GB2338953A - Making glass articles using high conductivity gases - Google Patents

Description

1 2338953 IMPROVEMENTS IN aM RELLATINQ TO GLASS- MANUFACTURE

This invention relates to the field of the manufacture of glassware, in particular glassware made using moulds 5 and/or plungers.

In the type of process used in the manufacture of glass containers and pressed ware, go bs of glass are fed from the forehearth of the furnace to the machine at a temperature which is typically in the range 1050 to 1100 C (viscosity ca. 10'Pas.). The mould and/or plunger surface temperatures are typically in the range 400 to 450 OC. As well as giving shape to the glass, the mould and plunger extract heat from the glass surface at a very is high rate (initially 10' to 107 W/M2). On removal from contact with the plunger and mould, the glass surface reheats for two reasons (a) the great reduction in the rate of heat loss from the glass surface to its surroundings on removal from contact with colder metal and (b) the very large temperature gradients in and near the glass surface which drive heat toward the surface.

In the forming of glassware by hand, the gather is rolled in contact with a marver plate, a horizontal metal plate.

This chills the glass surface and allows a further gather of glass to be made on the previous one. Even in hand production, a reduction in time required to carry out relatively unskilled parts of the process would be regarded as valuable.

In container manufacture, two moulds are used (the parison mould and the blow mould) After forming the parison, the glass must be given such time to reheat 2 that it begins to extend under its own weight. At this stage the parison can be blown using relatively small blowing pressures to take up its final shape in the blow mould. Further heat is extracted from the glass whilst 5 its surface is in contact with the blow mould. Thereafter, on removal from the blow mould, a second reheat takes place which must not be such that the final container begins to deform under its own weight, This is a more complicated situation than occurs in simple pressing operations in which only one mould station is involved. Heat is extracted from the glass during pressing, but reheat must not result in the glass deforming after it is removed from the mould.

In all these situations, the period of contact between the glass and colder metal surfaces must be long enough for sufficient heat to be extracted from the glass. How long this should be depends on the thermal properties of the glass and the metal and also on the thermal resistance at the interface between them. Usually in-the scientific and technical literature, values of the thermal conductance or the interfacial heat transfer coefficient, HTC, are quoted (units W/M2 K). This value is simply the reciprocal of the thermal resistance.

The results of some measurement work, giving values of the HTC are available in the literature. They depend on important forming parameters such as applied pressure and mould and/ plunger surface temperatures. However, in general, relatively little work has been done in this 3 field and practically no effort has been directed towards controlling the value of the JHTC other than by purely empirical process adjustments on existing plant. Moreover the basic knowledge required to make these adjustments in 5 a sensible way has been lacking.

In glass pressing operations, e.g. in the manufacture of parisons in container manufacture, it is known that the HTC decreases considerably whilst the glass is in contact with the metal parts. This was first shown by McGraw in 1961 (D.A. McGraw 'Heat transfer in glass during forming'. J.Amer. Ceram. Soc. 44 (1961) 353-362) and later in a laboratory study by Fellows and Shaw (C.J. Fellows and F.Shaw 'A laboratory investigation of glass to mould heat transfer during pressing, Glass Technol. 19 (1978) 4-9).

It has been shown by the present inventor (Figure 1) that the decrease in HTC during pressing observed by McGraw can be accounted for by the increase in the thickness of the air gap at the interface due to the thermal shrinXage of the glass. In order to show this, it is necessary to take account of the decrease in the thermal conductivity of air with decreasing temperature and to remember that 2S it is the dependence of glass volume on temperature which is the fundamental factor controlling the thickness of the air gap. The calculations are also based on the assumption that contraction of the glass in directions parallel to the interface is very limited and that the whole of the volume decrease is by the glass contracting in a direction normal to the interface. (Far better 4 agreement between measured and computed values is obtained if one examines the basic experimental data, which consists of measurements of heat content of the pressing at various stages of the pressing operation.) It is proposed that the HTC may be increased by a partial or complete replacement of the air layer at the meltmould interface by gases of higher thermal conductivity than air.

