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  1. #11
    Nicholas Lindan's Avatar
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    Very nicely done.

    A cheap source of high voltage, though it is at line frequency (or DC, if you wish), is the transformer, rectifier and capacitor from an old microwave oven. With a pair of voltage doublers made from more microwave rectifiers and capacitors it is possible to get 8kV at pretty hefty power levels.
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  2. #12
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    Do a patent search for "corona" and "kodak" and you will find about 1700 hits with extensive descriptions and a list of the problems that must be solved such as uneven corona and ozone removal. The wear of the discharge system and type of "wire" used is also important. Just some thoughts you may want to look into. I don't want to discourage you, just inform you of the mass of data out there and the lists of potential problems.

    Again, best wishes and it was a job well done! Congratulations.

    PE

  3. #13
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    I tried microwave oven transformer first as a transformer, but it went up only to 1 kHz (probably because of core material selected -- it's normally used at 50...60 Hz), which would be quite a low frequency for efficient corona treatment. That's why I had to make my own with ferrite core and double, layered windings.

    CRT TV Flyback transformers would go to high frequencies, but they have the rectifying diode incorporated that cannot be removed. What I need is AC. All my DC experiments, up to 30 kV, failed and the commercial unit we measured gives AC.

    I think that uneveness is easily addressed by doing many passes "just in case", which is not possible or easy in fast, continuous web machinery. Ozone indeed does smell, but for small scale treatment probably does not matter.

  4. #14
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    With the high power units at EK, I could smell the ozone even with an efficient exhaust. The RF generated would cause a lot of noise if anyone had a radio in their lab. The unevenness could be eliminated by having more than one discharge unit. At EK we used a wire IIRC. In fact, one patent refers to renewable wire corona discharge units with the wire coming off a spool slowly as it is wearing away due to use. The metal being discharged onto the film due to this wear can be a problem.

    PE

  5. #15
    AgX
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    Quote Originally Posted by hrst View Post
    I think that uneveness is easily addressed by doing many passes "just in case", which is not possible or easy in fast, continuous web machinery. Ozone indeed does smell, but for small scale treatment probably does not matter.
    Instead of several passes of the material one can employ several electrodes on a rotating head with the material passing in a longitudinal path.

  6. #16
    hrst's Avatar
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    Old patents I read almost a year ago were using, for example, seven electrodes in a circle, the web passed them all. This of course allows higher speed for web than one electrode, and increases evenness. I can well imagine how big problem ozone is on bigger units, as it smells quite strongly even when treating just one 21x30 cm sheet so that it is completely treated.

    Some patents talk of using a metal net instead of single wire, for larger surface area and many points for discharge.

    Luckily, I find currently no need to make any big unit that would use rollers to move a long web, or do a quick job with single pass. This sheet version with manual electrode is just fine to treat, say, up to 1 meter long sheets.

    I think that, from the possible pitfalls mentioned, the metal from electrode and possible fogging in case of overtreatment may be the ones I should be aware of.

    I thank you PE for pointing out any possible problems; by no way I take that as any kind of discouragement! I'm very happy about the positive response I get from all of you, and all the ideas I get.

    Actually, I'm currently winding a new transformer; I got the previous one to arc out, but that was all due to my very clumsy handling that caused some of the insulation came off very badly. Winding a decent HV transformer is possible, but quite difficult and tedious, especially if you want it to be completely corona-free to last for years. Mine is not, and corona discharges happening inside can shorten the life, but in my very intermittent usage this is not a problem.

  7. #17
    Photo Engineer's Avatar
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    Well, having wound a few myself, I know how cranky a home made transformer can be!

    As for corona treatment, we would never use support beyond about 24 hours corona treatment. It dissipates.

    Very best wishes and if you get to a stumbling block let me know.

    PE

  8. #18
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    Are corona discharge and electron bombardment the same thing? Can anyone give some kind of an explanation for how this can make raw PET/Melinex accepting of coatings... cough cough, for the electronically lay person?

    Also, if this treatment dissipates after 24 hours, how does Photo Formulary's subbed melinex store? Was another subbing layer applied at the time of treatment?

    This thread is a continuation of this thread, btw.

    I'm just thinking ahead to the day when PF's stock runs out.

    Hrst, you say this works of course, so in other words a sheet of PET without this treatment will repel an emulsion, so how good is adherence after running it through this machine?

  9. #19

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    As well as flyback transformers, auto ignition coils are another source of high frequency/high ratio transformers. Microwave oven transformers won't handle high frequency, but will deliver lethal currents.

  10. #20
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    Corona Discharge = Electron Bombardment! They are identical.

    The Melinex from the Formulary has been bombarded at the plant and then a subbing layer is applied that will adhere to both the Estar support (Melinex) and to the overlying coating whatever it may be, as long as it is water based.

    There you have it.

    PE

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