Screen printing exposure unit
In screen printing, we use a screen printing exposure unit, which transfers the individual colour channels created by the colour separation (our film), onto the screen as a positive. Thus, through the colour separation, we obtain a positive film, which transmitted through the exposure device, through the light-sensitive emulsion screen.
The importance is how much UV radiation is emitted from the exposure unit. The emulsion coated screen reacts only by UV. The black positive film blocks the UV radiation, and where no UV reached the emulsion, it washes out. And so we transferred the film to the screen.
Back to the exposure unit
There are different versions, as mentioned before, the most important thing is how much UV does an exposure unit transmit. If the UV radiation is weak, one must expose longer, the stronger the UV radiation, the faster it goes. This shorter timing is of course, noticeable in the production.
DIY versions, take a very long time to expose a screen, a metal halide unit reduces the time. But not only how much UV radiation a light source emits is essential, but also how the UV radiation hits the screen with its film.
So we distinguish from DIY versions to high-end exposure devices.
And I present here about different possibilities.
Screen printer exposure DIY Version 1
Firstly, You can take a strong lamp, put the screen with its coating upside down, fix the film on it and irradiate the whole thing with the light from above. This way to exposure your screen is the easiest and probably the most primitive version.
The result is accordingly, at the light strength to make out, but usually also that these screens do not develop well. The light dispersion is not equal all over, and most of the time the film will be exposed sideways (undercut), for example, fine lines, won’t be developed well. Forget about CMYK or even Index separation.
But as I said, it is possible for spot colours and if someone needs this only as a hobby, feasible.
Table Version 2 Screen Printer Exposure DIY
The second DIY version is the self-made light boxes, where below usually the light source is placed, with several neon lamps or led strips. Then there is a glass on it and the glass of the film than the frame.
The whole thing is usually weighted down with a weight so that the film presses against the screen. This way is better than the first version. Because here the film is pressed somehow, so in a way the film has direct contact with the coated screen.
But also here it depends on which light sources we used. You must always keep in mind that we want UV radiation. Neon lamps are not very UV intensive and also with LED you have to pay attention to how much they emit.
For full-colour prints, this is one way to develop your screens, but when it comes to more delicate parts, the system has many limitations.
Version 3 Direct Sun Screen Printing Exposure
The sun, it is a first-class source of UV!
The procedure is as follows. In his darkroom, the coated screen is turned over as in DIY version 1, fixes the film on it and covers the whole thing with a glass pane, which corresponds to the size, of the screen frame. It is advisable to fill the inside frame with a stiff foam, which ideally fits the exact inside dimensions of the frame. Now cover the whole thing with a lightproof cloth. And go with it outside in the sun. Hold the prepared frame in the sun and remove the fabric. And so you expose with one of the best UV sources we have. BUT how long should you expose? How intense is the sun right now? Do clouds get in the way?
In Fact, as you can see, this method has a lot of variables that we can’t assess at this moment of exposure. I’ll never forget that I once developed a CMYK separation this way and it was first class.
But seriously, we were lucky that the frames lasted until the end of the production. Because the frame would have had to be replaced during production. It would have been almost impossible to determine exposure time and Uv strength as precisely as with the first frame what I want to say here.
Yes, you can develop silkscreen frames in the sun, but you have no consistency with this way.
For hobby screen printers, this may not matter, this setup is cheap too. But if we go to the professional screen printing. Then clearly a NO GO!
Screen Printer exposure unit as Table with a vacuum unit
These are the most used method, exposure table with a vacuum device.
Also, The setup is a bit like DIY version 2. We have a box; there is the light source,
glass over the light source, the film, and on it the frame. Instead of weighing it down with any weight, there is usually a neoprene fabric over the gauze, attached to a frame, which is lowered airtight onto the glass. If you turn on the vacuum pump, the air escapes, and now the glass, film, and screen sandwich together really tightly. Now, absolutely no light can penetrate through the sides of the film.
Moreover, most vacuum exposure tables also have a built-in timer. Where you can set how long the exposure should be. So you have a consistency that always remains the same. But even here, it is crucial which light source used. But since it is already about more professional Screen printing exposure unit. Is there at least a metal halide unit used or now also special LEDs or even special neon lamps, with a high UV radiation.
Is this UV Light is dangerous for us Screen Printer?
Here I should mention that this UV (Ultraviolet) is not as dangerous as we work here only some minutes with it.
It is not necessary to approach this device with gloves and sunglasses. I saw when I consult at one screen printing factory.
However, on a Solarium, I also do not keep my eyes permanently open, as well as one does not look directly into the beaming sun!
Metal Halide Units with a stand
Also, we have metal halide units, which stand on their own and away from it a rotating frame, where you clamp on the glass the film, and the frames and the whole thing, as on the picture, pressed with the neoprene cloth and vacuum.
The entire thing is then turned upright, and the metal halide unit exposes from a further distance. The distance plays a positive role here because if the light source is too close, this is not advantageous. A certain distance should be maintained so that the light hits the glass straight on.
The future in Exposure in Screen Printing
The future in screen printing exposure unit Are the combined versions of DTS (direct to screen) or named CTS (computer to screen) with an exposure unit incl. Or as a Metal halide unit by the site.
The development today goes in the direction of DTS or CTS, where the film is now printed directly onto the mesh on the frame.
The usual film no longer exists. Thus again, some variables getting excluded, what is great. As we want to minimize the variable we have in Screen printing.
Some advantages of the CTS exposure units
- Films can not be damaged, as they are not existing
- The print is directly on the coating of the screen, no more vacuum needed
- No need for a storage place for films
- No dirty glass, which could lead to pinholes in the screen
- no film output device needed
These combi versions, which print the film and immediately expose them in the same unit. The prices for units like this are very high but if you are an advanced screen printing shop; you should put this in your next budget.
Furthermore, by any of these exposure units, you should check with time, if the amount of UV does not decrease. As many light sources diminish the UV ratio with time. This means your exposure time will change.
Screen Printing Exposure Unit summary
The DIY versions are more or less for hobby screen printers. The homemade exposure table could be used for a small printing shop with a low budget. Just be sure that your light source gives a max. on UV.
The table with a vacuum is the most used version in screen printing shops, and it is great to control the variables. As a proper timer is included and the time for exposure will be reduced. Mostly the price varies on the amount of UV they give and of course what size of screen they can take.
For the Professional Screen Printer
The future, however, with a high price tag, is for professional screen printers, who do many screens a day, the combination of these screen printing exposure unit is practical and reduce many variables in screen printing.