The difference between Flat Sheet Litho and Web Litho

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Permalink Reply by Frances Metcalf on June 30, 2009 at 1:34pm

While I'm not an expert by any mean, it would seem to me that on a gross level, the only differences are in how the paper is fed into the press and how the ink is laid down.

For example, all presses use plates of some kind to hold the image (let's not get into a discussion about direct to press at this time!).

All presses use ink on those plates to create the image.

Other than that, the differences in the processes are negligible. It's the technique of applying that ink to the plate and the method of transferrring the ink from the plate to the paper that's different. And it is the type of paper and how the paper is fed into the press that is the most different.

Seems basic to me, but flat sheet litho uses pieces of paper that are cut to a particular size and fed into the press before the ink is transferred from the plate to the paper. Further, sheet fed litho sends one piece of paper at a time into the press so that the image can be transferred from the plate(s) to the paper.

Web litho uses rolls of paper that are cut to a predetermined size after all the plates are printed. The paper is threaded into the press in a continuous roll. Web presses can use more colors of ink at one time, providing greater detail and preciseness of color than sheet fed presses. Web presses can also add varnishes or other finishes after the image is created, eliminating the need for another set of machinery to varnish or finish. Highly sophisticated web presses can also do complicated binding "on press", again saving time and steps in the completion of a project.

But all those things are bells and whistles. If you're just talking about printing processes, I still contend there are no real differences. Both presses put ink on paper!

Anyone who can expand, challenge, or give greater knowledge, please, please feel free to comment.

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Permalink Reply by Mike Malmad on June 30, 2009 at 5:37pm

With a sheetfed press individual sheets are fed through the press, there are many sized presses and sheets, probably the most common size of press today 28 x 40 which is the maximum size that press can accomodate, but it can use smaller sheets say 17 x 23. Each sheet goes into the press, ink travels from the ink rollers and is applied to the plate, that image is transferred to the blanket and the sheet travels between the balnket and impression cylinder where the image is printed, Sheetfed presses can range from single to 12 colors and may have Aqueous and or UV coaters. We have presses up to 64" (45x64 sheets) and 6 colors and dual coaters.
Web presses are fed a continuous roll and have a fixed cut-off (usually 22 3/4 or 17 1/2) but the roll width can be varied to best suit the project ink is transferred in the same manner butboth sides are printed at once and it is dried by passing through an oven then into either a sheeter, to be cut into sheets or a folder. These presses operate much faster. We have both 22 3/4" and 17 1/2 im 5 and 6 colors
Please feel free to contact me if you have further questions.
Mike Malmad
mmalmad@gmail.com

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Permalink Reply by Erin Walter on July 1, 2009 at 9:47am

I think Mike covered the differences very well. I work in the high end commercial side of the printing industry, and we utilize both Sheetfed and Web presses. In my experience, the greatest determination of whether a piece should be printed on a Sheetfed press or on a Web press is the quantity. It is more efficient to produce very large quantity jobs on a Web, as it yields at least 4x the number of impressions per hour (between our sheetfed and web averages). The presses you have determine the number of colors/coatings you can use, as well as the quality of printing. We have up to an 8 color Web and up to a 12 color Sheetfed press. The web also allows you to fold, perf, and glue inline, with no drying or back-up time. It should also be noted, though, that flat sheets can come off the Web--they will just be printed on two sides when they sheet.

Hopefully this extra information helps. Let me know if I can be of further assistance.

Erin Walter
ewalter@hennegan.com

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Permalink Reply by Vikas Agrawal on July 6, 2009 at 9:51am

This in addition to What Ms. Erin said, with web you have a limitation of the paper, what i am trying to say is that its not possible to get a 28 Gsm reel and say 350 Gsm art coated card reel ready stock (atleast not in this part of the globe). Sheets on the other hand are easily available of any grammage and quantity.

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Permalink Reply by John Rawlins on July 6, 2009 at 10:19am

The mechanics of the offset printing process are the same for both press types. And as described already, the main difference is the paper transport mechanism, one being sheets, the other being rolls of paper.

The main advantage of web litho is the speed, up to 45,000 impressions per hour, versus conventional sheetfed presses operating at anywhere from 9000 to 16,000 per hour. The second advantage of web litho is the capability to do inline finishing thereby eliminating the need for conventional bindery. Additionally, web presses have the ability to run very light paper weights, like 35lb or 40lb coated papers suitable for catalog work. The final advantage is the paper cost itself. Roll paper costs about 25% less than sheets.

The disadvantages of web litho are the fixed cutoff length of the particular press. Although the roll width can be varied, the cutoff cannot. Due to this, some sizes just won't work economically for a web press. Most full webs, and half webs are geared for 51/2 x 8 1/2" or 8 1/2 x 11" formats. Due to this, a 9x12" book just wont' work on a web press. So finished size is a major consideration. The second disadvantage of a web is the job setup costs. Makeready of each new form is an expensive proposition since so much paper is consumed just to fit the job and get color. Because of this, webs are usually considered best for "long run" type print jobs. Primarily publications, flyers, coupons, catalogs.

Hope this helps.

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