Sheet Fed Printing – Delivery Units
The delivery unit takes the paper from the printing unit and places it on the delivery pile. There are two common designs for sheetfed press delivery units: gravity and chain gripper systems.
Typeical Press End With a Coater, IR Dryers, spray Powder system and the chain delivery
The gravity system is the simpler of the two but less dependable and functional. As the printed sheet leaves the printing unit, it is dropped into the delivery pile by gravity. The basic limitation is that the paper cannot be delivered faster than gravity can pull it into place. With light weight papers this can pose problems. Today this type of delivery system is found mostly on small inexpensive offset duplicators.
Most modern sheetfed presses today utilize the chain gripper system. With this type of system, the paper can be either pulled through the printing and delivery units by the same chain system or transferred from the paper grippers on the impression cylinder in the printing unit to a different set of grippers on the delivery chain.
As the sheet leaves the printing unit, a set of mechanical fingers or grippers grabs the leading edge of the sheet and pulls it out of the printing system. The gripper bar is attached to a continuous chain that moves the printed sheet to paper pile, releases it, and moves the grippers back to receive another sheet. The chain moves at the same rate and in synchronization with the feeder, registration, and printing units.
Like the feed system, the delivery pile is automatically controlled for proper height. As sheets are stacked, the pile table automatically lowers itself to accommodate new sheets being added. Adjustments to the pile height can be made by the pressmen for different types of paper (board to light-weight paper).
In the delivery section, most sheetfed presses, today, have static eliminators, Infrared dryers, decurlers and a starch powder system. Static eliminators are used to reduce any static the printed sheets may have or accumulated during the printing process. Infrared (IR) dryers Typeical Press End With a Coater, IR Dryers, spray Powder system and the chain delivery are equipped on many presses today to help drive off the solvents in the ink. The IR dryers don’t actually dry the ink but assist in removing the solvents in the ink allowing for the exposed varnishes to react quicker with oxygen. This results in quicker ink drying. IR dyers also are used to help quickly dry aqueous coatings that may be applied over the printed ink. Sheetfed presses can also be equipped with Ultraviolet (UV) coaters. If these types of coaters are used, the press must be equipped with a UV light source to 뱒et?the coating. These type of coating usually are used to give excellent protection to the printed sheet as well a develope high gloss. An example of this type of coating would be on a magazine glossy cover.
The use of decurlers is to reduce any curl the sheet may have which will hinder delivery or post press performance. The use of starch spray powder is done to help, on a microscopic level, separate the printed sheets so that air (oxygen) can reach the ink and begin setting and drying the ink. The heavier the paper weight, the larger the particle size of starch needed. Starch powders come in a variety of particle sizes.