A hardcover jacket has one job beyond carrying the title: it has to look new on a shelf after a lorry ride, a pallet, and a few hundred people picking it up. That is a film question, not a paper question.
What a jacket actually is
A jacket is a printed sheet wrapped around a case. It is usually 130 to 150 gsm coated paper, printed, then laminated or varnished on the outside, then scored at the spine and the turn-ins and folded by hand or on a small machine.
A sewn book is held by thread that passes through the fold of every section. The machine that does it is a Smyth sewer, and it is the reason a hardcover can be opened flat after eighty years.
What the machine does
Signatures arrive folded, gathered in order, and lie in a stack in the hopper. The sewer opens each signature at the middle, pushes a row of needles through the fold from the inside, and passes a shuttle or a hook to catch the thread on the outside.
A printing plate starts as a coated aluminium sheet and leaves the platesetter carrying an image nobody can see until it is processed. The coating decides how that image is written, and the two coatings in common use behave very differently.
Two wavelengths
A thermal plate is sensitive to infrared at about 830 nm. A violet plate is sensitive to light at about 405 nm, near the blue end of the visible spectrum.
A hardcover book does not leave the bindery with a flat spine. The block is rounded into a curve and the spine edge is backed into a shoulder, and both steps exist for reasons that show up the first time the book is opened.
What rounding does
The sewn or glued block is clamped between two curved plates and rolled, or passed under a rounding roller, until the spine takes a convex curve and the fore-edge takes a matching concave one. The curve is set with glue in the spine, and it holds once the glue dries.
Paper arrives wrapped in a moisture barrier, and the wrap is there for a reason. A sheet of coated paper sitting at 23C and 50% relative humidity holds a thin film of water on its surface, and that film drains electrical charge. Unwrap a pallet in a dry pressroom in February and the same sheet becomes an insulator.
Static builds where two surfaces separate. Every sheet that lifts off the pile at the feeder carries a charge, and the sheet below it takes the opposite one. Below about 40% relative humidity the charge has nowhere to go. The symptoms are easy to read. The feeder pulls two sheets and the second one prints a ghost. A single sheet clings to the feed board and arrives late, so the gripper takes it out of register. The delivery pile leans because each sheet lands charged and repels the one before it.
A tape bound book looks like a perfect bound book from the shelf. The difference is at the spine: instead of a glued cover wrapped around a milled block, a strip of cloth or paper carries the adhesive and wraps a few millimetres onto both covers.
Where the tape goes
The block is jogged and clamped, the spine edge is notched or left square, and a tape coated with hot melt is applied along the edge. The tape is wider than the block is thick, so it folds over onto the front and back cover by 3 to 5 mm.
A case cover comes off the casing-in line flat and comes back from the warehouse bowed. The bow is not a fault of the printing. It is a moisture imbalance built into the boards, the wrap and the paste, and it shows up hours or days later when the two faces of the case lose water at different rates.
A case is a sandwich with two different skins
Greyboard is made from recovered fibre, pressed and dried. It has a machine direction, and it takes up and gives off moisture through both faces. When the board sits in a room at 40 per cent relative humidity, it dries from the outside in, the surface fibres shrink, and the sheet curls toward the dry face.
A hardcover jacket has one job beyond carrying the title: it has to look new on a shelf after a lorry ride, a pallet, and a few hundred pe...
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