Letterpress Label Printing
Rotary letterpress roller assembly
Letterpress is one of the oldest printing processes and has evolved significantly through the years with many variations existing today. In its earliest form, letterpress printing was accomplished by means of a flatbed press. Individual wooden, and later metal type was manually set into a tray to form a mirror image of the final printed piece. It is from this process that the term "typesetting" is derived. Paper was then pressed onto the tray of lettering by a clamping action, hence the term "letterpress". Today, most letterpresses are rotary and web fed. Letterpress remains one of the key label printing technologies as a result of its exceptional quality and its efficient in-line process capability.
LGI's Letterpress Printing Process
Like flexography, letterpress is a rotary relief printing process, however, letterpress is an intermittent rather than a continuous feed process. In a continuous roll process like flexo, plate cylinders must be the same diameter as the plate length. In an intermittent process like letterpress, the plate cylinder is always the same diameter no matter how long the repeat (plate length) is. The substrate passes over the plate then re-inserts to the starting position to achieve a continuous print.
The printed image is achieved by means of a semi rigid plate which contains the graphics and text to be printed in relief, much like a rubber stamp. The plate is adhered to a flat rigid surface on some presses and on a cylinder on others. Each color of ink to be applied must have its own plate and inking assembly.
Each color station contains a shallow tray of ink which passes to inking cylinders and is then applied to the plate. The first cylinder is called the fountain cylinder which draws the ink from the ink try in a thick mass. The fountain cylinder passes the ink to the transfer cylinder where it passes through a series of metering rollers which thin the ink to an even layer before transferring it to the plate cylinder. The image is transferred to the substrate as it passes over the plate by means of pressure.