Controlling Flexographic Print Repeats

Easy to Check - Often Missed

Print repeats play an important roll in the performance of the finished product. You may have a beautifully executed award-winning package, however if your repeats are out of specification tolerances you may have to throw the product away. Worse yet, the product could make its way to the packaging company and cause downtime to their own production line. As basic as verifying print repeats may seem, oddly enough they are often overlooked, forgotten, or missed in the final job approval. In some cases, the repeat was within specification when the job was approved. However, machine adjustments made during the press run can alter the repeat; therefore continued monitoring of PR is necessary throughout the press run. Listed below are recommendations to help eliminate repeats from being out of specification.

• The pressman should be supplied with a steel ruler with a length being longer than the largest repeat produced. The ruler should have the full spectrum of measurements; some jobs are measured in millimeters, sixteenths, etc. Steel rules used press-side and in QC should be standardized and verified to be consistent with one another using one steel rule as the master. If your company is ISO certified consult your ISO team leader for compliance.
• Print repeats should be measured on a smooth surface with the substrate laid out completely flat. It may be also necessary to measure the cutoff (rpt) over multiple impressions around to determine that all cutoffs within the cylinder repeat are within spec. Naturally in this case cutoffs would be within +/- 1/16 or tighter. Some bagging machines require PR to fall within tolerance over multiple impressions.
• Plant quality control specifications sheets should list the minimum and maximum print repeats allowable for each individual production run. These specifications should also be recorded on the production run condition folders.
• A print repeat should never be measured after the press rewind has indexed a cut over. The roll should be marked with a flag prior to the rewind cutover and the layers of material removed down to the flag. The reason for this is the web tension may change while the rewind is indexing and alter the repeat.
• After change to the machine settings such as oven temperatures, infeed and outfeed tensions the press should be stopped and the repeat checked.
• The rewind brake plays a role that is often referred to as snapback. The more you increase the brake the more the material will snapback and shorten the repeat. The same is true if you decrease the brake the repeat will increase. This is true when running polyesters, polypropylenes, nylons, and other extendible films. But is not an issue when running paper and other more rigid substrates.
• With the nip open install a piece of film under the nip and close the nip. With forty pounds of pressure on the air gauge try to pull the film out of the nip. Repeat this on both sides of the nip and the center as well. If you can pull the film out you have a mechanical problem and or the nip is worn.
• If the nip is not completely flat on the substrate the material can slip through the nip and create sporadic repeats and registration problems.

One of the best solutions to maintain consistent repeats is to install a repeat micrometer on the press in the rewind area. This will monitor the repeats throughout the production run.

For More Help...

If you want to learn more about this subject, please feel free to reach out to us by calling 1-800-445-4017, or by e-mail at [email protected]. All Printing Resources has formed our Technical Solutions Group (TSG) 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 along with best practices. They are armed with the latest in diagnostic tools and are experienced in problem-solving that can achieve sustainable results. The TSG has walked in your shoes, and have felt your pain.