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Lesson 3.3DeformChapter 3 · Bulk forming

Extrusion (aluminum profiles and plastic pipes)

Push metal or plastic through a shaped die

Extrusion pushes material through a shaped opening (a die), like toothpaste from a tube, giving a long piece with that cross-section. Window frames and heat sinks are extruded aluminum; water pipes and cable insulation are extruded plastic. Once you have the die, you can make the shape in any length.

Extrusion (aluminum profiles and plastic pipes): a real photo
Photo: Ulfbastel, Public domain, via Wikimedia Commons
A ram or a screw pushes material through a shaped die, like toothpaste from a tubeWhatever comes out of the die keeps the shape of the hole, for any length you want.Aluminum profile(direct extrusion, cut-away side view)hydrauliccylinderram2ram pushescontainerhot billetdieprofile1billet heated to about 450–500 °C3metal squeezes throughthe shaped hole4cooled, stretched straightand cut to lengthdie, front viewprofile: same shapePlastic pipe(screw extruder, cut-away side view)motorplastic pelletsheated barrel, turning screwdie with a pincooling tankpullersaw567Key:material being shapedmachine parts and diesheatcooling water
Extrusion: an aluminum press and a plastic pipe lineTop: an aluminum profile (steps 1 to 4). Bottom: a plastic pipe (steps 5 to 7). Circled numbers match the steps below.

How it works

  1. Heat the billet. A billet (a short, thick log of aluminum) is heated to about 450–500 °C. It stays solid and does not glow, but it becomes soft enough to flow under pressure.

  2. Push with the ram. The billet is loaded into a heated container. A hydraulic ram pushes it with a force of hundreds to thousands of tonnes.

  3. Squeeze through the die. The metal flows through the die (a thick steel disk with a shaped hole). It comes out as a profile (a long piece with a constant cross-section). For hollow shapes, the metal splits around a central core inside the die and joins up again.

  4. Cool, stretch and cut. A puller guides the hot profile along a table while air or water cools it. It is then stretched straight, cut to length, and often heat-treated or anodized (given a protective oxide layer).

  5. Plastic pipe: melt with a screw. Plastic pellets fall from a hopper into a heated barrel. A turning screw (an auger, like in a meat grinder) melts them by heat and friction, and pushes the melt forward without stopping.

  6. Form the tube. The melt flows through a ring-shaped gap in the die, around a central pin that forms the hole of the pipe.

  7. Cool, pull and cut. The soft pipe passes through a water tank that sets its final size. A puller draws it along at a steady speed, and a saw cuts it to length.

Key facts

Materials
Aluminum alloys (by far the most common metal), copper, brass, magnesium; plastics such as PVC, polyethylene and polypropylene
Typical parts
Window and door frames, heat sinks, rails and tubes, water pipes, cable insulation, plastic trims and seals
Part size
Aluminum profiles from about a centimeter to tens of centimeters across, cut to lengths of several meters; plastic pipes from a few millimeters to over a meter in diameter
Precision
Aluminum: about ±0.1–0.3 mm on small sections. Plastic pipe: typically a few tenths of a millimeter
Surface finish
Smooth, with faint lengthwise die lines; aluminum is often anodized or painted
Batch size it suits
From short runs (dies are cheap) to plastic lines running nonstop around the clock
Tooling and machine cost
Aluminum dies: often a few hundred to a few thousand dollars. Presses and complete lines: hundreds of thousands to millions
Speed
Aluminum: a whole billet pressed in a few minutes. Plastic: continuous, typically meters per minute

Strengths and limits

Strengths

  • Complex cross-sections in one pass, hollow ones included
  • Cheap dies make even short runs of a custom profile affordable
  • Continuous and fast, with little waste (scrap ends are recycled)
  • Good surface, ready for anodizing or painting

Limits

  • Only constant cross-sections; holes or features across the length must be machined later
  • Very thin walls and sharp corners are hard to achieve
  • Long profiles can twist or bow and must be stretched straight
  • Hard metals such as steel need very high forces and wear dies quickly

Where it's used

  • ConstructionWindow and door frames, curtain-wall sections, railings, PVC drain pipes
  • ElectronicsHeat sinks, equipment housings, LED strip profiles
  • TransportTrain and bus body sections, roof rails, bumper beams
  • Plumbing and utilitiesWater and gas pipes, cable conduits, cable insulation
  • Consumer goodsCurtain rails, ladder rails, garden hoses
  • Food (same principle)Pasta, breakfast cereals, snacks

How to recognize it

  • The cross-section is exactly the same along the whole length; a cut end shows the full profile.
  • Fine straight lines running lengthwise (die lines), easiest to see on bare aluminum under a light.
  • Saw-cut ends, with holes and notches added later by machining.
  • On plastic pipes, printed text repeating along the length, added as the pipe leaves the line.

Compare with

Rolling3.2rolling squeezes metal between turning rolls; extrusion pushes it through a fixed die and gives far more complex shapes in one pass.Wire drawing3.4drawing pulls metal through a die instead of pushing it, which only works for thin, simple sections.Pultrusion10.3the composites cousin: resin-soaked fibers are pulled through a heated die.

Watch it

Hydro · The whole aluminum route, explained step by step

corematerials · A simple cut-away of ram, billet and die

Hydro · A real press turning a billet into a profile

Machine GPM · A plastic pipe line: extruder, cooling tank, puller and saw

See real photos

Remember

  • Extrusion pushes material through a shaped die; the product keeps that cross-section for any length.
  • Aluminum is pushed by a ram one billet at a time; plastic is melted and pushed nonstop by a screw.
  • Cheap dies make custom profiles affordable, even in small runs.

More in Bulk forming

All 100 processes
3.1ShortForging (open-die and closed-die)Squeeze hot metal between flat or shaped diesOpen the lesson3.2RollingPull metal through the gap between turning rollsOpen the lesson3.4Wire drawingPull a rod through ever-smaller diesOpen the lesson3.5Cold headingCut wire into blanks and punch a head, coldOpen the lesson3.6Thread rolling and tappingPress a thread onto a blank, or cut or press one in a holeOpen the lesson

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