CA1054371A - Apparatus for manufacture of flat glass by the float process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is a method and apparatus for manufacture of flat glass by the float process and in particular relates to an apparatus for making flat glass wherein the apparatus is of the type having a glass forming chamber having a bottom and side portions for containing a pool of supporting molten metal. Delivery means at the entrance end of the forming chamber deliver a supply of molten glass to the forming chamber to provide a layer of molten glass on the pool of supporting molten metal wherein the molten glass as it advances along the molten metal is cooled to form a continuous glass ribbon of a specified thickness wherein at least a portion of the cooling of the glass being by transfer of heat to the molten metal. Means are provided for continuously removing the glass ribbon from the forming chamber. Movable dam barriers are mounted on the bottom of the forming chamber, each of the dam barriers comprising an outer cover of material which is substantially non-reactive with the molten metal and an inner body of material having a density greater than that of the molten metal. The invention also pertains to the movable dam barriers per se.

Description

This application is ~ di~ision of Canadian Serial No. 21~,~22, ~iled November 26, 1974.
This invention relates to the manufacture of flat glass. More particularly the invention relates to the manufacture of thin ~lat ~ glass by the float process for example, float glass of thickness ; in the range 1.5 mm to 5 mm and more especially in the range 2 mm to 3 mm. -In the float process for flat glass manufacture, molten glass is delivered at a controlled rate on to one end, the ho-t end, of a ! 10 molten metal bath contained in an elongated tank structure. Usually the molten metal bath is of molten tin or of a moltèn tin alloy in -~ which tin pred~minatesO
- The ~inal ribbon of glass is discharged from the bath by trac-tion means, usually driven traction rollers, disposed beyond the ;~ outlet end of the bath, which traction means applies tractive force to advance the ribbon along the bath.
In some ways of operating the float process, regulation of the . i ~ . . :, ;
`~ applied tractive effort is effected along with regulation of the thermal conditions to which the advancing ribbon of glass is sub-~20 jected so as to attenuate the ribbon to a desired width and -thick- ;
ness. Outwardly and longitudinally directed marginal forces may be applied to the glass while it is being attenuated so as to con-trol gradual and progressive reduction of width and thickness of the ribbon until the ribbon of glass reaches a desired width and ~ thickness. The thermal control is such that when the desired width ;~ and thickness o~ the ribbon is achieved, -the viscosity of the glassJl iS at a value at which further dimensional change cannot take place der the applied tractive effort~ -Usually the thermal regime to which the glass is sub~ected is 3C such that the ribbon of glass passes along a zone of the bath where the viscosity of the glass is controlled to regulate the attenua-tion of the ribbon and in this zons of the bath the glass acceler-ates as the controlled attenuation takes place.
When operating lmder high load condi-tions for example at a rate of deliver~J o~ molten glass to the bath of 2000 tonnes per ,' '::'' ' ' ,',: : . ' 3'^~1 'week or ~ore, -the speed of discharge of the ultimate ribbon of glass from the bath ls high9 for example 10 me-tres per minute up to 40 metres per minute. Such high speeds are necessary when ' attenuating the glass under these high load conditions to thick-nesses below 3 millimetres, for example in the range 2 mm to 3 mm.
The advancing ribbon of glass, when it is accelerating during attenuation and when it is advancing at uniform high speed for dis-;'' charge from the bath, entrains an appreciable quantity of the molten metal of the bath along the bath surface towards the outlet end of ~' 10 the bath which surface flow induces an ups-tream return flow of '~- cooler molten metal from the outlet end of the ba-th along the -~ bottom of the bath towards the zone of the bath where the ribbon ' ~ of glass is being attenuated and is at a viscosity such tha-t i-t is ' ,~ particularly susceptible to temperature variations across the sur-,i,~ .
face of the molten metal bath. It has been found that disto~ion ''' .~ introduced into the underface of the ribbon of glass in this a-tten- ~;
uation zone is present in the ultimate ribbon. `
., ..~ .
~- One way of minimising distortion in the underface of the ribbon ~ ;
when producing thin float glass at high speeds is to create mixing 20; currents in the molten metal of,the bath in the a-tt~nuation zone .,~,. . . .
so as to combat localised temperature gradients along the glass- ~ '~
supporting molten metal surface and thereby minimise surface dis-tortion in the attenuated ribbon. These mixing curren-ts have been created by electroma~netically inducing transverse surface flows of-molten metal across the bath surface in that zone.
It has now been found that when producing float glass at loads ' above 2000 tonnes per week the undersurface of the glass is par-ticularly subject to dist~tion known as "bands" which takes the ' form of corrugations in t,he glass which may be associated with .
some thickness variation. These so called "bands" may be a few ,-' centimetres wide and a metre or more in length.
~' It has also been found that a ribbon of thin float glass travelling at a relatively high speed along the outlet end of the .

