Heavy Metal

Metallized, "special effect" films have long been popular for decorating racecars, billboards and short-term, promotional signage. Some signmakers, though, shy away from specialty films, because older products often lacked long-term durability and were difficult to cut and apply.

The good news is, newer films feature several remarkable improvements that merit a second look. Better colorants and UV inhibitors extend the outdoor life of more films. Improved release liners greatly minimize tunneling during plotter cutting, and more forgiving adhesives make repositioning during graphic application much easier.

This month’s column covers metallized-film selections, how these films are made, and how you might use these eye-catching films in future signage projects.

Incredible illusions

I recently watched a woman repeatedly stroke her hand over the smooth surface of a metallized specialty film that appeared to be a rugged hunk of metal diamond plate. The amazement on her face revealed her disbelief. The 3-D illusions, which specialty films create, can certainly make one ponder how these graphics were fabricated.

The actual manufacturing process, however, is rather commonplace. Basic production steps — compounding, color matching, extruding, metallizing and slitting — are similar to those used to make other films. And, like any other adhesive-coated material, metallized films comprise sandwiched layers: a plastic film, a very thin metal layer, adhesive and a siliconized release liner. $image1

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Choices

Several plastic-film varieties, including acrylics, polycarbonates and polypropylenes, are metallized for various everyday applications, ranging from plastic potato-chip bags and wrapping paper to solar window film. In the sign industry, two of the most common metallized films are polyester and vinyl. Each film is a different type of plastic, with unique physical properties, performance characteristics and applications.

Vinyl, acrylic, polycarbonate, polyethylene and polypropylene are thermoplastics, solids that can be heated and extruded into a film, and embossed with an engraved roller.

Metallized vinyl is quickly becoming a popular polyester alternative. Its softness makes vinyl easier to plotter- and thermal-die cut, creating a more receptive printing substrate for thermal-transfer printing; airbrushing; vinyl inks; or solvent-based, inkjet inks.

In contrast, polyesters are thermosets, or plastics that start as liquids and cure with heat. Once they’re cured, they can’t be reheated, reformed or thermal-die cut. Compared to vinyl, polyester is harder, more durable and can resist chemical spillage.

Polyester’s toughness is both its strength and weakness, because the facestock’s hardness makes it more difficult to plotter-cut than vinyl. Still, metallized polyester films are offered in most sign-supply catalogs. These films are well-suited and cost-effective for labels, nameplates and short-term signage that must simulate the look and texture of metal.

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Extruding and embossing

With metallized vinyl or similar thermoplastic films, the colorant is an integral film component that extends the product’s fade resistance. Because the color’s in the film, it’s protected from hazards encountered with harsh cleaning chemicals, abrading car-wash brushes and day-to-day driving. In contrast, polyesters are surface-dyed, which subjects the colorant to everyday wear and tear, and the sun’s bleaching effects.

Prior to extruding vinyl films, the colorant is blended with the resin in the extruder’s hopper, to evenly disperse the ingredients. This creates color consistency throughout the roll’s entire web. The mixture is then melted and extruded through a slotted die, using a process similar to extruding acrylic or polycarbonate sheet.

When the film is hot and malleable, the vinyl’s second surface — or underside — is micro-embossed, or "coined," creating a textured film pattern. Prior to adhesive coating, you can actually feel the embossing on the film’s underside. The top surface, though, is smooth as glass, which is critical for printing without imperfections.

To create the micro-embossed texture, a pattern is etched into a flat, metal mold or shim. This flat plate is then wrapped around a cylinder, which serves as the stamping die.

Hot embossing creates a deeper, better impression. However, not all films are hot embossed. Polyesters, for instance, are cold embossed after the film has already been metallized. This technology provides an acrylic coating on polyester films, with the embossing within the coating.

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Patterns fall into one of two classes. The first class comprises smooth, textured, metallized films. Some relatively simple patterns duplicate the appearance of brushed or leaf metals. The second class of patterns — diffraction films — is micro-embossed with fine-line prisms or images that create 3-D illusions or scatter light into a rainbow of colors.

Film pretreatment

To improve the metal layer’s bond to the plastic facestock, the films typically require pretreatment, such as corona treatment, prior to metallization.

During corona treatment, an electrical discharge oxidizes the film’s surface, making it rougher and giving the metallization layer something to "bite." Corona treatment increases the plastic’s surface energy, making it easier for the aluminum to wet out and form a uniform layer of metal, allowing it to better adhere to the film.

