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Structural and seamless tube are both extruded. That means a billet of material is forced through a hole in a die shaped to form the two dimensional cross-section you need. (We’ve called it the “toothpaste process” in the past and compared it to decorating a cake.) Sometimes you want that extruded material to have a hole down the center. The simplest example is circular tube or pipe. for an explanation of the difference. Porthole and seamless are different ways of forming that hollow internal section. In porthole, metal is forced around a shape that matches the hollow section you want to extrude. Physics being what it is, the tooling needs ribs to hold that shape in place, and the metal has to flow around them. That separates the metal as it extrudes, so a second die forces those sections back together. At the micro-structure level, the surfaces weld to each other, which means there’s a seam. As the name suggests, seamless extruded tubes avoid this. The difference is that a forming mandrel is inserted into the billet of material from the rear, and pushed thorough until it’s very close to the opening in the die. The material flows through the gap, emerging with both internal and external dimensions fixed and without any seams.
