Quality control is paramount
IT IS important, says Tom Kerchiss of RK Print Coat Instruments, not to underestimate the role that quality control and product monitoring plays in the coating process; devices such his company’s K Printing Proofer play a part in resolving process issues.
These devices are especially useful when a converter or some other user is up against the clock and when product quality must be at it highest. Long established devices such as the K Printing Proofer, or the VCM (Versatile Converting Machine) and a companion system, the Rotary Koater can be used for pilot runs, for small-scale production, for product development and much more.
What if, asks Kerchiss, a two-ply bonded laminate comes off the laminator with a hazy appearance. This is a repeat job using the same adhesive and substrate, so what can be the problem? Could it be excessive drying temperature? A tension imbalance, a low adhesive weight or maybe the haze is due to substrate absorbing solvents? The correct answer is low adhesive weight. The hazy appearance is the tip off; if an inadequate amount of adhesive is applied to the primary substrate, high and low spots will allow air bubbles to form in the laminate. By increasing the adhesive coat weight the hazy appearance should disappear.
To take another example: as laminated paper/PET sheets cure, the edges of the sheets begin to curl, a situation that seems to get worse as the laminate ages. The cause of the problem is probably too much curing agent in the adhesive or the adhesive viscosity is too high? Or is it that the laminator tension is too high? Most likely the latter is the case. When the tension is too high, even a material such as PET can stretch. Following its travel through the nip the film will naturally relax and shrink. Most adhesives also shrink as they cure; with both the material and the adhesive shrinking sheets will curl.
A tension guideline listing can be obtained from web handling manufacturers or compiled in house for various machines and jobs and can be helpful.
Many problems can be avoided through good housekeeping practices. For instance, a commonly run laminating job that normally functions smoothly suddenly throws up the problem of low initial bonds. An operative suggests a badly adjusted doctor blade or a tension imbalance. But is he right? Could it simply be something as simple as a dirty gravure cylinder? If the adhesive has dried and cured in the gravure cylinder, the amount of adhesive transferred to the substrate will drop. Even though the amount of adhesive used seems adequate, the coating weight is reduced and results in the low initial bonds.
To overcome this problem, cylinders and laminator should be cleaned following each run, while coating weights should be routinely evaluated. Other problems include wrinkling of the film; an issue arising from not enough supply roll tension and the appearance of bubbles or silvering between plies of paper and film. Large bubbles on the leading edge of a sheet being laminated are characteristic of what happens when the item is pushed into the nip faster than the speed of the laminator.
The Rotary Koater and VCM assist companies in developing new products and in resolving the various issues surrounding production. The Rotary Koater is a multi-tasking pilot printing, coating and laminating machine, available as either a single or two-station machine. It consists generally of a bench mounted space frame onto which are fitted various printing heads, web handling equipment as well as drying/curing units.
The ease of use and interchangeability of coating/laminating and print heads together with drying/curing options makes the Rotary Koater a system worthy of consideration for those converters, R & D departments and formulators with daily changing requirements.
For those customers seeking a bespoke machine to meet known needs, the VCM concept is an attractive option.
Built to order each and every VCM is designed and constructed using state of the art components and is engineered to tight tolerances. Customers have the option of choosing from more than 15 different types of coating and printing systems, with drying options ranging from hot air, infra-red or UV curing.
Different web path configurations are available and RK Print Coat Instruments VCM can be made explosion proof, configured for clean room operation and fitted with corona treater or other operational critical components. Coating technologies include hot melt extrusion, knife-over-roll, reverse roll, slot die and meter bar; print options include flexography, gravure, gravure-offset and rotary screen play in resolving process issues. These devices are especially useful when a converter or some other user is up against the clock and when product quality must be at its highest.
C = RK Print Coat Instruments: VCM