There are, however, a number of basic considerations one should keep in mind for satisfactory parts. A few of those are as follows: 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. Therefor, 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 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, it follows, to a great extent, the laws of fluid dynamics. Simply put, what happens is that it 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 that area.

Long, narrow tongues (as are common 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 also cause the die to break and require rebuilding and/or redesign of the part.     

Large, deep voids, which are mostly surrounded by metal creates 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”, causing it to wobble and, possibly break.

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, there are nearby large masses of metal on one side and not on another which may cause the heavier flows to push the hollow mandrel out of position, there are small and large voids close to one another, and other similar problems.

As most mills are only able to cut extrusions off at lengths of about 8’ at the extrusion presses, they will charge extra for shipping them in shorter lengths. Cut to length pieces may be your best answer, but you might wish to consider stock lengths as an option, as well, particularly if you will be re-cutting anyway.

Sharp, pointed, “needle” types of feature, whether they be made of metal, or a pointed void into the metal are not practical. In the case of being made of metal (a finger), the problem of “filling” the finger with metal becomes significant, and it can probably stated that a complete fill would not occur. In the case of the pointed void, the problem is that the heat build up from friction of the metal flow, the friction of that metal flow, and other factors will cause problems. it can equally well be stated that the pointed end of the part of the die will almost surely burn off, thereby being unable to fulfill its function in creating the sharp, pointed void.