Correcting an Aluminum Extrusion Die: What to Change, and in What Order
No die comes off the wire EDM running the shape its drawing promised. The metal flow, the die deflection under load and the temperature along the bearing are not fully predictable, so the die is tried at the press and corrected. The correction is where a die shop earns its reputation, and it follows an order: check the press and the stack first, then the eight process variables, then the die itself, ports before bearings on a hollow die. This page sets out that order and the reason for it.
First: things that look like a die fault and are not
Before a cutter goes near the die, the press side has to be ruled out. The published list of press faults that present as die problems is short and specific, and every item on it has been blamed on a die at some point.
| What you see | What to check before touching the die |
|---|---|
| Profile runs out of square, or one side is heavy along the whole length | Press alignment and adjustment; the flatness of the pressure ring |
| Metal escapes at the die face, or a flash line appears | Sealing between die and container liner; a washed-out liner bore |
| Back end of the billet behaves differently from the front | A worn dummy pad that no longer seals against the liner |
| The stack moves or the profile changes when the press loads up | An improper tool stack; insufficient support behind the die, so it deflects |
| One part of the section is short of metal every run | Die or container temperature; whether the die is seated and centred the same way each time |
The eight variables that change how a die runs
The same die behaves differently across shifts because the process around it moves. These are the variables the correction has to be judged against, and all of them are cheaper to change than the die:
- Die temperature
- Billet temperature relative to container temperature
- Taper heating along the billet
- Extrusion speed
- Press alignment and adjustment
- Deflection of the die stack
- Rotating the die to change its position in the press
- Lubricant, and the use of a canister guide
Two of those are worth more than the rest in practice. Rotating the die is free and it tests whether the fault follows the die or the press: if the heavy side moves with the die, it is the die; if it stays on the same side of the press, it is the press. And taper heating tests whether the fault is thermal: a billet that is hotter at the front extrudes differently from one heated evenly, and the difference shows up as a shape that changes from the front of the run to the back.
Solid dies: cut the pocket or the bearing, and measure the cut
On a solid die the correction is either a modification to the pocket or feeder plate, or a change to the bearing length, worked from the front of the die or from the exit side. The method matters as much as the location. The published practice is to cut with a milling machine rather than a flexible grinder, because a milling cut can be measured and recorded. A grinder removes metal that nobody can quantify, and the next correction starts from an unknown.
The recorded number is the point of the exercise. If the correction is a bearing length, the figure goes back to the die designer; if it is a dimensional change, the figure goes back to the die maker so the wire EDM program changes on the next die of that shape. A correction that is not recorded is paid for again on the next order.
Hollow dies: ports before bearings
A hollow die is harder because the metal passes three stages: through the ports in the mandrel, through the weld chamber, and through the bearing. Ports decide how much metal each leg of the profile receives, so they set the balance. Bearings only decide how much resistance that metal meets at the exit. Correcting the bearing first on a die whose ports are unbalanced is the classic wasted afternoon: the bearing will not fix a leg that is being starved.
The faults that send a hollow die back to the bench are specific, and each points at a stage:
| Fault | Where to look first |
|---|---|
| Twist along the length | Port balance between the legs, then bearing distribution |
| Angularity out of tolerance | Port volume feeding the two walls that form the angle |
| Split corner or open corner | Weld chamber and the port that feeds the corner |
| Convex or concave wall | Bearing length across that wall, and support behind it |
| Uneven wall thickness | Port balance first; bearing second |
One more variable belongs to the hollow die alone: the depth of the bridge and the shape of the weld chamber. Both are set at design and both change how the two streams meet. If the faults repeat across dies of the same design, the correction belongs in the design, not on the bench.
The die corrector belongs at the press
The published procedure is blunt about this: the person correcting the die should be standing at the press during the trial, not reading a report afterwards. Being there gives them the press, billet and die variables at the same time, which is what separates a correction from a guess, and it lets them tell the die maker what to change when the fix is beyond the bench.
Practically, that means two records per trial: the process settings the die ran under, and the shape of the first piece. Without the settings, a die that ran badly cannot be distinguished from a die that ran badly because the press was cold.
Where this leaves a die steel order
Correction is metal removal, so the die has to be able to take it. The extrusion die grade is chromium hot work steel, AISI H13 and its European equivalent 1.2344, supplied annealed so a bearing can be re-cut and a pocket remachined; the working hardness and the temper that reaches it are set on the die steel page, and the nitriding that follows a correction on the nitriding page. The 1.2344 ESR programme for extrusion tooling is on the 1.2344 ESR H13 extrusion supply page, the grade on the H13 tool steel page, and the case on the tool steel nitriding page. The bearing that gets recut is described on the die bearing length page, the stack around it on the die types and tooling page, the defects that are not the die’s fault on the pick-up and die lines page, and the press window on the extrusion temperature page.
The liner and the dummy block that seal against that die face are on the container liner and dummy block page.
Before you cut a die from this page
These are the published principles and the fault categories, not a correction schedule for a specific die. The amount to remove from a bearing or a port is set by the die, the alloy, the press and the trial, and no table transfers between shops. The order of checks is the part that transfers. The final correction is confirmed on the press.
Source, Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000, section 4 die correction, pages 105 to 108.