Analysis of gravure and offset printing quality comparison

Because gravure printing and offset printing are different, if you look at the printed matter, it will be very different. The representative differences are as follows:

Field text outline

The field part in the offset printing does not need to have a dot, so the outline of the printed text is as smooth as the bottom plate; however, the gravure printing mode requires an ink groove regardless of the occasion, so the outline of the text is also due to this groove (grooves form independent holes). The existence of a sawtooth. In order to make this serration as small as possible, it is necessary to consider a suitable plate making method.

Field ink movement

This phenomenon is unique when using gravure ink with low viscosity. When the deep ink in the depth of the groove is transferred to a substrate such as paper, a uniform density is not obtained and slight unevenness occurs. As a countermeasure, the viscosity of the ink, the angle of the ink tank (screen angle), the change in the depth and shape of the groove, and the like can be adjusted.

Hue changes

The printing ink is the same, but the hue will be different due to the different thickness of the ink film. In offset printing, the dot size is different. In principle, the thickness of the ink film is constant (strictly speaking, there are differences). However, in the gravure printing method, particularly when the depth of the groove is used to express the layer, the thickness of the ink film also changes due to the depth change, so that the hue change that does not need to be considered in the offset printing occurs with the level change. However, with a constant depth, this phenomenon is difficult to occur. This phenomenon, no matter what kind of method is used for color proofing, the same chromatic aberration problem as the production machine's printing will appear.

The impact of static electricity

Gravure inks require special attention to fire protection due to the use of highly volatile, flammable solvents. Static electricity also has a great influence on the transfer of ink. Due to the electrostatic effect, the ink overflows the image area and hinders the transfer of the ink. However, the use of an electrostatic inking device is very effective for the transfer of ink.

Plate making and proofing

Compared with before, in the production of film masters, there is no need to continuously adjust the bottom version, and the production method is also different from offset printing. In particular, the production of dot bases, proofing can be as easy as offset proofing, and the process is shortened.

In the previous method, a drum for proofing was produced by the same method as that used in the printing on the production machine, and it was printed with a special gravure proofer. However, because of the difference in the printing environment between the proofer and the production machine, the printing quality is also different. Now, in order to get good quality printed products and satisfactory results, there are also proofing on the production machine. If the print quality is satisfactory, use it as an official printing plate.

Recently, platemaking has been digitized, and digital color proofing printers have been mostly used in order to be able to develop to filmless. Publication of gravure platemaking and color proofing The gravure platemaking method was published. From the preparation of the master plate to the imposition process, the plate making process and the quality management of the offset printing were exactly the same. Therefore, the work of gravure printing can be turned into offset printing if it is urgently needed.

Its plate making method, like offset printing, is divided into the method of making the film bottom plate and the method of making it completely using digital plate making data. With color proofing made with film masters, starting from the bottom plate, proofing plates were made with PS plates, and proof printing was done like offset printing. With regard to proofing inks, it is also considered to use the same color inks as gravure inks. However, as described above, if the printing method is different, ink characteristics are different, and it is difficult to achieve the same color reproduction.

In the proofing method using a digital color proofing printer, it is more difficult to maintain the consistency of the color reproduction of the gravure ink. Because printers currently used in the market are almost always aimed at offset printing, the color reproduction range of the used color materials is made to conform to the color reproduction range of the offset printing ink as much as possible even if a color matching technology is used, but it is difficult to achieve a sense of transparency. And the sense of concentration of gravure ink color reproduction. There are also better dye-based ink-jet printers, but in general different from the printing method, and the printed body is based on the printer and uses special paper. Therefore, when selecting a printer, it is necessary to decide based on satisfactory results.

Packaging gravure plates and color proofing

The method of packaging a plate and publishing a plate is basically the same. However, if a plastic film is added to a printed object different from the publication, a spot color ink is often used. Generally, six colors and seven colors are used for printing. Therefore, the plate making method that meets these conditions must be considered in the plate making, which is different from publishing printing.

Packaging prints are usually printed in multiple impositions on the same screen, but a single plate is made in the platemaking, and color proofing is also performed in a single frame. The problem of proofing can be said to be the same as that of publishing, but there are many problems with spot colors. In the printing proofing, the spot color ink can be used to correspond, but a color printer is used, and only the basic ink is used together with the method of expression. In such cases, too high quality cannot be expected.

As described above, in the platemaking software unique to packaging plates, there are (1) multi-format imposition function; (2) no-joint ring-and-band image connection function; (3) spot color separation function; (4) trapping function (corresponding to misalignment during printing, outlines of patterns are slightly overlapped, especially during film printing).

Plate making and upper printing plate

No matter how good the film master and digital plate data are, if the accuracy of the printing roller cannot be guaranteed, the quality of the printed matter cannot be guaranteed. This is considered to be the most stringent in gravure printing in various printing plate formats. The following are the contents of the process and processing from the preparation of the roller to the final production of the printing press.

roller

The substrate of the drum is generally an iron pipe, which is due to its low cost and robustness. Weights range from tens of kilograms (related to packaging) to hundreds of kilograms (related to publishing, building materials), and depending on the application, some are nearly one ton. Recently, aluminum has been used. More recently, plastics have also been used for the manufacture of packaging rolls.

The wall thickness of the iron pipe is from 20mm to 300mm, and the large-diameter drum wall is thicker. No shaft in the drum (for packaging purposes) or with shaft.

Roller copper plating

Regardless of the substrate, copper plating is applied to the surface of the newly manufactured roll. This copper layer is not an engraving and etching layer, but uses this copper plating layer to make the precision of the drum. For example, for cylinders with diameter accuracy within 10 μ and parallelism within 10 μ, the copper plating thickness is 600 μm. In this copper plating layer, the copper layer required for etching and engraving is usually plated, and the thickness is about 80 μm depending on the pattern of the plate. When using the engraving method, hard copper plating is required.

Image formation

After copper plating on the drum, image formation processing was performed on the copper surface. The processing methods include: (1) When a resist film (carbon paper, photosensitive resin, black paint) is used, the resist is transferred or applied onto the copper-plated surface. Image copying is performed on carbon paper or photosensitive resin, an image is formed on a high-sensitivity resin and black paint by a laser, and then chemical etching is performed by using a ferric chloride or the like through a resist film; (2) an engraving method. Engraved directly on the copper surface. Alloys and resin layers have been used instead of copper plating to form images using lasers, but most of them are under development.

The depth of the formed ink groove is generally from several μ to 60μ. The density of the grooves is from 2500/cm2 to 10000/cm2. Laser can be used to make higher density.

Chrome plating

After the ink tank is formed on the copper plating surface, it can be used as a printing roller. However, the ink falling on the outside of the groove during the printing easily scratches the copper surface by the scraper. Therefore, chromium is plated on the copper surface (about 10 μm). After this series of processes, the intaglio printing cylinder was made.

postscript

Gravure printing technology has indeed evolved to high efficiency and high productivity in the prepress digital environment. In recent years, full automation of roller production has become a research topic. At this point, depending on how the ink tank is formed, the development is not the same. The introduction of a chemically etched laser provides excellent detail reproduction, but chemical etching may no longer be used in the future.

In this regard, the engraving method does not have a corrosion process, but it is inferior to the laser method in reproduction and productivity of details. As various ways are still under development, this is also the hope for the future.