Circumscribing Circle Diameter and Minimum Wall Thickness

What the circumscribing circle diameter sets

In the direct process the metal at the centre of the billet flows faster than the metal at the edge. The wider the CCD, the more of that uneven flow the die has to correct, and the harder the shape is to hold to tolerance. Three things follow from the CCD. It decides the dimensional stability of the shape. It decides how much of the billet skin, and with it the oxide and the nonmetallic inclusions, can flow into the extrusion toward the end of the run. And it decides how thick the butt has to be, because a larger CCD needs a thicker butt to avoid waviness at the end of the extrusion. The clearance between the CCD and the container bore is therefore a design decision, not a leftover.

Two ratios turn the shape into a difficulty number. The form factor is the CCD divided by the minimum wall thickness. The shape factor is the perimeter of the section divided by its weight per unit length. Both rise as the section gets more complex and the wall gets thinner, and both feed into the production rate, the manufacturing cost and the design of the die and its tooling.

Minimum wall thickness against CCD and press size, soft and medium alloys

The table below is the working relationship for the soft and medium grade alloys, the 1000, 3000, 5000 and 6000 series that make up most extruded tonnage. Read a row across to find the thinnest wall a shape of that alloy and section type can carry at a given CCD, and read the bottom row to find the press that CCD implies. Section type A is a solid or semihollow shape, B is a hollow shape with a uniform wall including tube, and C is a hollow shape with walls of different thickness.

Relationship between minimum wall thickness, circumscribing circle diameter and press size for different soft and medium grade alloys. The wall thickness values are in millimetres and the press sizes are in meganewtons. Source layout preserved.

The same relationship for hard alloys

Hard alloys carry the same table with tighter limits, because they resist flow and their hot working range is narrow. A 2014 or a 7075 shape needs a heavier press and a thicker wall than a 6063 shape of the same CCD, and the gap widens as the CCD grows. The values below are for guidance. Actual billet temperatures and extrusion speeds also depend on the final shape and the extrusion ratio, and it may be necessary to start with a lower temperature than the alloy would normally allow.

Relationship between minimum wall thickness, circumscribing circle diameter and press size for various harder grade alloys. Where the table gives only section type A for a grade, the hollow version of that shape is extruded over a mandrel and needs a thicker wall again. Source layout preserved.

Extrudability decides which alloy goes on which press

Extrudability is measured by the maximum extrusion speed an alloy will take before it cracks, and it is the second half of the die design question. An alloy with a high rating flows easily and can be pushed into a complex shape on a modest press. An alloy with a low rating resists flow, runs at a low speed and needs a heavier press, a thicker wall and a die that can take more load. The source groups the soft and medium alloys into three bands. Above 50 is easy to extrude, 30 to 50 is moderately difficult, and the hard alloys below 25 are difficult. The table merges the two published ratings into one list so the two families can be read against each other.

Relative extrudability of the alloys covered by the two wall thickness tables. For the soft and medium grade alloys a rating of 50 to 150 is easy to extrude and 30 to 50 is moderately difficult. For the hard alloys a rating of 25 or below is difficult.

Where this leaves a die steel order

The CCD sets the load, the wall sets the bearing, and together they set what the die steel has to hold. The die body is the chromium hot work steel AISI H13 and its European equivalent 1.2344, covered on the H13 tool steel page, and the extrusion die supply programme is on the 1.2344 ESR H13 extrusion supply page. The grade by component is on the best tool steels for aluminum extrusion dies page. The bearing that the wall thickness dictates is on the die bearing length page, the tooling around it on the die types and tooling page, and the way the die is consumed on the aluminum extrusion die wear page.

Source, Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000.