Grafica News - June 2016
Maximizing Profits with Screen Reclaiming - Part 2
Dawn M. Hohl-Nowlin
Member, Academy of Screen
& Digital Printing Technology
(ASDPT) & Technical Trainer
and Consultant Uncommon
Enterprise LLC, USA
Reclaiming a screen is both an end-and-beginning stage of the screen-printing process, and as such is an essential part of the cycle. Simply stated, what happens in pre-press directly affects what happens on press. While there is more than one way to effectively reclaim a screen, there are procedures and tips to make it as efficient, cost effective and safe as possible. Screen reclaiming can be broken down into three stages; ink removal, stencil removal and ghost/haze removal.
One overlooked aspect of reclamation is the press wash used to open up/clean the screen during printing (Figure 1).
Petroleumbased 'hot' solvents that evaporate quickly such as acetone, mineral spirits, xylene and lacquer thinner can actually lock up emulsion/inks. The difficult stains that result require caustic haze removers which weaken the mesh. Safer, non-oily, fast-evaporating and low VOC press wash products specially designed for screen printing inks are a much better alternative. Press washes are also referred to as ink removers, screen openers, or on-press cleaners, and are applied using a rag.
After the screen comes off
press, the first important step
in reclaiming is carding off as
much residual ink as possible.
Leaving too much ink behind
will require more chemicals
and labor during reclaiming.
For dip tanks this step is
essential to minimize ink
build up in the tank and
extend the life of the
Once the bulk of the ink is scraped off, the remaining ink must be removed with an ink wash/remover, also known as screen wash or ink degradant. These products are not petroleum solvents, but rather safer water soluble chemicals containing biodegradable emulsifiers to dissolve or suspend all components of the ink (Figure 2). They are drain safe and have higher flash points than solvents, and therefore safer to store and handle.
Water soluble ink washes minimize the need for rags and do not present an oily barrier to hinder subsequent reclaiming steps. While the ink wash can go down the drain the ink it carries often cannot. Filtration systems should be in place to remove the ink residue as well as emulsion solids from entering the water system.
Ink wash is applied to both sides of the dry screen and brushed in well. The screen is then rinsed on both sides, first with low pressure (to minimize chemical/ink blow back on technician) and finally with a pressure washer to remove as much ink residue as possible. Ink wash application can be done with a spray bottle, bulk product sprayer or brush. Chemical recirculation systems are cost effective at this stage if many screens are processed every day. Be sure to replace the chemistry at appropriate intervals so ink isn't re-deposited back on the screen.
To minimize ink stains, select a press wash and ink wash chemistry compatible with the ink type being used. Comparative testing can determine which product produces the best results and is well worth the time. Spending a little extra money on the best reclaiming chemistries can provide benefits such as using less chemicals, shortening the reclaiming process, and reducing the use of haze removers.
Ideally, screens should be cleaned immediately after they come off press. The longer the ink sets in the screen the greater the stain. If it is not possible to remove the ink right away, pre-treat the screen with an ink degradant and keep screens out of direct sunlight.
Once the ink has been completely removed, the stencil must be removed. The active ingredient in traditional emulsion removers is sodium metaperiodate, which comes ready to use or in a concentrate/powder (Figure 3). While ready to use versions offer convenience, you end up buying and shipping water. Concentrates can easily be mixed with water on site, but must be mixed in the proper ratio as indicated by the manufacturer. Note, higher mixing ratios are not better and will not make the emulsion remover faster or more effective.
To remove the stencil, first wet both sides of the screen (the water actually helps the process), then apply stencil remover to both sides of the screen with a spray bottle or brush. Allow the chemistry to sit on the screen for 2 minutes to let it work, then use a dedicated reclaiming brush to rub both sides of the screen (Figure 4). A low pressure rinse can be used prior to high pressure washout to minimize blow back on the operator. High pressure BOTH side of the screen to remove all emulsion and blockout. DO NOT let emulsion remover dry on the screen as it will lock up the stencil making it very difficult to remove.
If stencil removal proves difficult the most likely cause is stencil underexposed. Underexposed emulsion is not completely cross linked and blocks the action of the stencil remover chemistry. Determine the correct exposure with and exposure calculator as described in the June 2014 issue of Grafica News, and examine exposure variables as discussed in Grafica News May, August & September 2015 articles.
A final note, there are also combination or one step products on the market which break up the ink and stencil at the same time, and are rinsed away with high pressure water. They are typically used with dip tanks and automatic reclaiming systems.