Stopper rods, or stoppers, are well known for use in controlling molten metal flow from a vessel, e. g. a tundish, into a mould. A stopper rod is raised and lowered by a suitable mechanism positioned adjacent the vessel and can thereby be used to close fully or partially or to open fully the outlet from the vessel, thus controlling flow of the metal from the vessel.

Conventionally, a stopper rod is formed as an elongate one-piece bonded alumina-graphite composition by isostatic pressing in a suitable mould. The stopper rod usually has a hollow structure in that it is formed to have a longitudinal, central bore by use of a suitable removable core in the mould. The lower end of the stopper rod bore may be closed or it may be vented to allow gas, e. g. argon, to be passed through the rod.

Thus in the conventional manufacture of a stopper rod, the desired alumina-graphite composition is placed around a removable core in a deformable mould, i. e. the composition fills the mould space between the core and the deformable mould. The deformable mould is then placed in a tank of liquid, e. g. water, and the pressure of the water is increased by known means to the pressure required to convert the alumina-graphite composition to the desired self-supporting structure. After removal of the isostatically pressed product from the mould, the core is removed to provide the hollow stopper rod.

The upper end of the stopper rod must be provided with means by which it can be attache to the lifting and lowering mechanism. Various prior proposas have been made, including co-pressing a threaded bush in the upper end of the stopper rod to receive a threaded shaft; forming the upper end of the bore of the rod with an internal thread to receive a ceramic or metallic insert, and placing a retaining pin through coterminous holes drilled through the stopper rod and an attachment rod.

In the particular embodiment in which the upper end of the bore of the stopper rod is formed with an internal thread to receive an insert, this is conventionally achieved by positioning an externally-threaded annular insert in the mould at a location to form the internally-threaded portion of the stopper rod bore so that it extends along a portion of the bore from the commencement of the bore at the upper end of the rod. It has been found, however, that a number of problems can arise from such an arrangement.

For example, cracks may be formed in the pressed stopper rod body. These may be discovered after stripping the tooling from the pressed product but some cracks may only be visible after the pressed product has been fired. Other cracks that are formed internally of the product can only be detected by X-ray examination. In all these cases the presence of cracks leads to the scrapping of the expensively- formed product. Moreover, the stopper rods may have low strength as measured by the torque used to tighten the stopper to the fitting mechanism, they may show inconsistent performance due, for example, to breakage at a relatively low torque or during transport after assembly, and they are liable to failure during use.

The present invention aims to provide an improved means of making a stopper rod having a body with an internally-threaded bore portion and, hence, also aims to provide an improved stopper rod.

Accordingly, in one aspect, the present invention provides a method of making a stopper rod in which a composition to form the body of the stopper rod is charged into a mould, a removable core being positioned in the mould to form a longitudinally-extending bore in the stopper rod body, an externally-threaded removable insert being positioned in the mould in correspondence with the removable core to form an internal thread on a portion of the bore and a plate being positioned in the mould to define a flat upper end of the stopper rod body, wherein a spacer is positioned between the externally-threaded insert and the plate whereby the internal thread in the bore of the stopper rod body is formed spaced from the upper end of the body when the composition is pressed in the mould.

It will be appreciated that by"upper end"of the stopper rod body is herein intended to refer to the stopper rod in its use position.

In another aspect the invention provides a mould for the manufacture of a stopper rod body, the mould comprising a removable core corresponding to a desired bore in the body, an outer shell to surround the removable core to define a cavity in which the body is to be formed, an externally-threaded removable insert to be fitted in the mould in correspondence with the core to form an internal thread on a portion of the to-be-formed bore, a plate to be positioned in the mould to define a flat upper end to the body and a spacer to be positioned between the externally-threaded insert and the plate, whereby the internal thread in the bore will be formed spaced from the upper end of the body.

Thus the amount by which the internal thread is recessed into the bore is equal to the thickness of the spacer.

In a yet further aspect the invention provides a stopper rod having a body formed by the method or by using the mould of the immediately preceding paragraphs.

Thus the invention also provides a stopper rod having a body containing a longitudinally-extending bore, the bore opening out at a flat upper end of the body and having an internally-threaded portion spaced from the upper end of the bore.

When attaching the stopper rod of the invention to a raising and lowering mechanism, the annular recess left at the upper end of the stopper rod by the removal of the spacer can be filled with refractory cement around the part of the raising and lowering mechanism that passes through the recess when the end of the mechanism is threaded into the threaded internal bore portion. Alternatively, in a preferred embodiment, a ceramic ring may be fitted into this recess. The attachment of the raising and lowering mechanism may then be facilitated with easier and more accurate centreing of the attachment rod. Moreover, a better sealing effect may be achieved. The ceramic ring may conveniently be made of the same composition as the stopper rod body, although this may not be found to be essential.

The basic means used to mould the stopper rod body of the invention may be any convenient means and may, therefore, be the conventional method using isostatic pressing in a deformable mould Thus the mould shell may be deformable as described above.

The lower end of the stopper rod bore may be closed or it may be vented to allow gas, e. g. argon, to be passed through the rod.

The threaded insert may be a solid cylinder or a cylindrical annulus. It may be attachable to one end of the removable core or be formed as an extension of the core so that it can be inserted into the mould in one piece with the core.

