The present invention is concerned with a method of making a cavity for holding an object, such as a foundry riser sleeve, and cavity former and sleeve for use therewith.

Although the method may be used for other purposes it is particularly useful in the production of metal casting moulds having risers, and it is in this context that the method is described.

The use of riser sleeves in metal casting moulds is well known. Up to now, they have been located either by moulding directly on the pattern used to make the mould or subsequently by insertion into the top or cope part of the mould into a cavity formed by a loose pattern piece which has been removed from the top of the mould.

It has also been proposed to insert riser sleeves into open half moulds where the mould has a vertical parting line.

Because of the increasing automation of methods used for the production of casting moulds from ^* moulding material such as sand, there is less access to the pattern plates at the moulding station, especially in the production of repetition castings. As a result, it is no longer possible to apply riser sleeves directly on the pattern plate, nor is it possible to locate a loose piece on

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the pattern plate to form a cavity into which is later inserted a riser sleeve.

When it is no longer possible to obtain access to the . pattern plate an alternative procedure may be possible with ^{' •} automatic moulding plants which are equipped with a cope mould line which is synchronised with a drag line where cores are set into the drag. On the moving cope line, inverted cope moulds are accessible for additional work, so riser sleeves may be inserted in the inverted cope mould.

With riser sleeves which have been used hitherto this is either impossible or only,partially successful. Known riser sleeves have a cylindrical or almost cylindrical outer surface for" reasons partly concerned with their production technique and partly concerned with their function. Because of dimensional variations inherent in their method of production such riser sleeves cannot be inserted into a preformed cavity with sufficient confidence that they will remain in place securely.

Another group of riser sleeves, especially those which are closed by means of a cap at one end, have a positive taper from their base going up towards the cap, i.e. the outside diameter becomes larger from bottom to top and therefore they cannot be used for the subsequent insertion into the inverted cope mould.

One object of this invention is the creation of a new method for the production of metal casting moulds made of finely divided moulding materials such as moulding sand which makes it possible to insert riser sleeves readily and so that they remain securely in a preformed ^' cavity.

According to the invention there is provided a method of making a cavity far receiving and gripping an object therein, which

comprises locating a cavity former in a body of particulate moulding material, compacting the material about the former, and removing the former, characterised in that the former is rrade oversize relative to the object to be received in the cavity and the former has recesses therein whereby the cavity is formed with at least one inwardly projecting rib-like formation of moulded material for gripping the object when received therein.

The former is preferably tapered so that the cavity has a - wider mouth than inner end. The former may have recesses to form side ribs or end recesses tα form ribs in the blind end wall of the cavity.

When objects such as riser sleeves are rrade in quantity to a predetermined nominal size, i.e. height and diameter, in practice the sleeves deviate from that nominal size. Such sleeves can still be used provided that they can be inserted in the mould cavities ^• which are to receive them, and that once inserted they will remain in place.

By checking and recording the actual height and diameter of a particular nominal size of sleeve during the course of product- ion it is possible to calculate the standard deviation in height and ^' diameter from the mean values for those parameters for that sleeve.

The invention also includes for carrying out the method a former whose exterior has a plurality of radially spaced apart grooves extending between the top and towards the bottom of the former and whose outside diameters are larger than the corresponding mean outside diameters plus three times the standard deviation of he riser sleeves which are to be used. In this way the outside diameter of the former is larger than the outside diameter of the riser sleeves which are to be inserted in the cavities. Retention of the sleeves in the cavities results -from the grip on the outside of

the riser sleeve of the sand ribs which are moulded from the recesses in the former. This results in a clamping ef ect because according to the invention the circles which can be geometrically iπcribed inside the base of the recesses have diameters which are smaller than the corresponding mean outside diameters less three times trie ^• standard deviation of the riser sleeves which are to be used-

According to a preferred feature of the invention the former has at its top end a raised rim which is so dimensioned that the total height of the former thus constituted is greater than the mean height plus three times the standard deviation of the actual riser sleeves which are to be used. Thus there remains after the insertion of the riser -sleeve an annular cavity into which any sand, which has been loosened during the insertion of the riser sleeve, may fall, without in any way hindering the desired depth of location of the riser sleeve.

