DEVICE FOR STIRRING MOLTEN GLASS

Abstract

Stirrer (1) for stirring molten glass (16), whereby the stirrer (1) comprisesa shaft (2) having a tip (4) and having a central longitudinal axis (L), andone or more inner stirrer blades (5,6) which are attached to the shaft (2), andone or more outer stirrer blades (7,8) which are attached to the shaft (2), whereby the inner stirrer blades (5,6) are attached closer to the shaft (2) than the outer stirrer blades (7,8), whereby, when considering the stirrer in a cylindrical coordinate system (11), both the one or more inner stirrer blades (5,6) as well as the one or more outer stirrer blades (7,8) are disposed at an angle (?, ?) to the central longitudinal axis (L), whereby said angle (?, ?) is between 0? and 90? not including these values, and are disposed having a least a blade section with a normal vector (N, P, Q, R), on the side directed towards the tip (4), with an angular component (N.sub.A, P.sub.A, Q.sub.A, R.sub.A).

Claims

1. Stirrer (1) for stirring molten glass (16), whereby the stirrer (1) comprises a shaft (2) having a tip (4) and having a central longitudinal axis (L), and one or more inner stirrer blades (5,6) which are attached to the shaft (2), and one or more outer stirrer blades (7,8) which are attached to the shaft (2), whereby the inner stirrer blades (5,6) are attached closer to the shaft (2) than the outer stirrer blades (7,8), whereby, when considering the stirrer in a cylindrical coordinate system (11) with the longitudinal axial coordinate (z) of the cylindrical coordinate system (11) defined to coincide with the central longitudinal axis (L), whereby the cylindrical coordinate system (11) is further defined by a radial coordinate (?) and an angular coordinate (ep), both the one or more inner stirrer blades (5,6) as well as the one or more outer stirrer blades (7,8) are disposed at an angle (?, ?) to the central longitudinal axis (L), whereby said angle (?, ?) is between 0? and 90? not including these values, and are disposed having a least a blade section with a normal vector (N, P, Q, R), on the side directed towards the tip (4), with an angular component (N.sub.A, P.sub.A, Q.sub.A, R.sub.A).

2. Stirrer according to claim 1, wherein the one or more inner stirrer blades (5,6) are mounted against the shaft (2) and are preferably executed as helicoidal blades or a helicoidal blade.

3. Stirrer according to claim 1, wherein the one or more outer stirrer blades (7,8) are mounted on rods (9) or tubes extending at least partially in a radial direction from the shaft (2).

4. Stirrer according to claim 3, the one or more outer stirrer blades (7,8) each have two ends, whereby each of these ends is mounted on a different one of said rods (9) or tubes, whereby the rods (9) or tubes used for mounting the ends of an specific outer stirrer blade (7,8) have a different angular position and/or axial position on the shaft (2).

5. Stirrer according to claim 3, wherein the one or more outer stirrer blades (7,8) are each mounted on two or more of said rods (9) or tubes, whereby the rods (9) or tubes used for mounting an outer stirrer blade (7,8) are placed at angular positions on the shaft which are mutually different by at least 30? and which are preferably mutually different by 90?.

6. Stirrer according to any claim 3, wherein at least one of the outer stirrer (7,8) blades is an open flat ring-shaped elliptical segment, preferably, each of the outer stirrer (7,8) blades is an open flat ring-shaped elliptical segment, said at least one open flat ring-shaped elliptical segment being placed following a line formed by the intersection of a cylinder and a plane making an angle (?), between 0? and 90? excluding said values, preferably between 10? and 80? including said values, more preferably between 20? and 70? including said values, with the central axis of that cylinder, said central axis of the cylinder being co-linear with the central axis (L) of the shaft (2) of the stirrer (1).

7. Stirrer according to claim 3, wherein the outer edge of at least one of the one or more outer stirrer blades (7,8) is provided with a raised edge (10), whereby preferably all of the mentioned outer stirrer blades (7,8) are provided with such a raised edge (10).

8. Stirrer according to claim 1, wherein the angular component (N.sub.A, P.sub.A, Q.sub.A, R.sub.A) of said normal vector (N, P, Q, R) of at least a blade section of one or more of said inner and outer stirrer blades (5, 6, 7, 8) is negative and the angular component (N.sub.A, P.sub.A, Q.sub.A, R.sub.A) of said normal vector (N, P, Q, R) of at least a blade section of one or more of the inner and outer stirrer blades (5, 6, 7, 8) is positive.

