Volume 46, Issue 5 - June 2011

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Tips from theTrade
GANA Volunteers Provide Entry-Level Education at Glass Fabrication Event
by Megan Headley

It all starts with sand. The presentations given during the Glass Association of North America’s (GANA) Glass Fabrication Educational Event, which took place in May in Kansas City, Mo., started very much at the beginning: with silica sand.

The biannual conference is geared toward professionals entering the glass industry, or those looking for a best practices refresher on various aspects of fabrication. In that regard, AGC Flat Glass North America’s Ivan Zuniga helped provide his listeners with the perfect starting point, as he walked them through the float glass manufacturing process, from charging raw materials (including silica sand) to melting in the furnace, floating along the tin bath, to annealing and, finally, cutting. Zuniga related the process to his audience’s experience, explaining, “For those of you who contact us and say ‘well I’ve got an architect and he’s got a LEED project and he wants to know the number of regional materials I have for this product ...’” Zuniga explained manufacturers are able to look at the source of each of the products in the glass’ raw batch: silica sand, cullet, soda ash, dolomite, limestone and other products. “The other thing you guys ask about is recycled material,” Zuniga continued. “Well, we do use crushed glass, which is recycled.” However, the way LEED reads, he explained, recycled content must be from a post-consumer product. “So according to LEED and according to the ISO definition, cullet does not comply as a recycled content.”

From Making to Breaking Glass
Following Zuniga’s presentation about making glass, Pilkington North America’s David Duly took the podium to discuss breaking glass. Duly, too, discussed raw batch materials, noting that the addition of ingredients such as calcium and sodium are what gives glass its structure and strength.

Duly went on to explain that there are very high stress concentrations in glass. For starters, Griffith Flaws are stresses invisible to the eye but inherent throughout the glass. “When glass is strained, the strain can be concentrated due to this Griffith Flaw. When the strain exceeds the strength of the atomic bonds, the glass breaks.” He pointed out, these are just the flaws existing in glass—then there are the flaws added to glass, such as scratches, etc.

Next, Duly went on to answer the question of what causes glass to break. A big concern today, Duly said, is thermal stress. He noted that the prominence of energy-efficient products absorbing heat can lead to thermal stress issues. He explained that a classic thermal stress fracture always begins at the glass edge. It starts at a 90-degree angle before meandering away from the edge of glass in any direction to escape the stress. “When you see a single fracture, in most cases the edge is damaged and the stress at the fracture was low,” he said. More fractures could mean a good edge simply couldn’t handle the extreme temperature difference between the center and edge of glass.

Duly ended his presentation by asking what we can do to improve the strength of glass. “The most common way to improve the strength is to heat-treat glass,” he said. “When you heat treat glass you’re putting the outer surface in compression and the inner surface under tension.” The compressive forces counter the opening of crack tips, he explained.

Chuck Beatty of Edgeworks offered information on properly cutting glass. He opened his presentation on “Principles and Fundamentals of Glass Cutting” with an explanation of cutting glass as overcoming the compressive stresses on the surface. As
Beatty explained, cutting glass means trying to separate the surface without destroying it. “In the glass cutting process the first rule would be never, ever destroy the surface of the glass,” he said.

Beatty explained how to release the tension within the glass in a controlled fashion. He offered his audience a number of tips, emphasizing the importance of the choice of cutting wheel that’s appropriate for a given thickness of glass. As Beatty explained, “It wasn’t very long ago everyone was using one wheel to do everything ... that’s incomprehensible to me now.” Cutting wheels are dimensioned for each different glass cutting application, with different finishes available for different results.

The angle and footprint also are important, he added.

“In choosing the right wheel angle it’s important to remember, the less energy you send into the glass while scoring the cut, the smoother the resulting edge after the break out,” Beatty said.

He reminded his audience to pay attention to the appropriate psi needed to make a cut. If an operator opts to turn up the pressure to complete the cut, the heat also is being turned up. This can lead to more sub-surface pressures—and potentially destroy the glass. Rather than turning up the pressure, Beatty advised changing the cutting wheel first, as that should be done at a regular frequency anyway. He noted that the occasional “shark’s tooth” flaw on the edge, should be a warning sign to “change something pretty quick if you don’t want major problems.”

Taking Care with Coated Glass
Dave Cooper of Guardian also had some tips for cutting, specifically when it comes to high-performance coated glass. He noted the importance of having the coated side of these glasses facing up during cutting so shards aren’t scratching that surface, although, he advised, the table surface should always be kept clean. He also recommended using only as much approved cutting fluid as needed and moving lites one at a time to prevent scratching.

Cooper reminded his audience of the importance of carefully storing coated glass prior to cutting. For starters, he said, “These high-performance glasses reflect solar energy, but they also absorb it, so if it’s stored in sunlight, it might break.” These high-performance glasses should be stored out of direct sunlight and away from water, in a stable and well-supported pack.

According to Cooper, though, the most important issue with handling high-performance glass is washing. “It’s such an important step; it’s repeated throughout the process often, and having the washer set up and running to the manufacturer’s specs is critical,” he said. If the glass is not appropriately washed, any subsequent fabrication is not likely to be successful. He added, “It’s especially important not to touch the coating as it is being removed from the washer.”

