Curing of heat sensitive materials

Package Printing & Converting International magazine
May/June Issue 2008 – Feature: Inks & UV Curing of Heat Sensitive Films.

Radiated and conducted heat can distort films in both the x and y directions during the printing process. This distortion can then go on to cause problems throughout the production cycle and in particular affect registration and die cutting. UV curing systems manufacturer, GEW (EC) Ltd. offers a number of ways to manage heat generated during the curing of inks and coatings on filmic substrates. This can be done electronically, through precisely managing the UV power output; optically, using “cold” reflectors and filters to reduce the amount of infrared (heat) radiation incident on the web; or physically, by integrating air- or water-cooled hardware to cool and support the substrate itself. These solutions can be specified for integration on a new press purchase or fitted as an upgrade to existing machines.

Lamp Power Control
Heat issues are often resolved by the press operator adjusting the lamp power so that the ink is cured without excessive heat build up in the web. GEW have developed a simple touch screen control panel (HMI) which is graphical, simple to learn and easy to use. Once set up, the HMI has its own memory to record the precise settings at which a job was successfully run, making it very quick to re-run a job with the same UV settings.

GEW have introduced across their range of products e-Bricks, highly efficient electronic power supplies that boost UV output with a lower energy input. Their precise, stepless power control and increased UV output further assist printers in processing heat sensitive materials.

A critical part of any UV system is the reflector. Materials, surface roughness and geometry all combine to form reflectors with different characteristics. Glass dichroic reflectors are ideally suited to printing UV flexo where excellent cure with moderate heat output is required. Highly polished glass is coated with dichroic material to give outstanding reflectivity and long life. The maximum UV output is achieved using dichroic-coated polished aluminium reflectors. Such reflectors are ideal for thick ink coatings such as silkscreen and inkjet, as well as for cationic inks and where high press speeds are required. The company’s CC ‘Cool Cure’ cassette (Fig. 1) features a semi-focussed, glass dichroic reflector with high UV, low IR reflectivity to optimise cool UV curing. A rotating shutter automatically insulates the substrate from the heat when the press stops. The CC’s design is ideally suited to UV flexo, offset, and letterpress printing on a wide range of substrates including synthetic materials and thermal papers.

Cold Filters
Cold filters (Fig. 1) are designed to protect the more sensitive substrate from direct infra-red radiation. They also protect the UV lamps and reflector assembly from dust and other contaminants, particularly when the lamphead is inverted. The cold filter is manufactured from material that is naturally transparent to UV, but reflective to infrared radiation.

Film Adaptation
In 2003, GEW launched VCP Film, a low drag, water-cooled chill roller built into the UV lamphead. By wrapping the heat-sensitive material around a water-cooled roller as it passes under the UV lamp, any heat build-up or web distortion is eliminated (Fig. 2). Unsupported film down to 30 microns can be processed with virtually no temperature rise through the dryer. Web temperatures can also be elevated under control, which is important when printing with cationic inks. Additionally, the VCP Film chill rollers have a low inertia, low friction design which means they do not need to be mechanically driven.
Waterless Chill Rollers

A recent development from GEW is AIRFilm, an air-cooled version of VCP Film which enables processing of a wide range of heat-sensitive materials without the need for water-cooled rollers. The AIRFilm system allows the majority of films to be printed without distortion and provides traditional label printers with the opportunity to enter new market areas in film packaging. AIRFilm incorporates static heat conductive rollers immediately adjacent to the UV lamp-head on the press where air is directed through the inner cores of the roller, removing the heat. AIRFilm offers an alternative for some film applications that does not require the installation of plumbing, pumps and a refrigeration unit.

Water-Cooled Heatsink
The water-cooled heatsink (Fig. 3) is designed to protect heat sensitive substrates in the 25 – 30 micron range. The heatsink is fitted behind the moving web inside the standard UV lamphead and is effective during slower printing runs or when frequently stopping and starting. For both water-cooled solutions, cooling water can be supplied from an existing factory supply or from a stand- alone chiller unit.

There are very many alternative methods of curing on heat sensitive material, the use of Inert Gas Curing further extends the capability to print and cure on heat sensitive materials. New technologies such as LEDs and even lasers may have a part to play in years to come.