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FLEXO Magazine : October 2009
28 FLEXO OCTOBER 2009 www.flexography.org There is a key difference in the table 17.9a that appears in the version 4.0, which is the single most important change in the Printing Plates section. Table 17.9a from FIRST 4.0. Printing Plate Measurement & Control Properties to Measure Measurement Device/Tool Durometer Shore A Gauge Only for material < 0.250" (6.35mm) Thickness Uniformity Plate Micrometer Plate Relief Plate Micrometer Halftone Dot Size Flexo Plate Analyzer, Microscope Bulb Output Radiometer Line Width Microscope "Bulb Output" was added to the list of properties to mea- sure with a "radiometer," commonly referred to as a "light meter," being the measurement device/tool to use. This device is easy to use and, with the exception of a handheld micro- scope, is the least expensive measurement tool that FIRST recommends be used in platemaking process control. As stated in the document, "Exposure is the most important step in platemaking because it is the image formation step." It also controls the thickness of the plate floor and hence, relief and the long term durability of the plate. Under exposure creates missing or ill-formed image ele- ments and over exposure can leave plates brittle and prone to cracking. Therefore, measuring the lamp intensity and making exposure dosage adjustments per the equation in the Radiometers section on page 144 is vital in producing consis- tent, high-quality plates. Energy (millijoules/cm2) = Irradiance (milliwatts/cm2) x Time (seconds) This is important because, for years prior, anyone in our industry directly involved with printing plates spoke in terms of time. A face exposure for plate product A might be 10 minutes, while the back exposure would be 35 seconds. It was, in es- sence, the language used to communicate platemaking. It's a simple concept that everyone understood and could relate to. The truth of the matter is that photopolymer does not cure with time, it cures with energy, so we must learn to commu- nicate in a different language or in different terms that are truthfully a little harder to understand and relate to. Com- municating and understanding platemaking in terms of mJ (millijoules) and mW/cm2 (milliwatts per centimeter squared) is different and will take a little time to become mainstream. This is already a part of FIRST 4.0's printing plate measure- ment and process control protocol that we need to create the predictable and repeatable printing process that our end- user customers demand. The main exposure frames, post exposure beds, and detack units all utilize a "bank," or multiple numbers of UV lamps to provide the necessary energy to perform their requisite function. The lamps within a particular bank rarely, if ever, have the same intensities so it's very important for good exposure control to map out the exposure frame. Using the radiometer, determine the lamp intensity in mW/cm2 in at least six but preferably nine places on the bed. Based on those readings, it may be necessary to move individual lamps around and re-measure the bed to achieve the most balanced output. This is very important in maintaining image integrity on plates with an image or images that are stepped across the plate. It is also why understanding the concept of energy dosage is needed. As an example, if the lamps on the left side of an exposure frame are on average outputting 13mW/cm2, and those on the right side average 8.5mW/cm2, with 10 minutes of exposure the stepped images in the plate on the left receive 7,800mJ of energy (10 x 60 x 13 = 7,800) while those on the right receive only 5,100 mJ (10 x 60 x 8.5 = 5,100). With this differential, it is highly unlikely that the entire plate will have held the same level of detail and will print the same on both sides. EXPOSURE OPTIMIZATION Now that the bed has been mapped out and lamp locations adjusted for optimum output balance, the exposure optimiza- tion process begins. For both back exposure and face expo- sure the process is identical to what has been done before: doing a stepped exposure test. The difference is the frame of reference. Now we will measure energy (dosage) versus time (the way it was done before). Going back to the equation mentioned earlier where E (energy in mJ) = I (lamp intensity in mW/cm2) x Time it becomes easy to determine the amount of energy delivered to the plate because two of the three variables in the equation are solved. Intensity was determined in the mapping process, and we had a time established from doing things the way it was before. Set the light meter function to dosage if it doesn't output it simultaneously with the intensity, place the probe near the strip to be measured, expose for the selected time and document the reading. Repeat this for all of the steps on the strip. Process the plate as normal and measure each strip to determine the desired floor height for the back exposure or utilize a plate analyzer or microscope to determine where the required minimum dot and minimum rule width are fully held for the face exposure. The dosages given to the selected strips are now optimized for their particular imaging step. All UV lamps do lose some intensity over time, but it is a gradual loss. It isn't necessary to utilize the light meter for every plate exposure. Depending on the number of plates being produced, measuring the lamp intensity should be done once each week, with exposure energy adjustments made when necessary. As the intensity loss becomes greater, by default, the exposure times will need to increase to maintain the neces- sary energy dosage. The time to change out the lamps will be when the corrected times are so long that productivity is lost. The light meter is also used to measure the UV energy dosage for the post exposure and detack process using dif- ferent probes designed for UVA and UVC wavelengths. The lamps in the post exposure bed and detack unit should also be mapped out and balanced. These exposures are also important to producing a consistent high quality plate, but TECHNOLOGIES & TECHNIQUES
Sustainable Fall 2009