Systems for optimum drying and curing

If the ink is not dried properly there is the danger of ink transferring to the back of the film or paper, or onto the printing press or any subsequent processing step.

Good adhesion of ink to the printed substrate is necessary to achieve the final label or package properties such as handling, transit resistance and robustness in use, and this can only be achieved and optimized by being fully dried.

In addition, drying is important to enhance the decorative elements of the ink, particularly the level of gloss. Incorrect drying leads to inconsistent gloss levels, and may also affect other properties such as slip properties. 

Drying is also critical in achieving the lowest possible print odor and ink component migration to comply with the legal and brand requirements for food packaging.

OIL-BASED INKS

Sheet-fed printing accounts for around 40 percent of global label volume. Drying of oil-based inks involves a physical separation of the ink, leaving solid components on the surface and penetration of various parts of the ink formulation (see Figure 4.2). Some types of offset inks dry further by chemical oxidation, which enhances the physical properties of the ink film. The drawback is that the whole process can take up to 24 hours.

Conventional sheet-fed printing requires the use of spray powder to prevent set-off in the stack and converting cannot be carried out until the ink is fully dried. This delay can be shortened by the use of an overprint varnish (such as a water-based coating) over the ink.

Drying on narrow web in-line presses is either forced air for solvent-based or water-based inks or with energy curing using UV or EB radiation (see Figure 4.3). 

Each printing process has a different range of printing film weights. Solvent-based gravure single film weight is generally the highest and in the range of 3 to 4 g/m2. Up to 1.6 to 1.7 g/m2 is the upper limit for offset printing (see Figure 4.4). 

Water-based flexo printing has a slightly lower film weight than gravure solvent-based. In general UV and EB inks are printed at lower wet film weights than solvent and water-based inks.

This is because solvent-based and water-based inks are not 100 percent solids. We are evaporating part of the ink so a solvent-based ink may only actually be 25 percent solid material and a water-based ink could be 50 percent solid material. 

EB and UV curing inks are 100 percent solids. Everything that is on the print is cured and goes through the further processing steps. 

SOLVENT-BASED INKS

Solvent-based printing – both gravure and flexo – has been around for many years and is the bedrock of today’s flexible packaging industry. It is a cost-effective process and the print quality in gravure is high. We can print multiple substrates and applications. In particular they are very good at printing on a wide range of different films, which can be an issue for both water-based and UV/EB inks.

Solvent inks are under attack from a growing list of regulations around volatile organic compounds, VOCs. Solvents are highly flammable and fire is a very significant risk that has to be managed in solvent-based packaging plants.

WATER-BASED INKS

Water-based printing is widely used globally on paper substrates in wide web printing. In narrow web printing there is a divide between Europe – primarily a UV market – and the US, where run lengths tend to be longer and there is a much wider use of water-based flexo. UV has however made major strides in the US market in the last 15 years.

Printing with water-based on to film presents challenges. This is not simply a matter of surface tension and ink adhesion, but also the volume of hot air required to remove the water. Water requires a higher level of heat for evaporation than most solvent-based inks, so requires properly specified dryers. This may also impact the thermal properties of unsupported films.

Control of humidity of the air used in the drying system is important, because the drier the air, the more water it can carry away from the print (see Figure 4.5). 

Bad experiences with water-based printing are often down to not giving enough thought to the drying capacity on the press – particularly where converting from solvent to water-base on the same printing press. Inks can also be optimized. Inks that are more intense have a lower ink film weight so require less water to evaporate them.

ENERGY CURING

The main value proposition of energy curing is that prints emerge fully dry from the press. Curing is a virtually instantaneous process, allowing in-line processing – cutting, folding and forming of the package. This is a key productivity advantage. 

A second key part of the value proposition is that energy curing is a solvent-free technology – ‘What you print is what you get’. 100 percent of what you put on the substrate remains on the substrate. 

