Get a Printer-Friendly Version of this Article
Anilox Sleeves - The Evolution Continues
Author: Art Ehrenberg, V.P., Harper Corp. Green Bay Division
Anilox sleeves have been around for about eight years now and have dramatically made their presence known over the last two years or so. Many press manufacturers are going gearless and all are understanding or at least exploring the benefits of sleeved technology for many of the same reasons that plate sleeves have become so popular. There are obvious reasons to be attracted to gearless press technology such as ease of register, overall quality control, maintenance, etc., and just as many to be attracted to the sleeved anilox such as ergonomics, storage, shipping costs and so on.
I have personally been involved with anilox sleeve technology from the very beginning when we as an anilox manufacturer were struggling with the earliest versions of sleeves which, for one thing, had no metallic covering over the outer sleeves composite materials. This made for a hit and miss success rate in the anilox manufacturing process due to many factors. First of all the shear physics of the ceramic coating application is not a good match for composite materials such as carbon fiber or fiberglass. The resins in these materials literally boil when they come in contact with the hot ceramic spray. The boiling and gasses emitted make for terrible bond properties of the ceramic coating not to mention the thermal coefficient of expansion and contraction of the sleeve and the ceramic materials being applied. The coatings eventually could be applied but only with a special metal sprayed pre-coat which often posed problems of its own.
To combat the problems associated with spraying metals and ceramics directly onto the sleeve materials, one manufacturer began to use an electroless copper plating over the sleeve. This did in fact help the situation but did not cure it. First of all copper is not a good material to be spraying ceramics onto. It’s oxidation rate plays havoc with getting the thermally applied ceramics to adhere. Secondly copper is very malleable and during the machining and surface preparation operations prior to the ceramic coating, the copper shell would often buckle and deform and release from the sleeve.
We then had the idea of “cladding” or covering the sleeve with an aluminum or steel shell. Basically a sleeve over a sleeve. We were directed to a company who had experience with just such a process for another industry. After experimenting with various metal clad thickness’ we found success and thus was born early versions of anilox sleeves we see today.
But, there was more to the anilox manufacturing process than just getting the ceramic to adhere to them. Most anilox sleeves are installed on, and adhered to, the press mandrels by an inner compressible layer, or “bladder” as I will refer to it, that is actuated by air. With air on and exiting small holes in the mandrel, the sleeve slides on it as its bladder expands under the air pressure. When air is removed, the bladder contracts and the sleeves is fit snug on the mandrel. Sounds innocent enough and in practice works very well. However, this technology introduced several variables that needed to be conquered by the anilox manufacturer.
In the anilox manufacturing process there is “fuss” so to speak put on the O.D. (outer diameter) of the sleeve. Machining, grinding and polishing all exert pressures on the sleeve which can compress the bladder. So in essence we are attempting to hold precise tolerances on the sleeves but having to work through a potentially moving target. Earlier sleeve versions had rather thick bladders which made the problem much worse.
Today, anilox sleeve manufacturers that utilize bladder technology have made tremendous strides in the material and construction that has made dramatic improvements in their stability. But problems still occur from time to time. As an anilox manufacturer we have to understand sleeves in their entirety and know how they react to our processes in order to provide a high quality, reliable product.
One major factor to understand is the TIR, or Total Indicated Runout, of the sleeve. TIR in simplest terms is a measurement of the comparison of the eccentricity of the engraved area of an anilox roll to the absolute center axis of the sleeve or mandrel to which it is affixed, when rotating. TIR is an important dimensional tolerance on all anilox rolls, but just as importantly, and more so with sleeves, is circularity. Is the sleeve circular or round once mounted to a mandrel? (You can understand these dimensions further by referencing a two part article I wrote called “Anilox Tolerances- The Missing Link” which can be found at www.harperimage.com.)
