When it comes to offset print scumming, I will assume you know enough about ink and water balance to know how to make it go away. I would like to go into some of the deeper reasons as to why scumming can happen and what remedies can solve this plague. First of all, let’s define scumming.
In offset printing, when the non-image areas of a plate become receptive to ink, that’s scumming. Too much ink or not enough water is not a definition. Those are causes. Since the causes are so many, that definition is broad enough to encompass most reasons.
Causes of Scumming
Let’s now look at a few causes of scumming beyond the simple ink and water balance solution.
- Plate is sensitized by piling.
- Dirty metering or water pan roller.
- Poor plate development.
- Poor ink strength.
- Bad roller settings.
There are several different areas that we can look at from this point. Let’s examine each consumable one at a time.
1. The press itself. Start here. Check all the roller settings and get them set them exactly to the press manufacturers specs. Take your time and check the settings when the roller are engaged. There is a difference.
2. The plate. We have found that sometimes our plates are too sensitive. It turned out once that gum wasn’t properly being applied by our plate room department. Whatever the case, a very narrow window can be created if plates are not processed properly. If the press is down for any significant time, gum your plates. We have found that plates made by computer-to-plate can oxidize fast and must be gummed quickly.
3. Chemistry. Start with your fountain solution. Double check that you are mixing it according to the manufacturers specifications. Consult with your supplier that your paper is made compatible with your paper and plate. Poor chemistry can cause piling which leads to the next area.
4. Piling. The buildup of piling can be abrasive enough to the plate that is will cause it to desensitize and scum. There are so many causes of piling that are difficult to troubleshoot that we cannot discuss them all here, but in our experience, paper is the place to start. Paper that is too abrasive will desensitize the plate quickly. A higher grade of paper may help.
5. Ink. In our experience ink is responsible for 3% of the cost but 97% of the quality. Good offset printing ink is worth its weight in gold. A liberal ink and water balance window is critical to prevent scumming. Work closely with your ink supplier and discuss values like ink and water pickup rate. Ink manufacturers take shortcuts too. Call them on it!
I do not necessarily go in this order. My first suspect is always setting and then ink, but you make the call and let your experience be your guide.