Aluminum extrusion: the formation of a "mushroom" ("hat")

When extruding (pressing) aluminum grades and "soft" aluminum alloys, such as alloy AD31 (6060/6063), often there is a problem of metal squeezing into the gap between the container sleeve and the matrix mirror.

Mushroom or hat?

Aluminum extruders call this phenomenon differently: who is the "mushroom", and who is the "hat" – it looks like that, and on the other (figure 1). The cause of this problem is insufficient sealing - for various reasons - of the joint between the container and the die. English-speaking aluminum extruders call this phenomenon "flare".

ekstrusiya-alyuminiya-grib-shlyapaFigure 1 - Formation of "mushroom" (hat ") when pressing soft aluminum alloys [2]

Three causes of the "mushroom" ("hat")

Flatness of container sleeve end

Violation of flatness of the end of the container sleeve due to adhesion of aluminum to it, container liner wear or dents in the container liner. This reduces the sealing of the joint between the container and the matrix..

Die diameter too small

Too small die diameter compared to the outer diameter of the container. Small-diameter dies gradually develop a matrix imprint on the end of the container sleeve. If then a die with a large, normal, diameter, then it is easier for aluminum to find its way out and form a "mushroom" ("hat").

Insufficient clamping of the container to the die

Insufficient clamping of the container to the matrix occurs due to a decrease in pressure in the cylinders for moving the container.

At the beginning of pressing, the end face of the container sleeve is reliably pressed against the matrix mirror due to the combined action of the pressing pressure and the braking force of the workpiece against the container wall (Figure 2).

ekstrusiya-alyuminiya-bolshaya-zagotovkaFigure 2 - The beginning of the pressing cycle: a long workpiece and a significant braking force of the workpiece against the container wall [1].

As pressing continues, the workpiece becomes shorter, and the braking force of the workpiece against the container wall decreases, until it reaches a minimum when the length of the workpiece reaches the thickness of the press cake (figure 3).

ekstrusiya-alyuminiya-grib-shlyapa-2Figure 3 - End of the pressing cycle: short workpiece and low braking force of the workpiece against the container wall [1].

In this position, the sealing of the container sleeve with the matrix mirror is provided only due to the pressing pressure of the container by the container moving cylinders. Moreover, the remainder of the workpiece is in a more plastic state due to an increase in its temperature at the end of the pressing cycle, which also contributes to the extrusion of metal through the gap. In such cases, before replacing the container bushing, the formation of "mushroom" ("hat") can be avoided by temporarily increasing the thickness of the press residue..

How to avoid the formation of a "mushroom" ("hat")

Several preventive measures are taken to prevent the formation of a "mushroom hat", such as:

"Correct" die diameter, as it shown on the picture 4.

Follow, so that the end of the container sleeve is always clean.

Periodically check the value of the container closing pressure by the container movement cylinders.

ekstrusiya-alyuminiya-normalyy-matrichnyy-komplektFigure 4 - "Correct" die diameter helps to prevent the formation of a "mushroom" ("hat") [1].

Sources:
1. Saha P. Aluminum Extrusion Technology, AMS International, 2000.
2. Bello L. Die Corrections for Changing Flow Characteristics, Aluminum Extrusion Technology Seminar, 1977.