. . .
'' ' ' . ., ":, ' . "

:
~ ~5~3 bath may be subject to instability in position on the bath by movement of the ribbon from side to side in a periodic manner.
; This phenomenon known as "snaking" may build up to an amplitude of up to 30 cm on either side of the centre line of the bath with a periodicity of about 5 or 6 minutes. Such movement of the ribbon is undesirable, particularly when the process is operatin~
with automated cutting at the outlet end of the lehr.
- It is a main object of the present invention to provide an improved method and apparatus for manufacturing flat glass on a molten metal bath which minimises these difficulties associated ~ .
with the high speed production of thin float glass.
Disclosed is a method of ~anufacturing flat glass which comprises advancing a ribbon of glass along a molten metal bath, control~ing the speed of advance of the glass to a zone of the bath where the viscosity of the glass is controlled to regulate attenuation of the ribbon to a desired ;~
width and thickness as it is accelerated. The molten metal flow is constrained at a first location in the region of the downstream end of the attenuation zone, to forward flow of molten metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon from downstream of the first location.
Molten metal f low is further constrained at a second location spaced upstream from the first location and in the region of . .
; maximum acceleration of the glass, to forward flow of molten `~ metal entrained beneath the accelerating glass and counterflow ~ of molten metal alongside the ribbon from downstream of the . ~ ~
;- second location. The method further includes establishing ;- ~ lateral access into the region of the bath supporting the ribbon ,... .
between the first and second locations for the counterflow of ` 30 molten metal at the first location to ensure replenishment of the molten metal of the bath in the attenuation zone between the first and second locations by molten metal drawn inwardly from the counterflow at the first location.

.. . .

~5~3~
Because the counterflow at the first location is directed alongside the ri~bon, upstream flo~ o cooler molten metal beneath the ribbon from the region downstream of the first location is prevented. The upstream re~urn flow is instead diverted into a counterflow feeding the sides of the bath in the region between those locations. The glass is accelerating in that region, the depth of the entrained forward flow of molten metal progressively increases, and the constraint of molten metal flow beneath the ribbon to forward flow only at the first location is accompanied - ~ -by continual diversion upstream of a return flow beneath the forward flow thereby setting up a continual/circulation of molten metal in that region~ into which circulation the counterflow of ~ ~;
;~ molten metal is drawn to replenish the molten metal of that region ~' of the bath.
; More particularly, the disclosure provides a method of manufact~
uring float glass of thickness in the range 2mm to 3mm, comprising ~, :
advancing a ribbon of glass along the molten metal bath, con-trolling the speed of advance of the glass to a zone of the bath where the viscosity of the glass increases through the range 10 poises to 107 poises and the ribbon is attenuated to a desired -width and thickness as it is accelerated to a speed of at least .
:, i, , 10 m/minute, and constraining molten metal flow, at a first location in the region of the downstream end of said zone, to ~5, forward-flow of molten metal entrained beneath the ribbon andcounterflow of molten metal alongside the ribbon to replenish the molten mekal of the bath between that location and a second location spaced upstream of said first location in the region of ~; ~ maximum acceleration of the glass where molten metal beneath the ribbon is also constrained to forward flow entrained by the 30 accelerating glass.
- A modified way of operating the invention comprises transversely dividing the zone between the first and second locations into a plurality of cells, and constraining molten ,'~ ' ' '" .

~t....................... ~LVS~L~3 7~
metal flow between each pair of ad~acen-t cells to the forward -flow entrained beneath the ribbon and counterflows of molten metal alongside the ribbon from the downstream cell of that pair to the upstream cell of that pair, The method may also comprise electromagnetically inducing flows of molten metal rom the counterflows to enter beneath the ribbon. This may be effected in one or more of the cells of the zone when the zone is so divided.
Also disclosed is an apparatus for manufac~uring flat glass comprising an elongated tank structure having side walls for containing a bath of molten metal, means for delivering r glass to the bath at a controlled rate and advancing the glass in ribbon form along the bath. Thermal regulators in the tank structure are provided for cooling the advancing ribbon in a zone of the bath through a viscosity range in which the glass can be attenuated, and means are provided for applying traction ~,~, . . .
to the ultimate ribbon of glass. A first transverse barrier 1s on the floor of the tank structure at the downstream end of the zone and extends beyond the position of the edges of the ribbon but short of the tank side walls. The top of this ~;
barrier is positioned below the level of the bath surface by a distance which is effective to constrain molten metal flow beneath the ribbon to forward floW of molten metal entrained beneath the ribbon and counterflow of molten metal alongside the ribbon to replenish molten metal ~n the attenuation zone.
A second tran~verse barrier is spaced upstream from the first , barrier in the region of maximum acceleration of the glass in the attenuation zone, the top of which second barrier is ', positioned below the level of the bath surface by a distance which is effective to constrain molten metal flow beneath the ribbon to fo~lard flow entrained by the accelerating glass.
The first and second barriers are arranged and spaced apart to establish lateral paths for inward molten metal flows from , '~ the counterflow to replenish the molten metal in the attenuation ,~
: ~i , S
" -- .

