I would like to continue the subject of essential tools for a web offset printing press with another device that has proved very helpful for us. Pictured above is our paper moisture content reader. As the name suggests, it can tell you the moisture content from a range of 1-15% for whatever paper you are running. Much has been written about the effect of moisture content, but I would like to talk about one designed specifically for offset printing.
How It Works
Here is another picture with the lid open. Notice the winding hollow in the lid (called a wave guide). A sample of paper that you are running is put on top of this guide. Upon closing the lid, a microwave signal is sent through it. Whether the product has printing on it or not makes no difference. Based on the strength of the microwave signal that reaches the end of the guide, an accurate reading of the moisture content is given.
Tip: Put four or five sheets in instead of one. You will simply need to divide the result by the number of sheets. This way you get a more averaged reading.
Why Is It Useful
We find it useful for the following reasons:
Fighting Web Shrinkage (or growth): In heatset printing, this instrument is essential for troubleshooting. Each offset printing oven, depending on it's temperature and design, will shrink the web as it passes through. This is a natural result of the moisture content of the paper being reduced. The higher the moisture content, the more shrinkage. Most heatset presses are capable of adding moisture back into the paper either through the silicone applicator or a separate system. The paper moisture content reader will enable you to quantify how much is being put back in and adjust accordingly. To reclaim some of that paper width, you can quantify how much moisture is necessary.
As a rule, we ensure that the paper has 2/3 of the original moisture content that it had in the roll.
Monitoring Paper Companies: The paper you receive has a certain amount of moisture in it.
This is why it is important for printers to monitor this. When the paper is being made, the manufacturer will reduce the moisture to a certain amount. This is fine, but if there is too much moisture, you will get more shrinkage in your oven. The result is essentially narrower paper.
Folder Performance: In my experience, the press folder is like a woman with pms. She has her good and bad days. She has many buttons of many different colors and types. Adjusting the wrong button on the wrong day can have consequences. With that many variables in the folder you need control of the paper moisture content. Without this measurement, you are shooting in the dark. I'm sorry if you are a woman who is reading this - that's just my take on it.
Paper Stretch: Most pressmen who has done four color offset printing knows why web growth happens. The principle is that a paper grows from printing unit to unit as water is added. The tension on the paper causes this. The pressman tries to control this fanout by means of bustle wheels or air bustle. However if you paper moisture content is high, get ready for even greater growth. The paper moisture content reader helps monitor this.
Additionally, this device has helped troubleshoot problems recently related to losing our compensation mark at a splice. The moisture content between two rolls was enough to stretch differently in the press and cause the automatic compensation to lose the mark.
So the question: Is it worth it to go out and purchase one of these? I would recommend it if you do a lot of heatset printing. These devices run into the thousands of dollars. I can only testify it's usefulness in our pressroom.
Heatset Oven Troubleshooting - Web Shrinkage
Web Press Troubleshooting Guide