When developing a new aluminum profile sooner or later have to face restrictions, which the molding process itself imposes on its size and shape - the extrusion process - aluminum. These limitations depend on both press equipment and press technology., and from the choice of aluminum alloy.
Rules: written and not written
Manufacturers of extruded aluminum profiles have long established general rules for the design of aluminum profiles, the implementation of which provide a new profile for successful extrusion pressing. Some of these rules are general in nature, while others relate to profile sizes and, naturally, Depend on aluminum alloy and process equipment.
However, the only absolute restrictions are maximum profile sizes, which are dictated by the size of the container. Very few such profiles, which in principle could not be pressed. However often, that the quality or price of such problematic profiles may not suit the customer.
More profile asymmetry - lower quality
The key word in the development of extruded aluminum profiles is the word "symmetry". The asymmetry of the profile gives an uneven flow of the metal and therefore requires a complex - and expensive - matrix design. Cross-sectional metal flow rate should be controlled, and this often leads to a decrease in pressing speed. Moreover, if the matrix experiences asymmetric loading, then the risk of breaking it increases, particularly during pressing high-strength aluminum alloys.
Asymmetric profiles can also cause temperature differences., in the matrix, and in the profile itself during cooling during quenching. Another problem with such profiles is, that parts of different thicknesses receive different thermomechanical processing and as a result a large difference in microstructure and mechanical properties. Another problem with asymmetric profiles is the complexity of bending and editing operations.. Examples of deviations from profile symmetry can be seen in the figure below..
The large ratio between the thickest and thinnest profile walls causes difficulties in transferring it along the press line, and a large eccentric cavity leads to an unfavorable flow of metal.
Larger profile sizes – more problems
If the profile is too large - has a large diameter of the circumscribed circle - there may be problems with getting into the metal profile from the surface of the workpiece. This leads to poor profile surface quality and defects such as bandedness., special, on anodized profile.
Moreover, if large profiles are pressed on solid dies, then they (dies) are usually not sufficiently supported by an auxiliary pressing tool - shims and bolsters. When pressing large hollow profiles on the mandrel of the matrix, excessively large forces arise. In both cases, the result is more significant dimensional deviations, as well as poor surface quality due to matrix traces (scratches) and visible stripes.
On the other hand, too small profiles give too high a pressing ratio (stretch ratio). In this case, the press may not be able to give the necessary effort to form such a profile.. The solution to this problem is often pressing several stitches (threads) of such a profile at the same time with an increase, in addition, productivity.
Cavities and languages of aluminum profiles
Simplicity is another key word in the aluminum extrusion process., although its main feature is, conversely, therein, to produce complex, maximum functional profiles. Like hollow profiles, and profiles with large "languages" tend to complicate production and therefore should be avoided if possible. matrix, on which such profiles are made, are more complex and, Consequently, more expensive.
However, the biggest problem is that these matrices, that they create more significant efforts on the matrix due to additional obstacles, which they create for the flow of aluminum. On the other hand, they have a more complex - and weaker - design, which reduces their ability to resist loads. Kinks of the matrix and large size deviations are characteristic phenomena for such matrices. To solve this problem, languages are often made with the lowest possible length-width ratio, which provides the language with more reliable support. Hollow profiles are sometimes performed in the form of two solid profiles, which then join together.
More details - harder to press
Often, many special parts are included in one aluminum profile., so that he was able to carry a large number of functions at once. However, adding even a small part can lead to a big change in productivity and product quality. Therefore, you must always evaluate, is this detail so necessary and is it possible to execute it in the form, which will be more “friendly” for the pressing process.
Sharp corners Profile
Sharp corners can, In many cases, cause incomplete matrix filling, and in others, the occurrence of hot cracks. Moreover, the nucleation of cracks and their growth occurs precisely in those elements of the matrix, which form the sharp corners of the profile. Therefore angles, indoor and outdoor, must always be rounded. Smallest radii, which are usually used, have a value from 0,4 by 1,0 mm depending on wall thickness. for instance, the smallest minimum radius of rounding have walls up to 3 mm: 0,3 mm according to GOST 22233-2001 and 0,5 mm according to EN 12020-2.
Sharp transitions of profile thickness
Sudden transitions of wall thickness and, Consequently, sudden changes in the length of the working belt of the matrix. With a sharp transition in thickness, two types of metal flow problems arise.. One side, if working belts are not carefully designed, then there are difficulties with acute angles near such abrupt transitions in wall thickness. On the other hand, due to a sharp change in the conditions of deformation and cooling between thin and thick sections of the profile, sharp differences arise in the microstructure of the metal. These differences in the microstructure lead to the formation on the surface of the profile of visible bands with different reflectivity – defect, which is called “flatness“.
The solution to this problem is the smoothest transition from a thick-walled section of the profile to a thin-walled. If this is not possible, then sometimes at the border between the thick and thin section of the profile perform special decorative risks, to mask these stripes whenever possible. However, this does not solve the problem with differences in mechanical properties in thick and thin sections.
Thin profile wall
In a sense, pressing thin-walled parts of aluminum profiles is always a problem.. Very high friction forces occur in such areas and the flow velocity is difficult to control.. As a result, complete filling of metals of some sections of the cross section and, of course, higher pressing force required.
If thin wall aluminum profile is in addition also internal (see. drawing), it is even more difficult to achieve full filling of this section of the matrix. Moreover, the pressure in the welding chamber may not be enough to produce a good weld. A weak or incomplete internal weld is a serious defect in the profile., and finding it is very difficult. For this reason, internal welds should be avoided.. It is often more reasonable to apply a thicker wall., to provide conditions for a good weld, despite the increase in the cost of material. Another problem with internal welds is, that in general, the matrix design becomes less durable and therefore the risk of breaking it increases.
Overly complex cross-sectional details cause problems, which are associated with a violation of the uniformity of the metal flow and increased heating from friction. As a result, pressing performance is reduced.. Moreover, additional problems are profile surface breaks and incomplete filling of matrix channels. In any case, these parts should be massive enough., not too long and really necessary. In the same time, all sharp corners should be rounded. You should always remember, that parts inside a cross section are usually always more problematic, than the outer parts.
In addition to the shape limitations of aluminum profiles, which are related to the pressing process itself, there are many other limitations, which are dictated by the conditions of use of finished products. One of these limitations may be, for example, the, that the shape of the profile should prevent corrosion of its surface. To do this, the profile must not contain elements, which lead to the accumulation of water or interfere with its drainage.
Источник: Handbook of Metallurgical Process Design / George E. Totten et al, Marcel Dekker, 2004