UV and EB ink systems are very open. There is no problem of inks drying on the machine. With evaporation-based inks evaporation can take place in the press, meaning the inks start to dry in the machine. UV and EB inks stay open on a printing press indefinitely. A converter can leave the press over the weekend and come back on Monday morning and the inks will still be in the same state. 

UV and EB inks generally demonstrate good adhesion to a wide range of substrates, and have a high quality and resistant finish. The footprint of UV curing is another key reason why the narrow web industry has adopted it.

Modern UV curing systems are highly compact compared to, for example, a big solvent-based drier or water-based drier.

UV CURING PRINCIPLES

UV inks rely upon polymerization. Starting with a normal viscosity ink, a photoinitiator absorbs the UV light and converts it into free radicals that then polymerize to form a molecular network – an interconnected polymer which is 100 percent solid (see Figure 4.6).

This process is very fast. If everything is optimized on the printing press, it is possible to print at hundreds of meters a minute in UV or EB curing with a print that comes out basically dry.

UV inks cure in the region of 200 to around 400 nanometers wavelength (see Figure 4.7).

Above that is the visible light region and then the infrared region which produces heat. This is something which has to be taken into account as UV lamps also emit partially in these areas of the spectrum. Although modern UV lamps are designed to prevent this heat from reaching the web, there will always be some heat reaching the substrate. This may not be an issue for self-adhesive substrates, but will need to be taken into account when printing on unsupported films, and particularly for heat-sensitive materials like shrink sleeves. In these cases, a chilled roll will usually be required (usually the impression roll on a flexo press) to absorb radiated heat energy. Quartz windows in front of the lamp can also be used to reduce incident heat on the substrate.

The ultraviolet wavelengths used to cure the ink are also divided into shortwave, medium wave and long wave UV (see Figure 4.8). These have slightly different properties. The longer wavelengths tend to be better at through curing the ink and the shorter wave lengths better for surface curing. So, a mixture of wavelengths is an advantage.

This broad spectra means we can cure with different types of photoinitiators and achieve through cure and surface cure. The medium pressure mercury lamps most commonly found on narrow web label presses consist of a vacuum tube containing a mercury vapor which comes from a bead of mercury over which a very high voltage causes a discharge. That discharge is in the UV wavelength area.

Manufacturers can adjust the properties of the UV output by ‘doping’ with different metals. Mercury lamps doped with a very small amount of iron compounds shift the spectrum in different ways. This tailoring of the spectrum to different types of inks means for example that lamps can be produced that are slightly better aligned for printing and curing with opaque whites. 

The UV lamp reflectors have a significant impact on the curing process. The vacuum tube radiates in all directions, so reflectors are required to focus energy on the print. Parabolic reflectors collect as much of the light as possible and direct it towards the print.

To ensure optimum cure UV lamps must be properly cleaned and maintained. Because of their exposed position on the press, lamps can get dirty with dust or ink deposits, which cut down the light reflected to the substrate.

ELECTRON BEAM

Electron beam (EB) curing systems create a very high voltage in a vacuum using an electrode. Electrons emitted from the electrode are accelerated by a very powerful magnetic field and go through a titanium window to directly polymerize the ink.

These energetic electrons knock electrons out of the raw materials and produce free radicals that then cross link. The big difference compared with UV is that photoinitiators are not required – we are curing the resins directly. This eliminates one of the possible causes of odor and migration of ink components.

EB systems always work under inert atmosphere using a nitrogen blanket, as oxygen dramatically inhibits the EB curing process.

The EB unit is electronically controlled and linked to the web speed, so at all web speeds the EB dose is being managed. Typical speeds are up to 400 m/min, with curing exactly the same when running at 50 m/min.

