Understanding the Differences Between Digital and Offset Finishing
One of the interesting things that I run into in my travels in the commercial (and digital) print world are the differences between finishing in offset and digital. Both processes differ from each other, and it’s vital to have an understanding of the digital finishing process when planning a digital print purchase or installation.
Offset finishing begins with the process of creating signatures. On offset web presses, a folder on the end of the press will “signaturize” the web by folding it into 8-, 16-, 32- or even 64-page signatures (depending on the web width and imposition size). The signatures sections of the book or magazine are collected on skids as “WIP” or work-in-progress, then transported to the binder or saddle stitcher. The offset process has a number of advantages over digital. First, offset presses run WAY faster than their digital counterparts - the top-of-line models from Goss or KBA can produce 20 finished magazines or newspapers every second.
And offset finishing is also fast. Because you’re loading discreet sections of media into a perfect binder or saddle stitcher (with non-variable content), the machine is only limited by how fast the individual feeders, or “pockets” can cycle. The latest generation of perfect binders and saddle stitchers can run between 18,000 - 22,000 products per hour. There’s nothing in the digital finishing world that can reach even half of those speeds. So, for those long, long runs, offset shines.
But ... that’s where offset's advantage ends. In the new world of short-run and on-demand, it’s digital all the way. In digital finishing, there are special considerations. Like offset, the base product is created from a digitally-printed web. That’s where a host of postpress systems come in. Depending on your application, they will simply produce a stack, or folded and glued “signatures.” The difference in digital is that stack is a complete book block. For digital saddle stitching, the web is sheeted into four-page single, or folded eight-page sheets, then fed into the stitcher at very high speeds. These book blocks or sheets for saddle stitching can all be personalized and consist of different page counts within a run.
There are some obstacles, however. Digital book blocks consisting of single sheets may not jog down in a perfect binder the way that signatures do. If the binder operator is not careful, a sheet of two pages may hang up, not drop down and be bound properly. The fusing chemicals used in some toner presses and deposited on the paper also refuse to bond with standard hot-melt adhesives. This created a whole new market for PUR adhesives, which are now widely used in bookbinding.
Designing the ideal digital finishing module requires a lot of analysis of both the postpress web handling systems and the various binding and stitching systems. Will an in-line (connected to the digital press) or near-line configuration be most productive and efficient for you? Or do you simply want to create book blocks or cut sheets to feed a variety of finishers?
For offset veterans, this may be a whole new world. I’ve been at it for 10 years now and the constant introduction of new print and postpress technology has forced me to be a constant student of new application possibilities. I’m not complaining though. I’m hoping this is keeping my brain in relatively decent shape!