GLASS FORMING & PROCESSING: Coating in Reverse
Polyester film, tinted to match the color of vision glass when viewed from the exterior, can be factory-applied to glass with an adhesive. Although this method was once widely used, problems with film application and durability have caused its usage to decrease substantially over the past several years.
Shadow box construction behind non-opacified glass using uniformly dark materials can also be employed, but the use of non-opacified glass can result in appearance problems related to condensation and staining. A better solution is to use heat-strengthened insulating glass with a dark frit opacifier on the fourth (innermost) surface.1
Coating spandrel glass is the most popular method of opacification and is typically performed by glass fabricators. However, even this method is not without its problems. If a fabricator applies insufficient amounts of coating to the glass substrate, or applies the coatings unevenly, "shadows" of the building's inner spandrel structure might be seen from the street. Quality assurance procedures during the coating process usually catch these defects, but corrective steps can add substantially to the fabricator's material and labor costs-which are already high in most traditional coating processes. A new method is needed to increase the quality and efficiency of the coating process.
Coating MaterialsCeramic frit coatings are among the oldest materials used to opacify spandrel glass. The original formulation was lead-based, but in 1990 Ferro Glass Systems (then Drakenfeld) introduced lead-free frit coatings. Frit basically consists of glass particles, inorganic pigments and chemicals that aid in curing. After the frit is applied, the glass is subjected to curing temperatures of around 1400°F to physically bind and fuse the coating into the glass prior to tempering. The result is an extremely strong and permanently colored spandrel glass.
Silicone coatings, which are also widely used, have what Kris Vockler, marketing manager of ICD High Performance Coatings, aptly calls, "a close kinship to the actual glass which is made of silicone and oxygen elements. This creates a natural tough bonding of the coating to the glass surface."2 Lead-free silicone coatings are applied cold to pre-tempered glass, after which the coated glass may be run through a curing oven (up to 400°F). The elastomeric nature of silicone coatings also provides an additional safety benefit: The applied coating cures to a rubber-like film that meets all ASTM criteria for fallout protection against damaged glass without the need of additional taping or film application.
The test of time has proven both ceramic frit and silicone coatings to be highly durable. They maintain their initial adhesion, color and appearance through years of global use. When appearance problems do arise, they usually result from aberrations in the coating process. Fabrication flaws of lesser importance, such as pinholes and striations, might not be visible from the exterior and are usually not considered defects. However, some defects arising from the coating process-such as an insufficient or uneven coating application-can be quite serious and costly for the fabricator.
Conventional Application ProcessesCeramic frit and silicone coatings are commonly applied to the glass substrate by spraying, screen printing, curtain coating or roller coating.
Spray coating is a process involving handheld or automatic airless spray systems. It has advantages in coating uneven surfaces and is used in those instances when bent or irregular-shaped spandrel glass is specified. For flat glass coating, however, spraying has numerous disadvantages. According to Ferro Glass System's publication, Glassworld, spray coating is very wasteful. Roller coating, for example, "can cover almost twice as much spandrel, some 18 to 20 square feet with one pound of lead-free color compared to ten square feet of spandrel per pound applied by spraying."3 If hand spraying is used, there is also a problem of thickness variations in the coating, as well as greatly increased labor costs.
Screen printing/coating (the transfer of a pattern onto a surface by forcing the coating through a screen with a squeegee) is used primarily for making patterned spandrel glass. Most screen printing is manual or semiautomatic, with higher associated labor costs compared to completely automated processes.
In curtain coating applications, the glass is passed under a thin curtain of liquid coating falling by gravity or pressure. The unused liquid circulates continuously, and the coating thickness is directly proportional to the coating flow rate and inversely proportional to the conveyor speed. Curtain coating creates a very uniform coating; however, for smaller production runs, the time spent in cleanup and preparation for the next run can be considerable. Spray heads, circulating plumbing and pumps need to be cleaned regularly, which typically requires an hour or more of downtime between runs. Moreover, curtain coating can take up to 15 gallons of coating just to start up-a large amount for small runs.
Roller coating is a method in which the coating is transferred to a flat substrate by a roller or series of rollers. The process is capable of applying a wide range of coating thicknesses while requiring a very low volume of coating. Cleanup and preparation for the next run can be performed in less than 15 minutes. With ordinary roller coaters, however, uneven coatings or striations can occur as the moving glass substrate moves past the coating roller. While such slight coating aberrations are inconsequential in many applications, they can be serious defects on spandrel glass.
Moving Forward by Going in ReverseSince 1942, The Union Tool Corp. (UTC), based in Warsaw, Ind., has made machines that handle and process flat sheets, boards and panels made of wood, metal, plastic, fiber, foam and glass. Among those machines are roller coaters used in dozens of industries and hundreds of companies worldwide.
"One of our customers challenged us to develop a roller coater that would coat spandrel glass absolutely evenly and without any visible striations," recalls President Charles T. Simpson. "They wanted the labor and material savings available with a roller coater, but they didn't want the problems."
UTC engineers knew that the locus of the coating problem was the application roller. As the roller deposits coating onto the substrate, even slight irregularities on the roller surface can affect the disposition of the coating. Moreover, minor variations in the surface viscosity of the coating can also affect the evenness of its disposition onto the substrate.
UTC was aware that a different type of roller coater was being used in Europe for coating light-sensitive lithographic plates. Coatings on those plates had to be absolutely even and free of striations-exactly what the spandrel glass industry was looking for.
"What we found," Simpson recalls, "was that those roller coaters did not employ complex technologies to do their job. They just turned things around." In other words, they reversed the rotation of the application roller.
While the application roller in an ordinary roller coater rotates in the direction of the moving substrate, the application roller of a reverse roller coater rotates against the direction of the moving substrate (see Figure 1). Rotating against the direction of substrate travel, and very slightly out of contact with the substrate, a "wave" or meniscus, of liquid coating is created. Though hardly visible to a naked eye, the meniscus deposits an extremely smooth and even coating on the substrate.
Additionally, the reverse roller coater maintains the benefits of conventional roller coaters, including a minimal amount of coating material required for startup, the ability to leave the edges of the glass clean (both sides, leading and back edge), a maximum of 15 minutes cleanup time, less wasted coating material during operation and cleanup, and ease of changeover from one coating to another.
A Proven Coating TechnologyInstallations in dozens of glass fabrication facilities have proven the quality and efficiency of the new technology. Companies have reported substantial savings in labor and materials compared to the coating methods they formerly used. Fabricators have also noted that overspraying and underglass coating leakage (both of which can cause product damage and harm to conveyors downstream) are eliminated by reverse roller coating. And for high volume runs (especially at production speeds up to 30 feet per minute), multiple pieces of spandrel glass can be coated side by side, rather than one piece at a time as in screen printing.
As glass fabricators continue to look for ways to improve quality and efficiency, technologies such as the reverse roller coater are helping an increasing number of companies meet their objectives.
For more information about reverse roller coaters, contact Chuck Simpson, The Union Tool Corp., P.O. Box 935, Warsaw, IN 46581-0935; (574) 267-3211; fax (574) 267-5703; e-mail email@example.com ; or visit http://www.uniontoolcorp.com .