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Photo Emulsion and Exposure Troubleshooting Tips.

Posted by Rory Atton at

We've all been there. We think our screen looks great only to find that we have a bunch of problems when it comes to exposing them. 

A few of the problems can have the same causes so if you can sort those you are on to a win. 

With the super high quality screen printing products available it is rarely the products that are faulty, its almost always in the process. 

Here are some common issues, the cause's and remedies. Whilst this can all sound quite negative, you can expose your own screens easily and we're here to help you make it happen!

 

Emulsion washing off or breaking down in wash out. 

Not just the bits you want to come out but all of it or more than the exposed part. 

  • Under exposure.  Check exposure time using a Step Test
  • Water pressure is too high during wash out. Don't blast the emulsion out too quickly, turn down the pressure and keep the water flow or jet moving about the screen. Give the screen a little soak for 30 secs before starting the wash out. 
  • Screen not properly coated. Too thick or too thin a coating of emulsion causing thick or thin spots that have under or over exposed leading to washout problems (which you are then tempted to blast out.) For a nice smooth even coat make sure your scoop coater is in good condition. Print-Lab Pro Tip: use a sponge scouring pad to give your scoop coater a once over leaving a nice clean edge.
  • Screen prep. Make sure to clean up your screen properly using Mesh Prep / Degreaser. Dirty or contaminated screens could add to emulsion problems. Be sure to fully rinse it off though. Coat the screen as soon as it if fully dry. This prevents dust and dirt settling back on your nice clean screen. If it is still a little damp the emulsion won't stick properly.
  • Out of date or contaminated emulsion. Print-Lab Pro Tip: Check the manufacturers guidelines, then write a started by and use by date on your emulsion tub. We recommend you don't scrape unused emulsion back into the tub. It can get contaminated with dirt and dust and just helps toward deterioration of the product. 
  • Emulsion not fully dry before exposure. Resist the temptation to get your screen done asap. It might feel dry but may well not be. 

Pin Holes and Air Bubbles in exposed screen.

  • Dirt and dust on the screen during coating. Clean and prep screen properly. Once dry, coat before the dust can settle back on it. 
  • Air bubbles in emulsion. Let the emulsion settle for a few hours after mixing.
  • Dirt or dust on transparency or glass during exposure. Keep your exposure area clean and dust free. 
  • Pin holes are hard to get rid of completely no matter how careful you are. If you do find some and the other tips don't work you can use some screen filler to block them, or like most people just use some screen tape to cover them. Place it on the print side. We use this tape.

 Premature stencil breakdown

  • Under exposure. It might have looked good and somehow washed out ok to start with, but an under exposed screen will just not be as tough. Use our guide to find your exposure time with a Step Test
  • Emulsion coating might be too thin. See the tips on coating and exposing your screen. 
  • Aggressive inks and aggressive cleaning.  Most waterbased inks still contain some solvents which can contribute to premature screen breakdown. Especially if ink is left to dry on the screen. Good screen management helps. Clean gently. Sometimes ink hazing is confused with being stuck in the screen. A little hazing isn't really a problem, just don't over scrub it. 
  • Mesh Tension. If your mesh is not tight it might ripple excessively when printing leading to early breakdown. 

Difficult washout. 

  • Over exposure. Too much light "cooking" the emulsion. This will make it hard to wash out. This can lead to the temptation of blasting the screen with a higher water pressure. All this will do is break down the screen or lead to a sawtooth or jagged edge on your stencil and loosing detail.
  • Possible fogged screen. Ensure correct storage of coated screen. Keep in a light proof area. 
  • Poor transparency to screen contact during exposure. Tape transparency to the screen or use extra weight to ensure contact.  
  • Transparency print out is not opaque enough. Read our guide on art work preparation for transparencies
  • Old Screens. Screens that have been left coated for long periods of time will be harder to work with. The chance of fogging and fluctuations in storage conditions can lead to exposure problems.

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