CAN I USE BLEACH TO RECLAIM MY MESH?
I've discussed your question about using bleach as stencil remover with our senior chemist. He says that bleach is both slower than SMP and environmentally harsher.
Controlling the dilution of bleach as it contacts the stencil during decoating is difficult. If the concentration is too high, there's danger of a lock-in from an oxidation-reduction reaction; too low a concentration, and there's no effect.
The optimal concentration varies for each emulsion blend. Certain types of emulsion, such as 925WR, are hard to reclaim with bleach of any concentration.
So there it is: bleach is slower, less green and, most importantly, considerably less reliable than SMP.
The most common reclaim chemical in the screen industry is sodium metaperiodate. This is a salt that is related to household chlorine bleach, but without the harmfull characteristics associated with bleach.
Typical emulsion removers contain sodium metaperiodate or salts of periodic acid.
Problems during decoating or reclaiming of the screen occur when the screen was underexposed or if the emulsion is extremely water resistant. Solvents may cause underexposed emulsion to harden and "lock in", resulting in difficult reclaiming and excessive emulsion haze. The higher the water resistance often emulsion the more difficult it is to reclaim. Emulsion that is only solvent resistant should reclaim easy. Never let the reclaimer dry on the screen.
Why am I having trouble reclaiming? The old emulsion won’t come off the screen. Many things can cause reclaiming trouble. The most common cause, however, is underexposure of the stencil and/or under cured emulsion. If the emulsion is not completely dry before exposure, the soft, wet emulsion remains inside the dried outside surface like a sandwich. This uncured emulsion will react with inks, solvents and other chemicals during the printing process and chemically lock onto the screen. This will make it very hard to reclaim the screen later. Other factors that promote easy reclaiming are proper exposure and cleaning the ink completely and then removing the ink wash with degreaser. Lead with - The basics of cross linking
Reclaiming The reclaiming problems are one of the biggest culprits of improperly exposed screens. If a screen is underexposed the subsequent stencil will have good detail but will be soft or slimy. This soft stencil will not reclaim as easily because any solvents in the ink mix with the soft emulsion and stain the mesh. If a screen should need to be underexposed for some reason always be sure to post expose the screen after it is washed out and dried. While this can be accomplished by placing the screen back on the exposure unit and exposing without the film positive, it is often just as easy to place the screen in the direct sun-light for a few minutes. Locked by chemical which is NOT breakable
Difficult reclaiming if Under exposed
Mixing stronger is not necessairly better.
Only a minor amount of the Sodium Metaperiodate in reclaimers is needed to breakdown the stencil. The large amount of water is what allows the chemicals in reclaimer to penetrate the emulsion coating so they can do their job.
Stencil remover has little effect on 'underexposed' stencils. The sodium metaperiodate in stencil remover attacks the cross links that hold the stencil in the mesh. If you underexposed, there aren't lots of links to break, and you have to use brute force to punch the stencil from the mesh, instead of the smooth breakdown of the stencil like turning a key in a lock.
So fully harden your stencil, mix reclaimer properly and apply as directed, and absolutely use the power washer to blast it off after the reclaimer breaks it down.
The longer a stencil remains in a screen the harder the emulsion is likely to become. Get in the habit of reclaiming your screens as soon as you are finished with them. Emulsions can also become excessively hardened through contact with certain strong solvents. If your exposures are bang on and you’re still running into reclaiming problems, try switching to a different wash-up or screen-opener.
Sometimes stubborn bits of stencil hang on despite your best efforts. Try reapplying the stencil remover or dehazer to that spot. Wait a few minutes and hit it with your pressure washer. Adjust the nozzle to deliver a more finely focused spray. Remember that as the water jet becomes narrower its power increases. Don’t let it stay aimed at one small area. Keep it moving.
Although, we've talked mostly about direct emulsions, the reclaiming process for films is similar. In fact, films can be easier to reclaim. Just remember that indirect films are gelatin-based and require the use of special stencil removers.