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Packaging Laminating Films – A Closer Look

Film lamination continues to be an area that many trade finishers have either added as a new service or made expansions to existing laminating services. Laminating is a natural transition for many finishers because it is a dry process, similar to foil stamping. As with hot stamping foils, there are many laminating film choices. Inside Finishing has solicited experts on the subject to help explore some of the questions that frequently surface in regard to laminating films from selecting the correct type to new film products on the market.

There are three basic types of films utilized in graphic arts laminating, which are available in a variety of thicknesses and finishes. All have their advantages depending on the application.

Polypropylene (OPP)
        Polypropylene provides the highest gloss levels of the three main types of films available. However, the user trades high-gloss for durability. OPP is the softest of the three clear base films, creating a greater potential for the finished piece to scuff or scratch. Polypropylene is the least expensive film type, and it also runs very easily through the laminating process. It bursts well and processes nicely for diecutting and other binding operations. It also is receptive to certain varnishes and coatings for other post lamination finishing operations. It is most commonly used on book covers and jackets.

Polyester (PET)
        Polyester is the most popular type of film in the marketplace. It combines durability with reasonable cost, although it is not the least expensive film. Polyester based films have excellent scuff resistance and durability, so they are a great choice for applications where the printed material will have a long shelf life. Polyester also has good gloss levels, although not typically as high as polypropylene films. Most films that are manufactured with enhanced surface treatments to accept foil stamping, printing, and/or gluing are made of polyester. If the right precautions are taken, polyester can run smoothly, although challenges can occur with sheets bursting cleanly and some slip problems can occur.

Nylon
        Nylon is an excellent choice for applications where curling of the sheet is of great concern (i.e. soft covers and paperback books). It is the only film type that is porous, so it can absorb moisture with the paper stock, helping to eliminate curl problems. In addition, nylon has excellent resistance to heat, as utilized in the thermal laminating process, so it won’t stretch or expand. It also will not shrink once the sheet is cooled. Nylon is the most expensive type of film available and can be as much as 30 to 40 percent more expensive than PET, and as much as 50 to 60 percent more expensive than OPP. However, there are applications where nylon is simply the best choice. It is mostly utilized for thinner sheet applications where scuff and scratch resistance is of great importance. Nylon films are generally more difficult to run because they require high temperatures to laminate and nylon does not burst cleanly.
         Besides clear laminating films, there are a variety of specialty films that can be laminated for covers and other applications. Metallized films in gold and silver as well as other colors are available, and holographic films are now becoming more popular in many patterns. Most often, these films are then over-printed or even foil stamped once they are applied. There also are iridescent films that have unique color shifting abilities and are translucent, so they can be applied over printed graphics.
        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.
         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.
        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.

         The type of application and the desired thickness of the end user certainly play a role in the thickness of the film. However, the type of laminating equipment on which the film is being run is also a factor in the thickness of the film used. Generally, the films range in thickness from1.2 mil to 1.8 mil for most common applications. High-speed units with large heated drums and heavy nip pressure generally use thinner material due to lower cost, faster productivity, and easier separation of the sheets. Older, slower laminators with limited nip pressure and imprecise temperature controls will require thicker gauge material in order to achieve a proper laminated finish.
        All films should be stored in a cool, dry location away from direct sunlight and in the box it was shipped in, if possible. Also, consider using a first-in, first-out inventory system. Nylon should always be stored in a concealed wrapper to help maintain its inherent non-curl properties. Typically, summertime presents additional storage considerations. The higher humidity in certain regions of the United States will require partial rolls of film to be re-bagged to minimize the humidity’s effect on film. Another situation to be aware of involves the exposure of film to chemicals. Polypropylene has good resistance to chemicals – polyester and nylon do not. It is recommended to keep polyester or nylon as far away from the chemical source as possible to avoid exposure.
        Because there can be slight differences between different lot numbers of a roll of film or potential changes in adhesion properties after a splice, it is recommended to perform standard tests on the adhesion of the laminated film. The following are a few recommended tests:
The Tear Test
        This is done by pulling the corner of the film away from the edge of the paper stock once it has cooled. Watch for the lamination pulling at the fiber of the paper. On some coated stocks, the laminate will pull ink directly from the sheet.
The Crease Test
        This test is done by simply folding the laminated sheet and checking the adhesion in the crease. Look for any signs of the film releasing from the stock.
The Crush Test
        This test is done by crumpling a laminated test sheet and then checking the adhesion in the same fashion as the crease test. Although it is not always a feasible option in today’s ‘need it yesterday’ environment, it is recommended to wait a full 24 hours before sending the substrate onto the next finishing process. This will help improve the bond and should make it easier to emboss and/or diecut. Each operator will have to determine whether he/she needs to wait before continuing onto the next finishing state. Successful bonding will depend on the substrates and inks used combined with the environmental conditions.

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