Coating & Laminating Trust the Pilot for going thin
Tom Kerchiss, chairman of RK Print Coat Instruments, says whether a coating or laminating process is judged a success or not is dependent on the quality of the coating and adhesives and on how well process fundamentals are controlled.
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n other words: what we coat the web with and how we coat and then dry or cure are all of equal importance, they’re process quality interdependent. An inconsistency in any one area negates all of the effort that went into the job prior to coating – or laminating.
Let’s begin at the beginning. Coating mixes as with application areas vary enormously; one can only generalise. For flexible packaging for example, an anti-fog coating may be required to provide a specific resistance or barrier to moisture, light (UV) or to gases and to micro-bio contaminants. If the coating is for an application where food contact is possible, migration, contamination and a risk to health must be given priority above all else. Coating or laminate layers and components must be optimised for the process. Heat sensitive materials for example would best be avoided in a situation where elevated temperatures may arise either when additional processing is undertaken or at point of use. Coating requirements are very different from the 1950s when simple waxes were used to protect liquid gable cartons. In the modern production environment with the focus on containing costs the coating practitioner is charged with processing thinner webs for what in many cases are thinner products. Everywhere you look designers are designing thinner versions of their client’s products and its not just packaging grades of material that are manufactured thin. Automotive panels, insulation materials, electronic and electrical goods, smart phones and tablets are all thinner and lighter than their predecessors.
Rising energy costs and operational outlay as well as the need to conserve material resources and reduce the amount of waste destined for landfill are incentives to produce products to maximise the use of materials as far as possible without compromising on performance. Processing thin webs at high speeds can be challenging. Material inconsistencies such as calliper variations or machine rollers that are ever so slightly misaligned increase the risk of processing problems such as wrinkling and coating voids. Thinner webs are often more sensitive to heat and moisture.
Tension ranges and tolerances may be tighter which in turn makes inconsistencies more noticeable. If the web has a tendency to be brittle then the risk of web breaks increases. Even a ductile material will break if the web goes momentarily slack due to fluctuations in tension. Drying can present problems. In some cases thinner coatings must be of a low viscosity in order that a uniform coat weight is achievable. Thin coatings can be highly sensitive to being hit by dryer impingement air velocity. Where this is an issue indirect air velocity impingement maybe the answer or alternatively other forms of drying or curing may be considered.
requires modification, perhaps adding an additional roller. Gravure on the other hand is well suited for the accurate deposition of thin coating where precision is a must.
Gravure coating and its many variants are able to deliver a pre-metered amount of coating solution onto a substrate. Coat thicknesses in the range of less than 2 micron on up to 50 microns are obtainable. Gravure differs from other roll coating methods in that one of the rolls is patterned with a surface engraving. The shape and size of the pattern can be engineered in order that a uniform and very accurate coating can be produced. Gravure printing differs in the respect that it provides good resolution or sharp definition of print and good edge resolution. Direct gravure coating is where the fluid flow is deposited as implied directly from the gravure roll onto the web. An alternative that converters may consider is reverse gravure. The difference between direct and reverse is that with reverse a speed differential is created between the applicator roll and the web being coated, with the applicator roll rotating in the opposite direction to the web.
Advocates of the reverse gravure
Care needs to be exercised when selecting coating techniques and technology. The complexities of the coating formulations, the individual components, viscosity, the range of solids necessitate different coating approaches and different coat weight thicknesses. Consider the coating of a pressure sensitive adhesive. The actual choice of adhesive and which coating technique to employ can be dependent upon the desired thickness of the PSA, the viscosity and whether the adhesive is an emulsion, hot melt, solids or something else.
Coating speed, web width to be coated and the characteristics of the web material and its surface properties need to be determined. Once these issues have been addressed the most suitable coating technology can more or less be decided upon. All coating techniques offer advantages and disadvantages; some coating techniques are not suited for applications that involve low coat weights. For instance, knife coating presents difficulties and roll coating may in some instances
coating method recommend this technology for the application of thin coatings with zero reticulation. Other gravure coating options include offset gravure. While coating and substrate choices have opened up new opportunities for bespoke products, formulators, converters, technologists and designers must undertake pilot runs, testing and trialling to ensure that the product and processes used meet expectations. Conventional production line coaters are configured for the high-speed output of materials within relatively limited material parameters. Production machines lack the flexibility of pilot coaters and, this limits the ability to experiment.
The VCML Lab/Pilot coating with a working web width of up to 300 mm offers short run capability and can coat, print and laminate on a wide range of flexible substrates. The ease of configurability makes it ideal for those companies engaged in developing and the bringing to market of more environmentally friendly products.
34
March 2023
www.convertermag.com
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