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Re: [TCML] Secondary form material
Hi Andrew,
Hmmm... perhaps you were using thin-wall PMMA tubing, or cracks were
created during cutting or from solvents. PVC and PMMA (acrylic) have
been found to make excellent coil forms since both have very high
dielectric strength and they are readily available. However, they are
subject to a few limitations.
For smaller coils, PVC is preferred by most coilers because of its low
cost and availability. However, make sure you get SOLID wall, _not
foamed_, pipe. Lighter (SDR) grades or schedule 40 PVC work best. The
fresh-cut wall surface should be a uniform color and appearance, and the
pipe should be made from virgin PVC. Schedule 80 can be used, but it
will add unnecessary weight to your secondary. In order to be compatible
with fittings, all 4" PVC pipe actually has an OD of 4.5". Be sure to
sand off any trace of lettering on the pipe before sealing or winding to
prevent possible electrical tracking.
If you _truly_ need a 4" OD secondary, then you'll need to buy 3.5" PVC
pipe. This is a relatively uncommon size, and you may need to obtain it
from a larger plumbing supply house instead of Lowes or Home Depot. You
may find the following table of PVC pipe dimensions useful:
http://www.apiplastics.com/pvc_pipe_dimensions.html
You can also use PMMA tubing. This IS available in 4" OD. Cast PMMA has
the best appearance, but it's also the most expensive. Extruded acrylic
will work just as well electrically, but it may show slight optical
non-uniformities, and it is not available in larger diameters. Be sure
to avoid cleaning the PMMA with certain solvents, such as alcohols,
acetone, toluene, or ammonia, since these can cause surface crazing or
lead to outright fracturing of the tubing. Do not overheat the material
during polishing, since this can also cause crazing over time. Because
of residual internal stress, crazing seems to be more of a problem with
extruded than cast tubing. Use thicker (1/4") wall tubing since it will
be more resistant to cracking during cutting or from handling/shock.
I've had good success with various polymers from US Plastics, and they
carry both cast and extruded PMMA tubing:
http://www.usplastic.com/catalog/category.asp?catalog%5Fname=USPlastic&category%5Fname=21315&Page=1
Good luck,
Bert
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Andrew Robinson wrote:
As many have already discovered, PVC is apparently a terrible material
to use as a secondary coil form. About a week ago my engineering group
also discovered that acrylic is extremely hard to work with. We ended up
cracking our only secondary coil form which we slaved over to find! So
my question to everyone out there is what on earth should we use for the
secondary coil form. We need a 4" OD coil form and if possible, if you
can name a material that can also be an 1/8" thick we have a nice
cutting tool that makes the most perfect end caps. So ideally 4" OD and
3.75" ID is perfect for what we need. I've called numerous plumbing
companies with no help what so ever. Thanks everyone!
Regards,
Andrew Robinson
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