,,/--- - ; , . . .

~ft zone, and ~he ends of the barriers are spaced from the side walls of the tank structure to define channels for the counter-flows of molten metal alongside the ribbon edges.
The invention to which the divisional application is directed is to a movable dam barrier adapted for mounting on a bottom of ~, a glass forming chamber of apparatus for making flat glass, comprising an outer cover of material which is substantially non-reactive with molten metal and an inner body of material having :. -a density greater than that of the molten metal.

; 10The invention also comprehends an apparatus for making flat ~ : -:
glass wherein the apparatus is of the type having a glass forming ~-chamber having a bottom and side portions for containing a pool of supporting molten metal~ Delivery means at the entrance end of the forming chamber deliver a supply of molten glass to the forming chamber to provide a layer of molten glass on the pool of ~ ;
. ..~; . .
' - supporting mol-ten metal wherein the molten glass as it advances along the moIten metal is cooled to form a continuous glass ~ -ribbon of a specified thickness wherein at least a portion of the , . - ~: , ,, - cooling of the glass being by transfer of heat to the molten metal.
Means are provided for continuously removing the glass ribbon from the forming chamber. Movable dam barriers are mounted on the bottom of the forming chamber, each of the dam barriers comprising an outer cover of material which ~s substantially non-reactive with the molten metal and an inner body of material having a , density greater than that of the molten metal.
,~ Preferably the dam barrier or barriers comprise a cylindrical :i ~
body and the cover is made of graphite with the body of tungsten.

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: .. ..
: - . .. . . ..
.. . .

; 1~5~37~ -; ~ .,, The invention also comprehends flat glass produced by a method as set out above; in particular flat glasis of thickness in the range

2 mm to ~mm. ~ ; ;
In order -that the invention may be more clearly understood --some embodiments thereof will now be described, by way of example, with reference to the accompanying drawings in which~
Figure 1 is a plan view of an elongated tank structure containing a bath of molten metal for use in the i . float process for the manufacture of thin flat ~10 glass by the method of the invention, .
Figure 2 is a section on line II-II of Figure 1 showing transverse barriers in the tank structure, Figure 3 is an enlarged detailed view of one of the transverse barriers of Figures 1 and 2 keyed in-to a groove in the floor of the tank structure, ~;~ Figure 4 is a plan view of the outlet end of the tank structure in another embodiment of the invention .~ in which the region between the two barriers of ` Figures 1 and 2 is divided into a plurality of ~20 cells by additi.onal transverse barriers9 Figure 5 is a section on line V-V of Figure 4,with Figs. 2 & 3, Figure 6 is a section on line VI-VI of Figure 4, ~ -Figure 7 is a transverse cross-section through a tank structure containing a bath of molten metal , . j.~ :
showing a transverse barrier of cylindrical form seated on the floor of the tc~nk structure beneath the ribbon bf glass, and Figure 8 is a section on line VIII-VIII o~ Figure 7.

~ 30 -7-, ': ' ` ' : - .
~ 3~ ~
Referring to the drawings Figure 1 illustra-tes in plan an elongated tank structure of the manufacture of thin glass by the floa-t process. The tank structure comprises side walls 1, an end wall 2 at the inlet end of the -tank structure, and an end wall ~ at the outlet end and contains a bath of molten tin. The geometry of the tank structure is such that it will acco~mo~ate the maximum possible width of ribbon of glass produced at the hot inlet end of the -tank structure by the unhindered lateral flow of molten glass on the bath ~-surface.
` 10 Molten soda-lime-silica glass is delivered onto the bath, which -is usually of molten -tin, at the inlet end of the tank structure by ~ ~pouring from a spout 4 which extends over the inlet end wall 2 of ~ ;
.~
the tank structure. A regulating tweel 5 controls the rate of flo~
of molten glass over -the spout onto the bath surface 6, which rate of flow in the preferred example to be described is 2100 tonnes per week.
In manner well known in the float process temperature regula-tors9 not shown, are provided in the roof structure over the molten ~"~
metal bath. Temperature conditions at the inlet end of the bath are such that the molten glass 7 arriving on the bath is permitted to flow freely, laterally unhindered, to the limit of its free flow during the first part of its advance along the bath.
-The te~lperature regulators in the roof s-tructure set a tem-perature regime to which the advancing glass is subjected, which regime maintains the glass in a deformable state over a longitudin-ally extending region of the ribbon in which the glass is progressi-vely attenuated as its velocity increases under the influence of tractive effort applied to the ultimate ribbon of glass 8 which is provided by driven rollers 9 located beyond the outlet end wall 3 of the tank structure~
The temperature of the glass is about 990C when maximum ~- spread is achieved and the glass thickness is about 7 mm. This layer of molten glass is advanced in ribbon form and the ribbon is . ' ~,.. .. . . ..
,. . . .

~4;37~
~- constituted by low viscosity glass, e.g. a~k a viscosity of about 104 ~ poises. This glass is gradua:Lly cooled during its initial ~ ,', ' , advance along -the ba-th and its viscosity slowly increases. As the '' viscosity of the glass increases so does -the influence of the longi-tudinally directed tractive force, originating from the rollers 9, , ' .l in stretching the ribbon of glass. Gradual and progressive reduc-tion in width and -thickness of the glass is controlled by the use of top rolls which engage the upper surfaces of the margins of the glass.
Initially while,the glass is at low viscosity a pair of in-' 10 clined to~ rolls 11 mounted on shafts 12 driven by motors 13 are ' ~ ' ~, mounted through the tank sid'e walls -to engage -the margins of the ~ ~`
ribbon. The -top rolls 11 are knurled graphite, stainless s-teel, or mild steel rolls which are internally water cooled and are inclined , at an angle of 3 to an axis at right angles to the direc-tion of advance of the ribbon of glass along the bath. The rolls 11 are ,~
driven at a speed of 1.5 metres per minute . Outwardly and longi-. ~ .
~ tudinally directed forces are thereby applied to the margins of the ? ' nascent ribbon. ~he outward force components provide restraint against undue loss in width. Slight attenuation of the ri~bbon is `

begirning to occur in this region.

~, Further pairs of top rolls 14, 15 and 16 are provided spaced ', ~ along the tank structure, being mounted on respective shafts 17, J ' 18 and 19 and driven by motors 20, 21 and 22. ~, The top rolls 14 are se-t at an angle of 4 to an axis at right , angles to the direction of advance of the ribbon and are driven at a rate of 1.8 metres per minute.

' The top rolls 15 are set at an angle of 6 and are driven at ~ , , a ra-te of 2.4 metres per minute.

, The top rolls 16 are set a-t an angle of 9 and are driven at a rate of 3.4 metres per minute.

, These sets of top rolls act to avoid undue loss of ribbon , width as the glass is being accelerated from about 1.5 metres per minute to about 3.4 metres per rninu-te. As the glass passes beyond _g_ ..

