Web offset printing machines work based on a complex process that incorporates the speed, accuracy, and efficiency. These machines have been created to print on a continuous roll of paper (called web) and they are common for high volume printing like newspapers, magazines, catalogues, and books. Here is a brief outline of their functioning mechanism:
Paper Feeding (The Web Roll)
The process first starts with a huge roll of paper fixed on the unwind portion of the machine. Tension controls will ensure that the paper flows properly and evenly without wrinkles and without slack.
Plate Cylinder: Image Transfer Begins
The machine has printing units which possess a plate cylinder, which contains a thin metal plate. The image is made using the principle of lithography in which image part is receptive to ink and non-image part repels the ink with a water based solution.
Inking and Dampening System
Dampening system sprays a fine layer of water to the plate. The non-image areas draw the water and prevent the ink from coming in.
Inking system applies oil-based ink. The ink is absorbed by the areas of the image while the water is rejected.
This selective printing is the basis of accurate printing.
Blanket Cylinder: Image Offset
The rubber blanket cylinder is imprinted with the plate’s picture. This is the “offset” portion of offset printing; instead of going straight onto the paper, the image is first transferred to the blanket and then to the paper. This process enhances durability and enhances image quality, especially in rough paper surfaces.
Impression Cylinder: Final Image Transfer
Between the blanket cylinder and the impression cylinder, the paper web passes. The image is then printed on the paper from the blanket in a smooth process, with uniform pressure, well maintained for clear and consistent print.
Multi-Color Printing (CMYK Process)
The web is passed through several printing units for full-color printing, each of the four process colors – Cyan, Magenta, Yellow, and Black (CMYK) are applied to make up the full color. This alignment of these layers is controlled by a system known as registration.
Drying process (heatset web offset only)
In heatset web offset machines, the printed paper gets into the drying ovens where they are subjected to hot air or infrared heat to make the solvents in the ink evaporate very fast. This is very important especially while printing on coated or gloss papers.
Cooling with Chill Rollers
After drying, the process of cooling down the paper by utilizing chill rollers comes in. This holds back curling, drives the ink in, and primes paper for the following process.
Folding and Cutting
The printed web is then folded into parts or signatures after passing through the folding mechanism. It can then be cut, bound or sent for further processing such as stapling, trimming and binding.
Optional Features
Web offset machines of a modern day may have such elements as:
- Automated quality control
- Waste reduction systems
- Color consistency monitoring
- In-line perforation, glueing, or varnishing
In Summary
The operation of a web offset printing machine is an activity which is highly refined and is comprised of the following;
- Continuous paper feed
- Precision ink and water control
- Indirect image transfer
- Fast drying and cooling
- Automated finishing
Such a process makes web offset printing the most appropriate option for cost-effective, high-speed, and high-quality printing for large runs.