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Instructions for building an exposure unit?

Posted on Tue, 20 May 2003 at 14:07



I'm just getting my feet wet with silkscreening - got a dandy little 4-color benchtop press. I'd like to build my own exposure unit (since I can't seem to locate any for under a grand). Can someone post a link to a site that may have instructions for such a project? Or maybe point me to a cheaper exposure unit? Thanks!

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Anonymous says: Instructions for Exposure Unit ~~~~~~~~~~~~~~~~~~ 1 lightsource 1 piece of glass 1 frame 1 possie 10-20 gallons of emulsion 3-4 months to get times right (from 5 min. to 5 days depending on lightsource ...

Instructions for Exposure Unit
~~~~~~~~~~~~~~~~~~

1 lightsource
1 piece of glass
1 frame
1 possie
10-20 gallons of emulsion
3-4 months to get times right (from 5 min. to 5 days depending on lightsource and size of screen)

Enjoy!!!!

or

You can buy one, be trained on how to use it, and start producing your own screens.

or

You can pay someone to create the screens for you.

Just a few suggestions, I too have travelled this road on the search for the perfect exposure unit...... unless you can spare the time (months) to get it right, spend the cash to save the time.

posted on: Sun, 06/15/2003 - 4:37pm
Anonymous says: Hanky spanky is right, it probably is cheaper/quicker in the long run to get a 'factory made' unit to produce nice screens, but here's how to do it if you can't afford one. 1. Determine your common frame ...

Hanky spanky is right, it probably is cheaper/quicker in the long run to get a 'factory made' unit to produce nice screens, but here's how to do it if you can't afford one.

1. Determine your common frame size.

2. Get a piece of 2-3" foam rubber (cushion material) this needs to be flat and cut 2" smaller than your inside frame dimension. It needs to be thick enough that when a screen is laid over it upside down, the screen mesh rests on the foam and the screen frame is not touching anything.

3. Get a piece of regular plate glass 1/8" thick (or close) and either tape the edges or get the glass place to bevel or sand the edges so you don't slice your fingers off. For size, this needs to fit the back of the screen to the frame edge, no bigger. It's ok for it to be a bit smaller.

4. Place the foam on a board. Place a coated screen over the foam, squeegee side down. Place your positive face down on the screen, or tape in place before you put the screen down. Place the glass over the positive. You can pick up the board to move this around.
Your glass should weight the posi into the mesh and stencil - not a perfect contact like what you get on a vacuum frame, but good enough for solid colour work.
You could build this all into a compression frame by hinging a wood frame that held the glass to a base board with the foam mounted in place.

5. Coat your screen with a coater and good emulsion - you only need a quart or gallon to start, not 20 gallons. Dry it flat, with the squeegee side up, in the dark. This must be kept in the dark till used - If you don't have a little lite proof drying box with holder guides, then dry the screen in the dark, and then wrap in black plastic bags and store in the closet.

5. Light source - this is the tricky part. You need UV in a certain range (330-440nanometers) You can use the sun, but you have very little control if you don't have a light integrator. Easiest and cheapest is to go to a hydroponics store and buy a 1000w metal halide unit. I paid $240 for one here, they vary in price. Local police might give you a deal if they bust a grow-show, but maybe that's not common where you live - we are the home of BC Bud.... Other sources that will work are murcury vapour (used in farm situations) or quartz halogen, although it isn't very good with the UV and will take longer. Some tanning lamps give off UV too, but flourescents need to be closer to the glass...

6. Suspend the lamp in position over the frame. Distance back from the glass to the light sould be at least 1.5 times the diagonal of your screen frame, so you don't get undercut stencils and it's not too hot in the centre.

7 If you keep this distance constant for each exposure, you can test until you get a good exposure time - as long as the lamp has been warmed up to the same intensity, this will stay constant until the bulb starts to die (5-10,000 hours!) Remember though, the lamp has to warm up before you get good UV. I would prepare the screen/posi/foam and glass away from the lamp and light, then place it below after the lamp is warmed up and then time from that point. Be careful about heat and exposure to these lamps, they are hot and will burn you. Don't look at the bulb!

8. To test for time, either borrow an exposure calculator (they are expensive to buy, but your screen supply store may loan you one with your emulsion purchase) or get a large film positive with halftones and fine lines, and a piece of rubylith or very black paper. The ruby or paper is for your step mask, and needs to be able to cover the entire surface of your screen. Start an exposure for one minute, and then every 20 seconds, move the ruby one inch until the positive and screen is covered- If you get 10 steps, it will represent intervals from 1 minute up to 4 minutes20 seconds. When you wash this exposure out, you can count the seconds until you find the section on the stencil that has the following characteristics:

- The surface isn't slimey - underexposed will be soft and visibly thinner and the stencil will give off colour and be sticky even after washout. In extreme underexposure, the stencil will wash right off the mesh. Find the spot where this starts to change and the stencil is harder and solid looking.

-The image is not closed in or lines aren't hard to washout - be careful, this also happens when you don't have a vacuum. When it starts to close in the detail and the surface is hard, this is overexposed

9. Calculate your exposure time from the results of this test. if the whole thing is slimey, then do another test and start the exposure with your maximum time and increase the intervals if necessary. If after 10 minutes you are not getting anything working, your light source is not high enough in UV. In the future, if the lamp is warmed up, and the distance doesn't change, and your emulsion and coating is the same, and your mesh is the same count and colour - you should be able to use the time you figure out and get good stencils each time.

The best is to have a proper metal halide lamp with cooling fans, a light integrator to count the UV hitting the stencil, and a vacuum frame to hold the film tight to the emulsion.

This is better than nothing, and will give you consistant results if you keep your processing steps consistant. Don't expect to make nice halftones or print process colour work, but this will get you going with your printing adventure.

When you are ready to step up to purpose built equipment, check the 'for sales' on this site, or hound your local print shops and suppliers - there's always good deals out there on small self contained exposure units.

visit my website for more info on exposing and other related screenprinting info
www.squeegeeville.com - if you click the big print on the front you can see how we do it with solar, and I think under the training and workshops there's a pic of a student just stepping out in the sun and burning a screen with a set-up like what I described above.
All the prints in the group shots in the training section were exposed on a metal halide lamp borrowed from the city works yard up in Whitehorse - they use these for outside floodlights.

Andy

posted on: Mon, 06/16/2003 - 9:45pm
Anonymous says: has anyone had any luck with uv blacklights. the party ones dont work. The others are only 40w ea. Maybe 3 of em would do the trick? posted on: Fri, 08/19/2005 - 8:20pm

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