WO1998017449A1

WO1998017449A1 - Continuous production of green ware articles

Info

Publication number: WO1998017449A1
Application number: PCT/AU1997/000689
Authority: WO
Inventors: Neil George House
Original assignee: Caroma Industries Limited
Priority date: 1996-10-17
Filing date: 1997-10-15
Publication date: 1998-04-30
Also published as: AUPO304896A0

Abstract

A method and apparatus for the continuous production of green ware articles, e.g. lavatory pans (20), from slip are disclosed. Plaster of paris moulds (10) at least partially reinforced with acrylic, fibreglass, polypropylene or carbon fibres pass through first and second microwave drying ovens (8, 16) on a conveyor system (2). In the first oven (8) a green ware article contained in each mould (10) is at least partially dried. In the second oven (16) the empty re-assembled mould (10) is dried prior to being refilled with slip. Also disclosed are a method of fabricating the reinforced moulds and a self-centering clamping cradle with opposed arms hinged at the base, and a releasable latch, for transporting and holding the mould. The cradle is tiltable.

Description

CONTINUOUS PRODUCTION OF GREEN WARE ARTICLES

The present invention relates to the continuous production of at least partially dry green ware articles, such as sanitary ware articles, from slip. The invention also relates to a method of fabricating moulds for use in the production, the products of the methods, a self-centring clamping cradle for holding slipware moulds, and a method of clamping.

BACKGROUND ART

Articles made from slip include sanitary ware articles such as toilet pans, basins, cisterns (or flush tanks as the devices are known in the U.S.A.), bidets and the like and tableware articles including plates, dishes and so forth. Traditionally these ware articles are made in a batch process in which liquid slip comprising essentially a mixture of clay and water is poured into a plaster of paris (gypsum) mould which absorbs moisture from the slip and thereby forms a layer of formed-up clay against the side of the mould. After a predetermined amount of time, which can be from 20 mins to, say, two hours, the remaining liquid slip is drained from the mould to leave the "green" article formed against the mould surfaces. When this green article is sufficiently dry, and therefore sufficiently strong to handle and be self-supporting without distorting, the article is removed from the mould by dis-assembly of the mould parts. The mould is then allowed to dry sufficiently to be ready for use on the next occasion. Conventionally this drying is by means of evaporation of moisture from the mould over a drying period which may be as long as 24 hours.

Because of the lengthy time periods involved, considerable space on the factory floor is occupied by partially dried moulds. Also the rate at which the mould dries is very much dependent upon ambient conditions and therefore there is considerable variation between summer and winter, and particularly during rainy and humid periods.

It has long been known that microwaves permit many articles to be quickly dried and in theory the plaster of paris moulds used in the production of slipware articles could be dried quickly by means of microwaves. Indeed, this is disclosed in U.S. Patent No. 4,292,262 (Tobin). However, despite the disclosure of this patent (which was filed in 1979), in practice microwave drying of slipware moulds has not gained commercial acceptance. The basic reason for this is thought to be the effect of microwaves on the plaster of paris moulds themselves. In the present applicant's experience, attempts to rapidly dry plaster of paris moulds repeatedly by use of microwave energy result in the moulds crumbling or cracking after only a few cycles. This crumbling is thought to be due to the removal of chemically bound water from within the plaster of paris when the mould reaches a low moisture content at hot spots or hot surfaces e.g. sharp corners exposed to high evaporation rates. In addition the mould itself crumbles due to the presence of cracks. All in all, it is a false economy to achieve rapid repeated drying of the mould itself, if the consequence is that the mould life is dramatically shortened.

Over the years, many attempts have been made to improve the nature of the basic plaster of paris mould. The basic plaster of paris mould suffers from a number of disadvantages including that it is not particularly strong mechanically, it is relatively heavy, and the life of the mould is relatively short, typically being less than 50 to 100 consecutive moulds for large sanitary ware articles. Better quality (and thus more expensive) gypsum results in mould lives at the longer end of the range of mould lives. The main advantage of plaster of paris is that it is inexpensive and abundant.

Furthermore, another factor which has a bearing on mould life is the quality of the clay used in the slip. The cheapest clays cost in the vicinity of A$ 30-35/tonne which essentially reflects the cost of digging up and transporting the clay. Such "unbeneficated" clays leave a brown gum like deposit on the outside of conventional plaster of paris moulds. This deposit is thought to be due to various salts present in the clay which are dissolved into solution when the slip is formed. However, these salts remain on the outer surfaces of the mould as the mould dries and progressively coat the outer surface of the mould. This coating substantially reduces mould efficiency and shortens the mould life. Stratagems such as cutting back the exterior of the mould with an angle grinder, or the like, are known, however, these are generally only of short term benefit and are generally not cost efficient.

