Every screen printer wants to create durable stencils that don’t degrade over the course of a print run. Stencils that soften or break down during long press runs or when used with water-based inks can slow production and lead to misprints and waste. To combat this, many screen printers turn to post-exposure. That is, exposing screens coated in photosensitive emulsion to more UV light after they have been exposed and washed out. But does post-exposure really increase stencil durability? How can you take advantage of post-exposure to create more durable screen printing stencils?
The value of post-exposure depends on your emulsion
Post exposure can, in fact, create more durable screen printing stencils that will better withstand long press runs or prolonged contact with water-based inks. When you expose your screens, the UV light emitted by your exposure unit causes a reaction within the emulsion that hardens, or cures, your stencil. Depending on the type of emulsion you use, exposing your stencil to more UV light after the screen has been washed out can cause more of the photoreactive chemicals to link, strengthening their bonds and strengthening the stencil.
In general, post-exposure is most valuable for photopolymer emulsions. That’s because even when a photopolymer is exposed adequately enough for use, not all of the polymers have bonded. When exposed to more UV light, the photopolymers will continue to react and bond for a stronger stencil. Diazo emulsion, on the other hand, fully bonds during exposure. If the emulsion has been properly cured, the emulsion won’t continue to react and strengthen with additional exposure. The same is the case for dual-cure emulsions.
How to strengthen stencils with post-exposure
Post-exposure is a very simple process. The screen should be exposed, washed out and allowed to fully dry as normal. For quick post-exposure, return the stencil to the exposure unit for a minute or two, depending on the thickness of your layer of emulsion. It’s wise to have the ink side of the screen facing the light source, as that side of the emulsion is more likely to wear down and more likely to have been under exposed in the first place. A simpler, but slower, way to post-exposure your stencils is to place them outdoors on a sunny day, or to place them next to a window in bright, direct sunlight.
Post-exposure won’t replace poor exposure!
While post-exposure can make more durable stencils if you are using a photopolymer emulsion, here is one word of caution: Post-exposure won’t cure an inadequately cured stencil. If your screen wasn’t fully cured to begin with, you run the risk of losing detail, having blurred or jagged edges, or having your stencil break down as you print. If your stencils are breaking down prematurely, having washout issues or problems maintaining detail, you should be examining your exposure unit and your exposure times rather than turning to post-exposure to fix the problem. Before you experiment with post-exposure, it can be valuable to test your exposure unit with your emulsion to determine your optimal exposure time for a full cure.
Post-exposure can be a valuable tool in your screen printing toolbox. Post-exposure can strengthen your stencils so they’ll stand up during long press runs or when you are screen printing with water-based inks. If you are confident that your screens are fully cured but you would like to see added strength from your photopolymer emulsion, try experimenting with post-exposure to see if it improves the quality of your stencils and the quality of your prints.
For more tips on properly exposing your screens, check out these blogs: