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Question and Answer

Solving Film Laminating Challenges

by Staff

November-December, 2003

As with other graphic finishing processes, film lamination can create its share of challenges on press. And, as with other processes, if careful planning and communication are involved with the laminating project, many of these challenges can be avoided. Of course, this is not the real world, so InsideFinishing called upon a few experts in the industry to provide some insight on commonly asked questions about film laminating.

Why is it important for the sheet to be completely dry before laminating, and does that mean the ink is set and cured?
There are various types of printing methods and inks in the marketplace today, so there are many variables to consider depending on the specific application. Typically, it is a good general rule to make sure that the inks on the printed job are completely dry before laminating. However, in the reality of just-in-time delivery in today’s market, this is difficult to ensure. Utilizing quick drying inks with minimal residual solvents is the best choice for laminating jobs and a good place to start.

With water-based or solvent-based ink systems, uncured inks can create several challenges when the sheet is being laminated with a thermal laminating process. The heat of the laminating machine as it applies the film can cause wet sheets and inks to bubble and begin gassing, resulting in air being trapped between the laminate and the printed sheets. The trapped air, depending upon the severity, can show up as blistered areas on the print. Many times, ink manufacturers add specific components to enhance or speed drying times, which also can create adhesion problems with laminating films. Additives such as waxes, surfactants, oils, and silicone can aid in reducing drying times but impede the ability of the laminating film to stick to the surface of the sheet.

UV-cured inks, which have become more and more popular, can create their own set of challenges with laminating. With UV inks, the sheet is printed and must fully cure in-line during the printing process through the exposure to UV light. If the ink is not fully exposed to the UV light and does not fully cure, adhesion will initially look good with the laminating film. However, after 24 to 48 hours, the film can begin to delaminate. The reason for this is that within uncured UV inks, monomers are present that essentially rise to the surface if not fully cured, disrupting the bond of the laminated film and creating the delamination.

With thermal lamination applications, the film is heated up to 220 to 260 degrees when applied to the sheet. With wet lamination, raw laminating film is used and the sheet is not exposed to the type of heat as with thermal lamination. This can help decrease the possibilities of delamination on press. Digital printing, which is becoming more common, can actually re-heat when laminating a thermal laminating film and cause blistering and delamination. With a wet laminating process, this is not a problem, utilizing a water-based or hot-melt adhesive.

Are there special films that need to be used to foil stamp over, emboss after laminating, or when the sheet will be glued? Is there one film that can work for most all applications?
When a sheet is going to be foil stamped or glued, the laminate must have a high dyne count or low surface tension to accept the foil or glue. For most applications, a chemically-treated polyester-based (PET) film is recommended. Polyester, by nature, has a higher surface energy than Polypropylene (OPP) films and when enhanced with a chemical treatment, the dyne level can be raised and maintained to adhere foil and glue. There has been success with corona treating OPP films before foil stamping that can be a less expensive alternative if it works properly.

When embossing a laminated sheet, it is recommended to wait 24 hours after the sheet is laminated to avoid the potential of air bubbles. The key here is to make sure the embossing die is not so deep as to crack the film during the embossing process. A very deep embossing die is not usually recommended with a laminated application.

There is not one film that can do everything. It is always best to work closely with the film supplier to determine the best film for the particular application.

Do all laminating jobs eventually cause curling of the sheet? If so, what can you do to prevent it?
Curling of the sheet can happen with both one- or two-sided lamination jobs. With two-sided lamination, curl is typically stretch-induced, which usually can be easily corrected by increasing or decreasing the unwind tension of either the top or bottom roll of film, depending on the direction of the curl. With upward curl, there needs to be less unwind tension on the top roll of film or increased tension on the bottom roll to effectively flatten the sheet. The opposite would be recommended with downward curl.

With a one-sided laminated job, curl is most commonly caused by stretch or moisture. Stretch induced curl can be controlled by several techniques, including lessening the unwind tension of the roll to decrease stretching the film; lowering the temperature to lessen elongation of the film that may occur due to overheating and the softening of the film; and adjusting nip pressure as the film is being laminated to the sheet. In addition, if the machine is equipped with anti-curl devices such as a breaker bar, adjustments with this can help decrease curl. In many situations, a combination of several of these techniques is necessary to ensure a flat sheet.

Moisture induced curl in one-sided laminated sheets is caused by the unlaminated side of the paper absorbing moisture and expanding while the film laminated side remains stable. The result is curl towards the film laminated side. Avoiding humidity and keeping the sheets dry prior to and after lamination will, of course, prevent this type of curl. This is easier said than done, meaning some curl with one-sided lamination is always a possibility. There are some types of films that can grow with the paper if moisture is present, but this may not prevent curling completely, because the paper may grow differently than the film.

It is worth mentioning that applying film through a wet laminating process versus a thermal laminating process provides less stress on the film and can help prohibit curling. Wet laminating does not expose the film to the type of heat that is necessary to apply a thermal film, therefore applying less stress on the film and decreasing the chance of curl. Raw films used in wet lamination applications are also much less costly than thermal films. In addition, machines such as the Billhöfer, include larger circumference rollers that press the sheet out better and provide a flatter sheet. Although this type of machine may run at slightly lower speeds, it provides an excellent end product with few rejects.

InsideFinishing would like to provide a special thanks to Bill Gaspelin with Protect-All Inc. (262) 724-3292 for his assistance with this article. We would also like to thank John Ducan with Bryce Thermal Finishing Films (800) 238-6563, Walter Dean with Billhöfer USA (678) 947-5433, and Mike Wilbins with alco print finishers (918) 664-8245 for their input.