Read on to learn what two industry suppliers have to say about thepros and cons of dip-tank use.
By Gary M. Valasek, ICC Chemicals
Dip tanks sound like an easy way to clean screens. You just soak screens in a tank of chemicals, then rinse the remaining ink and emulsion away. In practice, however, dip tanks don't work as well as advertised, and they present some significant problems.
Dip tanks are ineffective when it comes to tasks such as removing and degrading inks from screens.
Using a dip tank involves filling a polyethylene or stainless-steel tank with a mixture of chemical concentrate and water. A typical ratio is 1:3 (for example, five gallons of chemical concentrate to 15 gallons of water). After a three- to five-minute soak, the screens are subjected to a high-pressure rinse in a conventional washout booth. Some dip-tank suppliers recommend the use of a separate ink degradent at this point, followed by another rinse. In many cases, application of caustic haze remover is then required to re-move stains.
Most dip-tank suppliers claim the process removes ink, emulsion, block-out, and adhesive-tape residue in one step. But ink and emulsion, the two main substances being removed, have fundamentally different chemistries and, therefore, very different removal needs. Although dip tanks will soften emulsion, like a bath in plain water, they are ineffective at chemically affecting inks. Blasting off the softened emulsion with a high-pressure spray carries ink off and deposits it all over the washout booth and down the drain. But the sticky ink, especially plastisol, is chemically unchanged--the ink's adhesive qualities remain. The ink can cling to pipes, traps, and filters, then reform into clumps and clog the drain system. When this ink builds up, it is nearly impossible to remove with drain-cleaning equipment. Taking up concrete floors to replace plumbing is not a positive experience, especially in an active business.
Dip tanks can't properly treat the ink because the chemistries of ink degradents and emulsion removers are incompatible. Ink degradents are solvents with additives, and emulsion removers are oxidizers. Because of these chemical differences, it is impossible to combine the two and maintain their effectiveness. When combined into a single solution, the two chemistries counteract each other and can't accomplish their separate tasks. The oxidizing action of the emulsion remover attacks the ink degradent, significantly reducing the effectiveness of both. As a result, very little ink degradation actually occurs in dip tanks. In fact, some dip-tank suppliers actually recommend the use of a separate ink degradent, negating the idea of a one- step approach altogether.
Poor ink treatment is one reason dip-tank suppliers stress the necessity of thoroughly carding ink off every screen prior to soaking. In practice, printers rarely card ink thoroughly before reclamation. Excess ink causes several problems. Ink left on the screen prevents the emulsion remover from contacting the emulsion. Because much of the ink is left on the well side of the screen, the emulsion is softened only from one side, making it difficult to effectively remove. And most of the excess ink ends up being pressure-rinsed into the drain system in an untreated state. From an environmental perspective, any problem in the drains results in a problem downstream as well. Regulatory officials in water treatment are becoming more aggressive in seeking out and fining violators.
Dip tanks use more chemicals, which increases operating costs.
When screens are removed from the dip tank, so are chemicals. Each screen carries out two to four ounces of chemical solution. The chemicals carried out are wasted and must be replenished, and evaporation from the tank adds to the loss. Total chemical usage can triple or quadruple with a dip tank, compared to light-spray techniques.
It is difficult to maintain effective chemical concentrations in dip tanks.
Although the contents of a dip tank may start out in recommended concentrations, treating the first screen begins to alter that ratio. The significant amount of carry out when screens are removed necessitates topping off the dip tank frequently. Chemical concentrate and/or plain water is added, usually with little regard to recommended ratios.
Chemical interaction with the emulsion on the screens--and the emulsion accumulating on the bottom of the dip tank--also changes the effective concentration over time. If the dip tank's chemical solution contains any ink degradent, its ingredients are af-fected by the oxidizing action of the emulsion-remover component, further decreasing effectiveness. Often, more and more chemical concentrate must be added to compensate.
Hollow-metal frames can retain chemicals after soaking and ruin subsequent stencils.
