Coating and Laminating Coating Web Control By Tom Kerchiss, chairman of RK Print Coat Instruments
differential heated gravure-offset head for the application of filmic laminating adhesives at coat weights evenly and as low as 0.5 gsm –wet is a recent development and is one of a number of selectable coating technologies that can be supplied with RK Print Coat Instruments VCML-Lab/Pilot coater. It offers short run capability, while allowing for products and materials to be trialled for commercial and product viability, feasibility and other purposes.
D
esigners appreciate the decorative possibilities that many coatings afford. Retailers and brand owners applaud not only the
looks that coatings can provide, but also the functional properties: durability, moisture and gas barrier resistance, scuff, chemical resistance and flexibility. Eco aware and with sustainability in mind, coating practitioners need to be technically astute, output precise and able to adapt to a market that is rapidly changing as alternative materials to fossil fuel derivatives come on stream.
Coating success depends upon the control of process fundamentals. In order to minimise process transients that may occur in a fast-moving web, axial dimensional stability, coating thickness, roll quality, line speed and drying effectiveness must be controlled in order that the product produced is coated uniformly with no variation in the transverse or machine direction of the web. The selection of the correct coating methodology is, of course, important, but equally accurate tension is needed to ensure that a product is free from wrinkles, bagginess and with lamination free from curl.
Controlling a high-speed moving web onto which a coating is to be laid can be challenging enough, more so when ultra-low coating weights are involved. A
Drying or curing a coated product is not always problem free. Some coatings do not take well to being hit with a lot of heat. If the product for whatever reason contains solvents, too much heat makes them skim; too much heat can trap solvents. If the substrate is a thin gauge structure and the coating solution must be applied thinly and with caution, then there is the risk that the coating might dimple or pucker and that the substrate will be marred.
Hot air impingement drying systems are an option, but so too are UV, EB and IR (infrared). With hot air, controlled turbulence is generated in order that the air breaks through the laminar zone of the boundary layer of air associated with fluid dynamics (Bernoulli principle) to promote drying efficiency. Infrared provides certain benefits over conductive and convectional methods. With infrared, energy is transferred not heat. The substrate converts the received radiated energy to heat by absorption and it is said that IR provides for thorough through heating or a defined depth of drying. IR is often selected for spot drying applications, where coatings and adhesives are applied selectively to precise areas. Compact in size, it is easy to fit an infrared drying system in with an existing process line.
Unlike convection ovens, used to drive off water, or solvents and to apply heat to cure coatings, UV energy cures by polymerisation rather than evaporation, therefore VOCs and HAPs are virtually eliminated. Better heat management, energy savings and a resistance to scratches, scuffs and chemicals are obtainable. Gloss, durability and the possibility of faster processing are touted benefits, though the risk of post cure issues in stored rolls warrants care and attention.
12
November 2024
www.convertermag.com
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