and poured into small portions in water: in particular, weights for fishing can be made. But this method is relatively widely used even in the arms industry! From the top of a special tower, resembling a cooling tower in outline, molten metal also begins to pour out in a dosed manner. The height of the structure is such that a perfectly formed droplet that has already cooled has reached the ground. This is how fraction is produced in industrial volumes.
Earthy casting method
The simplest and most ancient way is casting metal into the ground. But its “simplicity” is a relatively conventional concept, since this work requires extremely painstaking preparation. What is meant by it?
First, a full-size and maximally detailed model of the future casting is made in the model workshop. Moreover, its size should be slightly larger than the product that should turn out, since the metal will settle during cooling. As a rule, a model is made detachable from two halves.
Once this is done, prepare a special molding mixture. If the future product should have internal cavities and voids, then it will be necessary to prepare rods, as well as an additional molding composition. They should temporarily fill in those areas that are “empty” in the finished part. If you are interested in metal casting at home, be sure to remember this circumstance, as otherwise the already filled flask can simply be broken by pressure, and the consequences of this can turn out to be most sad.
What are molding compounds made of?
The basis is various grades of sand and clay, as well as binders. Natural and synthetic oils, drying oil, resin, rosin, and even tar can play their role.
Next comes the time of molders, whose task is to manufacture foundry molds. If it is easier to explain, it is done like this: a wooden box is taken, half of the mold is placed in it (it is also detachable), and the gaps between the walls of the model and the mold are clogged with the molding compound.
Do the same with the second half and fasten both parts with pins. It is important to note that two special cones are inserted in that part of the form, which will appear at the top when filling. One of them serves for pouring molten metal, the second - for the exit of expanding gases.
The end of the preparatory phase
And now the time has come for almost the most critical part of the operation. The flasks are very carefully disconnected, trying not to violate the integrity of the moldable mixture. After that, two clear and detailed prints of the future detail remain in the ground. After that they are covered with a special paint. This is done so that the molten metal does not come into direct contact with the mold sand. This technology of casting of metals should not allow, since otherwise the quality of the finished product can significantly deteriorate.
If there is a need for this, at the same time, an additional gate passage necessary for pouring the melt is cut. The flasks are folded again and connected as firmly as possible. Once the moldable mixture dries slightly, you can start casting.
Start casting
First, cast iron billets are melted in cupolas, that is, special furnaces. If it is required to cast steel, the raw materials are smelted in blast furnaces, open-hearth furnaces, invertors and other furnaces. To bring non-ferrous metals into a state of melt, specialized melting devices are used.
Everything, you can start casting. If the form is one, then the melt is poured into it with a ladle, individually. In the remaining cases, as a rule, a conveyor is organized: either the tape with the blanks goes under the bucket, or the bucket moves over the rows of flasks. It all depends solely on the organization of production. When the time comes and the metal cools, it is removed from the mold. In principle, this method is ideal in cases where metal casting is required at home (for forging, for example). Something more perfect under such conditions will still fail.
Sandblasting or grinding machines remove the scale and adhering molding mixture from the finished product. Incidentally, this method was actively used in the production of tanks during the Great Patriotic War. This is how cast towers were made, and the simplicity and manufacturability of this process made it possible to produce a huge number of military vehicles, which the front needed so much. What other types of metal casting exist?
Chill casting
But now they use much more advanced and technologically advanced methods of manufacturing cast products. For example, casting metal in a chill mold. In principle, this method is largely reminiscent of the one described above, since casting molds are also used in this case. 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 the formation of voids), and then firmly fasten them together. Everything, you can get to work. The peculiarity of this method is that here the molten metal hardens extremely quickly, there is the possibility of forced cooling of the molds, and therefore the release process is much faster. With the help of the chill mold alone, you can get hundreds, if not 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 casting method is the fact that it is suitable only for those types of metals that are characterized by increased fluidity in the molten form. For example, only pressure casting is suitable for steel (see below), since this material does not have good fluidity at all. Under the influence of compressed air, even the most "malleable" grades of steel take much better the required shape. The bad news is that a regular chill mold of such extreme production conditions simply cannot stand it and fall apart. Therefore, we have to use a special production method, which we will talk about below.
Injection molding
How is injection molding of metals carried out? We have already considered some aspects above, but nevertheless it is necessary to disclose this issue in more detail. Everything is quite simple. Firstly, a casting mold of high-quality steel grades is required, which can be multi-stage, complex internal shape. Secondly, it is necessary to pump equipment capable of issuing from seven to seven hundred MP.
The main advantage of this method of smelting is high productivity. What else provides injection molding? In this case, the amount of metals is much less, and the surface quality of the finished product is very good. The latter circumstance involves the rejection of a complex and rather dreary procedure for cleaning and grinding. What materials are most preferable to produce finished products and parts with this production method?
Most often, alloys based on aluminum, zinc, copper and tin-lead are used (casting of non-ferrous metals). Their melting point is relatively low, and therefore achieved a very high adaptability of the whole process. In addition, this raw material has a relatively small precipitate upon cooling. This means that it is possible to produce parts with very small tolerances, which is extremely important when producing modern equipment.
