“The demand for tempered glass is increasing,” says Chad Simkins, Vesuvius’ Commercial Manager for Fused Silica Business in North and South America. “Tempered and laminated glass is being used to meet hurricane codes. Architects are calling for larger sizes of tempered glass panels for commercial and residential towers.

“For manufacturers, it’s an opportunity to fulfill larger glass orders while delivering glass that’s free of imperfections, such as roller waves or scratches, which can really stand out.”

Publicly owned and traded on the London Stock Exchange, Vesuvius delivers engineered solutions for the steel, metal, and glass industries. Its global footprint includes 55 manufacturing facilities, with U.S. headquarters in Charlotte, South Carolina, and a facility in Dillon, SC and one in Cleveland, OH.

While its primary market is iron and steel production, the company also manufactures and supplies fused silica glass tempering rollers used to convey flat glass through horizontal tempering furnaces.

Roller performance is key to delivering defect-free glass. What makes Vesuvius’ Tempered Glass Rollers stand apart is their ability to withstand the intense temperature changes within the tempering oven without warping – to help ensure perfect glass and extend roller life.

The science behind tempered glass heating and cooling

Tempered glass differs from annealed glass mainly in how it breaks when shattered. Annealed glass, or plate glass, will break into large – and fatally dangerous – shards when struck.

Tempered glass, on the other hand, breaks into pebble-sized pieces when shattered; it’s why this glass is used in the automotive and architectural industries. If you’ve been in a serious car crash, you’ve seen first hand how windshield or side windows shatter on impact; the resulting small bits of glass cause little, if any harm.

Tempered glass is annealed glass that’s been heated to 1292°F ( 700°C ) in a tempering oven, and then rapidly cooled. This heating-cooling process creates a tension or stress pattern within the glass, which it holds until it’s broken. This tension or stress increases its mechanical strength by up to 5x over annealed glass.

Vesuvius fused silica Tempered Glass Rollers

Unlike standard metal conveyor rollers, tempering oven rollers are quite large – approximately 84” long by 12” or more in diameter. (Roller size depends on OEM oven specs.)

The large planes of glass float on these rollers as they undergo the tempering process, so uniformity across all rollers is key to creating defect-free tempered glass.

If even one roller expands slightly in size, it creates a “chattering” effect on the glass, which can lead to wavy lines. Slight chips or dents also cause problems, including scratches and scuff marks on the glass surface.

“TIR – or true indicated rollout – is a key measurement with glass manufacturers,” says Simkins. “It means, ‘how flat is the glass coming out of the oven?’”

Vesuvius manufactures its Tempered Glass Rollers using fused silica, which is sintered into place. (A thermal process, sintering causes powdered materials to bond without fully melting for enhanced strength, durability, and density.)

“Because they’re made from ultra-pure fused silica, our Tempered Glass Rollers have very low CTE or coefficient of thermal expansion,” explains Simkins. “This means that unlike steel rollers, which can expand by 28% under high heat, our rollers will maintain their circumference through the tempering process. Other roller materials simply can’t handle the thermal shock, which is similar to what materials undergo in aerospace applications.”

Vesuvius is focusing extensively on the quality of its silica raw material supply, with strict control of the silica mines and partial integration of the production process (refining, filtering, removing impurities, and crushing). Even small impurities will alter the material’s performance under extreme heat.

End caps spec’d for every OEM oven manufacturer

Developed in 2008, Vesuvius’ Smartly Driven® and patented end cap attachment system has made Vesuvius a market leader in the glass industry.

According to Simkins, competitor roller manufacturers use glue to attach end caps to the roller. The glue is then exposed to the intense heat in the oven and burns off, which causes the end cap to move, which in turn causes the roller to jiggle or “chatter.”

Or, roller manufacturers will grind down the roller ends in order to properly fit the inside diameter of the end cap. Because ceramic materials are very brittle, the grinding causes a weak point in the roller and the potential for breaking or snapping during operation or roller changes.

“Several things make our end caps different,” says Simkins. “First, we have end caps manufactured to spec for every OEM tempering oven on the market. You tell us which tempering oven you have, and we’ll supply the rollers with the correct end caps for it.

“And, because they’re made to OEM specifications, our end caps are pressed on – meaning no glue, no mechanical fastener, no grinding. As a result, our Tempered Glass Rollers are 10x stronger than the competition. They’re also designed to withstand the wide temperature ranges associated with tempering ovens.”

Simkin’s advice: Change rollers before they break

Because they’re made to order, lead time for Vesuvius’ Glass Tempered Rollers is 18 to 20 weeks.

Simkins advises that oven operators change out a roller or rollers if they see chips, scratches, warping or loose end caps. Even small imperfections can become big issues over time – such as when the glass no longer comes out of the oven.

“Extremely hot molten glass causes damage to rollers,” he says. “They have to keep moving. If one or more need to be replaced, and you don’t have replacement stock on hand, then your oven is down for many weeks. We recommend having 5 – 10 replacement rollers on hand for every machine with 50 rollers.”

As a result of Vesuvius’ partnership with ISC, [click here] glass manufacturers can work with ISC to determine the exact rollers needed for their specific tempering oven. ISC sales reps can also perform line audits, or bring in a Vesuvius engineer or technical support, to solve any production or performance issue.