EB is a ‘cold’ process, in that it does not radiate any heat onto the film. On the other hand, there is so much energy going into the film that it can warm up by maybe five to 10 degrees. These powerful electrons can also damage films by changing their physical characteristics – for example increasing the heat sealing temperature of the film or producing unexpected odors. So, it is always important in EB printing to validate very carefully the substrates being used, because not all substrates are compatible with EB printing compared to, for example, solvent-based printing or even UV printing. UV generally doesn't damage the film. On the other hand, adhesion can be helped because we're curing through to the bottom of even thick ink film, which can help adhesion of the ink. This means that in EB we often get slightly better adhesion than with UV on certain film types. 

A recent development is a miniaturized EB unit launched at Labelexpo Europe 2017 on an EB inkjet press.

Another difference with UV printing is that in UV the pigment absorbs some of the UV light. This means that blacks, for example, are harder to cure than yellows because black absorbs across the whole spectrum.

EB by contrast doesn't ‘care’ what the color is. The electrons are not affected by the pigment so inks do not have to be formulated separately. In UV, blacks are formulated differently to yellows and magentas to compensate for the black pigment absorbing a lot of the UV.

LED-UV

An LED (Light Emitting Diode) is a semi-conductor. When electricity is passed through it photons are emitted. This will cure an ink with a photoinitiator tuned to that frequency.

Up to now the biggest penetration of LED-UV has been to the sheet-fed printing market for commercial printing. That has been driven by the fact these are quite compact, very simple units and a conventional sheet-fed press can be relatively easily converted into a UV press. This has lowered the barrier to entry for sheet-fed printers into UV printing. This market has also seen new developments of mercury lamps with an adjusted spectra referred to as low energy UV lamps. H-UV, for example, is a Komori brand name for its particular system. These lamps have a slightly shifted spectra, reduced in the ozone generating area.

The problem for sheet-fed presses is that LEDs, like all light sources, lose power the further you move away from the print. The sheet transport on a sheet-fed press means the lamps have to be kept some distance from the sheet. This means the LEDs often have some kind of focusing of the LED light to take this bigger distance into account. In narrow web, by contrast, the lamps can be very close to the substrate.

The advantages claimed for LED-UV include:

1. A key environmental benefit is the elimination of mercury. The World Health Organisation has been moving to eliminate mercury from industry and the environment as mercury is a dangerous substance. So far, however, the UV industry has not been specifically targeted.
2. There is no ozone. One of the side effects of UV radiation is that it affects the oxygen in the atmosphere and creates ozone. Extraction is required to take that ozone away from the printing press. LEDs (and some new types of doping used in low energy mercury lamps) do not have this effect.
3. LEDs can be turned on and off like a domestic light switch without any warm up or cool down period. UV mercury lamps take a short time to warm up to full power and require shutters to protect the web during idling or make-ready. LEDs are at full power immediately.
4. LEDs emit light at a single wavelength, typically 385 or 395 nm. This has advantages and disadvantages. Ink manufacturers need to ‘tune’ their inks to this wavelength, which means using more photoinitiator in the ink – and this makes LED ink more expensive than standard UV ink (see Figure 4.9a).
5. No infra-red emission. With mercury UV lamps measures must be taken to reduce heat at the substrate surface using quartz windows and heat sinks. LEDs by contrast do not produce any heat coming out of the front of the lamp.
6. The claimed lamp lifetime is in excess of 20,000 hours.
7. Sections of the LED lamp can be turned off to match the web width, thus saving power. On intermittent offset presses the LEDs can also be switched off during the backward movement of the web.
8. No reflectors to worry about as all the light comes out the front of the LED lamp. Reflectors are required for conventional UV lamps because light is emitted from all sides of the lamp.

In terms of ink formulation, LEDs do pose some challenges because there is only one single wavelength to play with, and because the best aligned photoinitiators are not suitable for food packaging. Migration compliant formulations are however now coming to market.

LED requires the use of highly reactive vehicles to ensure cross-linking from a narrow wavelength. This can have consequences – for example it can make the ink film more ‘brittle’, and because short wave lengths are absent it is quite difficult to get the cured ink tack free. These issues can be solved, but it requires different formulating strategies and more photoinitiator to get the ink to cure properly. 

