Converting - coating & moving webs

14 November 2018

Converting is a no brainer; there are never any problems, after all what could go wrong? In truth and as we all know plenty can go wrong. Tom Kerchiss explains.

Converting is a no brainer; there are never any problems, after all what could go wrong? In truth and as we all know plenty can go wrong. Sometimes a moving web seems to have a mind of its own. Webs can be flighty and from time to time seem almost wilful creatures. Substrates, inks and coatings don’t always get along, and indeed much time can be spent in taking corrective action or deciding upon the best processing method to use.

Apart from some internationally recognised standards such as series ISO 12646-7 that sets out to establish certain process control parameters and tolerances for those engaged in printing and converting there is comparatively speaking little that comprehensively deals with how to regularise the overall print and converting process for quality and profit. True there are many peer to peer and academically written papers that cover specifics such as colour management, tension regulation, anilox selection and so on, but no one, including trade associations and government regulatory bodies have so far come up with anything that could be regarded as definitive in setting out how to set benchmark standards or meet quality assurance objectives.

It is hardly surprising that it is difficult, some would say impossible to standardise many print and converting processes. For one thing they are not like many other manufacturing processes with conveyor flow line output systems and where input parameters once set are generally not touched for long periods. Print and converting is different in that job changes are frequent, so too are the materials used, the inks, the coatings, the design layout, graphic components, colour requirement and substrates. With job changes and materials changes, variables change also. Obtaining consistency of colour and perfectly wound rolls of material are frequently influenced by variables introduced into the process and many are not directly attributable to the obvious dictates of make ready and run.

Take wrinkles as an example of a variable attributable to a number of factors. Wrinkles are a cause of defect in many web-based operations especially where filmic materials are concerned. Web tension must be precisely regulated from unwind through the intermediate zone to rewind to avoid as far as possible the formation of wrinkles, creases, curling and problems associated with the formation of a well defined roll. To the uninitiated the process of producing and winding up a roll may seem straightforward but this is not always the case.

When it comes to rolls of adhesive tape and labels they might be wound with straight edges but later these rolls may telescope sideways, they may also be prone to becoming egg shaped when core supported during storage or when being mounted on the unwind or generally when being handled.

Rolls that change shape or telescope do so primarily because individual layers are slipping and sliding. While many products with high levels of interlayer friction are prone to slippage, even when they possess a low C.O.F wound adhesive products can be susceptible to telescoping, due to factors such as variation in temperature and changes in viscosity.

While some of the problems associated with winding up an adhesive coated roll are due to adhesive shear and Newtonian fluid factors, this is not always the case, sometimes they are due to wound in tension. Hence the need for rolls to be wound tight at the core and then wound with less tightness as roll diameter builds. With regard to our coated adhesive example if the coating applied to the web when dry and not tacky the roll can be rewound in a normal manner and within constant tension parameters. The exception is on rewinders where a reducing tension on the rewind is needed for good reel. For example when the coating is silicon or another slippery material and when the reel ratio build up is too high. In this case the tension needs to be reduced as a function of rewind diameter and is known as taper tension.

Web tension affects so many areas of performance. It affects the accuracy and consistency of coating thickness, axial dimensional stability of the web, inaccurate print registration and much more. An additional problems that can be time consuming is that sometimes tension control related issues may be due to other factors, which is why many converters, printers, consumable providers (inks suppliers, etc.) utilise quality control devices and operate to a series of check lists enabling them to rule out obvious problems.

Some confusion can be caused by what happens prior to the converter or printer even begins to run a web; by factors completely outside the control of the printer of converter, by what happens during material and consumable manufacture.