A good bond between the metallized and adhesive layers becomes important during application when repositioning the graphic material. A good bond prevents adhesive/metal and metal/film delamination.

Vacuum metallization

After the film’s second surface is micro-embossed, the pattern is coated with a thin layer of metal, which acts like a mirror to reflect light and create special illusionary effects. A frequently used metallizing process is called vacuum metallization. It’s not the only way to metallize, but it’s the preferred method to coat a uniform, thin layer of metal.

Vacuum metallizing involves evaporating a metal, such as aluminum or gold, in a vacuum chamber, so the metal vapor condenses on the film. Although several different metals are used for metallizing, approximately 90% of plastic films are coated with aluminum.

Before the metallizing process begins, the chamber must be pumped out to create a vacuum. Remaining gases will collide with the metal vapor and inhibit the metallization process. Under normal atmospheric conditions, the molten aluminum would only form a pool at the bottom of the chamber.

The aluminum turns into a vapor when it touches a heating element, called a boat or crucible. Its temperature becomes extremely hot, approximately 3,000° F. As the metal vaporizes, a coil of aluminum wire continually feeds into the vacuum chamber to replace the metal that has evaporated. To maintain consistent metal thickness, the rate at which the wire is fed, the chamber’s pressure, and the heating element’s temperature, are tightly controlled.

The cooling drum is positioned directly above the vaporization area. As vacuum metallization begins, the roll of film is fed into the chamber, where it travels around the main cooling drum. The formed vapor rises straight up to the drum, where it condenses on the film’s textured second surface. The evaporation and condensation processes mimic the hot steam from your shower that fogs up a cooler bathroom mirror.

The amount of metal deposited on the film depends on several variables: the aluminum’s temperature in the melting process, the speed of the film as it travels through the metallization chamber, and the atmospheric pressure within the chamber.

The aluminum, which condenses on the film’s second surface, is ultra-thin (roughly four-millionths of an inch). The thin layer acts as a reflector to create visual effects. Because the metallization layer is so thin, it’s susceptible to abrasion, oxidation and corrosion. This layer is sandwiched between the transparent plastic film and adhesive coating, thus protecting it. Unsupported material — a film without adhesive — usually needs a protective coating over the metallized surface.

Best uses

Many sign people are fascinated by the look of specialty films, but they don’t know how to use them. Applications range far beyond good racecar numerals.

Less is sometimes more with metallized films, because a small amount can go a long way. When used with opaque vinyl films, airbrushing and handlettering, specialty films are excellent for lettering and logos.

Last week, I watched a friend create some interesting effects by airbrushing transparent paints onto silvered, hammered-leaf vinyl. For those interested, my friend was using the Createx Auto Air Color line of automotive paint. For more information, go to www.createxcolors.com.

Drop shadows and white contours can provide contrast between lettering and backgrounds. If you’re using an older film construction with rubber-based adhesive, try overlaying the film atop a cast vinyl.

This is especially important when decorating a banner. The cast film will act as a barrier and protect the rubber-based adhesive from the banner material’s plasticizers. Using the cast film as a base layer can also aid in the metallized-film removal.

Dark-colored specialty films and certain textured, metallized films can produce eye-catching backgrounds for signage or tradeshow graphics. Try using specialty materials for automotive striping or decorative sign borders. Finally, consider some of the smooth or textured silver patterns for print media.

Metallized-Film Tips

* Some specialty films are printable; some are not. Shop carefully before selecting a film to use as a print medium. You also need to follow the manufacturer’s fabrication recommendations. Before going into production, test; don’t guess.

* If you decide to airbrush a specialty film, make sure it’s compatible with the paint system. Polyesters typically require a special topcoat, whereas vinyl doesn’t. Before you get started, wipe down the film to remove any oils or other contaminants. That’s also good advice before printing onto these films. Between coats, use a hair dryer to dry the paint. After airbrushing, protect your work by spraying on a clearcoat.

* If printed graphics will be subjected to gasoline spillage or abrasions, protect them with an overlaminate.

* After completing your application, always seal the edges to protect the perimeter. Clearcoating the entire graphic provides added insurance.

* When all else fails, read the product information bulletin and application instructions provided by the film manufacturer. These should cover appropriate application surfaces, substrate preparation and the minimum application temperature.