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Why Web Growth Occurs In Offset Printing


Web growth happens mainly for two reasons.  First, the application of water on the web.  Second, fiber directions.  Web growth is sometimes referred to as fan out, a topic I have written about before.  But this time lets look at at each of these reasons to help understand why a good prepress department and bustle wheels on the press are necessary in dealing with this phenomenon.


1. Water applied in the offset printing process.
The nature of web printing requires that the paper is under tension.  This tension causes the paper to stretch, and it stretches in both directions: circumferential and lateral.  Pass that paper through four printing units that are each applying a thin layer of water, and you have the recipe for web growth.  As the web passes through each printing unit and is stretched by the tension from one to the other, it literally grows larger.


2.  The direction of fiber in the paper making process.
When paper is made, it is goes through a process that causes the fibers to lay in a certain direction.  At the start, the paper is 99% water and 1% fiber.  It is literally poured onto a screen called a fourdrinier table.  This table is like a screen that can be travelling at 3000 feet per minute.  As it falls on this screen, the fibers tend to lay in the direction of the passing screen.  See video below.




Since the fibers lay down in the direction of the screen, the strength of the paper that comes from the length of the fiber, tends to be in the direction of the running web.  With less fibers in the cross direction, the paper tends to allow itself to be stretched more in that direction.  Hence, when it passes through the printing units, the tension in the cross-direction of the paper sees the most stretch.


How to deal with web growth
1.  Good prepress software.  Even before the plates get to the press, a web growth profile can be created.  For example, on a four color press, all images on the plate would be set to match the last unit - yellow (or whatever your last color is).  Black, cyan and magenta would have their images shrunk slightly to allow for the stretch that occurs between the units.


To calculate your web growth for the sake of preparing your plates properly, I have put the steps in another article.  


Bustle wheel
2. Bustle Wheels.  Most web presses (especially double wide) have bustle wheels and they are usually installed between the printing units.  They are simply designed to push against the web and cause a stretch in the paper that will allow the next printing unit to line up properly. The only drawbacks are the slur that they can cause as well as the stress of them can sometimes cause web breaks.


More popular lately are air bustles.  They are simply a jet of air that blows against the paper and creates the same phenomenon, but without touching the web.  This helps to control the problem of setoff.
  
Related posts:

Heatset Oven Troubleshooting - Web Shrinkage
Printing Units Troubleshooting - Register

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