• Extrusion: push a billet through a die (like toothpaste) → constant cross-section (railings, frames); cut into discrete parts. Room- or elevated-T. Common metals: Al, Cu, steel, Mg, Pb.
• Drawing: pull a rod/wire/tube through a die to reduce/change its cross-section. Rods (shafts, fastener stock) are larger than wire (wiring, cable, springs, paper clips).

Extrusion Process

Direct (forward): a round billet in a chamber is forced through a die by a ram. Variables: die angle $alpha$, extrusion ratio $R = A_0/A_f$ (usually 10–100), billet temperature, ram speed, lubrication.

Extrusion force:

$F = A_0\,k\,\ln\!\left(\frac{A_0}{A_f}\right)$

$k$ = extrusion constant (chart; depends on metal & T), $A_0$, $A_f$ = billet and product areas. Usually done hot to raise ductility and lower force; the hot billet may form an oxide film.

Hollow sections: spider/porthole/bridge dies split the metal around a mandrel; strands re-weld under pressure in a welding chamber (Al only; no lubricant, or welds won’t form).

Lubrication: important in hot extrusion. Glass is an excellent lubricant for steels/high-T alloys (Sejournet process uses a glass pad). Jacketing/canning encloses a sticky billet in a softer metal (Cu, mild steel).

Cold Extrusion & Defects

Cold extrusion (often extrusion + forging): ↑mechanical properties (work hardening), good tolerances/finish, no billet heating — but very high tool stresses.

Defects:

• Surface cracking — too high T/friction/speed (Al, Mg, Zn alloys); fix by lowering billet T and speed.
• Pipe defect — flow draws surface oxides toward the center (funnel-like); minimize by controlling friction and temperature gradients, or etching oxides first.
• Center (chevron) cracking — hydrostatic tension at the centerline; ↑ with die angle and impurities, ↓ with extrusion ratio and friction.

Equipment: horizontal hydraulic press (constant force over a long stroke); vertical presses for cold extrusion (less floor space).

Drawing

Variables like extrusion: die angle, $R = A_0/A_f$, speed, friction.

Drawing force:

$F = Y_{\text{avg}}\,A_f\,\ln\!\left(\frac{A_0}{A_f}\right)$

$Y_{\text{avg}}$ = average true stress in the die gap.

• Maximum area reduction per pass ≈ 63% (e.g. 10 mm → 6.1 mm, since $6.1^2/10^2 approx 0.37$).
• Intermediate annealing between passes restores ductility. High-carbon spring/instrument wire can reach UTS ~5 GPa.
• Bundle drawing pulls many wires at once for productivity.
• Die design: entering + approach angles, typically 6–15°. A draw bench has a single die (like a long tensile machine).