Note that fabric works very well as vacuum conductor. So you could
conduct your vacuum around with sheets of fabric (different sheets for
different sizes of boards).
However that only works for single sided exposure.
I don't think any vacuum distribution is really needed. The commercial
machines i have seen just suck the cover against the glass and it
pulls the whole surface just fine. Maybe put a wick around the
perimeter if you are really worried.
ST
On Sun, Mar 22, 2009 at 4:12 PM, William Alford <walford@...> wrote:
> At 09:44 PM 3/21/2009, DJ Delorie wrote:
>> converted my UV exposure box to vacuum hold-down.. . . the glass bow in so
>>much I feared it would break!
>
> i've built several vac hold downs over the years and the trick is to
> have the substrate supported everywhere. one way i did this was to
> take a piece of good quality flat plywood and cut parallel groves
> with a table saw in both directions about 1/8 inch deep on one side
> across the entire face of the board. (say, 1/2" apart) spray it with
> acrylic or some other sealer to seal the wood, drill a 3/4" or
> whatever size hole you need to connect to the vacuum line in the
> center of the board and then glue a piece of Formica on the top of
> the grooved side carefully so's not to get glue in the grooves (i
> used contact cement on a 2 ft x 3 ft table that lasted over 20 years
> for screen printing). now you can see from the edges where the
> grooves are and draw pencil lines across the formica and use these
> guides to drill 1/16" holes across the entire surface over the
> grooves beneath. now tape up the edges to seal the ends of the
> grooves and attach the vac hose (maybe with a 90 degree connector).
> you will have to put wooden feet on the bottom so the hose can exit
> from beneath and keep the board sitting level. now whatever size
> board you place on the top, just tape around it's edge to seal off
> the uncovered vac holes and any size board can be accommodated. if
> you want to expose negs to photoresist, you can also place a piece of
> flexible clear plastic over the top of the board/litho neg sandwich
> that covers the entire hold-down surface and get perfect contact for
> exposures. i used to use sunlight to expose and place a toy dart
> stuck on the top edge of the board to orient to the sun. when you
> move the board so that the shadow of the darts shaft disappears, the
> board is exactly perpendicular to the sun's rays. i now have access
> to some free corian and will build one from that which has a
> perfectly flat and smooth surface.
>
>
> William Alford
>
> GI Motility Medical Research Page
> http://alford.grimtrojan.com/
>
> [Non-text portions of this message have been removed]
>