UV Reflector Geometry for Curing Inks and Coatings
When selecting a UV curing system for curing inks and coatings it is essential that the lamp head reflector is matched to the printing process since this is critical for optimum ink curing. The materials selected, surface profile and geometry of the reflector all combine to form different characteristics. Up to 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.
“To fully understand reflector performance and provide the correct reflector profile for the job intended, it is essential to accurately measure the delivered UV intensity of the various designs available,” says Malcolm Rae, managing director of GEW. “Even though GEW has conducted many bench top reflector studies, we concluded over a year ago that we would conduct a detailed scientific investigation of reflector optimisation under closely controlled conditions. We brought in-house a highly trained scientist to design our experimental parameters and conduct thorough testing under a single variable environment. A summary of our findings and conclusions are presented below.”
Focussed Reflectors:
The first step in understanding reflector performance is to understand the aspect UV lamp output irradiance focussing 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 focussed reflector array is used in order to obtain the depth of cure and press speeds common in the printing industry today. This is particularly the case when used with flexographic and screen printing where thick ink coatings along with metallic pigments and varnishes are used. The new XC ‘extreme cure’ reflector from GEW is designed with fully-focussed, dichroic-coated aluminium reflector with integrated clam shell shutters to protect the substrates and is ideally suited to thick film coatings including screen, cationic inks and where ultimate curing is needed at high speeds.
Diffused Output Reflectors:
Reflectors giving a diffuse output are normally used to reduce the intensity of the reflected IR (heat) impinging on heat sensitive substrates. Most all UV emitting lamp output consists of around 60 percent IR energy. Not only do reflectors focus UV energy but also IR energy. The down side of a diffused reflector array is the reduced curing efficiency in terms of depth of cure and also a reduction of the cure rate which is partially temperature driven.
Independently Controlled Reflectors:
A third reflector design has been pioneered by GEW. Here each reflector is 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 to construct the test reflectors ranged from a basic polished aluminium material to more exotic non-metallic substrates coated with a number of specialized coatings. A standardized UV spectrophotometer was used to measure the entire UV energy output of each reflector construction. The energy measured included UVA, UVB, UVC and UVV, which were then summarized for total UV energy output. The bar graph table below summarizes the relative total UV irradiance measured.
The results obtained have a number of significant ramifications. First, the total UV output of a reflector array can be optimized for obtaining the most efficient curing ability. UV curing involves use of significant electrical energy. Knowing there is as much as a 40 percent 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 UV curing systems are specified with a reflector type best suited to optimise performance to a particular application. Flexibility is provided by a fully modular design and all reflectors are mounted on interchangeable cassettes. A simple quick-release latch mechanism enables differently configured cassettes to be swiftly mounted into the lamphead as required.
Glass Dichroic:
The latest development in reflector technology from GEW is featured in its new ISO-cure model, a new concept in UV curing with water-cooled reflector profile designed for processing a variety of substrates on printing presses in the mid and wide web range; from 450 mm (18”) to 1450 mm (57”). The ISO-cure design is optimised for situations where the UV lamp head has to be integrated in a compact space on machines such as central impression, flexographic printing presses for flexible packaging.
ISO-cure features the energy saving, e-brick electronic power supply and new lamp reflector design from GEW with all electric shutter operation for maximum performance and cure efficiency. Rae concludes, “The design of the reflector on the ISO-cure includes a specially developed glass dichroic surface that enhances UV output whilst maximizing the absorption of IR. Research has shown that this reflector construction gives the best performance for optimising the ratio of UV reflection to IR absorption. Additionally, the life of the reflector has been shown to be extended, further reducing operating costs.”
- Companies:
- GEW, Inc.