337Glass Processing Days, 13–16 June ’99ISBN 952-91-0885-0fax +358-3-372 3180

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Maximizing Ceramic Furnace RollPerformance

Mr. Renald D. BartoeVesuvius McDanel

Abstract

While ceramic roller properties anddimensional specifications are critical to the glasstempering process, many product and processvariables influence roller performance and glassquality. The manufactured quality of the roller isessential, but the environment in which the rollersoperate and the processes and procedurespracticed before and during tempering directlyinfluence the life and performance of the rollers.The combined influences of the roller quality, theenvironmental effects on the rollers and the roller-to-glass interface, and the methods and frequencyof cleaning the rollers directly relate to the life andcost effectiveness of the rollers. These influencesultimately effect the productivity and profitabilityof the glass tempering operation. Additionally,scientific identification of microscopic defects intempered glass is essential to isolating the causeand resolution of such defects.

Introduction

Maximizing ceramic furnace roll performancerequires an understanding of the total temperingsystem. This presentation addresses the criticalproperties and specifications that are required offused silica ceramic rollers and the influences ofthe glass tempering plant environment on therollers. Roller maintenance and cleaning methodsare discussed along with methods for identifyingmicroscopic defects in glass using optical micros-copes, scanning electron microscopy, and energydispersive spectroscopy.

A comprehensive understanding of the fusedsilica rollers, the glass tempering environment, theglass tempering furnace, and the interaction ofeach of these systems will improve roller

performance and the productivity of the glasstempering operation.

Ceramic Furnace Rollers

The History: The first fused silica rollers wereintroduced to horizontal tempering furnaces in1972. Since their introduction, Vesuvius Zyarock™brand fused silica roller applications haveexpanded into float glass annealing lehrapplications. Roller manufacturing and sizecapabilities have increased from the original32mm (1.25in.) diameter to over 305 mm (12in.)today. Length capabilities also increased from1.14 meters (45in.) in 1972 to over 6.25 meters(246in.) today. Zyarock™ rollers have effectivelyreplaced asbestos and steel rollers in horizontalglass tempering furnaces.

The Material: Fused silica is uniquely qualifiedfor tempering roller applications due to its physicaland chemical properties. It has a non-crystalline,amorphous structure with very low thermalexpansion, low thermal conductivity, and avery high resistance to thermal shock. It iscompositionally a glass (SiO2) which is inert andresistant to corrosion, making it very compatiblewith tempered glass.

Manufacturing: There are many critical steps inthe manufacturing process that can and will effectthe performance of the fused silica temperingrollers. The process requires tight controls fromthe raw materials through the casting andfiring processes to produce a homogenous andthermally stable blank.

The blank quality will directly effect the surfacefinish, the strength, and the hot TIR of the finishedroller. TIR is very critical especially when therollers are at operating temperatures. An inferiorroller blank may have a higher thermal expansion

338Glass Processing Days, 13–16 June ’99ISBN 952-91-0885-0fax +358-3-372 3180

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resulting in bow or warp at temperingtemperatures. The blanks are machined withdiamond tooling to achieve the tight tolerances ofthe finished roller.

Surface finishes are critical to the roller-to-glassinterface and effect the heat transfer from therollers to the glass. Rollers may be polished,textured, or spiral machined. The surface finishspecification may vary with each furnacemanufacturer. Be certain that replacement rollersmeet the OEM specifications.

A variety of end cap designs are availabledepending upon the application temperature,furnace design, and drive system. These includepopular RTV silicon bonded caps, customdesigns, patented high temperature caps for useup to 750°C (1380°F), and for certain furnacedesigns, rollers without end caps. The end capsshould not induce mechanical stress on theceramic. They must be concentric to the roller toassure good TIR and the attachment method mustbe carefully considered.

The Quality: To assure the quality of yourrollers, all Vesuvius manufacturing locations areQuality Assured ISO 9001. We recommend that allspecifications be confirmed in writing andpreferably with a detailed drawing. Certifiedquality inspection reports for each roller areavailable. It is suggested that incoming inspectionbe conducted on rollers if they are not purchasedfrom an ISO certified supplier.

The Environment

The manufacturing environment andproduction practices influence the temperingoperation, specifically the roller life andperformance. Plant cleanliness is of highestimportance. Dirt and dust that is on the glass orthat passes into the furnace cavity will deposit onthe rollers. This debris can result in bottom surfacemechanical damage or it can be picked up andre-deposited onto the glass. The sources of thisdust seem endless.

