Perfect binding: What you need to know
Perfect binding: What you need to know
For many printers, perfect binding is now a key component of their business. If you have a perfect-bound job to complete, or are looking at purchasing a perfect-bind machine, you should understand the basics.
What is perfect binding?
Perfect binding is the process where either flat sheets or signatures (a sheet with multiple pages printed on it) are bound together using a flexible adhesive with a wrap-around cover that creates a square-shaped spine.
The equipment can range from small tabletop machines to million-dollar systems. With the increase in on-demand bookbinding, dozens of affordable machines are now available that include many of the features found previously only on high-production machines.
Perfect binding machines have one common element – a glue applicator. Almost all hot-melt machines use a contact roller or multiple rollers to apply the glue to the spine of book blocks. Then a cover is pinched around the spine and held in place for up to 10 seconds to allow the glue to set. The amount of glue being applied is adjustable. More advanced machines will also apply a side glue to the edges of the book block before adding a cover. Side gluing is important because it binds the edge of the cover to the side of the book block (scored hinge area), allowing the book to open and close properly. This helps to prevent the spine lining and the book’s end sheets from becoming partially unglued from the cover.
In order for the glue to have maximum adhesion, most commercial-grade perfect binders have a grinding wheel that grinds notches into the spine of the book block anywhere from 1-3 mm deep. This allows the glue to flow up into the spine to obtain maximum adhesion. Smaller perfect binders require the operator to place the covers in position, while more automated binders have integrated cover feeders that automatically feed the next cover into position.
The paper and grain direction
The cover and the interior stock should not be more than 100 GSM apart. Although it’s common to attempt to create a feel of quality with a heavier cover stock, this must also be accompanied by a heavier interior book block stock. Grain direction is the final alignment of the paper fibres during the manufacturing process. In perfect binding it’s essential that the grain runs in the same direction or parallel to the spine of the book. If it doesn’t, the book will not open and lay flat correctly, and the additional pressure placed on the spine will increase the chances of warping, sheets falling out – or worse, complete sections of the book block separating.
Hot-melt EVA (Ethylene Vinyl Acetate) glues have long been the staple of the small commercial bookbinder. They’re inexpensive and can be left in the machine and reheated at any time, making them the choice for almost all small-format perfect binders under six pockets. The main limitation to these hot melt glues is their inability to bind some digitally printed and coated stocks reliably. Due to the rapid set-up speed (five to eight seconds), it’s difficult for the glue to penetrate into the fibres of these stocks because the coating or toner acts as a barrier. Without proper penetration into and between the sheets, a perfect-bound book will have a limited ability to stay together. A cold glue or PUR (Polyurethane Reactive Adhesive) adhesive will solve this problem.
Although more expensive, PUR is becoming increasingly popular in binding coated stocks and digitally produced toner output. PUR is different from EVA glues in that it’s dispensed through an applicator head instead of being applied through a traditional roller. This applicator allows the glue to be forced up into the book block (with pressure from a heavy-duty pump) penetrating the spine and creating a much stronger bind. It also prevents the escape of fumes and any premature moisture curing of the PUR adhesive by exposing it to the atmosphere. PUR binding, however, requires up to 24 hours to completely cure.
A third type of adhesive PVA (Polyvinyl Acetate) is applied as a cold liquid with a look and viscosity similar to ordinary white glue. Being cold, the glue has a much longer drying time, allowing it to absorb into the fibres of the paper, facilitating the binding of both heavier and coated-paper stocks. PVA’s are more difficult to apply and consistent chemistry is needed for consistent results. As a result, only large binding lines currently use this adhesive.
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