Aluminum Products

In porthole, metal is forced around a mandrel that matches the ID of the tube you want to extrude. The mandrel needs webs to hold it in place, and the metal must flow around them. That separates the metal as it extrudes, so a second die forces those sections back together. That forms a longitudinal seam running the length of the extrusion. This weld seam is somewhat weaker than the base metal.
Seamless tubes insert a mandrel into the rear of a billet and basically punch a hole through the billet until it’s very close to the opening in the die. The material flows through the gap between the mandrel and die, emerging with both internal and external dimensions fixed and without any seams. This process is slower and generates more scrap so it is more expensive. The tolerance on the wall thickness cannot be held as tightly as they are during the porthole extrusion process.If the application involves internal pressure or is in a life critical application, choose seamless tubing. If neither of those apply, however and if the highest priority is keeping the cost down, choose porthole.