Handbook of Materials for Product Design Part 11 pps

As is typical in most industries, new designs are aimed atlower overall cost of operation the calculation of which includes initialcost, melter lifetime, and costs of repairs as well as

refractory materials themselves Crowns are often self-supported fractory structures The refractories must withstand or resist hightemperatures, heavy loads, abrasion, and corrosion.

re-Types of oxide refractories used in glass melters are as follows:

1 Clay refractories (generally used for insulation)

■ Fireclay: kaolinite (Al2O3 · 2SiO2 · 2H2O) plus minor nents; classified as low-, medium-, high-, or super-duty; (25 to45% Al2O3)

■ Mullite (3Al2O3 · 2SiO2)

■ AZS (alumina-zirconia-silica) containing 30 to 42% zirconia

■ Zircon (ZrO2 · SiO2)

■ Chrome-magnesite and magnesite-chrome (combinations of

fir-■ Fusion cast (cast as blocks from arc-melted raw materials); also

re-ferred to as fused or fused cast

Many factors affect the choice of refractory for a particular tion These include melting temperature, thermal conductivity, me-chanical strength, creep resistance, and resistance to corrosion andspalling, to name a few Generally, different refractories are used indifferent regions of a glass-melting tank, because the requirements

applica-are different However, the ultimate design consideration is resistance

to corrosion by molten glass and by the hot gas atmosphere within themelter Corrosion determines tank lifetimes and affects the rates at

which certain glass defects (such as stones) are generated In selecting

a refractory the considerations are, in order,

■ Front surface attack (frontal attack). This is direct attack at theglass/refractory interface Its mechanisms include alkali diffusioninto the refractory with consequent fluxing and dissolution of the re-fractory crystals Porous refractories are generally more susceptible

■ Melt line corrosion (attack where glass surface, air and refractory

meet) Corrosion at this location is enhanced by localized tion currents and fluctuations in glass level within the melter

convec-■ Upward drilling. This form of corrosion occurs where bubbles formunder horizontal refractory surfaces, such as throat cover blocks orsubmerged horizontal refractory joints Corrosive vapor species con-centrate in the bubbles As with melt line corrosion, the greatest cor-rosive activity is believed to take place where the vapor, refractory,and glass touch

■ Downward drilling. This type of corrosion results when droplets ofmolten metal settle on the bottom of the tank Sources of metal can

be contaminants in batch raw materials or cullet, chemical tion of certain glass components (such as lead oxide), and even tools

reduc-or metal parts accidentally dropped into the tank

Glass contact refractories. The most common glass contact refractoriesinclude the following:

■ Fused AZS (alumina-zirconia-silica) This is the most common today(41% ZrO2 in high-wear areas and electrode blocks, as opposed toless expensive 34% variety) The oxidation state is critical (the re-fractory contains a residual glassy phase which, if produced in re-duced condition, will oxidize in use, swell, and exude from the brick)

■ Dense sintered zircon (ZrO2 · SiO2) This is used in some low sion borosilicate melters.

expan-■ Clay, fused alumina, bonded AZS, and dense sintered alumina.These are used for lower-melting specialty glasses

Typical glass contact refractories used for melting various glass typesare:

■ Container glass. Fused AZS, life 8 to 10 years; also, fused α-β mina; finer bottoms are sometimes bonded AZS, zircon, and clay

alu-■ Float glass. Fused AZS in melting zones; fused alumina in tioner zones, life 10 to 12 years

condi-■ Hard borosilicates. Fused AZS and zirconia, sometimes dense tered zircon

sin-■ Fiberglass wool. Highly corrosive, melted electrically in fusedchrome-AZS or fused alumina-chrome refractories (coloration due tochromium is of little consequence in this application)

■ E-glass (textile fiber). Less corrosive, melted in dense sinteredchrome oxide

■ Lead crystal. Tendency to electric melting in AZS

Several refractory sidewall design considerations are based on rosion concerns First, the thicker the wall, the lesser the heat lost andthe lesser the energy consumed But thicker walls create a smallertemperature gradient, allowing chemical attack to penetrate moredeeply into the refractory Thus, the design thickness of a wall must be

cor-a compromise between hecor-at loss cor-and wecor-ar To give cor-all sections of thewall approximately equal lifetimes, thickness, and type of refractoryare often varied from location to location These techniques are some-

times referred to as zoning by thickness and zoning by type.

To avoid melt infiltration of horizontal refractory seams and the sequent increased opportunity for upward drilling of corrosion, glasscontact wall refractories are often large, full-height blocks arrangedadjacent to each other in a “soldier” course fashion However, if multi-ple courses are required, close-fitted diamond ground horizontal jointscan help minimize melt infiltration Similarly, to avoid horizontal

con-joints, the paver blocks composing the top layer of the tank bottom are

butted up against the side blocks, not placed under them

Superstructure and crown refractories. The superstructure, which cludes all furnace walls above the melt line and the crown, are subject

in-to corrosion by aggressive vapor species such as NaOH, KOH, PbOand HBO2, batch dust particles, liquid condensates and liquid reaction

products running down from refractories higher up Superstructuretemperatures in fuel fired furnaces are often 60-100°C hotter than the

glass Typically, walls and ports made of fused AZS from the back wall (where the batch enters) to the hot spot; fused β-alumina is useddownstream But these are not hard and fast rules Crowns typicallyconsist of sintered silica block Although silica crowns are attacked byalkali vapors, the drips are homogenized into melt Crown life is more

of a concern This is especially true with gas-oxy firing (more sive vapors, see Sec 8.3.3.5), in which case more costly refractoriesmay be justified Silica and alumina blocks should never be in directcontact, for example at the joint where the crown and superstructurewalls meet (they will react); zircon is used as a buffer

aggres-Regenerative heat exchanger refractories. Special refractory ations are needed because of the large temperature gradient (top–bot-tom) within the checker chambers and the corrosive nature of theexhaust gases As the gases cool, a temperature is reached at whichthe corrosive vapors condense on the refractory surfaces, enhancingthe corrosion Fine batch particles carried over with the exhaust gasesalso tend to react with and corrode the regenerator refractories Highthermal conductivity and heat capacity are also important character-istics Checker construction for a typical soda-lime-silica melting tankis: top third, bonded 95 to 98% MgO bricks; middle third, lower mag-nesia content bricks; bottom third (where alkali vapors condense), sin-tered chrome (chromic oxide) or magnesium-chrome bricks

consider-A relatively new approach to checker construction, especially in rope, is with special interlocking shapes (cruciforms) of AZS or high-alumina fused cast refractories

Eu-8.3.3.4 Electric boosting and all-electric melting. Crucibles, pots, andday tanks for glass melting can be heated in electric furnaces wherethe heat is generated by resistance heating in windings or bars Income cases, heat is produced by the flow of electricity through themetal crucible itself, and in others by the flow of electricity throughthe molten glass, which is a moderately good ionic conductor at hightemperatures, between submerged electrodes

These principles are applied to varying degrees in large continuousmelters as well In some fuel-fired furnaces, electrodes are installed inthe walls below the glass line so as to provide a source of heat belowthe batch layer, thus the batch is actively melted from below as well asabove By such means, the melting rate for the tank can be increased,

or boosted, leading to the term electric boosting Resistance heating by

external windings or bars is often used to control temperatures at theorifice or delivery tube by which the molten glass leaves the melter

Sometimes, all the heat required for melting is supplied electricallywithin the molten glass In this case, electrodes are positioned asplates at the walls of the furnace, or as water-cooled metal rods ex-tending upward through the bottom of the furnace The electric cur-rent passes from electrode to electrode, through the glass, with theamount of heat generated depending on the applied voltages, theshape and spacing of the electrodes, and, very importantly, the electri-cal resistivity of the molten glass In this case of all-electric melting,the batch components are melted solely by heat flow up from below It

is possible and desirable to maintain a continuous layer of batchacross the top of the melt, eliminating the need for a refractory roof orcrown to contain and reflect the heat However, in many cases, thecrown is there for other reasons, but it is cold in temperature Hence,

all-electric melting in this manner is sometimes referred to as cold-top

or cold-crown melting.

