The harder an alloy is, the less easily it will flow during the extruding process, making it more difficult to extrude. Accordingly, typically, the harder the alloy, the less detailed the design should be, the more uniform the wall thicknesses should be, and the more hollow shapes should be avoided, being generally more difficult than solid shapes.

Harder alloys are usually more expensive alloys. They are used less, harder to process, and, accordingly, more expensive per pound of product. Therefore, it is generally best to try to stay with the softer, more common alloys such as 6063 and 6061 whenever the special properties of the harder alloys are not specifically required (for whatever reason). Please see the “Alloys” pages for further information.

Shapes with particularly thin walls are often difficult to produce, because the metal must be forced through a smaller gap and there is more flow resistance. This creates a higher “back pressure” on the die and makes it much more likely to break. The thinner the gap, the more true this is. This consideration is especially true with the “hard alloys”.

Significantly different wall thicknesses are a problem. The metal is somewhat fluid as it is being extruded. Accordingly, the metal follows the laws of fluid dynamics, to a great extent. Simply put, what happens is that the metal will all try to flow through the die at the widest gap (the thicker wall area), and “starve out” the thinner wall, creating voids, thin spots and/or gaps, in other words, an incomplete part in the thinner wall area.

Long, narrow tongues (long “fingers” on the die, as are common between the fins on many heat sinks) are a problem in several ways. One is that the tongues often try to wobble back and forth as the metal flows through the die, just like trying to hold your hand steady outside the window of a fast moving car. This can cause the die to break and require rebuilding and/or redesign of the part. Another cause of breakage of the tongues is that their bases are often narrow, and, thereby, weak, adding to the above problem.

 Large, deep voids, which are only mostly surrounded by metal create a problem in that the die is essentially the reverse of the finished extrusion. Accordingly, the void in the part must be a solid mass on the die, and, in this case, creates an enormous stress on the “neck” of that part of the die (the neck of the “mandrel”), causing it to wobble and, possibly break (similar to the tongues” in the heat sinks, discussed above).

In both of the above cases, even if the poorly supported portions of the die don’t wind up breaking off, they may well move around enough to create tolerance problems regarding the thicknesses of the adjacent walls. In this case, one side will be too thin, and the other will be too thick. These conditions may well vary from side to side (reverse themselves) as you go along the extrusion.

 Multiple voids, or hollows, in a single shape can create problems in some cases, especially if they are too close together. Other problems crop up if there are nearby large masses of metal on one side if a void and not on another. This may cause the heavier flows on the thicker mass side to push the hollow area mandrel aside, forcing it out of position. This will cause wall thickness anomalies. If there are small and large voids close to one another, the planned shape may not be producible. Other, similar problems can also pop up in these circumstances.

As most mills are only able to cut extrusions off at lengths as short as about 8’ at the extrusion presses, they will charge extra for shipping them in shorter lengths. Shorter “cut to length” pieces may be your best answer, even though this may require a “secondary operation” at the mill. You might also wish to consider longer “stock lengths” as an option, however, particularly if you will be re-cutting at your facility, anyway. Most mills can handle and ship lengths of up to 20’ – 30’, if you wish. You can also usually specify “odd lengths” (as an example, 17’ 8.75”), if you wish, but be advised that “on line” (at the press) cutting usually neither produces as clean a cut, or as accurate a cutting length as the secondary operation saw lines, so the “on line” cutting length tolerances are usually “-0”, +0.25”. The latter is usually true regardless of the “on line” cutting length specified. Much more accurate cutting lengths are usually available with secondary operation cutting, sometimes below +/- .005”.

Very sharp, pointed, narrow “needle”, types of features, whether they be metal, or a pointed void into the metal are usually not practical. In the case of being made of metal (a finger), the problem of “filling” the finger with metal becomes significant. It can be stated that a complete fill will not occur in many of these cases. In the case of the pointed void, the problem is that the heat build up from friction of the metal flow, the frictional wear of that metal flow, the long “lever arm” on the finger of the die creating that void, and other factors will likely cause the pointed end of the part of the die to burn off, or wobble around, thereby being unable to fulfill its function in creating the sharp, pointed void and/or keep it in the proper location at all.