According to another pre erred feature of .the invention the former has at its tap end an annular depression which is so dimensioned that the height of the former up to the base of the annular depression is smaller than the mean height less three times the standard deviation of the actual riser sleeves which are to be used.

By using such a former there is formed after moulding an annular rib of moulding material. This rib is of such depth that it touches even the lowest or shortest inserted sleeve at its top end and when longer riser sleeves are inserted the pad becomes partially compressed. A sealing means is achieved with this arrange¬ ment.

A further example of a former according to the invention is characterised in that the annular depression adjoins a surface lying above the level of the annular depression whereby the height

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of the former up to this surface is greater than the mean height plus three times the standard deviation of the actual riser sleeves which are to be used. By this means, using riser sleeves which are open at the top, an additional riser volume is achieved, while when using a riser sleeve with a closed top an air gap remains' between the surface and the' top of the cap of the riser sleeve.

The invention also includes a riser sleeve of exothermic, exothermic and heat-insulating or heat-insulating material. According to the invention such a riser sleeve is characterised in that its outer surface exhibits from its base to its top a negative taper having an angle from 2 to 20 with respect to the vertical. Preferably, the inner and outer surface of the tapered riser sleeve are parallel one with another. In another embodiment the sleeve also has a height to diameter ratio (with respect to the lowermost internal diameter) in the range 1 : 1 to 1.6 : 1.

According to a further embodiment a riser sleeve according to the invention is characterised in that it has a closed top in which a blind recess is provided which reduces the thickness of the top and which permits the controlled venting of the riser.

Because the cavity is oversize by a predetermined amount, when a riser sleeve is inserted, several air cavities are formed which are separated from each other by the vertical sand ribs and which are connected one with another by the gap at the bottom of the cavity. This has several advantages. Firstly, heat transfer to surrounding moulding material or moulding sand is reduced, and the modulus extension factor of the riser sleeve is increased. Secondly, the atmospheric oxygen contained in the air cavities can lead to a reduction in the necessary quantity of oxidising agents in the sleeve material or by retaining the same amount of oxidising agents the ignition and burning rates of the exothermic sleeves can be increased.

Further, in the case of green sand moulds the transfer of moisture to the riser sleeve is reduced. Thereby the ignition and burning characteristics in the case of exothermic sleeves will become more consistent.

Preferably a breaker core is present at the lower end of the riser sleeve and the core has an outside diameter larger than that of the riser sleeve so that the cavity formed by the former can be sealed by the projecting edge of the breaker core. By choosing suitable dimensions for the breaker core the breaker core can be made to seal the cavity formed by the former.

The former of the invention may be made from any suitable material which will retain its shape during mould production.

The invention is illustrated with reference to the drawings in which Figure 1 is a cross .section through a closed tapered riser sleeve fitted with a breaker core.

Figure 2 is a section through a tapered open riser sleeve fitted with a breaker core.

Figure 3 is a top plan view of a former. _

Figure 4 is a section along the line A-B of Figure 3.

Figure 5 shows schematically a riser sleeve fitted with a breaker core inserted into a casting mould.

Figure δ shows schematically a riser sleeve without a breaker core inserted into a casting mould.

Figure 7 shows a cavity in a sand mould for the later insertion of a riser sleeve the cavity being formed by a former according to the invention.

Figure 8 is analogous to Figure 7 but shows a riser sleeve fitted with a breaker core inserted into the cavity in the sand mould.

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In Figure 1 a riser sleeve 1 has a closed top 2 and a breaker core 4 fitted by glueing, whose outside diameter is larger than the outside diameter of the riser sleeve 1 . As a result the breaker core 4 exhibits with respect tα the riser sleeve 1 a pro- j ecting rim 4 ' . ~

In Figure 2 a riser sleeve 1a is shown which is open at the top and to which a breaker core 4 can be fitted as is indicated in the representation of Figure 2.

The riser sleeve 1 or 1 a is made from exothermic, exothermic and heat-insulating or heat-insulating material . The breaker cores 4 are made of refractory material .