Clearly, the spacer will be of a thickness determined by the amount that it is desired to space the commencement of the threaded portion of the bore from the upper end of the rod body. For example, the spacer may be from 10 to 50 mm thick, although it will be appreciated that thicknesses outside this range may well prove suitable in certain circumstances. It may be attache to the threaded insert by any convenient means.

The spacer is preferably of metal. e. g. steel, but may be of any other suitable material, e. g. a ceramic material.

The externally-threaded insert may be the same as has been conventionally used.

Thus, by way of example only, it may be of steel and of external diameter from 50 to 150 mm and of length from 25 to 150 mm.

We have found that stopper rod bodies formed according to the invention are surprisingly and significantly less prone to cracking than prior stopper rod bodies in which the threaded portion of the bore extends to the upper end of the stopper.

Although not wishing to be limited to any particular theory, it is believed that a summary of some of the avantages obtained and of the reasons for some avantages is as follows.

1. Cracking at the end of the bore during pressing is greatly reduced or eliminated due to the absence of thread in this critical region.

2. Higher torque values can be tolerated because the thread area is consistently under compression during assembly.

3. The thread area is better protected from oxidation during firing of the pressed body.

4. Better gas tightness can be obtained.

Moreover, further avantages include the fact that conversion to the design of the invention from conventional designs is very simple and requires a minimum of tooling changes. It may also be found possible to eliminate the need to use a ceramic or other insert threaded onto the threaded portion of the bore, i. e. the stopper fittings to the lifting and lowering mechanism may be directly threaded onto the threaded portion of the bore.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a portion of a conventional mould for the manufacture of a stopper rod body; Figure 2 is a similar view of a portion of a mould according to the invention; Figure 3 is a longitudinal section through a stopper rod of the invention; Figure 4 is a similar view to Figure 3 but showing a stopper rod with a ceramic ring insert at its upper end; and Figure 5 is a longitudinal section through the upper end of a further stopper rod of the invention.

In Figure 1, a mould cavity 10 is defined between a removable core 12 and a deformable shell 14. At one end of core 12 is positioned an externally-threaded insert 16, which at its end remote from core 12 is in contact with a plate 18. Insert 16 has an externally-threaded portion 16A, a central stepped portion 16B and a tail portion 16C. Plate 18 defines the end of mould cavity 10 that corresponds to the upper end of the stopper rod body that is to be formed in the cavity. It will be appreciated that the thread 20 on portion 16A of insert 16 will be moulded into the stopper rod body that will be formed when cavity 10 is filled with a suitable composition and pressed between shell 14 and the core 12 and insert 16. This thread will be on the wall defining the bore that is formed in the body and will extend along the bore from its end corresponding to plate 18.

The use of stepped insert 16 enables a number of cores 12 of different lengths to be used depending on the overall length of the stopper required.

In Figure 2 like parts are given the same reference numerals as in Figure 1.

In this embodiment of the invention mould cavity 10 is again defined between core 12 and deformable shell 14. An externally-threaded insert 16 is again attache at one end of core 12. However, according to the invention, insert 16 is separated from plate 18, which again defines the end of mould cavity 10, by a spacer 22.

The threaded end portion 16A of the insert is attache to spacer 22 by screw-threaded means 22A.

Thus it will be appreciated that the pressed body to be formed in this mould will also have a threaded portion on its bore wall but this portion will be spaced from the end of the body by an amount equal to the thickness"t"of spacer 22.

In Figure 3, a stopper made in the manner described with reference to Figure 2 is shown. The stopper 30 has a longitudinally-extending bore 31 which emerges as a narrower bore 32 at its lower (in use) end 33. As is conventional, a porous plug 34 has been inserted in bore 31 near end 33.

At its upper (in use) end 35 the bore emerges as an unthreaded recess 36, wider than bore 31 and corresponding to spacer 22. A threaded portion 37, corresponding to threaded portion 16A, lies between recess 36 and main bore 31.

In Figure 4 is shown a similar stopper rod to that of Figure 3 and in which the parts are numbered identically except that recess 36 of Figure 3 has had a ceramic ring 38 inserted into it to enable fitting to a raising and lowering mechanism to be facilitated.

The ring 38 need not be adhered into recess 36 but may advantageously be a relatively tight fit.

In Figure 5 the stopper rod 40 has a longitudinally extending bore 41 having an unthreaded recess 42 at its upper end and a threaded bore portion 43 immediately below recess 42.

A threaded ceramic insert 44 is threadingly engaged in the threaded bore portion 43 and a ceramic ring 45 lies in recess 42 in contact with the upper surface of threaded insert 44. Threaded ceramic insert 44 ensures a tight seat in the stopper body for the connecting rod 46, the lower end 47 of which lies within bore 41. Rod 46 has an annular collar 48 which seats on a ledge 49 in bore 41 immediately below threaded portion 43 of the bore. The rod 46 is inserted in the bore before the threaded insert 44, the lower end of the latter locating on the upper surface of the collar 48 and thereby locking the rod firmly inside the bore 41.

A fibre gasket 50 is positioned on rod 46 and lies in contact with the upper surface 40A of stopper rod 40, although it is shown spaced from that surface for clarity in Figure 5. A screw nut 51 on the rod 46 can be screwed downwardly onto a washer 52 above fibre gasket 50 to lock the components tightly together.

This arrangement provides a good air-tight assembly at the upper end of the stopper rod, which is highly desirable as there may be a vacuum inside the bore 41 during use.