Therefore, according to a first aspect of the present invention there is provided a method of manufacturing glass articles by pressing or blowing characterised in that a gas mixture of higher thermal conductivity than air is present at the glass-mould interface during formation of the glass article. Preferably, the method includes the steps of: 9 evacuating air from the volume around the plunger 20 and/or moulds; and back-filling said volume with said gas mixture prio? to formation of a glass article. Preferably, the gas mixture includes helium or hydrogen. 25 In a second aspect of the invention there is provided apparatus for use in the manufacture of glass articles comprising at least one mould into which molten glass can be introduced and means for introducing a gas mixture of higher thermal conductivity than air to the glass-mould interface during formation of a glass article.

Preferably, the or each mould is provided with a plurality of throughholes of small diameter, which holes are in communication with both a gas supply chamber, for 5 storing said gas mixture, and the glass-mould interface. Ideally, said gas supply chamber is machined or cast into the body of the or each mould.

Preferably, said apparatus is enclosed in a substantially gas-tight chamber including an air-lock for the removal of finished glass articles.

Preferred embodiments of the invention will now be more particularly described, by way of example only, with reference to the accompanying drawings in which:

Figure 1. Comparison of McGraw's experimental results (loc. cit. 1961) with results computed given an initial Heat Transfer Coefficient of 10 kW M-2 K-1 Both relate to the changes in the values of the HTC whilst the glass is in contact with the metal.

Figure 2. Computed curves assuming that for the gas mixture the initial HTC is determined by the initial gas 2S gap thickness, here assumed to be 3.0 micrometer, hence giving a value of about 11000 kWm -2 K-' for pressing in air.

Figure 3. Temperature distribution through the thickness of the sheet at the end of the pressing. Pressing in air.

Figure 4. Temperature distribution through the thickness 6 of the sheet at the end of the pressing. Pressing in the helium-nitrogen gas mixture. Note the general similarity in shape to Figure 3, but the lower temperature level.

Figure 5. Variation with time of the centre and surface temperatures during pressing. Pressing in air. Initial HTC = 10 - kW M-2 K-1.

Figure 6. Variation with time of the centre and surface temperatures during pressing. Pressing in hydrogennitrogen mixture. Initial HTC = 19 kW M-2 K-1.

Note the almost constant surface temperature throughout most of the time of contact with the metal.

one general feature which is apparent from the results so far obtained is that the benefits of using a higher conductivity gas mixture are not directly proportional to the thermal conductivity of the gas. Clearly the benefit will be limited by the rather high thermal resistance of the glass itself.

Figure 7. Variation with time during pressing of -the average temperature of the glass, which is directly proportional to its heat content. Pressing carried out in nitrogen- hydrogen gas mixture. Note that by pressing using either of the high thermal conductivity gas mixtures, approximately the same amount of heat has been extracted from the glass in 1.1 sec as is normally achieved in 1.5 sec. when pressing in air.

Figure 8. Shows the reheat effect at both the glass 7 surface and the centre after pressing for 1.5 sec. using the hydrogen- nitrogen gas mixture. A value for the natural convection HTC of 10 W M-2 K-' is assumed to apply at the glass surface Figure 9. Shows the reheat effect at both the glass surface and the centre after pressing for 1.5 sec. in air. A value for the natural convection HTC of 10 W M-2 K-' is assumed to apply at the glass surface Figure 10. Shows the variation with time during reheat (after pressing in air for 1.5 sec) of the rate of extension under its own weight of a 0. 1 m long sheet of glass held vertically. Calculated using viscosities obtained from the Fulcher equation with parameters characteristic of container glass.

Figure 11. Shows the variation with time during reheat of the extension under its own weight of a 0.1 m long sheet of glass held vertically. Calculated using viscosities obtained from the Fulcher equation with parameters characteristic of container glass. The curve is for glass pressed in the hydrogen-nitrogen mixture for 1.2 sec. Note the greater rate of extension even though more heat 2S has been extracted from the glass.

Figure 12. Shows the variation with time during reheat as for Figure 11. Calculations have been carried out for three gases indicated. Approximate values of the natural HTC are:

Air 10 W/m'K 8 Argon 5 W/M2 K Helium 30 W/M2 K The effect of using different gases is relatively small. However, circumstances could arise when it is desirable to decrease the rate of extension during reheat by using a high conductivity gas mixture.

