Binding - the last link
Binding is the last link in the chain of operations which transform the paperboard into attractive and functional covers for brochures, annual reports, manuals, books, or magazines. This last link must be as strong as the others, so it is equally important to choose of paperboard product that suits the binding methods used.
The most common binding methods for paperboard covers are saddle stitching, wire binding, glue binding, thread binding, and fadensiegel binding. In all of them, the fibre direction in the binding operation is a key factor. Please note that the folds of the cover (as well as the insert) must always be parallel to the fibre direction of the paperboard. This is necessary to achieve a durable bond, narrow and permanent fold lines and low folding resistance, and to avoid waviness of the binding. Thanks to its long and strong fibres, a fully bleached paperboard gives the best results under difficult binding conditions.
In this section we will concentrate on the gluing operation. But first we will just say something about trimming. To ensure a good cutting result without risking delamination of the edges, parameters to be closely controlled are the pressure on the book, the knife angle, sharpness and lubrication, and the wear of the counterpart.
With well controlled trimming you avoid delamination of the edges. Click to enlarge.
The gluing operation
Gluing is not difficult but negligence in performing it can be costly. Either you use glue binding (in which the sheets are folded, bundled, and then milled to adhere better), or if you use thread or fadensiegel binding (in which the sheets are stitched, folded and bundled). The insert is then glued directly to the back of the cover.
Types of surface
The type of glue and results of the gluing process depend on the type of surface to be glued.
We can classify surfaces into three different types:
- easy surface – uncoated or lightly coated surface
- demanding surface – fully coated, printed surface
- difficult surface – UV varnished, film laminated or PE coated surface
Types of glue
The most common types of glue for bookbinding are cold glues:
- polyvinyl acetate (PVA) for easy surfaces
- ethylvinyl acrylate (EVA) for easy, demanding or difficult surfaces
- co-polymers for difficult surfaces.
Important factors to consider when gluing are:
- the glue must wet the substrate and adhere to it
- the glue must be applied in the right place
- there must be enough glue to form a good bond but not too much so that it squeezes out
- the time between glue application and joining the surfaces must not be too long
- the pressure must be maintained until the bond is strong enough
- the glue must meet the demands of the binding machine regarding correct temperatures and clamping time before the book is removed from the machine
- the folding resistance in the spine creases must be low to avoid failure of the side glue seams after clamping.
Side seam gluing
Side seam gluing is mostly used in packaging applications but also for CD covers. In this method the glue is applied to the first paperboard surface with an applicator. The glue wets the surface and starts setting. The second paperboard surface is applied under pressure and the glue bond starts to develop. If the glue is water based the paperboard first absorbs the water, enabling the glue to set.
The paperboard properties generally required to achieve successful binding are uniform strength, consistency of flatness and stability, together with good cutting, creasing, folding, and (when applicable) surface properties suitable for gluing. When using saddle stitching or wire binding, strength is especially important. Saddle stitched operations put a lot of strain on that small piece of paperboard that holds the cover to the insert by the staple. When glue binding, a paperboard with a lightly coated reverse side is the most suitable choice.
Otherwise special precautions when choosing the
glue are required, as mentioned above.
The production of almost any graphical product is carried out in more than one operation. The most common situation is the combination of printing, creasing and folding. However, some products are more complex and demanding than others and can sometimes undergo all possible finishing stages after being printed.
Implementing a complex and demanding design is far from being an easy process. It requires skill plus a paperboard that provides perfect register in multi-step processing, that is, a paperboard with extremely good dimensional stability and exact sheet squareness. To arrive at an excellent result, it is of the utmost importance that the paperboard retains its original moisture characteristics throughout the whole sequence of production steps from start to finish. Otherwise the sheets may deviate from their original stability and exactness (please refer to the chapter “Handling” ).
Like we said before, paperboard is a forgiving base material. This becomes particularly obvious when it comes to the finishing operations. Below the strong, smooth and white surface you find all the strengths you require for carrying out the most demanding applications. Whether you are just adding a varnish to highlight the graphic presentation or producing the most advanced designs in multi-step processes, you can rely on obtaining excellent results.
A complex and demanding design requires skill and a paperboard that provides perfect
register in multi-step processing.