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Jan 1, 2004 12:00 AM
In “Laminators: Looking beyond image protection and preservation” (January 2003), we offered an overview of the laminating process as well as wide-format equipment/material options from Autobond, Advanced Grieg Laminator, Billhofer Maschinfabrik, D&K International, GBC Films Group, Seal Graphics and Transilwrap. We also featured some tips from David Goetter, director, Digital Graphics Consulting (Pensacola, FL), and mentioned the consultant's free limited-edition CD-ROM, “I Could Just Scream: Troubleshooting the Top 5 Laminating Mistakes.”
Goetter, a former screen and digital printer, developed the wet transfer process in wide-format electrostatic printing and has helped design laminators. His CD certainly touched a nerve with our readers — we were swamped with requests. So stop screaming and start reading: This article covers boat wakes, infeed waves, print curl, silvering and blistering.
“Boat wakes are nothing more than film distortion,” explains Goetter. The name refers to distorted lines in film, which radiate out as a wake would behind a boat. “Whether you're using heat-activated or pressure-sensitive films, these films will stretch,” says the consultant. “They're subject to the forces that are exerted when processing them through the laminator.”
There are three types of boat wakes: those that point away from the operator, toward the operator, and actual stretch lines, where too much pull pressure has been exerted on the film.
Solution: How are you webbing your laminator? Don't bypass the chill rollers, urges Goetter. “I don't recommend webbing the film straight through the back of the laminator,” he says. “The chill rollers, in combination with the cooling fans, are the best way to avoid boat wakes. As a print enters the front main rollers and heads toward the back pull rollers, there are a lot of forces that can stretch or distort the films. This includes the amount of pull-pressure that you've set and how hard the pull rollers are driving the print out of the machine.”
According to Goetter, it's far easier to use a laminator's cooling mechanisms than to make infinite adjustments on the back pull roller or clutch pressure. If your laminator doesn't have cooling rollers or fans, try the following. For boat wakes that point away from the operator, decrease downward roller pressure on back pull-rollers. If the boat wakes point toward the operator, increase the downward roller pressure. If you're driving the film too hard, you'll stretch it, resulting in long lines running the entire length of the print. “If that's the case, decrease the clutch pull-pressure on the back pull-rollers,” advises the consultant.
Infeed waves — a line of bubbles — typically result from excessive downward roller pressure. To visualize what causes this problem, Goetter suggests pressing your thumb and index finger together to form an “OK” sign. “This point of contact [between your two fingers] is referred to as the nip,” says Goetter. “It's the same principle as when the laminator's upper roller makes contact with the bottom roller. The harder you press your two fingers together, the greater the area of contact. The same thing happens when you apply more pressure from upper roller onto the bottom roller.”
If this area of contact or “footprint” varies in width, the print will travel through the laminator at varying speeds. A print subjected to the subsequent buckling action will come into contact with the adhesive prior to entering the laminator's nip. “This traps air in between the overlaminate and the print, causing bubbles in the backside,” notes Goetter.
According to the consultant, if a footprint varies in width, it probably isn't parallel. “If you apply enough downward pressure, you can create roller deflection,” he explains. “In other words, you can bend or bow the roller. This would cause greater pressure on the roller's right and left, with less pressure in the middle, thereby creating a footprint that is wider on the right and left but narrower in the middle. This uneven/unparallel footprint will pull the print into the machine at different rates of speed, causing infeed waves.”
Solution: Some laminators feature crowned (barrel-shaped) rollers to compensate for roller deflection. Use your laminator's “magic window” to determine if a print's footprint is parallel. “Between the front main rollers and back chill rollers, there's a window-like area that should be wrinkle- and bubble-free,” explains Goetter. “What's magic is what you can see. Advance the film a couple of inches to see a visual representation of the footprint. It should be parallel across the web.”
If the footprint is narrow in the middle and wider on the two ends, reduce downward pressure on the main roller. If the footprint in wider in the middle, but narrower on the sides, increase the pressure.
Print curl is exactly what it sounds like. The culprit is typically film tension, webbing path or incompatible film combinations. Goetter says many operators attempt to compensate for print curl by adjusting brake tension. “This is a common mistake,” relates the consultant. “An operator will often see a few lines and immediately apply more brake tension to try to pull them out. That's not the best answer, because excessive brake tension can lead to [more] print curl and a host of other problems. The best thing to do is slow down and let the film react to the heat.”
Solution: As with boat wakes, Goetter says it's best not to thread film from the front main rollers straight through to the back of the laminator. Film sets, or forms, as it exits the front rollers. “You can change the film's form by changing the webbing,” notes Goetter. Webbing options include running the film over the first chill roller and under the second, under the first and over the second or under both. While the optimum webbing path will vary by film combination, the consultant says less film tension is generally best.
“If you've adjusted brake tension and tried different web paths, and you're still having problems, you could be combining the wrong films,” adds Goetter. Rather than leaving film choice to a salesperson — who may simply spread out samples in front of a client and let the client choose the prettiest one without realizing the inherent production challenges — Goetter suggests a three-point analysis. What type of adhesive will be used? Is this adhesive compatible with the application? Finally, evaluate the film and its properties against the application. Gloss films, for example, can pose a problem under certain lighting conditions.
When air is trapped underneath a laminating film, silvering results. There are three main causes: insufficient downward roller pressure, leading to poor contact between the adhesive and print, incorrect roller temperature/speed, and film that lacks enough adhesive to cover the surface area of the print.
Solution: Increasing downward roller pressure will solve some silvering problems (but don't overdo it or you'll end up with boat wakes). Evaluate film temperature. Your adhesive should be soft and pliable before it enters the nip — the point where the two rollers come together. “Wetting out of the adhesive is important for getting a good flow rate,” says Goetter. “Ultimately, the flow rate prior to the nip [determines whether] you get good contact between the printed surface and the adhesive.” Thicker films may require a little more processing time than thinner ones.
Finally, consider film construction. Thermal films have three parts: the finish, the film itself and an adhesive layer. “A polyester film might have three mils of polyester and two mils of polyethelene adhesive,” notes Goetter. “Depending on the surface you're laminating, the adhesive might be insufficient to flow out and fill all of the voids, leaving small air gaps that result in silvering.”
Cold (pressure-sensitive) film may have air trapped in it. “This air is very difficult to get to flow out prior to going into the nip, because it's trapped underneath the film and over the surface of the print,” relates the consultant. He suggests applying a small amount of heat to change the viscosity of the adhesive prior to its entering the nip.
As a print exits a laminator's front rollers, hot spots can cause a blistering effect. “Heat rises,” submits Goetter. “Depending on how long rollers have been stopped, heat can rise to the center of your top roller, creating a hot spot. Roller temperature can increase by as much as 30 degrees.”
Solution: Unweb your machine when you've finished a job. Ultimately, the print media used will determine the optimal temperature.
Katherine O'Brien joined AMERICAN PRINTER in 1996. Her e-mail is firstname.lastname@example.org.