It should be noted that the results of the reheat calculations can only be expected to give a rough indication of the likely behaviour of the glass. During reheat, one face of the glass is exposed to radiation from the remainder of the container. This effect can only be assessed approximately. It is not considered appropriate to discuss this in detail here.

Each of the above graphs show the effects of the use of gas mixtures having about twice the thermal conductivity of air i.e. those containing 40% helium and 30% hydrogen. The information should not be interpreted as indicating that a mixture containing as high a percentage of hydrogen can be safely used. All the data giverf is computed.

The means for providing the gas mixture at the glass25 metal interface will obviously depend on the nature of the process to which the invention is to be applied. Thus in the case of hand made articles, a marver plate may be provided in which the glass contact surface is made from a porous sintered metal and means are provided for supplying the gas mixture to the rear face of this porous layer. In order to ensure a uniform supply of gas to the 9 glass-contact surface, it is necessary that this porous layer should have a higher resistance to gas flow than any other part of the gas supply system between the outlet from the gas cylinder and the atmosphere. Such structures are used in transpiration cooling systems sometimes used in gas turbine blade technology. In this case however the main purpose is to provide the required high conductivity gas mixture at the glass-metal interface and is not intended primarily to cool the metal.

As a further example, in container manufacture means already exist for introducing air or gas to either mould. These means may be sufficient for the method of the is present invention. If not, fine holes must be drilled through to the internal surfaces of the moulds and the plunger from a gas supply cavity machined or cast within the bodies of the moulds and plunger. These holes must be drilled in such a way as not to impinge on the air or water cooling passages provided in the moulds and plunger.

It is understood that in either manual or machine operation the pressure in the gas supply (and hence the flow of gas) will be adjustable either by hand or automatically according to the requirements of the process in order to conserve on gas supply.

In automatic machines, the method of providing the surfaces of the plungers and mould with a porous surface layer, gas being supplied at the rear surface of the layer may also be used. However the provision of such a layer and method for supplying the high conductivity gas must be designed in such a way as to be compatible with the cooling provisions used to extract heat from the 5 plungers and/or moulds.

An alternative approach which may be particularly convenient in the manufacture of glass containers is to use any existing (or easily made) provisions on the machine for the supply for example of blowing air or settle blow to feed the gas supply to the moulds immediately before the glass is introduced. In this method, it will be beneficial to first evacuate and then back fill the mould with high conductivity gas.

It will be appreciated that, in automatic machine operation, a falling gob will carry with it a momentum boundary layer of composition corresponding to the atmosphere through which it is moving. Hence to ensure that such a layer does not form so as to interfere with the operation of the high conductivity gas, it may- be preferred to house the entire machine in a substantially gas tight enclosure so that the gob falls within an atmosphere of the high conductivity gas. In such a system, some type of air lock mechanism will be required to allow the glassware to be extracted from the enclosure.

It is unlikely however that the interface atmosphere need consist of a pure high conductivity gas, such as helium. This would be likely to extract heat too rapidly from the glass surface and lead to a difficulty in controlling the process. Hence a relatively imple method of providing a high conductivity gas at the glass-metal interface may be sufficient.

It is understood that hydrogen may be preferred to helium on the grounds that it has a higher conductivity than helium and is also cheaper. However the amount of hydrogen in the gas mixture must be limited by considerations of safety. The gas mixture used must be neither explosive nor combustible.

Nevertheless an amount of hydrogen (up to ca. 20%) is likely to be acceptable and is even desirable in that (provided the gas mixture is dry) the use of hydrogen will reduce any oxide forming on the metal surface. This will in turn reduce the tendency of the glass to stick to the metal, possibly even eliminating the need to dope the mould with graphite containing lubricants, The use of hydrogen may also reduce the tendency for small particles of plunger material to be transferred to the inner surface of glass containers, thereby significantly increasing the strength of the containers particularly to impact stresses.

