Re: LDPE source and thickness
There are a lot of sources for plastic in all thickness' and flavors. I
just posted my favorites to the list a week ago.
It is generally better to use multiple sheets of plastic, the theory
being that an imperfection in one, won't be in exactly the same place as
the next. Multiple sheets of plastic do leave more places for air to
become lodged in the material, so it is a mixed blessing. Thin sheets
of plastic do seem to have more imperfections also.
The vacuum pump is to pull any trapped air and moisture from the rolled
cap and to allow oil to fill the spaces. Air trapped in a poly cap will
ionize and the ozone, and corona will attack the plastic and cause it to
fail. Someone suggests using a palm sander to shake the cap while
pulling a vacuum - good idea.
Keep your work space free of dirt and avoid handling the materials
directly, wear gloves. Work on a piece of some 6 mil plastic for a
clean surface. It is a job for four hands.
1/16" may be a little hard to work with, but you'll only have one
thickness to contend with. It will withstand a 10 kV transformer, if
there are no serious imperfections. The .062" is a nominal dimension.
It will be thicker and thinner in places. A tightly rolled cap will
thin and mineral oil may cause it to weaken.
Leave plenty of margin at the edges of the insulation. The corona is
more severe at the edges of the plates, more air is likely, and it only
takes a little dirt on the plastic to break down and arc.
Any vacuum is better than no vacuum. Borrow a pump if you have to. "The
Bell Jar" site showed using a refrigerator compressor to evacuate
vessels. A vacuum will also turn up any flaws in the mechanical
structure of the casing.
One commercial manufacturer (in a cap I dissected) uses a layer of
tissue paper between the plate and dielectric plastic. I don't know
what the purpose is, but it may aid in drawing oil up between the plate
and dielectric, or the buffer of paper may keep air pockets from being
formed in the tightly rolled cap, or the oil just may be a better
dielectric than the plastic.
Extended foil caps (making connections along the tops and bottoms of the
foil, along the entire rolled edge) makes for a very good performing cap
(low ESR) but there is a better chance to have air trapped inside.
Polyethylene absorbs mineral oil and swells. Pull a vacuum as soon as
you can, once the oil is added. It is a good idea to pull a vacuum
several times over the course of a week (an hour or two at a time).
Elevating the temperature of the cap slightly will aid in air removal.
Tilt it around its axis, while evacuating it, to allow the bubbles to
Polypropylene works very well also. It has very low dissipation. It is
not as flexible as LDPE. It is more resistant to oil.
Engineering is compromise. Mistakes are the price of tuition. "any
engineering solution contains the seeds of the next problem"
Tesla List wrote:
> Original Poster: "Samuel Rosset" <samr-at-chez-dot-com>
> I would like to make rolled caps, and I'm looking for LDPE sheets. I checked
> Mcmaster-dot-com, and they have 1/16'' and 1/8'' which is 62.5 MIL or 125 MIL. I
> read somewhere that the best thickness is 90 MIL, which is impossible to
> obtain with these thicknesses.
> But I read somewhere that LDPE would resist 1200V/MIL, which means that a
> 62.5MIL sheet would resist a 75000 V tension. My xformer is rated 12KV/30ma.
> Can I use the 1/16''-thick sheet for my caps??? And what do you use a vacuum
> pump for in the building of a rolled cap???
> Thank you for your help