Aluminium extrusion

Extrusion butt

The butt in direct aluminium extrusion

During the extrusion process, sticking friction usually occurs between the billet and container. It results in inhomogeneous flow of material towards the die. The centre of the billet flows towards the die faster than the billet surface (figure 1). At the end of the extrusion stroke, the actual flow of alloy into the die is tapered. This leaves a dead metal zone at the end of the container. In this area oxides, and other inclusions and impurities from the skin of the billet are accumulated. So the extrusion must be stopped before this contaminated alloy is carried through the die, and into the product. This residue then forms the extrusion butt. After each billet has been extruded, the container is opened to expose the extrusion butt. The extrusion butt must then be sheared off before the container is closed, and the next billet is loaded (Figure 2 and 3).
Figure 1 [1]

Figure 2 – Shearing of the butt [2]

(a) end of the extrusion of the first billet;
(b) shearing the billet butt off the first billet;
(c) loading the second billet into the container;
(d) beginning of extrusion of the second billet
Figure 3

The thicknes of the extrusion butt

When the length of the billet in the container decreases and approaches the specified length of the butt, aluminum starts to flow more radially towards the center of the billet. This is because, that when the extrusion cycle ends, friction between the billet and the container wall is reduced to a minimum. The change in friction between the blank and the container during the extrusion cycle is shown schematically in Figure 4 [1].

Figure 4 – The change in friction between the workpiece and
container during the direct extrusion cycle [1]

Setting butt thickness

The thickness of the press residue is usually assigned from the following considerations, so he:

  • separated from the metal in the matrix as easily as possible
  • does not damage the metal remaining in the matrix and
  • removed from the press.

It is known, that the surface layer preform – the surface of the aluminum ingot – It contains a significant amount of pollution, including, various oxides of the alloying elements of the alloy. At the end of the pressing cycle, these surface contaminants accumulate at the end of the workpiece and begin to flow inwards. extruded profile.

This process begins when the length of the discard about 15 % of the original blank length. This is much more than that press residue, which is removed in most extrusion presses. However, considered, These inclusions are usually completely fall into the aluminum profile and therefore do not affect the quality of its surface. Except in cases, when etched profiles, for example, when anodizing. The scheme for calculating the thickness of the press residue is shown in the figure. 5.

Figure 5 – Relationship between dead zone dimensions and
thickness of the press residue [1]

Butt separation

  • Design considerations of the shear blade are necessary depending on the alloy and butt lengths typically in use on the press. Therefore, blade type changes may be necessary at product changes.
  • In the case of softer 6xxx alloy extrusion, the cut and curl type blade needs to be provided with a sharp edge and a scooped rear face to enable effective butt removal.
  • As with any cutting blade the clearance between the shear blade and die face needs to be controlled.
  • Lubrication of the rear face of the blade encourages flow of the butt across the blade, and helps butt detachment.
  • Shear blades should be removed from the press at frequent intervals and inspected and replaced if necessary.

Figure 6 – Knife Shear Blade [4]
The most effective geometry, but requires good extrusion practices.
The 2-piece design allow the cutting edge to be changed on the press and can be adapted to all blade types

Figure 7 – Scoop Shear Blade [4]
Soft alloys (6060, 6063).
Typical butt length 20-35 mm.
Provides a “cut and curl” action. Behaves as a knife blade, to cut through the butt/die interface


Figure 8 – DELTA Shear Blade [4]
Medium alloys (6061, 6082).
Typical butt length 30-60 mm.
Provides a “cut and split” action to cut through the butt/die interface

Figure 9 – 90° Shear Blade [4]. Hard alloys (7075).
Typical butt length longer than 60 mm.
Behaves as a shear, rather than a knife, and shears through the butt/die interface.

Sources:

  1. Saha P. K. Aluminium Extrusion Technology – ASM International, 2000
  2. E. Giarmas and D. Tzetzis – 6th International Conference on Manufacturing Engineering, Greece, 5-6 October 2017
  3. Modeling the Formation of Transverse Weld during Billet-on-Billet Extrusion / Yahya Mahmoodkhani et al – Materials, 2014, 7, 3470-3480
  4. Castool.com