The following data is relevant to assessment of the invention:

Thermal conductivity of gases at 900 "K in W m-1 k-1. data from Eckert and Drake 'Analysis of Heat and Mass Transfer' McGraw-Hill (1972) Appendix B. 12 Air 0.063 Nitrogen 0.061 Helium 0.298 Hydrogen 0.412 The thermal conductivities of gas mixtures may be calculated using equations quoted by Eckert and Drake. Appendix B. p.767 and 770. The following values were obtained for the thermal conductivities of mixtures of helium and nitrogen and of mixtures of hydrogen and nitrogen, also at 900 OK and in the same units.

Helium-nitrogen mixtures helium 0.0707 0.0821 0.0952 40 0.1104 so 0.1281 0.1493 0.1750 0.2065 90 0.2464 Hydrogen-nitrogen mixtures hydrogen 10 0.0782 0.0979 0.1200 0.1450 0.1736 0.2067 70 0.2452 0.2908 0.3454 In the simpler pressing processes in which only one mould is involved and there is no reheat phase between two steps of the forming process, the reduction in the process time would be as much as 50 %.

Clearly the difference in pressing atmosphere has little effect on the extension during reheat, provided that approximately the same amount of heat has been removed in the previous pressing operation.

It should be noted that the results of the reheat calculations can only be expected to give a rough indication of the likely behaviour of the glass. Du-ring reheat, one face of the glass is exposed to radiation from the remainder of the container. This effect can only be assessed approximately. it is not considered appropriate to discuss this in detail here.

The shaping of glass sheets for automotive glazing involves first heating the cut glass sheets from room temperature to as uniform a temperature as possible (say 750 OC). The glass may then be just beginning to deform under its own weight. The glass is then removed from the 14 heating furnace and is pressed to shape between metal platens usually coated with some high temperature textile. Although the process is well understood and errors can be corrected by using a computer model of the 5 process, defects do arise even in making standard shapes,_ whilst certain shapes involving rapid changes in curvature may not be possible.

The use of a high conductivity gas mixture in the heating furnace will reduce the time necessary to heat the glass to the pressing temperature. Here the use of forced convection heat transfer would also be helpful in accelerating the heating phase. This may be achieved by motor driven fans in the furnace or by directing jets of recycled high conductivity gas at the glass surface.

The use of a high thermal conductivity gas mixture also increases the uniformity of the temperature over the interface between glass and mould during pressing, thus producing a higher quality product by reducing the incidence of shape defects and making possible --the manufacture of more complex shapes.

In general it will be appreciated that apart from opening up the possibility of significantly increasing production rates by increasing the rates of heat exchange and increasing the uniformity of temperature across the glass surface, the invention also gives the process operator an extra degree of freedom in choosing operating conditions so as to optimise the yield from his process.

Claims (1)

1. Method of manufacturing glass articles by pressing or blowing characterised in that a gas mixture of higher thermal conductivity than air is present at the glassmould interface during formation of the glass article.

2. Method as claimed in claim 1 including the steps of:

9 evacuating air from the volume around the plunger and/or moulds; and back-filling said volume with said gas mixture prior to formation of a glass article.

3. Method as claimed in claim 1 or claim 2 wherein the gas mixture includes helium or hydrogen.

4. Apparatus for use in the manufacture of glass articles comprising at least one mould into which molten glass can be introduced and means for introducing a gas mixture of higher thermal conductivity than air to-'the glass-mould interface during formation of a glass article.

5. Apparatus as claimed in claim 4 wherein the or each mould is provided with a plurality of through-holes of small diameter, which holes are in communication with both a gas supply chamber, for storing said gas mixture, and the glass-mould interface.

6. Apparatus as claimed in claim 5 wherein said gas 16 supply chamber is machined or cast into the body of the or each mould.

7. Apparatus as claimed in any of claims 4 to 6 wherein said apparatus is enclosed in a substantially gas-tight chamber including an air-lock for the removal of finished glass articles.

S. Apparatus for use in the manufacture of glass articles substantially as described herein with reference to the accompanying drawings.

9. Method of manufacturing glass articles substantially as described herein with reference to the accompanying drawings.