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MMD > Archives > March 1998 > 1998.03.19 > 08Prev  Next

Properties of Delrin
By Dennis Mead

Hello All -- Just thought I'd add my two cents to the Delrin
definitions that have been posted earlier.  First of all, Delrin is
an acetal resin.  It can be specified that way since it is now made by
several manufacturers.  It also comes in two forms; a copolymer, which
is Delrin, and a homopolymer which I believe is known as Celcon.
Delrin as black and Celcon is white.  So, there is a white version
after all!  They're both fairly slippery, especially against polished
steel, which is why they're used for bushings a lot.

Acetals are thermoplastics and are routinely used for parts that are
injection molded.  Acetals are available in a wide variety of sizes
in the form of rods, bars and plates for those parts that are to be
machined.  Delrin also comes in a few alloyed flavors: silicones and
Teflon are added to make it an even better material for bearings.  One
such alloy is called Delrin AF (anti-friction??)  Talk to a plastics
sales engineer for more specific info.

Now some tales about machining Delrin.  Yes indeed, it machines like
butter, but with some caveats.  Someone said earlier that it is a
stable material, but failed to mention which characteristics are
stable.  Mechanically, I've found that this material is _not_ so
stable.  Rods suffer from micro-porosity and density differences in
the center portion, so when you try to drill a tiny hole in the center
(as in the lathe), the drill will wander off center due to those
density differences.

Delrin has a rather high coefficient of thermal expansion when being
machined.  If a 1/2" diameter hole is reamed to size, it will come out
a half thousandth or so smaller than the reamer.  This becomes very
frustrating when you are trying for a nice close fit as in a bushing
or bearing.  A brand new, dead sharp tool is needed to minimize this
effect.  I use a cold air blast while machining to help minimize this
even further.

Larger parts (approx. 12" long) machined from plate will also warp
after machining, and warp even more while lying there on the shelf
waiting to be used.  We had a terrible problem where I work with parts
such as these.  They checked out okay during incoming inspection, but a
month later were all pretzelized from time warps.  There are annealing
procedures for Delrin to relieve stresses and minimize this effect, but
for parts with critical dimensions, it is no cure.

The solution to the above problem was to change the material to
Ertalyte, a  P.E.T. plastic with similar properties to Delrin, but
totally without el warpo supremo.

Regards,  Dennis Mead

(Message sent Thu 19 Mar 1998, 07:32:36 GMT, from time zone GMT-0800.)

Key Words in Subject:  Delrin, Properties

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