It should be noted that the electrical resistivity of a pile of melted batch, or even a glass melt at temperatures well below1,000°C, is too great to allow for efficient heat generation Conse-quently, all-electric melters are started in a more or less traditionalway using fossil-fuel burners and a hot crown Once the molten glasshas reached sufficient temperature, the burners (and sometimes thecrown) are removed

non-Cold-top melting is valuable for two reasons First, the batch layer

acts as a thermal insulating blanket (the batch blanket), which helps

reduce heat loss out the top of the melter (thus enabling it to operatecold-top) Second, the top layers of the batch blanket, being muchcooler than the molten glass below, act to condense volatile vapor spe-cies that might otherwise escape into the atmosphere This is espe-cially valuable for fiberglass and other specialty glass melting wherethe compositions contain fluorides and other very volatile, and some-times unhealthy, components

Electrodes are made of materials such as carbon, tin oxide, num, and platinum, with the choice depending on the temperature ofoperation and the composition of the glass being melted Melt temper-atures, temperature gradients, and convection currents are greater atand near the electrodes; therefore, better refractories (higher temper-ature, more corrosion resistant) are required at these locations

molybde-8.3.3.5 Oxygen for combustion. Over the past decade, there has been

a trend toward the use of oxygen instead of air, in combination with

natural gas, to heat fuel-fired furnaces This is called oxy-fuel firing It

has several advantages First, since one is not using air with its 80%nitrogen content, much less polluting NOx gases are produced In the

face of increasingly stringent air quality legislation, this factor alone

is often sufficient to justify conversion to oxy-fuel firing Second, sincethere is not the large volume of nitrogen to heat and expel from thefurnace, much less waste heat is generated than with gas-air Someoperations have been able to eliminate the massive, costly regenera-tors as a consequence Third, higher flame temperatures are possible.Fourth, as claimed by some manufacturers, more stable furnace oper-ation, with an associated improvement in glass quality, is achieved.This is especially the case when regeneration, with its inherent peri-odic reversals of gas and heat flow, is eliminated

There are some disadvantages to oxy-fuel firing One is the need forliquid oxygen storage or oxygen generation on site A second is that,without the large volumes of air moving through the furnace, the con-centrations of water vapor (a product of combustion) and corrosive vol-atile species from the melt (e.g., NaOH) are much higher, in some casesleading to increased deterioration of the refractory superstructure

8.3.3.6 Furnaces for specific applications. Furnaces designed for cific applications include the following:

spe-■ Container glass. Typically cross-fired regenerative; maximum melttemperatures ~1600°C; large, up to 500T/day

■ Float glass. Typically cross-fired regenerative; no bridge wall, but

rather an open surfaced narrow region called a waist to keep

inho-mogeneities running parallel to the surface of the glass sheet; mum melt temperatures ~1600°C; larger, up to 800T/day

maxi-■ Fiberglass. Smaller gas-fired recuperative or all-electric

■ Lead crystal. Small electric boosted or all-electric

■ Hard borosilicates. Tending to all-electric or heavily boosted erative; melt temperatures > 1600°C

regen-■ Aluminosilicate glass-ceramics. Regenerative gas-fired; tures near 1700°C required for efficient fining

tempera-■ Optical glass. Small fuel-fired or electric heated; fining and tioning often done in platinum tubes to avoid refractory contact andresulting inclusions and inhomogeneity

condi-With the advent of oxy-fuel, or more specifically gas-oxygen, firing,many of the above listed regenerative and recuperative furnaces havebeen converted to use this new technology However, as of this writing,float glass manufacturing is just beginning to convert to gas-oxygenfiring

Trends. As is typical in most industries, new designs are aimed atlower overall cost of operation (the calculation of which includes initialcost, melter lifetime, and costs of repairs as well as the daily operatingcosts) less energy consumption and less overall environmental impact.Lower cost almost always must be achieved in combination with im-proved glass quality and less adverse environmental impact.

The term forming collectively refers to all the processes of glass

mak-ing used to form a solid object or product from the molten glass torically, all glass objects were formed by hand using relatively simpleimplements Over time, the techniques were modified, automated, andscaled up While several glass forming methods in use today have noprecedent in early glass history, most still bear important resemblance

His-to their forbearers Due His-to space limitations here, this section will scribe only processes used in today’s manufacturing plants and, whenrelevant, the early hand-forming operations Little attention will bepaid to the many processes that have intervened We will first discussprocesses involving molds

de-8.4.1 Blowing

By far, containers (bottles and like products) account for the largestvolume of glass production Almost all these products are manufac-

tured using some form of a blowing process.

Historically, glass containers have been blown to shape by gathering

a gob of molten glass on the end of a hollow iron pipe, the blowpipe orblowing iron, and blowing a puff of air into the soft glass to form a

bubble, which is gradually expanded and worked into shape by the

combined effects of gravity and the forces of tools pressed against it.Generally, the blowing iron, with the soft glass attached, is rotated tobalance the effects of gravity and provide an axial symmetry to theproduct While useful containers of remarkably repeatable shapes anddimensions can be created in this manner, for rapid and precise pro-duction, it is preferable to use a two-step process First, a hollow pre-

form, called a parison, is prepared using a simple blowing process.

Second, the parison is blown to the final shape in a mold

This process has been automated to a very high degree in moderntimes, to the point where more than a dozen containers per minute

can be generated from each mold Generally, rotating split molds are

used for shapes involving bodies of revolution whenever visible seamlines from the molds are undesirable, such as for light bulbs or high-quality drinkware Stationary split molds must be used for containers

having handles, flutes, or other nonrotationally symmetric shapes.

The rotating split molds are generally paste molds, called that because

their molding surface is coated with a thin layer of cork or similarlypermeable substance, which is saturated with water after each mold-ing cycle When the mold surface is contacted by hot, molten glass, asteam layer results, which provides a low-friction layer between glassand mold, giving the product a highly polished appearance withoutseam lines

The stationary molds are generally hot iron molds These metal

molds are operated at a temperature hot enough to keep the molten

glass from being chilled so quickly that surface cracks or checks result,

but cold enough to quickly extract heat from the glass and allow it tobecome rigid before removal Any metal mold surface defects, as well

as the mold seam lines, are transferred to the ware, but productionrates can be much faster than with paste molds Also, on the plus side,intentional designs such as logos can be molded or embossed into theglass surface

When blowing by a hand-type operation, the final product must beseparated from the blowing iron, usually by cracking it off This leaves

a rough surface that must be properly finished by grinding or fire ishing, a process step that involves locally reheating the glass to a

pol-point at which it will flow to a smooth surface under the influence ofsurface tension In modern automated container production, free gobs

of glass are handled in the molds, so separation from a blowing iron is

not required Two common processes are called blow-and-blow and press-and-blow, depending on the method used to form the parison.

Blow-and-blow is generally used for narrow-neck containers such asbeverage bottles The parison is blown in one mold in a way that formsthe neck and then, held by the relatively cold newly formed neck, it istransferred into a second mold to blow the body of the container One

of the more common machines featuring these operations is Hartford

Empire’s (now Emhart Corporation’s) Individual Section (IS)

ma-chine This mechanism may have as many as 12 sections driven intandem by a cam with overlapped timing or, more recently, by elec-tronically synchronized operation Each section operates on as many

as four gobs Processing speeds are about 10 s per section In addition

to speed, an advantage of the IS machine (as opposed to a rotating ret machine) is that the machine can be programmed to run the re-maining sections while one is being repaired The operation of a singletwo-mold IS section is shown in Fig 8.12

tur-Press forming of the parison before blowing to final shape is used forwide-mouthed containers such as food jars Press forming will be de-scribed in the next section For container manufacture, while pressing

of the parison is complicated by the need for an additional tool (the

plunger), this disadvantage is offset by yielding a product of more form wall thickness, hence a more efficient utilization of glass and alighter-weight product than produced by blow-and-blow.

uni-A very high-speed process for blowing light bulb envelopes and the

like, known as the ribbon machine, was developed in the 1920s by

Figure 8.12 The H.E IS (individual section) blow-and-blow machine The gob is delivered into a blank mold, settled with compressed air, and then preformed with

a counter-blow The parison or preform is then inverted and transferred into the blow mold where it is finished by blowing.27

Corning Glass Works (now Corning Inc.) and is still in use worldwide.