Substrates and inks can produce some baffling results for the non-aware. Optical brighteners added to certain papers can affect spectrometer readings. In paperboard/corrugated applications and when providing a customer with a colour match it is essential that the sample shown is exactly the same as will be commercially printed upon. Printing a colour on Kraft, mottled or a clay-coated sheets will yield different colour shades despite the fact that the printer is using the same colour. The reason being that sheets with a high level of absorption properties will produce a weaker looking colour while sheets with lower absorption characteristics will produce a stronger looking colour. If a clay coated substrate is used the ink may not even penetrate the surface.

Colour communication, sample preparations or proofing devices such as the FlexiProof for users and producers of flexo inks and the K Printing Proofer, which produces proofs from gravure, gravureoffset and flexo inks are useful for determining substrate/ink interaction, colour matching and for resolving print related issues. 

In the coating of a moving web coating uniformity across the width of the web is an important operational parameter. To produce a product that is coated uniformly with no variation in the transverse or machine direction of the web depends upon every element functioning correctly. There are many types of coating applicator technologies to choose from and they all have advantages and disadvantages, no one method is suitable for every application. They differ with regard to optimum coating deposition, viscosity and line speed capability. An unsuitable coating method will result in a high level of material waste and product reject.

I will return to the subject of applicator technology a little later. Let us just consider some of the problems that can arise and which are related to substrate, adhesion, winding and drying of a coated product, in this instance a heat seal coating. In this example of an out of line application a heat seal coating is applied to foil in order to provide adhesion to polyester (when needed). The heat seal coating is applied to one side of the foil using an appropriate coating applicator. The coated foil then travels through an oven and then the dried coated foil is rewound. So far so good - the roll is then stored until needed. At this point the roll is unwound and the polyester and foil are combined. This is achieved by passing the two substrates through a heated nip to effect s bond. Again the material is rewound with the foil/heat seal coating/polyester and is ready for shipment to the customer.

The converter may feel secure and believe that that is the end of the job and all has gone well but this is not always the case. A number of variables can influence the effectiveness of the process. For instance, the coating or primer must be thoroughly dried. Retention of any vehicle associated with the coating supplied could make the material tacky and when wound up on itself it will adhere to the backside making unwinding impossible.

When jobs are landed on the slimmest of profit margins; when competition is often global in nature the need to produce a quality product with minimal downtime, waste and as quickly as possible many converters, printers and consumable providers rely to a greater extent than in previous times on product development, quality control, pilot coating, print and laminating systems to help prevent or to resolve potential processing issues. 

To return to the importance of selecting the right coating technology, print laminating or indeed drying technology the most suitable choice for reasons of both quality and economics are not always obvious or clear-cut. In areas such as research and development, when trialling new materials prior to a production run or when engaged in running a specialised product, a degree of experimentation may be required, perhaps trying pre and post metered coating systems or different drying techniques such as hot air, infrared or UV curing. In many situations it makes commercial sense not to take a machine out of the production flow but to employ a pilot coating, print, laminating device, which can provide the level of flexibility needed to evaluate different processes.

Two pilot coating, print/laminating systems developed by RK Print Coat Instruments, each aimed at a different category of customer. The Rotary Koater is suitable for those converters and other customers who face the challenge of frequently changing product changeovers. The VCM on the other hand is a customer bespoke system, each VCM being designed and constructed to meet specific requirements: therefore no two VCM systems are the same. 

The Rotary Koater enables users to conduct trials prior to bringing a product to market. It can be used for trialling, testing formulations and evaluating different methods of coating. It is often used for small-scale production, often of specialized label and tape products.

Selectable web path and a choice of almost two-dozen coat/print head technologies make the Rotary Koater a popular system for those converters facing frequent product changes. The Rotary Koater also allows for hot air, infrared drying and UV curing. Wet and dry laminating is also available. Because of the complexities of modern day product development and production some converters, product developers, etc., have difficulty in finding a standard machine that meets their individualistic requirements.

Custom designed the VCM ensures that the converter/manufacturer/ink/coating provider and substrate producer obtains a high tech machine that perfectly meets their requirements. The system can be flame proofed/explosion proofed and made suitable for clean room running.

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