ANILOX CLEANING SYSTEMS: PRESERVING ROLL LIFE

Transcription

1 ANILOX ROLLS ANILOX CLEANING SYSTEMS: PRESERVING ROLL LIFE By Stan Seelig Proper cleaning is critical in eve ry segment of the printing industry, but nowhere is effe c t i ve cleaning more i m p o r tant than when it comes to a n i l ox rolls. Improper cleaning of a n i l oxes can lead to poor print quality, plugging and eventually the need to r e s u r face the rolls. Taking the time to d e fine cleaning procedures and implement them, can lead to better quality p rinting and longer lasting rolls. And yet, many printers are still cleaning their va l u a ble anilox rolls with floor c l e a n e r s, oven cleaners or even hards u r face cleansers. While most existing cleaning systems wo rk when used properl y, the p rinter must take into considera t i o n (a) wo rker safety and env i r o n m e n t a l e f fects; (b) the ove rall economics (costs) relating to the implementation of a cleaning system and (c) whether the cleaning process does a thorough job while minimizing anilox damage. O ver the course of the past two d e c a d e s, the United States E nvironmental Protection Agency ( U S E PA) and the United Nations Environmental Program (UNEP) have been eliminating solvents and VO C s ( volatile organic compounds) from major industrial applications because of the potential harm to the o zone laye r, contri bution to global wa rming and/or smog potential of these emissions. Using energy intensive cleaning systems can also generate carbon d i ox i d e, which may contri bute to global warming, as well as increasing costs. The use of water-based cleaners maximizes chemical energies and minimizes those energy intensive steps. This paper will discuss current cleaning methods as well as a new type of cleaning system that (1) provides protection to the anilox roll; (2) decreases the use of corrosive chemicals and (3) minimize s the energy requirements for achieving a high level of cleanliness. How Clean is Clean? The age-old question asked by all i n d u s t ries is How clean is clean? The answer in too many cases is,

2 If it looks clean, it is clean. In the case of anilox rolls used in flex o- graphic printing, the removal of ink from the surface is not complete just because the color (or pigment) has disappeared. Ink components such as acry l i c s, surfa c t a n t s, binders, catalysts and various other additives are transparent to the naked eye and remain on the anilox roll after cleaning. Any ink contamination left on the roll becomes further embedded into the cells of the anilox, making subsequent cleaning eve n more difficult. S u rp ri s i n g l y, most anilox rolls require resurfacing due to plugged cells rather than wo rn or damaged s u r fa c e s. This is usually a result of poor cleaning and maintenance. Paying attention to cleaning will prolong the life of anilox rolls and therefore, prevent unnecessary expenses related to resurfacing. The actual answer to this question is that Cleanliness is in the eye of the beholder. What this means is that the printer must define his or her own cleanliness standards. This can be related to visual microscopic t e c h n i q u e s, but must be related to p e r fo rmance cri t e ria. If the pri n t e r plans on resurfacing anilox rolls eve ry so often, cleaning becomes less important. Where print quality and the additional cost of resurfa c- ing are important, then good cleaning is critical. S u m m a ry of Cleaning Methods to Date In the course of rev i ewing va ri o u s p a p e r s, perusing list servers and discussion groups on the Internet, and meeting with hundreds of customers, it has become clear to this author that most people are cleaning their a n i l ox rolls using blasting, ultra s o n i c s and/or chemical cleaners. A summary of these techniques is shown in Ta ble 1.

