Offset litho printing
Principle of offset printing (sheet fed). Click to enlarge images.
Offset lithography is widely used for graphical applications. The method is different from other printing techniques because it is indirect. The ink is transferred to the paperboard via a compressible rubber blanket, which adapts to the surface structure, enabling good contact and ink application . With its relatively low pre-press costs, fast make-ready, and high production speed, this process is commercially attractive for a wide range of run lengths.
The two main types of lithographic inks are based on conventional drying oils and UV drying respectively. To finish the printed surface, varnish is often used. Since there are three different types of varnishes for inline or offline applications, the possible combinations enable a large number of printing and varnishing options. To avoid problems and achieve the best result it is always important to consider the interplay between ink, varnish and paperboard surface. The most suitable combination should be chosen in consultation with a skilled printer.
Pros and cons of offset litho printing
- achieves the highest screen ruling of all printing methods
- reproduces the largest colour gamut
- gives superior colour matching in multi-colour presses
- enables superior register between different colours
- is economically viable in a great span of run lengths (about 300 to 60,000 impressions)
- most presses can print a wide range of paperboard grades (from about 0.08 mm to 1 mm thick)
- special presses can handle thick paperboard grades and also large formats (up to 220 x 180 cm).
- reproduces less brilliant metallic colours than e.g. flexography (some offset presses are therefore equipped with a flexo-like printing unit)
- consumes more paper waste during start-up when setting the ink moisture balance
- gives a thin ink layer which often has to be varnished to achieve rub resistance
- the process is sensitive to temperature fluctuations (this can be prevented using a press with temperature controlled rolls).
Key paperboard features
Whiteness, smoothness and surface finish are the most crucial paperboard features for achieving excellent colour reproduction. The exceptional stability of multi-ply paperboard is also important for doing high quality offset printing. So is the flatness of the paperboard, especially when running at high speeds. Compared to other printing methods, offset inks are tackier and therefore demand somewhat higher surface strength.