by Renald D. Bartoe, Frederick Caillaud, Dr. John Dodsworth and Jerry Osele
Ceramic roller properties and dimensional specifications are critical to the glass tempering process. While the manufactured quality of the roller is essential, the environment in which the rollers operate and the processes and procedures practiced before and during tempering directly influence the life and performance of the rollers. The combined influences of the roller quality, environmental effects on the rollers and the roller-to-glass interface, along with the methods and frequency of cleaning the rollers, directly relate to the life and cost effectiveness of the rollers. These influences ultimately affect the productivity and profitability of the glass tempering operation. Additionally, the scientific identification of microscopic defects in tempered glass is essential to isolating the cause and resolution of such defects.
Maximizing ceramic furnace roll performance requires an understanding of the total tempering system. This article addresses the critical properties and specifications required of fused silica ceramic rollers and the influences of the glass tempering plant environment on the rollers. Roller maintenance and cleaning methods are discussed along with methods for identifying microscopic defects in glass using optical microscopes, scanning electron microscopy and energy dispersive spectroscopy.
The first fused silica rollers were introduced to horizontal tempering furnaces in 1972. Since their introduction, Vesuvius Zyarock fused silica roller applications have expanded into float glass annealing lehr applications. Roller manufacturing and size capabilities have increased from the original 1.25 inches in diameter to over 12 inches today. Length capabilities have also increased from 45 inches in 1972 to over 246 inches today. Zyarock rollers have effectively replaced asbestos and steel rollers in horizontal glass tempering furnaces.
Fused silica is uniquely qualified for tempering roller applications due to its physical and chemical properties. It has a non-crystalline, amorphous structure with very low thermal expansion, low thermal conductivity and a very high resistance to thermal shock. It is compositionally a gas (SiO2) which is inert and resistant to corrosion, making it compatible with tempered glass.
There are many critical steps in the manufacturing process that can and will affect the performance of the fused silica tempering rollers. The process requires tight controls from the raw materials through the casting and firing processes to produce a homogenous and thermally stable blank.
The blank quality will directly affect the surface finish, strength and the hot Total Indicated Runout (TIR) of the finished roller. TIR is critical, especially when the rollers are at operating temperatures. An inferior roller blank may have a higher thermal expansion resulting in bow or warp at tempering temperatures. The blanks are machined with diamond tooling to achieve the tight tolerances of the finished roller.
Surface finishes are critical to the roller-to-glass interface and affect the heat transfer from the rollers to the glass. Rollers may be polished, textured or spiral machined. The surface finish specification will vary with each furnace manufacturer. It is important that replacement rollers meet OEM specifications.
A variety of end cap designs are available depending upon the application temperature, furnace design and drive system. These include popular RTV silicon-bonded caps, custom designs, patented high temperature caps for use up to 138°F, and for certain furnace designs and rollers without end caps. The end caps should not induce mechanical stress on the ceramic. They must be concentric to the roller to assure good TIR and the attachment method must be carefully considered.
We recommend all specifications be confirmed in writing and preferably with a detailed drawing. Certified quality inspection reports for each roller are available. It is suggested that incoming inspection be conducted on rollers if they are not purchased from an ISO certified supplier. All of our manufacturing locations are certified to ISO 9001.
The manufacturing environment and production practices influence the tempering operation, specifically the roller life and performance. For this reason, plant cleanliness is of the highest importance. Dirt and dust on the glass or that passes into the furnace cavity will deposit on the rollers. This debris can result in bottom surface mechanical damage or be picked up and re-deposited on the glass.
While glass seaming and fabricating can be significant sources of dust, the quality of these operations is also critical. Edge work must always be complete, smooth and free of chips or small shards protruding from the glass, as they will break away during the heat treating process. These particles may stick to the rollers and result in bottom surface defects. When viewed by scanning electron microscopes (SEM), the defects are bottom surface break outs or chips in the glass and deposits on the glass surface. Always check the quality of the edge work, seaming and drilled holes and be sure the glass is clean.
Debris and airborne dust in the tempering plant will cause defects in the glass and limit the effectiveness of the rollers. Debris may be carried into the furnace from different operations such as silk-screening, painting operations and fabricating. Paint located on the edge of the glass may drip on the rollers. Airborne paint can deposit on the rubber rollers in front of the furnace and track into the furnace cavity. Ceramic-based frit paint bonds to the fused silica rollers and cannot be washed from the roller surface. Paint defects on glass may appear as white haze or scuff marks while it is typically a deposit that can be viewed by SEM. Energy dispersive spectroscopy (EDS) reveals the unique components of the paint including lead and chromium.