It is also known to overcome this problem by using various treatments to beneficate the clay in order to remove such salts and impurities. Typically such beneficiation involves washing and press filtering the clay. Whilst this substantially lessens the problem of short mould life, the cost of the clay is substantially increased to something approaching A$ 1000/tonne.

Because of this very substantial cost increase in the raw material from which green ware articles are made, the present invention seeks to avoid the need for benefication and instead uses unbeneficated clays in its slip.

Various attempts over the years have also been made to increase the ability of the mould the absorb moisture and thereby reduce the amount of time required for the slip to be in the mould. U.S. Patent 4,307,876 (Shannon/Owens-Corning Fiberglass

Corporation) discloses the use of chopped fibreglass filaments in a plaster of paris mould in order to increase the ability of the mould to absorb water.

Similarly, other attempts have been made to try and increase the mechanical strength of moulds. U.S. Patent No. 4,552,329 (Hattori/Noritake KK) discloses the use of chopped carbon fibres (including those made from poly aery lonitrile, rayon or lignin- poval materials) in order to increase the mechanical strength of the gypsum or plaster of paris mould. In this disclosure the moulds are dried for up to three to seven days at

50 °C in a hot air dryers.

DISCLOSURE OF THE INVENTION

Somewhat surprisingly, the present inventor has discovered that the addition of chopped fibres to plaster of paris and like material moulds can overcome the tendency of such mould to crumble when subjected to microwave drying. This is thought to be due to the presence of the fibres terminating small cracks at the location of the fibres, rather than allowing the cracks to propagate. OBJECT OF THE INVENTION

The object of the present invention is to provide a method of continuous production of green ware articles from slip, which method utilises microwave drying so as to enable the time of production to be substantially reduced and thereby enable continuous production methods to be employed. A further object is to provide a mould and a method of fabricating same which enables the mould to withstand the rigours of microwave drying. A still further object is to provide apparatus to enable the above production to be achieved.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there is disclosed a method of continuous production of at least partially dry green ware articles from slip, said method comprising the steps of:

(1) preparing a plurality of multi-part moulds from plaster of paris or equivalent material which is at least partially reinforced with chopped fibres;

(2) placing the moulds on a closed loop conveyor system having a plurality of stations and first and second microwave ovens through which said conveyor passes;

(3) filling the moulds with slip at a filling station; (4) conveying the slip filled moulds to a draining station whilst allowing the moulds to absorb water from the slip and thereby cast a green ware article;

(5) draining excess slip from the moulds at the draining station;

(6) at least partially drying the moulds and included green ware articles in said first microwave oven; (7) dis-assembling the moulds to release the at least partially dried green ware article therefrom;

(8) re-assembling the moulds;

(9) at least partially drying the assembled moulds in said second microwave oven; (10) conveying the at least partially dried moulds to said filing station; and

(11) repeating steps (3) - (10).

In accordance with a second aspect of the present invention there is disclosed a method of fabricating moulds for use in the continuous production of at least partially dry green ware articles from slip utilizing microwave drying of the moulds and/or moulds and cast green article, said method comprising the steps of:

(a) forming a slurry of plaster-of-paris or equivalent material;

(b) combining the slurry with a plurality of fibres formed into strands;

(c) agitating the slurry to disintegrate the strands and disperse the fibres whilst maintaining the mixture under reduced pressure to reduce the entrapment of gas within the agitated slurry, and

(d) fabricating the mould in conventional fashion from the de-gassed agitated slurry containing chopped dispersed individual fibres.

Preferably the inner surfaces of the mould are fabricated from fibre free plaster of paris or equivalent material.

In accordance with a third aspect of the present invention there is disclosed a self-centring clamping cradle for holding slip ware moulds, said cradle comprising a base, a pair of sides each pivoted to said base in opposed fashion to define a set of jaws formed from said paid of sides, a like plurality of contact projections extending from each said side and facing the corresponding contact projection of the other side, and a releasable latch means interconnecting said sides, whereby with said latch means released a mould having at least two parts can be placed on, or lifted from, said base, and with said latch latched said sides simultaneously clamp said mould parts together by clamping forces applied by said contact protrusions and centre said clamped mould between said sides.