In this machine, a stream of molten glass is continuously fed between

a set of rollers, one flat and the other with pocket-like indentations.These rollers form a ribbon of glass several inches wide, containingregularly spaced circular mounds of glass down the centerline Theparison for each light bulb is formed by inserting a synchronously

moving blow head (analogous to a blowpipe) into each mound of glass

and blowing it through a synchronously moving orifice plate As theribbon travels horizontally along the machine, the parison is enclosed

in an also synchronously moving rotating paste mold, and the blowingprocess is completed The moving molds open and swing away to allowthe finished glass envelope to be cracked off the ribbon at the machineexit The operation of the ribbon machine is illustrated in Fig 8.13 In-candescent lamp envelopes (for example A-19, 60-W bulbs) can bemade at speeds in excess of 1,200 per minute on a single machine us-ing this technique Small automotive and other specialty lightingbulbs can be made at rates exceeding 2,000 per minute

8.4.2 Pressing

In simplest terms, pressing or press forming of glass involves placing a

gob of molten glass in a hot metal mold and pressing it into final shapewith a plunger Sometimes a ring is used, as illustrated in Fig 8.14, tolimit the flow of glass up the side of the mold and produce a rim of well

Figure 8.13 The “ribbon machine” used for light bulb envelope manufacture U.S patent 1,790,397 (Jan 27, 1931), W J Woods and D E Gray (to Corning

Inc.) (Courtesy of Corning Inc.)

controlled shape The process steps can be performed entirely by hand

or fully automated It produces more accurate and controllable wallthickness distributions than blowing but is generally limited to open,moderately shallow articles such as dinnerware, cups, baking dishes,sealed-beam headlamp lenses, and television panels and funnels, orfor solid objects Pressing is capable of generating intricate and accu-rate patterns in the glass surface, such as found in sealed beam spot-

Figure 8.14 Pressed glass, mold types and pressing operations (Courtesy of Hill)28

McGraw-light, floodMcGraw-light, and automotive headlamp lenses and in street andtraffic light refractors and lenses Large objects, such as 27- and 35-in.(diagonal) color television bulb panels weighing more than 25 pounds,can be made using automatic pressing equipment.

Glass is generally pressed at a viscosity between 2000 and 3000 Pwith an applied pressure of about 100 lb/in2 of article surface area Forlarge television panels, the total force on the plunger can exceed 20 T.Temperature control of the molds and plunger is crucial; too cold leads

to brittle fracture of the glass under the pressing forces, and too hotleads to sticking of the glass to the mold surface, requiring it to bephysically broken free Vents within the mold body, through whichcooling air or water may flow, are often used to maintain uniform tem-perature distribution across the mold surface

8.4.3 Casting

Casting is a relatively little used process, found mostly in hand shopsand for the production of very large pieces of glass such as glass sculp-tures and astronomical telescope mirrors For the large pieces, glass ispoured into hot ceramic refractory molds (often sand with a smallamount of binder) that are slowly cooled after the mold is completelyfilled Alternatively, chunks of rigid glass may be placed in a cold moldand raised in temperature until the glass is sufficiently fluid to flowand fill the mold This latter method is more susceptible to entrap-ment of bubbles Generally, slow cooling and long annealing times arerequired The mold can be used only once The glass surfaces in con-tact with the mold are generally rough

8.4.4 Centrifugal Forming

Centrifugal forces have often been utilized by the glassmaker A glassbubble on the end of a blowing iron can be elongated by swinging theiron back and forth to aid gravity in elongating the bubble to generatethe parison A thick-walled bubble on the end of a rod can be cut open

at the point opposite to the rod, and the rod rotated to generate cient centrifugal force to open the bubble and spin it into a relativelyflat, circular sheet of glass This is one of the earliest flat glass manu-

suffi-facturing methods, the crown process Glass made this way is often

found in old European churches A droplet of very fluid glass placed atthe center of a rotating turntable will also spread under centrifugal

force, a process utilized in spin coating or spin casting The latter is sometimes simply called spinning.

If molten glass partially fills a rotating container such as a mold or acrucible, the molten glass will tend to climb the walls, propelled by the

centrifugal forces, giving the glass surface the shape of a paraboloid of

revolution This method, called centrifugal casting, is used to form the

parabolic shapes for thin astronomical telescope mirrors It has alsobeen used to spin, rather than press, large, deep television tube fun-nels and glass-ceramic missile radomes Six- to 8-meter diameter mir-ror blanks are spun at about 10 rpm, a television bulb funnel at about

200 rpm In the 1960s, Corning Glass Works used this spinning cess to make large, 56-inch diameter glass hemispheres of 1.5-inchwall thickness for use in undersea exploration

pro-A continuous centrifugal process of forming tubing from very short(steep viscosity) glasses or easily devitrifiable glasses has been de-vised It is somewhat analogous to the Danner tubing process (see be-low) except that the stream of glass is fed into the open end of aninclined rapidly rotating pipe The very fluid entering glass is flat-tened against the wall and the adjacent layer of previously depositedglass and is maintained in position by centrifugal forces until it iscooled to sufficient rigidity to be withdrawn from the end of the pipe

8.4.5 Rod and Tube Drawing

Drawing is the term for a process in which a preshaped blank, or glass

flowing from an orifice, is elongated (stretched) in one dimension whilediminishing in orthogonal dimensions without losing its cross-sec-tional characteristics

The above statement is exactly true for the drawing of cane (rods) or

fiber It is not so for tubing or sheet, where the ratios of inside to side diameter or width to thickness are not the same as they were at

out-the root (The solid section of out-the blank or out-the glass at out-the orifice is

of-ten referred to as the “root.”)

Redrawing is the specific case of drawing from a solid preform (or blank) rather than from a melt This involves reheating the end of the

blank to provide glass sufficiently fluid to be stretched and attenuated

In a continuous process, the blank is replaced at the volume rate atwhich it is used up by gradually feeding it into the hot zone of the re-draw furnace

In a steady-state process, the volume flow per unit time, the

quan-tity Q, is constant and equals A (area) × v (velocity) at any point in the process As we will show, A is a very important parameter in the tube

drawing process

Tube drawing processes. In a hand process, a gob of glass is gathered

on the end of a blow pipe, a bubble is blown within the glass, and anassistant attaches a rod to the side of the gob opposite the blow pipe(or grabs the gob with a pair of tongs) and walks across the room tostretch out the glass and the bubble within it The final diameter of

the resulting tubing, and its wall thickness, depend on several factors,including how fast the assistant walks (compared to how rapidly theglass cools) and how much pressure the blower maintains in the bub-ble If faster cooling is needed, a second assistant may fan the tubing

as it is drawn out The air pressure resists tubing collapse from thedraw forces and surface tension After the drawing step is completed,the hollow glass tubing is cut away from the bulky pieces at each end.There is only about 10 to 20% glass utilization The rest of the glassremains on the blowpipes or is of unusable dimensions and is gener-ally recycled as cullet The process is highly labor intensive

The Danner process, named after its inventor, Edward Danner, was

developed by the Libbey Glass Company It is one of the oldest uous tubing drawing processes still in use today and is the commonmethod for forming fluorescent lamp tubing The process is somewhatunique in the manner of preparing the root of glass from which thetubing is drawn See Fig 8.15 A ribbon-shaped stream of glass is fedonto a slowly rotating (~10 rpm) hollow clay (or metal) mandrel, in-clined downwards perhaps 15° from horizontal The glass stream flowsonto the mandrel at a viscosity about 1,000 P, wraps around the man-drel, and overlaps itself to form a cylinder, which is smoothed by theforces of gravity and surface tension The glass cools (to a viscosity of

contin-Rotating mandrel used in Danner tube drawing.

about 50,000 P) as it moves along the mandrel and is drawn off themandrel end in a horizontal direction forming a catenary Air is fedthrough the mandrel, so the latter acts somewhat like a hand blowpipe with a continually replenished supply of glass The tubing isdrawn (stretched to smaller dimensions) by a tractor device locatedmany feet from the mandrel and is cut into lengths after it passesthrough the tractor.

