Blanket piling is a fact of life for our pressmen and becomes a source of downtime for us during long runs. This requires shutting down the press to clean them sometimes. We wash our blankets at different intervals, depending on the paper. In our case it is mostly negative blanket piling - that is to say piling that occurs in the non-image areas. Lately when running a medium wieght coated stock, we wash our blankets with automatic blanket washers because of piling every 50,000 revolutions. Some papers we can increase this to 100,000. For us it depends greatly on the paper we run.
Why Blanket Piling Happens
Essentially, minute particles of ink are embraced by the water in the emulsion process that takes place during the printing process.
Let me explain. Ink and water mix in the rollers and water is deposited on the non-image areas of the plate. What happens though is that in this process, very small particles of ink are accepted by the water. This water then carries them to the non-image area of the plate. Then where does the particle go? In some cases, the paper will take it away. For the most part though, it is deposited right on the blanket and the process continues each revolution.
Some refer to this process as micro-scumming. This is not a bad term, but not entirely accurate either. The reason is that running more water does not help the process. In fact it increases the amount of ink carried to the blanket and deposits it. That is why piling gets worse when water is at the upper limit of the window. Learn to measure this window here.
Here is a list of factors that effect blanket piling in the order of influence.
1. The Role of Paper
We print on all types of paper. However when printing with certain papers that are uncoated, we can go long as 200,000 copies without cleaning our blanket piling. On some coated stocks, we cannot go more than 30,000. The reason? Paper that absorbs water has the tendency to pull most of the water and ink off of the blanket. Thus it tends to not build up. Coated stocks however absorb almost nothing, leaving it all for the blanket.
2. The Role of Ink
Ink manufacturers have the challenge of designing an ink that will absorb as much water as possible. Since many papers will not do so, it is up to the ink to effectively create an emulsion that allows the ink and water balance window to be stable. Thus a high water pickup is necessary.
3. The Role of Water
It is the pressman's job to run the right combination of water and ink to cause the minimal amount of piling. Usually the type of paper cannot be controlled. However, as we have experienced, increased water will increase piling. Low water will also increase piling because it encourages more ink particles to be deposited on the blanket. A balance must be achieved, not necessarily at the bottom of the window, but in a delicate balance between the two factors that cause blanket piling. I've also written about different strategies to keep blanket piling to a minimum.
Please leave me a message if you have other strategies. Here is also a link to help in troubleshooting piling problems. Also, please like me on facebook to receive any updates on this subject. I'm happy to share anything I know.
Troubleshooting Piling Prolbems
Troubleshooting Piling Prolbems