the manufacture of continuous glass fibres

7/28/2019 The Manufacture of Continuous Glass Fibres

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The Manufacture of ContinuousGlass FibresPRESENT TRENDS IN THE USE OF PLATINUM ALLOYSby K. L. LoewensteinFibertech Consultants Limited, Fleet, Hampshire

One of the most exacting applications of platin um is in the production ofglass fibre. Th is involves the rapid $ow of molten glass at temperaturesaround 1300C through a series of small orifices which must retain theirsiae and alignment. Current trends in the design ofpla tinu m alloy bushingsare toward s a greater output of fibre pe r unit weightof platinum employed.Almost all continuous glass fibres are

manufactured by the attenuation of moltendrops of glass exuding from nozzles locatedin the base of a special fibre drawing furnacecalled a bushing. Nearly all bushings areconstructed from platinum alloy despite thehigh investment cost involved; one basicreason is that, when expressed in cost perkilogram of fibre produced, the use of plati-num alloys is cheaper than the use of othermetals, e.g., nimonic alloys. A second reasonis that, at the operating temperatures required,metals other than platinum alloys do not haveadequate mechanical strength. The mainplatinum alloys used are 10 nd 20 per centrhodium-platinum. The current trend ofbushings design development aims at greateroutput of fibre per unit weight of platinumalloy employed.

Bushings are basically of two types :(i) Remelt or marble bushings operatingfrom cold glass marbles as the feedstock

and fulfilling the dual functions ofmelting the glass and conditioning it tothe correct temperature for fibre draw-ing (see Fig. 2).

(ii) Direct melt bushings, which are sup-plied with liquid glass near the operat-ing temperature for fibre drawing andare attached to a feeder channel of afurnace into which the raw materialsfor glassmaking are fed (see Fig. 3).

Because of their dual function, remeltbushings are larger and weigh about twicethat of direct melt bushings for the sameoutput of fibre. For economic reasons, themajority of continuous glass fibres are nowmade from direct melt bushings and designdevelopment of remelt bushings, except forthe production of special fibre products, hasceased. This article concentrates on directmelt bushings, although much of their de-velopment can be applied to remelt bushingsalso.Bushing Design

Bushing design is always in a state ofdevelopment. With an average operating lifeof one year it is rare that an old bushing isreplaced by one of identical design; thenozzle sizes may have been increased toachieve an increase in output, or somestructural weakness may have been eliminated.But of all the changes over recent years themost important is related to the size, spacing,and the manufacturing technique of nozzles.This has led to much closer spacing of nozzlesand also to a reduction in the use of platinumalloy per unit weight of fibre produced.

Originally, nozzles were made by takingthe base plate, pressing small indentationsin it where the nozzles were due to be located,and then building up solid nozzles by meltingplatinum alloy wire and placing it drop by

Platinum Metals Rev., 1975, 19, (3), 82-87 82


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drop on the apex of each indentation.Periodically, during this build-up, eachnozzle would be inserted in a small press-tool and the outside smoothed. When thenozzle build-up was completed the holeswere drilled individually, including a counter-bore if required. It can be readily appreciatedthat the manufacture of base plates forbushings by this technique was extremelylaborious. Technically it placed a limit onthe minimum distance between nozzles sinceaccess was required not only for the welderbut also for simple tools needed for shapingthe outside of the nozzle as well as forcentering tools required for ensuring that theholes drilled in the nozzles were central tothe nozzle itself.The consistency of nozzle shape and loca-tion was improved by pre-manufacturingsolid nozzles and inserting them into holesin a base plate followed by welding aroundeach joint. But this method also called foraccess around each nozzle on both sides ofthe base plate and did nothing to enablemore nozzles to be placed in a base plate ofgiven area.

The objective of placing more nozzles ina given area of bushing base plates wasbrought about by the coming together oftwo separate developments. These were thedemonstration that a nozzle counterborewas not necessary in order to achieve efficientproduction rates, and a new method of nozzlemanufacture.Nozzle Manufacture

As referred to above, most nozzles wereoriginally made with a counterbore (seeFig. 4 (a) ) : This stemmed from the earlyperiod when bushings were made from pureplatinum; due to the low glass/platinumsurface tension the nozzles were frequentlywetted by glass which travelled up the outsideof the nozzle and across to adjacent nozzles,thus covering the underside of the base platewith glass. Th is seriously interfered withfibre drawing operations. By providing acounterbore, that is by thinning down the

Fig . 1 Glass-JibreJilamentsare formed by draw-ing molten glass through hundreds of small ori$cesin a bushing constructed in a rhodium-platinumalloy. Great attention is given to the design ofthe bushing and to the method of manufacture of thenozzles to achieve maximum productivity. At thevery high temperatures at which the process isoperated only plati nu m alloys possess the necessarystrength, while their use is most econom ical in termsof the amount of jibre produced.