,.` , ~

~ 5437~
the top rolls 16 its te~perature is cooling below 880C correspon-ding to a viscosity of about 105-2 poises~ The glass thickness is about 4.5 mm.
The ultimate ribbon of glass 8 which is 2.5 mm thick is dis-charged from the bath by the rollers 9 at a speed of 10.4 metres per minute and after the glass passes downstream of -the last pair of top rolls 16 it is subjected to varying acceleration which reaches a maximum at a distance just downstream of the top rolls 16. The glass is rapidly thinned to 2~5 mm and the attenuation takes place against the reaction provided by the upstream part of the ribbon of glass engaged by the top rolls 11, 1~, 15 and 16 as well as by the entrainment by the undersurface of the ribbon of a forward flow of molten metal of the bath along the bath surface beneath the acceler-ating ribbon. As the ribbon speed increases so force is dissipa-. ~ .
~ "r ted in accelerating the forward flow of molten metal entrained by i.i,.... .
the ribbon and molten metal is drawn inwardly beneath the ribbon from the sides of the bath to compensate for the entrai~ment of -molten metal. T~s forward flow of molten metal along the surface of the bath becomes considerable as the ribbon speed increases, for example up to a discharge speed greater than 10 metres per minute, and there is engendered a return flow along the bottom of the bath of cooler molten metal. It is this generalised return flow of cooler molten metal along the bottom of the bath which produces top to bottom temperature gradients through the depth of the bath which have been shol~n to be particularly troublesome in the region of the bath where the rapidly accelerating ribbon is being attenuated. ,~
The side walls 1 of the tank structure are inclined inwardly near the outlet end of the bath to form shoulders 25 which connect the wider part of the tank where the formation of the ribbon and its attenuation takes place, to a laterally constricted part of the tank at the outlet~end having side walls 26. The narrowing of the tank structure at the outlet end brings the tank width to a dimension greater than the maximum width of the ultimate ribbon of thin glass ,~', , ' '. ~, ' , . - . ' . , 1~5f~3~
8 to be produced and the use of a tank structure with a laterally constricted outlet end intensifies the pumping effect of the entrained forward flow o~ molten metal and enhances the upstream bottom flow of cooler molten metal.
Under the influence of tractive force which discharges the ribbon ~ which ls 2.5 mm thick from the bath at a speed of about 10.4 metres per minute, the glass is i.n a condition in which no further dimensional change can take place under the influence of that tractive effort, when the viscosity of the glass is about ~, 7 10 10' poises corresponding to a temperature of about 750C for the .: usual soda-lime-silica glass. The downstream end of the attenu-ation zone is therefore in the region where no further change in ::~ width of the ribbon takes place and at a first location in the :
. ~ ~, . .
~: region of the downstream end of the zone there is located a first transverse barrler ~7.which is operative to constrain mol~
`~i ten metal flow in the region of the downstream end of the atten- :
uation zone.to.forward flow.of molten metal, indicated by the arrows 28 entrained beneath the high speed ribbon of glass, and ~; counterflow of.molten metal indicated by the arrows 29 alongside ~ 20 the ribbon. The generation of the return flow of cooler molten .'.:,' metal from the outlet end of the bath is indicated by arrows 30. ~ .
mis return flow along the bottom of the outlet end of the bath feeds the counterflow 29.
As shown in Figures 2 and 3 the barrier 27 is a carbon bar of upstanding rectangular cross-section and has a dove tail base ,:~ 31 which is keyed into a matching dove-tail groove 32 formed .~ transversely of the bath in the floor 33 of the tank structure.
The flat top of the bar i~ about 5 cm. long in the direction of .i. ribbon advance and is spaced from the level of the bath surface 6 by a sufficien;t depth to ensure that the lower layers of en- :
~: trained molten metal of the forward flow are directed downwardly ., ~ .
as indicated by the arrow 35 to feed a return flow 39 along the bath bottom in the reyion upstream of the barrier 27, which is not at a substantially lower temperature than the forward flow.
~ -lL-:

.
s~
Usuall~ the top surface 3L~ of the barrier 27 is from 6 mm to 15 mrn below the le~el of the ba-th surface, depending on -the speed and acceleration of -the ribbon, ~/hich distance ensures sufficien-t pressure gradient in -the forward flow 28 which is constrained over the top surface 34 of the barrier -to ensure effective obstruction of any upstream flow of cooler mol-ten me-tal along the bottom of the bath beneath the ribbon of glass.
In principle it is desirable -tha-t -the -top of the barrier 27 ` may be at a depth below the level of the ba-th surface 6 which is exactly such that all the entrained molten metal of the forward flow 28 travels over -the barrier but none of the mol-ten metal of the return flows 30 passes over -the barrier. In prac-tice, however, such an exact setting may be difficult to achieve and the barrier ~ height is therefore preferably set as described above to direct the ; lower layers of entrained molten rnetal of the forward flow down-wardly as indicated at 35 thereby ensuring tha-t molten metal flow ~;
over the barrier is constrained to forward flow.
The return flows 30 of cooler molten metal from the outlet end of the bath are thereby effec-tively directed by the barrier 27 into 20 the counterflows 29 alongside the ribbon.
The ends of the bar 27 are spaced from -the side w~lls 1 of r the tank structure to define channels 36 for -the counterflows 29 of cooler molten metal from the outlet end of the ba-th. These counterflows are therefore directed alongside the ribbon edges in-to . ... .. .. ............ .. . ... ...
the region of the bath upstream of the barrier 27 The channels 36 are preferably clearly clefined for high speed operation and for operating at -the lower end of such high speed ranges -the ends of the bar 27 rnay extend right up to the side walls 1 of the tank structure, the counterflows 29 of molten metal taking place over the top of the bar alongside the path of travel of the ribbon.
As the ribbon i5 accelera-ting in the attenuation zone there is progressively increasing entrainrnen-t of molten metal into the forward flol" since the acceleration of the glass is such -that the , .