As can be seen from Figures 1 and 2 the outer surface of the respective riser sleeve has a negative taper from bottom to top having an angle of from 2 to 20 with respect tα the vertical . Moreover the inner and outer surfaces of the tapered riser sleeve are parallel one to another.

The ratio of height to diamter of the riser sleeve 1 or

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1a lies in the range of 1 : 1 to 1.6 : 1 with respect to the lower¬ most inside diameter.

In the example according tα Figure 1 the riser sleeve 1 has a closed top 2 and in this closed top at least one blind recess 11 which reduces the thickness of the top 2 is provided. By means of one or more of such blank recesses a. controlled venting of the riser is possible.

In Figures 3 and 4 a former 5 is shown whereby the left half αf Figure 4 represents a sectioned view of line A-B in Figure 3 and the right half of Figure 4 represents the external view.

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a -

The former of Figures 3 and 4 has a tapered form corresponding to that of the riser sleeves 1 and 1a according to Figures 1 and 2, but, the outisde diameters of the former 5 are larger than the respective mean outside diameters plus- three times the standard deviation of the riser sleeves which are to ^' be used. By this means it is ensured that the former 5 has at any point on its height or length an outside diameter which is larger than the outside diameter of the riser sleeve which after moulding will be inserted or pushed into'the cavity formed by the use of the former 5.

The outer surface of the farmer 5 has distributed around its circumference, severalrecesses 6 which extend from the top of the former towards the bottom. The recesses 6 form ribs from the moulding sand. The depth of the recesses 6 is chosen such that the circles which can be geometrically inscribed inside the base of the recesses have diameters smaller than the corresponding mean outside diameters less three times the standard deviation of the riser sleeves which are to used. In this way the height of the sand ribs formed in the cavity by the recesses 6 is always such that a riser sleeve inserted into the cavity is gripped by the ribs. The riser sleeve is held fast by the sand ribs with suf icient force to withstand rough treatment likely to separate the sleeve from the cavity.

A raised rim 7 is present on top of the former 5 and is dimensioned so that the total height of the former 5 so formed is larger than the mean height plus three times the standard deviation of the riser sleeves which are to be used. By "mean height" of the riser sleeve the mean height of a particular type of riser sleeve is tα be understood. Therefore the former 5 has an added length or is oversize with respect to the length or height of the riser sleeve which is tα be inserted. As a result there remains after the

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insertion of the riser sleeve an empty annular cavity into which sand which is loosened during the insertion of the riser sleeve may fall. This ensures that the rigεr sleeve is always fixed at the desired depth.

An annular depression 7a is present within the rim 7 and is so dimensioned that the height of the former 5 up to the base of the annular depression 7a is smaller than the mean height less three times the standard deviation of the riser sleeves 1, 1a which are to be used. As a result an annular sand pad is formed in the cavity produced by the former 5 and the height of the pad is suff¬ icient for the pad to contact the top of even the shortest or lowest riser sleeve. With this arrangement a seal is achieved especially in the case of open riser sleeves, αr when using capped sleeves which have been vented in the foundry, so that when the sleeves are used and filled with metal, metal cannot flow from above, behind the sleeves.

The annular depression . 7a on its inside adjoins a surface 8 which lies above the level of the annular depression the height of the former tα this sur ace 8 is larger than the mean height plus three times the standard deviaition of the riser sleeves 1 , 1a which are to be used. By "mean height" here again the mean height of a particular type of riser sleeve is to be understood. As a result, after a riser sleeve 1 or 1a has been inserted into the preformed cavity an air gap or a volume of air remains between the top end of the riser sleeve and the surface of the cavity opposite to the sleeve.

Figure 5 is a schematic representation showing a pre¬ formed cavity 9 formed by means of a former 5 according to the invention and in which cavity 9 has been inserted a riser sleeve 1 . It is to be understood that the riser sleeve 1 is in close contact

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on its outer surface with the ribs which are formed on the inner surface of the preformed cavity 9 by means of therecesses 6 in the former. In addition the cap 2 of the riser sleeve 1 is in contact with the annular pad which has been formed by ^* the annular depression 7a in the former 5. Between the outside of the ^" insert- • ed riser sleeve 1 and the wall of the preformed cavity 9 there are several air cavities separated from one another by the vertical ribs but which are all connected one with another by means of the annular air cavity which is formed in the preformed cavity 9 by the rim 7 on the former 5. In addition, an air cavity exists between the cap 2 of the riser sleeve 1 and the opposite end wall of the preformed cavity 9 and is also to be seen in Figure 5.

