A few Extrusion Tips....
Aluminum extruding is a cross between science and
experience, and it is often difficult to know for sure where one begins and the
other leaves off.
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,
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
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.
The specialists at Materials Management, Inc. can help
you avoid these problems, and can usually suggest alternative design concepts
which will extrude more easily and reliably, may result in lower cost dies, and
may cost you less per lineal foot of material, and, even, work better for you.
How about faxing us your preliminary drawings and allowing us to assist you?