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, . . .
. .

5 ~

increase in sur~clce area o~ -the ribbon p~r ~llit tim~, and hence the amount o~ ~ntrained molten me-tal, incre~ses along the attenuation ~one. Molten m~-tal is therefore bein~ ~rawn'continuously un~er the ribbon to comp~nsate ~or that removed by the entrainment.
The counterflows 29 of mol-ten metal alongside either side of the ribbon replenish the mol-ten me-tal of the bath between the loca-tion of the firs-t barrier 27 at -the downstream end of the a-t~nuation zone and a second location defined by the position of a second barrier 37 similar in construction and arrangement -to -the barri~r 27 and lo-cated in the region of maximum acceleration of the glass. The topsurface of both barriers 27 and 37 may be a-t the same dep-th, for example 6 mm below the level of the ba-th surface, or the dep-ths may differ7 for example the top surface of barrier 37 at a depth of lO
; mm and -the top surface of barrier 27 a-t a depth of 6 mrn.
The barrier 37 is also effec-tive -to cons-train mo'lten metal flow beneath the ribbon to the forward flow 38 of molten metal entrained by the accelerating glass. There is counterflow 40 of cooler mol-ten metal from the region between the -two barriers 37 and 27 into the upstream region of -the bath where the glass is not so susceptible to 20 distor-tion as it is in the region between the two barriers.
The return flow along the bath bot-tom between -the two barriers ~' - 37 and 27 is indicated at 39 in Figure 2 and is direc-ted upwardly ' when it reaches the barrier 37, to feed the forward flow 38 down-stream of the barrier 37. A con-tinual circulation is thereby se-t .. ,, .. , .. .. . .. , .. .. , ~ . , up"in th'e"région between the two barriers in-to which circulation -the counterflows 29 are drawn to become entrained in that circulation and in the forward flow 28. Replenishment of the mol-ten me-tal supporting the accelerating glass is thereby achieved with -tempera-ture differences be-tween the top and bottom of the ba-th kept to a minimwm.

In this way cooler molten metal which is forced upstream from ,, the outlet end of the bath by -the pwnping effect of -the en-trained forward flow of molten metal moving wi-th -the ribbon of glass, is .

~, . . .

l~S43'7~
prevented from flowing direc-tly benea-th the rib'bon o~ glass which i.s being attenuate~ bu~ ra-ther ls diver-ted by -the barrier 27 and is hea-ted as it is diverted to a -temperature more nearl~ approaching that of the molten me-tal suppor-ting the glass being attenuated, so -that by the -time the counterflows 29 of cooler mol-ten metal have been drawn in-to the region of the bath supporting the glass being a-ttenuated, the temperature of tha-t mol-ten metal is not greatly ~ .
different from -the temperature of the en-trained forward flow of molten metal 28.
, 10 It has been found in practice tha-t tempera-ture differences between the top and bottom of the mol-ten tin in the region just up-stream of the barrier 27 are not more than 15C. ~:
In the same way any molten metal from the counter~lows 40 around the ends of the upstream barrier 37 which are drawn into the ,~
entrained forward flow 3~ of molten metal which is being accelerated with the accelerating glass, is molten metal which has been heated in the region between the two barriers to a temperature,not greatly ~ :
different from the tempera-ture of the glass upstream of the barrier 37.
~! 20 Thus the molten metal being draw~ into the entrained forward flow as the glass accelerates is at a -tempera-ture not grea-tly diff- ;~;
erent from that of the molten me-tal being carried along beneath the ~;.
ribbon; top to bottom temperature differences are substan-tially re-duced.and the possibility of bo-t-tom surface "band" distor-tion in -the thin float glass is grea-tly reduced.
In the embodiment described with reference -to Figures 1 and 2 top and bottom bath temperatures were measured using thermocouples in positions marked on Figure 1, namely posi-tion A jus-t downstream ''~
of the barrier 27, position B just upstream of -the barrier 27, position C midway between the barriers 27 and 37, posi-tion D just : ', downstream of the barrier 37 and position E just upstream of the barrier 37 between the barrier 37 and the edge rolls 16. All tem-perature measurements were made just alongside one edge o:~ -the '' ' . , , ' . . ..
, ... . . . . . .

