By Tim Reece, All Printing Resources
I think many would agree that it is easier to take steps to help extend plate life and minimize damage to the plate than to effectively recognize a worn plate in need of replacement. As a matter of fact, the odds are often greater that the plate will be damaged, lost, or require a change in artwork before it can be deemed “worn out”. One of the most difficult questions to answer in our industry is, “How long should my plate last?” or “How many feet or impressions should I expect to get out of my photopolymer plates?” If we all knew the answer to these questions, then throwing out worn plates would be as easy as tracking the footage. But considering the number of factors that must be taken into consideration this is not a reasonable expectation.
A few months have passed since we explored some of the more common but less controlled causes for replacing the photopolymer printing plate in Part 1 & 2 of this series. During the past few months, we have uncovered a tool that makes identifying a worn plate far easier than ever before. Part 3 will focus on how to properly identify a worn plate and take the “guess work” out of the decision to replace the plate.
How many times have you witnessed a long production run slowly drop off in image quality during the printing process? There is a common reaction to this circumstance. (1.) Take a deep breath, cross your fingers, and hope you can make it to the end of the run. (2.) Start making a deal with GOD, promising that if the plates just last until the end of the job, you promise to never use them again, and this time you really mean it. (3.) Stop the press…
It is after the press is stopped, that we come to a split in the road. Do we take a chance that replacing the stickyback will be enough to get us by or do we replace the plate? The truth is that it can often go in either direction. Unless the edges of the plates are clearly visibly worn, or the screens are breaking down, we just don’t know what to do at this moment. Replacing the stickyback could take 10-20 minutes, but it may not fix the problem. If it doesn’t correct the problem, we then have even more downtime without the proper action being taken. Depending on plate type, a replacement plate could take 1-3 hours and that’s if platemaking is on-site. Then there is the sickening thought of making or ordering a new plate only to find out the first one really wasn’t worn out.
The best way our Technical Solutions Group (TSG) has found to determine if a plate should be identified as “worn out” is through image analysis and/or 3D plate scanning. The most effective means found to determine wear on the plate was through 3D color mapping, as described below. As each of these plates wear, the surface also changes. Most often the wear shows at the leading or trailing edges. Depending on the type of plate, the wear can either begin to polish the plate, making it more reflective in worn areas, or on some polymer materials with an already smooth surface, wear can have a roughing effect. Regardless of the direction it takes, in each situation there is a measurable change and there is a definite reduction in plate height in the worn area(s).
Using a 3D microscope, the process of identifying plate wear becomes much easier to determine. 3D dot profiling allows you to check surface topology of solids, dot shape, relief depth (even between dots) and detect the first signs of plate wear. The AniCAM 3D Scanning Microscope with Flexo Plate QC is easily
positioned onto the photopolymer plate and physically does not touch the area analyzed. This particular device is portable so measurements can be taken in the press room, the plate room or in the plate storage area. 3D color mapping shows distinct color differences as the plate experiences wear in increments as small as a micron. Wear can be identified using this method far before differences in plate thickness could be detected on a plate micrometer. Note: 1 micron = 0.00003937″. An added benefit to this method is that it can be done while the plate is still mounted to the sleeve or print cylinder, further minimizing press downtime.
A Two-Step Process
The first step in establishing a system to identify worn plates is to establish a target that will most likely fail in the trim area where run control targets are likely to exist. Your first inclination may be to use an ultra-highlight dot patch, but a solid patch is often a better choice. Tonal patches often have the ability to absorb some of the abrasiveness of the anilox and substrate, whereas a solid polymer surface only has the compressibility of the stickyback and often show the first signs of wear. It’s kind of like the flexibility your hand has when the fingers are extended compared to rolled into a fist. The solid patch could be as small as a ¼” square area. The nice thing about wear first showing up on the edges of a solid is that you can establish your “fail” benchmark before process or screen areas are affected. You may decide to insert a fine .5 pt horizontal line ¼” wide, or even isolated dots as “fail areas”. The targets you design should be based on your experiences and what you see failing first.
The second step is to create a pass/fail benchmark. If using a conventional plate reading device, take a reflectivity reading on what has been established as “target(s) likely to fail”. Each reading only takes a moment, but takes into account roughly 700 reflectance % data points across region of interest (ROI) and they can be plotted in either the horizontal or vertical axis to avoid laser signatures that can give misleading results. If using an AniCAM with Flexo Plate QC, you will simply place the device on the plate on what has been established as “target(s) likely to fail” and with one click the device will build a 3 dimensional image of the plate surface along worn targets. Take these readings from the point the plate is new, through varying degrees of plate wear, i.e. skipping edges, loss of image sharpness, all the way up to the point of failure. We found that once wear was evident .002″ in from the plate edge that it would quickly become visually apparent and then spread to screen areas. Your pass/fail will likely depend on plate type, thickness, and durometer.
You will be tempted to want to associate and assign a “moderate wear” or a “fail” to the number of impressions or footage while performing this step. The amount of impressions and footage experienced is only one small factor of plate wear. DO NOT assume that the plate knows how long it has run and when it’s time to quit. There are too many changing variables to base plate wear on footage or impressions alone. What you will find is a pass/fail tolerance which has quantitative data to support your findings.
About the TSG
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 about determining plate wear through reflectivity readings and 3D color mapping or assistance in determining your plate wear pass/fail limits, please feel free to contact me at 847-922-0134 or email@example.com.