Glass Seaming & Fabricating: While glassseaming and fabricating can be significantsources for dust, the quality of these operationsis also critical. Edge work must always becomplete, smooth, and free of chips or smallshards protruding from the glass, as they willbreak away during the heat treating process.These particles may stick to the rollers and resultin bottom surface defects. When viewed byscanning electron microscopes (SEM), the defectsare bottom surface break outs or chips in theglass “Figure 1” and deposits on the glass surface“Figure 2”. Always check the quality of the edgework, seaming, and drilled holes, and be certainthat the glass is clean.

Plant Dust Control: Debris and airborne dust inthe tempering plant will cause defects in the glassand limit the effectiveness of the rollers. Debrismay be carried into the furnace from differentoperations such as silk-screening, paintingoperations, and fabricating. Paint that is on theedge of the glass may drip on the rollers. Airbornepaint can deposit on the rubber rollers in front ofthe furnace and track into the furnace cavity.Ceramic based frit paint bonds to the fused silicarollers and can not be washed from the rollersurface. Paint defects on glass may appear aswhite haze or scuff marks while it is typically adeposit that can be viewed by SEM “Figure 3”.Energy dispersive spectroscopy (EDS) reveals theunique components of the paint including leadand chromium.

The Glass Washer: Effective use of the glasswasher is critical to every tempering operation.The glass should be washed just prior totempering. Washing the glass and staging it forextended periods of time before tempering isineffective. Dust collecting on the glass will becarried into the furnace. The glass washer must

Figure 1. SEM image at 500X of bottom surface mechanicaldamage from glass debris on rollers.

Figure 2. SEM image at 500X of bottom surface glass deposit.

339Glass Processing Days, 13–16 June ’99ISBN 952-91-0885-0fax +358-3-372 3180

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be operating properly with the detergents,brushes, and rinse water at the manufacturer’srecommended settings. Maintenance of the glasswasher is equally important to ensure that theglass is clean and without residue or debris thatwill be carried into the furnace. Certain materialsused to separate the glass are difficult to removeif the washer settings are not correct. Thesedefects may appear as chips under an opticalmicroscope but under SEM they are deposits“Figure 4”. Energy dispersive spectroscopy (EDS)shows the spectrum of a high carbon component

furnace operators are encouraged to consult withthe furnace manufacturer for maintenanceschedules and the proper methods for handlingrefractory insulation, roll seals, heating elements,and Sulfur dioxide.

The Fiber Insulation: This is one of mostcommon and prolific sources for glass defects“Figures 6 & 7”. The furnace cavity must be kept

Figure 3. SEM image at 500X of paint deposit on glass.

typical of Lucor™ “Figure 5”. Proper settings andstrict adherence to the maintenance schedules ofthe glass washer will minimize certain glassdefects and protect the rollers from contamination.

The Tempering Furnace

Cleanliness continues as the fundamentaltheme for optimizing the life and performance ofthe Zyarock™ fused silica tempering rollers.Several sources of contamination can begenerated within the furnace. Glass tempering

Figure 4. SEM image at 200X of glass defect from separationmedia that was not washed off of the glass prior to tempering.

Figure 5. EDS spectrum of deposit shown in figure 4.

Figure 6. SEM image at 150X of refractory dust on glass.

Figure 7. EDS spectrum of alumino-silicate refractory dust fromfiber insulation on the glass as seen in figure 6.

340Glass Processing Days, 13–16 June ’99ISBN 952-91-0885-0fax +358-3-372 3180

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free of refractory fiber dust. This dust is oftencreated when furnace maintenance is performed,although it may also develop as the fiberdegrades with time and temperature. Thedetrimental effects of refractory fiber contamin-ation are compounded in the presence ofaspiration and forced air convection, which stirsup the dust.

The Roll Seals: The roll seals are important tomaintain heat balance in the furnace, energyconservation, and to protect the roller end caps,furnace drive, and bearings. Many roll seals aremanufactured with refractory fiber insulation thatis surface hardened. The roll seals may producedust if they are broken, damaged, or worn. Anydust should be vacuumed away and broken rollseals immediately replaced. Plugging gaps orholes in the roll seals with bulk fiber insulation isnot recommended, as it may introduce dust intothe furnace cavity. Alternative seal materials areavailable including Zyarock Mousse™, a lowdensity, hard fused silica refractory offeringexcellent thermal insulation, matched thermalexpansion to the rollers, and elimination of thealumino-silicate fiber contaminants.

