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