The Danner process is capable of drawing 1/16- to 2.5-inch diametertubes Because of temperature nonuniformity, coupled with gravity ef-fects, Danner tubes often exhibit some ovalness and wall thickness

(called siding) variations Solid rods of similar diameters can be

drawn by stopping the airflow through the mandrel or even drawing aslight vacuum Composition variations can produce hairpin-shapedcord defects

The Vello process, after inventor Sanchez Vello, was developed by

Corning Glass Works and dates back to the early part of the 20th tury Here, the glass is delivered from the glass-melting furnace, atabout 100,000 P viscosity, through an annular orifice created by the

cen-spacing between a conical bowl and a bell-shaped blowpipe, called the bell, centered within the bowl See Fig 8.16 The drawn tubing first

extends vertically downward then turns horizontally, following a nary curve as it stretches under its own weight, then is transported on

cate-a runwcate-ay of “V” rollers often severcate-al hundred feet long cate-as the glcate-asscools As with Danner, the tubing is cut into lengths after it passesthrough the tractor at the end of the runway

The Vello process allows drawing of precision-bore tubing, such asfor thermometers and burettes It is fast (for example, 800 52-inchsticks/min at a 2000 lb/hr flow rate) and can draw tubing of diameters

up to 3 in without significant oval

Control of diameter and wall thickness is based on a mathematicalequation relating volume flow of glass, tubing velocity, and tubingcross-sectional area, which determines how fast one must run thetractor pulling the tubing to give the requires cross-sectional area ofthe glass in the tubing This area is given by

where Q = volume flow of glass from the melter

v = speed of the tractor

= A = πw D w( – )

wall thickness interact, and their ratio is maintained within tion by the pressure of the air flowing through the tubing, just as withhand drawing.

specifica-The downdraw tubing process is essentially a vertical Vello specifica-The

glass tubing is cut off one or more floors below the draw It is useful fortubing that is too large in diameter or wall thickness to be successfullyturned horizontally (i.e., without breaking or deforming from cylindri-cal shape) For example, Corning has drawn 6-in dia., 3/8-in wallborosilicate tubing for PyrexTM brand pipe by this process Molds anddies can be added to give controlled cross-sectional shapes

The updraw tubing process is analogous to the updraw processes,

flat or cylinder, used to manufacture sheet glass, which will be scribed in the next section The air pressure needed to keep the tubebore open is supplied from below, through a refractory cone positioned

de-in the melt just beneath and on axis with the drawn tubde-ing Theheight of the cone helps control the tubing wall thickness Continuousupdraw of thermometer tubing with enclosed colored glass ribbons isonly slightly more complicated

Glass line

Drain

Air

Figure 8.16 Bell and bowl arrangement used in Vello tube drawing process From

lec-ture notes, E H Wellech, Corning Glass Works, 1963 (Courtesy of Corning Inc.)

8.4.6 Sheet Drawing

Sheets of glass can be drawn either upward or downward from a bath

of molten glass by a tractive mechanism, provided a method can befound to maintain the root of the draw in a fixed position at constant

dimensions For a downdraw process, the root is essentially a

rectan-gular slot in the bottom of the melter Molten glass flows from this slotunder the combined effects of hydrostatic pressure from above andtension from below Below the melter, the sheet is cooled to becomerigid, after which its weight is supported by pairs of horizontal rollers

As the glass is cooled by radiation beneath the slot-shaped orifice, therollers stretch it to the desired final thickness The sheet also tends tobecome narrower in width during stretching, but this effect can beminimized by the judicious use of edge coolers to help hold the edges ofthe sheet out These edgewise forces, while maintaining the desiredsheet width, also help maintain its flatness One is essentially pulling

on a stretched membrane The formed sheet is lowered through a

heated annealing zone (annealer) and cut into separate sheets below,

often in a subbasement or a specially excavated pit

A disadvantage of the process is that any nonuniformities in theslot, such as might be caused by erosion or corrosion, lead to verticalstreaks in the glass surface In part to overcome this difficulty, and inpart to have better thickness control over the resulting sheet, Corning

Glass Works (now Corning Inco.) developed their fusion downdraw

process In this process, molten glass is fed into one end of a slightlyinclined refractory trough at a viscosity of about 40,000 P and allowed

to overflow both sides, as shown in Fig 8.17 (Sometimes this trough is

called the overflow pipe for obvious reasons.) The outside of the trough

tapers to a line at the bottom where the two layers of overflowing uid meet and fuse together, forming the root of the draw, hence the

liq-name fusion The outside surfaces of the glass are generated from

within the interior of the melt and are therefore never subsequently incontact with other materials; thus, they are pristine and defect free Akey element of Corning’s initial patent for this process is the mathe-matical design of the tapering cross-sectional profile of the troughwhich, in combination with the incline of the top of the trough, assuresthat the volume flow of glass over the pipe is uniform along its length.Along with a method to precisely control the temperature along theroot of the draw, this assures uniform sheet thickness across its width.Below the root, the process is somewhat similar to a downdraw from

a slot One notable difference is that the glass at the root is far moreviscous Having been cooled to a viscosity of 500,000 P or more as itdescended the tapered lower refractory of the trough, it must be pulleddownward with greater tensile stress The pulling forces are provided

by the weight of the sheet and edge rollers only, so the glass surface isnot subject to roller marking A wide range of thicknesses may bemade, ranging from less than 1 mm to greater than 1 cm This process

is currently used to manufacture thin, flat glass of exceptional qualityfor active matrix liquid crystal displays (AM-LCDs) for flat-panel com-puter screens and televisions

A variety of updraw processes have also evolved The Fourcault

pro-cess, invented in 1910 by the Belgian Emile Fourcault, uses a partiallysubmerged refractory block containing a long machined slot to form

the root of the draw This block is called the debiteuse The molten

glass that forms the root of the draw is forced up through the slot bythe buoyant force of gravity The draw is started by lowering a metal

mesh, called the bait, to the slot and then, once it is wetted by the

glass, drawing it upward to form the sheet Once the process has beenstarted, the sheet of glass is pulled upward by pairs of horizontal roll-ers, drawn through an annealing zone, and cut into separate sheetsabove The bath of molten glass is held at temperatures providing aviscosity of about 100,000 P Water-cooled edge rollers, located some-what above the melt surface, help prevent the sheet from narrowing inwidth as it is pulled upward This process is still in use throughout theworld for the manufacture of window glass Process disadvantages in-

clude vertical streaks, called peignage or music lines, caused by

ero-sion of the slot in the debiteuse

The Pennvernon process, developed by PPG, uses a fully submergeddraw bar to control the location and straightness of the root Viscous

Figure 8.17 Illustration of Corning’s “fusion” overflow sheet drawing

process (Courtesy of Corning Inc.)29

forces confine the drawn sheet to a position over the center of the bar.

