By Tim Reece, All Printing Resources
This multi-part article will explore some of the more common but less controlled causes for replacing the photopolymer printing plate. Reasons for replacing an expensive photopolymer plates can vary greatly. In Part 1 of this series we focused on many advantages of deep plate cleaning with automatic plate washing systems, including how we extend the life of the plate. Part 2 will focus on minimizing damage during plate de-mounting, while the final chapter will address how to properly identify a worn plate and taking the “guessing” out of the decision to replace the plate.
- Part 1 – Plate cleaning
- Part 2 – Plate demounting
- Part 3 – Identifying when the plate is worn
Where Plate Damage Occurs
In this article we will turn our primary focus to plate demounting. Those who imagine plate and tape removal to be a relatively simple task, have most likely not performed this task and been held accountable for plate damage for an extended period of time. It seems that we take great care during the methodical steps of platemaking to insure a quality plate has been manufactured, free of pinholes and with optimal relief and exposure. After leaving platemaking these plates are handled with care and trimmed to size with care before being mounted and wrapped in opaque polyethylene to protect them from harmful UV, ozone and surface debris and scratches. Then it’s time for the plate to do what it was created to do; transfer a beautiful multi-colored artwork to the substrate. The marriage of plates starts
peacefully with a kiss impression. But after that optimal setup check and the thumbs up from the supervisor to “run it”, the plate that was handled so carefully prints what can be millions of feet of product and running sometimes in excess of 2,000 fpm, not to mention exposure to solvent mixtures that swell and abrasive white inks that wear on the plate surface. This sounds like a harsh environment, and it is, but this is NOT where most plates are damaged or worn out. More plates are deemed unusable after incurring damage during demounting, or the stripping of the plate from the stickyback. When polling convertors, an estimated (average) of 78% of plates are replaced due to damage versus the plates actually wearing out.
Oh, The Shame…
If it wasn’t bad enough that an expensive plate is damaged beyond use during the demounting process, it often gets worse. We have made it a point to inform employees how expensive a photopolymer plate can be. So when an employee inadvertently tears a plate during demounting, believe me they get a sick feeling in their stomach. People who would never consider damaging company property and take pride in their job, with one slip, just a little too much pressure, or pulling at the wrong angle tear the plate. The dilemma now is to report it, or quickly hide it in the trash and hope it will be classified as “lost” or maybe the job will incur a change in artwork and it will all work out. Of course the problem comes when the damage does go unreported and the platemaker is already behind and thinks he/she is going to just pull the “full” set from storage.
Avoiding the Tear
Plates damaged during demounting typically fall into two different areas, tearing the plate at a stagger (see image A) or creasing the Mylar backing by pulling the plate in an offset angle from the plate cylinder or sleeve. When carefully trimming a plate, often just prior to the mounting process, we often need to trim 90 degree cuts where the design nests or lanes are staggered. It is at the inner angles that the plate is most susceptible to tearing during demounting. No matter how careful the trimming, the two cuts intersect one another creating a small slit that allows tearing to easily occur when removing the plate from the mounting tape (see image B). The individual can do something so easy at this point to minimize this mousetrap. Simply take a commercial grade hole punch, and punch out the area of the intersecting cuts (see image C). When the plate is pulled from the tape the force will follow around the radius of the holes before changing in the Y to X axis. You will be amazed at how well this method works. Industrial handheld
hole punches cost around $50 and can punch a variety of holes sizes, not just the standard 3/4″ although that size does work well. The larger the diameter of the hole, the more that forces is spread and the better the result, but in some cases the convertor may choose something smaller, not to cut into the artwork. A Kongsberg (see image D) table takes an automated approach to plate trimming with the ability to trim multiple plates from a master sheet of polymer with radius at each critical area prone to incurring damage.
Everybody’s Got an Angle
The second area that damage occurs during demounting is creasing the Mylar backing by pulling the plate in a non parallel angle from the cylinder in hopes to get it to release from the mounting tape (stickyback). It’s difficult to tell the person demounting the plate to “just pull the plate away and parallel from the cylinder. The fact is that if it was that easy, that is exactly what they would be doing. When demounting a plate by hand, many of us refer to a method coined by 3M as the “low and slow” technique. Folding the plate over on itself or pulling the plate side to side inevitably will result in some damage to the plate. Removing the plate in a steady slow motion while using a low angle of peel (less than
90¡) will help minimize damage to the plate (see image E). Still we must consider that after a plate is mounted, the adhesive level typically increases. The reason for this is because when we mount the plate, we want to be able to easily reposition the plate. Once the plate is mounted we want it to stick aggressively until the order is run, then magically release. The truth is that it increases in adhesion, and impression in press only enhances this effect. The result is a plate that often doesn’t want to release, and a lot of pulling back and forth and side to side to get it to release. There have been cases that the employee pulling the plate or stickyback from the print cylinder actually pulls the cylinder off the cart or stand in which it is resting. Considering the awkward movement and amount of force required to get the plate to release by the employee, it isn’t hard to image damage to the plate, or worse yet, physical injury to the employee. It only makes sense that this would be a likely area for automation.
Automated plate demounters are designed to automate the removal of polymer plates and tape (stickyback) from sleeves or print cylinders (see image F). The effectiveness of these demounters is only enhanced by the plate punching methods and use of automatic cutters like the Kongsberg table. The auto demounter replaces the need of an operator to remove plates or mounting tape by hand, an operation that can be particularly time consuming especially for wide web plates. The demounters can be equipped with an expandable shaft, allowing the machine to work with all sleeve inner diameters or work with conventional plate cylinders. The cost savings in time and damage to plates can be great; the operation of these machines seems rather simplistic initially.
Once the mounted plate cylinder or sleeve is placed in the demounter, a key part of the demounting operation takes place. A specially coated pneumatic roller gently moves forward to prevent damage to the plate and locks on to the lead edge of the plate. Once this “clamp roller” locks down on an edge of the plate, the machine gently pulls the plate off of the sleeve or cylinder perfectly parallel and at a constant (controlled) speed and peel angle without tearing the photopolymer. The very same process can be used to then remove the mounting tape.
Current automated plate demounters do require the operator’s assistance in releasing the initial lip of the plate to start the removal process. The reason this intervention is required is to address butt fit plates, radical plate gap patterns and step and repeat staggers. The is most safely executed by using a plastic tool with a flat beveled edge to start to lift the plate without fracturing the mylar backing which can occur by using ones fingers.
More to Come…
In Part 3, we will look into minimizing plate costs by using a quantitative process determine how we can be sure a plate is worn and no longer suitable for use. We have formed our Technical Solutions Group to encompass our full range of expertise in all critical areas of the flexo process. This team is made up of industry professionals dedicated to being up to date on new technologies, armed with the last in diagnostic tools, and experienced in problem solving that can achieve sustainable results. The TSG have walked in your shoes, and has felt your pain. For any specific questions please feel free to contact me at 847-922-0134 or email@example.com.
We have formed our Technical Solutions Group to encompass our full range of expertise in all critical areas of the flexo process. This team is made up of industry professionals dedicated to being up to date on new technologies, armed with the last in diagnostic tools, and experienced in problem solving that can achieve sustainable results. The TSG have walked in your shoes, and have felt your pain. For any specific questions please feel free to contact me at 847-922-0134 or firstname.lastname@example.org.