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CDROM metal stripper

Discussion in 'Electronic Design' started by RST Engineering \(jw\), Feb 21, 2007.

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  1. I plan on using two junk CDROMs in my basic science class as a variable
    capacitor for a crystal set -- shim brass cut to cover a little less than
    half of each cdrom, glued to the surface, with a vinyl grommet in the middle
    to hold the disks together and a disk of mylar cut from a sheet protector
    between the brass plates as the dielectric.

    Each student is going to have to prepare two disks each, so it isn't a
    onesie-twosie job. I need a method that I can use fifty times a semester
    every semester.

    The problem is removing the metal "plates" that are there now. First
    thought was to soak the disks in drano or aqueous lye and dissolve the
    aluminum, but further reading and experimentation shows this isn't quite
    enough. Apparently there is a lacquer or epoxy coating over the aluminum
    and a nickel flash under the aluminum. This is only hearsay, but I know
    from experimentation that a caustic solution by itself isn't enough.

    Anybody KNOW what the various layers are on the disk and any suggestions on
    how to remove them without destroying the plastic of the disk itself?

  2. Phil Hobbs

    Phil Hobbs Guest

    If you use CDRs, you can usually delaminate the coatings by flexing the
    disc a few times. Peeling with strong tape is another possibility.


    Phil Hobbs
  3. D from BC

    D from BC Guest

    Brainstorming ideas:
    * Use orbital sander or try a scraper.
    * Sandblasting
    * Microwave the disk..this will create breaks in the coatings allowing
    NaOH to attack the Al
    * Drop the CD idea ...Buy poly carbonate sheets at the hardware
    store... (lexan)...Get it laser cut to any shape..
    D from BC
  4. Neither trick works with the Verbatim junkers I just played with, but thanks
    anyway. How "strong" does the tape have to be? All I used was plain old
    clear packing tape for one try and a really stuck-on paper label for

  5. Labor intensive. Getting 50 kids to sand two disks apiece will either take
    a lot of sanders or a lot of time.
    School doesn't have a sandblaster.
    THAT was a hell of an idea ...fireworks city, but if you only do it for five
    or ten seconds that just might work. It sure as hell crazed the surface.
    I'll drop it in the lye bath and report back...
    You wanna explain to the taxpayer why we have a $50 variable capacitor in
    our $5 crystal set?

  6. Rich Grise

    Rich Grise Guest

    I just took an old Philips that I've been using for a coaster, and just
    scraped some of the top layer off, and imagine my surprise when there
    was nothing else there - the rest of the CD was transparent. I was
    kinda surprised - I always assumed that the data stuff would be on
    the LED side, but apparently the LED sees through the plastic, and the
    data is on the underside of the label.

    So maybe some 100 grit wet sandpaper?

    Good Luck!
  7. D from BC

    D from BC Guest

    Maybe after the microwave, the disk might be easier to scrape instead
    of doing the NaOH bath..
    Also..I'm not sure about this but..the bubbling NaOH provides a
    lifting action so any form of damage to the coating might work.
    By microwave or scratching by sanding (say 80 grit).
    Heck...just grazing the surface with a single scratch might even
    work.. The NaOH bubbles may flake/erode the film until there's no

    Also, CDR peeling was mentioned..
    One time I got a bad HP CDR and I was able to peel off 60% of the film
    by hand.
    D from BC
  8. Joel Kolstad

    Joel Kolstad Guest


    Can you tell us where you're teaching?

    How'd you manage to convince the teachers' union to let an engineer teach a
    class? :)

  9. Sure. There's a molded polycarb 'plate' with the pits on top covered by
    metallizing, varnish and paint.



  10. James Waldby

    James Waldby Guest

    1. Have you verified that you need to remove anything? I.e., maybe they
    will work as-is, since you aren't connecting to the conductive reflective
    aluminized layer anyway.

    2. Five seconds in a microwave will disrupt much of the conductive layer.

    and have a little diagram
    that shows CD and CDR structure. Probably lots of other sites too.

    4. Packs of 25 and 50 CDR's come with a clear disk (like a CD with no
    metalization) at each end; if you know anyone who goes thru lots of
    media, ask them to save those up for you.

  11. James Arthur

    James Arthur Guest

    If the CDROMs are just there to support the brass plates--with the
    mylar dielectric in the middle--why not just face the intact
    metallized sides outward, whereby the large gap will minimize any
    affect on total capacitance?

