Extrusion – this is a plastic flow
Extrusion – pressing – is a plastic flow process, at which the flow rate of the material through the outlet must be the same at all points of the cross section of the extruded profile. The technological window of pressing is determined by several factors, one of which is wall thickness aluminum profile. To other factors, which affect the pressing process, include aluminum alloy, stretch ratio (the ratio of the cross-sectional area of the profile to the transverse area of the workpiece).
Factors for minimum wall thickness
Heat generation is a key factor.. Pushing aluminum through the matrix generates heat. The greater the degree of molding (the degree of deformation, drawing ratio and t. P.), the more heat is released. High temperature creates problems for the quality of the profile surface in the form of scoring and cracks..
Tolerances are highly dependent on wall thickness.. If the wall thickness is too small, it is very difficult to provide the necessary metal flow parameters and, Consequently, small tolerances.
The type of alloy greatly affects the minimum achievable wall thickness.. Alloys with a high alloy content are harder to press, than alloys with a low content of alloying elements and therefore they require thicker walls.
Cross Section Complexity
In general, it is easier to obtain thin walls on simple profiles., compared to complex profiles. In hollow profiles with multiple channels, it can be very difficult to fill cavities between mandrels with metal. it, in turn, may cause increased tolerances on such walls.
Smaller profile – thinner than the wall
It’s easier to get a thin wall on a small profile, than on the big.
A press with a high specific pressure (the ratio between the press force and the cross-sectional area of the container) is generally more suitable for pressing thin-walled profiles.
Minimum wall thickness of 6xxx alloy profiles
The main parameters, which control the minimum wall thickness when pressing aluminum profiles from 6xxx series alloys, are as follows:
- Pressing force.
- Maximum pressing speed at the exit of the die.
- More complex matrix design and more complex manufacturing.
- Geometric Wall Thickness Tolerances.
- Stability of geometric tolerances during the whole pressing process.
In addition to technological limitations, above, there is a very important economic aspect, which must be taken into account. Most extruded aluminum profiles sell "by weight" and thin-walled profiles may not be as profitable..
Tolerances on the wall thickness of aluminum profiles
Standards for aluminum profiles allow certain deviations of the dimensions of the profiles - above and below the nominal values. Usually, the larger the nominal size, the more tolerances on it. To understand this problem, it is convenient to consider the minimum wall thickness of extruded aluminum profiles.
On the image 1 the dependence of the minimum wall thickness for the alloy is shown 6063, as well as more durable and harder extruded alloy 6061 (AD33). It is seen, that with increasing alloy strength, the minimum wall thickness increases.
Figure 1 - Minimum achievable wall thickness of industrial aluminum alloy profiles 6063 (AD31) and 6061(AD33) .
The pattern of changes in the tolerances on the wall thickness depending on its thickness is shown in the figure 2. Profiles with tolerances for wall thickness ± 0,1 mm is very difficult to press in an industrial environment.
Figure 2 - Tolerances for wall thickness depending on its thickness
A major challenge is achieving dimensional stability and repeatability when extruding thin-walled aluminum profiles. This is especially true for hollow sections with large cavities.. Large cavities and differences in wall thicknesses are a difficult task to ensure uniform metal flow through the matrix.
Pressing thin-walled aluminum profiles
Theoretical and industrial experimental data on extrusion of thin-walled aluminum profiles give grounds for the following conclusions and generalizations:
- The main limiting factor for pressing thin-walled profiles is the lack of geometric stability during pressing. The lower limit for wall thickness is estimated at 0,6 mm.
- Pressing thin-walled profiles requires a higher pressing force, than for a regular profile with the same pressing ratio (drawing).
- In the case of pressing thin-walled profiles, it is almost impossible to achieve geometric tolerances for wall thickness better 10 %.
- For successful industrial pressing of thin-walled profiles, the critical conditions are the design of the matrix and methods for its manufacture.
A source: Aluminum Extrusion Technology Seminar, 2000.