Chapter 9
Exposing The Screen To Make The Stencil
Now that the art is taped to the screen in the appropriate place, we are ready to place it in the exposure unit and expose the emulsion to UV light. This is the step in which we cure the emulsion with light. Remember that your film positive will be near 100% black for the areas that will print ink. The black areas on the film positive that make up your design will block the light from hitting and exposing the emulsion. Those unexposed areas of your design will remain water soluble while the rest of the emulsion on the screen, being exposed to light, will be cured. Once cured, it will no longer be water soluble. This means that the emulsion that was blocked from the light by your film positive will dissolve and wash out when rinsed with water. The areas of emulsion that did get exposed to UV light will harden and will not washout of the mesh.
Simply place the screen in the exposure unit with the substrate side down, (with film attached), coming in contact with the glass. Close the rubber blanket top and turn on the vacuum. Allow the machine to finish the vacuum cycle to ensure the film is sandwiched between the glass and screen tightly. Now set the timer to the pre-determined setting and expose.
VIDEO: Screen Printing T-Shirts: What is The Exposure Time For Screens?
Your exposure time will depend on a few factors. The exposure unit or light source, the emulsion, and the mesh being used. Your film positive may also affect what exposure time you will need to use. The vendor of the emulsion should be able to provide a starting point for you based on your criteria. But let us rundown the variables.
Exposure unit - How much UV light does your exposure unit output? Industrial black lamp exposure units put out about 300 nm UV light and an LED will put out about 400 nm UV light. 500-watt work lamps have much less UV light and will require longer exposure times.
The emulsion - Emulsions have varying speeds meaning just like old fashioned film back in the day, some emulsions expose faster than others. A photopolymer will expose much faster that a dual cure emulsion and some photo-polymers are faster than others.
Your film positive - The denser and near 100% black your film is, the easier time you will have making screens. If the film positive is not opaque and allows some light to pass through, you will need to under expose the emulsion to make it work.
The mesh count - This is really related to how much emulsion the mesh count holds and not necessarily whether the mesh is dyed yellow or not. Lower mesh counts naturally hold more emulsion and generally take more time to expose. Higher mesh counts hold less emulsion and tend to expose fully at shorter times and since you are likely to be doing higher detail on a high mesh count, that is good.
There are many different emulsions available on the market today. I work with a pre-sensitized emulsion that has a very good shelf life. It is made by Saati and called Textil PC Blue. It is the “house dressing” here at Catspit Productions and if you hear me mention any specifics in videos, that is the emulsion I am referring to. I sell it on the ecommerce site of course. Other emulsions must be mixed with a sensitizer or catalyst and may not have such a good shelf life. You may need to do research on the emulsion that will be appropriate for your application. A standard pre-sensitized textile emulsion will hold up to solvents and plastisol inks very well. Remember, printing tee shirts is not rocket science. Most any high-quality textile photo emulsion should suffice.
VIDEO: Setting Up & Exposing 2 Color Screens
When the timer stops and the light has gone off, turn off the vacuum and remove the screen from the exposure unit. Take off all the film and tape.
Exposure units come in various sizes and this will limit your ability to do oversized work if you buy a very small unit.
For instance, if your exposure unit is only big enough to expose 20 X 24 frames maximum outer dimensions then you are limited to that size or smaller. If you want to take on printing signs, you may not be able to produce the size screens needed. Your exposure unit is one piece of equipment that you should purchase with your business goals in mind. Some exposure units can expose 23x31 or bigger screens and smaller.
VIDEO: Exposing Yellow Or Dyed Screen Mesh
VIDEO: Exposing 1 Color Halftone Screens With Spot Colors
VIDEO: Exposing & Washing Out 3 Screens
VIDEO: Spot Color Vs Halftone Screen Burns
Using The Sun As An Exposure Source
I do not recommend the Sun as an exposure source for exposing UV sensitive emulsions. The reason is that we ideally need 300 to 400 nano meters UV wavelength to properly expose screen printing emulsions.
Remember, I am not a manufacturer. I am only a screen printer turned supplier. I am very different than most suppliers in that I screen printed for most of my life. I am not selling equipment here. I am simply trying to teach the best and easiest way to make screens. The question here is twofold. First not only does the Sun vary in UV output throughout the year day by day but second, it is also a poor source of the UV light that we need to expose photo emulsions. At best I believe the Sun can output about 10 to 180 nano meters wavelength UV light. Emulsion is only responsive to UV light. It is blind to all other parts of the spectrum.
Industrial black lamp exposure units put out about 300 nm UV light and an LED like mine will put out about 400 nm UV light and the quality is such that it penetrates the emulsion more thoroughly. The closer you are to 400 + nm UV light, the better the burn and cure which results in a much easier time making screens with much higher levels of detail.
When time is money and you need detail, an exposure unit is the way to go. Now some sign printers use the Sun to make very large screens. But it takes great skill in understanding the exposure time for any given time of day and time of the year. Not to mention possible weather delays. While you can make screens with the sun, it is better suited for hobby and limited printing situations.
Unless you make signs in Phoenix Arizona or similar climactic regions and have some serious experience, I would recommend purchasing a UV exposure unit to make your life more flexible and easier. I can burn a screen with an exposure unit and wash it out in about 5 minutes or so anytime of the day or night no matter the weather or the time of year.
VIDEO: How To Screen Print: Can The Sun Be Used To Expose Screens?