In binding and finishing, a means of binding utilizing an adhesive to hold pages together. Perfect binding, also known as adhesive binding, applies an adhesive to the spine of gathered pages which, when dry, keeps them securely bound. Commonly, a soft paper or paperboard cover (or paperback) is attached over the binding adhesive. Perfect bound publications have rectangular backbones. Publications bound by perfect binding include paperback books, telephone books, catalogs, and magazines. (About 40% of national magazines are perfect bound.)
There are six basic units in a perfect binding line:
'Gatherer'. After printing and folding, book signatures are gathered together in the correct sequence, commonly using gathering machines comprising several pockets which hold individual signatures. At the gathering phase, any separately-printed inserts (such as advertising supplements, subscriber reply cards, etc.) or foldouts also need to be assembled and included. Gathering devices can be rotary or swinging-arm devices. Once signatures have been gathered together, the book-length pile of them is called a book block.
'Backbone Cutter'. In perfect binding—unlike other types of binding such as saddle-stitching or case binding—the signature folds at the spine need to be removed and the pages aligned squarely with each other at the binding edge, which facilitates the effect of the adhesive. After gathering, the book blocks, spine down, are carried by clamps to the binding section. The spine protrudes from the bottom of the clamps, where knives, saws, or shredders cut off or grind away the signature folds. After cutting away the folds, the spine must be roughened, so as to improve the application of the adhesive. In notch binding, large G-inch grooves are cut into the spine. The depth and spacing of the notches depend on the paper type and the composition of the adhesive. In burst binding, perforations are made in the crease of a fold either on press or on the folder. This eliminates the need to actually cut away the signature folds, as the punched holes allow for the penetration of glue. A variety of burst binding is punch perforation, in which slots are cut into the binding edge prior to the last fold.
After spine cutting, the book block is ready for the application of the adhesive.
'Gluer'. While still clamped together, the book block is carried to the gluing station, where applicator wheels force the adhesive onto the spine, a back spinner metering the thickness of the adhesive (which is usually about 0.020 inch). Often, multiple glue pots are used to apply separate layers of different adhesives. Paperback books, for example, have one layer of a low-viscosity hot-melt adhesive applied to bind the pages together, while a layer of a high-viscosity hot-melt adhesive is used to adhere the book block to the cover. Thicker publications, such as metropolitan telephone books or heavy catalogs, use three different types of adhesives.
Most of the adhesives used in perfect binding are hot-melt adhesives, a mixture of resins and polymers which become fluid at high temperatures and dry by cooling back to a solid state. Most hot-melts achieve their best combination of flow characteristics and bonding strength when applied at a temperature between 350:400ºF. In addition to hot-melts, polyvinyl acetate-based adhesives are often used. These do not need to be heated in order to be applied, but require special ovens to dry. They do, however, provide a more flexible spine than do traditional hot-melt adhesives. A third type of adhesive increasing in popularity is a polyurethane reactivate (PUR) adhesive. PUR-bound materials tend to lie flatter than material bound using other adhesives, PUR bindings tend to dry faster, and tend to be more durable. PUR, however, is more expensive and emits toxic vapors when heated.
'Cover Feeder'. After application of the adhesive, the cover is applied to the book block. A feeding mechanism scores the cover where it is to be folded around the book block, and the cover is pressed onto the backbone. Nippers pinch the cover around the spine, while clamps press the front, back, and sides securely around the book block. The bpund book is then dropped onto a conveyor belt where it is sent for trimming.
'Trimmer'. Once the adhesive is cool, the tops of the folded signatures of the book block need to be split, and other trimming around the other sides may also be necessary. Often, three-knife trimmers—located in-line or off-line—can trim all three unbound sides at once. In some cases, binding is done two-up, where two books are bound together as one unit. In this case, the two individual books must be split apart prior to trimming. Some books can be trimmed two-on, or one book on top of another. This is more effective when used with thinner books.
'Counter-stacker'. The final step in the perfect binding process is the counter-stacker, a device which counts the number of individual units coming off the finishing line and stacks them for shipping.
Perfect binding equipment can bind up to 18,000 units an hour, with trimming stages slowing the process down somewhat; three-knife trimmers operate only up to about 6,000 units per hour. Any overflow can be diverted directly to stackers and trimmed off-line.
Despite the name of the process, perfect binding is not truly "perfect." Inflexible adhesives can result in books not lying flat, and the spines of paperback books can often be distorted almost beyond recognition, primarily by sloppy readers. The Swedish textbook manufacturer Otava has invented the "Otabind process" of perfect binding which uses two applications of a quick-drying adhesive along the spine. The binding is reinforced with additional layers of hot-melt adhesive along both sides of the book block, which are topped with crepe paper or cloth, followed by another layer of adhesive to secure the cover. The cover, in turn, has been scored several times, which in effect creates "hinges" which make the spine very flexible.
Perfect bind is also used occasionally in conjunction with case binding, where an adhesive is applied to the spine of a book block after sewing. Many book publishers use the same book blocks for hardcovers and their corresponding trade paperbacks. If there is a significant number of hardcover books left in the warehouse, the trade paperback is produced by stripping off the cloth case and perfect binding a paperback cover onto the book blocks, rather than printing a whole new edition. This is an economical way of producing paperback versions of hardcovers which have not sold as well as had been anticipated, the only drawback being is that corrections or updates to the text cannot be made.
(See Binding and Finishing.)