Although most hollow-metal frames arrive from the factory with gaskets in place, the frames lose their watertight qualities with use. Chemical solution can infiltrate the frames during the soaking process. The frames may appear dry on the surface, but when subjected to a vacuum during exposure, emulsion-removing chemicals are sucked out of the frame tubes and onto the new stencil. This results in a waste of effort and expensive emulsion, as well as production delays while the exposure unit is cleaned and the screen is remade.
Sludge accumulating in the bottom of dip tanks presents a serious disposal problem.
As screens are soaked in the dip tank, some of the emulsion sloughs off, carrying with it relatively untreated ink that accumulates on the bottom of the tank. After a while, this sludge must be removed. Putting the sludge down the drain is not recommended. The sludge is sticky and of high viscosity--an obvious drain clog waiting to happen.
To further complicate disposal, the pH of the sludge is likely to be in the 1-3 range. That's far too acidic to be permitted to enter most municipal sewer systems, where pH guidelines for effluents are typically in the 6-9 range. Federal hazardous-material regulations apply to materials of pH 2 and below.
Most environmental regulations prohibit dilution or chemical treatment of an effluent to achieve safe disposal levels, unless the dilution occurs as part of the cleaning process. Adding water to the sludge does not qualify as part of the cleaning process. Even if it were permitted, the amounts of water required would quickly become impractical. Raising the sludge from pH 2 to pH 6 could require 1000 or more gallons of water to every gallon of sludge.
The dip-tank user must figure out what to do with the sludge if it can't be rinsed down the drain without plumbing risks or violating environmental regulations. The sludge can't go into landfills or dumpsters in most jurisdictions because it poses environmental problems, so the only remaining option is to pay a hazardous-material hauler to dispose of the waste. The risks of noncompliance include heavy environmental fines or cease-and-desist orders.
Dip tanks are a relatively ineffective method of ink and emulsion removal. While the approach promises easy reclamation, the process is, in fact, less efficient and less productive than proven screen-cleaning methods. Furthermore, the costs of replenishing chemicals and remaking stencils damaged by chemicals leaking from frames add up in both money and labor. In addition, it can be quite expensive to repair a plumbing system blocked by improperly degraded inks and pay to dispose of spent chemicals and residual sludge in an approved method. These major operational drawbacks clearly indicate that dip tanks are not the right solution for every screen-printing shop.
About the author
Gary M. Valasek is chief chemist with ICC Chemicals, a manufacturer of cleaning, reclaiming, and specialty chemicals for screen printing. Valasek has been with ICC Chemicals for more than 20 years, working in new-product development and technical support. A graduate of Marietta College, he holds a master's degree in chemistry from the University of Cincinnati.
By David Landesman, Lawson Screen Products
The dip tank is an effective tool. When complemented by the proper chemical solution in the tank, a proper workflow in the screen shop, and correctly selected ink and stencil systems, the dip tank can help users realize efficiencies that are otherwise difficult to attain. Dip tanks are useful not only for reclaiming purposes, but also in supporting the stencil-development process. This, in fact, is one of the best ways to use a dip tank because it makes stencil development after exposure much easier and much more consistent--especially for the novice screen printer. Let's explore some of the other benefits of working with a dip tank.
Dip tanks are useful.
Most people who are against using the dip tank as a one-step solution tend to think that is the product's main purpose. But other applications--aiding in stencil reclamation and developing--are truly helpful uses. The latter two functions work under almost all conditions. It's the one-step, or all-in-one, capacity that has issues, and these issues usually have to do with the chemical products. Some are better than others. It also has to do with a shop's internal processes--how thick the stencils are, how much ink they scrape out, and how old the screens are. Some people buy a dip tank, gather up all their screens that have been in storage for two years, and try to reclaim them. Then they say it doesn't work. Well, yeah! I'm not surprised! They should take that two- or three-year-old screen and try to reclaim it by hand and then tell me how easy it is.
Some stencil materials are just easier to reclaim than others. But if you take a water-resistant emulsion and try to use the same process you use for solvent-resistant stencils, it's much more difficult--partly because it's a water-resistant emulsion to begin with! Experimentation is key, but expect a decline in effectiveness if you attempt to treat stencils built up with heavy capillary films or thick-film (high-density) emulsions.