3 ANILOX ROLLS Ta ble 1. Common Cleaning Methods Cleaning Method A dva n t age s D i s a dva n t age s C o m m e n t s P hy s i c a l Baking Soda Blast Cleans cells well May damage cell walls Requires enclosed blast area; where LPI>600 may leave deposits behind Plastic Bead Blast Safe and effective; reusable May damage cell walls Requires enclosed blast area where LPI>600 Dry Ice Blast Removes dried ink; disappears May crack ceramic None known at this time High Water Pressure Should knock out ink residues May push ink residues in deeper Requires enclosed area and highly alkaline cleaner Ultrasonic Systems Newer units clean better ; LPI<800 May cause stress cracks; Requires chemical cleaner, lead to corrosion usually highly alkaline C h e m i c a l * Highly Alkaline (ph=12-14) Effective cleaners, in general Very corrosive to aluminum; Many products available; other metals will cause severe burns Mildly Alkaline (ph=10-12) Generally weak cleaners Removes pigments but May give a false sense leaves ink residues of cleaning Very Acidic (ph=1-3) Effective cleaners Unsafe to work with; may etch cell No known products * Chemical methods can clean better by combining with heat, ultrasonics, and/or high water pressure. The anilox rolls could also become damaged at an accelerated pace. Blasting - This cleaning method can be done either with baking soda, plastic beads, dry ice or other media. Blasting produces good results cleaning is not usually a probl e m. The abra s i veness of the materi a l s digs out the embedded ink from the pores of the anilox roll cells. This method tends to be safe, as the most common blast agents are non-tox i c and easy to handle. Blasting can damage cells, part i c u l a rly with cell counts greater than 600 lpi, and promote roll we a r. Ultrasonics - U l t rasonic cleaning is a ve ry effe c t i ve method for cleaning, and is used in many industri e s. Be cautioned the units must be tuned to the correct frequency fo r cleaning. (Using an old ultra s o n i c unit is not recommended!) Most p rinters who use ultrasonics are also using strongly corrosive solutions to m a x i m i ze their cleaning efficiency while minimizing their time. The ultrasonics themselves can cause stress c ra cks in roll coatings. Chemical Cleaners - Most cleaning systems used to fall under this c a t e g o ry, ranging from oven cleaner to va rious floor cleaners. In my ow n ex p e riences in the industry, it has been interesting, though distressing, to see how many cleaning fo rmu l a- tions have been taken from other i n d u s t rial and non-industrial applications and used on ex p e n s i ve anilox rolls for precision cleaning. Many wo rk, but often the operators are p o o rly trained or ill-equipped to deal with the caustic chemicals which t h ey have little knowledge of. Many p rinters reported caustic alkali bu rn s from operators that used these c l e a n e r s. Furthermore, many cleaners arri ve concentrated, requiri n g water for dilution. Many opera t o r s p e r c e i ved that the less diluted the c l e a n e r, the better it will wo rk. Another misconception is that hardwater environments can affect good cleaning. Of course, wo rke r s i m p r o p e rly trained in chemicals are i nviting trouble diluting these strong alkaline solutions. I n t e r e s t i n g l y, anilox roll manu fa c- turers are cautioning against many of the existing cleaning methods. These include blasting media due to the abra s i veness against the cell wa l l s, ultrasonics for stress cra ck s fo rmations and highly alkaline solutions for corrosive n e s s. Although any of these methods can be used to clean anilox rolls, great care must be exercised to prevent damage to the a n i l oxe s. Many times the operator in charge of cleaning does not have the proper training required.

4 Unique Chemical Cleaning Method P rinters that use anilox rolls fo r f l ex o gra p hy include, among others, l a b e l e r s, corrugated box printers and p a ck a g e r s. Anilox rolls can ra n g e from 6 inches to more than 10 feet in length. Inks include water-, solve n t -, s oybean- and UV-based inks. The rolls are usually ceramic or chrome but the base metals may va ry if lightweight aniloxes are desired. All these factors influence the method of cleaning to be used. Traditional methods of cleaning a n i l ox rolls have all invo l ved removing ink from the cells of the roll by force whether physical fo r c e, sonic force or chemical fo r c e. An altern a t e approach to cleaning anilox rolls of ink without risking damage invo l ves a measure of preve n t a t i ve maintenance before the ink is applied to the roll. This method consists of applying a thin coating to the roll surface protecting the ceramic (or chrome) surface from dried-on ink deposits. Using this preve n t a t i ve medicine approach, a clean surface remains clean rather than cleaning a surfa c e that has increasingly more ink deposits left behind on the roll. At least one anilox roll manu fa c- turer has developed a method of applying a vacuum-deposited coating to protect the anilox surfa c e. This thin coating protects the roll surfa c e by filling the pores within the cells with a hydrophobic polymer that would help expel the ink from the roll. The roll would then be easier to clean as mentioned in the litera t u r e. The draw b a ck is that strong alkaline c h e m i c a l s, ultrasonics and bl a s t i n g would remove some or all of the coating. Even if a small amount of the coating is removed, printing quality deteri o rates quickly and all of the coating would have to be removed to r e e s t a blish quality pri n t i n g. S u b s e q u e n t l y, the special roll wo u l d become a regular anilox roll that costs ex t ra and suffers the usual cleaning probl e m s. A coating that could be reapplied by the customer would be better. But what if too much was applied? H ow would the printing quality be affected? How should the ink on top of the coating be removed without damaging the coating? Can a printer still clean plugged rolls without this coating? T h e o ry To understand the cleaning of a n i l ox rolls requires an understanding of the ceramic (and chrome is ve ry similar) surfa c e. Ceramics are porous materials that are strong and heat resistant. The porous surface is the key to poor cleaning. Ink bu i l d - u p (not the pigment found in the ink) occurs at the pores found inside the cells of an anilox roll. The continu o u s e n t rance and exit of ink into the cells produces a va rying pressure diffe r- ential, which means that the ink is being dri ven into the pores of the cell. Since printers cannot see what is happening at the surface of the a n i l ox roll, they may be unaware of this probl e m. Alkaline cleaners penetrate these cells and wo rk on removing the ink residues from the pores completely. The use of heat softens the ink residues and is helpful but it still doesn t completely remove the ink.

