> When I played with this kind of thing on an old HP plotter, the
recommended
> resist pen ink was a staedler red felt marker.  I replaced the felt
soaked
> core of the standard plotter pens with a scavanged staedler pen.  You
can
> get a bottle of the same ink (although it's special order around here.).
> Might try to see if it will work in an ink jet nozzle.

Back in 1980 I was right out of school and working for HP.  I wanted a way
to make my own circuit boards for personal projects like you guys are
talking about.  I even wrote a simple layout program on the desktop
computer I had.  You entered coordinates for net points in a text file and
it would compute the tracks and mark whatever it couldn't route.  The
problem was how to get the information out of the computer onto a circuit
board.

There was a pen plotter available that could be hooked to the computer, so
I programmed it to draw filled areas by using a raster pattern.  At first
I just had it draw onto a piece of paper, then converted that to a
Kodalith negative in the darkroom, then used that to expose boards.  At
the time there was photoresist available that came in two parts.  It ended
up being a thick greenish liquid that you painted on the copper then let
it dry.  I think it was called "Resolve" or something like that.  It was
not lacquer based as everything I've seen since then.  After exposing the
board, you rinsed away the unexposed areas with plain water, then baked
the board to cure the remaining stuff onto the copper, then etched
normally.

This process worked OK, but there were problems.  The worst one was that
it would use up one or more of the little felt tip plotter pens per board.
I tried to fix that by cutting the top off one of them and manully filling
it with ink before a run.  That helped, but it made large blotces in the
beginning, and needed refill steps along the way.  Then I modified a
"liquid ink" pen mount on the plotter.  These have no ball point or felt
tip, just a cappillary with a tube to an ink reservoir.  The cappillary is
thin enough so that the ink doesn't come pouring out until the bubble
touches the paper.  This worked much better, but was quite a mess to use
and clean.

Then I began experimenting with using dark ink and writing directly onto
the photoresist layer.  That sortof worked, but the ink apparently didn't
block the light that well or otherwised messed up the photoresist.  Then I
tried "writing" a resist layer directly to the copper using the liquid ink
pen.  I changed jobs and lost use of the equipment before I found an ink
that would both adhere to copper and stay there during the etching
process.

If you guys really want to try direct writing resist onto a copper board,
I suggest looking into a liquid ink pen driven by a plotter.  Liquid ink
pens will take just about any ink, and can deliver large quantities of it
unlike any other pen.  It will be hard enough to find a suitable ink
without also requiring it to work in an inkjet printhead that is finely
tuned for a specific type of ink.

Since I was experimenting with this 22 years ago, I've noticed some
substances that might be worth a try as ink.  One of these is a liquid
plastic you can get in toy stores.  I think it's meant to immerse wire
loops in, then pull them out and let the "soap bubble" film dry into a
plastic membrane.  I bet this stuff stands up to etchant, but I don't know
how well it might adhere to copper.  I'm sure it is way too gloppy for an
injet head.  The next thing I was going to try was polyurethane paint, but
never got to it.


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