Aluminum Extrusion Die Types and Tooling
An aluminum extrusion die is not one tool. It is a stack, and every part of the stack carries a different load. The die makes the shape, the backer stops it collapsing, the bolster passes the press load down to the pressure ring, and the liner protects a container that costs far more than the die. Which type of die a press runs, and which alloy it runs it on, decides what the die steel has to survive. This page sets out the parts of the stack, the die types used for solid and hollow shapes, and the way the tooling changes when a press moves from 6063 to a hard 7000 series alloy.
The parts of the die stack and what each one does
In the direct process the billet is pushed through a stationary die by a stem, and the whole load of the press passes through the stack behind the die. Each part has one job. The die forms the profile. The backer sits immediately behind the die and carries it so that it does not collapse or crack. The die ring holds the die and the feeder plate together. The bolster spreads the load from the die onto the pressure ring, which passes it into the press platen and resists the deflection of the bolster itself. The die slide carries the complete set into and out of the press. Behind all of that sits the liner, which is the wearing surface of the container, and the dummy pad, which is the wearing surface of the stem.
| Tool | Function |
|---|---|
| Die | Makes the shape of the extrusion |
| Die holder or ring | Holds the die with the feeder plate and the backer |
| Die backer | Supports the die to prevent collapse or fracture |
| Bolster | Transmits the extrusion load from the die to the pressure ring |
| Pressure ring | Transmits the load from the bolster to the press platen and prevents bolster deflection |
| Die carrier or slide | Holds the complete die set, the die ring and the bolster, in the press |
| Bridge, spider or porthole die | Special die to make a hollow shape with a welding joint along the length of the shape |
| Feeder plate | Sits in front of the die to balance the metal flow and to make a continuous extrusion |
| Liner | Protects an expensive and large container from thermal and mechanical stress |
| Stem | Fitted with the main ram to push the billet through the container and liner |
| Dummy pad | Protects the expensive stem, fitted to it or floating in front of it |
Functions of the individual tools used for extrusion of soft and medium grade alloys with the conventional direct process. Every part in the list is a wearing part, and the grade chosen for each one is set by the load it carries rather than by the die itself.
Solid dies and hollow dies
Extrusion dies divide into two families. A solid die has a plain opening and makes a solid or semihollow shape. A hollow die has a welding chamber, and it makes a closed shape by splitting the metal into separate streams and rejoining them inside the die. Within the solid family the difference between one die and the next is how the metal is fed to the opening. A flat-face die feeds it straight through. A recess or feeder plate die puts a chamber in front of the opening and lets the metal fill that chamber before it enters the bearing, which evens out the flow and lets the press run billet on billet without stopping.
| Die type | Shapes made | Note |
|---|---|---|
| Solid flat-face die | Solid and semihollow shapes | Plain bearing on a flat die face |
| Feeder plate die, also called housing die | Solid shapes larger than the billet | A welding chamber in front of the die balances the flow and allows continuous extrusion |
| Recess die | Solid shapes | A single stage cavity, called the recess, sits in front of the die opening |
| Single-bearing die, equal bearing die | Solid shapes at higher speed | Several converging cavity stages hold one bearing length over the opening |
| Porthole, bridge and spider die | Hollow and semihollow shapes | The metal splits into ports, flows round the bridges, and rewelds in the welding chamber |
The die types described in the chapter. Whether a shop uses a flat-face, feeder plate or recess die for a given solid shape comes down to the shape, its size against the billet, and the flow balance the die maker wants.
The tooling changes when the alloy changes
The same press runs different tooling for a soft 6000 series alloy and for a hard 2000 or 7000 series alloy. Soft and medium grade alloys are extruded in the 1100, 3000 and 6000 series, they flow easily, and they are usually run with a feeder plate in front of a flat-face die so that the press can run continuously and handle several shapes on one die at once. Hard alloys are extruded in the 2000, 7000 and some 5000 series. They resist flow, so the die is made thicker and stiffer, the bearing is longer, and the die is run as a solid die without a feeder plate, one extrusion at a time. The table below lines up the two practices.
| Item | Soft and medium grade alloys | Hard alloys |
|---|---|---|
| Die | Solid flat-face, feeder plate, recess or single-bearing, often run as a multihole die | Solid flat-face die of greater thickness with a larger bearing and a small choke in front of it |
| Dummy pad | Fixed expandable type dummy pad | Loose solid dummy, or loose hollow dummy for seamless tube |
| Die set | Feeder plate, die, backer, die ring, bolster and pressure pad | Solid die, backer, die ring, bolster and pressure pad, with no feeder plate |
| Run style | Billet on billet, so the press runs continuously, usually with a puller system | Each extrusion is run separately |
| Hollow sections | Welding chamber die, which is the porthole, bridge or spider type | Seamless type, over a pierced or fixed mandrel carried through a hollow stem |
Tooling practice for soft and medium grade alloys against hard alloys. The change from a feeder plate die to a plain solid die is the largest single difference between the two practices, and it follows directly from how readily the alloy flows.
How metal moves through a hollow die
A hollow die works in three stages, and each stage is a place where the flow can get out of balance. In the first stage the metal leaves the billet and enters the ports in the mandrel. In the second it crosses from the ports into the welding chamber. In the third it passes from the welding chamber through the gap between the mandrel and the die cap, and that gap is the bearing that sets the final wall thickness. A recess on the entry of the mandrel speeds up the flow round the outside of the die relative to the centre, and a recess on the top of the mandrel balances the flow in a thin wing of the shape. The volume of metal is the same at every stage, so the area of the port, the area of the welding chamber and the area of the final opening set the three speeds against each other.
Flow balance is also where a hollow die cracks. A mandrel bridge of unequal stiffness, or a bridge with a sharp radius on top, is the shape that fails, because the load pushes the bridge over and the mandrel shifts against the cap. A bridge with a larger radius is the fix the source records.
Where this leaves a die steel order
The tooling decides the dies, and the dies decide the steel. A flat-face die for 6063 is the least demanding part in the stack, a thick solid die for a 7075 aircraft shape is one of the most demanding, and the liner, the dummy pad and the stem are wearing parts that sit against hot metal all day. The grade that carries the die itself is the chromium hot work steel AISI H13 and its European equivalent 1.2344, which is covered on the H13 tool steel page, and the supply programme for extrusion die stock is on the 1.2344 ESR H13 extrusion supply page. The grade by component is set out on the best tool steels for aluminum extrusion dies page, and the wider hot work selection on the hot extrusion die steel page. The bearing that forms the shape is discussed on the die bearing length page, the way the die wears during a run on the aluminum extrusion die wear page, and the size of shape a press can hold on the circumscribing circle diameter page.
Before you use this as a tooling specification
This page is a reference summary of published practice and it is not an Aobo Steel specification. The tool functions and the die types are reproduced from the source. The tooling a particular press uses also depends on the press, the container, the shape and the alloy, and no two dies of the same design, material, hardness and surface finish are truly identical. Final die design is confirmed on the job.
Source, Aluminum Extrusion Technology, P. K. Saha, ASM International, 2000.