5 ANILOX ROLLS The ink residues get embedded further into the pores. When so many cleaning cycles occur, all the cell pores get filled and cleaners become less effe c t i ve until the aniloxes have to be resurfaced or cleaned in a different manner. Scru bbing action with the proper brush or rag also remove s some ink residues and pushes others further into the cell wa l l. U l t rasonics achieve better penet ration into the pores of the cell wa l l s and remove the ink residues, but if used ex c e s s i ve l y, can cause stress c ra cks under the ceramic surfa c e a l l owing for corrosion to occur (similar to the way a cavity in a tooth provides an entra n c eway for bacteri a ). Proper blasting methods also r e m ove ink residues from the pores and the cells but would tend to etch the cell wa l l s. Dusting and noise are additional issues the printer mu s t deal with. By applying a thin coating prior to p rinting, the ink assists in pushing the coating into the pores of the c e l l s, preventing the ink residues from getting in. Excess coating material is ex t racted by the ink but doesn't a f fect print quality as already eva l u a t- ed in many tests. This particular coating also lowe r s the surface tension of a typical anilox from approximately 40 dynes/cm. to 25 dynes/cm. This means ink will we t out from the cells faster and prov i d e better release when needed. The quality of print is unaffected regardless of whether water-, solvent-, soybean- or UV-based inks are used. The protective nature of this coating is why many anilox roll manu fa c t u r- ers are considering pretreating their a n i l oxes before leaving the manu fa c- t u ring site and instructing the pri n t e r s to reapply the coating as needed. Typical Cleaning Procedure Using the New Cleaning System When using this cleaning system, it is impera t i ve that the printer start with a new or thoroughly cleaned a n i l ox roll. Any ink residues wo u l d reside under the coating and not be r e m oved. This procedure also applies for narrow - web rolls bu t could be modified for wide rolls. S p ray the diluted coating (pretreatment) material onto the anilox roll in a unifo rm manner. This is best applied while rotating the roll; allowing excess material to drip off. Applying too much will not affe c t p rint quality but, applying too little will cause poor cleaning. Continu e rotating the roll until the surface is r e l a t i vely dry. Then use the anilox roll as usual. The next step invo l ves cleaning on-line; where the anilox roll is in the press and a change in color is required or the rolls have become plugged in the middle of a process run. Not all flex o graphic pri n t e r s h ave this capability and should just go on to the off-line cleaning step. At this point, the roll should be dra i n e d of ink and flushed with fresh, clean water for cleaning off most of the pigment. After draining the colored wa t e r, flush the roll with a non-corros i ve alkaline cleaner (ph<11) to r e m ove ink residues residing on top of the coating. The cleaner mu s t contain some of the coating chemical to prevent the removal of the coating. At this stage, someone can wipe d own the roll in a circular motion with a rag or proper brush. Drain and s ave the cleaner for future use. When the roll is clean, rinse it thoroughly again with fresh wa t e r. After d raining the wa t e r, refill the system with ink and start up the process line. If the anilox roll is removed from the press for off-line cleaning, then wash the roll first with fresh, clean water. Place the roll in a soak tank filled with a non-corrosive cleaner (ph<11) and allow the roll to rotate. The cleaner must contain some of the coating chemical to prevent the r e m oval of the coating. Ultra s o n i c agitation, water spray and/or heat (120 F) may be used to assist in cleaning. Use ultrasonics conservatively and as a last step to minimize damage to the anilox roll. Drain and s ave the cleaner for future use. When the roll is clean, rinse it thoroughly again with fresh wa t e r. Reapply the protective coating by s p raying it on the anilox roll. Place the anilox roll in storage or return it to the press. By completing multiple ru n s, the coating becomes reinforced and more unifo rm, making the cleaning easier with time. At this point, applying the coating could be done after eve ry other cycle. The Future of Cleaning in the Printing Industry This is simple. Use less chemicals (or more water). Use safer chemic a l s. Minimize energy usage. Protect the anilox roll surfaces and gain

6 ANILOX ROLLS longer life. All of this will allow fo r s m a l l e r, narrower cells that will give fin e r, more precise printing capabilit i e s. Labelers and packaging people will find print quality improve d. E PA s Design for the Env i r o n m e n t and Pollution Prevention concepts apply to this cleaning procedure by p r oviding a safe wo rking env i r o n- ment and easily disposed of chemic a l s. Wo rking with extremely corros i ve chemicals should be left to the c h e m i s t s. Va rious local-area unive r- sities are looking at additional eva l u- ation studies on this cleaning concept. Protecting the surface is better than hitting it with harsh chemicals or e n e r g y. Printers have been making the change to this altern a t i ve cleaning method. The protection of the a n i l ox roll, operators and the env i r o n- ment has all been considered. Ke e p in mind, it s also important that this process is cost-effe c t i ve. About the Au t h o r Stan Seelig is president of Seelig and Associates, Carmel, Ind., a consulting group specializing in industrial cleaning systems. With 20+ years of experience in the specialty chemical industry, Stan has investigated, formulated, developed and promoted CFC-alternative cleaning technologies. He holds an MS in Chemistry from the University of Kentucky and a BS in Chemistry/ Mathematics from the George Washington University. Stan is an active member of the American Chemical Society (ACS), the American Society for Testing and Materials (ASTM), a Fellow of the American Institute of Chemists (FAIC), and a Senior member of the Society for Manufacturing Engineers (SME). He is credited with more than 30 publications, presentations, and patents. Superior Solvents & Chemicals, a manufacturer and distributor of cleaners and supplies relating to the printing industry, sponsored this work. Superior specializes in developing products and services that are environmentally sound, and has invested in hazardous waste recycling and reclamation. For further information on, please contact Superior at