Offset printing is the process most commonly used in commercial printing. In addition, offset printing is the standard most used in the printing industry for the ability to print in high quality, giving the final product a superior presentation.
Offset printing is a process that consists of the interaction between water and fat (the offset ink is of greasy consistency). The offset printing process is indirect, that is, the image is transferred from the matrix to a printing roll (blanket) and only then is passed to the paper. Therefore the matrix (offset sheet) is readable even before printing, unlike the direct processes where the matrix is mirrored (inverted written text).
The offset printing process:
Offset printing passes through basic processes, which are standard in the printing industry. Small variations or order may occur depending on industry, type of equipment, etc. The offset printing steps are: matrix recording, assembly, printing.
Recording of the offset plate:
The offset printing matrix is usually metallic (made of aluminum) and sensitive to light. In the traditional process, the recording starts from a photolitho (which can be positive or negative) that is placed on the offset plate and exposed to light for image fixation. After exposure, the matrix is referred to chemical development, similar to the photographic development process.
With the offset plate revealed, the offset roller for the installation of the matrix is mounted on the roll. The flexible plate is mounted on the cylinder intended for the die. If the form is colored, a plate is required for each color of the chromia process, that is, at least 4 plates will be used.
In both flat and rotary offset printers, the system works the same way. The difference between them is paper input, which can be done in reels or sheet by sheet. Offset cylinders drive ink and paper in a linear flow. As already mentioned, printing is done indirectly. The ink is transferred to the matrix and, although the matrix is completely smooth, the pigment adheres only to the engraved area, since the water used in the process repels the paint. The die transfers the ink to the rubber blanket cylinder, and the cylinder transfers the image to be printed onto the final media, which may be paper or plastic (depending on the equipment). The indirect printing process aims to keep the paper dry and also extend the life of the matrix.
There are several types of offset printing equipment. The most common are flat offset printers, which print sheet by sheet and are widely used in the commercial printing segment; and rotary offset printers that print from paper reels and, because of their configuration, allows high print speeds.