Ghosting is an age old problem of offset printing. In our printing plant, some presses have more of a problem with it than others. This is due in large part to press design. I would like to share a few pointers that help us to minimize this problem.
First, let’s define ghosting. When some areas of the plate that draw large amounts of ink are directly beside areas that require minimal amounts, mechanical ghosting occurs. This same problem can also occur if the plate “starves” part of an form roller of ink and then upon the next revolution transfers that low ink film thickness to the next revolution. Either way, ink film thickness is is left insufficient to properly ink another image on the plate.
There are several factors that can cause ghosting. Here are just a few:
- Short ink train.
- Inconsistent ink feed.
- Insufficient ink density.
- No form oscillation.
- Poor product design.
- Bad roller settings.
- Poor ink and water balance.
Look at the picture of a hypothetical printed sheet at the top. If the ghosting is caused by a form roller, you will be able to tell which form roller is doing it. This is accomplished by measuring the distance between the darker printed image and the image below where the ghosting occurs. This length will be the diameter of the form roller that is causing the problem.
Of course, just knowing the causes does not solve the problem. And in most cases, you cannot change the length of the ink train or the ink feed design. So let’s consider some steps you can take to make the best of the offset printing job that you’ve been given.
1. Run sufficient density. It may seem simple, but often running the minimum amount of black will not help you in this situation. Give it a little more ink and see if it helps.
2. Use oscillating rollers. Perhaps your press will allow you to put these in. Likely not. However if you do have oscillating rollers, try changing the latitude that they are allowed to move. Sometimes increasing or decreasing it can make a difference.
3. Change the product design. This doesn’t necessarily have to be done by the customer. Perhaps the way you are folding the job or collating it can be changed. At any rate, both the designer and the customer should know what effect their layout has in offset printing.
4. Check your roller settings. I have often found that sometimes an ink form roller isn’t even touching in the area with ghosting. Don’t eliminate this possibility, it happens.
5. Run minimal water. So many problems result from poor ink and water balance – ghosting is just one of them. The best result will come with the minimal amount of water. It may not be your comfort zone, but in this case it is necessary.
6. Put the product on a different press. Perhaps you are running the job on a press with a ductor roller for inking. Such an intermittent system can contribute to ghosting. A continuous feed inking system may be more appropriate. Or another press may have a longer ink train or oscillating rollers. This option of course should be a last resort.
Really though, this problem should rarely come to the press because you’ve already educated your design team and customer about it, right? With good planning, these sorts of problems can be kept to a minimum. After all, doesn’t a pressman have enough variables to deal with?