This can be misleading when comparing sleeved anilox to steel anilox. What we have witnessed is that some sleeves bladder material has a better memory than others. Which means once deflected some bladders spring back to their original shape and some do not, and at varying rates. If the sleeve does not respond or return to its original shape after one of our processes, we might measure a TIR condition that typically would well be out of the OEM tolerance. But, the real question is, is the sleeve circular? If the sleeve is circular to say within .0005” and, due to the memory of the bladder, the TIR is out up to say .003”, (which is three times most OEM specifications), this will in most cases not effect the sleeves performance. This is due to the fact that once under load of the plate and doctor blade assembly, the bladder has equally displaced pressure and any irregularities in the bladders memory is negated . Once we understood this, we proved it in many press room trials. However, if this situation does exist prior to laser engraving, the key is to be able to offset the TIR during the engraving process. If this is not or cannot be accomplished there will be irregularities in the engraving around the circumference of the sleeve and you will see this on press typically as a banding effect in the print. So, during trouble shooting we must be careful when measuring TIR, on the press with a dial indicator, and be sure not to use this information alone to determine if the sleeve has problems. Again the real key being circularity and not necessarily TIR.
If the press or anilox manufacturing mandrel has TIR beyond the manufactures specifications, this is a totally different story and will definitely cause problems
Obviously the best case is one of where we do not have to deal with sleeves moving around and thus all tolerances stay in check. There are sleeves today that provide the necessary expansion with out a compressible bladder and other ideas are being developed that address some of these potential issues including total weight, but none to date are 100% fool proof. The situation of problematic sleeve fits is under much better control with today’s newer sleeve technology, however, we still need to carefully monitor the sleeves dimensions through out the anilox process.
It also not good practice to measure the circularity on a sleeve when not mounted on a mandrel due to possible deformation of the sleeve while un-mounted. All dimensional measurements should be done on mounted sleeves. Years ago some sleeves used to deform so much that after several fits on and off the press, the ceramic, which is not ductile, would crack horizontally across the sleeve. Those days are gone now with new sleeve technology.
The mandrels used to perform the anilox manufacturing process are a critical part of the processes success. We must maintain diameter, circularity, concentricity and TIR. We also must protect any exposed bladder material and the inner diameter of the sleeve from contact with solvents, cutting, grinding and polishing solutions to help maintain the sleeves original condition. An anilox vendor has a huge investment in running mandrels for sleeves which is a necessary tooling cost.
There are a few unanswered questions that exist with anilox sleeves. One I am most often asked is, can sleeves be reconditioned.
There are two factors to consider here. The first is the thickness of the metal or aluminum cladding on the sleeve which the ceramic is applied to. As with steel anilox rolls, at each reconditioning when the old ceramic is removed, a small amount of the base, or in the case of sleeves the cladding, is also removed. So depending on how thick the cladding is and how much of it is removed during the reconditioning process will equate to how many times the sleeve can be reconditioned and hold its stability. (Usually a maximum target for base or cladding removal is .010”, on diameter, with traditional machining.)
If the cladding were ever cut through during reconditioning, re-cladding a sleeve could be considered to extend the sleeves life but the timing and all the associated costs of the re-clad versed purchasing a new sleeve would make this not feasible.
A thermally sprayed metal also is a possible life extender to replace the cladding but that coating would not again be reusable and the cost of re-applying it may well exceed an effective economic level.
For sleeves without a metal clad, reconditioning can be accomplished but as stated earlier this is a bit tricky and the adhesion and damage resistance of the ceramic is sometimes questionable.
I believe the more important question on the reuse issue is one of the expansion devise materials holding up, or retaining their memory over time. I have to say I have witnesses an unexplainable case or two where we processed sleeves with absolutely no problems in house only to get a report that the sleeve slipped in the press. Or in a couple cases the sleeve would not fit all the way on the customers mandrel. We immediately launched an investigation to see how this was possible. The mandrels used in our process all checked out, as did the customers. And the rolls seemed to retain their dimensional stability but still in a few very rare cases they slipped on the press, or would not properly fit?
