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Rotogravure (or gravure for short) is a type of intaglio printing process, which involves engraving the image onto an image carrier.
The modern day rotogravure printing press uses a printing cylinder which has been laser engraved with minute cells capable of retaining ink, the size and pattern of which reflect the required image. These cells are forced to transfer their ink directly onto the substrate by a combination of pressure and capillary action, so producing the printed image.
The process, also commonly called gravure printing, is used in the manufacturing of food and non-food packaging, as well as labels, magazines, cardboard, wall coverings, transfer printing, and has a variety of further applications in the security printing, industrial, and tobacco segments of industry.
Rotogravure printing is the most important printing techniques for flexible packaging. No matter it is single layer packaging printing, or multi-layer packaging, most of the printing companies are choose rotogravure printing as their first choice. The printing quality is good and the printing speed is very fast which can meet large quantity printing demands.
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The trapping accuracy of rotogravure printing is 0.2mm maximum, which means text and graphics (especially for text ) with strokes less than 0.4mm can only be one color printing instead of being printed by multi-colors such as CMYK or 2 to more spot colors, otherwise it’s easy to cause ghosting phenomenon.
When you need printing for small words/characters and patterns, it should use one color printing. It’s not suitable to use multi – color overlay stencil printing, let alone printing directly with the background color of the photograph. As a matter of fact, stencil printing is not appropriate to apply on small size text and patterns. (strokes less than 0.4mm)
We should also pay attention to the size of words/characters, and the thickness of the strokes.
We might not print it out if a word/character is too small and a stroke too thin. It will cause printing mistakes such as incomplete or missing text.
If the text with is with too small size front, it’s easy to cause a blur. Especially the strokes are big and dense while the lines are thick or the printing plate is too deep.
Normally the thickness of lines should be making no less than 0.2mm; with no compound color for thin lines; and no yellow text and lines on a white background. Likewise, yellow background should include no white font and lines.
In shallow screen printing, the plate cylinder has less and small mesh pits. The mesh dots could easily be blocked by the ink, and the ink cannot be transferred onto the printed material, so that the printed products would appear reverse type.
For reverse printing, shallow net should with no less than 13% mesh dots. If the pattern color area has less than 13% mesh dots, the mesh dots should e removed or increased.
One color printing should be with no less than 15% mesh dots and two-color overprint should be with no less than 12% mesh dots.
For surface printing, mesh dots for paper and foil should not be made less than 30%.
Too shallow the mesh dot is not good for printing ink transfer, even though it works at first. Plate cylinders would wear and tear in the process of printing, thus affect the printing color accuracy and shorten the lifespan of printing plates.
For printing large area of light color we would suggest to use solid color. It’s difficult for screen printing to transfer if the ink colors are gold, silver, and white, so the mesh dots should increased 30%.
In theory, CMYK is ok for all the printing designs. But as a matter of fact, for some sharp colors with high purity; or some special colors such as fluorescent color, transparent color, gold color and silver color; we must use spot colors.
For some very complicated artwork designs, CMYK might be too difficult to print out the need effects, so we will also suggest using spot colors.
The patterns and lines should avoid to be too dense at somewhere vertically. Otherwise it will have ribs when rewinding the roll films (stress concentration phenomenon). The high pressure will cause ink anti-adhesion, and wrinkles after aging due to stress concentration after laminating. The solution is we can reduce the thickness of ink layer by proper screening.
For reverse printing generally it’s printing from dark colors to light ones. For surface printing is the other way around.
It sounds like there’s quite a different between them. But if you take a look at the product you will see the same color sequences: the dark colors always come first in your eyes.
For CMYK, the three primary colors should be printed continuously instead insert any other colors between them, otherwise it’s not good for the three primary color for overprinting.
The principle of rotogravure printing is the color change composed by a number of different size dots on plate cylinder.
When the dots turning from dark tones to shallow screen, the space of each single spot is getting smaller. Due to the impact of ink and dots on the plate cylinder, the color patch will appear some dot – like color-deficiency phenomenon, we called it Crawling. Which is especially obvious in some dark colors such as green、red、blue、brown. It also happens in light colors like yellow, but obvious.
Normally the are range of crawling is around 75% ~ 95%; 80% ~ 90% belongs to quite a severe range. Before confirming the flat screen color in a set of plates, we must pay attention to the flat screen color in this range.
Because different printing equipment have various performances, we should determine the colortrapping according to the trapping accuracy, printing speed, stability of the printing equipment.
For surface printing, with its equipment’s poor trapping accuracy, then the colortrapping should be no less than 0.5mm.
For reverse printing, with its advance equipment and good trapping accuracy, the colortrapping can be around 0.1mm or even no colortrapping needed.
Normally, colortrapping should be controlled between 0.1~0.3mm.
Aim to avoid cutting off the effective pattern, we will widen the size in the process of automatic cutting, pouch-making process.
Normally, enlarge 1~2mm of the printing material would be fine.
But for die cut bags for products such as jelly, beer, label, the film width should be widen 2~3mm.
Normally the printing films should be widen when it comes to the following situations:
1) the unit size on the layout drawing is edged,
2) die-cutting unit bottom has color pattern,
But no width widening needed if the background color is transparent.
Normally for printing, unit pattern are connected repeatedly from end to end or from left to right. The text and pattern at the interface connect naturally and continuously, we called it seamless.
The trapping for seamless plate with more than 2 colors should also be based on trapping line so that to ensure the printing accuracy.
This article is written by Longdapac.
Longdapac is the leading flexible packaging manufacturer from China. We focus on sustainability packaging as you can see below:
Longdapac is the first high barrier MDO-PE film supplier in China.
Longdapac is the first Chinese factory who can make high barrier mono-material PE pouch.
Longdapac recyclable high barrier mono-material PE pouches can meet the recyclability of
Longdapac is a BRC and ISO certified factory with annual capacity of more than 25,000 tons.
Basically Longdapac can satisfy all your recyclable packaging needs, both standard and high barrie