Blanket piling is a long studied phenomenon in offset printing. The reasons as to why it happens are just as complex. If you're like me, you've tried to control it without really understanding why it happens for a long time. I've recently taken the time to understand the causes and remedies behind it by consulting with ink chemists and paper manufacturers. And it turns out that the reasons are not so complex as first thought. I speak about reverse piling in this entry, that is, piling on the non-image areas of the blanket. However some principles can be equally applied to normal piling.
Causes of Blanket Piling
Let's start with a list of the most common causes of blanket piling. Some are related to the raw materials consumed in the press room, others from the way the press is run. Here are the main raw consumables that cause piling
- Type of dampening solution used.
By listing those things above, we've covered almost every major aspect of the printing process. We'll break each down shortly and talk about their effect on piling. Processes that are controlled by the pressman that cause piling include the following.
- Dampening solution mixture.
- Ink and water balance.
- Press settings (bearers, rollers)
- How the image is laid out on the plate.
- Water pickup of the ink.
How Blanket Piling Happens
It is really not that complicated. Ink unavoidably emulsifies. Particles of ink become suspended in water that covers the non-image areas of the plate. This is normal. However there must be enough water conveyed from the plate to the blanket and onto the paper to "flush" it out. After the water system adds water to the plate, it then splits when it contacts the blanket. There must be enough water remaining on the blanket to keep those particles suspended. Then the water film contacts the paper. A proper transfer to the paper then depends on the papers properties. If the paper absorbs the ink vehicle too quickly, it will deposit the pigment on the blanket. Hence, it really boils down to how well your water and ink flow along those points.
What Can Be Done To Minimize Piling?
1. Start with your plate. The plate topography will determine how well it carries water. It may seem like a minimal point, but under a microscope, you can see how porous the surface is. If it can carry water, then it can carry enough for those suspended ink particles to transfer properly to the blanket. I do not know of any measuring tool in the industry that does this yet, but your plate supplier will be able to tell you more about this property.
2. Run ample water. Probably the simplest thing you can start with. If you run the minimal amount of water then you are a good pressman. However this aggravates the problem of piling. Running slightly above the minimum will allow the anti-piling agents in the water to do their job. But most importantly it will supply an adequate amount of water to allow suspended ink particles to transfer right off the blanket. Not too much though, or you will start piling in the image areas.
3. Analyze your water pick up rate. Your ink should come with a water pickup number. It is done with a duke water pickup tester. Ink that can carry water properly will adequately distribute it over the plate. Unavoidably, some areas of the plate will have heavy coverage. These areas will tend to see more piling, particularly around the edges of the solids. If you have a good water pickup value on your ink, this will take care of it. Consult your ink manufacturer for this information.
4. Test your paper. Quite possibly, your piling is caused by mineral filler from the paper (or coating from glossy paper). The water added while passing through several units can cause it to break down and deposit on your last printing unit. To figure out if this is the problem, you must change your paper. This is not so easy as paper manufacturers have come under heavy pressure to reduce costs. As a result, the spectrum of good quality paper has ballooned in recent years.
Remember this too: a paper that absorbs too quickly will cause piling. The ink vehicle absorbs into the paper and leaves the pigment on the blanket. If this is the case with the paper you use, then you must analyze whether the cost of washing blankets all the time really outweighs the savings in buying cheap paper. This is all the more so true if you have paper piling which is caused by fibers being picked off the paper onto the blanket.
5. Blankets. I consider this the least likely cause when I am troubleshooting as it has less of a bearing on how water is transferred as do the other four points. Nonetheless, if your blanket has a poor release, then it will not transfer the water or ink properly. Check your bearer pressures and your blanket heights before taking on this venture. It could likely be a combination of products in this case.
These five reasons are by far not comprehensive. You can add to these other press room variables such as temperature, water chemistry, ink flow rate, tack etc.
Lastly, I will add that there are some water additives out there that act as anti-piling agents. These can be added to the water. However I think the jury is still out as to whether there is a significant enough cost savings. You can experiment with that one.
Pressmen for the most part have just come to accept this problem as a part of what offset printing is. Hopefully we will see more progress in eliminating piling all together. I also recently had a problem troubleshooting piling that you can read about here.
Troubleshooting Piling Prolbems