A Tacky Subject
Plate mounting product manufacturers offer some suggestions and tips to help smooth out the plate mounting process.
TRAPPED AIR BUBBLES. Plates lifting during printing. Damaged plates from residue. Constant press adjustments. Low print quality. … Plate mounting has potential to be a nightmare for some press operators, but help is here to overcome some of the stresses brought on by the simple act of plate mounting. packagePRINTING spoke with some plate mounting material manufacturers to find out some tricks and hints, and a few new products that may soothe a frazzled plate mounter.
Be gentle to your plates
Flexographic plates, although tough, do need some tender loving care when being mounted onto a cylinder or sleeve. Some mounting tapes offer cushioning support, which lends itself to faster startups, higher line speeds, and fewer press adjustments.
One such product line, Rogers R/bak® mounting tapes, offer cushioning support much different from other companies, according to Timothy S. Wilson, senior printing product specialist with Rogers Corporation. He said that many tape products use a closed-cell polyethylene material in their construction, which has the potential of breaking down during press operation. Wilson explained that the closed cells in the polyethylene material can be compared to balloons. After all of the pounding that the plate and material takes during the printing process, the cell walls tend to burst like a balloon, leading to print quality problems in the long run.
"Closed-cell polyethylene tapes will break down during the press run, creating more press adjustments, slower press speeds, and compromised print quality," said Wilson. "Using an open cell urethane, like R/bak, will provide the most consistent and the highest quality print results on the press."
Joe Prunier, strategic market manager, print industry for tesa tape inc., said if a printer is noticing differences in pinholing or dot gain in the same print job each time they print it, this could mean a problem in the density of the foam of the plate mounting tape. "Every plate mounting tape has a target thickness and a tolerance, and there are two ways of accomplishing this—either extrude the foam to very tight tolerances, or have foam cut or shaved to meet tolerances," Prunier said.
Cutting or shaving the foam can destroy the foam's cell structure, Prunier added, and may create soft and hard spots in the cushioning of the foam. His advice to plate mounters is to look for plate mounting tape with foam that is extruded, not shaved, to reach the desired thickness.
The 3M Co. offers its Cushion-Mount™ Plus Tapes with Easy Mount Adhesive which, according to Gerald L. Vogler, product marketing manager for converter markets at 3M, is designed to reduce air entrapment because of the microchannels built into the tape. The trapped air escapes through the channels on both sides of the tape, so the plate mounter has an easier time of bleeding the bubbles out of the tape between the tape and cylinder or sleeve, and also between the tape and the flexo plate.
Keep it clean
As with most anything else, it seems the best practice is to start with a clean plate and cylinder/sleeve. But watch out; sometimes what you clean the tooling with can be detrimental to the printing process.
Using any kind of solvent for cleaning plates, cylinder and sleeves, such as acetone or acetate can lead to the ultimate demise of the equipment, which can result in plate lifting problems and downtime, said Prunier. He warns users that even though it looks like the solvents may be completely evaporated, there are still some residues that may remain on the surfaces. He said the best practice is to use a 50/50 mixture of alcohol and water for cleaning.
"Most, if not all, acrylic adhesives are manufactured in a solvent medium. Solvents are what makes the adhesives liquid and what gives them the ability to flow and allows them to be coated on a substrate in the manufacturing process. The adhesive cannot properly bond to the surface, and there are usually plate lifting problems and downtime," Prunier said.
"Or, if the tape is mounted to a solvent-contaminated surface long enough, the solvent can have sufficient time to evaporate, re-cure, and bond to the surface even stronger. The result is torn, stretched or otherwise destroyed plates, or the tape can leave pieces of split foam or adhesive residue on the plates and sleeves/cylinders. This all adds up to additional cost for cleaning, new plates, and downtime," commented Prunier.
Keep your eyes on the plate
An otherwise successful printing job can become an instant headache if just one piece of the equipment isn't working correctly. To keep many problems from surfacing, Brian Powell, sales engineer for E.L. Harley Inc., said a preliminary check of the plates prior to mounting should be the first step in the plate mounting process.
The caliper of the plates should be checked with a table micrometer, he said. The plates should be matched to the artwork, and checked for color separation, graphics, locator marks, and eye marks to make sure all the plates are available for the print job.
Powell also emphasized the importance of inspecting the cylinders and/or sleeves before mounting the plates. "Over the years, I have seen and continue to see printers mount a nice new set of plates on a bad cylinder or sleeve, and send it out to the press only to have a bad print job. The plate mounting department is then blamed for a bad mounting job because nothing registers," said Powell.
General tips
Some plate mounting material manufacturers offered suggestions to follow for good plate mounting practices.
Prior to mounting:
• Always clean the plate and cylinder or sleeve, but make sure to use the recommended cleaning agent, usually a 50/50 solution of alcohol and water, said Wilson;
• When using sleeves, use the minimum amount of air pressure to get the sleeve on and off of the mandrills, as this will minimize sleeve distortion and prevent tape wrinkles, Wilson added;
• Prunier said that all elements (tape, plates, and sleeves/cylinders) should be handled either at room temperature, or at equal temperatures;
• Anthony Foley of Converting Solutions, Inc., representing Edward Graphics, said exposure to humidity can decrease the tack level on the tape and cause the plate to lift from the tape, and the tape to lift from the cylinder or sleeve. He suggests storing mounting tape products at a relative humidity of about 45 to 50 percent;
• If a sleeve is scored or cut up, Wilson said, press operators should allow plenty of time for the sleeve to dry—greater than two hours—prior to mounting the tape and plate. Allowing at least two hours should insure all of the solvent has been removed;
• Powell and E.L. Harley Inc. emphasized the importance of pre-press proofing of the plates.
While mounting:
• Prunier suggested leaving a small fringe of tape around the plate, and to make sure the joint of the plate is not positioned directly above the joint of the mounting tape;
• "When cutting plate-mounting tapes, use a clean sharp knife," offered Wilson. "This will allow for added control and cleaner cuts that will improve tape lie down and flat seams;"
• Plate mounters should avoid using their hands to apply the tape to the cylinder, and the plate to the tape, suggested Foley, because the pressure sensitive mounting tapes require a specific and consistent amount of pressure. He suggests using a flexible applicator or a low durometer application roller to increase bonding;
• Prunier said to only fasten the plate on a narrow area to avoid premature contact between the plate and the tape. He suggested using a liner material to help the plate mounter;
Demounting:
• Many manufacturers agree it is important to demount the plate smoothly and slowly at an acute angle, or an angle less than 90 degrees. A steady downward motion should prevent plate damage.
Following these rules and employing these tips may lessen the headaches, waste, and press downtime that can come from faulty plate mounting practices.
By Megan Wolf
Assitant Editor
