Printing techniques for packaging

Various  techniques  are used  by packaging material  manufacturers for packaging . Once the art work  
is approved  the packaging material supplier works on printing  that art work on  packaging material  it
could be  carton boxes , Laminates or wrappers  , Display boxes  etc .


Off set Lithography







The function of the original stone printing surface is now served by thin aluminum plates, although other
materials, such as stainless steel and plastic, can also be used. The plates are wrapped around the
circumference of the printing cylinder and make direct contact with the rubber blanket cylinder. Rubber
rollers carry ink and water to the plate surface. The ink is transferred first to the blanket cylinder and then
to the paper.

Lithographic plates are the least expensive printing surfaces available today, and this fact has
contributed greatly to the success of the process. Aluminum plate materials have a thin surface coating of
light-sensitive material, such as a photopolymer, that undergoes a solubility change when exposed to an
intense source of blue and ultraviolet light. Images are transferred to the surface by exposing the plate
through a film positive or negative (see Photography). Some materials can be exposed directly, as in a
graphic-arts camera or by a computer-controlled laser beam, thereby eliminating the expense of film and
speeding up the platemaking process.

Modern offset lithographic presses range in size from small sheet-fed duplicators—used for small, single-
color jobs such as brochures and newsletters—to massive web presses capable of printing millions of
copies of magazines, catalogs, mailing pieces, and packaging materials in full color. No other process has
such a broad range of applications.




Flexo graphy






The soft plates and highly fluid inks used in flexography make the process ideal for printing on nonporous
materials such as foil laminates and polyethylene. Originally, all flexographic plates were made of molded
rubber, which is still the preferred material when multiple copies of the same image are needed on a
single printing cylinder. Rubber plate molds are impressions of original relief surfaces, such as type forms
or engravings, and are normally used to make several duplicate rubber plates. The preparation of a
printing cylinder using molded rubber plates is a time-consuming process because many rubber plates
are mounted on a single cylinder and each plate must be carefully positioned in relation to the others.

In the 1970s photopolymer plate materials were introduced, and the time required to manufacture and
mount a set of plates was reduced significantly. This has allowed the process to enter new markets, most
notably newspaper printing. In addition, water-based inks can be used in flexography, eliminating the
need for toxic solvents.

Flexographic printing presses are simple in design because the fluid ink is easily distributed to the
printing surface without an elaborate inking system. Printing is usually done on rolls or webs of substrate
rather than on cut sheets, and the printed rolls are then converted into finished products in a separate
manufacturing process.


Rotogravure  printing








Gravure, also called rotogravure, is a high-volume printing process employing an ink transfer mechanism
that is fundamentally different from that of relief printing. The printing surface is a polished metal cylinder
covered with an array of tiny recesses, or cells (as many as 50,000 per sq in), that constitute the images
to be printed. The cylinder, which can be 2.5 m (8 ft) or more in length, is partially immersed in a reservoir
of solvent-based fluid ink. As the cylinder rotates, it is bathed in ink. A steel blade called a doctor blade
running the entire length of the cylinder wipes the ink from the polished surface, leaving ink only in the
cells. The ink is then transferred immediately to a moving web of paper forced against the cylinder under
great pressure.

Gravure cylinders are constructed of steel with a thin surface layer of electroplated copper. The copper
can be either chemically etched or electronically engraved to form the cells that will transfer ink. Once the
cells have been created, the cylinder is electroplated with a thin layer of chromium to produce a hard
surface for the doctor blade. Each cell transfers a tiny spot of ink to the paper. The cells can be made to
vary in depth from one part of a cylinder to another, causing the darkness of the resulting ink spots to
vary also. This enables gravure to print a wide range of gray tones and thus to render excellent
reproductions of photographic originals.










Color printing is accomplished by using separate printing cylinders for the cyan, magenta, yellow, and
black inks. Each cylinder is housed in a separate printing unit. The web is transported by rollers from unit
to unit and can reach speeds of close to 900 m (3000 ft) per minute. After each color is printed, the web
passes through a dryer, where the solvent base of the ink is evaporated. The solvent is either reclaimed
or burned to produce energy. Some gravure printers have begun to use water-based inks. This trend is
likely to continue because of health and environmental threats posed by the use of hydrocarbon-based
solvents.

The expense of manufacturing a set of gravure cylinders has restricted its use to long-run jobs (millions
of reproductions). Mass-circulation monthly magazines, mail-order catalogs, and packaging are natural
markets for the process. Gravure is also used to reproduce a variety of textures and patterns on
decorative materials. Computer-controlled electronic engraving machines have reduced the time required
to prepare a set of cylinders, but they are still far more expensive than lithographic printing surfaces.


Reference
Frank Cost, M.S.
Assistant Professor and Coordinator, Printing and Applied Computer Science Program, School of
Printing, Rochester Institute of Technology.
Further Reading

"Printing Techniques," Microsoft® Encarta® Online Encyclopedia 2009
http://encarta.msn.com © 1997-2009 Microsoft Corporation. All Rights Reserved.
















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Printing  Techniques  For Packaging
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