In the example according to. Figure 5 a riser sleeve 1 with breaker core 4 is used. Since the diameter of the breaker core 4, already described above, is larger than the lowermost outside diameter of the riser sleeve 1 a seal is formed between the projecting rim 4' of the breaker core 4 and, according to Figure 5, the lower end of the wall of the preformed cavity 9 when the riser sleeve 1 is inserted into the preformed cavity 9.

Figure 6 is an analogous view to Figure 5 but where a riser sleeve 1, which has no breaker core, is inserted. In order to achieve a seal between the outside of the riser sleeve 1 and the lower end of the wall of the preformed cavity 9 by the use of such a riser sleeve the outside diameter at the lower end of the former 5 is so narrowly shaped that when a riser sleeve 1 is inserted a sealing contact results between the outside of the riser sleeve and the wall of the preformed cavity 9.

Figure 7 is a schematic view of a section of a sand mould with a preformed cavity 9 in compacted moulding sand 10 and having sand ribs 11 farmed by the recessεsδ in the farmer 5. An annular rib 12 is formed by the rim 7, the cavity 7a and the surface 8 in the

shape of the former 5.

Figure 8 is an analogous view to Figure 7 in which a riser sleeve 1 has been inserted into the preformed cavity _, 9.. It may be seen that to a certain extent during insertion of the riser sleeve • 1 the sand ribs 11 have been compressed or pinched together so that a secure fit is ensured between the outside of the riser sleeve 1 and the ribs 11. 13 indicates that part of the sand ribs 11 with which the riser sleeve does not come into contact. Sand 14 loosened during the insertion of the riser sleeve 1 has fallen 'into the des- cribed annular cavity where it does no harm.

Figure 8 shows once again the seal at the wall of the preformed cavity 9 between the breaker core 4 and the compacted moulding sand 10.

As has been said, the riser sleeves may consist of exothermic, exothermic and heat-insulating or heat-insulating material. Examples of such materials are given below:-

EXAMPLE 1

Exothermic riser sleeves were made having the following composition by weight : - Silica sand 51 .5%

Aluminium grindings 26.0%

Sodium cryolite 5.0%

Clay binder 2.0%

Resin binder 3. 5% Sodium nitrate 12.0%

The materials were mixed with 3-4% by weight water and compacted in core boxes by ramming by hand or using a core shooting machine. The "green" sleeves were stripped from the core boxes and dried at 180 C .

The sleeves had a density of 1 .3 g/αn .

EXAMPLE 2

Exothermic and heat-insulating riser sleeves were made having the following composition by weight:-

Aluminium powder and grindings 24.0%

Boric acid 1 .0%

Sodium cryolite 7.0%

Resin binder 6.9%

Iron oxide 11 .0%

Silica sand 17.0%

Alumina ^* 18.5%

Organic fibres 3.6%

Lightweight silica 11 .0%

A slurry of the materials was produced in water. Formers for the sleeves were iπrmersed in the slurry and the solid materials in the slurry were sucked on to the formers by means of pressure. The formers were removed and the "green" riser sleeves were dewatered.

The sleeves were then stripped from the formers and dried at 180 C.

3 The sleeves had a density of 0.85 g/cm .

EXAMPLE 3

Heat-insulating riser sleeves were rrade having the following composition by weight :-

Aluminium 5.5%

Alumina - 13.0%

Resin binder 9.0%

Aluminium sulphate 1.0%

Colloidal silica sol 6.5°

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Aluminium silicate fibres 6.0%

Silica fibres 55.0%

Organic fibres 4.0%

The sleeves were produced using the method described ^" in Example 2.

3 The sleeves had a density of 0.45 g/cm .

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