Lead Pipe Extrusion

Rolling squeezes metal between two rolls and can only make sections open to the outside. Extrusion pushes it through a hole instead — and if you put a mandrel in the middle of that hole, what comes out is a tube, seamless, in one continuous length. Joseph Bramah patented a lead pipe press in 1797 and Thomas Burr built on it, replacing pipe that had been made by casting sheet, bending it round a rod and soldering the seam. The seam was the weak point and the failure point; extrusion abolished it. The metal behaves almost like a fluid under enough pressure, and the die's shape becomes the product's shape — which is why extrusion can produce complex hollow profiles that no rolling mill could ever form.