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There's usually more than one way to do something. Sign makers, for example, have devised many different techniques for laminating prints. What follows are some tips that have worked for some industry friends. Hopefully, these words of wisdom will help you prevent laminating mishaps, save production time and, most importantly, improve profitability.

Dry Run

Before applying an overlaminating film to your print, be sure the ink on the print is completely dry. If the ink has not cured properly some of the problems that can ensue are bubbles under the laminate, yellowing of the media and overlaminate, and migration of solvents through a vinyl film and into the adhesive, causing adhesion failure.

Two good rules of thumb are:

• Wait 24 hours before laminating the print, so the inks can cure properly
• Before mounting or rolling up the laminated print, let it lay flat for 24 hours.

Print-drying time varies, depending on the print's ink density, the ambient temperature and your shop's humidity.  Even if the print feels dry to the touch, its ink may not be completely cured.  Usually, inks take longer to dry when temperatures are cooler and the humidity is high. Consult the manufacturer's recommendations on drying time.

Read & Heed

When all else fails, read the instructions!  Before threading the overlaminate through the machine, study the roll-laminator diagram in your owner's manual. The laminated print's finished appearance depends upon several variables, including the feed roller's unwind tension, the nip rollers' pressure, laminating speed and temperature. All laminators are different, so you must determine your machine's optimum settings. After you load the overlaminate roll onto the unwind shaft and secure it in place, make sure the roll can't shift from side to side. The tension on the unwind roll should be low. As a general rule, use the least amount of tension to accomplish the job. Don't allow the unwind roller to free-wheel. A slight amount of tension ensures that the overlaminate feeds into the nip rollers evenly. Some back tension will prevent wrinkling and trapped air bubbles. High unwind tension stretches the overlaminate. Any stretched film, especially a stiff film like polyester, has a memory and tends to return back to its original shape. This can cause the laminating film to either curl in the direction of, or delaminate from, the print. Excessive tension can also contribute to delamination of the print from the substrate to which it's mounted. High unwind tension and high laminating pressure facilitate print curling.

Lamination

Printers have developed many different laminating techniques; there is no one right way to laminate! If you find a technique that works for you, continue to use it. Here's one that you might try.

• Slit the release liner, 18 to 24 in. from the overlaminate's lead edge, across the web, without cutting through the film facestock. Don't remove the liner from these first couple of feet of overlaminate. This lead edge will aid film feeding and prevent the adhesive from sticking to, and wrapping around, the bottom roller.
• From the slit you've made, peel back enough liner so you can tape it to the take-up shaft (if your laminator is equipped with one). As you guide the overlaminate through the rollers, ensure that the material is feeding evenly, and that no wrinkles or bubbles are forming. During startup, you likely wasted a few feet of overlaminate.
• Once everything is running smoothly, you can feed the prints into the laminator. Laminating prints is always easier if one person stands in front of the machine and feeds the print, and another person guides the laminated print and prevents the laminate from wrapping around the rollers. Apply laminating film to a print in one continuous pass. And, whatever you do, don't stop in the middle of a print to see how it's going. Starting and stopping during this process will pick up the impression of the release liner — at the point where the liner and film separate — causing silvery lines in the adhesive and over the print. This is especially noticeable over the print's dark “shadow” areas.  (It's worth noting that in most cases this problem can be easily fixed by burnishing the line with your thumb nail to aid the adhesive in wetting out.)

Speed and settings

Pressure-sensitive films usually laminate better at slower speeds. Thus, set your laminator's machine speed to 3 to 5 ft. per minute. Then set the machine's pressure-control gauge, which governs the nip rollers' pressure.  Pressure-sensitive overlaminates require pressure so the film's adhesive properly flows out and makes complete contact with the print's surface. Insufficient pressure can result in silvering and air-bubble formation. Conversely, too much pressure can cause print wrinkling and curling. The required pressure varies, but overlaminates generally require between 30 and 50 psi of pressure. If air bubbles appear, you can increase the nip pressure, slow the laminating speed, and, if worse comes to worse, apply a little heat.

Thicker overlaminates, such as 5-, 10- and 15-mil polycarbonate films, typically require higher pressure settings. When laminating graphics with these films, using a “sled” (which comprises an 1⁄8-in. sheet of polycarbonate or acrylic) underneath the print can increase pressure and even out the pressure across the laminator's web. Should you use heat when laminating a pressure-sensitive film? Some people argue that with a little heat on the top roller — from 80 to 110°F — the overlaminate's adhesive flows out better, which creates a better bond to the print and can prevent silvering.  An opposing argument says that heat can cause the overlaminate to stretch.  As the overlaminate cools, it then contracts, causing the laminating film to shrink and in some cases delaminate from the print. The shrinking film quite often causes the print to curl to the print side. Heat can cause other problems too, including waviness, tunneling and wrinkles. Basically, unless you have a serious problem, it's best not to use heat. 

Rolling Laminated Prints

Whenever possible, store and ship prints flat, especially when using polyester overlaminates. Generally, a flexible overlaminate, such as vinyl, is preferred for rolled graphics. If you need to roll a print, place the printed image on the outside of the roll. Never roll a print too tightly, or you'll end up with tunneling between the overlaminate and print. When rolling up a printed vinyl film with a vinyl overlaminate, the core's outside diameter should be no less than 6 in. When rolling graphics protected with a thick (5 to 15 mils) vinyl or polycarbonate overlaminate, the interior roll should be no less than 12 in. in diameter. Basically, the thicker the overlaminate, the larger the core diameter should be.