~ 5~L37~
ribbon and the measuremen-ts made are set out in -the following Table.
TABLE

Position Top Ba-th Bottom Bath _ Teml~erature (C) l~m ~ 812 797 10 E 82L~ 820 These results show that between the two barriers thermal con-trol is achieved by the invention to produce top to bottom tempera- .;
~: .
ture differences in the bath metal of 15C or less and generally in the range 5C to 10C. In the region of maximum acceleration as represented by posltion D a temperature difference of 12C was measured. Temperature regulation may be improved by the use of additional transverse barriers on the floor of the tank s-tructure in the attenuation zone dividing the region between the first barrier 27 and the second barrier 37 into a plurality of cells.
Such an arrangement with three additional barriers 42 9 43 and 44 is illustrated in Figures 4 and 5. These barriers are of similar construction to the barriers 27 and 37 with a gap usually in the range 6 mm to 15 mm between the flat top of each barrier and the bath surface. Because the depth of gap to constrain molten metal flow to forward flow depends on the speed of -the glass advan- ~ , cing over a particular barrier it will be appreciated that a differ-;~ ent gap may be employed over the barrier 37 than over the barrier 27, and the gap between each b~arrier and the ba-th surface may pro-gressively change from the upstream end to the downstream end of the attenuation zone.
The ends of each of the barriers are spaced from the tank side walls so that counterflows take place around the ends of each barrier. l'he attenuation zone is thus divided into four cells, ~15-~ 5~3'~
which number is effective -to main-tai.n a -temperature dif~erence, preferably in the range 5C -to 10~ between the top and bot-tom of the tin depth in each cell. The flow of molten metal between each pair of adjacent cells is constrained to the .forward flow entrained beneath the ribbon over the barrier between those adjacent cells and the coun-terflow of molten metal taking place alongside the ;~ ribbon from -the downstream cell of that pair to the upstream cellof that pair. , -The length of each cell in the direction of ribbon advance is such that there is an internal molten metal circulation in each cell as shown in Figure 5, and the counterflows around the ends of each barrier are entrained into the molten metal flows within the ups-tream cell o~ the pair and does not substantially by-pass a cell., In this ' way molten metal entrained into the forward flow in the region of each cell is obtained from the downstream cell, -that is the molten metal entrained is already at a,temperature near to that of the molten metal already in the cell and no substantial temperature ., dlfference will exist at any location in the attenuation zone between ~' the forward flow under the ~ibbon and the molten metal being en-trained into that forward flow as the glass accelera-tes.
A further advantage of using a plurality of barriers, as in Figures 4 and 5, is to accommodate different operational set-tings which will cause a shift in the position of maximum acceleration ~' :
of the glass. When operating according to the Example which has ,:~ ' been described with an output speed of about 10~4 me-tres per minute the position o~ maximum acceleration of -the glass is in the region ,, of the barrier 37.
As the speed of the ultimate ribbon is increased either to accommodate higher load operation or to decrease the thickness of ' 30 the glass, the position of maximum acceleration may move downstream ',~ and the provision of the plurality o~ barriers ensures that for all expected operational settings one of the transverse barriers will be in the region of maximum acceleration of -the at-tenuating , -16-' .:"' ' ' ' ' '- -, glass. ~5~3~1 The ribbon of glass may be discharged from -the bath at a tem-perature of about ~50C so that the cooler molten metal moving up-s-tream along the kottom of the bath is a-t a temperature of about 650C to 700C. The deceleration of this upstream flow as it en-ters the widening part of the bath between the shoulders 25 and is diverted by the barrier 27, assists thermal exchange due to the high conductivity of the mol-ten tin to accomplish the required heating of the cooler molten metal so that i-t is at a ternperature of abou-t 770C to 775C by the time it is flowing into the counterflows 29 around the ends of the barrier 27.
Thermal exchange may be further assisted by defining a tortuous path for the counterflows between the cells, by means of lateral baffles 45 of carbon which are illustrated in Figures 4 and 6 which e~tend from the side walls l between the barriers 27 and ~4 and the barriers 43 and 44. These barriers intersect the bath surface and thereby ensure that the counterflows cannot by-pass the cells. ;
Additionally the mixing of the counterflows wi-th the flows in -`~ the region between the barriers may be assisted by employing linear induction motors 46~ Figure 1, mounted above the bath surface to induce flows of molten metal from the counterflows 29 to enter beneath the accelerating ribbon.
Linear induction motors may also be positioned as indicated at ~
47 in Figure 4 to assist diversion of the return flows 30 into the ~;
counterflows 29. Further linear induction motors may be posi-tioned , as indicated at 48 and 49, to direct the counterflows. Immersed heaters may also be employed in conjunction with or in place of the linear induction motors at positions 48 and 49 to assist fur~
ther the achievement of thermal homogeneity in the region where ~ 30 acceleration of the glass is high.
; Barriers of cylindrical form located on the floor of the tank ~` structure may be used as illustrated in Figures 7 and 8.