Photograph by courtesy of Owens-Corning Fiberglas

Platinum Metals Rev., 1975, 19, (3), 83


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wall of a nozzle at the exit, this problem wasreduced. Although the subsequent introduc-tion of rhodium-platinum alloys increased theglass/metal surface tension, the nozzlecounterbores remained until the underlyingreason for meniscus stability at the base of thenozzle was more clearly understood. It is nowsufficient to say that, for rhodium-platinumalloys, provided the wall thickness a t the baseof a nozzle is 0.2 mm, then the glass will notnormally wet the outside of the nozzle.Nozzle counterbores are therefore no longerneeded. The fact that nozzles could now bedrilled in one operation and that a counterbore

was no longer necessary meant that the overallwall thickness of nozzles could be reduced.

The next development was a new methodof nozzle manufacture based on coining anddeep-drawing (see Fig. 5) . The startingmaterial was a base plate of such a thicknessthat it contained slightly more than all themetal required for the base plate of finalthickness plus the metal needed for thedrilled-out nozzles. By a process of coining,the metal in excess of that required for thefinal thickness of the base plate is concen-trated in those positions where nozzles willbe located; these are then deep-drawn in



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stages, with annealing between the stageswhen necessary to prevent fracture. Thisgives nozzles which have very accurateoutside and inside dimensions as well as avery smooth bore; however, they are stillclosed at the outlet end. The outlets areopened by punching foIIowed by smoothingthe ends of the nozzles by surface grinding.A typical bushing of this type is shown inFig. 6 before mounting in refractory brickand its metal frame.Rate of Flow of Glass

This development clearly involves con-siderable work on special tooling and anunderstanding of the behaviour of platinumalloys when subjected to deep-drawing. Theprocess itself could clearly be made simplerif the nozzles could be reduced in length.Having demonstrated that, for the stabilityof the fibre drawing process, the counterborewas no longer necessary and that metal couldbe saved by thinning down the nozzle walls,then the next stage was to aim to makeshorter nozzles, which would, however,maintain a given rate of glass flow.

The rate of flow of a liquid through a pipeis given by Poiseuille's equation:

r4hF'X-

F is the rate of flow, r is the radius of nozzlebore in its narrowest cylindrical section, 1 isthe length of the cylindrical section, h is theheight of the liquid above the nozzle and qis the viscosity of the glass.

It is clear that if dimension 1 is reduced,then, in order to maintain the flow rate,dimension r must be reduced; if it is assumedthat the clearance between adjacent nozzlesis at a minimum for bushing manufacturethen the distance between nozzles can also bereduced. Since several hundred nozzles arelocated in a bushing, even a small change indistance between adjacent nozzles can lead toa significant change in the number of nozzlesthat can be accommodated in a given baseplate. For example, the base plate originallyholding 400 of the longer nozzles, couldreadily be changed to accommodate about 600of the shorter ones.



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uddering. This is entirely due to creepat the operating temperature under the smallload imposed by the load of glass above thebase plate. There are several techniques forminimising this defect. The first is to use analloy of low creep; for this reason 20 per centrhodium-platinum is now preferred to the10 per cent rhodium-platinum alloy. (Thecosts per unit volume of the two alloys arepractically identical.) Alternatively, the use ofzirconia-stabilised platinum will reduce creep.A second or additional method is to placeinternal stiffeners on the bushing base plateto prevent the sagging commonly experienced(one may be seen in Fig. 3). In some cases,indeed, the base plate has been provided withan inverted V in the longitudinal direction toprovide stiffening. However, this method hasa drawback since the base plate requires moreplatinum. As in most cases, a compromisehas to be struck between bushing life andoperating efficiency, But, even so, bushings

With improvements in the operating effi-ciency of fibre drawing processes it becameclearly desirable to increase still further thenumber of nozzles in the base plate of a givenbushing since this leads to savings in the useof platinum alloy as well as to other produc-tion cost savings. However, even afterplacing nozzles as closely as possible togetheron the base plate, this inevitably leads to anincrease in the size of the base plate and thebushing overall. Since platinum alloys, atthe operating temperatures of bushings areliable to creep, larger bushings lead to increas-ing danger of distortion of the bushing duringoperation.

Distortion of the bushing is most seriouson the base plate, since this can affect theefficiency of fibre drawing. The most seriousdanger is what is colloquially referred to as



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Fig. 6 A completed 40 0nozzle bush ing, fabric atedin 20 per cent rhodium-platinum alloy, beforemounting in i ts supportf rame . The two pair s ofcoiled wires are the thermo-couples

with over 1000 nozzles are now widely em-ployed for many fibre products.

A further use of a platinum metal inassociation with fibre drawing is the use ofnozzle shields. It was found some years agothat the stability of fibre drawing could beimproved by placing radiation-absorbingshields between adjacent rows of nozzles.It was also found that the placing of theseshields between alternate rows of nozzleswas almost equally effective. Originally thesefins were made of silver and attached to a


water-cooled manifold acting as a heat sink.In some cases coolers made of flattened metaltubing were preferred, especially for use withbigger bushings, and have come into wideuse. A suitable metal for these flattened tubesis rhodium-platinum alloy; a cheaper andequally effective metal is palladium.AcknowledgementThe author wishes to thank Elsevier ScientificPublishing Company for permission to reproducethe diagrams from his book The ManufacturingTechnology of Continuous Glas s Fibres.

the manufacture of continuous glass fibres

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