Drying, Curing & UV
Drying curing options By Tom Kerchiss, chairman of RK Print Coat Instruments
C
hange is the one constant when it comes to printing and converting. Consistent colour and coating uniformity, the ability to protect
goods such as fl exible papers and fi lms with barriers to prevent spoilage is expected. Every element associated with production is interconnected and is of equal importance and must function within clearly defi ned and set parameters. Drying and curing is no exception. Drying printed or coated mono web, bio-based and many other materials can be challenging. Looks count but performance counts also. Inks and coatings must be environmentally compliant and comply with circularity requirements.
Colour consistency is a good indicator of process stability when the converter is engaged in narrow web width LED UV fl exo printing. Brand owners expect colour to be consistent for all given product lines and from one run to the next. Colour builds brand identity so if the cured appearance diff ers in any respect there is a real risk of product rejection.
UV energy cures by polymerisation rather than evaporation, so VOCs and HAP emissions are virtually eliminated. UV and LED UV provides a superior fi nish, resistant to scratches and scuff s. Printing, curing and proofing inline with the integrated UV or LED UV equipped FlexiProof captures the proof at that specific moment in time. Blemishes and pinholes are readily identifiable as printing and curing is undertaken in a seamless operation. Colour matching and determining how inks and coating components: additives, resins, etc., interact with a substrate are possible with these colour communication devices.
The LED UV equipped FlexiProof off ers minimal heat/energy transfer onto the substrate and uniform and consistent radiation across the width of the web for the given life time of the bulb. Mention must be made of the fact that LED UV bulbs off er a far lengthier period of bulb life (measured in many thousands of hours) than a mercury lamp, which can burn out after 1,000 of run time.
The lamps for the LED UV FlexiProof off er a tailored spectral output either at 385 or 395nm. UV emitting at a spectral wavelength of 395nm evade the TiO2 titanium dioxide particles in white inks, enabling more UV to be absorbed by photo initiators.
LED UV curing emits no ozone and there is no warm up period, they’re ready to go as soon as the need arises. There is no cool down time either. LED UV addresses many of the issues surrounding the use of thin heat sensitive materials such as substrate shrinkage, which can cause registration and other diffi culties. Inks formulated for LED UV also provide low migrationary performance.
As with any technology there are issues to consider. LED UV inks have a shorter shelf life than water-based or solvent inks and technology costs tend to be more expensive. While lamps do have a long life performance subtle aging of the diodes may aff ect gloss levels and colour shifts on press. Colour communication proofi ng devices such as the LED UV FlexiProof can monitor colour and other elements over time, enabling an operator to evaluate and spot irregularities as they arise.
The conversion of a liquid or semi liquid coating of an ink or an adhesive or a varnish or indeed anything else at a commercially
acceptable speed from a pilot or production coater or from a fl exographic or gravure press depends to a large degree on matching the hot air dryer or curing technology to the application. Get it wrong and the fi nal product might have poor rub resistance, will scuff easily and generally will not perform well or look good. If the substrate is of a lightweight gauge and is hit by hot air too aggressively the coating may pucker or dimple and the substrate may be marred. Caution is needed with drying many substrates and materials. A VCML pilot coater is being used to trial a very sensitive material dried by steam rather than conventional drying or curing system.
Inadequate or uneven hot air drying of printed or coated PE and PP can result in spots and streaks; inadequate drying weakens the bond between coating and substrate and this can cause poor adhesion.
Infrared or IR drying makes use of radiant heat to evaporate solvents and other chemistries, it can be ideal for spot drying but precise calibration is needed.
Water based inks dry by evaporation and depending upon the type of substrate may require a signifi cant amount of heat to drive off the water and leave the substrate with a suffi ciently cured fi lmic layer of ink that is rub resistant and has other desirable properties. Printing packaging fi lms may require powerful dryers with good airfl ow. Each colour laid down must be thoroughly dried before any additional colours are added. VCML pilot coaters provided by RK Print Coat Instruments for the surface coating of fl exible papers and fi lms and have been confi gured with additional drying units to ensure optimum performance.
16
May/June 2026
www.convertermag.com
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