Printing engineers are frequently involved in heated debate regarding the factors that influence the results achieved by a particular printing method, especially in offset printing. This debate largely centres on the press and the relevant printing process because this is where all the components needed for printing come together. However, the print result also always reflects the interplay between the materials and resources used and the operating personnel. This series of articles will explain the various factors separately and describe them in detail.
The diagram below shows the most important factors. The pressroom climate is an additional factor, but this will not be considered due to the significant regional differences.
Each of these factors has a significant impact on the print result and is directly linked to the other parameters. While the last edition of Gallus In Touch took a closer look at printing plate parameters, this issue will focus on blanket parameters.
Rubber blankets
The diagram below shows the most important blanket parameters that influence print quality and the print result.
Blanket structure:
The structure of a rubber blanket, which must also be able to absorb tangential thrust forces (flexing), has a key impact on its tear strength, stretchability, compressibility and resilience. That's why compressible blankets are normally used.
Sample structure of a UV blanket (Phoenix Ruby Carat)
[Stabilisierungsfasern = Stabilising fibres, Mikrofein geschliffene Oberfläche = Microground surface, Kompressible Schicht = Compressible layer, Nicht dehnbares Gewebe = Non-stretchable fabric]
The weft and warp direction are also important. The warp, which is indicated by lines on the back of the blanket, always runs in the direction of printing
Rubber blanket requirements in UV offset printing:
Blankets are always selected according to the relevant area of application. UV inks are the main inks used in label printing, so the blanket must be resistant to these inks and the associated cleaning fluids. It is also important for suppliers to comply with the specified thickness tolerances. Compressible rubber blankets must have good resilience. Adhesive and release properties are also important for the self-adhesive blankets mainly used on rotary label presses. Last but not least, good print run stability also plays a big role in production.
Ink transfer and printing pressure adjustment
Ink transfer from the printing plate via the blanket to the substrate is a key factor in ensuring good print quality. The blanket's surface texture is particularly important in this respect.
Adequate pressure adjustment between printing plate cylinder and blanket cylinder and between blanket cylinder and impression cylinder (substrate) is essential for good ink transfer. In halftone printing, the deformation/enlargement of screen dots must be minimised. This dot gain can vary depending on the blanket used, as the following printing characteristics show.
FD = Coverage
[Druckkennlinien Gummitücher / 60L/cm / Offsetpapier = Blanket printing characteristics / 60 l/cm / offset paper, %FD Druck = % coverage (print), %FD Daten = % coverage (data), 0% Tonwertzunehme = 0% dot gain, Gummituch X = Blanket X]
Printing situation between printing plate and blanket
The diagrams below show the flexing behaviour in the printing zone between plate cylinder and blanket cylinder. With a compressible blanket, pressure adjustment compresses the compressible layer. With conventional blankets, the rubber layer is compressed on the surface, which creates a rolling bulge. A similar thing happens in the printing zone between blanket cylinder and impression cylinder. When using compressible rubber blankets, the screen dots in the printing zone are subject to the least deformation and also adapt better to the substrate surface. They are therefore not affected by different thicknesses in the substrate web, e.g. glueing points that could compress the blanket.