9. Stirrer according to claim 8, wherein the stirrer comprises two or more of said inner stirrer blades (5,6), whereby the angular component (N.sub.A, P.sub.A) of the normal vector (N, P) of at least one of said inner stirrer blades (5,6) is negative and the angular component (N.sub.A, P.sub.A) of the normal vector (N, P) of at least one other of said inner stirrer blades (5,6) is positive and whereby the number and size of said inner stirrer blades (5,6) having a normal vector (N, P) with a negative angular component (N.sub.A, P.sub.A) is the same as the number and size of said inner stirrer blades having a normal vector (N, P) with a positive angular component (N.sub.A, P.sub.A).

10. Stirrer according to claim 8, wherein the one or more outer stirrer blades (7,8) in totality comprise two or more blade sections, whereby the angular component (Q.sub.A, R.sub.A) of the normal vector (Q, R) of at least one of said blade sections is negative and the angular component (Q.sub.A, R.sub.A) of the normal vector (Q, R) of at least one other of said blade sections is positive and whereby the number and size of said blade sections having a normal vector (Q, R) with a negative angular component (Q.sub.A, R.sub.A) is the same as the number and size of said blade sections having a normal vector (Q, R) with a positive angular component (Q.sub.A, R.sub.A).

11. Stirrer according to claim 8, wherein at least a blade section of one of said one or more outer stirrer blades (7,8) extends over a certain axial section of the shaft (1), whereby at least one of said one or more inner stirrer blades (5,6) is placed in the same axial section, whereby the signs of the angular components (N.sub.A, P.sub.A, Q.sub.A, R.sub.A) of the normal vectors (N, P, Q, R) of this inner stirrer blade (5,6) and this blade section of the outer stirrer blade (7,8) are opposite.

12. Stirrer according to claim 1, wherein most, and preferably all, of the mentioned one or more inner and outer stirrer blades (5,6,7,8) are disposed at an angle (?, ?) to the central longitudinal axis (L) which is between 10? and 80? including said values, and which is preferably between 20? and 70? including said values.

13. A plunger (18) for gobbing molten glass, said plunger comprising a stirrer (1) according to claim 1, said stirrer (1) comprising on its tip (4) a gobbing element (19).

14. Stirrer (1) according to claim 1, wherein said stirrer (1) is made of platinum or a platinum alloy or of molybdenum or a molybdenum-based alloy, or of iridium or an iridium-based alloy.

15. Plunger (18) according to claim 13, wherein said plunger (18) is made of platinum or a platinum alloy or of molybdenum or a molybdenum-based alloy, or of iridium or an iridium-based alloy.

Description

[0053] In order to explain the invention, any without limiting the invention in any way, examples of preferred embodiments are given below, referring to the following figures:

[0054] FIG. 1, showing a schematic view in perspective of a stirrer according to the invention;

[0055] FIG. 2, showing a side view of the stirrer of FIG. 1;

[0056] FIG. 3, showing a cross-section according to III-III of the stirrer of the previous figures;

[0057] FIG. 4, showing a cross-section according to IV-IV of the stirrer of the previous figures;

[0058] FIG. 5, showing a cross-section according to V-V of the stirrer of the previous figures;

[0059] FIG. 6a, showing the use of the stirrer of the previous figures;

[0060] FIG. 6b, showing the global molten glass displacement generated when the stirrer of the FIG. 6a is operating under stationary conditions;

[0061] FIG. 7, showing a schematic view in perspective of another stirrer according to the invention;

[0062] FIG. 8, showing a schematic view in perspective of yet another stirrer according to the invention;

[0063] FIG. 9, showing a schematic view in perspective of yet another stirrer according to the invention;

[0064] FIG. 10, showing a schematic view in perspective of yet another stirrer according to the invention;

[0065] FIG. 11, showing two side views, in directions which are mutually different by 90?, of yet another stirrer according to the invention.

[0066] FIG. 12, showing a schematic view in perspective of a plunger according to the invention;

[0067] FIG. 13, showing a schematic view in perspective of yet another plunger according to the invention;

[0068] FIG. 14, showing a schematic view in perspective of yet another plunger according to the invention; and

[0069] FIG. 15, showing a schematic view in perspective of yet another plunger according to the invention;

[0070] The stirrer 1 shown in FIGS. 1 to 5 consists mainly of a shaft 2, which is provided at one extreme with a connector 3 for connecting the shaft 2 to a drive, and nearer to the other extreme, further to be called the tip 4, with a number of stirrer blades. The shaft 2 has a central longitudinal axis L which during use will be the axis of rotation of the stirrer 1.

