Narrow Web Plate Mounting

For narrow web flexo printers the need for accurate plate mounting has never been more real than it is today. For many years now the quality standards for flexographic printing have been steadily rising. This translates into smaller traps and tighter registration requirements for printers. To meet these new standards plates need to be mounted precisely, generally within a few thousandths of an inch. It simply is not possible to consistently do this by hand with the unassisted eye. By using video cameras to magnify your registration eye-marks video plate mounting let’s you mount plates many times more accurately and can contribute greatly to improving your print quality.

Print to print registration is a critical component of quality printing. When colors print out of registration images become blurry, edges become ragged and solids miss their traps, creating white outline shadows. To protect against these print flaws pre-press artists employ a technique called trapping in which the colors overlap. Provided the printer can stay within the amount of trap the damage from mis-registration is neutralized.

In flexo printing traps have traditionally been large- as much as .012". But large traps also hurt print quality because you can see the trapped outline in the finished product. Other types of printing have been able to hold tighter registration and print with minimal traps. In order to be more competitive versus these other types of printing narrow web flexo press manufacturers have designed presses that hold dramatically improved registration, typically +/-.003 inches. The result is that today’s flexo jobs are being designed with less trap, and that means less room for error on press.

When mounting plates by hand registration marks on the plate must be lined up with a line scribed on the plate cylinder. It is not unusual to occasionally mount a plate with as much as ten thousandths of an inch of plate skew. This means that if one image is in register on press the image on the opposite side of the plate will be as much as ten thousandths out of register. The problem is compounded when another color on press has a plate that is cocked the opposite direction. These two colors are then as much as twenty thousandths out of register with each other. Many jobs cannot tolerate this level of inaccuracy, and the job will have to be pulled from production and re-mounted. The lost productivity due to press is down-time can have a serious effect on a printer’s profitability and disrupt production schedules. Alternately, the job will be nursed through the run and will result in inferior product being shipped to your customers.

Video plate mounting provides a simple solution to the problem of crooked printing plates. Two aligned video cameras magnify the registration eye marks on the printing plate and display them on a video monitor. This magnification makes it possible to mount the plates with much greater precision. The cameras and plate cylinder are positioned in aligned fixtures along with a table that holds the plate square to the cylinder. The unit is designed to minimize the opportunities for human error. This all combines to mount plates faster and with less than .003" of error (a 300% improvement over hand mounting.)

Propheteer developed video plate mounting in cooperation with DOW Industries of Massachusetts in the early eighties. DOW Industries is one of the printers on the forefront of pushing flexo quality standards higher. That’s why they ran up against the need for more accurate plate mounting earlier than most printers. The video plate mounter was designed to meet their requirements.

How Plates Are Mounted

Plates are mounted in three steps (refer to accompanying photo).

1. First you place the print cylinder in the plate mounter and usethe built in sticky-back dispenser to mount the sticky-back tape on the cylinder. The cylinder rests in a fixture that is aligned and centered under the cameras. A motorized drive assists you in raising and lowering the cylinder so that it is level with a table that holds the printing plate.
    
2. Next you adjust the plate to bring the register marks into alignment on the monitor. The video cameras magnify the register marks to 20X actual size and the split screen let’s you display the marks side by side so that marks can be lined up with great accuracy. A vacuum on the plate table holds the plate while you adjust the alignment, the amount of vacuum pressure being controlled by a foot pedal so that both hands can stay on the plate. When alignment is perfect you switch on an air bladder that grips the plate securely without deforming soft plate material.
    
3. Now you are ready to mount the plate. Peel the liner off your sticky-back and secure the plate to the cylinder. Next you turn off the air bladder and roll the plate on to the cylinder. Once plates are mounted they can easily be inspected to verify accuracy. You simply spin the cylinder under the cameras and check each set of register marks around the cylinder. This second look before going to press can catch a lot of operator error.

For plate mounting purposes we recommend three sets of register marks inset into the bearer bars along the outside edges of your printing plates. That’s a set on each end and a set in the middle of the repeat. This gives you your best visual inspection after mounting. However, on press some operators prefer a single set of marks so that when they see the marks they know they are always looking at the same point on the repeat. If only one set are used we recommend that they be in the middle of the repeat so that if there is any skew it will be split between the two ends of the plate.

The next step up from visual inspection would be a proofing system that allows you to do a test print off press, but that’s beyond the scope of this article and calls for a commitment beyond the scope of what’s appropriate for many narrow web printers. At the start of this article I specified video proofing as a technique for narrow web printers. That’s because it is the narrow web industry that has traditionally mounted plates by hand- wide web printers are much more likely to already have a mounting and proofing department. Video plate mounting is an approach that offers greatly improved accuracy while reducing the amount of labor required for mounting. This approach is most compatible with shops that don’t have or want full-time mounting and proofing personnel.

It should be noted that the accuracy of video plate mounting depends on the mounter being properly calibrated. If the mounter’s components are not in accurate alignment with each other then that degree of error will be passed on to every plate you mount. But every plate will be cocked in the same direction- you will be very consistently inaccurate.

Crooked plates are simply not acceptable in a modern flexo operation. When printers are working to keep quality high and meet delivery schedules, jobs need to be ready to run when they go on press. Video plate mounting is a simple way to quickly and consistently mount plates with the accuracy that today’s tough jobs demand.

For more information on this product, please contact: info@teamflexo.com

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