Real stuff on plastics for high voltages-- (% Water absorption 24 hour per
ASTM D570 test)
There are a lot of types of plastic around but the list narrows a lot when you
go looking for thick blocks to machine.
Don't use--
--Nylon 6 and 6-6. Water absorption 1.5-1.6. Hard to hold tight machining
tolerances.
--Nylon 6-12. Water absorption 0.4. Hard to find in machining block
thicknesses.
In the design engineering world Nylon is notorious for its water absorption.
OK for low voltages though I don't think I'd use it around sensitive
electronic stuff at least in thin sections.
--Polyimide (Vespel) This is what we used in Semiconductor equipment high
voltage power supplies. For what you would pay for a distributor cap sized
block of this stuff you could buy an MSD ignition and have enough money left
over to pay a machinist for a day's work to make the distributor fit.
--Phenolic laminated blocks -- the cloth they laminate into this stuff to
reinforce it is pretty unpredictable for high voltage insulation. Nice to
machine, looks nice; but I'd save it for low voltage applications and things
like heat barriers for carb mountings.
--Polyethylene and Polypropylene. Negligible water absorption. Not available
in thick blocks
--Acrylic (Plexiglas). Water absorption 0.2. Easy to machine. Cracks easily.
Hard to find in thick blocks. You could glue up thinner sheets to get the
thickness you need; but you'll fight air bubbles between the layers (visible
through the clear material). This is a real last resort.
Consider--
--Acetals including Delrin Water absorption 0.2. Easy to machine. Readily
available in thick blocks.
--Ultra High Molecular Weight Polyethylene (UHMWPE) Negligible water
absorption. Easy to machine
Available to 2 inches thick. A bit pricey. Soft compared with Delrin but real
tough. Neat other applications in race car construction. See one source's
link:
http://k-mac-plastics.net/uhmwpe-square-rectangle-bar-stock.htm There are
others on the web.
--Teflon. Negligible water absorption. Expensive, hard to machine with
accuracy but might work for small round inserts especially if you have a cheat
surplus material source.
--Build an RTV mold and experiment with epoxy and microspheres (turns the
epoxy purple in color) for filling to get some strength and dimensional
accuracy. You could cast in metal parts. Use a convenient shaped stock cap
with what ever pieces added or removed as the master for the rubber mold. Any
of you guys familiar with the old HC dual coil flathead ignitions know about
the problems with broken plastic parts. I've got one and I think this is how
I'd make a replacement part for it.
Ed Weldon
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