Bob Lang of Billco had more tips about washing glass of all types in his presentation on “Understanding and Maintaining a Glass Washing Machine.” Lang began by explaining how the pre-spray system minimizes the contamination in the washing zone portion, among other things, as it removes loose debris from glass before passing through the washer. He also stressed the importance of using clean water so that minerals—such as those commonly found in city water—don’t end up contaminating the glass. In addition to using clean water, regular maintenance of the machine can impact the cleanliness of the glass. Lang also noted that it’s important to maintain the drying section rolls so they stay clean and there is no contamination on the bottom of the glass.

As Lang explained it, “The thing to remember is if the inside of the glass washer is dirty, the glass can’t get clean.”

Preventing Delamination
Philip Bradbourne of DuPont provided tips on “Troubleshooting Laminating Glass Manufacturing,” noting as he opened his presentation that producing good quality laminated glass is “important to have that edge over your competition.

“A number of different things can cause delamination … High moisture can cause problems with delamination; PVB likes to absorb moisture,” he added. “The good thing is it sucks it up but you can also recondition it back.”

Other causes of delamination can include rippled or dirty glass, having the wrong adhesion grade and pinch points in the autoclave, among other factors. As far as solutions to delamination, Bradbourne advised seeking lab analysis to know the exact cause.

Bradbourne also reviewed problems such as PVB blocking; where the PVB interlayer sticks to itself. He noted this can occur when the interlayer storage temperature is too warm, and that suppliers must be sure the interlayer is transported in a refrigerated truck on the way to customers. The problem also can occur if the roll is wound too tightly by the supplier or is too old.

He also discussed causes of bubbles, gas pockets in the interlayer or between the glass and interlayer. A large number of small bubbles along the edges could mean the autoclave pressure releases at too high a temperature. Glass imperfections, such as caliper variations, can cause a large number of bubbles at the glass-PVB interface.

Bradbourne also stressed the importance of keeping the lay-up or clean room clean. He suggested using filtered air condition, and making sure employees use special lint free hats and clothing to prevent static or contamination—and only allowing authorized personnel entry into the lay-up room.

To minimize problems, Bradbourne advised having clear written directions, a troubleshooting model and experienced employees handling the laminating.

Tools for Tempering
Chuck Wencl of Viracon addressed an audience interested in tempering on the topic of “Roll Wave Distortion.”

He opened by noting there are a number of different definitions for this problem, but GANA’s Roller Wave Subcommittee offers this one of roller wave: a repetitive wave-like departure from flatness related to the heat-treated process, excluding edge effects, distortion influenced by assembly or installation.

Wencl also showed images of glass installations with severe distortion, including one project that won an award (for energy efficiency he pointedly added). “If this is what the market will take, this is what the market gets,” he commented. However, today many fabricators are seeing requests from the market for thicker lites of glass in an attempt to get flatter glass. Wencl reviewed a number of tools available today to indicate when this distortion is present and notify the operator that it’s time to check the performance of or maintain the rollers or tempering furnace itself.

The Zebra Board
Wencl went through a list beginning with the zebra board, a subjective test for monitoring roller wave, to the various options available in flat bottom gauges. He called the 3-point gauge “very effective,” noting for this particular device, “it doesn’t matter what kind of a surface you put them on they target that wave only.” In recent years, he noted, digital grid photography has been introduced. This method uses a grid board and if “you look at the reflection in a piece of glass you’ll see the variation.” In addition, a number of automated solutions are available to provide on-line visual inspection for optical distortion.

Edgetech’s Mike Burk introduced the group to “Triple Glazed Insulating Glass Units (IGUs)” or, as he suggested calling it, “multiple cavity IGUs.” As Burk pointed out, it’s not the glass that provides the performance boost, since more glass means more conductive surfaces—what’s really important is the extra number of cavities. “It might be better to talk about cavities rather than the layers of glass,” Burk suggested.

Whatever it’s called, he could safely say of this product segment, “I think everyone in the industry believes this is going to grow.”

Burk noted that there are people who say triples aren’t the answer to improving the energy efficiency of window products. He pointed to concerns about decreased light transmittance, acknowledging that by adding an additional lite there may be some decrease. He agreed, too, that there could be some extra weight by adding an extra lite and producing frames that fit (although products such as suspended film negate that argument). And, he noted that the concern for a higher risk of glass seal failure on a triple than a double-lite unit makes some sense, since the product is going from two to four seals. But that solution, he added, “comes down to workmanship and training and building good IG.”

Overall, Burk said the three main advantages of triples—thermal performance, condensation resistance and sound dampening—can outweigh the negatives, when produced appropriately. He advised his audience to carefully consider the steps of determining what type of gas to use and in which cavities, the impact of selecting an appropriate glass type and coatings on light transmittance, and considering structural components to be sure the frame can handle the added weight.

More Information
For more information on these and other events, visit the Glass Association of North America at www.glasswebsite.com.


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