According to this modification each barrier may comprise a .`,''~' ' , . .
. " ' ' . .

1~5437~L
carbon cylindrical sleeve 50 machined from harcl grap}lite rnaterial which surrounds a tungsten core 51 in the form of a tungsten rod.
The diameter of such a barrier is such as to leave the appropriate gap for example in the range 6 mm to 15 mm between the top of the barrier and the surface oE the molten metal bath so that the con-strained forward flow 28 takes place over the cylindrical surface of the barrier.
` The barrier may be slid into the tank structure from the side and then simply rolled into the location which achieves an optimum result in the sense of a minimum temperature difference ` from top to bottom of the bath depth and then held in position for - ~
example by means of carbon Eorks indicated at 52, extending down- - -wardly over the ends of the barrier alongside the ribbon of glass.
With a change in operational conditions the barrier can then be ~' moved easily by rolling along the floor of the tank structure.
Such a barrier enhances streamlined flow in the entrained forward ~ ~.
flow 28 of molten metal and in the diverted return flow 35. The tungsten core, being a metal which is of greater density than molten tin, ensures that the cylindrical barrier seats firmly on to the , 20 floor of the tank structure, because the density of the core 51 and sleeve 50 in combination are effectively greater than that of ' the molten tin. The barrier therefore effectively obstructs up-stream flows of cooler rnolten metal along the bottom of the bath.
The invention thus enhances the provision of conditions . I A
of thermal homogeneity across the molten metal surface supporting and contacting the bottom surface of the ribbon of glass when it is in the critical condition of being accelerated to attenuate the ribbon, at the same time as the viscosity of the glass is increasing to a value at which any distortions introduced into -that surface are retained in the ultimate ribbon. Further the symmetrical ~ ., ~
control of molten metal flows in the region of the attenuation zone and at the outlet end of the bath, by employing the method and apparatus of the invention, has enhanced the stability of the ~' ''' - ' ' , ~ ~ ' ' .

5~37~L
process by avoicling "snaklng" of the ribbon of glass and maintain-ing the physical location of the ribbon on the discharye rollers 9 as it is delivered to the annealing lehr.
The invention can be applied -to any method and apparatus for float glass manufacture, employing either a parallel-sided bath without restricted outlet end or a bath with a restricted outlet end as illustrated in Figures 1 and 4, and in which a glass ribbon is advanced through an attenuation zone in which the glass is ~ ;
accelerated to a high speed for c7ischarge from the bath. For example the invention may be applied to a process in which the advancing ribbon is stiffened and gripped prior to being reheated to a viscosity at which it can be attenuated.

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Claims (7)

The embodimen-ts of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In an apparatus for making flat glass which is of the type having a glass forming chamber having a bottom and side portions containing a pool of supporting molten metal, delivery means at the entrance end of the forming chamber for delivering a supply of molten glass to the forming chamber to provide a layer of molten glass on the pool of supporting molten metal wherein the molten glass as it advances along the molten metal is cooled to form a continuous glass ribbon of a specified thickness wherein at least a portion of the cooling of the glass being by transfer of heat to the molten metal and means for continuously removing the glass ribbon from the form-ing chamber, the improvement comprising movable dam barriers mounted on the bottom of the forming chamber, each of said dam barriers comprising an outer cover of material which is substantially non-reactive with the molten metal and an inner body of material having a density greater than that of the molten metal.

2. The apparatus as set forth in Claim 1, wherein at least one of said dam barriers comprises a cylindrical body, and wherein means associated with said body prevents rolling motion of said body.

3. The apparatus as set forth in Claim 1, wherein the cover is made of graphite and the body has a density sufficiently greater than the density of the molten metal to increase the density of the body and cover in combination to be effectively greater than that of molten metal.

4. The apparatus as set forth in Claim 1, wherein said means for continuously removing the glass ribbon from the forming chamber comprises means for horizontally removing the glass ribbon from the forming chamber.

5. A movable dam barrier adapted for mounting on a bottom of a glass forming chamber containing molten metal of apparatus for making flat glass, comprising, an outer cover of material which is substantially non-reactive with molten metal and an inner body of material having a density greater than that of the molten metal.

6. The dam barrier as set forth in Claim 5, wherein the dam barrier comprises a cylindrical body.

7. The dam barrier as set forth in Claim 5 or 6 wherein the cover is made of graphite.