How are the plates for offset printing?

The premium quality offered by offset technology starts with the preparation of the aluminium plates where the original image is to be transferred

One of the reasons why offset technology provides such high quality is because of the plates onto which the image is transferred, and then printed on the substrate. This is an indirect printing system, as the substrate does not come into contact with the plates; the image transfer is carried out by means of rollers, thanks to the chemical treatment to which the matrix plate is subjected. Let’s get to know this process:

Offset printing is monochrome; each plate only transfers ink of one colour, so to print an original four-colour image, four plates are needed: one for cyan, one for magenta, one for yellow, and the plate that will transfer the black (CMYK colour process). This process provides a colour rendering very close to the actual image. You can also follow the Pantone model, which offers a very wide range of shades. Each colour is identified with a code and can be transferred to the plate so that the print reproduces it exactly. In this case, you do not have to prepare four plates, but as many as you need to reproduce each Pantone colour.

How are offset printing plates made?

The plates consist of a base and an emulsion. The base can be made of various materials, but the most common is aluminium, because it offers an optimum combination of quality, strength, and malleability. The emulsion is a very thin, lipophilic, photosensitive layer, i.e. it retains the ink.

During the plate manufacturing process, the aluminium is subjected to physical and chemical treatments that give it the necessary qualities to transfer the images to the printing rollers. These treatments include graining, which is currently usually carried out by anodising, an electrochemical treatment with subsequent oxidation. In this way, among other things, the plate can retain the water in the hydrophilic areas, where the ink must not adhere.

The plates are very thin —the ones used in ROTATEK machines measure between 0.15 and 0.2 mm— which allows them to be easily mounted on the press cylinder. The original image is transferred to them by photochemical, photomechanical or laser engraving processes.

¿How is the image transferred to the substrate??

The emulsion layer is barely a micron thick. It is usually made of a photopolymer which, under the effect of light, takes up the ink and fixes the image on the surface.

Actinic light (which includes UV rays in its spectrum) acts on the emulsion depending on its type and structure. Thus, when developing the printing plate, there are two possible photochemical reactions: 

  • Hardening of the light-sensitive layer (negative plates).
  • Decomposition of the light sensitive layer (positive plates).

For positive plate production with conventional printing plates, a positive film is used, i.e. the opaque and blackened sections of the film match the ink-accepting surface elements of the plate.

For negative plate production, a negative film is used, i.e. the ink-accepting areas of the printing plate matches the translucent and light areas of the film.

To ensure quality during the platemaking process, control elements are copied onto the plates.

Waterless printing plates

The waterless printing plate uses an ink-resistant silicone coating to avoid the use of a dampening solution. The silicone layer masks the area where the image is not to be fixed, while leaving free the area where the image is to be reproduced, so that when the receptive photopolymer is developed, the ink avoids the silicone and accumulates in the image areas. When the plate is coupled to the press cylinder, the image is transferred to the blanket and from there to the substrate.

Plates that do not require dampening allow the printed dot to be more securely fixed. This avoids unwanted ink movement and minimises dot gain to create a sharper image, in ultra-fine resolution, with improved solid ink density and less colour variations throughout the print run. This leads to better contrast and more vibrant colours.

Registration is improved because the paper is not stretched by the dampening solution. It is also a more environmentally friendly printing process, because it reduces the need for chemicals as well as emissions of volatile organic compounds (VOCs). In addition, paper waste is also reduced, as makereadies are faster and more accurate.

ROTATEK rotary and semi-rotary presses are designed to maximise the advantages of offset technology. We invite you to discover them. We invite you to discover them.