Single-pass inkjet printing systems are the work-horse of industrial inkjet printing as applied to manufacturing applications. For the last 20 years the most common choice for many of these applications has been UV-curable ink. In this blog we talk a little about why and look at the wider uses of so-called “energy curing” inks, also sometimes known by physicists like the author as “radiation curing”.
WHY UV?
The most often used phrase for UV inks is that you can “print on anything”. This is probably true, but the reality of the situation is that it is harder to make it stick to everything! In a complete turn-around from aqueous inks, standard UV-curable inks are effectively 100% solids since no part of ink evaporates. This makes them great for managing print head performance, but only if those expensive heads are not exposed accidentally to UV light. Building bigger print width bars, even ≥1m is easier to handle than for some other ink technologies. Looking historically, Inca Digital really set the bar in the big printer space with their FastJet single-pass printers developed with Sun Chemical and first installed in 2005. Since then the continued evolution in printheads has meant an explosion in applications from flooring to corrugated sheets & labels to bottles and 3D printing.
From the application viewpoint, the biggest advantage of UV inks is their very high level of tunability to different requirements, due to the incredible choice of reactive monomer and oligomer materials. As demonstrated in the graphic below, it is the blend of these that make up the bulk of the formulation that results in the cured film behaviour.
The breadth of formulation scope means it is possible to create an ink for most applications, from 300% stretchable thermoforming inks and highly weathered vehicle graphics to automotive coatings & scratch-resistant glass ink. As we mentioned in our previous blog on ink technology for production inkjet, the Konica Minolta KM1 uses a UV ink. Curable inks have also been patented for making simulated organ tissue through 3D printing, although some of these are what we call “hybrid” inks, which will be the topic of our next post.
Over the last 10 years we gave seen the steady rise in UV-LED curing replacing conventional mercury lamps. This generally results in increased efficiency, lifetime/process stability, reduced heat generation and no ozone. For industrial applications there are still good reasons for using the older technologies, or in combination. To understand this, it is important to realise that the wavelength of the curing light is closely linked to the photo initiator materials selection and the ability to achieve those finely-tuned properties.
BEYOND UV
One big challenge for UV inks is health and safety, as more and more materials are classified as hazardous under the EU’s Registration, Evaluation, Authorisation and restriction of CHemicals (REACH) regulations. This effects the reactive monomers and the photo-initiators & has led to constant reformulation to avoid risks to printer operators, as well as the costs of distribution due to the resulting labelling. For sensitive applications, like food-contact, or skin-contact, UV inks are often avoided and much of that comes down to the risk of material migration following incomplete curing.
Electron beam curing is one way to get around this. In these inks, the cross-linking reaction is caused by the interaction of high energy electrons rather than the lower energy UV photons and means those more hazardous photo-initiators are not needed. As shown below, the penetration of the electrons is not influenced by the pigmentation and so e-beam is well suited to highly pigmented coatings applications, whilst more sensitive inks can be cured by tuning the electron energy. In both cases extremely fast curing is possible.
The disadvantage is that it is harder to make the electron sources needed for e-beam, especially at size, so they are usually substantially more expensive. Electrons absorbing into materials also have enough energy to make x-rays so electron beam curing systems need to be shielded. There is normally only one e-beam system on a printer so the inks have to be trapped wet-on-wet, which leads to big issues with bleed when using the low inkjet viscosities needed for the latest high-speed Si-MEMS print heads.
Although substantially more niche, a few exotic materials for 3D printing are curable by heating (i.e. thermal energy) often after being pinned or pre-cured by UV in order to provide green strength to the 3D-printed parts prior to post-processing.
PRINT HEADS
A big factor for UV formulation is the viscosity requirement of the print head, since even the thinnest UV monomers are much thicker than water. Getting a UV that can print 100+kHz and cure at 300m/min to match the latest breed of water-based inkjet printers is still a challenge, but one where development continues at a good pace. A recently-developed UV label printer now offers in excess of 100m/min.
Suppliers of raw materials are constantly innovating new material to help formulators meet new challenges and the growing 3D printing market is a big beneficiary of these efforts, which have enabled inkjet deposition of functional polymeric materials. In these applications the choice of inkjet head is critical since the accepted limits of inkjet viscosity are pushed to the limits. UK head manufacturer Xaar have disclosed printing of BASF materials at over 50cP at elevated head temperatures.
THE FUTURE
UV inks will continue to form the backbone of inkjet manufacturing due to the huge breadth of possibilities it offers. With the high growth in 3D printing ,that should become a increasing part of the inkjet puzzle with more innovations sure to come.
Penetration of e-beam into inkjet applications is mostly a factor of the cost and the availability of inks to run with. Although there are a few practical difficulties to overcome with printing wet-on-wet the considerable advantage of speed offers clear potential for future applications, including hybrid water-based-UV inks that we shall discuss next time.