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Discussion Starter #1
If you are using an array of black-lights or other diffuse light source you may find that your ability to print fine detail is restricted, here's why:
This is a schematic diagram of an exposure unit.



The mask is in perfect contact with the flat screen. If there was only one small tube the system would approximate to a point source and the light would cure the screen so that the image is faithfully transferred to the screen.

Compare that to the effect of the light from the tubes at the extremes, they are going to undercut the mask and result in a loss of detail and closing of halftones.

If you do not use a vacuum the chances of good contact between mask and screen are nil, and that includes a piece of plywood and some bricks.
Once contact is lost the undercutting is made worse.




  • So point light sources are good, but if you have a point source then most of the energy will be wasted and result in long exposure times. Even commercial exposure units will make a reasonable compromise which leads to the exposure distance being about the same as the diagonal of your screen.
  • Vacuum units are essential for fine detail and halftone printing.
  • The screen in the area between point source and diffuse source will receive a variable amount of light which means that some may wash out other bits won't. You will have variable detail depending on your washout.
  • Over-exposure will make the problems worse because more of the semi-cured emulsion will resist washout.
  • I've shown a flat screen, screens aren't flat, they follow the contours of the mesh. Even with a good vacuum contact will not be perfect.
  • Light travels in straight lines - except when it hits anything, the first thing that happens is light-scattering so that even a perfect system would have some undercutting due to scatter. Tinted mesh absorbs uv light and reduces scatter.
 

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@%1;

i agree. my emulsion is dark colored to better absorb light and lessen double light exposure due to bouncing light rays.
 

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There's a few things this schematic is missing - one could the amount of reflected light, that goes through your screen, bounces off whatever is on the other side and then is reabsorbed by the screen.

Also, those same rays of light that are hitting your screen at extreme angles, will also hit the screen where the mask is.

Also, if you're using, say, a mylar sheet of some type, that also has some dimension to it, so if you're exposing the sheet with your mask on the opposite side of your screen, you'll have a similar effect of not having the mask firmly placed pressed to the screen.

I don't know if a vacuum unit is essential - although it does help. We're screen printers! We're nothing but resourceful, frugal and crafty :)

Do you have a test graphic of what you find are adequate results with a vacuum unit? I'd love to see what my hobbled together exposure unit can do, in comparison, given the same screen mesh and perhaps, emulsion.
 

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Discussion Starter #4
There's a few things this schematic is missing - one could the amount of reflected light, that goes through your screen, bounces off whatever is on the other side and then is reabsorbed by the screen.
The light going through the screen should be filtered by the screen, exposure blankets tend to be matt black rubber so won't reflect much light.

Also, those same rays of light that are hitting your screen at extreme angles, will also hit the screen where the mask is.
Sorry, don't quite understand this?

Also, if you're using, say, a mylar sheet of some type, that also has some dimension to it, so if you're exposing the sheet with your mask on the opposite side of your screen, you'll have a similar effect of not having the mask firmly placed pressed to the screen.
The mask always goes ink-side to emulsion. You will, of course, scatter light that hits it and give some undercutting.

Do you have a test graphic of what you find are adequate results with a vacuum unit? I'd love to see what my hobbled together exposure unit can do, in comparison, given the same screen mesh and perhaps, emulsion.
No ;)
 
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