According to a still further apsect of the present invention there is disclosed apparatus for the continuous production of at least partially dry green ware articles from slip, said apparatus comprising:

(a) a closed loop conveyor system having a plurality of stations, (b) first and second microwave drying ovens through which said conveyor passes and each including steam extraction apparatus,

(c) a slip filling station located alongside said conveyor between said ovens and downstream of said second oven , (d) an excess slip draining station located in said conveyor system downstream of said slip filling station and upstream of said first oven, and

(e) a mould dis-assembly and re-assembly station located in said conveyor system located between said ovens and downstream of said first oven, whereby empty re-assembled moulds pass through said second oven to be dried prior to being filled with slip at said slip filling station, slip within said moulds consolidates to form a green ware article before each mould reaches said excess slip draining station, said green ware article and mould are at least partially dried in said first oven, and said at least partially dried green ware article is released from said mould at said mould dis-assembly and re-assembly station. Also disclosed are slipware articles, and moulds made in accordance with the previous methods. A method of clamping and self-centring a mould is also disclosed.

Preferably the size of the fibres is from 2 to 12 mm in length (preferably 6 mm in length) and from 10 to 20 microns in diameter (and preferably 11 microns in diameter). The preferred fibres are acrylic fibres (a polymer formed from at least 85% acrylonitrile) manufactured by HOECHST and used as precision and random cut flock.

Since such chopped fibres are of very light weight, they are capable of being inhaled or ingested by personnel who have to work with the fibres. Such fibres are manufactured and supplied in bundles of individual filaments called strands and in this condition are safe. Therefore in accordance with an aspect of the present invention the strands (bundles of fibres) are entrained in the slurry of plaster of paris from which the mould is to be formed and the slurry is agitated under reduced pressure or a vacuum so as to disintegrate the strands and disperse the filaments whilst covered by the slurry. The presence of a vacuum, or partial vacuum, above the agitated slurry ensures that air is not entrained during the agitation process and that any entrained air in the slurry is reduced as much as possible. The consequence is fewer voids or bubbles formed in the resultant mould.

A consequence of adding the filaments to the slurry is that its viscosity is increased. This has the effect of making the slurry hard to pour to form the mould. This disadvantage can be to some extent overcome by adding a wetting agent such as DURASAR (trade name) manufactured by Hardy Chemicals Ltd of 745, Ste-Rose, La Prarie, Quebec, Canada J5R-IZ2. This substance is a calcium salt of napthalene sulphonate polymerised with formaldehyde and is mainly used as an ingredient in wet cement or concrete to improve workability.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention will now be described with reference to the drawings in which:

Fig. 1 is a perspective view from above of the production line apparatus for continuous production of green slipware articles;

Fig. 2 is a block diagram illustrating the sequence of operations in the production methods;

Fig. 3 is an exploded perspective view of the four-part plaster of paris mould used to create a lavatory pan; Fig. 4 is a perspective view a clamping and self-centring cradle for the mould of Fig. 3;

Fig. 5 is a cross-section view along the line V-V of Fig. 1 and showing the filling station at which moulds are filled with slip;

Fig. 6 is a cross-sectional view through a mould showing it filled with slip; Fig. 7 is a view similar to Fig. 6 but showing the slip being initially drained from the mould;

Fig. 8 is a side elevational schematic view of the drain opening and closing apparatus; Fig. 9 is a side elevational view illustrating the tilting of the cradle so as to drain slip from the mould;

Fig. 10 is a view similar to Fig. 7, but illustrating the complete drainage of slip from the mould; Fig. 11 is a view similar to Fig. 10, but illustrating the removal of unrequired slip;

Fig. 12 is a transverse cross-sectional view through the first microwave oven showing the drying of the cast-up article within the mould;

Fig. 13 is a side elevational view of the clamped mould; Fig. 14 is an end elevational view of the clamped mould being released prior to removal from the cradle;

Fig. 15 is a view similar to Fig. 14, but illustrating the full retraction of the sides of the cradle;

Fig. 16 is an end view, partly in cross-section, illustrating the removal of the mould from the foot of the mould;

Fig. 17 is a view similar to Fig. 16, but illustrating the removal of the top of the mould;

Fig. 18 is a plan view illustrating the first step of the removal of the sides of the mould; Fig. 19 is a plan view illustrating the final de-moulding stages;

Fig. 20 is a view similar to Fig. 14, but illustrating the completion of the reassembly of the mould following reversal of the previous de-assembly steps;

Fig. 21 is a transverse cross-sectional view through the second microwave oven, Fig. 22 is a cross-section view through a vacuum agitator in which the slurry and fibres are mixed to form the mixture from which the mould is produced, and

Fig. 23 is a schematic cross-sectional view through a portion of a mould of a further embodiment. DETAILED DESCRIPTION

Turning now to Fig. 1 and 2, the general arrangement of a continuous production system 1 for lavatory pan production is illustrated. The system 1 consists essentially of a closed loop conveyor 2 which includes a number of stations or sections.

