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The quality of flexographic print has improved greatly over the years and should continue to do so. The inks used today are better than those used just a few years ago. Anilox rolls are made with volume predictability and line counts that were once unimaginable. Registration tolerances and trap margins are approaching those of offset lithography. Plate properties can be measured and controlled with greater precision; recent developments in plate imaging and measuring technologies promise even greater control. Flexography is moving steadily from craft to science.
Among the advances in flexo are innovations related to setting printing pressure, or impression. Today, many presses, particularly in the wide web category, come with motors that move print stations from a fully retracted position for changing cylinders, to a slightly out-of-print position, requiring only a few adjustments by the operator. Some presses have dials or indexes to make adjustments in precise increments. Still others make minute adjustments to impression at the press of a button. In addition, special plates have been developed for judging impression more objectively. Mounted inconspicuously, and often included on color control bars, these visual aids can help operators monitor changes in impression and parallelism. Improvements and advances in plates and mounting tapes have also helped to improve impression quality.
However, even when an operator has started a press run with seemingly excellent impression, the settings may sometimes drift out of adjustment during the course of a run. Print quality may look great at the start, but change gradually due to variation in impression. Often, an operator must “chase” impression, making many adjustments during a job, or over-impress so that the print does not begin to skip as the station backs out. Losses due to skipping or over-impression sometimes result from these changes. Many printers know the horrible feeling of scrapping rolls of product with skipping or over impressed UPC codes, changes in color of process work, etc..
Press wear can be a major factor contributing to changes in impression. The problem may be less severe with presses of greater precision and heavier construction, but even robust printing presses show signs of wear over time. Among the components that wear are some associated with impression, including screws, fittings, bearings, bushings, and flanges. When they wear, or become loose, gauge readings and impression presets can become less reliable. More importantly, impression can vary by as much as several thousandths of an inch, even with lock-down clamps holding the print station in place.
A sure sign that there is wear or play in these critical components is having a “dead” spot in your adjustment when changing directions. Turn the knob left and right in short twists. If there’s a portion of the twist that barely resists efforts at turning, you may want to closely inspect for worn or loose parts. Make note of the amount of “play” you find here. In the adjusting procedure described later, the amount of play will be referred to as “free twist” and will be taken into account during the final adjustments.
Presses vibrate while they are running. The vibrations come from shafts turning, gears meshing, plates contacting aniloxes and drums, etc.. The interplay of vibrations and gravity can contribute significantly to the problem of varying impression, particularly with the heavy components on wide web presses. Vibrations can work with gravity to move worn print stations out of adjustment. This can be especially true if the final adjustments to impression are in the direction of the downward slope of the printing station (CI presses), because room is left for the station to slide down further. However, if the final adjustment is in the upwards direction, slack is collected in the station leaving little room for it to slide down.
On most CI presses, the upper printing stations slope down towards the drum, and the lower stations slope down away from the drum. This arrangement allows plate cylinders to approach the drum close to perpendicularly. It’s due to these slopes that I believe the following observations are true:
Impression settings tend to increase in upper stations.
Impression settings tend to decrease in lower stations.
It’s like cereal settling in a box. With vibrations, loose parts will settle and pack. Loose printing stations are no different; they, too, will settle to the lowest point that they can, taking up all of the play in the station. On upper stations, the result is an increase in impression; on lower stations, the result is a decrease in impression. Armed with this knowledge, an operator can consciously work with gravity and anticipate this trend, lessening its impact on quality.
Setting impression on upper stations:
On upper print stations, anilox rolls can gradually slide on their tracks down towards the plate cylinder and plate cylinders can gradually slide on their tracks towards the substrate. In anticipation of this, I make my final impression adjustment to upper stations in the upwards direction. This helps to “collect” or remove any slack in the parts critical to holding impression, keeping the station from sliding down. Here are the steps that I take when setting impression on upper CI print stations:
- Adjust the anilox roller to the plate, making equal adjustments to each side.
- Once the plate is inked, adjust the plate to the substrate. Again, make equal adjustments to each side to insure parallelism.
- Once the printing plate inks the substrate, make minute adjustments in and out of impression until you are certain that the drum, plate cylinder, and anilox roll are parallel to each other and touching very lightly, with no skipping.
- Back out slowly on the anilox impression until it just starts to skip evenly across the plate. Make note of the knob positions.
- Turn each knob back in by at least the amount of “free twist” you found earlier, until you get complete impression again. Turn back out until just before the point where it began skipping. Experiment with how close to the skipping point you can get.
- Back the plate cylinder out away from the substrate slowly until it begins to skip evenly across the image. Make note of the knob positions.
- Turn each knob back in by at least the amount of “free twist” you found earlier, until you get complete impression again. Turn back out until just before the point where the print began skipping in the previous step.
- Make certain that your final adjustments are in the backing out direction.
Setting impression on lower units:
On lower print stations, anilox rolls will gradually slide on their tracks down away from plate cylinders, and plate cylinders will gradually slide on their tracks away from the substrate. In anticipation of this, I make my final impression adjustment to lower stations in the upwards direction. Again, this collects slack, keeping stations from sliding. Here are the steps that I take when setting impression on lower CI print stations:
- Adjust the anilox roller to the plate, making equal adjustments to each side.
- Once the plate is inked, adjust the plate to the substrate. Again, make equal adjustments to each side to insure parallelism.
- Once the printing plate inks the substrate, make minute adjustments in and out of impression until you are certain that the drum, plate cylinder, and anilox roll are parallel to each other and touching very lightly.
- Back out slowly on the anilox impression until it just starts to skip evenly across the plate. Turn each knob back in until you just get complete impression again.
- Back the plate away from the substrate until the print begins to skip evenly across the plate. Turn the knob back in until you just get complete impression again.
Once satisfied that both anilox and plate impression are parallel and minimal, closely examine print quality. If you need to make more adjustments, follow the same steps, ending with more or less pressure, as needed. Finally, a simple rule to follow is to make the final adjustments in the upwards direction, whether setting impression on upper or lower printing stations.
Despite all of the advances in flexo, and the many innovations that make adjusting impression easier, the quality of impression is still largely determined by an operator’s experience, judgement, and dexterity. Yet, even the most experienced operator can be caught unaware by changes in impression during a job run. By gaining a better understanding of one of the greatest causes of variation or changes in impression on wide web CI presses, and anticipating likely trends, an operator can greatly minimize the magnitude and frequency of these changes, job after job.
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