DRYING METHODS FOR LABELS AND PACKAGING
Ultra-violet curing is used for many processes including flexographic printing of folding cartons but in relative terms, its use in flexible packaging has been limited to varnishing processes. This is slowly changing as UV is specified more and more for multi colour applications.
There are many economic justifications for using UV curing for package printing. By using the term package printing we are also including labels, folding cartons and flexible packaging. In the label printing industry, figures released at Labelexpo, Chicago, showed that there are more than 2000 label presses in Europe using UV curing. The presses included not only flexo but all kinds of printing technologies. The United States are following the same pattern, with just over 1000 narrow web presses.
Dot gain is a significant consideration with wide web flexo printing, especially when high quality four colour work is involved. Up until recently it was necessary to print either solid areas of colour or fine screens. Now, with the help of foam or cushion mounts this is changing.
However, both Starna and their associates, SY Technologies acknowledge a need for more fine tuning in order to resolve remaining difficulties in wide web flexo. This drying technique will become an accepted process as it is with labelling, screen, sheetfed, forms and many other printing methods.
The concept of UV curing involves using an energy source to bring about a physical-stage shift of an ink from a liquid to a solid and opaque form. Or to simply manipulate light energy to penetrate a thick pigmented ink film which is actually designed to prevent the penetration of light.
It is important to fully understand light energy when dealing with UV systems. UV radiation forms part of the spectrum of the electromagnet radiation. It is adjacent to the short wave part of the visible light and reaches to the ionising radiation. Within the wave range of 380 – 100 nanometers, radiation is termed UV.
Visible light forms part of the spectrum between 400nm to 700nm. Therefore, UV light is the energy form used to penetrate an ink film designed to prevent the penetration of visible light, this is referred to as opacity.
UVB range is sub-divided into: UVA = 380 to 315nm (long wave radiation), UVB = 315nm to 280nm (medium wave radiation) and UVC = 280 to 100nm (short wave radiation).
The efficiency of an ink system to react with UV energy to form a cross linked network is dependent on a number of factors. For example, the quantity (thickness) of UV curable material depends upon the customer and his needs and the intensity of the light source. Concentration of photo-initiators and pigments is the responsibility of the ink manufacturer.
Only the emission of the UV light source is controlled by the lamp and drying unit manufacturer.
There are many benefits to be gained from switching to UV flexo inks, cost savings derived from reduced ink waste and unlike water based and solvent based flexo inks, UV flexo inks do not evaporate and the ink does not dry on the plate, therefore as soon as the press is in register, the job can be run. There are also a number of effective proprietary cleaners now on the market and inks can be left for a reasonable amount of time, which reduces labour input.
In wide web the cost, the environmental implications and the quality of UV flexo is attractive. It offers excellent rub resistance when printing on polybags and although solvent wide web flexo does not have the print quality of solvent gravure, it can print equally well and sometimes even better.
There are many applications in which UV offers advantages over other curing methods. The UV flexo printing of folding cartons offers superior graphics when compared to offset and the ability to print heavy reverses and small type on line work and colour consistency. There is also excellent scuff and run resistance as well as no clean up between shifts.
UV inks and curing is also environmentally friendly. Curing is virtually instantaneous and inks do not contain any VOC’s. Wasted ink is kept to a minimum and paperboard which comes with UV inks can be sold to scrap recyclers. De-inking is a relatively simple process and most scrap with UV ink can be recycled into fine grade paper.
SY Technologies Cure Tec UV lamp heads use dichroic reflector technology. The advantage of SY Technologies ‘patented’ dichroic coating is that it is said to reflector back to the substrate while passing through the IR (infared) energy. This heat is then extracted by means of an exhaust fan. In operation this results in 98 per cent of available UV reaching the substrate, with a significant reduction (down by 40 per cent) of total IR.
Using the dichroic method the user has two options, either he can cure at a higher press speed or he can extend the range of substrates on the press. With less heat build up in the machine more delicate/heat sensitive substrates can be used.
The specially formulated dichroic coating is applied to a ceramic or metal base and in many instances this provide a competitive edge. Dichroic is equally suited to short print runs (as in many flexible package orders) or for longer runs of two to four hours.
Trials and customer experience has shown that this new method of dichroic reflector technology provides systems with a life expectation of three to four years. Indeed, all Cure Tec lamp heads remain in optimum condition. The only proviso that SY Technologies stipulate is that when a replacement Starna SyTec lamp is fitted, the surface of the reflector should be wiped down with either a dry cloth or an alcohol wipe. Apart from being able to run the press at higher speeds and use a broader range of substrates, the long life dichroic reflector means that shutting down the press every month to replace aluminium or glass reflector is not necessary.
