What is Seamless Tube? from luis huang's blog

Chal seamless tubes are produced using two methods. One method pushes a hollow aluminum billet through a Die and Mandrel Press with tremendous force at high temperatures. The other method pushes a solid billet through a Piercer Press and then a mandrel pierces and extrudes the billet in a second forward stroke. Regardless of the method used, the tube will have no weld or seam, making it ideal for anodizing and other finishing procedures.

Advantages of seamless tube and pipe:

No weld seams, preferred for pressure vessels

More uniform anodizing appearance, especially on heavier wall sections

No weld seams that could split in forming operation

Increased structural integrity

Structural Tube:

Hydro structural tube is produced with either the porthole or bridge die method. Solid aluminum billet is pushed through the die with tremendous force at high temperatures. The tube separates during the process and is formed back together by an inherent welding process while passing through the extrusion die. There is a slightly visible seam that may be noticeable to the naked eye if the tube is anodized or "finished."

Advantages of structural tube and pipe:

Improved control of wall thickness eccentricity

More ability to use multi-hole dies for smaller diameter sizes to improve productivity, decreases costs

Seamless Tubing Applications

Seamless pipe and tube is typically used for pressure vessels, hydraulic cylinders, compressed gas cylinders, drive shafts, lighting applications and bus conductors. They are recommended for applications requiring:

Critical pressure ratings

Demanding forming applications

Critical strength requirement

Uniform anodizing appearance

Aluminum Tubing Options

As a widely used aluminum profile shape, aluminum tubes are available in square, rectangular and round shapes in a range of sizes based on the outside dimensions. The tube’s strength is a factor of the aluminum alloy used, the thickness of the walls, and the size of the tube. Round tubes are available in the widest variety of sizes, from an outer diameter of 5/8 inch to 16 inches and wall thicknesses of 1/16 inch to 1 inch.