Gravure printing

Gravure printing is normally a reel-fed printing method although sheet-fed machines exist in small numbers. Gravure is used mainly for large runs for magazines and directories on thinner paper. However, a significant number of applications are run on paperboard for high volume packaging such as cigarette cartons and large volume confectionery/liquid packaging. The presses used for printing packaging applications are different from the publication presses in two ways: they are narrower and print units are almost exclusively set up in a straight sequence horizontally (magazine printers can have print units set up vertically). The advantages of gravure printing lie in the high, consistent and continuously reproducible print result. Further, the advantage of fast ink drying (by evaporation) contributes to an immediate post-finishing of the printed goods. The process is ideally suited to situations where there is a range of print designs but a constant carton construction and size.

Description of the gravure printing technique

Click to enlarge.

The gravure printing process uses a metal cylinder with the image engraved or etched on to the surface in the form of a pattern with cavities, called “cells”. The gravure cylinder surface varies depending on the engraving or etching system. The surface can be made of either copper protected by a final chrome finish or zinc. The cylinder rotates through an ink pan where the cells pick up ink. The excess ink on non-image areas is scraped off the cylinder by a doctor blade before the ink is transferred directly to the paperboard surface. The inks are based on low viscosity solvents or water. After each ink unit, the ink is dried using high velocity air nozzle dryers in hoods, which are either heated or not depending on the application and ink system.

The transfer of ink from the cylinder can be aided by electro­statically charging the impression roller. This is called Electrostatic Assistance (ESA) and is particularly used when half tones are being printed. However, ESA cannot be used with printing inks that have metallic pigments which are conductive. In this case, the printability properties of the paperboard must be sufficient for printing without ESA. In many cases where metallic inks are used the ESA can be run on the units prior to the metallic ink being applied, for instance in the case of pictorial health warnings on tobacco applications. Similarly, ESA cannot be used on foil/metallised polyester-laminated boards.

The principle of gravure printing. Click to enlarge images.

Gravure in practice

Due to the high volumes and high speed in the gravure printing machines the operation is demanding when it comes to reel presentation. There must be tight tolerance in width, clean edges, well aligned cores and high quality joins (if any joins are present). Issues relating to web tension and dimensional stability are critical for achieving good register in printing and finishing. They therefore require consistent properties with regards to the tensile strength and stiffness of the paperboard. The choice of paperboard will affect the natural abilities for dimensional stability (see the section on flatness and dimensional stability).

The hardness of the cylinder also demands a smooth surface for best contact and ink transfer since the cylinder does not adapt to the surface structure in the same way as, for instance, a rubber blanket in offset printing. The thickness of the paperboard must be consistent. This also means that the gravure method is relatively more sensitive to surface defects like blade lines and indentations than other methods. The failure in ink transfer results in missing dots.

In food packaging, the paperboard’s solvent retention properties affect the speed at which the cartons can be wrapped without having to “air” the pallet to achieve acceptable levels of solvents trapped in the board. Factors governing the degree of solvent retention include the specific ink system used, plus well-managed  and well-controlled dryers that are in good condition.

Freedom from visual defects is clearly of crucial importance and a high proportion of modern presses are equipped with a unit that will scan for print defects (including those caused by the board) and reject defective cartons. Clearly, the paperboard’s freedom from any surface defects, whether visual or physical is a key property.

Key paperboard characteristics

• thickness
• tensile strength
• stiffness
• surface smoothness
• flatness and dimensional stability
• moisture level in the board to control solvent retention.
Finishing in gravure printing
In gravure printing most products are finished up to the point of blanks inline. This means that varnishing, embossing, die-cutting and, at times, foil blocking, is done at the end of the press with rotary or flat bed tooling.

Misregister can occur not only in the printing process itself but also in all subsequent finishing stages.

Overview of crush cut, dual knife cut and the embossing principle.



Principle of three-stage finishing in gravure packaging printing. 
Click to enlarge images.  


Creasing and die-cutting at these high speeds therefore require consistent properties in terms of thickness, tensile strength and plybond in order to avoid variations in crease performance and registration between creases and embossings.

Due to the high tooling costs for rotary converting, the issue of tool wear in die-cutting is an important factor to which the paperboard properties can contribute to some extent. Composition of the coating may provide more or less abrasion and lubrication and the baseboard finish will to some extent also affect the resistance to cutting.

Rotary cutting can be done in two ways: crush cut, in which the knife meets a flat impression cylinder, and dual knife cut, in which two knives separate the paperboard, one from above and one from below. The latter is the more common approach in high volume tobacco applications and has the advantage of minimising the number of adjustments that must be made to the tooling during production, resulting in consistency of creasing, etc.

Rotary conversion also has the advantage that no “nicks” are required to hold the cartons together to take them for­ward to separation of the blanks.

Some web-fed gravure printing presses are fitted with flat-bed cutting and creasing equipment. This is most suitable for plants that print small to medium runs in which the variation of layout/shape size in the process and possibly also the shorter make-ready of the flat-bed technique offers advantages. It is possible to incorporate a foiling unit at this stage: however, this is often done by sheeting the printed web and doing the foiling as a separate process.

Key paperboard characteristics for print finishing

• thickness
• tensile strength
• stiffness
• surface smoothness
• flatness and dimensional stability
• freedom from surface defects.




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