In the Colburn-LOF process, the soft glass is bent 90° over a roller ashort distance above the melt surface This roller, along with water-cooled edge rollers, serves to keep steady the position where the glass

is drawn from the melt surface In all these updraw processes, thepulling rollers and the bending rollers, when used, tend to mar theglass Another disadvantage of updraw is that whenever the sheetbreaks in the rolls, the broken glass falls back into the machine andinto the melt The updraw processes are more difficult to restart thanare the downdraw processes With the possible exception of Fourcault,these updraw processes are in relatively little use today, primarily be-cause they have been superseded by float (see Sec 8.4.8)

to the rollers The speed of the operation and the diameter of the rollsmust be sized to the thickness of the sheet being rolled The glass en-ters the rolls relatively fluid, but it must be considerably less fluidwhen it leaves so as to maintain its shape The thicker the glass, themore heat that must be removed by the rollers Generally, thick sheetrequires large rollers, maybe 4 to 10 ft in diameter, and forming rates

of only a few feet per minute Very thin ribbon can be made with smallrollers, a few inches in diameter, at rates of several feet per second Af-ter rolling, the continuous sheet is transported horizontally through aheated annealer

The surface finish quality and thickness uniformity is generally sufficient for mirror, automotive, and architectural applications Prior

in-to the development of the float process, described below, rolled glasssheet was ground and polished on both sides, sometimes simulta-neously, to meet these requirements These products were known as

plate glass The process was inherently wasteful and expensive Patterned glass is made by applying texture to one or both glass sur-

faces using suitably embossed rollers Applications include showerdoors, furniture tops, room dividers, and windows There is even an

application for roller-applied Fourcault-type sheet texture for use inrestorations of nineteenth century homes.

As a variation, wired glass, such as used in fire doors and building

skylights, can be manufactured by continuously feeding wire mesh tween the rollers along with the molten glass

be-8.4.8 The Float Process

Grinding and polishing of rolled glass was very expensive, labor sive, and wasteful of materials In the 1950s and 1960s, the Pilkingtoncompany in England developed a much more economical process based

inten-on floating a cinten-ontinuous ribbinten-on of molten glass inten-on a bath of molten tin

as the glass cooled and solidified This process is illustrated in Fig.8.18 A detailed description of the process and the difficulties thatwere overcome in its development lie outside the intent of this hand-book, but some key points should be made The glass product, known

as float glass, has excellent surface properties, the upper surface

hav-ing flowed freely without contact with rollers or any other formhav-ing

Tin

(b) (a)

Float process: Pilkington-type tin bath: (a) side view and (b) top view.30

vices before its solidification, and the lower surface similarly havingbeen in contact only with a flat, smooth liquid metal surface that wasincapable of marring it The product also has exceptionally uniformthickness.

Regarding thickness, a freely spreading puddle of glass suspended

on molten tin (by buoyant forces in a gravity environment) will reach

an equilibrium thickness determined by the tin and glass densitiesand the various surface and interfacial tensions For soda-lime-silicaglass on tin in the Earth’s gravity, this thickness is between 6 and

7 mm, approximately that of traditional plate glass Several niques have evolved to make thinner and thicker float glass These es-sentially involve pulling the glass off the bath at a rate faster orslower than would maintain the above-defined equilibrium thickness,and doing so in a manner that preserves the thickness uniformity.Several techniques for “stretching” the glass in this manner haveevolved All employ gripping the edge of the spreading glass puddle onits top surface with knurled rollers to assist, restrict, or redirect theglass flow One method is illustrated in Fig 8.19

tech-It should be noted that many early references to the float process scribe the glass sheet as being formed by rolling between two rollersbefore it is fed onto the tin bath This approach was tried initially by

de-Figure 8.19 Decreasing the thickness of float glass by both lateral and longitudinal stretching, with knurled wheels pressing on the edges of the ribbon.31

Pilkington, but it proved unsuccessful (It led to surface defects in theglass.) In the commercialized processes, the molten glass is fed ontothe tin bath by flowing it over a refractory block The Pilkington andPPG designs differ in how this is done.

The Pilkington and PPG float processes have proven so effective and

so economical that they have virtually replaced all plate glass facture (ground and polished rolled sheet) throughout the world, andmost of the drawn sheet products as well Most of the flat glass pro-ducers in the world have been licensed to use the float process Today’slargest float glass plants can produce about 1,000 T of finished glassper day at widths up to about 12 ft and thickness between about 2 and

manu-25 mm The overall length of the production line, including melter, tin

bath, and annealing lehr can exceed 700 ft, with the tin bath itself

oc-cupying between 100 and 200 of those feet

Specially designed float lines can produce glass less than 1 mmthick While initially developed for the manufacture of soda-lime-silicaglass, several manufacturers have successfully applied the techniques

to borosilicate glasses However, because of temperature limitations ofthe tin bath, and its required chemically reducing atmosphere, not allcommercially useful glass compositions can be manufactured by thefloat process

8.4.9 Fritting

Techniques used for making glass frit (granules) include dry gauging

or dry gaging (drizzling or pouring a stream of molten glass into cold

water) and rolling as very thin ribbon, followed by particle attrition orcomminution (size reduction) processes These techniques involve arapid quenching of the melt and can be used to vitrify (make glassfrom) compositions that tend to crystallize readily Cooling the glassquickly, directly from the melter, creates high thermal stresses, whichshock and often break the glass into small pieces suitable for charginginto a ball mill Dry gauging sometimes forms clinker-like pieces thatare difficult to mill and may require an intermediate process step.Thin rolled ribbon often provides the better, more uniform mill feed

8.4.10 Spheres, Marbles, and Microspheres

Glass spheres can range in size from a few nanometers in diameter to

a meter or more They can be solid, porous, or hollow, in all but thesmallest sizes The application range includes

1 The extremely small solid precursor particles used in Types III, IV,and V fused silica manufacture

2 The small, hollow spheres used to contain fuel in inertial ment nuclear fusion research

confine-3 The small, <0.2 mm microspheres used in reflective signs and

pro-jection screens or as fillers in plastics and elastomeric composites

4 The ~1/2-in marbles used in games

5 Fish net floats

6 Deep ocean submersible vessels of military interest

Because of the wide range of size and the need for solid, porous, andhollow variations, manufacturing techniques necessarily vary Direct-ing a high-temperature, high-velocity flame across a vertically de-scending stream of molten glass can generate small solid spheres Ifthe glass is sufficiently fluid, strands of glass are formed that quicklybreak apart into droplets These droplets spheroidize under the forces

of surface tension and cool as they leave the flame The resulting ticle size distribution is not easily controlled Similarly, molten drop-lets can form and detach from an orifice at the bottom of a crucible orglass-melting tank If the droplets have sufficiently low viscosity andare released from a great enough height, they will spheroidize andcool before reaching the ground, where they are collected Particle sizecontrol is better

par-Microspheres (<0.2 mm dia.) of controlled size and composition canalso be prepared by fritting, sieving, and injecting into a heated region

to remelt, spheroidize, and cool, somewhat as described above Theprecursor particles can be fed into the top of a tall column having athermal gradient decreasing downward and collected at the bottom, orthey can be injected into an upward-directed flame, whereby the mol-ten droplets are drafted to cooler higher altitudes and collected

To mass produce marbles (solid spheres about 1/2 in in diameter), a

more viscous stream of glass is delivered from the melter and ically cut (sheared) into mini-gobs having the required volume Thesoft gobs fall into the space between two counter-rotating cylinders inwhich there are machined opposing spiral grooves The gobs of glassare simultaneously rolled into spheres and cooled to temperaturesnear their annealing point as they are transported down the length ofthe rolls Streams of different colored glass can be partially mixed to-gether before gobbing to give variegated appearance Less sphericallyperfect marbles can be generated by dropping the mini-gobs into cylin-drical holes in vibrating molds, sometimes mounted on a conveyorbelt Steady vibration, maintained until the gobs have been wellcooled, generates near-spherical shapes Of course, decorative marbles

mechan-of varying sizes can be created as novelties or works mechan-of art by studio orhand-shop techniques

Perfectly spherical spheres of a wide variety of sizes with optical ish can be produced from near-spherical starting blocks by grindingand lapping on optical finishing machines

fin-Hollow spheres of moderate size can be hand blown freely or inmolds, but the location where the blowpipe is separated after forming

is seldom perfect Hemispheres can be pressed and then pairs fusedtogether Very large, thick-walled spheres can be made from pairs ofcentrifugally cast hemispheres

At the other end of the size spectrum, very small, hollow glassspheres such as used in inertial confinement nuclear fusion (ICF) re-search are also prepared by a variety of techniques, all relying on gen-erating small volumes of precursor materials that are injected into ahot zone The surface of the precursor particle melts or otherwise re-acts to generate a viscous liquid layer As the interior material heats,

it evolves gasses that serve to blow the hollow sphere Solid precursorscan be formed as small aggregates of batch material, sometimes byspray drying or sol-gel techniques; alternatively, the required chemi-cal species can be dissolved in an aqueous or organic solution with pre-cursor droplets of the required size being generated by an appropriate

means such as an ultrasonic nebulizer.

