What you don’t know about ink systems, Part 4
Episode 4: Filters, Magnets and Stuff in Your Ink
The big dog food bags running on your big CI press look a bit darker than they should and the press operator has spotted a couple of score lines. He stops the press and finds scoring on the black anilox roll. So he changes the roll, taking the $750-per-hour press off line for an hour and a half. The job proceeds well and all seems fine. But what caused the scoring?
It reminds you of a problem a couple months back on one of your narrow web presses, except that time the press was running a bit too light. The first reaction was to increase the pressure on the anilox and plate cylinders to put down a bit more ink. An anilox roll and the doctor blade were changed and all was good. But this only cured the symptom. That day, the real trouble was some foreign matter caught under the edge of the doctor blade. This lifted the blade slightly and deprived a few anilox cells the amount of ink they should be putting on the plate—so a portion of the image printed lighter than planned.
And now you’re thinking, could these two problems be in any way related?
The answer is yes.
“Customers call us saying their press is running light or dark, or say they are seeing some scoring in one of the decks,” relates Randy Carter, Technical Sales Rep at Provident. “If the press is running dark they are probably also seeing some scoring on the anilox roll because a foreign object has come in contact with the roll and scored the ceramic coating. The scoring shows up in the print but the scored area is also collecting more ink. It can be like a little canyon, like a long cell running around the circumference of the roll. That puts more ink onto the anilox roll, then the plate, so the overall label or package prints darker than intended and can show up as a banded area on the label or package.”
Changing the roll, and perhaps the doctor blade, may “fix” the immediate problem but there’s more to resolving these issues than just a parts swap. So what else is going on? To find out you need to dig into the press a little bit.
Filters and Magnets
Somewhat buried in your press are two fairly critical parts of your ink system—filters and magnets—that are often overlooked and in some cases, never even installed.
“These turn up as trouble spots in nine out of ten print shops reporting inking troubles,” notes Mr. Carter. “Filters and magnets aren’t highly visible like a doctor blade or an anilox roll, and they’re not moving parts. Many converters don’t give them much thought because they only need occasional servicing (if they are even installed), but they can be absolutely critical to achieving consistent print quality.”Filters are a fine mesh designed to catch all manner of non-metallic objects such as bits of dried ink, tiny fragments of plates or even sand. Wait. Sand? Yes, from the boot sole of the pressman who went outside for a cigarette break on second shift, came back in, climbed up on top of the press and some sand from his boot scraped off and landed in an ink bucket. The gain of sand (which can be up to 10,000 microns in size) migrated with the ink, winding up wedged under the doctor blade where it wore a tiny canyon into your pricey anilox roll. The grain eventually wore down or was dislodged, but your anilox roll was scored and now requires recoating. A fine-grid filter would have caught the sand and your pressman would never have had to stop the press. That same filter, by the way, would also capture the bit of debris that lifted the doctor blade and reduced the ink reaching some of the cells. Then there’s the magnet or more accurately, a “rare-earth” magnet. This is a super-strong magnet about one inch wide by eight inches long that lives near the inlet hose in each ink bucket and attracts any metallic particles before they enter the ink supply. “But,” you’re probably thinking, “my ink comes out of new drums and is always clean!” That may be, but what about the ink being re-circulated? Your doctor blades, for instance, wear constantly as the press runs, producing a very fine metallic residue can become suspended in the recirculating ink. The rare-earth magnet attracts this residue, accumulating a coating of ultra-fine metal particles that you don’t want in your ink—and being applied to the printing plates.
Keep your credit card away
Why a rare-earth magnet? Because they are many times stronger than a conventional magnet: one will wipe your credit card from about a foot away. The term “rare earth” can be misleading, because they are made of metals about as abundant as tin or lead. Those elements are, however, unevenly distributed around the globe, with China being the major source. This has led to some export restrictions and prompted other nations to look at other ways to develop and provide especially strong magnets.
For flexo shops, though, filters and magnets are just one more part of your ink system that requires your attention. These silent, unseen parts—which are not necessarily standard items on new presses—can help you improve productivity, avoid some frustrating problems, and do so at a low cost compared to having to recoat or even replace expensive anilox rolls. They are just a couple more parts of your inking system to keep in mind when making sure your operation keeps running as productively and profitably as possible.
>>Read Episode 1 on Shrinking Cell Volumes
>>Read Episode 2 on Cleaning Anilox Rolls
>>Read Episode 3 on End Seals