[0071] The stirrer blades can be grouped in two groups, more specifically inner stirrer blades 5,6, which are attached directly against the shaft 2 and which are helicoidally shaped, meaning shaped like a part of a helicoid, and outer stirrer blades 7, 8 which are attached to rods 9 which are attached to the shaft 2.

[0072] As depicted in FIG. 2, the outer stirrer blades 7,8 are flat blades, in particular open flat ring-shaped elliptical segments, which are mounted on the shaft 2 following a line formed by the intersection of a cylinder C and a plane P, said plane P forming an acute angle ?, between 0? and 90? excluding said values, preferably between 10? and 80? including said values, more preferably between 20? and 70? including said values, with the central axis of the cylinder, said central axis of the cylinder being co-linear with the central axis L of the shaft 2 of the stirrer 1.

[0073] In a further preferred embodiment depicted in FIG. 2, the central axis L of the shaft 2 is coinciding with the central axis of the cylinder C.

[0074] In particular the central L axis is the central axis of both the cylinder and the shaft.

[0075] In the framework of the present invention, the open flat ring-shaped elliptical segment corresponds to a flat segment fully defined in the plane P and having a predetermined thickness.

[0076] The flat segment 7 has a frontal face 7a directed to the connector 3 and a dorsal face 7b directed to the tip 4, the frontal and dorsal faces being parallel to each other and being further parallel to the plane P (see FIG. 2).

[0077] Such an embodiment presents the advantage of allowing a continuous and homogeneous grasp of the molten glass due to an improved local pumping effect and shearing effect obtained due to the specific shape of the outer blades.

[0078] Furthermore, the outer stirrer blades which have a raised edge 10 at their outermost edge. The inner stirrer blades 5,6 make ? turns round the shaft 2 and the outer stirrer blades 7,8 make ? turn around the shaft 2.

[0079] In this embodiment, but not necessarily, the raised edged 10 is present both above as well as below the main body of the outer stirrer blades 7,8. This raised edge 10 serves to reinforce the outer stirrer blades 7,8, but also has a function to improve stirrer performance, as will be explained below.

[0080] The stirrer geometry will further be considered in a cylindrical coordinate system, similar to the cylindrical coordinate system 11 drawn in FIG. 1, but with the longitudinal axial coordinate z of the cylindrical coordinate system defined to coincide with the central axis L and having an increasing value from the connector 3 to the tip 4, and further having a radial coordinate ? and an angular coordinate ?, defined to have an increasing value in clockwise direction when looking from the connector 3 to the tip 4.

[0081] There are four inner stirrer blades 5, 6. All are placed forming an angle ? of circa 70? with the central longitudinal axis L, whereby due to the helicoidal shape of the inner stirrer blades 5, 6 the angle ? can vary locally.

[0082] The two inner stirrer blades 5 closest to the tip have a normal vector N on the side of the tip 4 with a negative angular component N.sub.A, so with a direction opposite to the defined direction of the angular coordinate ?, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, these inner stirrer blades 5 will create an axial displacement of molten glass away from the tip 4.

[0083] The two inner stirrer blades 6 furthest away from the tip 4 have a normal vector P on the side of the tip 4 with a positive angular component P.sub.A, so with a direction coinciding with the defined direction of the angular coordinate ?, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, these inner stirrer blades 6 will create an axial displacement of molten glass towards the tip 4.

[0084] As all inner stirrer blades 5,6 are of the same size and shape, during use the combined inner stirrer blades 5,6 will not create any, or at least an insignificant, net displacement of molten glass, at any rotational speed.

[0085] The outer stirrer blades 7, 8 are placed on rods 9 which are attached to the shaft 2 at different angular and axial positions, more specifically at angular positions 90? apart, in line with the outer stirrer blades 7,8 making ? turn around the shaft 2. Each outer stirrer blade 7,8 is attached at both its ends to a different rod 9, and whereby some rods 9 are attached to the ends of two outer stirrer blades 7,8, and some other rods 9 are only a attached to the end of a single outer stirrer blade 7,8.

[0086] Note that in the present example the outer blades 7,8 are attached at or at least close to the ends of the rods 9. It is however also possible that outer blades 7,8 are attached to the rods 9 at a point between the attachment point to the shaft 2 and the free end of the rods 9.