    Namely, in cross-section: (view using Courier font)

    .. ||xxxxxxxxx #### mmm #### xxxxxxxxxx|| <--aluminized layer
    .. \_________/ \__________/ facing outward
    .. B M B
    .. r y r
    .. a l a
    .. s a s
    .. s r s

    If the resulting minimum capacitance is acceptable it'll save a lot
    of effort stripping.

    James Arthur
  12. Sierra Community College, Rocklin CA.

    Simple. I'm the VP of the freakin' union local and a statewide union
    officer as well {;-)

  13. Well, the math isn't difficult. The plates (half-round on a CDROM) have an
    area of about 9 square inches (which makes the whole CDROM surface about 18
    sq in). If I use a 5 mil mylar (er = 2 or thereabouts) sheet between the
    plates and some reasonable estimation of the air gap, I come up with a
    fully-meshed capacitance of about a nanofarad.

    Polycarbonate has an er of 3, and the CDROM plates are about 50 mils thick.
    The capacitance of the brass plate to the aluminum surface is something on
    the order of 150 pf, and the capacitance from the outside of one of the
    aluminum surfaces to the other is about the same. So, 3 150 pf capacitors
    in series has a plate-to-plate capacitance of 50 pf. Compared to 1000 pf of
    the meshed plates, that's not a whole lot, but there will be fringing and
    all of that stuff, so the more we can keep the strays at bay, the better I
    like it.

    Besides, the kids can see inside the capacitor and can have a feel for what
    happens when the plates rotate with respect to each other.

  14. DaveM

    DaveM Guest

    My $0.02 USD worth...
    Here's an idea that came to me as I was opening a new spindle bulk pack of
    DVD-RW disks. On the top and bottom of each spindle is a very clear spacer of
    the same dimensions of the DVD blanks. No metallization, no printing... just
    clear (I assume)polycarb.
    My thinking is for the OP to contact some of the bulk CDR suppliers and see if
    they can sell a multiyear supply of these spacers. No pain, no chemicals, no
    burning plastic. And the price should be right... maybe a few pennies apiece.
    I did a cursory Google search, but didn't see any of these spacers listed on any
    of the bulk suppliers' web sites, but you gotta know that they have them or can
    get them.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    Some days you're the dog, some days the hydrant.
  15. john jardine

    john jardine Guest

    Was just thinking that myself but am calcing about 50pF max across the
    variable capacitor but a constant 30pF in the parasitic shunt paths
    Suggests something like a 40 to 80pF variable cap.
    Better still and in the interests of science (i.e. the bits were at hand),
    spent 5 minutes making one. (the grommet idea works well).
    Min C=64pF, Max C 102pF.
    But with finger pressure making the adjusment results in 88pF to 330pF

  16. Why not use thin fiberglass PC boards? Etch the plate areas you
    want. Another approach is the old "Book Capacitor", or a homemade
    piston trimmer capacitor.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  17. James Arthur

    James Arthur Guest

    Dang, actual data! Good show. But, I calculate/estimate the
    capacitance of two full-disc plates through two CDROMs (two
    thicknesses of 1.3mm polycarbonate, dielectric coefficient=2.9) as:

    k * e0 * A 2.9 * 8.85e-12 * pi * (0.060m)^2
    C = ---------- = ------------------------------ = 112pF
    d 2 * 1.3e-3 m

    so this real-world result bugs me.

    Hmmm. Why wonder when you can measure? I must try it.

    James Arthur
  18. That wasn't my battle plan. My idea was to have the polystyrene "platters"
    simply there to have a place to glue the plates onto.

    The plates themselves are on the INside of the disks with a thin sheet of
    plastic (kitchen wrap, saran wrap, a sheet protector cut in half...) between
    them. Something like this, and an ASCII artist I ain't:

    @@ grommet through the center hole
    ******************* top cdrom disk
    --------- brass shim stock glued to top disk
    =================== thin plastic dielectric
    --------- brass shim stock glued to bottom disk
    ******************* bottom cdrom disk
    @@ grommet coming through from top

    I calculate somewhere between 500 and 1000 pf for the capacitor with 3 mil
    mylar and a 3 mil airgap allowance on both top and bottom plates.

    I'm sort of curious why I was off nearly an order of magnitude from the
    experimental results.

  19. Cost. Cost. Cost.

    But the last two sound interesting if I can figure out a cheap method of

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