Geometry key to curing

When selecting a UV system for curing inks and coatings it is essential that the lamphead reflector is matched to the printing process, for optimum ink curing. The materials selected, surface profile and geometry of the reflector all combine to form different characteristics. Until now little information has been available on lamp efficiency and the role that reflector shape, focus and surface properties play in the curing process. GEW recently completed exhaustive tests on different types of reflectors and all aspects that affect their efficiency and performance.

GEW managing director Malcolm Rae comments: “To fully understand reflector performance and provide the correct profile for the job, it is essential to measure accurately the delivered UV intensity of the various designs available. We concluded over a year ago that we would conduct a detailed scientific investigation of reflector optimization under closely controlled conditions. We brought a highly trained scientist in-house to design our experimental parameters and conduct thorough testing under a single variable environment.”

The first step in understanding reflector performance is the aspect UV lamp output irradiance focusing plays. By simply adjusting the position of the reflectors it is possible to obtain a highly focused energy impingement on the substrate or a broadly diffused output. Generally, a focused reflector array is used to obtain the depth of cure and press speeds common in the printing industry. This is particularly the case with flexo and screen printing, where thick ink coatings with metallic pigments and varnishes are used. GEW's new XC 'extreme cure' reflector has a fully focused, dichroic coated aluminium reflector with integrated clamshell shutters to protect the substrates. It is said to be ideal for thick film coatings including screen, cationic inks and where ultimate curing is needed at high speeds.

Reflectors giving a diffuse output are normally used to reduce the intensity of the reflected IR (heat) impinging on heat sensitive substrates. Most UV emitting lamp output consists of around 60 per cent IR energy. Reflectors focus UV energy and IR energy. The down side of a diffused reflector array is the reduced curing efficiency in terms of depth of cure and a reduction of the cure rate, which is partially temperature driven.

Control

Another GEW design has each reflector independently controlled where the focus can be varied or even having one of two reflectors kept in the closed position. This again is a heat management technique used during jogging or low speed job set-up on a press.

The next phase of the reflector research involved measuring the spectral output of 15 different reflector constructions. The materials used in the test reflectors ranged from a basic polished aluminium to more exotic non metallic substrates with specialized coatings. A standardized UV spectrophotometer was used to measure the entire UV energy output of each reflector - including UVA, UVB, UVC and UVV, which were then summarized for total UV energy output.

GEW says the results obtained have a number of significant ramifications. First, the total UV output of a reflector array can be optimized to obtain the most efficient curing ability. UV curing involves use of significant electrical energy. Knowing there is as much as a 40 per cent difference between the most and least efficient reflectors can result in a very significant difference in operating costs. Secondly, since it is known that certain inks and coatings rely on different UV wavelengths to achieve optimum cure, the selection of the appropriate reflector construction can maximize the curing process. The various reflector materials studied identified a shift in the UV wavelengths emitted. This shift can also be enhanced with certain additives within the lamp.

GEW's latest reflector development is featured in its new ISO-cure water cooled profile for processing a variety of substrates on mid and wide web presses from 450-1,450mm. On show at ICE next month (see preview feature), it is aimed at situations where the UV lamphead has to be integrated in a compact space on machines such as CI flexo presses for flexible packaging.

Malcolm Rae explains: “The design of the reflector includes a specially developed glass dichroic surface that enhances UV output whilst maximizing the absorption of IR. Research has shown that this construction gives the best performance for optimizing the ratio of UV reflection to IR absorption. Additionally, the life of the reflector has been shown to be extended, further reducing operating costs.”