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What Are the Differences Between Seamless and Structural Aluminum Extrusions?


When working with aluminum extrusions, it’s important to understand the differences between seamless and structural extrusions.  Manufacturers and designers are tasked with fulfilling material requirements without sacrificing quality and they need to rely on their supply streams.  Aluminum extrusions have a proven track record of being reliable and offering consistent performance over the lifetime of the project.

Aluminum, despite becoming a commercially viable material barely a hundred years ago, offers a tremendous amount of flexibility and versatility to the market in the year 2020. With so many high performance alloys available on the market today, there’s almost no industry or use case scenario that can’t benefit from aluminum.

It’s important to work with a knowledgeable and dedicated material supplier who has a strong understanding of all the various benefits of aluminum extrusions and can help match the alloy with the application.

What is an extrusion?

The extrusion process is a technique used to form objects that have a fixed cross-sectional profile.  A desired hollow or solid profile is machined into a die made of hot work tool steel.  A heated aluminum billet is then forced through the die in an extrusion press, creating a lineal piece of aluminum in the shape of the profile cross section.

Among the many benefits of the extrusion process is its flexibility, allowing for any number of different shapes.  It also enables manufacturers to work with brittle materials, as the only forces applied to the work piece are compressive and shear stresses.  Another plus is that extrusions allow for an excellent surface finish. Moreover, the process of extrusion can cause the aluminum to increase in strength, making the finished product more durable.

Although similar, extrusion should not be confused with drawing, which differs in that the material is pulled through the die rather than pushed, which limits the process to simpler shapes than what’s possible from extrusion.  In theory, an extrusion can be infinitely long, if you have material to keep pushing through.  In practice, extrusions tend to be limited in length according to how it will be transported.

What is a seamless extrusion?

One of the main methods of extrusion is known as seamless extrusion.  This entails a fully extruded hollow tube that does not have any welding or bracketing along the entire lineal length of the profile.

The method of creating a seamless aluminum tube involves what is known as a piercing extrusion.  Starting with a solid round aluminum billet as raw material, it is heated to the desired temperature and then a small hole will be punched at one end to act as the starting point.

Rotary piercing is then applied.  This fast rolling process buffets the billet between a pair of barrel-shaped rolls.  The rolling induces high tensile stress to occur at the center of the billet, and the piercing tool forces the material to flow along the roll. The profile shell forms as this force is applied.  During the entire process, the metal will be reheated as necessary, and then afterwards it is further stretch reduced and cooled.  At this point the final product can be cut to the desired length and any final shaping or finishing can occur before it’s ready to ship.

The main advantage of the seamless extrusion process is the product’s lack of a welded seam, which is the mechanically weakest point of the profile’s structure. Therefore, the finished product has greater structural integrity and is more immune to failures along weld lines; seamless extrusions are the preferred material for pressure vessels.  Seamless tube is also ideal for anodizing and other finishing procedures.

In addition to pressure vessels, seamless extrusions are also found in a variety of applications including hydraulic cylinders, compressed gas cylinders, drive shafts, lighting applications and bus conductors.  Anything that requires critical pressure ratings, has demanding forming applications, high strength requirements or needs a uniform anodized appearance is likely to be a seamless extrusion.

What is a structural extrusion?

In contrast to seamless extrusion, a structural extrusion is designed to be attached to another extruded part via welding, bracket or fastener of some kind.  This means that no matter how strong the extruded material might be, the weak point of the extrusion will be the weld point or connector holding the pieces together.

Hollow profile structural extrusions, including tube, are formed using a porthole or bridge die method.  Like a seamless extrusion, the aluminum billet is forced through the die using tremendous force at high temperature.  Since the profile wall will separate during the extrusion process, it must be pressed back together within the die.  This will take place as the material is passing through the extrusion die within the extrusion press.

A pipe that has been extruded can be recognized from a seamless product because there should be a visible seam separate from any die lines if the pipe has been finished or anodized.

Among the advantages offered by extrusion is an improved control of the wall thickness eccentricity.  It’s also possible to use multi-hole dies that allow for smaller profiles sizes. Structural extrusions also tend to be faster to produce while at the same time decreasing costs.

There are many ways to connect structural extrusions, including t-slots, angles, tracks, fasteners and brackets.  Applications include aluminum frames to which you can attach panels, wire mesh or screens.  The industry has evolved to the point where these extrusions can be purchased in kits and quickly assembled.  Typical use cases can be found in the manufacturing, retail, automotive and medical industries.  Applications include office workstations, machine bases, enclosures, industrial racking, fixtures, safety guards, stairs, platforms and material handling carts.

As the Midwest’s leading aluminum supplier, Clinton Aluminum is an expert in aluminum extrusions.  Contact us today to learn more.


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