Coating and Laminating Coating dynamics
Tom Kerchiss, chairman of RK Print Coat Instruments, says the only certainty when it comes to developing and bringing a product to market is that ‘uncertainty’ is certain.
packaging applications, reverse gravure provides an extremely homogenous coating without pinholes. Reverse gravure also facilitates the application of low and high coat weights simply by changing the anilox roller.
Thinner webs present challenges for drive systems and other devices associated with web handling. Tension ranges and tolerances may be tighter, which in turn can make inconsistencies more noticeable. If a machine is at the edge of its performance envelope or if the coating applicator technology is mismatched for the job, there will be a noticeable rise in waste, particularly at the rewinder due to bagginess, blocking, ridges and coating defects.
C
oatings may be chosen for aesthetic reasons, but more generally these days they are chosen for the degree of functionality that a specifi c coating applied in a certain way brings to the product. Other determinants need to be factored in when deciding upon the coating and technology to employ. The conditions under which the product to be coated will be manufactured, the potential robustness of the coating solution over a given period of expected lifespan of the coated substance, economics, the costs involved, the environmental implications, sustainability, recyclability and feasibility of production. They are all up for consideration. Feasibility is of great concern. Will it process? Will it coat? The introduction of fossil-fuel replacement chemistries and materials more than ever underlines the need for coating devices and pilot/production systems for trialling, the monitoring of quality and short run capability.
Controlling costs can pose problems. It can be particularly diffi cult to quantify, especially when coating those materials needing extra care, the ultra-thin fi lmic web or laminated structure or the bio-based polymers, bio-compostable materials that many converters may not have had any experience with. Running thinner webs in their own right can be diffi cult. They are more prone to be less uniform and can be aff ected by heat and
26 May 2025
With the deposition of thicker coatings, there can on occasion be more of an opportunity to hide or mask minor defects. This is defi nitely not the case when laying down thin coatings where every blemish shows. For this reason, there is often a need for clean room conditions to be put in place for both the coater and the general operating environment. Diffi culties in processing come from a variety of sources. Substrate inconsistency can mean that no two rolls are exactly the same. One roll could be thinner in one spot than another. When placed in a processing context, a variation in gauge may impact on quality, on line speed, and appearance and functionality of the roll at wind up. There is evidence to suggest that high and low spots in the substrate increases the risk of web breaks.
moisture, warranting care when drying, curing and during the actual coating or laminating process.
The move towards using water based inks; water wash inks, adhesives and coatings have meant that dryers must be optimised to take into account the necessarily longer drying times. RK have provided a number of fl exible packaging customers with VCML coaters with three drying tunnels, especially for applying water-based coatings onto paper. Selectable coating applicator technology available for the VCML includes reverse gravure. In fl exible
A variation in the thickness or non-uniformity in the transverse directional profi le can aff ect various processes. In coating and in knife-over-roll uniformity can be undermined. In knife coating the height of the blade is determined and fi xed at the beginning of the run so that substrate and coat interface is critical.
The way in which an air knife coating is applied can induce an anomaly. When the coating is applied at high speed and onto a moving web the fl ow is subjected to an alternating fl ow regime. Laminar fl ows on a moving web have a parabolic profi le with the fl uid on the surface keeping pace with the moving web. Above the web and at the air interface, the coating fl uid behaves diff erently in that it is just a bit slower. To summarise, volume fl ow up exceeds volume fl ow down.
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