The point of pinning
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By Sophie Matthews-Paul
7 June 2012
EFI's new VUTEk HS100 Pro incorporates pinning as part of its curing process
A term which has started to crop up in digital circles more frequently in recent times is pinning or pin curing, but what does it mean? As a technique, this isn't particularly new but for optimising efficiency and obtaining the best quality, pinning is likely to become more commonplace as precision droplet placement gets more important across all sorts of UV-curable print, from narrow web through to display platforms. It is a feature of EFI's recently announced high-end wide-format platform, the VUTEk HS100 Pro, and it's also to be found in the new Epson concept printer which many of us saw at drupa with the working title Surepress X, which uses LED curing.
But what is the point of pinning? As UV-curable inks are now being used more frequently for fine-quality work so the need for accuracy in droplet placement becomes a higher priority. Conventionally, there is a race once the ink has been laid down onto the substrate so that it can be dried before oxygen gets to it and, particularly prevalent in high-speed machines, there is a need to eradicate drift or ghosting caused in the time between jetting and curing. The trick has been to devise an interim solution which results in a thickening of the ink, with lamps positioned close to the print-heads, prior to the final cure.
Today we are seeing more LED curing lamps coming into the wide-format display sector. But these have found a place of value in platforms where the ink needs to be pinned, effecting a partial cure immediately after printing. This allows any drop spread to be controlled accurately and, where needed, penetration can be restricted on absorbent and porous surfaces. The use of LED as a light source is sufficiently powerful to be able to cure inks partially so, as a result, a better finish is generated which affects the overall quality after the final cure has taken place.
Incorporating pinning into UV-curable printing means that, in effect, the ink drops are frozen with the benefit that additional colours can be over-laid prior to a full cure being effected. This technique is also advantageous when producing gloss or matte finishes. What this partial curing achieves is the ability to control the spread of the droplets which, in turn, makes it easier to stop the ink spreading in its uncured state. As a result, because of greater control and reduction of dot gain, final images appear sharper with improved quality.
Pinning has resulted in increased integrity of printed jobs, providing better overall sharpness and droplet accuracy in the finished output. Manufacturers pursuing its efficacy also state that finer detail and edge sharpness are apparent because their colours lose their tendency to bleed; using this method they retain clearer characteristics and rendering is more precise. Another point is that pin curing has made it easier to work with black ink which, traditionally, has been prone to problems. Likewise, smaller sizes of text logically become more viable with cleaner edges and fills.
This capability has proved to a be a good candidate for LED curing lamps because, while technologically they were seen to bring about benefits for working with UV-curable inks at lower temperatures, their power tends not to be sufficient to generate the energy required to produce a full cure, particularly at higher speeds. The relevance of pinning becomes more practical when materials are brought into the picture. Mercury arc lamps have a reputation for producing high levels of heat which can damage some substrates and cause problems with adhesion and buckling. By pinning the ink immediately after the jetting process, final curing can be carried out further down the line. This has two benefits; firstly, the risk of overheating can be minimised and, secondly, the chance of ink accidently being cured onto the heads is removed.
Pinning is actually an interim process now made practical in ink-jet platforms by the use of LED curing lamps that need to be sited close to print-head arrays so that the process can take place efficiently. Logically a device needs to be cool and suited to being located in a small, enclosed area. This is difficult to achieve with mercury arc lamps which generate heat, require warm-up time and are intrinsically less stable.
Most users tend to take for granted the curing process which goes on in their UV-curable print platforms. But manufacturers are continuing to fine-tune the performance of inks and accompanying technologies, and these developments will enhance quality without compromising speed.
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