The Lightweight Paper Challenge
Printers have had to contend with an entire universe of substrates since Gutenberg. One of the major challenges has been the very lightweight papers used for financial, pharmaceutical, and legal printed pieces. Printing on these is no joy, as they don't have the tensile strength or body of heavier grades. As for the finishing end, work destined for either saddle-stitching or perfect binding is folded into signatures. Either at the back end of an offset web press, or using a buckle folder for sheetfed presses. This gives additional body to light papers so that they can be better handled by both operators and machinery.
When it comes to digital print and lightweight paper, the ante is upped. First, these may be subject to "show through" when running through an inkjet press. Since inkjet ink is absorbed into the paper, the ink may bleed through to the reverse side. The paper companies have attacked this issue by applying a variety of coatings, and the inkjet print manufacturers have also built coating stations onto their presses.
But problems remain in finishing. Finishing vendors have responded by building book block modules that will signaturize the Web, similar to what happens in offset. These can deliver a book block of one, as is often required for digital runs. The last "gotcha" is in saddle-stitching. Digital saddle-stitching machines differ from their offset brothers. The offset saddle machine feeds signatures from multiple feeders on to the saddle, one of top of the other to make a complete book. Most of the digital machines do not handle signatures at all, but collate individual digitally printed sheets at high input rates. The collection of lightweight sheet stock at a high input rate is a challenge because they will collapse at any stop point.
One vendor, IBIS Bindery Systems has solved this problem by adding an inline buckle folder to their Smart-binder digital saddle stitcher. The buckle folder type is specified according to the paper weight. In a recent installation, a Herzog + Hermann M8 folder was used. These buckle folders are widely used for the multiple folds with lightweight paper required for pharmaceutical booklets.
This configuration allowed the Smart-binder to successfully run 20.9-Lb. ground wood directory paper, in-line with a continuous digital press. Folding lightweight sheets gives them the body needed to enter the Smart-binder at very high input speeds.
Another example of digital challenges solved by creative thinking!