Effective use of the glass washer is critical to every tempering operation. The glass should be washed just prior to tempering. Washing the glass and staging it for extended periods of time before tempering is ineffective because dust collecting on the glass will be carried into the furnace. The glass washer must be operating properly with the detergents, brushes and rinse water at the manufacturers recommended settings. Maintenance of the glass washer is equally important to ensure the glass is clean and without residue or debris that will be carried into the furnace. Certain materials used to separate the glass are difficult to remove if the washer settings are not correct. These defects may appear as chips under an optical microscope but under SEM they are deposits. EDS shows the spectrum of a high carbon component typical of Lucor . Proper settings and strict adherence to the maintenance schedules of the glass washer will minimize certain glass defects and protect the rollers from contamination.
Cleanliness continues as the fundamental theme for optimizing the life and performance of the Zyarock fused silica tempering rollers. Several sources of contamination can be generated within the furnace. Glass tempering furnace operators are encouraged to consult with the furnace manufacturer for maintenance schedules and the proper methods for handling refractory insulation, roll seals, heating elements and sulfur dioxide.
This is one of the most common and prolific sources for glass defects. The furnace cavity must be kept free of refractory fiber dust. This dust is often created when furnace maintenance is performed, although it may also develop as the fiber degrades with time and temperature. The detrimental effects of refractory fiber contamination are compounded in the presence of aspiration and forced air convection, which stirs up the dust.
The roll seals are important to maintain heat balance in the furnace, energy conservation and to protect the roller end caps, furnace drive and bearings. Many roll seals are manufactured with refractory fiber insulation that is surface hardened. The roll seals may produce dust if they are broken, damaged or worn. Any dust should be vacuumed away and broken roll seals replaced immediately. Plugging gaps or holes in the roll seals with bulk fiber insulation is not recommended, as it may introduce dust into the furnace cavity. Alternative seal materials are available.
Maintaining the heating elements and other metallic components, and following the OEM furnace builders specifications for replacements, may prevent metallic contamination in the furnace cavity and on the rollers. Certain metal alloys or failure to properly install or maintain the heating elements may result in metal contamination on the rollers and on the glass.
Sulfur dioxide is useful as a dry lubricant to form a coating on the glass and a barrier between the glass and the rollers. Debris on the roller surface will often leave an impression or defect in the glass surface. Sulfur Dioxide provides the barrier to prevent or minimize defects, but it will react with the sodium ions in the glass at elevated temperatures to form sodium sulfate. Excessive use of sulfur dioxide leads to the formation of sodium sulfate deposits or nodules on the rollers. While these deposits are water-soluble, the nodules may pull out grains of fused silica from the roll surface leaving pits in the roller. The pits have more surface area and become the sites for more build-up on the roller surface. Sodium sulfate nodules can transfer from the rollers to the glass surface resulting in defects. Limited use of sulfur dioxide or dosing the gas often reduces the frequency and concentration of sodium sulfate nodules.
The ceramic rollers should only be washed with water. Never use detergents! Detergents will react chemically with the rollers, resulting in denitrification and roller failure. Due to the extremely low thermal expansion, the rollers can be washed hot. Glass debris on the roller surface can be shocked off when cold water is applied properly to a hot roller. Precautions should be taken to prevent the end caps from getting hot during the cleaning process. Rollers can also be effectively cleaned at room temperature. Lint-free towels and certain abrasive pads such as 3Ms Scotch Brite are effective. The rollers should be dried following cleaning. If the rollers are soaked with water and heated too quickly, steam may develop in the roller and cause it to fail.
Other cleaning methods include the use of tack cloth to remove surface dust. Sandpaper can be used, but should be avoided if possible. Sanding may induce waves or an out-of-round condition to the roller surface. On certain rollers, the surface specification requires a textured finish. Sanding the rollers may remove the finish and polish the surface. If sanding is required, take great care and be absolutely certain to avoid introducing dust into the furnace environment.
The frequency of roller cleaning varies greatly and is usually determined by the cleanliness of the tempering environment. As discussed, the plant environment, furnace cleanliness, fabricating practices, glass cleanliness, sulfur dioxide usage and other factors will influence roller cleanliness. There are methods and measures for determining the cleaning frequency for each furnace.
Refurbishing may also be performed on rollers that have excessive surface build-up and a moderate amount of surface damage. This usually involves re-machining the rollers and removing up to .005 inches from the diameter. Before proceeding, consider the TIR of the rollers, wear and end cap condition. A roller with excessive TIR may look better after refurbishing, but it may not perform better. It is recommended that rollers be re-machined only one time to avoid significant diameter changes.
Maximizing the performance of ceramic furnace rollers requires a total systems approach. Roller quality and stability are essential as are the cleanliness of the factory and the furnace, the operating practices and glass cleanliness. Defects in tempered glass can usually be identified, isolated and resolved through the use of optical microscopy, scanning electron microscopy and energy dispersive spectroscopy.
The above article was presented at Glass Processing Days 99 in Tampere, Finland.
Renald D. Bartoe is product line manager, Americas for Vesuvius McDanel; Frederick Caillaud is Zyarock development manager for Vesuvius France; Dr. John Dodsworth is technical manager for Vesuvius McDanel; and Jerry Osele is research engineer for Vesuvius McDanel.
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