SCREEN Product decoration
Digital printing inks are designed for particular applications, including an increasing number of textiles.
Sublimation inks are well developed but many of the digital inks do not provide the range of adhesion and resistance
achievable with screen and pad printing inks or the hot foils with their protective layers. Digital printing is excellent for short run attractive graphics where survival of the print in difficult working environments is not crucial to their success. Inks are improving, but an ink's suitability for the jetting mechanism can compromise its long term operational resistance. For the specialist product printer, digital is a useful add-on, but is unlikely to become their core production process.
The other aspect of product printing that is often ignored, is packing and unpacking. Product is often supplied in cardboard boxes, strapped to pallets, bound in adhesive tape and often shrink wrapped. Cutting open and unwrapping creates dust and a static charge, both of which are an anathema to efficient print production. Once printed and dried, items then have to be repacked. This unpacking and repacking often represents the majority of the cost in a print job.
All shapes and sizes
Product printers also have to be able to print on many different shapes. Selecting the printing process is normally determined by the shape and the
equipment the printer has available.
If it is possible for the squeegee to stay in contact with the surface to be printed, then screen printing is possible. If the surface is uneven or difficult to get at, then pad printing is more suitable. Pad printing does not give the same ink deposit as screen printing, but pad printing is better at fine detail and multi-colours. Batch quantities and the type of equipment available can swing it one way or another. Hot foil printing can apply metallics, wood grains and pearlescent effects as long as there is a foil to suit. For larger quantities multi-coloured transfers can be applied.
Look around and it soon becomes apparent that print is everywhere, which can set you wondering how it was produced and admiring the print specialist that did the work. In this age of digital displays on phones and games, print is ubiquitous. Without companies who have mastered these printing techniques, today's hi-tech devices would not exist.
Peter Kiddell of PDS International. Process Experts in Screen Pad and Digital Printing
peter.kiddell@
pdsinternational.com www.pdsinternational .com
the first two methods, sometimes corona discharge and very occasionally plasma pre- treatment. Plasma equipment is expensive but can sometimes be the only solution. The purpose of these techniques is to make the substrate wettable by the ink. If it is not wettable, then the ink won’t stick! Screen printing, pad printing and digital printing all require the ink to wet the surface to form a bond. With hot foil printing, the temperature of the die will assist adhesion by melting the surface to be printed, but even then pre- treatment may be necessary.
Due to the variability of substrates, the most popular ink systems are two-component - that means they have a catalyst added to enable polymerisation to take place. The resulting cured ink film is a thermoset plastic that adheres well to a range of substrates and is resistant to abrasion and chemical attack. Sweat can be very aggressive on ink so products that are handled have to be resistant. Other aggressive materials are natural oils, plant extracts, perfumes and after-shaves, all of which can attack ink.
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April/May 2010 l SCREEN PROCESS AND DIGITAL IMAGING
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