How Waterless Printing Works

( 1 ) The Toray System

	Waterless vs. Conventional Printing Plates

The Toray waterless printing system is comprised of three main components, these are the waterless plate, specially formulated waterless inks and press equipment outfitted with a temperature control system.

The Toray Waterless Plate is based on a laminate design. Aluminum is used as the base material. Light sensitive photopolymer material is bonded to the aluminum and a two micron coating of silicone rubber is applied to the photopolymer.

Depending upon the type of Toray plate used, run lengths for the plates range from 150,000 to more than 600,000 impressions. These run lengths are based on use with a #1 or #2 coated paper. The use of more abrasive stocks will significantly reduce maximum run lengths. Toray waterless plates are recyclable and are not differentiated from conventional aluminum plate material in recycling. Toray plates will fit all popular sheetfed and web presses.

Exposure of the plate is done using conventional vacuum frames and light sources. Exposure times for the plate material is comparable to most conventional plates. Under exposure, UV light, controlled by the film carrier, passes through the silicone layer of the plate and strikes the photopolymer layer beneath. UV exposure activates the photopolymer, causing a break in the bond between the photopolymer and the silicone layers. The photo reaction is very precise, and the plate easily achieves resolutions as fine as six microlines, supporting a dot range from .5% to 99.5% at 175 lines per inch.

After exposure, the plate is ready for processing. Processing equipment for the waterless plate is unique to this system, using specialized chemical and mechanical treatment of the plate. The finished plate now has a non-image area composed of ink repellent silicone. In the image area, the silicone has been removed to expose the ink receptive photopolymer material. This design allows the plate to selectively attract and resist ink without the use of any water, etches or alcohol.

Simple additions can be made to the waterless plate by scratching or scribing the silicone surface to expose the ink receptive layers beneath. Deletions are made with the application of a liquid silicone solution which is used to replace silicone in any area where it has been removed (either by photo-imaging or by scribing).

The main difference between waterless and conventional inks is in the resins or vehicles that are used. Vehicles for waterless inks are selected for their rheological properties and tend to have higher viscosities than resins used in conventional ink systems.

The theory behind waterless printing is that the silicone material which makes up the non-image area of the plate has a very low surface energy. This material will resist ink provided the ink's viscosity is such that it has a greater affinity for itself than it does for the silicone.

One factor that will affect viscosity is temperature. By removing the water from the offset process, there is a loss of it's cooling effect on the surface of the plate. This will tend to cause a primary temperature increase at the plate cylinder due to friction. Because of the higher initial viscosities of the waterless inks there is also a secondary heat generation in the roller train caused by the friction of milling the ink through the rollers.

	A typical press color unit temperature control system. Systems and presses will vary in terms of the number of ink vibrators which receive temperature control.

This heat is why press temperature control systems are required to allow for the precise control of temperature within the printing unit. The most popular systems use vibrator cooling, in which coolant is pumped through hollow core vibrator rollers in the inker. This type of system has been in use in high speed web presses for many years. The technology has been refined and is now being applied to sheetfed equipment as well. Almost all sheetfed press manufacturers offer hollow core ink vibrators which can then accept the after-market temperature control systems.

The function of the temperature control system is to circulate enough coolant through the roller train to carry away the heat that is generated by the mechanical actions in the printing unit.

It is important to note that this type of system is not designed to chill or refrigerate the inker, but merely to maintain it's temperature at a constant throughout the pressrun. By maintaining constant temperature, the viscosity of the inks can be maintained at their optimum levels.

Proofing for waterless printing is handled directly from film. It is important to note that not all analog proofing systems are capable of rending the low dot gain associated with the process. Experienced waterless printers offer highly accurate proofs. Some waterless printers are using carefully calibrated digital proofing systems successfully.

( 2 ) The Presstek System

The Presstek PEARLdry printing system is comprised of two main components, these are the plate and specially formulated waterless inks similar, if not identical, to the inks used with Toray’s waterless plates.

According to Presstek, press equipment outfitted with a temperature control system is optional for the printer. The maximum rated run length capability of the PEARLdry plate is 50,000. With run lengths of less than 10,000 impressions, press temperatures are said not to be a factor. Longer print runs will require some form or press temperature control. The use of more abrasive stocks will reduce maximum run lengths. PEARLdry plates are also recyclable. Maximum plate size is 23.75" by 29.4" (A2/four up).

The PEARLdry plate, like the Toray plate, is made up of a top layer of silicone that repels ink. A middle light absorbing (image forming) layer and an aluminum or polyester base layer that accepts the ink and gives the plate dimensional stability.

	Exposing the printing plate. (obtained from Presstek.com)

Plates are imaged or exposed without the use of film and film processing, photographic plate exposure and processing. Plates are produced with an imaging technique called ablation. An IR-based laser diode array is used to remove the silicone image forming layers from the plate, thereby exposing the base layer of aluminum or polyester which accepts the ink. During imaging, the lasers write image areas by rapidly heating the image layer. The resulting gas expansion causes the image and upper silicone layers to break away (ablate) from the plate. After the plate is imaged, it is wiped down and mounted on the press.

The system is capable of imaging plates on press (two-up A3 format GTO-DI or Quickmaster DI) or off-press using the Presstek PEARLsetter plate imager. Off-press PEARLsetters are available in two-up A3 size (15.75" x 20") or four-up A2 size (23.75" x 29.4"). Plate material for 40" (102 cm) is available for imaging with thermal platesetters.

As with other PostScript based systems, proofing can be handled by a variety of digital proofing options such as the Iris ink jet and the 3M Rainbow.

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