[0087] The outer stirrer blades 7,8 are placed on the same axial section of the shaft 2 as the inner stirrer blades 5,6.

[0088] There are eight outer stirrer blades 7,8. They are all placed forming an angle ? of circa 45? with the central longitudinal axis.

[0089] The four outer stirrer blades 7 closest to the tip 4 have a normal vector Q on the side of the tip 4 with a positive angular component Q.sub.A, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, these outer stirrer blades 7 will create an axial displacement of molten glass towards the tip 4.

[0090] The four outer stirrer blades 8 furthest away from the tip 4 have a normal vector R on the side of the tip with a negative angular component R.sub.A, meaning that if the stirrer is rotated in a positive angular direction ? during use, these outer stirrer blades 8 will create an axial displacement of molten glass away from the tip 4.

[0091] As all outer stirrer blades 7,8 are of the same size and shape, during use the combined outer stirrer blades 7,8 will not create any, or at least an insignificant, net displacement of molten glass, at any rotational speed.

[0092] The shaft 2, inner and outer stirrer blades 5,6,7,8, and the rods 9 are all made of dispersion-hardened platinum.

[0093] The use of the stirrer is simple and as follows, and as illustrated in FIG. 6a.

[0094] The stirrer is placed, connected with its connector 3 to a drive 12, in a stirring chamber 13 having an internal wall 21, which is provided with an inlet 14 and outlet for molten glass 15. The stirrer diameter d is only slightly smaller than the chamber diameter D, which is the diameter D of the internal wall 21 of the chamber. Molten glass 16 is made to flow through the mixing chamber and the stirrer is rotated, as shown in FIG. 6a, in this example clockwise as indicated by arrow A when looking from the drive 12 towards the tip 4.

[0095] In use under stationary conditions, two global cycling flows 17a, 17b of molten glass 16 are now established, both going repeatedly through the volumes swept by the stirrer blades 5,6,7,8 so that inhomogeneous regions are repeatedly elongated and chopped up and thereby become smaller and better dispersed in the molten glass 16. In particular the raised edge 10 is believed to play an important role in smearing the molten glass 16 close to the wall of the stirring chamber 13, thereby elongating impurities, to be chopped later by other actions of the stirrer 1.

[0096] The edge 10 also plays a role of stability of the outer blades movement during rotation of the stirrer.

[0097] By the presence of said edge 10, the mechanical stability of the outer blade is indeed preserved during use so that it is not subject to bending when cooperating with the molten glass material during stirring.

[0098] Only limited mixing between the two flows 16 occurs at the vertical level in the stirring chamber 13 where they meet. No significant cycling flow is established between the stirrer 1 and the wall of the stirring chamber 13, due to the narrow gap between them.

[0099] Due to the overall flow of molten glass from the inlet 14 to the outlet 15, molten glass is forced slowly from the upper cycling flow to the lower cycling flow, and then to the outlet 15.

[0100] In FIG. 6b, is provided the displacement of the fluid (or molten glass) for two units, a first unit a, and a second unit b, of the stirrer 1 along a I-I (for unit a) and a II-II (for unit b) sections.

[0101] In a particular embodiment disclosed in FIG. 6a, each of the units a, b, comprise three rods 9, and is defined between a first and a second rods parallel to each other. A third rod, perpendicular to the first and second rods, is placed between said first and second rods.

[0102] In the first and second units, inner stirrer blades 5,6 are attached directly against the shaft 2 and are helicoidally shaped, meaning shaped like a part of a helicoid, and outer stirrer blades 7, 8 are attached to rods 9 which are attached to the shaft 2, so that the first and second units a, b forms a stirring element of the stirrer as depicted in FIGS. 1 to 5.

[0103] A first cycling flow 17a (FIG. 6bsection I-I) of molten glass 16 is established in a first volume defined in the first unit a.

[0104] In the first unit a, an intern region 22a is substantially defined by the diameter of the inner blades 6, and by a distance d between a first and a second rods defining said first unit a.

[0105] In this intern region 22a of the first unit a, when the stirrer is rotated clockwise as indicated by arrow A when looking from the drive 12 towards the tip 4, the molten glass is displaced along the shaft 2 in direction of the connector 3 (upwards).

[0106] Moreover, in the first unit a, an outer region 23a is substantially defined by the diameter d of the outer blades 7, and by the distance d between said first and second rods defining said first unit a.

