Coating & Laminating COATING SPECIFICS By Tom Kerchiss, chairman of RK Print Coat Instruments Ltd C
ustomers of coating practitioners expect that they will take delivery of a product that is of a uniform thickness, free from blemishes and optimised to meet
the aesthetic and functional requirements of the application. But coating, as with laminating, slitting and other fast web fed roll-to-roll and roll-to-sheet processes, can at times be unpredictable, especially when the customer and the design and marketing personnel want to make substantive changes to materials and consumables.
Converters, whether for package and label products or for wound care and medical devices or electronics and disposables or indeed something else, may have guidelines, set standards and tolerances to run through at moments of crisis. However, defects that appear constant or intermittently impact on the ability to run the coating machine at high speed, to high quality and on time.
Poor winding properties, poor adhesion, poor planarity, poor lay-flat and absence of uniformity may be attributed to imperfections or lack of symmetry in the surface of a filmic or composite material. On the other hand the fault or faults may lie elsewhere.
Coating viscosity; the thickness or resistance to flow, the C.O.F or co-efficient of friction. That is the slipperiness or skid resistant properties of the coating and how it interacts with the substrate need to be considered. In addition, there are variables to contend with such as those that affect drying, for example, film thickness of coating, or difference in stock, like SSB board (which is very absorptive) or foil (which is non-absorptive). The ability to coat and uniformly wet the
substrate surface is controlled by the surface tension of the coating solution and the substrate. In order that a coating solution and substrate surface interface optimally, it is often necessary to modify surface conditions via flame/electrical discharge or plasma treatment. Another approach is one taken by the chemist whereby surface tension characteristic are measured and the formulae is manipulated within certain parameters. For some applications, dished rolls, wrinkles and even web breaks are symptomatic of poor control of tension at the unwind. The same problems may arise when tension is not precisely controlled at the rewind. Inappropriate tension control manifests itself in many other ways, with wrap around driven rollers amongst the most common Converters finding that they are unable to run different web widths and thicknesses.
Coatings applied should be of the appropriate weight and have that all important, uniformity of thickness. Defects that can interfere with
the ability to achieve a quality product include pinholes, localised regions of the substrate that remained uncoated. This is due to the coating not being laid down uniformly, but may also be caused by bubbles that have formed in the coating fluid as a result of air entrapment. Streaks and drips may also arise as a result of air entrapment or because of irregularities in the coating filmic flow. Transverse variations in film thickness can also give rise to a regular striated pattern or ribbing on the coated film. Coating requirements are becoming more demanding with new materials being trialled and developed as more environmentally favourable in order to reduce our reliance on petroleum and derivatives. Coating thin, often on ultra-thin materials is increasingly important. In many cases coatings and indeed products need to be thin for reasons of functionality. Every day items such as smart phones and tablets are precision coated with conductive coatings for a variety of purposes, including screen presentation and protection. The most suitable coating technology for a given application may not always be obvious. Different products require different pre-metered or post metered coating approaches. Coating is a fast moving, situation specific affair. This can be difficult for product manufacturers, research and development departments and converters to be knowledgeable across all areas. With all of this in mind pilot/bespoke coating systems are of some merit.
26
March 2024
www.convertermag.com
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