Sometimes it can seem that converting is an exercise in correcting imperfections at every stage of the process; sometimes it can seem as though there are too many variables to contend with that one can be left wondering why you took this or that particular job on in the first place. Fortunately, at least when it comes to inline and out-of-line coating and laminating, many problems can be resolved or even avoided in the first place using an appropriate quality-control and product-monitoring device together with a modicum of care and attention.
For example, in an out-of-line application a heat-seal coating is applied to foil for the purpose of providing adhesion to polyester. The heat-seal coating is applied to one side of the foil using a standard coating unit. The coated foil then travels through an oven, and then the dried coated foil is rewound onto rolls. That roll is then stored until a subsequent operation takes place, whereby the roll is unwound and combined with the polyester foil by passing two substrates through a heated nip to effect the bond. Again, the material is rewound with the foil/heat-seal coating/polyester ready for shipment to the customer.
A number of factors can influence the effectiveness of the process in this example. To begin with, any coating or primer must be dried completely. Retention of any vehicle in which the coating was supplied could make the material tacky. When wound up on itself, this material will adhere to the backside making subsequent unwinding for further processing impossible.
Another problem arises when the coating material or primer is not fully compatible with the substrate. If any ingredients such as plasticizers or low molecular weight materials migrate from the substrate to the coating during storage, the coating may once again become tacky. This "blocking" effect will once again make unwinding a stressful problem.
The migration of an ingredient in a coating onto the surface of the substrate can also be problematic when rolls are stored. This will contaminate to some degree the backside of the material, in many cases rendering the substrate useless.
Laminating also throws up its own challenges. For example, in situations where solvents from the adhesive are being absorbed into the laminate inks, coating, or substrate, the solution generally is to change the adhesive system.
Although changing substrate is often the right way to go, usually this is not an option as the type of application dictates what type of substrate has to be processed. Experimenting and changing to a less aggressive solvent or water-based adhesive is the preferred option. If for any reason this option is unavailable, it may be possible to resolve the problem by increasing dryer air speed. This is generally regarded as the least favourable option as many substrates are temperature sensitive. When solvents are absorbed from an adhesive into the substrate they take a long while to dissipate.
Operators can come across a problem of a hazy appearance when a two-ply, dry-bonded laminate comes off the machine. This may be perplexing, especially if the same substrate and adhesive is being used as in previous runs. The tip off is the hazy appearance. If an inadequate amount of adhesive is applied to the primary substrate, high and low spots will allow air bubbles to develop in the laminate. If the adhesive coating weight is increased, more than likely the haziness will disappear.
A substrate's physical characteristics impact on failure or success. Consider paper—depending on the grade or thickness of the paper, it might not be the best coating option as it may be prone to tear. Alu-foil is another example. Again, as with paper, if the thinnest foil is used, the slightest nick will result in web breaks. Metallized films may seem the best choice for a brand owner looking to make a big impact, but it can become badly abraded during travel through a laminator.
Substrates must have good planarity and not curl, otherwise damage will occur in the coating applicator or in the dryer. Substrates should also have straight sides and be free from telescoping.
The coating solution is of course important, meaning that mixing time and temperature profiles must be right. The correct raw materials must be added at the proper level and at the right points, and solution/dispersion properties—percentage solids, pH, particle size and purity—must be within defined parameters.
Other elements to consider include the selection of the correct coating applicator, which is not always immediately obvious, especially when undertaking research and development or when contract coating; or, indeed, when asked to print, coat, or undertake some other process on an unfamiliar substrate or one known to be temperamental, examples being co-polyester are non-porous and thus impermeable to liquids such as blood, water, and bacteria.
Coating uniformity, the desired coat weight, and the methods to achieve this are vital for many reasons, one of which is that when it comes to expensive materials, no one wants to apply more than is needed. All coating methods provide an inherent uniformity that they can achieve. If the wrong coating method is used, desired uniformity might not be achievable.
Coating has to be less of an art and more of a science. It's not just a case of deciding whether to employ reverse gravure coating technology or meter bar; every element must function in unison and every component must work synchronously. For example, unwinding a web at one end of a machine, then drawing a web through the machine so that it can be printed or coated, then winding a web up at the other end for storage and final shipment needs to be considered and undertaken carefully. Rolls must not only be dried and wound, with the right degree of hardness; rolls must look good and be of the right shape and consistency; rolls must be wound carefully to avoid problems such as blocking, etc.
RK Print Coat Instruments Ltd are headquartered in the United Kingdom but their colour communication and print/coat/laminate product development and quality control systems are sold worldwide. Further information is available by visiting www.rkprint.com |
