Keep it cool

We all understand the problems that overheating a substrate can cause, from distortion and shrinkage leading to register issues, through drying out and web breakage to, in extreme cases, fire. But converters are still under pressure to turn products round faster and to produce complex finishes on increasingly sensitive materials.

This has fuelled the recent growth in curing systems such as UV and NIR (near infra red), which can offer much higher throughput speeds, better quality finishes and lower reject rates. However, especially with UV curing, there have still been some issues around heat, so you need to know what will suit your needs best.

Mercury vapour bulbs used for UV curing have an external temperature in excess of 800degC; the reflector systems focus the UV energy (intensity), but also gather heat energy and direct it to the substrate so, depending on the process speed, substrate temperature can rise rapidly.

That directly radiated heat energy is not required in the UV curing process, so over the last few years various methods for removing it have been identified, some more successful than other. So which are the best available technologies?

Absorption: absorbing the heat energy in the reflector system through multi layer laydown of dichroic layers to absorb heat energy. This provides a reduction of around 50 per cent of the heat energy from the lamp systems – but 50 per cent of the direct energy still impinges on the substrate.

Reduction of bulb diameter: the IR (heat) energy radiated from the lamp surface is directly related to the surface area, so reducing bulb size does reduce the radiated heat energy. However, the surface temperature still exceeds 800degC and the direct energy still radiated onto the substrate needs to be controlled.

Interference: simply positioning a heat absorber such as a cooling tube or hot mirror between the lamp and the substrate. Water cooling tubes: various methods are used, from one or two tubes to complete enclosure of the lamp - but the loss of UV energy is high and makes it difficult to achieve high intensity.

Hot Mirror: placing a quartz plate in front of the lamp which can have a special dichroic coating that allows UV through and reflects IR back. There are still some losses of UV energy, little of the longer wave length IR is reflected back and the temperature of the mirror can rise, adding more heat in the curing area rather than eliminating it.

Indirect radiation: by redirecting the UV from the lamp to the substrate with an angled reflector coated with dichroic layers, the IR (heat) passes through whilst the UV is reflected to the substrate. However, it can be difficult to focus the UV energy and achieve high intensity, and the lamp housings can be very large.

This provides one of the lowest temperature solutions, but with poor UV energy to the substrate. However, with clever engineering it has been possible not only to obtain indirect UV energy, but also to focus it onto the substrate, providing very efficient low temperature curing. The latest design is a double lamp system providing two peaks of energy and a high degree of dose energy.

This system provides good curing of UV inks and coatings on a whole range of heat sensitive substrates and is particularly useful for applications with temperature controlled drums, where lower amounts of heat energy need to be absorbed by the drum.

Already well established for drying water based inkjet ink on mailers and for continuous inkjet applications, NIR is now finding many applications in the converting industry.

NIR technology enhances the very high power density found close to the visible light spectrum at around 800nm. This drives out water and/or solvent from the coating and can be achieved in a very short time, reducing dwell times and dryer length associated with normal convection and IR drying systems. It provides high speed drying of both water and solvent based coatings with a lower delta T of the substrate than conventional drying technologies.

It also provides for drying of thick films of water based coatings on flexible packaging and high speed webs where substrate temperature control is an issue. Additionally, when printing on CI flexo presses, with or without temperature control drums, NIR allows the drying width to be controlled to the web width in 40mm increments, reducing both the heat energy into the drum and the total energy required for narrower webs.

Where water based high gloss coatings are being considered, it is often difficult to obtain very high gloss with normal laydowns, but if the weight of coating is increased, the amount of water carried means that the normal press or coater dryer is undersized. NIR can either boost the existing dryer with only a small increase in dryer length or can replace the complete unit, providing a versatile, economic and flexible drying system.

For re-moistenable adhesives and special patch applications of coatings or adhesives, where drying is only needed at certain points on the sheet or web, NIR can be supplied in modules to be positioned only where required - a much quicker and more economic cure than conventional options. When a new job is run, the modules are simply repositioned to suit.

By using NIR, which is focused on a very short wavelength, maximum heating of the water molecules is achieved. As a high energy density can be applied, the space envelope compared to conventional dryers is reduced.