Porous spheres can be prepared by leaching one glassy phase from

a two-phase spherical product or by processes similar to those usedfor hollow microspheres whereby the surface layer never forms in afully continuous manner; i.e., the blowing bubbles are exposed at thesurface

8.5.1 Development of Permanent Stresses

in Glass

Stresses in an unconstrained elastic solid develop only if there is anonlinear temperature gradient across the body Such stresses aretemporary, or transient; they exist as long as the temperature gradi-ent exists Liquids, on the other hand, cannot sustain shear stressesfor any finite length of time; such stresses relax by viscous flow.Glasses behave like a liquid when heated into the liquid state; i.e., allstresses relax due to viscous flow However, upon cooling through theglass transition range into the solid state, stresses are likely to de-velop within the body, and such stresses no longer relax in the absence

of a viscous flow The various mechanisms for such permanent stressdevelopment are as follows:

1 Cooling from the outside results in a “frozen” temperature ent with a higher temperature in the interior (Fig 8.20) Inner lay-

gradi-ers continue to relax from fluid flow while the outer laygradi-ersgradually freeze In effect, the stress-free state is a solid with atemperature gradient The need to shrink the inside more relative

to the outside at room temperature and the enforcement of theelastic compatibility criteria between the layers cause the appear-

ance of compression on the outside and tension on the inside The magnitude of the stresses so developed is related to the linear ex- pansion coefficient of the solid This mechanism is also called the viscoelastic mechanism.

2 The outside layers cool faster than the inside layers during normalcooling Hence, the outside layers tend to possess a “faster-cooled”structure having a higher volume in the free state (see Ref 8,

p 15) This, in principle, is a permanent structural heterogeneity.

Again, the enforcement of the elastic compatibility criteria causesthe outside layers to develop compression and the inside layers to

develop tension The magnitude of the stresses so developed is lated to the difference between the volumes of the fast-cooled and the slow-cooled solids.

re-3 The fact that the various layers travel through the glass transitionrange at different instants in time causes the development of a

“frozen fictive temperature gradient.” This is a transient structural heterogeneity As in (1), the removal of the transient fictive temper-

Figure 8.20 Simplified concept of permanent stress production in glass due to a zen temperature gradient.” (a) Glass with no temperature gradient well above the transition region has no stress (b) Temperature gradient develops on cooling; how- ever, no stress develops because of rapid relaxation while the glass remains well

“fro-above T g (c) Cooling to below T g while maintaining the same “frozen temperature gradient” produces no stress yet (d) Final removal of the temperature gradient pro- duces stresses that are now permanent 32

ature gradient causes the appearance of compression on the

out-side and tension on the inout-side The magnitude of the stresses so developed is related to the configurational (or the structural) contri- bution to the linear expansion coefficient of the supercooled liquid

(= linear expansion coefficient of the liquid – the linear expansioncoefficient of the solid)

During glass forming, various regions of glass indeed go throughvarying cooling rates due to the nonuniform applied heating/coolingand the usually nonuniform thickness; hence, stress generation islikely There is little chance of fracture while the glass remains some-what viscous fluid but, once the material starts solidifying, stressesbegin to accumulate For a common soda lime silicate glass productcooled through the glass transition region at “normal” cooling rates,nearly 60% or so of the total stress is due to the mechanisms (1) and(2) Warping may result to relieve uneven bending moments across thebody of a glass product In turn, the warping generates bendingstresses, some of which end up being tensile on an outer surface Thisthen poses a potential risk for glass fracture during the manufactureand, worse yet, during service There is additionally a longer-term riskfor property and dimensional changes due to the continuing stabiliza-tion of glass (Optical glass components, particularly those that arespace-based, need to have minimized refractive index and dimensionalchanges.) Glass products are therefore customarily reheated in theproximity of the glass transition range to allow the release of any in-ternal stresses that might have developed, and cooled at a rate slow

enough that prevents their rebuilding This procedure is termed nealing An additional objective of annealing is to homogenize the

an-thermal history across the body

Tempering (also called toughening, which is a misnomer in this

case), on the other hand, implies strengthening and is accomplished

by introducing surface compression into the glass Since flaws thatcause strength degradation usually occur on the surface, the introduc-tion of surface compression strengthens a glass product Unless thevolume-temperature diagram for the glass at hand is somewhat un-usual (like that of fused silica), one expects to have some degree oftemper obtained by normal cooling through the glass transition, which

is clearly beneficial for ordinary handling of glass products

8.5.2 Stress Profiles in a Symmetrically Cooled

Glass Plate during Annealing and Tempering

If a glass plate is cooled symmetrically from both sides, then the perature distribution across the section is a parabola (for low cooling

tem-rates); see Fig 8.21 The magnitude of the temperature difference ∆T

between the outside and the inside layers of glass is given by

where d = half-thickness of the section

where E = Young’s modulus

v = Poisson’s ratio of the glass The center tension is proportional to ∆T/3 (= half of the surface com- pression magnitude) and is therefore given by one-half of the above quantity The above equation may also be used to calculate a con-

stant cooling rate that would give rise to a specified center tension atroom temperature For instance, for a soda lime silicate glass with α

parabola of the form y = ax2 but may be approximated by y = ax n Itcan be shown that the ratio of the magnitude of the surface compres-

sion to that of the midplane tension is simply n Depending on the Biot number (= hd/k; where h = the heat transfer coefficient and k = ther- mal conductivity), n values as high as 4.5 have been obtained (see Fig 8.22) Most commonly, n = 2.2 to 2.4 For a rectangular parabolic

distribution of stress, the zero stress (“neutral”) line is about one-fifth

of the glass thickness below the surface from each side (see Fig 8.21).The thickness of the compression layer from the surface up to the

* A typical value of κ for soda lime silica glass is 0.0084 cm 2 /s.

† Note that the calculations assume that the room temperature stress is the inverse of that established during fluid state when the thermal expansion coefficient was roughly three times that in the solid state.

∆T d2R

2κ -

nearest neutral line is called case depth Thus, a 6 mm plate may have

surface compression layers roughly 1.2 mm thick on each side, whichrepresents a significant depth of protection

One of the fascinating examples of tempered glass is the Prince pert Drop Teardrops, generally 4 to 10 mm in diameter, of molten soda

Ru-lime silicate glass with a tail are allowed to fall into water The highdegree of surface compression that develops into the teardrop allowshammering on it The drop explodes into flying fine powder of glass

Figure 8.21 A parabolic temperature gradient produces surface

com-pression that is twice the interior tension in magnitude; d = glass

thickness (From Ref 32, p 304)

when the tail is snapped (Caution: such demonstration should be ried out under protective wrapping or behind a protective shield.)

car-8.5.3 Standards of Annealing

Commercial glass products are generally called annealed if the center

tension is no greater than about 500 psi In a stress-birefringencesetup (see Sec 8.5.9), and with the assumption that the stress-opticalcoefficient is 3 brewsters, this stress would roughly correspond to a re-tardation of 100 nm/cm of light path For most optical glasses, the op-tical birefringence in the middle should be no more than 5 nm/cm or,even better yet, roughly 2.5 nm/cm

8.5.4 Annealing Practices

Coarse annealing may be accomplished, particularly in a laboratorysetting, using the following guidelines:

1 Hold the glass product isothermally in a box furnace at a

tempera-ture within the annealing range The holding time t may be

esti-mated by the KWW relaxation formula:

(8.2)

Figure 8.22 Ratio of “plateau level” surface compression to midplane

tension as a function of Biot number Curve 1 = experimental results.

