When Coating Fails to Impress25 October 2019
Tom Kerchiss of RK Print Coat Instruments writes about
Although the act of depositing a smooth and uniform coating of a prescribed thickness onto a substrate using an appropriate coating technology may go well – it doesn’t always pan out that way. Thin substrates for instance can create unexpected manufacturing problems such as tearing, wrinkling, creasing, stretching and much more, all of which impact on yield, product quality and product viability.
A relatively recent trend is the move towards ever thinner coatings; coatings deposited on ever thinner substrates and often, for very thin end products, such as OLEDs (Organic Light Emitting Diode) displays; batteries; catalyst layer electrodes; climate control components (such as solar reflective films); photo masks and electronic communicative packaging devices, etc. These thinner layers need to be deposited with precision in order to ensure end product performance is not compromised.
Many types of defects can arise including pinholes; localised areas of a substrate that remain uncoated. Sometimes this is attributable to a coating flow that lacks uniformity and/or an inappropriate coating applicator selection but not always. Occasionally the problem is more to do with bubbles in the coating liquid as a result of air entrapment.
A product’s appearance can be marred by streaks and drips due to irregularities in the coating film. Ribbing may also occur. This term refers to the transverse variation of film thickness in a web. This gives rise to a regular striated pattern on a coating film. Other performance limiters may be linked to factors associated with web handling and tension control. Without precise control of web tension, it can be difficult to maintain close coating weight tolerances and also difficult to avoid the various tension transients that limits machine speed, generates higher levels of waste and affects overall quality.
Tension requirements differ according to the zone of the machine, for example: unwind, processing zone and rewind and therefore need to be regulated accordingly. In processes such as laminating, curling, creasing and wrinkling can occur as a result of tension transients. Tension measurement and control systems must be appropriate for the materials to be processed and for the required speed. Watchfulness, mindfulness, attention to detail is necessary. Or to put it another way – don’t take anything for granted. While many aspects of processing can be controlled or resolved through experience, some such as airborne contamination or particulate contamination of a base film can be a major source of product waste, downtime and poor quality.
Unlike many process disturbances that affect a web at selected points, the effects of contamination may be intermittent or may manifest at all points throughout the process. Airborne contaminants such as dust, hair, insects, clothing fibres and even skin shed by the coating operative may be a contributing factor in contamination.
Contamination can occur for many reasons. For instance, during a slitting process dust particles and fibres contaminate mainly the edges of the substrate and remain there during the winding process. In relation to their weight these particles have a big surface area, making it easier for these particles to become attracted by electrostatic forces or be caught and carried in the boundary layer of the substrate surface. Commercially available web cleaners, static discharge systems can be of assistance.
Good housekeeping can resolve many difficulties. For instance, when laminating unwanted matter can to stick to engraved cylinders often resulting in the appearance of a continuous line in a laminate. Cylinders need to be routinely cleaned along with coating components and applicators. Substrates must be handled carefully from manufacture to shipment and delivery to the converter where the adding of value on the coating, printing and laminating takes place. At each stage cost increase as value is added. Simple procedures such as for instance not removing the wrapping from material stock such as films until needed helps to maintain the integrity of the roll. Even when unwrapped, a roll of material, which may have been kept in cool storage conditions, should be allowed to rest so it reaches the same temperature as the working surround. Filmic materials must be stored carefully and off the floor and ideally in a moisture free environment. Moisture can wick into wrap layers and can be a cause of later problems such as blocking.
The coating solution, the mixing and temperature profiles must be right. The correct raw materials must be added at the proper rate and at the right points. Solution/dispersion properties, the percentage of solids, pH percentages, particle size and purity must be within defined parameters and monitored.
Drying technology must be appropriate for the application. Line speed and web temperature must be in agreement and coating/adhesives suitable. Drying processes if not correctly and thermally controlled can result in many serious defects to a product surface. This can happen if the substrate’s transition temperature is above that of the dryer temperature resulting in distortion, variation in flatness parameters as well as other changes in form, function and appearance. Insufficient drying and excessive drying conditions can often be the root cause of blocking especially when processing centres around the production of items such as heat seal coatings.
Another element to consider is whether the correct coating applicator has been selected, which is not always obvious, especially when an unfamiliar material is to be run and when a material is known to be temperamental. Roll coating technologies such as forward roll, reverse roll and gravure, both direct and indirect may be the systems of choice; but so too could other methods of application, perhaps a pre-metered and contained or enclosed method such as slot die. One of the many advantages of this technology is that being enclosed or contained there is no risk of contamination of the coating solution or exposure to outside environmental sources. A popular method of coating is direct gravure, a method whereby a web passing around an elastomer covered impression roller is coated in a nip formed with an engraved applicator roll. Gravure is suitable for a wide range of viscosities. With direct gravure the operative can apply a continuous and specified amount of coating onto the web. If significant changes need to be made to coat weight a different applicator roll may be requited.
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 applicator roll and the web being coated, with the applicator roll rotating in the opposite direction to the web. Gravure can be ideal when engaged in the manufacture of electronic goods and items that require a thin and uniform coverage of large areas with precise edge definition.
Having the right systems in place; pilot coating/print and laminating systems and equipment optimised for the process speeds products development and helps decision making. RK Print Coat Instruments have introduced the VCML Lab/Pilot Coater and for pilot and demanding production purposes the advanced and customer bespoke VCM coater.
The VCML-Lab/Pilot Coater enables operators to print, coat and laminate on all types of flexible substrates and on a reel to reel basis. It has the ability to apply various inks, varnishes, adhesives and paint using environmentally acceptable formulations and where necessary solvent-based materials as well. The VCML-Lab/Pilot Coater also offers short run production capability, making it ideal for speedily bringing products to market; for monitoring quality and for undertaking many types of tests and procedures.
Touch screen controlled and with a web width of up to 300mm the system is equipped with a cantilevered unwind and rewind, head mounting station with tray lift and trough and a laminator station with pneumatic nip.
Flexography, offset gravure and gravure, knife over roll, reverse coating, meter bar, slot die and many other processing options can be made available. The VCML Lab/Pilot Coater can be configured for hot air drying, infrared, UV curing and for corona treatment. Edge guide and heated laminator and ATEX coating zone can also be integrated in with the system.
Finally, the VCM or Versatile Converting machine can be designed and configured for customers with specific needs operating across many industries. Systems currently employed are involved in applications where processing accuracy is essential including: the production of polymeric semiconductors, medical diagnostic technologies, solar reflective films, fuel cells and batteries, printable electronics and even aerospace components.