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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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