Ultraviolet curing is a proven technology with a wide range of applications. It works with flexo, litho, letterpress, gravure, inkjet, inks, coatings and varnishes and can also be used to cure coatings on circuit boards, fibre optics, floor varnishes, furniture and many other uses. When all the costs are accounted for, such as the price of the equipment and the expense of clean up, then UV is usually the most cost effective drying technology.
There are a number of alternative drying methods to UV, including infrared drying, electron bean drying and hot air drying.
Infrared drying works in two ways, it heats the stock and speeds up the evaporation of the ink.
It is slower than other forms of drying. The stock temperature rises by 20 percent above ambient temperature and the release of solvents (VOC’s) can take seconds rather than milliseconds, depending on ink formation and substrate material. Environmental considerations now require that VOC’s be removed and not dispersed into the atmosphere. The slower release of VOC’s by the IR process makes their removal easier.
The infrared method, despite being faster than hot air drying, is still slow as the energy absorbed by the printed image is low, which means a longer dwell time. Therefore it is only practical for sheet fed presses with paper stock and as an alternative to hot air. However on the positive side it can help drying and under the right conditions can virtually eliminate the need for spray powder on a sheet fed press.
Electron Beam curing was introduced approximately 25 years ago, about the same time as UV. EB is undoubtedly one of the best forms of curing, but it is still not one of the favoured choices. Technology has only recently been developed to achieve its true potential.
The polymerisation process works with inks but does not contain solvents. It can cure thick films, even through opaque surfaces, so it is ideal for curing adhesives used in lamination. EB is colour blind and dries all colours at the same speed (the Vari Tec power supply from SY Technologies is able to address this problem, to some extent, with UV). EB curing bombards the printed surface with electrons, causing the ink to polymerise.
To be fully effective, EB must operate in a sealed, pressurised chamber using nitrogen or carbon dioxide. EB drying speeds are virtually instantaneous, there has to be space between the last printing station and the dryer in order for the ink to level.
Unlike most drying methods, EB generates very little heat and works well with specific chemistries.
The drawback to EB is very simple; expense. Capital investment costs are high and recouping that expense is probably beyond the average printer. Operating costs are equally high.
Hot air drying
Heat in the form of hot air, enhances the evaporation by steaming the solvents (VOC’s) out of the ink. The solvents then keep the ink open on the press. Both the substrate and the ink are heated sufficiently to release solvents into the atmosphere.
Hot air drying has been used for many years, but has several drawbacks. The ink can lose moisture, causing shrinkage or curl, and dries so fast that it chalks, losing adhesion and gloss. Chill rollers are required to reduce web temperature which sets the ink quickly. VOC’s are prevented from being freely dispersed into the atmosphere, for environmental reasons. In addition to these disadvantages there is also the cost of heating the ink and the subsequent cooling of the web, especially as these are linked to the speed of the press.
Starna and SY Technologies have been engaged in a research and development programme to make UV more cost effective and efficient. The result is the EPS VariTec solid state electronic power supply, designed to replace the traditional ballast or transformer type power supply. Whichever model is selected, benefits for both the press manufacturer and the press user include installation flexibility. Cubic volume is about one-third of a transformer type power supply, therefore only a minimum amount of floorspace is required.
The basic unit is the EPS1, which has a variable power level and is suitable for use with up to 7kw lamps.
The compact, top of the range EPS3 model has the ability to operate from 2kw to 7.5kw from a three – or single phase, 208 to 240AC, 60HZ input with a reduction to 25 percent of full power on standby. It incorporates many functions, such as pick up for shutter open/close with indication from the press, a system trip indicator and control as well as stand by and full power indication control.
With the 25 per cent power stand by feature, energy consumption in this mode is more than reduced by half compared with conventional power sources that operate at around 60 per cent of full power in stand by mode.
The adjustable gain facility enables the press user to match power output according to colour being cured at the time.
On multi-colour presses, the station with the most difficult to cure colour determines maximum power output. With the adjustable gain, the power necessary to effect cure rate on each of the remaining print stations can be set appropriately and in relation to the speed determined by the most difficult colour.
This result in a saving in heat from station to station and the overall heat build up in the press is reduced.
This means that the most difficult colour to be cured, co longer has to be at the last print station to minimise heat build up.