Few Design Tips
extruding is a cross between science and experience (and with a little “witch
craft” thrown in), and it is often difficult to know for sure where one begins
and the other leaves off.
are, however, a number of basic considerations one should keep in mind for
satisfactory parts. A few of those are as follows:
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.
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.
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”.
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.
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.
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).
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.
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.
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”.
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.
The specialists at
Materials Management, Inc. can help you avoid these problems (and many others),
and can usually suggest alternative design concepts which will extrude more
easily and reliably, may result in lower cost dies, avoid excessively rapid (and
costly die) wear, may cost you less per lineal foot of material, make a mill
more willing to accept your shape for production, and, even, work better for
about faxing or emailing us your preliminary drawings and allowing us to assist