NuArc vacuum control and good practices for contact printing?

[Tin Can]

I have just started using a NuArc 16x20 effective size vacuum table for contact prints. Recently someone posted they were controlling vacuum to -8". I am seeing a problem with -20" where the pinhole rubber mat impresses onto the print.

I am thinking of using a 4 ply hot press satin Arches board for a bottom board, and think I should turn down the suction.

I am running NuArc filters both in and out of the oilless pump, so a bleed off is really all I need.

Any advice on inexpensive needle valve vacuum controls for this application?

[Harold_4074]

If you put a choke between the pump and valve, you can use even a globe or gate valve to tweak the working pressure. I would recommend a length of small-diameter tubing (think of a coil of poly drip line, for example) and whatever hardware-store valve is compatible with the size fittings that are already there. Set the valve at about midpoint, start with 50 feet or so of 1/4 inch tubing, and then shorten the tube until you are reasonably close. You would probably want some adjustment range if only to allow correction for pump wear.

That said, I'd be surprised if it really matters. 20" of mercury is only around 10 psi, a pressure that you can probably exert with your thumb. Will any hot press board collapse at that pressure? My guess is that it wouldn't be affected at all.

[Tin Can]

I'll try it with just the mat board and see what I get, I agree 4 ply should not compress and conform to any rubber pinholes. I was also seeing rubber casting lines when using release paper. I used release paper first to help with dust and the pinholes were marking the glass cover, making another source of image degradation.

[Harold_4074]

Ah, now I see your concern.

If the board is "thick" compared to the diameter of a pinhole (even 1:1 and ideally 3:1 or more) then is extremely unlikely that the pinholes will "telegraph" through.

[Jim Fitzgerald]

Randy, I use 2 ply mat board and keep the pressure at around 7 psi.

[Tin Can]

Thanks Harold and Jim, the satin hot press is the smoothest mat board I could find, I don't want to 'telegraph' a texture.

I'll make a bleed off hole tonight with a valve. I have done that with Bimba linear actuators under high pressure.