The Heating Elements: Maintaining the heatingelements and other metallic components, andfollowing the OEM furnace builder’s specificationsfor replacements may prevent metallic contamin-ation in the furnace cavity and on the rollers.Certain metal alloys or failure to properly install ormaintain the heating elements may result in metalcontamination on the rollers and on the glass“Figures 8 & 9”.

defects. However, sulfur dioxide reacts with thesodium ions in the glass at elevated temperaturesto form sodium sulfate (Na2SO4). Excess usage ofSO2 leads to the formation of sodium sulfatedeposits or nodules on the rollers. While thesedeposits are water soluble, the nodules may pullout grains of fused silica from the roll surfaceleaving pits in roller. The pits have more surfacearea and become the sites for more build up onthe roller surface. Sodium sulfate nodules cantransfer from the rollers to the glass surfaceresulting in defects “Figures 10 & 11”. Limited useof SO2 or dosing the gas often reduces thefrequency and the concentration of sodium sulfatenodules.

Figure 8. SEM image in the compositional mode at 1000X ofa metallic deposit on glass.

The Sulfur dioxide: SO2 is useful as a drylubricant to form a coating on the glass and abarrier between the glass and the rollers. Oftenbuild up or debris on the roller surface will leavean impression or defect in the glass surface. TheSO2 provides the barrier to prevent or minimize

Figure 9. Chemical analysis by EDS of metallic contaminationon glass. “Chatter” chemistry is from figure 8.

Figure 10. SEM image at 97X of sodium sulfate deposit onglass.

341Glass Processing Days, 13–16 June ’99ISBN 952-91-0885-0fax +358-3-372 3180

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Roller Cleaning and Maintenance

The ceramic rollers should only be washed withwater, never use detergents! Detergents willchemically react with the rollers, resulting indevitrification and roller failure. Due to theextremely low thermal expansion, the rollers canbe washed hot. Glass debris on the roller surfacecan be thermal shocked off of the rollers whencold water is properly applied to a hot roller.Precautions should be taken to prevent the endcaps from getting hot during the cleaning process.Rollers can also be effectively cleaned at roomtemperature. Lint free towels and certain abrasivepads such as 3M’s Scotch Brite™ are effective.The rollers should be dried following cleaning. Ifthe rollers are soaked with water and heated tooquickly, steam may develop in the roller andcause it to fail.

Other cleaning methods include the use of tackcloth to remove surface dust. Sandpaper can beused, but should be avoided if possible. Sandingmay induce waves or an out-of –round conditionto the roller surface. On certain rollers, the surfacespecification requires a textured finish. Sandingthe rollers may remove the finish and polish thesurface. If sanding is required, take great careand be absolutely certain to avoid introducing thedust into the furnace environment.

Figure 11. Chemical analysis by EDS of sodium sulfate depositas seen in figure 10.

The frequency of how often rollers should becleaned varies greatly and is usually determinedby the cleanliness of the tempering environment.As discussed throughout this presentation, theplant environment, furnace cleanliness, fabricatingpractices, glass cleanliness, SO2 usage, etc. willinfluence the roller cleanliness. There are methodsand measures for determining the cleaningfrequency for each furnace.

Refurbishing may also be performed on rollersthat have excessive surface build up and amoderate amount of surface damage. Usually thisinvolves re-machining the rollers and removing upto 0.13mm (.005 in.) from the diameter. Beforeproceeding, consider the TIR of the rollers, wear,and end cap condition. A roller with excessive TIRmay look better after refurbishing, but it may notperform better. Refurbishing should include 100%inspection of the rollers, surface reconditioningto the OEM specification, end cap replacementif necessary, and final inspection. It isrecommended that rollers be re-machined onlyone time to avoid significant diameter changes.

Summary

Maximizing the performance of ceramicfurnace rollers requires a total systems approach.Roller quality and stability are essential as are thecleanliness of the factory and the furnace, theoperating practices, and glass cleanliness.Defects in tempered glass can usually beidentified, isolated, and resolved through the useof optical microscopy, scanning electronmicroscopy, and energy dispersive spectroscopy.