Extrusion

Definition (what it is)

Extrusion is a manufacturing process in which a material is forced through a shaped die to produce a long product with a constant cross-sectional profile. It is used for metals (ram/press extrusion), polymers and elastomers (screw extrusion), ceramics (plasticized or paste extrusion), and composite feedstocks. The process yields continuous or semi-continuous lengths that can be cut, finished, and assembled, and it enables solid, hollow, and multi-void profiles that would be difficult or impossible to make by rolling or machining alone.

Function and purpose (key technical characteristics)

  • Die-defined geometry: The die opening controls the profile, allowing thin walls, deep channels, internal webs, and other complex features while maintaining uniform cross-section along the length.
  • High area reduction: Large extrusion ratios (initial area to final area) are possible, enabling efficient consolidation and shape formation.
  • Modes and temperatures: Metals may be extruded hot, warm, or cold; polymers are extruded above their melting or softening temperature; ceramics and some composites are extruded as plasticized pastes that are later sintered.
  • Process variants: Direct (forward) and indirect (backward) extrusion; hydrostatic extrusion for brittle or hard-to-deform materials; impact extrusion (a high-speed cold process for cups/cans); porthole/bridge dies for hollow profiles; seamless tube extrusion with mandrels; co-extrusion for multilayer or multimaterial profiles; reactive extrusion for in-line polymerization or compounding.
  • Property control: In metals, hot working refines microstructure, breaks up segregation, and can improve strength and toughness; subsequent heat treatment (e.g., solution treat and age for certain aluminum alloys) tailors final properties. In polymers, melt temperature, shear, and cooling rate influence orientation, crystallinity, and dimensional stability.
  • Throughput and scalability: Capable of continuous or semi-continuous operation with high line speeds (especially for polymers and aluminum), good material yield, and near-net-shape production that reduces machining and scrap.
  • Surface and dimensional quality: Good surface finish and tight tolerances are achievable with proper die design, lubrication, and cooling; calibration and sizing equipment help control distortion and ovality.

Common equipment and process steps

  • Metals (ram extrusion): Billet casting and homogenization → billet preheat and surface preparation → extrusion in a hydraulic or mechanical press through a heated die and container (with dummy block and discard) → controlled exit temperature, quenching/cooling → stretch straightening → cutting → heat treatment (as applicable, e.g., T5/T6 for aluminum) → machining, joining, and finishing (e.g., anodizing, powder coating).
  • Polymers and elastomers (screw extrusion): Resin handling and drying/conditioning → plasticizing and pressurization in single- or twin-screw extruders with mixing, venting, and melt filtration → die forming → calibration (e.g., vacuum sizing for tubes/profiles) and controlled cooling → haul-off → cutting or coiling → optional post-treatments (annealing, surface activation, printing) and co-extrusion for multilayer structures.

Design and quality considerations

  • Die design and flow balancing: Bearing length, choke, and manifold design govern fill uniformity and dimensional control; porthole/bridge dies create longitudinal weld seams in hollow profiles whose strength must be validated and properly oriented in the design.
  • Process window: Extrusion ratio, ram/screw speed, exit temperature, and lubrication affect flow, surface quality, and defect risk (e.g., surface tearing, chevron/centerline cracking in metals; sharkskin, melt fracture, die swell in polymers).
  • Shape limits: Very thin walls, high tongue ratios, sharp internal corners, and extreme aspect ratios may require special alloys, dies, or process adjustments; distortion (twist/bow) is mitigated by cooling strategy, puller tension, and straightening.
  • Joining and fabrication: Extrusions are readily joined by welding (MIG/TIG, laser), friction stir welding (aluminum), brazing, mechanical fastening, and adhesive bonding; post-extrusion drawing or forming can further refine dimensions and properties.
  • Inspection and standards: Dimensional tolerances, surface finish, seam integrity, mechanical properties, and heat treatment temper must meet applicable specifications.

Materials commonly extruded

  • Metals: Aluminum alloys (notably 6xxx; also selected 7xxx/2xxx with constraints), magnesium alloys, copper and brasses, steels (hot/warm), titanium and specialty alloys (often hot or hydrostatic).
  • Polymers and elastomers: PE, PP, PVC, PA, PET/PBT, PC, TPU/TPE, fluoropolymers, rubber compounds for seals and weatherstrips.
  • Ceramics and composites: Clay and technical ceramics (e.g., honeycomb monoliths), metal- or ceramic-filled compounds, fiber-reinforced thermoplastics (including co-extruded tapes and profiles).

Applications and relevance

  • Structural profiles: Beams, channels, tubes, and multi-chamber sections for transportation, building, machinery, and consumer products.
  • Thermal and fluid management: Heat sinks, micro-channel plates, manifolds, ducts, and piping.
  • Functional profiles: Seals, gaskets, cable insulation, conduits, glazing profiles, trim, and protective covers.
  • Example in modern EVs: Aluminum extrusions for battery enclosures, crash rails, rocker and roof reinforcements, subframes, and heat sinks; polymer and elastomer extrusions for high-voltage cable jackets, seals, gaskets, conduits, and coolant hoses. Complex extruded geometries enable functional integration (mounting features, channels, seal grooves), reducing mass, part count, and assembly time. Aluminum extrusions also support sustainability goals via high recyclability and closed-loop scrap systems.

Related terms and distinctions

  • Direct extrusion, indirect extrusion, hot/warm/cold extrusion, hydrostatic extrusion, impact extrusion, co-extrusion, reactive extrusion.
  • Porthole/bridge die, spider die, seamless tube extrusion, mandrel, die land/bearing, billet, breaker plate and screen pack (polymers), puller, quench.
  • Extruded product is often called a profile, section, or shape. Pultrusion and drawing are related but distinct processes. In additive manufacturing, “material extrusion” is a different, layer-wise process and should not be confused with bulk extrusion.

Selected standards (examples)

  • Aluminum extrusions: ASTM B221; EN 755 and EN 12020 for tolerances and precision profiles.
  • Plastics and elastomers: Relevant ISO/ASTM standards apply to specific materials and profile tolerances for extruded products.