By Tom Kerchiss, RK Print Coat Instruments Ltd
Co-operation and shared goal setting has become increasingly prevalent in label and package printing and converting as all supply chain providers and operatives strive to meet benchmark standards while endeavoring to make a profit. Regardless of whether the process step that you are engaged in is printing at the start of a run or it’s a converting process at the end of a run it is in every one’s interest that process variables and what causes them are understood and that inconsistencies are bought under control in the shortest possible time, with minimum inconvenience and with as little waste generated as possible.
It goes without saying that process quality is only as strong as its weakest link. For example, if print is of the highest quality this will count for nothing if a subsequent coating process, slitting or rewinding goes awry. Apart from product throwaway the printer will have wasted time, energy and material turning out a product with value only to see it scrapped.
When it comes to print processes such as flexography there are devices now such as the FlexiProof for identifying, resolving and controlling ink variables; for colour matching and determining printability: gloss, durability, rub resistance, etc. Other devices, such as the hand held portable trouble shooting device, the EsiProof were covered earlier in an editorial entitled Part 1 of Troubleshooting.
Coating and laminating are process areas that are theoretically straight forward but are becoming more complex as time goes on. Coating and laminating has its own set of challenges, some of which the coating technologist seemingly has little control over.
The substrate onto which the coating is applied is often responsible for a range of defects inclduing: poor adhesion, poor planarity and lay-flat. Substrate associated issues of note include winding irregularities as well as the defects that arise as a result of airborne and other forms of contamination. These defects can occur during substrate manufacture, product shipment and storage and during on machine processing including, when the product is being fed through the machine or on being unwound and rewound. In many instances the problem is exacebated by the presence and generation of electrostatic forces. Even the operator may introduce contaminants such as skin, hair and fibre from apparel.
An inability to coat a substrate uniformily or to wet the substrate effectively warrants mention. In order to deposit a fast flowing fluid coating onto a given substrate with no voids or inconsistencies its necessary that the surface tension of the coating solution and the substrate surface interface optimally.
To achieve the best of surface conditions it may be necessary for the surface to undergo modification via flame/electrical discharge or plasma treatment. Another approach is one taken by the manufacturing chemist whereby surface tension characteristics are measured and the formulae is manipulated within specific parameters.
As with many of the problems that arise during processing, regardless of whether its flexo printing, slitting, coating or laminating there is rarely a clear cut solution, its often a case of eliminating possible causes one by one. Coating uniformity or lack of it can sometimes be due to substrate manufacturing variability or an innapropriate choice of coating applicator. For instance, variation in the thickness in a non-transverse direction can, particularly if knife coating is the selected applicator method affect coat uniformity. The reason that knife coating technology is susceptible to substrate variation is that the blade is set at a fixed height and therefore substrate/coat contact interface is critical.
Staying with knife coating for the moment its worth noting that the way in which an air knife coating is applied can produce some unexpected results. When a coating is deposited on a web at high speed the flow of fluid is subject to an alternating flow regime. All laminar flows on a moving web have a parabolic velocity profile with the fluid at the surface of the web moving at the speed of the web. The fluid at the air interface however, behaves differently and is just a bit slower than the speed of the actual web. In effect the volume flow up exceeds the volume flow down.
With regard to air knife coating the thicker flow builds up just below the air impingement zone or meniscus. Once the air knife allows the thinner coating flow to pass, any excess liquid collects below the meniscus and will not flow on until the lower layer builds up and becomes thick enough to balance the upward shear force that derives from the web. This ebbing and flowing produces tiny fluctuations in the flow of coating just above the air knife which can result in the coating looking flawed. Sometimes the coating fluid smooths down and has a desirable varnish or enamel looking finish. Sometimes it does not smooth down and the coating shows signs of chatter or streaking. If this occurs the coating operative has little choice but to experiment, find an appropriate suficant, one that will promote post coat smoothing without jeapordising wetting or adhesion properties.
Laminating throws up its own share of problems. Sometimes when a two ply dry bonded laminate comes off the machine it has a hazy appearance. This often perplexes everyone, especially when the job has been run previously without any problems using the same substrate and adhesive. In reality the clue to what’s causing the problem is in the hazy appearance. If too little adhesive is applied to the primary substrate high and low spots willl allow air bubbles to develop and with it the hazy appearance. If the adhesive coat weight is increased the haziness should disappear.
As with most processing functions attention to detail pays off. For example coatings and primers must be dried properly. Lets consider an out of line heat seal coating application as an example.
This heat seal coating is applied to foil for the purpose of providing adhesion to polyester. The heat seal coating is deposited on one side of the foil. This coated foil travels through an oven and the dried coated foil is wound up in rolls. The rolls are then stored until they are needed. The roll is unwound and combined with the polyester by passing the two substrates through a heated nip to effect the bond. The material is then rewound with the foil/heat seal coating/polyester ready for shipment to the customer.
Any coating or primer must be dried completely. If not the material could become tacky and when wound up in on itself the material will adhere to the backside making subsequent unwinding impossible
Good housekeeping is a must. The flow rate to roll coating pans should be monitored and adjusted; solution and applicator temperatures must also be watched and the gap between applicator and coating roll measured with substrate in place. Consideration must be given to the correct coating applicator. A slot die applicator may well give good results when used with a closed surface substrate such as polyethylene as the fluid will not penetrate and will be uniform. However the same applicator if used on a substrate with an open porous surface would produce a less uniform result as some of the fluid will penetrate into the pores. In this instance a knife coater would be the best option because as the blade is fixed at a set height from the web there will be uniformity of coating fluid and the pores or cells of the substrate will be filled.
Coating viability depends to a large extent on selecting the most appropriate coating method for a product to be processed – not always easy. The coating operative must take into account product function, desired coating weight, quality level needed, raw materials required, and of course the material onto which the coating is to be applied needs to be considered. There is a wide range of pre and post metered coating technologies that can be used. But which one will give the best reproducible quality? Sometimes its trial and error to determine which technology to use and this is especially true when it comes to developing new products.
The Rotary Koater
The Rotary Koater, a pilot printing, coating and laminating system designed and developed by RK Print Coat Instruments is an ideal system for those faced with daily coating/laminating challenges. As a research and development tool it enables the manufacturer, the printer, the converter and others, to undertake R & D on an economical scale and under precisely controlled conditions. It can be used to test different formulations, substrates and processes. It is also highly effective as production machine for the small-scale production of specialized materials.
On the other hand there are those organizations that for one reason or another need specialized machines; a standard machine simply won’t suffice. The VCM or Versatile Converting Machine meets the needs of this niche market.
Each VCM is purpose built and unique to each customer. A precision system, each is built under conditions of close commercial security. Customers are able to select from best available technology including 15 different printing/coating technologies.