Bearing Length Rules for Balanced Metal Flow

In an extrusion die, every part of the profile opening must release metal at the same speed. If one wall runs faster than its neighbour, the profile bends, twists, waves or tears. The bearing, the short land that gives the profile its final shape, is the designer's main tool for equalizing speed. This article explains how bearing length controls flow, gives starting values, and lists the factors that lengthen or shorten each bearing.

How the bearing controls speed

Aluminum sticks to the bearing surface and is sheared as it passes. The friction force grows with the length of the bearing, so a longer bearing slows the metal and a shorter one lets it run faster. Three shapes are used.

The angles involved are small, from a fraction of a degree to about two degrees, and their effect is large. Behind the bearing, the die is undercut (relief) so the profile touches nothing after it leaves the bearing.

Why the speed is uneven in the first place

Without bearing control, metal would leave the die at different speeds because:

1. Thick walls flow more easily than thin walls. Thin sections have more friction surface per unit of area.
2. Metal near the centre of the billet flows faster than metal near the container wall, where friction against the liner holds it back.
3. Free ends and tips are slow. They are fed from one side only and have friction on three sides.
4. In hollow dies, areas under a bridge are slow and areas directly in front of a port are fast.
5. In multi-hole dies, the openings nearer the centre run faster than those nearer the edge.

Bearing lengths are assigned to cancel these differences.

Starting values

For 6063-type alloys, a workable starting procedure is:

1. Find the slowest point of the profile: usually the thinnest wall, at a free end, farthest from the die centre. Give it the shortest bearing, typically 2–3 mm and not less than about 2 mm, which is the practical minimum for strength and wear.
2. Increase the bearing in proportion to wall thickness. A common starting ratio is a bearing length of about 2 to 3 times the wall thickness.
3. Increase the bearing toward the die centre, and reduce it toward the edge of the die.
4. Reduce the bearing at tips and free ends, typically by 20–30 % compared with the same wall away from the end.
5. Limit the maximum. Beyond roughly 10–15 mm, extra length adds little control and increases heat and pick-up. If a section still runs fast with a long bearing, use a choke or restrict its feed instead.

In the example, the 1.5 mm flange ends get 3.0 mm, the flange grows to 4.0 mm as it approaches the centre, the 4 mm web gets 8.0 mm, and the free tip of the web is reduced to 6.0 mm. The junction is blended at 7.0 mm.

Factors that change the bearing

Transitions between different lengths

Abrupt steps between two bearing lengths cause streaks that show after anodizing and create a line where the profile can tear. Rules:

• Blend from one length to the next over a distance, not at a single point. A slope of about 30–45° on the bearing back edge is common.
• Keep the difference between adjacent bearing lengths moderate; if a large difference is needed, the feed (pocket or port) should be changed instead.
• Put transitions at corners or junctions where possible, not in the middle of a visible flat face.
• On the two sides of one wall, keep the bearing lengths equal, or the wall will curve toward the longer side.

Bearing is not the only control

When the wall thickness ratio in one profile is large, bearing length alone cannot balance the flow. Use the feed in front of the bearing:

A good layout and feed design leave the bearings with only fine adjustment to do. A die that depends on extreme bearing differences is hard to correct and sensitive to temperature and speed.

Manufacturing quality of the bearing

A correct length on the drawing only works if the bearing is made correctly.