|Left: Hickey on plate. Right: Hickey on blanket|
Hickeys are a fact of life in offset printing. That much I've made peace with. However left uncontrolled, they cause us massive amounts of waste. That's why we are constantly trying to troubleshoot and control the sources of offset printing hickeys. Here are a few sources of hickeys and some methods to solve this problem. First of all though, let's define a hickey so that we know we're on the same page.
Hickeys - What Are They?
A hickey is any particle that sticks to the blanket or plate that transfers an imperfection to the printed page. Usually they are a dot with a ring around it. Sometimes they are just a dot of missing color. In any case, there is missing image. Whether it is around the dot or the dot itself that has not image. I'm sure that could only make sense to a pressman.
Sources Of Hickeys
In my years in printing on sheet fed, heatset and coldset web presses, most of the time a hickey will come from the following sources:
- Dust in the environment
I don't think it's an exaggeration to say that these three account for almost all offset printing hickeys. It is always possible that they may be coming from a sleeve in your dampening system or from the rollers themselves breaking down, but these are rarities in our press room. Nine times out of ten we can isolate one of these three sources.
Finding the Source
Most of the time I have found that that you can tell the source just by looking at the hickey. Often a hickey that comes from the ink will be well defined. By that I mean that the image in the doughnut shaped ring has clearly defined edges. In contrast, a hickey from paper fiber will have fuzzy edges and even bits of fiber extending out from it. Examine it closely, it may be that simple.
If it is not that evident, we have performed the following steps to figure it out. Here they are:
- Examine the blanket. If there is much white dust on it, likely your paper bond is weak.
- Shut down the press and apply a piece of tape to the hickey.
- Draw it off the plate and examine it under a powerful magnifier.
- If you still cannot identify it then it could be connected with how the paper was made. Send a sample to your paper and ink supplier for identification.
That is our last resort of course. Most of the time step four is not required.
What Causes Hickeys?
Lastly, let's talk about the properties of ink and paper that cause hickeys. With ink, there are numerous properties that can cause hickeys. Poor grind or dried ink skin in the fountain are just a couple. For us lately, it has been our high ink tack. We've been trying to reduce our dot gain by manipulating the viscosity of our ink. The resultant higher tack has caused us more hickeys as the ink draws fibers off the paper. Hopefully your problem isn't that complicated.
I couldn't write something about controlling hickeys without talking about dust. If you do not control dust in your press room environment, this cancer will slowly grow until it is out of control. Stay on top of it! Housekeeping in offset printing is critical part of preventative maintenance.
Lastly, how do you get rid of a hickey. Officially, I do not encourage wiping them while the press is running. With a sheet fed press, it is easier since the start ups and shutdowns are easier. On a high speed web press where much is wasted, this decision is a little more difficult. Hickeys can come and go so we have a standard number of copies that we wait to see if it goes away. If it is still there we take care of it. If it is on someones face we take care of it right away. You must establish your own standard in this regard.
Again, hickeys are a fact of life and they're probably not going away completely any time soon. Constant diligence is needed to keep them under control.
Web Press Troubleshooting Guide
Web Press Troubleshooting Guide