Gravure printing is a highly versatile and efficient printing process that is widely used for various applications, including magazines, packaging, postage stamps, and more.
In gravure printing, the image is engraved onto a cylinder in the form of cells, which are then filled with ink.
This detailed article will delve into the intricacies of the gravure printing process, covering the printing unit, inking system, doctor blades, impression rolls, drying systems, gravure cylinder preparation, halftone processes, press control systems, substrates, inks, and applications.
The Printing Unit
The heart of a gravure press is its printing unit, which consists of several key components:- Ink Duct : The etched cylinder rotates in a solvent-based ink in the ink duct.
- Doctor Blade : A metal blade reciprocates across the cylinder's surface to remove excess ink, ensuring only the cells contain ink.
- Impression Roller : This rubber-covered roller supplies the pressure needed to transfer ink from the cells to the substrate.
- Substrate Feed : The substrate is fed from reels into the nip between the engraved cylinder and the impression roller.
- Drying System : After printing, the substrate passes through a heated drying system where solvents evaporate, and the ink dries. Each printing unit typically has its own integral drying equipment.
The Inking System
The choice of inking system is crucial in gravure printing. Various methods are employed:- Ink Trough : The simplest method involves placing the cylinder in an ink trough. However, it has limitations, such as poor ink agitation and unsuitability for high speeds.
- Transfer Rollers : These rollers, not in direct contact with the cylinder, are used in some gravure machines printing from plates.
- Enclosed Duct Designs : Introduced in 1924, these designs allowed the use of more volatile solvents, leading to increased press speeds.
- Weir System : This refinement of enclosed duct designs enables even higher press speeds by utilizing a circulation pump for continuous agitation.
- Spray System : Ink is sprayed onto the cylinder, preventing it from drying out, and is suitable for high speeds.
- Upward Ink Flow : Some systems pump ink upwards through an open-mouthed jet directly under the doctor blade, suitable for speeds of up to 25,000 revolutions per hour.
There are variations and combinations of these systems to optimize ink application and control.
Doctor Blades
Doctor blades play a critical role in gravure printing by removing surplus ink from the cylinder's surface, leaving ink only in the cells. Key points about doctor blades:- Blades are typically made from high-carbon flexible steel, approximately 0.15–0.25 mm thick.
- Backing blades provide extra support to doctor blades, typically 0.76 mm thick.
- Blades can be sharpened by hand or using a special grinding machine.
- Blade angles and pressure exerted on the doctor blade influence the ink left in the cells.
- Doctor blades may be adjusted using screws, air pressure, weights, or springs.
- High-speed presses may use pre-doctoring systems for improved results and reduced cylinder wear.
The Impression Roll
The impression roll is a critical component in gravure printing:- It has a steel core with a rubber covering, 12–20 mm thick, typically made of relatively hard rubber (60–100 Shore A).
- Roll hardness varies depending on the compressibility of the substrate.
- Flexible rolls may be used to even out pressure across the width of the web.
- Electrostatically assisted ink transfer can be employed to overcome speckling issues on rough papers.
- The electrically conductive rubber in the impression roll encourages ink transfer even with imperfect contact.
Drying System
Drying is essential in gravure printing to ensure the ink is set properly. Various drying systems are used:- Slow-speed sheet-fed machines may use cold air blowers or extended delivery transports.
- High-speed web machines require more sophisticated dryers between each unit.
- Web tension control is crucial in preventing solvent carryover.
- Solvent recovery systems are common to capture and reuse solvents from drying processes.
Press Configurations
Gravure printing is predominantly used for web-fed presses, which offer advantages for long runs and in-line finishing.Various equipment configurations are used, including manual and automatic splicing, input feed control systems, folding equipment, rewinding systems, and more.
Sheet-fed gravure presses are also used for certain applications.
Gravure Cylinder Preparation
The gravure cylinder is a critical component, and its preparation is meticulous:- Typically made from a steel tube, it is copper-plated through electrolytic deposition.
- Copper plating involves careful control of bath constituents, current, and anode configuration.
- The base cylinder is often reused, either by turning off the image and regrowing the cylinder or using the Ballard skin technique.
- Achieving a consistent deposit with uniform properties is vital for mechanical engraving.
Halftone Processes
Gravure printing can employ various halftone processes to achieve different effects:- Conventional etching is capable of producing fine results but is challenging to control.
- Double positive halftone gravure uses a screened positive along with a continuous tone positive, resulting in cells with varying depth and area.
- Mechanical engraving, such as the Hell organization's technology, uses diamond stylus engraving to create cells with an inverted pyramid shape, varying in depth and area.
- Laser engraving and other methods have been explored but are less widely adopted.
Press Control Systems
To maintain stable and efficient printing operations, gravure presses are equipped with various control systems:- Viscosity control is crucial, especially for high-speed presses, to ensure proper ink transfer.
- Web tension control ensures consistent substrate movement through the press.
- Solvent recovery systems are used to capture and reuse solvents from drying processes.
- Register control systems help maintain precise alignment of colors during printing.
Substrates and Inks
Gravure printing utilizes very fluid solvent-based inks that dry through solvent evaporation. Key considerations include:- Solvent-based inks offer rapid drying, making them suitable for non-absorbent materials like plastic films and metal foils.
- Water-based and UV-cured inks are being explored due to environmental concerns over solvent emissions.
- Ink properties are chosen to match various substrate surfaces.
Applications
Gravure printing finds applications across various industries:- Magazines : Gravure is commonly used for magazine printing due to its high-quality color reproduction and low waste.
- Packaging : Many cartons, plastic films, and metal foils for packaging are printed using gravure due to its in-line finishing capabilities.
- Specialized Printing : Gravure is also used for postage stamps, reproductions of oil paintings, and telephone directories.
- Competition with Heat-Set Web-Offset : In the field of magazine printing and similar applications, there is competition between gravure and heat-set web-offset printing, with factors like run length, print quality, consistency, and waste influencing the choice.
Conclusion The gravure printing process is a versatile and efficient method for achieving high-quality prints in various applications.
With precise control systems and ongoing advancements in technology, gravure printing remains a competitive choice for long-run projects, especially when consistency and print quality are paramount.
As environmental concerns drive ink formulation changes, the industry continues to adapt to more sustainable practices.