Ok, so this pertains to both extruded aluminum tube and formed steel tubing of any profile – round, square, rectangular, hexagonal, you get the idea.
In the old days, tube cutting was an expensive proposition and not all that accurate. You’d have loose tolerances and a lot of tolerance stack if the parts had to be unfixtured. Not any more. Laser tube cutters are amazing. You end up with parts that can be put together like Legos. I mean, you get 0.005″ accuracy on on the entire part. No more centering issues, no more mis-aligned assemblies and the process is fast and not labor intensive.
Modern laser cutters can cut 1″ thick steel and 1/2″ thick aluminum. For thinner materials, the cut is usually very clean. For weldments consisting of extruded aluminum tubes, this means smaller, stronger welds. If you’re in the bicycle business, you probably use this process exclusively for both prototyping and production. Production laser cutting can employ tube auto-feeders to make precision cut extruded product all day long (and even all night long for lights-out operation). Btw, you can also find machining information on a related machining blog we manage:
If you don’t believe me, watch this video. I find this absolutely amazing.
Finned heat sinks are produced using a variety of manufacturing techniques. Shape and cost most largely dictate what means of manufacturing to use.
Extruded heat sinks are the most commonly used today. Extrusion allows for a large variety of shapes to be used. Extrusion is also a relatively inexpensive means of producing fairly large batches of parts quickly and effectively. The fins can be relatively fine and closely spaced as well.
Since the transition from the base to the fins is relatively seamless the heat transfer out to the fins can flow quite freely, unlike with bonded or folded fin heat sinks where a separate piece of metal is affixed to the base. The some what homogeneous nature of the extruded part makes it ideal for seamless conduction.
Copper extrusions are ideal for heat transfer applications, but the cost of the copper itself makes them inherently expensive in most design applications. Additionally the copper needs secondary treatment against corrosion. None-the-less, copper is almost twice as thermally conductive as aluminum (401 vs 250 W/mK), so for design applications where heat transfer is crucial, copper has more heat transfer ability but you have to weigh that with the additional costs.
For extrusion, you can design fins or you can stack copper sheet. Extrusions are better however because the air gap incorporated in stacked sheets is a thermal resistor, big enough in some cases you can actually negate the affects of the additional thermal transfer obtained from using copper. When using sheets try welding the sheets (anyone who had done household plumbing knows how easily copper welds) to the substrate.
So you’re not an expert at designing aluminum extrusions. No problem. The trick is to get the design as far along as possible before you interact with an extruder. To do that, you need to become familiar with the process. Today we’ll discuss some common bad design elements.
Thin Walls
Thin walls are often difficult to produce because they require a lot of force to push through a die. This creates a lot of back pressure on the die. This is also why die construction is done by the extruder – they can calculate the required load on the die and then it’s corresponding material thickness. Thin walls with hard alloys magnify the problem. Sometimes your design can’t escape thin walls, especially if you’re designing heat sinks.
Significantly Different Wall Thicknesses
Aluminum during the extrusion process more approximates a fluid than a solid. So, when you have parts with wildly differing wall thicknesses the material tends to flow right into the large areas and starve out the thinner sections. The large sections offer low resistance, while the thin/small sections offer high resistance. The material follows the path of least resistance.
The 2000 series alloys are made up of copper and aluminum. They are strong, corrosion resistant and can be machined and extruded easily. 2000 series finds applications in truck paneling, aircraft structural parts and building curtain walls (outer floating facade on most modern buildings). The 2000 series is ideally suited for these applications because it gains structural rigidity by precipitation hardening (age hardening).
There are a few drawbacks however. The 2000 series is difficult to anodize due to the high copper content (up to 6.8%). It can be anodized, but coating thickness is minimal in comparison to 6000 or 7000 series aluminum. Also, parts made out of the 2000 series may suffer “stress corrosion cracking” – a sudden unexpected failure of a ductile metal.