[0107] In this outer region 23a of the first unit a, when the stirrer is rotated clockwise as indicated by arrow A when looking from the drive 12 towards the tip 4, the molten glass is displaced along the shaft 2 in direction of the tip 4 (downwards).

[0108] The first cycling flow 17a of molten glass 16 results in the combination of the inner and outer displacements, in the first unit a, of the molten glass along the shaft 2.

[0109] A second cycling flow 17b (FIG. 6bsection II-II) of molten glass 16 is established in a second volume defined in the second unit b.

[0110] In the second unit b, an intern region 22b is substantially defined by the diameter of the inner blades 6, and by a distance d between said first and said second rods defining said second unit b.

[0111] In this intern region 22b of the second unit b, when the stirrer is rotated clockwise as indicated by arrow A when looking from the drive 12 towards the tip 4, the molten glass is displaced along the shaft 2 in direction of the tip 4 (downwards).

[0112] Moreover, in this second unit b, an outer region 23b is substantially defined by the diameter d of the outer blades 7, and by the distance d between a first and a second rods defining said second unit b.

[0113] In this outer region 23b of the second unit b, when the stirrer is rotated clockwise as indicated by arrow A when looking from the drive 12 towards the tip 4, the molten glass is displaced along the shaft 2 in direction of the connector 3 (upwards).

[0114] The second cycling flow 17b of molten glass 16 results in the combination of the inner and outer displacements, in the second unit b, of the molten glass along the shaft 2.

[0115] Note that in possible variants, especially in batch processing of molten glass, the stirrer may be significantly smaller than the stirring chamber, which is in that case formed by the vessel in which the stirring is performed.

[0116] In yet another variant the stirrer may be placed in a long channel through which glass flows, whereby the channel itself forms the stirring chamber.

[0117] The alternative stirrers shown in FIGS. 7 to 11 differ from the stirrer shown in FIG. 1 by having a different number and different arrangement of inner and outer stirrer blades.

[0118] The stirrer 1 shown in FIG. 7 has the same number and arrangement of inner stirrer blades 5, 6 as the stirrer of FIG. 1. The arrangement of outer stirrer blades 8 is different, in that this stirrer has six outer stirrer blades 8, which are all positioned so that they have a normal vector R on the side of the tip 4 with a negative angular component. This stirrer 1 has an outer pumping effect, when rotated in a positive angular direction ?, away from the tip. This pumping effect is dependent on the rotational speed of the stirrer 1 so that this stirrer 1 is mostly suitable for batch-wise processing of molten glass.

[0119] The stirrer 1 shown in FIG. 8 has the same number and arrangement of inner stirrer blades 5,6 as the stirrer 1 of FIGS. 1 and 7. The arrangement of outer stirrer blades is different, in that in addition to the outer stirrer blades 8 of the stirrer 1 of FIG. 7, the stirrer 1 of FIG. 8 has six more outer stirrer blades 7, which additional outer stirrer blades 7 are all positioned so that they have a normal vector Q on the side of the tip 4 with a positive angular component. This stirrer 1 has no net pumping effect.

[0120] The stirrer 1 shown in FIG. 9 has a different arrangement of inner stirrer blades 5 than the previously described stirrers 1. All four inner stirrer blades 5 are placed so that they have a normal vector N on the side of the tip 4 with a negative angular component, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, the inner stirrer blade 5 will create an axial displacement of molten glass away from the tip 4. The outer stirrer blades 7,8 are arranged similarly to the outer stirrer blades 7, 8 of the stirrer of FIG. 8, only their total number is eight, instead of twelve.

[0121] As the outer stirrer blades 7, 8 together do not have a net pumping effect, and the inner stirrer blades 5 do, this stirrer 1 has a net pumping effect.

[0122] The stirrer 1 shown in FIG. 10 only has a single set of two inner stirrer blades 5, both placed so that they have a normal vector N on the side of the tip 4 with a negative angular component, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, the inner stirrer blade 5 will create an axial displacement of molten glass away from the tip 4. The outer stirrer blades 8 are arranged like the outer stirrer blades 8 of the stirrer of FIG. 7, only their total number is four, instead of six. This stirrer 1 has a particularly strong pumping effect, so that it is mostly suitable for use in batch processing, although it may be used in continuous processes as well when there is a requirement for a pumping effect.

