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Offset printing: what it is and how it works

The offset printing is an issue that we hear a lot today in the world of graphic arts and printing online: this, because now it is one of the most common printing methods in the industry. Often, however, the news is confused or can generate perplexity in those looking for information: let's see here what is really meant by offset printing and what its characteristics are.

This word refers to an extremely widespread methodology given its advantages and strengths: to be precise, reference is made to what can be considered the most widespread printing methodology in the world by virtue of strengths such as speed of execution and quality of the final product. Why is offset printing so popular and popular?

 Offset printing: general definition

Let's start with a definition of the concept: the term offset printing refers to a printing process that already existed centuries ago and which, over time, has been widely perfected. To be precise, lithographic printing can be traced back to the 1700s.

Today the machines available have improved considerably and can guarantee high quality performance. The result of this evolution, offset printing is used by many professionals in all areas and is able to create a substantially infinite variety of products quality printing services, from the business card to the catalogs and magazines. The offset printing can be used on any type of paper in sheet or coil reaching run lengths up to 50,000 copies per hour and maintaining the print quality always constant from the first to the last sheet.

 Offset printing is classified as indirect printing in that the graphics printed on the matrix plate are transferred first onto a rubber support and only subsequently printed on the sheet. Factor that differentiates it from direct printing, for example the classic letterpress with movable type.

Why is offset printing defined as indirect and paleographic printing?

The term ' indirect printing ' refers to a process by which the sheet is printed not directly , therefore in contact with the matrix; but through a further passage which involves the use of a special cylinder covered with a light layer of rubber previously counter-molded by the matrix itself to subsequently transfer the image onto the paper support.

In this printing process, the various cylinders that make up the machine, such as the plate - holding cylinder, the rubber cylinder and the pressure cylinder, can guarantee extremely precise work and at high speeds, up to 50 thousand rpm.

 The high precision of offset printing derives from the lack of reliefs (for example the reliefs are typical in movable type and cliché of letterpress printing) on ​​the matrix, for this reason lithographic printing is also called pantographic printing: in this way the typographical inaccuracies due when the die is pressed on the paper they are canceled. To date, the screening on continuous tone images has been able to reach very high levels, going from 20-30 lines / cm of letterpress to 60-80 lines / cm of offset printing.

 How an offset printing machine works

The offset process in printing machines is divided into several phases, each fundamental for the perfect outcome of the final work. The first phase of offset printing is the sheet feeder, with particular reference to single sheet printing (not reel). This first step involves inserting the support on which it will then have to be printed and can be performed by loading more or less paper according to requirements: we will therefore speak of a low or high pile sheet feeder.

 Once the paper is loaded, the sheets must be taken individually to be detached one by one thanks to a particular mechanism that involves the use of suction cups and nozzles. In this phase, electronics are used to prevent two or more sheets from entering the press at the same time, a factor that would compromise the job as well as some mechanical parts of the machinery.

The sheet is advanced by the belts and stops on a plane called the "register plane". A print is perfect when each color exactly overlaps the other: this linearization is called the register. This is why, on the registration plane, the sheet must necessarily be aligned both horizontally (by the clamps) and vertically (by the team). These two supports guarantee both printing precision from one color to another, as well as a perfect cut in the next binding and finishing phase.

 The grinding of inks in offset printing

At this point the print job begins: obviously the inking of the plate (matrix) is missing , a fundamental aspect of the whole process. In this phase we proceed thanks to a group of steel rollers alternating with rubber rollers which, by turning and rubbing together, grind the ink making it more uniform and free of lumps.

It is absolutely important that the ink appears homogeneous in the final draft on the matrix, avoiding making the inking on the plate unstable and uncontrollable, a factor that would have a significantly negative impact on the final quality of the print. Once the entire inking has been completed we begin to enter the most important and central phase of offset printing.

Offset printing: how a sheet is printed

In this second phase, the differences between offset printing and other types of printing emerge even more clearly . Before being inked, the plate is moistened by the wetting rollers with a liquid solution based on water and alcohol. Due to the repulsion effect this allows the ink to fix only on the areas to be printed.

Once the ink has been ground, it passes to the inking rollers, which have the task of uniformly inking the plate. At that point, the inked image on the plate is ready to be imprinted on the rubber cylinder.

Once this particular procedure has been carried out, and after being impressed by the plate cylinder, the rubber cylinder begins to rotate around the pressure cylinder, applying pressure to the sheet itself . The sheet, passing between the two cylinders, is finally printed . This printing phase can be repeated several times depending on the quantity of colors to be obtained in the final result (ex: 4 CMYK colors).

 The advantages of offset printing

The offset printing company in Ghana is the ideal tool to carry out the printing of catalogs , books , magazines , packaging and any other product in the medium and long runs; this also in consideration of the fact that rubber is a material with a rubbery surface and allows printing on any support from classic hand or coated paper, but also on embossed and rough sheets, always obtaining the best results in terms of print quality.

This is why we still tend to prefer offset printing over other types of printing: this approach allows you to obtain a first quality print with a wide range of colors and paints, four-color or Pantone, on any paper support of any thickness up to 500-600 g / m2.

Read also: Company catalog, what it is and how to make it


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