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Stress on short turnaround timesThe challenge of drying UV ink

There’s been a steady rise in the use of UV inks for offset printing, driven mainly by improved technology and the demand for ever faster turnaround of jobs. The growth of digital printing, with its emphasis on short turnaround times, has forced the offset press world to speed up its own game. And one way of doing that is to reduce the drying times so that jobs can go straight into finishing. So, many printers have turned to UV-curable inks, which are touch dry as they leave the press and are relatively scratch proof. But conventional UV inks tend to leave quite a flat image that is not always suitable for commercial printing.

Ryobi was the first press manufacturer to introduce LED UV-curing, seen here on a Ryobi 920 press

In recent years two alternative approaches to UV curing have emerged – low energy and LED UV. Both of these rely on tuning the inks very precisely to activate the UV curing process with a minimal amount of light energy. The downside of course is that this puts more pressure on the inks to perform, leading to higher ink prices. The trade-off between paying more for the inks or for the energy differs from one region to the next and according to the products and substrates being printed plus the business models of each individual print company so there’s no easy answer as to which system is best.

There are some distinct differences. The low energy approach is a variation on conventional UV although it does use less energy. Most low energy systems only need a single lamp, another saving over conventional UV, which normally requires three to six lamps depending on the sheet format. However, low energy systems still use an ion-doped lamp and these lamps still use a mechanical shutter, and still take time to warm up and cool down.

Light Emitting Diodes (LEDs), on the other hand, are an array of semi-conductors that produce light when connected to an electric current. There are no moving parts and LEDs typically last for over 15,000 to 20,000 hours, while conventional lamps have an average life span of just 1,000 hours. Also, LEDs can be turned on or off instantly, so there’s no time wasted waiting for the lamps to come up to temperature. This also means that the lifespan equates directly to hours spent printing with no degradation of the lamps over time as they deliver their full performance up until the point they fail. Besides offset, LED UV curing is already used in many wide format and inkjet label presses.

Most manufacturers will quote impressive numbers of press installations, though most of these are in Japan. Conventional UV curing has been popular in the Japanese market, mainly because it obviates the need for a coater or sealing unit and the jobs can be turned around quickly with no need to store work in progress while it dries. But the Japanese have also embraced solutions that cut energy usage, following the 2011 Fukushima Daiichi disaster and the subsequent shut down of most of the country’s nuclear power plants.

Both low energy and LED curing can offer dramatic energy savings over conventional oil-based inks since there’s no need to dry the inks. Matt Rockley, marketing and product executive for Heidelberg sheetfed presses in the UK, estimates this could be as much as 45% on an SRA sheet with an average 70% coverage. However, he points out that this alone is unlikely to offset the higher ink prices. This is particularly true of European markets, where energy prices are currently falling. But the real saving comes from turning jobs around faster.

It’s not just about the energy use. Rockley points out that there are no low migration inks suitable for either low energy or LED curing and so most packaging printers are still using conventional UV curing. Instead the newer systems are aimed at commercial printers looking to turn jobs around faster.

These newer inks have a very different look to them as Rockley explains, “Conventional UV inks are mainly made with plastic photopolymers so when you cure it you get more of a matt finish so there’s normally a UV gloss coating to enhance the product. Whereas the LE and LED UV are mixed with different products that give a glossier effect when dried so they look more like a conventional ink.”
Most vendors agree that there’s little difference in image quality between the low energy and LED systems as they both cure instantly. However, Rockley points out that simply following ISO12647 will produce a different look from an oil-based ink so that you might have to make adjustments to compensate. This could be an issue in producing a glossy magazine where the cover will have been printed on different stocks so that matching the inside cover, page 2 to page 3, could be more tricky, but essential given that many magazines will sell a double page advertisement in this position.

Neil Handforth, sales and marketing director at Apex Digital, distributors for Ryobi in the UK, says that the LED inks will work well with most substrates, coated or uncoated, adding, “The image quality is outstanding because the ink is dried instantly on the surface of the sheet so there’s no absorption to the stock.” This gives a very sharp dot for crisp images and uses less ink – Handforth estimates 10 to 15% less ink than a conventional system. However, this can vary considerably as some substrates are more absorbent than others.

Ryobi was one of the first press vendors to look at using LED curing, showing its first system as far back as drupa 2008. Ryobi uses Panasonic LED arrays, which can be fitted to many of its presses from the 2-up 520 series to the 8-up 920.

Handforth says that the main attraction is that the sheets are immediately dry and ready for guillotining or reworking. He adds, “There’s no ozone and no heat so you are not ducting anything out of the machine. Also, there’s no spray powder and most finishing operators will tell you that gets everywhere and clogs everything.”

ABC Print Group, based in Hereford in the UK, is one of the largest B3 printers in the country. ABC was the first UK company to order Ryobi’s LED UV, with a 924 SRA1 press. Managing director Mike Greene explained that he wanted to cut energy costs, adding, “Most drying solutions that produce a dry-to-the-touch sheet tend to be a big draw on electricity, as well as being hungry on consumables such as coatings or varnish.”

The SRA1 format will produce 8-up A4 pages, which will be ready for finishing straight out of the press thanks to the LED UV. Greene noted, “The productivity gains of the configuration will be huge to a business like ours, and the savings on both maintenance and cleaning by not having to use spray powder are also important.”