For the past few weeks we've had serious problems within piling on our four color web press. We are running four units in the following order: black, cyan, magenta and yellow. What happens is that even after about 5,000 copies, thick piling occurs on the blankets and they quickly get smashed. This can vary and sometimes we can go much longer. However, the problem remains that the piling comes prematurely. There are of course many factors that cause piling, but here are a few facts that helped us to troubleshoot it properly.
1. The piling occurred on mostly the last two units.
2. Upon close examination, the piling was actually black that was backtracking on to successive blankets (see pic above - shown is yellow blanket printing last with blanket ink on it that was printed first).
3. Not all ink adjusters were having the same problem.
The immediate thought was that the ink and water balance wasn't quite right. If it is an ink problem, then all the pressmen should be having the same problem on each shift. But it wasn't that way. This was a learning curve for all of us. Here was the thinking that put us in the right direction.
Water not only keeps the plate clean but to a certain extent also controls piling. Increasing the amount of water on the last two printing units (magenta and yellow) drastically reduced the amount of ink piling up on those units. Some pressmen contested that it shouldn't be that way. We should be able to run minimum water. This is true. However it wasn't sufficient to stop our tackiest ink from sticking to blankets on successive units.
The problem is gone now, but it has given birth to another question. Is our tack too high on the black. This problem only surfaced when we had our ink manufacturer Flint increase the tack to help us reduce dot gain a little. We were successful, but now the piling problem. Given that our black has the highest tack, we have essentially reached the top of the 'tack window'.
This isn't of course the end of it all. Successive units should be able to run minimum water and not have previous units backtrack into them. This is a quick fix and we've got Flint working on it.
So now we are back to running full speed, albeit with waters higher, and the rookie ink adjusters have gained a little experience from it all.
Web Press Troubleshooting Guide
Troubleshooting Piling Prolbems