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  • Aqueous Coating Problems

    Hi,

    I have been trying to sort out offsetting problems.

    The Press has a Analux coater but no extended delivery.

    I'm running medium-hard dry Inks with Algan coatings.

    Sheets are ok during the run and stuck together the next day.



    Any input appreciated
    Print... because you don't need electricity to get the message out!

  • #2
    Re: Aqueous Coating Problems

    Tony:
    You might want to reduce the thickness of the coating, by diluting with water.
    You might try lowering the Temp in the delivery, with maximum air flow.
    Is the coating beading up on the edges of the sheet on the coating blanket? What is the substrate you are coating?
    John Lind
    Cranberry Township, PA
    724-776-4718

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    • #3
      Re: Aqueous Coating Problems

      Tony, I have a short delivery press also and the most important thing is getting all the moister laden air
      out of the delivery. The best thing we did was put a helper fan on the air evacuation vents that pull the
      air from the press delivery.
      If this moister laden air does not make it out of the press delivery and is still in the load the coating
      will not set and you will get some picking or load sticking together.

      Buzz Lee

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      • #4
        Re: Aqueous Coating Problems

        Hi i have operated presses for a few years now in the uk and this has happened to me . Firstly are you using double sided coating suitable for quick work and turns? the answer is probably yes. Secondly you say the problem only occurs when the sheets have rested, this leads me to believe the problem is BLOCKING. This occurs when the solvents in the ink re wet and the ink surfaces stick together leaving you with concrete slabs of substrate. If this is the case it is caused by too much heat in the loads, we never let a load get over 32 degrees celcius on the first side and max 34 on the second pass, this is because as the ink drys the solvents come out of the ink and this chemical reaction creates heat which can add up to 4 degrees celcius to a load. At 38 degrees the ink starts to melt and this is when the re wet inks sticks to each other. The press I run has a standard delivery which makes coating difficult, mainly i run my air knife at 70% and my IR turned off, but even with no IR and absoloute minimum air i still run some jobs a 4000-5000 sheets per hour to maintain a sheet surface temp of 32 first side and 34 second, occasionaly when the pile gets above these figures we open a fire door and leave the stack to cool down for a hour in the fresh air and this has saved the job on more than one occasion. I know that running the press so slow may be difficult to envisage as a reasonable solution but press manufactures sell extended deliveries for a reason and if you do a lot of coating work then a short delivery may not be suitable.
        If you keep the pile below these temperatures (34 max) then you should never block a pile ever again, check your air hoses to and from the air knife and check for holes which may be reducing your ability to provide a large enough volume of air to dry the coating or provide enough exhaust to remove the moist air.

        Good luck

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        • #5
          In my former life we converted 44 Komori to a Harris and Bruno analux coater. Komori has problems with running matte coatings. The coater ran great. We than bought two new presses. One was a 40 8 color UV and conventional the other was a 6 color conventional. The 8 color started up first.
          I offset almost every job we ran with coating. I tried different inks and only one did not offset.
          It was because the one ink that did not offset had a higher dot gain and my ink film thickness was less.
          The issue was not with the ink it was with the BCM of the coater roller. When we UV coated we also had dry back on heavy solids or builds. Also a sign of ink not curing or not enough coating applied.The BCM of the roller we started with was an 8 with I think a 220 screen. We changed to a 12 BCM and all issues disappeared.
          Check the BCM size.

          Comment


          • #6
            Reduce squeeze on the sheet till the coating does not cover evenly then increase till you have a good even coat. You should use a work and turn aqueous if doing two sided work. First side can run as hot as you want but second side needs to be less then 94 degrees Fahrenheit. Powder higher on second side. We had similar problems when we tried switching from Nicoat to a different brand except the sticking did not happen till after we tried to cut. Our application was to cut do some bindery and then cut again. Both times cutting caused the brick like sensation. The solution to this was to add more wax or go back to nicoat which we finally did. Ask you coating rep what wet on wet temp should be. This is what you need to know when backing the job up. Wet on wet means when you exceed a certain temp the aqueous re wets from the first side and comes in contact with the wet aqueous on the second side and then sticks together. Make sure you have a thermometer you can stick in your pile. You need to have a pile temp around 88 degrees a half hour after printing.

            Do you have a vent system attached to your coater. If so that may need to be opened or closed to help vent out the moisture. Too far open and you will not achieve the heat you need. Too far closed and you will not vent the moisture you need to vent out of the sheet.

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            • #7
              Hi Tony / Guys,

              Good to read such informed responses with some good practical solutions to this problem. Been there also from a technical point of view. Involved in aqueous litho coatings for over 20 years as well as oil inks.

              Hard dry inks may have long oil length / slower drying oils which contributed to slow drying and stickiness. They take longer to through dry. In these situations it would be a good idea to get setting/drying and blocking tests carried out on the ink using your substrate. Try if possible to use a faster setting ink with aqueous coatings .

              As far as the coating is concerned through the analux it may well be that the film thickness is too high (beading on the edges) or the viscosity of the coating needs adjusting to reduce the thickness. In these difficult conditions you may also require a coating with a harder polymer and less plasticizer - what is technically known as a higher glass transition temperature.. Also get lab blocking tests carried out on the coating/ink/substrate combination. Certain coatings can have special additives added to help prevent blocking.

              These are just my opinions based on past experience and specific situations will be dependant on the exact ink / coating / press conditions / air flow / temperature etc. involved.

              Any questions please get in touch - glad to help in any way I can.

              John Tierney.

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