The point is that one can try to make the same part or batch of parts by any known method of metal casting. The only difference is that while casting a part by use of different methods we would get different castings on quality, price and time spent for the casting process. There are sufficient variations, one can get ones of high-quality, however these are very expensive parts, they can be even more expensive and at the same time they would be not all satisfactory with respect to quality, they could be of medium quality by the suitable price, but one can correctly choose a kind of casting and accordingly get a high-quality part by spending for it a minimum amount of money and time.
Thus our goal will be to choose a correctly kind of casting and then to get an inexpensive and of high-quality casting. How will we do that? I do have the following plan. First we have to understand, what kinds of casting generally exist. And then we will have to define the criteria to the part on which we could determine the kind of metal casting that would be more suitable for achieving our main goal – to get a quality casting for the minimum amount of money spent.
So now let us go from simple to complex and get acquainted with all available to us of variants in the “menu” of the foundry.
As a determing factor in this process is that the moulds are once-only i.e. each of such mold one can use only once, then it would be destroyed and respectively for the next casting it is necessary to produce the next mould. And this is not so a fast process and it is quite labor-consuming. The sand-clay moulds are made, while compacting molding mixtures (based on quartz sand, clay and auxiliary materials) on the model of the product (usually made of wood) in the flask. Before consolidating of the molding mixture one installs on the model inside of the flask the gating system (through it the molten alloy is flowing and gases are being exhausted) and the rods (which are used for making holes in the casting). This kind of casting can be quite well automated or be completely non-automated however the essence would be the same – it is used to get simple specimens of the casting parts. One mould should be one part.
As advantages of this kind of casting will be to obtain parts of various moulds and mass.
As disadvantages are: low accuracy sizes of the obtained castings, a low surface quality as well a high labor-consuming of the process.
The essence of the process will be the same as in the previous example. Therefore let us stop at the differences. The casting mould (chill mold) is not one-off disposable and destroyable but reusable and indestroyable. Accordingly as a kind of casting it possesses a less labor-consuming process (the chill mold needs not to be produced again and again after making a previous casting). As a rule, the chill molds are being made from metall. Since the chill mold is metallic, so it has a great strength, heat capacity, thermal conductivity and there is practically no gas permeability. Because if this it has the following advantages respecting the quality of castings: greater dimensional accuracy and less surface roughness.
The gravity die casting is used practically for casting of all metallic alloys: cast iron, steel and non-ferrous metals, as well their alloys (aluminum, copper and magnesium). The castings of simple and medium shapes or with the medium and thick walls are obtaining from cast iron and steel.
It is difficult and impractical to get in the child molds castings with a developed surface, of a complicated shape, with a sharp difference of the wall thickness, with sharp angles or long protruding parts. Due to the fact that the costs for producing of the chill molds and tools significantly exceed the value of the same casting, so this kind of casting would not be suitable for obtaining of the simple castings. So it is worth to start with the production of the parts while using the gravity die casting only in the case if the minimum size of the batch would exceed 50 unites and more (depending on their mass).
For brevity and better understanding we will stop on the differences and similarities of the already considered types of casting. The main difference of this kind of casting from the previous ones will be the possibility to get any complex shape as well a developed casting outer surface. At the same time this casting has a high accuracy and a low roughness. While using this kind of casting one can obtain casting parts to begin with several grams (jewelry, for example) up to some tens of kilograms.
The cast process is described as follows. First a very precise model of the product is being made, with its help is produced a die casting mold in order later to produce many models from fusible materials. Each of such fusible models will be used for obtaining only of one casting part from metal. These melted models are made by pressing paraffin (or of other low-melted material, like: stearin, wach and so on or of salt composition for greater parts) into the mold. The model to be melted is being dipped into the pulverized refractory suspension in the binder liquid. This layer is dried and then applied by analogy from 5 up to 10 successive layers alternately dipping them into the binder suspension and sprinkling with a refractory composition of a different fraction. After completion of drying of the last layer the melted model is heated above the melted temperature point and thus it will be drained from the mold. The obtained shell (casting mold) is annealed at the high temperatures, then it will be installed into the oven and filled into it the melted metal. After cooling and crystallization of the casting the shell will be mechanically removed (destroying it in a convenient way, while breaking, for example).
Despite the indisputable advantage – possibility of obtaining a skinful complicated shape and a developed outer surface; there are disadvantages of the present kind of casting too. This is a large consumption of metal as well a high unit cost of consumables in the very cost of the casting part. Therefore it is mainly used for a mass production of castings of a high accuracy with a minimum of subsequent machining or generally without it when the criterion of the final cost will be secondary in relation to the criteria of their quality.
