Heat transfers may not be one of the more obvious uses for digital printing, but for years the innovative companies have been successfully building up a market in a variety of ways. From bar codes to plastic decorations using digital presses, the uses for thermal transfer printing have continued to expand. Today thermal transfer can offer a choice of finished printed items, pre-printed transfers or blank foil rolls for customers to apply for themselves.
The novel benefits of thermal transfer techniques include: instant availability with no need for plates or screens, short to medium runs with personalisation if needed, high resolution, wide gamut colours and opaque white undercoats from the latest generation of digital presses, plus transfer application from foils and machines developed by forward-thinking users.
As the transfers are applied dry with instant-setting adhesive, the printed mouldings can be handled, assembled and packed immediately afterwards. This is far simpler and much more flexible than the use of in-mould decorations.
Some users have gone so far as to use their own research chemists to develop their foil materials. The lacquer and adhesive can be varied for different substrates: for instance an alcohol-resistant lacquer can be used successfully on cosmetics bottles.
Being able to print small quantities in full colour at high resolution, with a white backing if needed, allows attractive designs to be developed for smaller quantities and with more variations than have been possible with conventional processes.
The very thin transfer gives a ‘no-label’ look, according to users. It appears to have been printed directly onto the surface. A paper or plastic label is always separate from the item. Brands, especially cosmetics and pharmaceutical companies, are increasingly worried about counterfeiting. They see a substantial advantage in the ‘printed-on’ transfers because of their permanence.
Before the digital method was developed, heat transfers had to be produced either by screen process with limited quality, or by gravure and solvent inks, which offer high quality but the cost of creating the print cylinders means that only very long runs are feasible. Digital printing of transfers allows variations within a run, so branded items can be produced with the names of individual stores, bars or restaurants, or sports events, for instance.
How it works
Heat transfers are built on a roll of plastic film that is discarded once the image is transferred to the final item. The film is coated on one side with a release layer that is heat activated. Over that is a very thin lacquer layer that forms the final top protective surface of the image. Blank foil rolls are loaded into the press, which prints onto the lacquer layer. Its heat process fuses the toner particles together and they adhere to the lacquer.
After printing the foils are re-wound and taken to a coating station, where heat-activated adhesive is applied. Different adhesives are used, depending on what the transfers will be used for. Originally the foils were only used with rigid plastics, but flexible plastics (such as polypropylene and polyethylene) are also an option.
Myriad uses possible
Techniques, such as heated roller heads and custom-designed application machines allow the thermal transfer process to be used for a wide variety of products. From large and small cylinders (ice buckets, drink containers, pens and cosmetic applicators) to phone and tablet computer covers, images can be completely seamless and look like direct printing, but with much higher full-color quality than screen process.
Other transfer processes
Promotional heat transfers are just one of several ‘indirect print’ processes that can be handled by these presses. The others are textile transfers, water slide decals and industrial transfers.
For textile transfers, color images are combined with a white undercoat and specialized glue and are applied by screen press. The images are transferred to textiles using widely available heat presses.
Water slide decals are used for difficult shapes such as sports helmets and ceramic plates and bowls. They are usually applied by hand. Here a paper carrier is coated with water-soluble gum, then the toner image is printed onto that. A protective lacquer is then applied on top.
While popular in uses around promotional products, a very similar process is also used for industrial work, such as labels for paint and chemical drums that must be tear-proof and chemical resistant.
Clearly, heat transfers hold considerable potential even beyond today’s use on plastics. Innovative users are working on foils suitable for metals, glass, ceramics and other materials, and working to automate more application systems. The future is certainly bright (and hot) for thermal transfer techniques using digital printing.