- February 01, 2011, By Timothy J. Walker Contributing Editor
The first time you see a web flip, you will wonder if you are looking at an M.C. Escher drawing. Escher was famous for his castles where you could walk on stairs mounted on the walls and ceiling - where up was up in the X, Y, and Z directions.
People go this way and that way, ways that seem unnatural. There is something fun and disturbing about losing your sense of orientation in this art.
Web flips are Escheresque. The web goes this way, then that way - ways that seem to be unnatural.
Web flips are fun and disturbing. They are especially disturbing if your web scratches, crashes, or wrinkles during one.
The purpose of a web flip is to - as the name implies - flip the web from side A up to side B up without reversing direction. (The easy way to flip a web is turn around a roller, but then you reverse from heading north to south.)
Web flips are added to dual coating or multi-station printing processes to provide the following two options:
- bypassing the web flip and putting two coatings on one side of the web
- using the web flip to coat on both sides
A web flip system includes three parts:
- the first web turn bar
- a return roller (or rollers)
- a second turn bar
By the time a web leaves the web flip system, it will have traveled in at least three different directions. Follow the web path in the top view and end view schematic of a web flip system as shown above.
The web enters traveling east with side A up, shown in blue.
The web helically wraps the first air bar and exits with a 90-deg turn heading south, now with side B up, shown in green.
The web wraps a roller (or rollers) for 180 deg and heads back north, with side A up again.
After another 90-deg turn on the second air bar, the web leaves the system, again traveling east, but now with side B up.
Fun and disturbing, right?
Where Do Web Flips Run into Problems?
There are several chances to mess up this system. If the air turn has insufficient supply air or a low pressure drop, the web easily may touchdown or crash into the turn, especially with a baggy web or if the air turn is poorly aligned.
If you try to force the web to make a 90-deg turn with improper geometry, the result will be severe crossweb tension variations, or worse yet, wrinkling and web breaks.
These are two common mistakes that lead to improper geometry:
- If the turn bar is installed off of 45 deg, it will create a short web path on one side and a long one on the other.
- Less obvious is the problem created if the elevation change from web paths 1 to 2 or 3 to 4 don't account for the air turn diameter and the air flotation height. If not, the air turn will be wrapped more or less than 180 deg and will over- or under-shoot a true 90-deg turn.
The last point on web flips is the same as any air turns: Avoid long spans into the air bar. However, unlike Escher drawings, web flips are real, and once installed correctly, they can run smoothly in your converting operations.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; email@example.com; www.webhandling.com.