Because the LED wavelength is quite close to the visible area of the spectrum and photoinitiators absorb in these frequencies, if these inks are left open in the sun or under factory lighting they are more likely to cure in the ink pan or open containers.

This means more control is required over stray light in the press room. LED coatings are particularly sensitive to light – if left open for even ten minutes a skin can form. 

UNDERSTANDING DOSE

Whatever the rated energy output of a UV/LED lamp, what is important is how much energy the ink is receiving and the printing speed – and therefore the dose of UV (in joules/cm2) that is actually being used to cure the print. This requires a proper analysis related to the number of watts/cm2 on the substrate and the speed of the web. This analysis is more important in LED because we are running with tighter margins with regard to cure.

We are looking at a complex formula of peak irradiance, total lamp power, ink cure speed and the speed of the web. So, this is not just about the power of the lamp – it is about adapting the process to each application (see Figure 4.10). 

Another variable is heat. Heat actually helps UV inks cure, so it is not a simple question of taking all heat away from the web to ensure optimal cure. To illustrate this, curing speed at 40 degC is twice as fast as at 30degC.

Viscosity is an issue because oxygen inhibits ink curing and lower viscosity inks tend to absorb more oxygen. This means it can actually be more difficult to cure low film weight flexo inks than offset inks, because offset inks are high viscosity and it's harder for the oxygen to get into the high viscosity ink film.

Inkjet requires very low viscosity ink systems, which gives challenges with oxygen inhibition of the cure because the inks can absorb more oxygen.

As noted above, the thermal footprint of the UV curing system must be optimized to ensure there is sufficient heat to support the cross-linking reaction.

A quartz window in a conventional UV system prevents heat getting into the substrate, but that may mean a more powerful lamp is needed to compensate for the lower temperature of the print surface. It is easy to make the mistake of putting in a quartz window on the lamp when printing film, then finding that the ink will not fully cure. The key is to test the application and preferably with all supplier partners present.

UV DOSE MEASUREMENT

Curing UV inks requires a certain amount of radiation dose at the correct wavelength, but is also impacted by a number of variables such as web speed (to ensure adequate dwell time to effect good cure) and lamp output (affected by the lamp power and how well maintained the lamp and reflectors are and substrate surface properties).

The critical factor is the amount of energy reaching the web surface, and there are several ways to measure this. The simplest is a self-adhesive test strip stuck onto the web which changes color as it goes through the press depending on dose levels. The strips have calibrated color patches and the press operator matches the color change of the patch to this color scale to give an approximate value for the dose of UV going to the web surface at that line speed.

Measurement ‘pucks’ are available but can only really be used on conveyor equipment installed by ink manufacturers with laboratory-scale equipment. They cannot be put through a normal printing press. 

A recent development is detectors that can go in front of the lamp, where they measure the UV dose in real time. The only issue here is the sensor is slightly above the substrate, so is not actually measuring dose exactly at the substrate surface, and a compensatory calibration is required. A key benefit of these systems is to see if the UV dose is degrading over time. Another key issue is that these sensors sit in front of the lamp, so are in UV light all the time and can degrade over time, which again requires regular calibration. This is a promising technology which will enable better control of UV curing.

Cure can also be tested off-line. Ink manufacturers and convertors use tests including solvent rubs according to defined FINAT test methods to assess the cure rate, while tape testing for adhesion can also detect poor cure. Because this requires only basic equipment, it is generally good practice in the press room to test if the ink is cured when it comes off the press.

The effects of substrate type and press speed are key factors to bear in mind when assessing correct UV system operating levels. Printing on paper, for example, requires different approaches to printing on film.

Care must also be taken in the storage of inks – particularly regarding shelf life. UV inks typically have a shelf life of up to two years, but LED-UV inks generally only one year since they are more reactive. This additional reactivity also means care must be taken to avoid ambient light leakage if LED-UV inks are kept on the press overnight or over a weekend.

UV absorbing films should be placed over fluorescent lighting and windows.