The complexity of this method is that when separating the finished products from the molds, they may be damaged. In addition, this method is only suitable for the manufacture of parts with a relatively small wall thickness. The fact is that a thick layer of metal will harden extremely unevenly, which will determine the formation of shells and caverns.
Varieties of installations for "pressure" casting
All machines that are used with 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. At the same time, the foundry chamber itself is immersed in hot metal. Under the pressure of air or a special piston, it flows into the casting cavity.
As a rule, a strong pumping force is not required in this case; there is enough pressure 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 molds, molten metal has to be "driven" deep into the casting chamber under especially high pressure. Moreover, 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 precisely by this method. It is especially widely used by engineering, including those directly related to the aviation and even space industry. The mass of cast parts can range from a few grams to 50 kilograms (and even higher). Can any other “processing” of metals be used by casting? Yes, and there are still many ways.
Lost wax casting
As in the very first case we examined, mankind has long been aware of the method of pouring molten metal into a pre-prepared model made of paraffin or wax. It is simply put in a flask and the gaps are clogged with a molding mixture. The melt dissolves the wax and ideally fills the entire volume of the primary preform. This method is good because the model does not need to be removed from the flask. In addition, it is possible to obtain parts of simply perfect quality, this metal casting process is relatively easy to automate.
Shell casting
If the casting is relatively simple, and “space” strength is not required from the finished product, a shell casting method can be used . They have been made from time immemorial, with fine quartz sand and resin used as the basis. Today, of course, various synthetic compounds are used as the latter.
Then, collapsible metal models, consisting of two halves, are taken and placed on a surface heated to approximately 300 degrees Celsius. Then the molding mixture is poured there (from sand and dry resin) so that it completely covers the surface of the metal models. Under the influence of heat, the resin melts, and a rather strong “flask” appears in the thickness of the sand.
As soon as all this cools down slightly, the metal ingots can be removed and the sand sent for “roasting” in the oven. After this, sufficiently strong forms are obtained: by combining their two halves, molten metal can be poured into them. What other metal casting methods exist?
Centrifugal casting
In this case, the melt is poured into a special shape, which rotates at a very high speed in horizontal or vertical projection. As a result of the action of powerful, equally applied centrifugal forces, the metal flows evenly at all intervals of the mold, due to which high quality of the finished product is achieved. This casting method is ideal for the production of various types of pipes. It allows forming a much more uniform wall thickness, which is extremely difficult to achieve using “static” methods.
Electroslag casting
Are there any methods of casting metals that can rightfully be called modern? Electroslag casting. In this case, liquid metal is first obtained by acting on pre-prepared raw materials with powerful electric arc discharges. The arcless method can also be used, when the iron melts from the heat accumulated by the slag. But on the last, powerful discharges act.
After that, the liquid metal, which has never been in contact with air throughout the entire process, enters the crystallization chamber, which, “in combination,” is also a casting mold. This method is used for relatively simple and mass castings, for the manufacture of which it is not necessary to observe many conditions.
Vacuum fill
It is used only in the case of "elite" materials, such as gold, titanium, stainless steel. In this case, the metal is melted under vacuum, and then quickly (under the same conditions) is distributed in the forms. The method is good in that when it is used, the formation of air cavities and cavities in the product is practically excluded, since the amount of gases present there is minimal. It is important to remember that the weight of the castings in this case cannot exceed hundreds or two kilograms.
Is it possible to get more weight parts?
Yes, such a technology exists. But it can be used only in those cases when one hundred tons of steel and more are simultaneously processed. First, the metal is melted in a vacuum, and then it is poured not into molds, but into special molding ladles, which are also protected from air entering their cavity.
After this, the finished melt can be distributed into molds, from which air was also previously pumped out by a pump. The steel obtained as a result of such a technological process is quite expensive. It is used for forging, as well as some types of all the same casting, when it is required to obtain blanks and parts of the highest quality.
Gasification (burnable) casting
In terms of casting quality and simplicity, this method is one of the most profitable, and therefore in modern industry it is used more and more widely. Such metal casting, whose production is increasing year by year, is especially popular in the PRC and the USA, since the industrial bases of these two countries have the greatest need for high-quality steel. The advantage of this method is that it allows casting without any restrictions on weight and size.
In many ways, this method is similar to the ones described above: for example, in this case, the primary model is used not from wax or plasticine, but from the now widely used foam. Since this material has its own specifics, a binder sand mixture is filled into a flask under a pressure of about 50 kPa. Most often, this method is practiced in cases where it is necessary to make parts weighing from 100 grams to two tons.
However, we have already said that there are no strict restrictions on the size of parts. So, using this method of casting, even components for ship engines can be produced, which never differed by their "modest" sizes. For each ton of metal raw material, the following amount of additional materials is consumed:
- Fine quartz sand - 50 kg.
- Special non-stick coating - 25 kg.
- Granular polystyrene foam - 6 kg.
- Dense plastic film - 10 square meters. m
The whole molding mixture is pure quartz sand without any additional additives and additives. It can be reused by approximately 95-97%, which significantly increases efficiency and reduces the cost of the process.
Thus, metal casting (the physics of the process was partially considered by us) is a “multifaceted” phenomenon, since today there are a lot of the latest techniques. At the same time, modern industry applies methods that were already in use several thousand years ago, adapting them to current realities.