Curve 2 = predictions by Indenbom’s theory Curve 3 = predictions by

Bartenev’s theory 33

η -

1 2–exp

=

where σt is the stress at time t, and η and G are the viscosity and

shear modulus of the glass at the holding temperature, tively The stress σt is normally taken as 5% of σo, the initial stress,

respec-and the high-temperature value of G is taken as 20 to 25% lower

than that at room temperature Thus, at the annealing ture, where viscosity is 1013 P, one needs to hold for 10 to 15 min.However, as much as 6 to 8 hr are needed for a comparable relax-ation at the strain point (η = 1014.5 P)

tempera-2 Cool the furnace usually at 5°C/min, which can be accomplished bysimply turning the box furnace off to cool overnight

On an industrial scale, a continuous belt lehr is often employed forannealing One of the several suggested heat treatment schedules for

the annealing of glass products is shown in Fig 8.23 In region A, the

glass is rapidly heated or cooled to about 5°C above the rated ing point, depending on whether the product is hot coming out of theforming machine or is cold coming out of storage It is held at that

anneal-temperature for a time period t (region B), following which it is slowly cooled to a temperature a° below the strain point (region C) Once the

glass is well below the strain point, any risk of rebuilding permanent

stresses is slight, and the glass may be cooled faster (regions D and E).

The limiting consideration for the cooling rate in the two regions is theavoidance of thermal shock The recommended schedule for the vari-ous regions is as follows:

Figure 8.23 Suggested schedules for commercial annealing of soda-lime-silica

glass-ware (Courtesy of Corning, Inc., from Ref 8, p 307)

A (heating rate) = 500/[αd2] °C/min

B (holding time) = 15z min for cooling from one side

C (slow cool) = 42.6/[αd2] °C/min

a° (°C below

the strain point) = 5°C for 0.3 cm thick plate

= 10°C for 0.6 cm thick plate

= 20°C for 1.3 cm thick plate

D (fast cool) = 2 times the slow cool

E (final cool) = no more than 10 times the slow cool

where α is the thermal expansion coefficient in × 10–7/°C units, z is the glass thickness (cm), and d = z when heated/cooled from one side only and = z/2 when heated/cooled symmetrically from both sides Compu-

tations for a few glasses with different plate thickness and cooling ditions are tabulated in Table 8.20 The total time for the completion

con-of a commercial quality annealing is expected to be about 20 min

8.5.5 Standards of Temper

According to ASTM-C1048, Kind FT (fully tempered) glass shall have

a surface compression no less than 69 MPa (10,000 psi) or an edgecompression of not less than 67 MPa (9700 psi) Such a glass is up tofive times as strong as a normally annealed product of the same thick-ness and configuration It is often recommended that a fully temperedglass should have roughly 100 MPa (14,500 psi) surface compression.This corresponds to roughly 45 MPa (~6,400 psi) midplane tension, orabout 1350 nm/cm birefringence in the midplane Glass should meetAmerican National Standards Institute (ANSI) standard Z97.1 orConsumer Product Safety Commission (CPSC) standard 16CFR1201.The ANSI standard specifies that a tempered automotive glass mustwithstand the impact of a 1/2-lb steel ball and an 11-lb shot-filled bag,both dropped from specified heights In the ball drop test, 10 out of 12specimens must remain unbroken The drop height is then increaseduntil all the specimens have broken Upon initiation of the fracture,the stored mechanical energy is released virtually instantly, causingrapid bifurcation of advancing crack fronts This then yields a dicingbehavior, where the pieces do not have acute-angled corners and henceare not expected to cause serious injury by sharp cuts The higher themidplane stress, the smaller the pieces For the glass to be acceptable,none of the diced pieces may weigh more than 0.15 oz (4.3 g) for a

TABLE 8.20 Suggested Annealing Schedules for Glass Products *†

6-mm thick glass The dependence of the break pattern on midplanestress is shown in Fig 8.24.

Kind HS (heat-strengthened) glass has a surface compression

be-tween 24 and 52 MPa (3500 to 7500 psi) for a thickness of 6 mm Such

Figure 8.24 Dependence of break pattern on midplane stress Exit

tem-perature, 660°C (1220°F) (From R A McMaster et al., Ref 7, Fig 10, p.

458, reproduced with permission of publisher)

a glass may be up to three times as strong as a normally annealedproduct of the same thickness and configuration.

The U.S Food and Drug Administration (FDA) requires that all wear (prescription or otherwise) sold in the United States pass safetyball drop tests for impact resistance described by ANSI Z80.1-1995and ANSI Z80.3-1996 This testing requires a fully finished and edgedglass lens to survive impact testing with a 16-mm dia steel ball,dropped free fall from a height not less than 1.27 m, onto the central

eye-16 mm diameter area of the outer surface of the horizontally placedlens Certain specialty lenses are exempt from this test, as are plasticlenses Industrial safety glasses must pass a more severe test Boththermal tempering and ion-exchange strengthening are used to meetthese impact requirements

8.5.6 Commercial Tempering Practices

Tempering is usually accomplished by heating the glass product, erally to a viscosity of about 109.5 to 1010 P for a short while, and thenquenching symmetrically from both sides using forced air jets For atypical soda lime silicate glass plate, these viscosity values correspond

gen-to about 640 gen-to 620°C The normal practice is gen-to run a thinner glasshotter; however, it must be remembered that higher temperaturesbring about noticeable optical distortion Air power requirement isroughly 40 kw · m–2 for a 3-mm plate and is only about 12 kw · m–2 for

a 6-mm plate The quenching to a viscosity below 1014.5 P takesroughly 5 s for a 3-mm plate and ~12 s for a 6-mm thick plate (see Fig.8.25) Cooling to room temperature occurs over longer periods A typi-cal tempering furnace is about 40 m long in which the heating zonemay be ~20 m long, the quenching zone ~5 m long, and the slowercooling zone ~15 m long With a width of ~1.5 m, the furnace can de-liver about 15 T of fully tempered glass per day

8.5.7 Limitations of Thermal Tempering

The most important drawbacks of the thermal tempering process are:

1 Surface compression magnitudes much higher than about 140MPa (~20,000 psi) are accompanied by a center tension of about

65 MPa At this magnitude of tension, a defect such as a stone inthe interior could cause the glass product to fail spontaneously

2 It is difficult to exceed cooling rates of about 100°C/s through theglass transition by forced air jets Use of Eq (8.1) suggests thatplates thinner than about 1 mm develop no more than about

24 MPa of surface compression, which is not much of a protection

Figure 8.25 Effect of glass thickness on (a) air power and (b) quenching time required for full temper Exit tempera-

ture, 620°C (1150°F) (From R A McMaster et al., Ref 7, Fig 5, p 456, reproduced with permission of publisher)

3 Because of the lack of symmetry in achievable cooling, glass tubes,containers, and other complex-shaped products cannot be mean-ingfully tempered using thermal means.