Printers also need to scrape out their inks before they use the dip tank for a one-step process. To those who don't do a good job of carding out their ink from the screen, I say dream on. It's not going to work because the one-step chemistry involves a mild ink degradent. If you use a really strong ink degradent, it competes with the stencil-remover solution.
Dip tanks are not complicated.
Dip tanks are probably the simplest pieces of hardware in any screen-printing shop. Most are polypropylene tanks on stands with a spigot to vacate the materials they contain. If you have a properly designed unit, it won't leak. Nothing will stick to the sides. They're maintenance-free items, and they're relatively inexpensive. And purchasing a dip tank that can accommodate several screens at once saves time.
There are plenty of economical benefits associated with using dip tanks.
Let's talk about stencil reclamation. The traditional way to reclaim a stencil is to brush on your stencil-removing solution and let it set. Well, the reality is that the stencil-removing solution runs down the screen and goes down your sink when you blast it out. When you use a dip tank, the chemicals are used again and again. And yes, you do have to change the solution periodically, but it is less expensive when you use a dip tank than when you simply pour a che-mical solution on a screen--or brush it on a screen--and send it straight down the drain.
When you use a one-step or all-in-one product, you get many uses out of it. It varies from shop to shop, but generally speaking, you can treat between 200-400 screens with a one-step cleaning solution before you have to change the chemicals. As far as stencil reclamation, the general rule of thumb is between 150-300 screens. And a lot of times, you just have to reactivate the stencil-removing solution, so you add in some concentrated solution or you add in more crystals to beef it up. You don't have to drain the whole thing--you just spice it up a little bit, like warming up your coffee. The frequency in which a shop should change out the chemicals depends on its typical throughput.
Dip tanks pose little to no risk to well-made frames.
Most retensionable and aluminum frames are welded well enough so water won't get in them. Roller-frame manufacturers, of which there are several, all claim that their rollers are watertight. However, if a roller frame has a leak, then you'll hear the water sloshing around in it, which can be a problem. Water and chemicals also may get into the frame during other processes, like when you take it to a rinse sink and rinse out all of the ink, apply your stencil-removing solutions, and then blast it out. If you use untreated wooden frames and allow them to sit in dip tanks for long periods of time, that could promote warping.
Dip tanks don't have much of an effect on safety and waste disposal.
Local regulations related to waste disposal apply whether you use a dip tank or not. If you're not supposed to send anything down your drain, a dip tank could help you because then you could drain the tank into a drum and have the drum hauled off. This method of disposal ties in with the economical benefits of using a dip tank that I described earlier.
The reality is the types of chemicals we're talking about are, generally speaking, drain-safe in most parts of the country. In fact, I've never heard of stencil removers and emulsions not being allowed to go down a drain. And if you've carded away excess ink, the degradent should successfully break down any residue.
Using a dip tank to remove the ink and stencil from a screen is more controversial than putting the unit to work in stencil reclamation and developing because it's much more dependent on the ink and stencil systems in use, as well as what types of press washes a shop uses. That's why some people may think that dip tanks don't work. Well, in some cases, guess what? They don't work well! But in many other cases, they do because the processes used in that individual shop lend themselves to a one-step type of product.
Many people only think of the one-step process, whereas the biggest marketplace for the dip tanks is really stencil reclamation and developing. The one-step process for the dip tank is another application, not the primary. And that's what people forget about. To entirely banish the concept of using dip tanks is just wrong. Dip tanks aren't meant for everyone. However, they are useful and helpful in most shops.
About the author
David Landesman is co-president of Lawson Screen Products, a manufacturer of screenmaking, screen-cleaning/reclaiming, and screen-printing equipment. He holds several patents related to industrial screen-printing machinery, has authored numerous articles relating to screen printing, and has given presentations about the process at various technical conferences. Landesman holds a master's degree in education from the University of Massachusetts.
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