Let us stop at this method of casting briefly at all. It differs from the previous method of casting of the lost wach casting through the fact that there instead of paraffin for making a model the foam polystyrene is being used which before pouring metal is not being melted from the shell mold and it is situated directly in the sand mixture under vacuum. Under influence of the high temperature of the melted metal which is to be filled in such a mold with a foam plastic model, so the model is being evaporated and the metallic alloy will fill this volume to be occupied before by this model. The main advantages of the method will be a wide range of castings mass: from 100 g up to more than 2000 kg and their overall dimensions: from 4 cm up to 1 meter.
The main feature of this kind of casting is that it makes it possible to obtain castings depending on complexity of the shape, accuracy and quality of surface similar with the obtained castings by the method of lost-wach casting quality. However at the same time the cost of castings obtained by the Croning – process is lower than that by the lost wach casting.
The essence of the process is as follows. The casting mold is made by pouring a mixture of quartz sand and phenol-formaldehyde resin or pulverous bakelite into a metallic model heated to a temperature of 300˚C. The mixture „sticks“ to the model, afterwards it will be cooled for some seconds and the excess is removed. Subsequently the mixture is heated on the model up to 350-360°C in this way while hardening the mold. There is an alternative option of applying a mixture to the model – it is blown into the gap between the model and the contour plate which is located outside of the model. Regarding this variant the sand has to be clad i.e. each grain of sand will be covered with a thin layer of resin. Having removed the mold from the model (in the form of 2 semi-mold) and having fastened them with the staples or adhesive composition on the base of the pulverous bakelite, receiving a shell casting mold ready prepared for pouring the melted alloy. The casting is obtained while pouring the alloy into such a mold.
Due to this kind of casting one can produce castings with a mass of 200 g up 30 kg. With the wall thickness from 3 up to 15 mm.
It is difficult describe this kind of metal casting by the example of the differences and similarities from any of examples considered above. That is why it is very different both regarding the technological process and concerning the casting quality and on the limitations for the outer and inner casting shape.
The die casting organization for a concrete part looks like as follows: First we have to produce a casting mold (for an aluminium die casting it is usually called as a die casting mold). Usually it is ordered at the enterprises, specializing in the production of the die casting molds. Designing and manufacturing of the die casting mould is a complex, responsible, precise and expensive process. The approximate term for designing of the die casting mold does make up about 2 weeks. 4-6 weeks more will be required for it producing plus the delivery time depending on the remoteness of the tools producer and the customer as well on a chosen transport kind. Totally as you see, it would take you from three months in dependence on the complexity. There is not any average value for producing the die casting mold. The calculation is carried out individually for a concrete part (casting) and concrete demands regarding casting from the customer. The cost usually varies in the range from 5 up to 100 thousand USD. The die casting mold for die casting is produced from the high-quality steels with a hardness of 37-48 HRC. The mold consists of some movables and fixed parts being necessary to ensure that in a closed state its cavity would form a required shape of a future metallic casting and however in an open state the casting could be freely extracted from the mold.
After having this die casting mold, so it is will be accordingly installed into the casting machine. Further on the die casting process would follow: the melted aluminum alloy (or other not-ferrous metal alloy) is pouring into the pressing chamber and afterwards, under action of a piston, by an enormous pressure (hundreds of times higher exceeding the pressure to the melted metal in case of any other casting kind) is being pushed out from the pressing chamber (in fact – of a large metal syringe) in a closed die casting mold, where it subsequently is being crystallized while forming a casting. Then the die casting mold is being open and the aluminum casting (or a casting from other metallic alloy – as a future casting part) will be pushed out from the mold and accordingly extracted for its further use.
Advantages of the present casting kind are as follows:
The disadvantages of this kind of die casting will be a great availability of restrictions regarding outer and inner shapes of the casting part in comparison with other methods where a main advantage is a developed surface and complexity of the shape.
In order not to get confused with all above mentioned kinds of casting the table below is given with the list of criteria in accordance with them one can preliminary determine if the aluminium die casting would suitable for you and if not to determine another one more suitable for your conditions as well metal casting kind.
|Sand casting||Gravity die casting||Lost wax casting||Lost-foam casting||Shell molding||Die casting (aluminium die casting)|
|Dimensional accuracy class||6-16||5-13||4-11||3-13||4-11||3-8|
|Surface accuracy class||7-21||4-14||3-13||3-13||5-16||2-11|
|Roughness (Ra), μm||8- >100||4-40||3.2-32||3.2-32||6.3-63||2-20|
|Minimal casting mass, kg||100000||500; 4000; 14000 (non-ferrous metal, steel, cast iron)||150||2000||300||32|
|Wall thickness range,||3 and more||3-100||0.5-6||0.5-6||3-15||0,6-6|
|Minimum batch shipped size, piece||1 and more||200 and more small
50 and more big castings
|Allowable mold complexity||High||Average||High||High||Complex||Average|
|Tools investments, USD||From 100||From 3000||From 1000||From 1000||From 3000||From 5000|