8.5.8 Chemical Strengthening of Glass

The exchange of large alkali ions from an external source such as a ten salt bath with comparatively smaller host alkali ions in a glass atlow temperatures leaves the glass in a state of surface compression that

mol-is an effective means of strengthening the glass The level of ing achieved could be 2 to 10 times that of an unstrengthened product.

strengthen-Large ions are essentially “stuffed” in glass network interstitial spacesoccupied previously by the small ions (Fig 8.26) Stress generation isclosely linked to the kinetics of the interdiffusion and the difference inthe size of the exchanging ions Although the diffusion rates increaseexponentially with temperature, the rate of stress buildup as the ex-change temperature approaches the glass transition temperature is ac-tually reduced by stress relaxation arising from glass fluidity and

perhaps some network plasticity Hence, chemical strengthening of glass must be carried out at temperatures well below the glass transition range A typical ion penetration and stress profile in a glass is shown in

Before ion exchange After ion exchange

Molten Salt Molten Salt

Si Si Si

Si

Si Si O

O

O O

O O

-

-

-

-

-

-

- -

-

Figure 8.26 Crowding from low-temperature exchange of K+ for Na+ ions (After

W H Dumbaugh and P S Danielson, Ref 5, p 120)

Fig 8.27 Surface compression on the order of 100 to 1000 MPa (=14,500 to 145,000 psi) can be generated using this technique in manyglasses having wall thickness as small as about 1 mm For consumerglass products, a minimum case depth of about 30 µm is generally rec-ommended to provide an effective protection from the surface flaws.

This, unfortunately, means that a successful ion exchange strengthening process would usually involve 2 to 100 hr of diffusion, which does not en- courage a large-scale continuous process Nonetheless, some commercial

Figure 8.27 Stress profile in a sodium borosilicate glass sample after ion exchange at

400°C for 12 h (After A.K Varshneya, J Non-cryst Sol 19, 355, 1975 Reproduced with permission of Elsevier Science, from Ref 8)

glass products are ion exchange strengthened: windshields for aircraft,borosilicate glass syringes, ophthalmic products, and large-capacity re-turnable beverage containers are some examples.

Ion exchange strengthening of glass is typically carried out by mersion of the glass in a bath of molten salt For instance, a commoncommercial soda lime silicate glass would be strengthened by immer-sion in a bath of molten KNO3 at temperatures higher than 328°C(melting point of KNO3) but lower than about 480°C (the onset of glasstransition) Lithium aluminosilicate glass is strengthened by immer-sion in NaNO3 similarly The alkali aluminosilicate glasses are thebest candidates for ion exchange; the soda lime silica glasses do notstrengthen well because of rapid relaxation processes, despite therather large alkali content in the host glass Low-alkali-contentglasses, such as the alkali borosilicates of the Pyrex type, make evenpoorer candidates for ion exchange strengthening

im-As the glass ions are continuously rejected into the bath, the tration of this contaminant builds up This usually has deleteriousconsequences on the percent surface exchange even at very low level ofcontamination Hence, the salt bath requires careful control for the

concen-“contamination” level

The strength is commonly measured in four-point bending mode or

a ring-on-ring method using a mechanical tester on suitably sized testcoupons, with or without surface abrasion (It is strongly recom-mended to employ edge-independent techniques.) A steel ball drop testfor ophthalmic lenses and a bird impact test for aircraft windshieldsare also carried out

To measure stresses in a chemically strengthened glass, one needs

to obtain a thin slice across the wall thickness, which has been mographically prepared to have sharp square edges at the surface

cera-The observed stresses in such a thin slice must be divided by (1 – v) to

obtain stresses in the unsliced plate It may also be possible to employtechniques such as the differential surface refractometer (DSR) orgrazing angle surface polarimeter (GASP) to measure surface com-pression and the laser scatter to measure internal tension without theneed of slicing (see Sec 8.5.9)

Although immersion in molten salt is the more popular technique ofion exchange, other techniques have been tried Examples of these arepastes using mixtures of inert clays, vapor phase, electrical field-as-sisted strengthening, multistep (such as thermal and chemical or two-step chemical), sonic-assisted, and plasma-assisted ion exchange

8.5.8.1 Standards of chemical strengthening. Chemically strengthenedglass is sold on the consumer’s expectation for the improvement of

strength; however, it is believed that the varying conditions for the plication, such as the level of abrasion to be tolerated and the edgeconditions, make it difficult to classify the products on the basis ofstrength For this reason, the ASTM’s recent standard for chemicallystrengthened flat glass products is based on the measurement of sur-face compression as well as case depth These are:

ap-Surface compression σc:

7 MPa (1,000 psi) < σc < 172 MPa (25,000 psi) Level 1

172 MPa (25,000 psi) < σc < 345 MPa (50,000 psi) Level 2

345 MPa (50,000 psi) < σc < 517 MPa (75,000 psi) Level 3

517 MPa (75,000 psi) < σc < 690 MPa (100,000 psi) Level 4

Case depth ε:

50 microns (0.002 in.) < ε < 150 µm (0.006 in.) Level B

150 microns (0.006 in.) < ε < 250 µm (0.010 in.) Level C

250 microns (0.010 in.) < ε < 350 µm (0.014 in.) Level D

350 microns (0.014 in.) < ε < 500 µm (0.020 in.) Level E

Thus, a glass plate that has 60,000 psi surface compression and a casedepth of 70 µm will be termed Level 3B chemically strengthened glass

8.5.9 Examination of Stresses in Glass

Stresses in glass are best examined using photoelastic techniques Inthe absence of external forces, a well annealed and homogeneous spec-imen of glass is isotropic However, when a nonhydrostatic stress isapplied to the glass or in the presence of unannealed internal stresses,perpendicular vibrations of an unpolarized wave of light travel at dif-ferent velocities along planes of principal stresses Glass develops two

refractive indices, also termed double refraction or birefringence The

magnitude of the stress-birefringence is measured using one of severalmethods and related to the stress using the stress-optic equation:

where d = thickness of the specimen

B = the stress-optical coefficient or the Brewster’s constant

δ = the optical path difference

σ33 Bδ

λd -

=

The optical path difference is the product of the physical path lengthtimes the difference in effective refractive index for each polarization

of light, and it describes the retardation on polarized beam will ence relative to the other The subscript λ indicates that B may vary

experi-with the wavelength

In a simple polariscope setup, light rays from a lamp are first lowed to pass through a polarizer whose sole function is to allow vibra-tions along only one plane When an analyzer is oriented such that itsplane of allowed vibrations is at 90° to that of the polarizer, all of thelight intensity is cut off, leading to a field of view that is dark (i.e., the

al-analyzer is crossed with respect to the polarizer to achieve extinction).

A stressed glass specimen is now inserted between the polarizer andthe analyzer The principal stress axes of the specimen are to be at 45°

to that of the polarizer The light rays entering the specimen may beconsidered to be composed of two equal-intensity, in-phase, and mutu-ally perpendicular vibrations The vibrations travel with different ve-locities through the specimen, and recombine upon exiting Depending

on the path difference δ (or the phase difference φ = 360° δ/λ) that isintroduced between the two vibrations while traveling through glass,the recombination yields some intensity along the 90° plane of the an-alyzer Consequently, the field of view is no longer dark For instance,

a phase difference of full 360°, i.e., a path difference of λ, brings thetwo vibrations in the same manner as they were upon entering, withthe result that the analyzer cuts off that particular wavelength λ (i.e.,extinction is achieved for λ) When a white light source is used, theparticular λ will be missing from the field of view The balance color oflight, therefore, may be judged to estimate δ from a Michelle-Levy chart, and the use of the stress-optic relation gives the particular

stress responsible for it Sensitivity is greatly increased if a “full wave”

retardation plate (sensitive tint plate for λ = 565 nm) is introduced tween the specimen and the analyzer With a zero-stress specimen, theanalyzer cuts off only the 565 nm wavelength, which is between thegreen and yellow color of the spectrum The absence of this lightcauses the balance reds and blues to form a magenta color Introduc-tion of stress that adds to 565 nm will cause wavelengths at the redend to be cut off, yielding a bluish field of view On the other hand, astress that subtracts from 565 nm will cause bluish light to be cut off,thus changing the field of view to orange-red colors Since the humaneye is most sensitive to changes of about 565 nm, even small magni-tudes of stresses can give noticeable color change The identification oftensile or compressive stresses by the shift of the color from the ma-genta tint to either side of the green-yellow line in the white spectrum

be-is the essence of strain viewing in glass The appearance of colors with

and without the sensitive tint plate is shown in Table 8.21