Members aluminum profiles require more light and at the same time, rigid aluminum profiles and at a lower price.
Aluminum profiles with thin wall
The desire to reduce weight and cost of aluminum profiles dictates the use therein of reduced thickness wall. This primarily refers to those profiles, which have little load. It belongs, for example, to profiles for LED lighting or, as they are often called, aluminum LED profiles.
Aluminum window profiles due to fierce competition constantly strive to have the lowest possible thickness. for instance, typical wall thickness profiles window frames is 1,6 mm and a width of some profiles to 90 mm. Windows are enclosing structures and do not belong to the supporting structures of the building. base load, they experience – This wind load. Baling wide window profiles, especially the outer frame profile elements with thermal – thin and wide – It represents a problem for aluminum Extruders.
slimmer profile, tighter control
To control the flow of material during metal pressing - aluminum alloy - and the cross-sectional dimensions of thin-walled profiles, you must pay special attention to the design of the pressing tool and the outlet of the matrix, and careful temperature control of the preform and the pressing tool during the pressing process.
The nominal dimensions of the matrix outlet rarely coincide with a cross section profile. And the thickness, and the cross-sectional shape may vary, and the differences between them can achieve one-tenth of a millimeter.
This is because, what, At first, always there is such a thing as the difference in thermal expansion coefficients of aluminum and a matrix tool steel.
Secondly, on the running web inevitably occurs aluminum matrix adhesion, nonmetallic inclusions and various contaminants, which are present in the preform. These narrow outlet adhesion matrix. They are rarely very thick, but they, Nonetheless, can significantly affect the flow conditions through the aluminum matrix.
The deflection matrix
Thirdly, pressure on the mirror matrix - its inlet side - can reach 500 MPa – near 50 kilograms per square millimeter. This can lead to significant deformation - deflection - the pressing tool, first of all, die. These deflections matrix may seriously affect the size of its outlet.
The matrix set is usually designed in such a way, to provide maximum support to the matrix and to minimize its deflection and, Consequently, distortion of its outlet. This usually makes the lining and bolster with a maximum narrow outlet openings.
The matrices for these profiles problematic, as U-shaped profiles or hollow profiles, the first to suffer from a lack of support and experience large deflections. Moreover, few of these matrices, for which it would be possible to completely compensate for their deflection during molding.
aluminum flow instability
In the case of thin aluminum profiles warping cross-sectional, and, to some extent, its thickness fluctuation may also occur as a result of the phenomenon of, as the instability of the metal flow. Experienced adjusters matrices can customize the length of the running web and their angles so, to facilitate metal flow there, where it is needed. In this they are out, that the metal velocity at the outlet of the matrix becomes close to uniform over the entire cross section of the profile. With the same purpose of the matrix is often done with feeders.
aluminum current self-stabilization
If the resistance to flow of metal along any flow lines change, the profile at the outlet from the die shear stresses occur, which counteract shear deformations. Profile plot, which moves faster, pulls the adjacent portions lagging. As a result, compressive and tensile stresses also appear in the profile (figure 1). Therefore, it can cause both thinning, and thickening Profile.
Figure 1 - For thin strips pressing step length used running web matrix. Within the vicinity of the edges of the profile is too fast so warped profile at the outlet of the matrix. This molding defect, which can be called "wrinkles", there, when the voltage profile reaches a certain critical level.
Due to the impact these shear stresses within the metal during pressing is substantially stable and the shape of the profile is distorted slightly. This mechanism is called the self-stabilization mechanism..
How does current instability
Limits metal flow resistance may be exceeded, if a:
- the design of the matrix and the flow of metal in the container contribute to the instability of the flow;
- profile is too thin and wide,
- metal (aluminum) is excessive for some reason “soft”.
This excessive “softness” metal can occur due to:
- its excessively high temperature at the outlet of the matrix,
- during pressing unalloyed aluminum or overly dilute - minimally doped - aluminum alloys.
molding defects “garmoška” and “wrinkles”
In such cases, the difference in the speed of their expiry aluminum matrix may cause warpage such forms, as shown for pressing wide thin strip.
In the first case a higher speed have a central web portion. This defect in English baling called «buckles», that in analogy with the rolled steel defects can be called "accordion".
In the second case the faster moving band edge. This defect is pressing in English called «wrinkles», but in Russian it is logical to call the "wrinkles".
Sustainable for a puller
In an industrial environment in pressing the aluminum profiles used puller, which leads - pull - profile from the exit of the die and before the end of the table, meters on forty or fifty. Small tensile force, puller that applies to profile, in many cases it can prevent the unstable flow of aluminum matrix.
If the flow is unstable metal from the exit from the die, the pressing in this case may not be possible. There are cases, that the outlet is fully blocked by the metal.
It must be emphasized, that the instability of metal flow is rarely a problem in the pressing of a sufficiently thick sections. Kits spotters matrices and the matrices based on their own experience and statistics on the result of the pressing profiles similar, in general, are quite workable matrix. These matrices require a relatively small number of compressions and adjustments to run their production.
Basically, pressing technology allows to produce very thin aluminum profiles - to 0,5 mm, and even thinner, depending on aluminum alloy. However, there is a limitation on the ratio of wall thickness to the width of the plate or shelf Profile, which depends on the technological capabilities of a particular extrusion press. This means that, that the larger the width of the profile, the greater the minimum wall thickness.
Flow instability thin strip
The thinner profile, moreover it is prone to triggering mechanism metal flow instabilities. Unstable for aluminum - it is over, when different portions of the profile at the outlet of the matrix are moving at different speeds. If the thickness is excessively small profile, the application of shear stress is not enough to develop the self-stabilizing mechanism, to prevent warping. The critical value of effort, which lead to warping profile as it exits from the die, also reduced, so thin profiles "warp" is much easier.
Announced the results of experiments on the compression of thin strips of width 78 mm and a thickness of 1,1 to 1,8 mm alloy 6060. When the thickness of the pressing strips 1,8 mm instabilities arose. When the thickness of the pressing strips 1,1-1,4 mm phenomenon of flow instability is already clearly present. Increasing the strip temperature at the exit of matrix 480 to 520 ºC "provoked" a clear manifestation of the instability of.
Factors unstable flow of aluminum
The problem of unstable flow of aluminum from the matrix closely associated with the rheological properties of the metal and metal friction characteristics of a pressing tool. The extrusion process is very complicated and difficult to mathematically describe. deformation zone and friction conditions in the matrix are in constant change. flow inhomogeneity can occur even in a container, and on the running web of the matrix it is manifested most clearly.
The instability of the metal flow is associated with such factors as:
- non-uniformity of pressure distribution;
- different conditions of metal flow;
- sliding conditions on the belts of the matrix;
- inhomogeneous friction on the matrix belts;
- deflection of the matrix;
- the degree of wear of the matrix;
- the nature of slip-sticking on the border of the dead zone of the matrix;
- dramatic change the stress state of the metal after exiting the die.
A source: P. T. Moe, PhD Thesis, 2005