[0123] The stirrer 1 shown in FIG. 11 has four inner stirrer blades 6 which are placed so that they have a normal vector P on the side of the tip 4 with a positive angular component, meaning that if the stirrer 1 is rotated in a positive angular direction ? during use, the inner stirrer blade 5 will create an axial displacement of molten glass away towards the tip 4.

[0124] The outer stirrer blades 8 are arranged in two groups of fours, which are both similar to the four outer stirrer blades 8 of the stirrer 1 of FIG. 1 that are placed away from the tip 4. These have a normal vector R on the side of the tip with a negative angular component, meaning that if the stirrer is rotated in a positive angular direction ? during use, these outer stirrer blades 8 will create an axial displacement of molten glass away from the tip 4.

[0125] For completeness it is noted that also the connector 3 is executed differently.

[0126] It is noted that in the above examples the rods 9 extend only in radial direction from the shaft 2 and are straight rods. It is of course also possible that the rods 9 additionally have an axial and/or angular direction and/or that they are curved.

[0127] It is further noted that in the examples above the outer stirrer blades 7,8 each have a single normal vector.

[0128] It is also possible that an outer stirrer blade has different blade sections with different normal vectors. It is also possible that an outer stirrer blade has several blade sections with a normal vectors with mutually different angular components.

[0129] Such an outer stirrer blade could for instance be a combination of the outer stirrer blades identified by numbers 7 and 8 in FIG. 9.

[0130] These two outer stirrer blades lie in the same plane, and could therefore easily be made as a single outer stirrer blade from a single straight sheet with a blade section corresponding to outer stirrer blade 7 and a blade section corresponding to outer stirrer blade 8.

[0131] This single outer stirrer blade would then be mounted on the shaft as in FIG. 9 but by executing the rod between outer stirrer blades 7 and 8 shorter than in FIG. 9, so that this rod would support the single combined outer stirrer blade only on one side, the side facing the shaft 2.

[0132] The present invention also concerns a plunger or a gobber 18.

[0133] The plunger 18 is made of the stirrer 1 according to the invention on which a gobbing element 19 or gobbing member is mounted on the tip 4 of the stirrer.

[0134] FIGS. 12 to 15 provides several embodiments of the plunger 18 according to the present invention.

[0135] The plunger depicted in FIG. 12 corresponds to the stirrer of FIG. 7, but on which a gobbing element, such a piston or gobbing piston has been bound.

[0136] The plunger depicted in FIGS. 13 and 15 corresponds to the stirrer of FIG. 8, but on which a gobbing element, such a piston or gobbing piston has been bound.

[0137] The plunger depicted in FIGS. 14 corresponds to the stirrer of FIG. 11, but on which a gobbing element, such a piston or gobbing piston has been bound.

[0138] During operation, the plunger or gobber 18 is placed in a gobbing chamber and is put into a movement of translation along the L axis for gobbing molten glass out of the gobbing chamber.

[0139] With the plunger or gobber 18 according to the present invention, if said gobber is put into rotation and translation simultaneously or alternatively, the benefits of said gobber/plunger when used in this way is twice: i) it allows a simultaneous stirring and gobbing of the molten glass, so that the molten glass can be continuously homogenized when being gobbed; and ii) it allows a stirring of the molten glass before the gobbing step, so that the molten glass can be homogenized during a certain period before being gobbed.

[0140] For each of these plungers, the gobbing member 19, or gobbing element, or gobbing piston is welded to the tip 4 of the shaft of the corresponding stirrer.

[0141] In addition, in a preferred embodiment of the plungers according to the present invention, the gobbing element 19 may have different shapes.

[0142] For instance, in the plungers of FIGS. 12 to 14, the gobbing element is a cone or truncated cone with its basis welded on the tip 4 through a cup-shaped connection means 20, the base of the cone being in contact with and welded to a first face of said connection means 20 and a second face of the connection means, opposed to said first face, being directly welded to the tip 4 of the stirrer.

[0143] In the plunger depicted in FIG. 15, the gobbing element 19 has a semi-spherical shape with basis that is directly welded to the frontal face of the connection means 20.

[0144] In a preferred embodiment of the stirrer and plunger according to the invention, the stirrer and the plunger are made of platinum or a platinum alloy or of molybdenum or a molybdenum-based alloy, or of iridium or an iridium-based alloy.

[0145] It is understood that the present invention is by no means limited to the forms of the above embodiments and that many modifications can be made without departing from the scope of the appended claims.