We then began measuring the inner diameter (I.D.) of these problematic sleeves and we found discrepant readings measuring in a circle on the same plain and at different locations in the sleeves I.D. Some readings were up to .010” larger than the diameter of the press mandrel but also the readings were not consistent. We have come to understand that with these particular sleeves this was normal due to the sleeves construction and naturally occurring deviations in the inner sleeve. Therefore in this case we have no way to use measuring instruments to gage the sleeves mandrel fit.
We also noticed that none of the sleeve prints we had on file had an I.D. tolerance. There was just a single reported size. We have since learned that this is common and that the I.D. size reported is simply the mandrel size that the raw sleeve itself was manufactured on.
In order for the devise to expand and contract in a manner to hold it in place on the press mandrel, the I.D. of the sleeve has to be smaller than the press mandrel. So a sleeve manufacturer may have a tolerance of say .0015” between the two which simply means that the mandrel the sleeve was manufactured on and thus the sleeves I.D. is .0015” smaller than the presses mandrel. This would supply the necessary interference fit to hold the sleeve secure.
The only thing we can conclude from these rare “mis-fit” situations is that either the sleeve I.D. was too large to begin with but not large enough to cause slippage in our anilox process. Or, that the bladder did not retain its original unexpanded size once removed from our process mandrels and thus provided the clearance necessary for the slippage? So we have had to institute new procedures which torque test the sleeves for fit before and after processing.
As the few cases of sleeves that did not fit all the way on the customers mandrels but did not have problems during our manufacturing stages, we slightly honed the I.D. with sand paper and the rolls performed fine but never really concluded why the problem existed. It could have been a very minor size discrepancy in the press mandrel? It has been stated by one sleeve manufacturer that it only takes very small size discrepancies (ten-thousandths of an inch) between the mandrels and the sleeve ID to make a fit difference.
We may find that the memory or resiliency of the bladder or other expanding devises at some point in time will not recover and the sleeve will have to be replaced due to this. The newness of today’s sleeve technology is such that there is not enough documentation to truly answer the longevity question on any type of anilox sleeve. We do have documented cases of sleeves utilizing expandable bladders being reconditioned several times with no quality issues to date.
Personally I feel that as this technology matures and is reflected in lesser prices that sleeves will eventually be seen as throw away anilox rolls.
A few tips on handling sleeves:
One of the benefits of sleeved anilox rolls is their weight. They can be installed by hand easily. But this may lend itself to handling practices that could cause damage. Be sure not to rest a sleeve on its end or rest it too heavily on any surface. This is sometimes a natural reaction when handling sleeves. The ceramic at the ends can chip. A small chip could also mean a small crack which could expand when the sleeve is being mounted. It’s best to use a rack arrangement for storage and handling.
Sometimes a slight worming action is used to start the sleeve on or remove it from the mandrel end. Be sure not to over exaggerate this movement as it can cause deflection of the sleeve which could result in damage especially if there is damage already in the ceramic on the end of the face.
Also we suggest that when using sleeves do not use ultrasonic tanks, chemical cleaning tanks, caustic solutions or in general any submersible methods to clean them. This will help keep the bladder and inner diameter material in its original condition. And since most of these sleeves are aluminum clad certain cleaning solutions can damage the aluminum and cause blistering of the ceramics.
In summary, anilox sleeve technology is here to stay and for good reason. Month by month our sleeves inventory continues to grow to a point where recently sleeve production represents up to 30% of our total anilox shipments. And there seems to be no end in sight. Sleeves provide fast change over times, great ergonomic value, shipping cost savings and easier, simpler storage.
Today there are sleeves that utilize various expandable bladder configurations but also some where the mandrels are not air activated but mechanically activated by a hydraulically expanding mandrel system. And there is new technology on the horizon. Cases of problematic sleeves have been very, very rare and represent such small numbers its not worth being overly concerned about. However, the more we know about the technology the better anilox rolls we can produce and the more continuous improvements we can provide as sleeves continue to become a very important part of the Flexographic world.
About the Author:
Art Ehrenberg is Vice President of Harper Corporations Green Bay division. He has been in the printing industry for 25 years and with Harper for 22 years working in various capacities.
© Copyright 2003; Harper Corp.; All Rights Reserved