Adhesive Binding 101
Adhesive (perfect) binding for soft cover publications is the most widely used form of binding in the print industry. As print shifts from offset to digital, and run lengths shrink to reduce inventories, a whole new generation of perfect binders has been introduced. These machines are built for short- and medium-runs, and can quickly adjust to changes in both the book block and cover sizes.
Hundreds of printers have installed these newer binders, including many digital printers that do not have a long history (and experience) in bindery technology. So they install the machine, fire it up, and begin binding books. But...hold on! It's not called adhesive binding for nothing. This means you have to know something about adhesive technology and proper practice in order to avoid problems.
The most common adhesive used is EVA (ethylene vinyl acetate) hot-melt. There are others, but we'll confine this to EVA at the moment. EVA is heated to a liquid form and applied to the spine of the book block just before the cover is applied. In most binders, there is a side-gluing apparatus which also applies glue to the sides of the book block, just below the scores applied to the cover. EVA is applied "hot' and very quickly cools to a solid right after application.
Lots of operators load up the binder glue pot with hot melt (which starts as a solid) turn on the heater and the machine, and away they go. But...adhesive, and especially application temperature must be carefully monitored. Let's look at the viscosity of the adhesive. Viscosity is approx. 10,000 cps at 350°, but drops to only half (5,000 cps) at 400°. Operators may let the glue pot go "low" before they add more solid hot melt. This will cause temperature variations within the glue pot, and potentially overheat what's left in the pot, causing it to "char."
The amount of adhesive being applied must also be carefully monitored. More is usually not better, and adhesive suppliers issue lots of specifications for application temperature and viscosity range, as well as the amount of adhesive for the application. Side gluing must also be carefully observed. Covers must be longer than the book block.
Side glue is applied either with angled wheels, or from an extrusion gun. Since side gluing uses considerably less glue, the side gluing tank may not get topped-up frequently. This may cause the glue in the side-glue tank to get "cooked" and stringy when applied to the book block spine. The only cure is to change out the glue in the side-gluing tank with fresh adhesive. So ensuring that your adhesive is "fresh" and not over-cooked—as well as careful monitoring of glue pot temperature (which controls viscosity), and application amount—are all critical in producing a quality book.
I want to credit Prof. Emeritus Werner Rebsamen (formerly) of RIT, as well as Andy Mehdyoun of IBIS Bindery Systems. Another great reference suggested by Prof. Rebsamen is "Bookbinding With Adhesives" by Tony Clark, available at Amazon.