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Aluminium Extrusion Process

Fletcher Aluminium in Auckland creates shaped extrusions that are designed to meet customer needs.

The extrusion process involves five major stages, which are explained in detail in this section:

1. Preheating and initial cutting of the alloy log
2. Extrusion of required section shape (profile)
3. Cooling (quenching)
4. Final shaping and cutting
5. Final hardening (by heating)

A finishing process such as anodising or powder coating may follow final heating, after which we test and despatch extruded products to customers.

1. Preheating and initial cutting

High quality alloy logs are produced at smelters in New Zealand or Australia, depending upon the type of aluminium alloy required. The logs may be up to seven meters long and are 200mm in diameter.

Each aluminium log enters a preheating furnace. Depending on the alloy and the type of extruded section being produced, the log is heated to a specific temperature between 400°C and 500°C.

After preheating, the log exits from the furnace and is cut into smaller pieces called billets. The billets may vary in length between 400mm and 800mm depending on the:

• Weight per metre of the aluminium section to be produced
• Customer’s required finished length
• Total length of metal required to fill the press run out table efficiently

2. Extrusion of required section shape (profile)

The cut billet is placed in the container of a horizontal hydraulic press. The billet is forced through a steel die that has the required cross sectional shape machined into it.

The extrusion now has the required cross sectional shape. This exits from the press and on to a run out table which supports the hot metal until it reaches a transporter, called a “puller”. The puller leads the long piece of extrusion to the end of the process line.

Once the metal from one billet is exhausted, another is placed behind it in the press container. The extrusion from the second billet is pressure welded to that of the first in the die cavity. This enables a continuous length of extrusion to be produced by extruding billet upon billet. The long lengths of metal efficiently fill the run out table.

3. Cooling (quenching)

After emerging from the press, the extrusion is cooled by various means such as ambient air, forced air jets or water sprays.

The required quenching method depends on the alloy, temper (degree of hardness and elasticity) and shape (profile) complexity. Each complex architectural section, or group of similar sections is likely to require its own set of quenching methods.

4. Final shaping and cutting

After final cooling to ambient temperature, the lengths of extrusion move across a storage and transfer table for final shaping and cutting.

Firstly the cold metal section is 'stretched' to straighten the lengths and bring the dimensions into specification. Moveable jaws clamp the section at either end and apply a tensional force to straighten the metal. Stretching elongates the piece of metal by about 1% of its original length.

The lengths of metal then move to a sawing machine that cuts the lengths required by the customer. Billet joins, crushed ends (from stretching) and other defects are removed at this stage.

5. Final Hardening

The cut lengths are stacked on metal frames and transported to an oven where heating and final tempering (hardening) takes place. This final heating process is known as 'artificial ageing'.  Some tempers such as F, T1 and T4 do not require this process.

6. Finishing and despatch

The sawn lengths of metal may be diverted to various finishing operators, for processes such as anodising, painting or precision machining. Alternatively, the lengths of metal can be despatched to the customer in 'as extruded' or 'mill finish' condition.

We test all our extruded products for strength and the various surface finish requirements before despatch to the customer.