5 The first station is a fill station 3 where plaster of paris moulds are filled with liquid slip. The next section is a consolidation section 4 where the slip forms into a cast and the cast consolidates. The next section is a drain section 5 where excess slip is drained from the tilted mould. Next a removal station 6, including a robot 7, is arranged to remove unwanted cast material and a rubber ring from the lavatory pan 20. ι o Next the mould 10 is passed through a first microwave oven 8 which includes a steam extraction apparatus 9 (not illustrated in Fig. 1 for clarity). After the oven 8, the mould 10 is undamped at unclamping station 11 and the mould 10 is dis-assembled by a robot 12 at a mould dis-assembly re-assembly station 13. The cast article itself (a lavatory pan 20 in the preferred embodiment) is moved to a fettling table 14.

15 The next station is a re-loading station 15 where the re-assembled mould parts are placed back onto the conveyor 2 before being conveyed to a drying oven 16 which includes steam extraction apparatus 19 (also not illustrated in Fig. 1), and leads onto a weighing station 17 and a mould conditioning section 18.

An outlet closing station 22 immediately before the fill station 3 (and

20 corresponding to an outlet opening station 23 immediately before the drain station 5) completes the loop of the conveyer 2.

It should be understood in relation to Fig. 1 that each element of the conveyor 2 normally carries a cradle 34 which is omitted in Fig. 1 in order not to clutter the drawing.

25 Each of the ovens 8,16 is essentially tabular with a screen door 25 at each end of the oven. The conveyor 2 consists of a number of pallets 26 and is arranged to move in steps separated by a dwell time. At each step of the conveyor 2, the doors 25 are raised and two pallets 26 move into each of the ovens 8 and 16 whilst a corresponding two pallets 26 move out of each oven 8 and 16. The dwell time is typically 2 minutes and a complete cycle is approximately 3 hours during which a particular pallet 26 returns to its original start position. The microwave ovens 8,16 are each provided with a magnetron and an associated waveguide (both substantially conventional and not illustrated) which supply microwave radiation to each oven 8,16. On each occasion when the doors 25 are to be opened, the magnetron is switched off and is not switched back on again until the doors 25 are closed. This prevents the escape of any microwave radiation. The pallet movement and indexing mechanisms for the conveyor 2 are essentially conventional and are not illustrated in detail.

Turning now to Fig. 3, the details of the mould 10 are illustrated. It will be seen that the mould 10 takes the form of a foot 27, side walls 28 and 29, and an open top 30. The mould 10 is also provided with an elastomeric rim ring 31 and outlet ring 32. The rim ring 31 is installed on and removed from, the mould 10 in the production process. The outlet ring 32 remains in place throughout. The mould 10 also has an outlet 42 and an inlet 43. The inlet 43 is able to be blocked by means of a chock 69. Fig. 4 illustrates a cradle 34 having a base 35 which is supported by, and is movable with, a corresponding one of the pallets 26. Pivoted on the base 35 are a pair of sides 36,37 each of which include projection arms 38 having an elastomeric tip 39 which comes into contact with the side walls 28,29 of the mould 10. All pivoting metal surfaces are bushed by nylon sleeves in order to avoid any close metal-to-metal air gaps which might cause arcing in the microwave ovens. Similarly, sharp metal corners or edges are avoided. The sides 36,37 are maintained in the position illustrated in Fig. 4 by means of a pair of latches 40. The full operation of the cradle 34 will be described hereafter particularly in relation to Figs. 14 and 15. For the present it is sufficient that the sides 36,37 in the latched position illustrated in Fig. 4 result in the tips 39 clamping the side walls 28,29 of the mould 10 tightly together so as to form a substantially slip impervious seal between the two side walls of the mould 10.

In Fig. 5, the right hand mould 10 is located at the fill station 3 whilst the left hand mould 10 is located within the consolidation section 4. The fill station 3 includes a pair of flexible slip hoses 45 supplied from a thermostatically controlled heated slip tank 59 (Fig. 2) and each mounted on a corresponding hydraulically operated arm 46. Each hose 45 also has a sensor unit 47 adjacent the hose nozzle 48. By operation of the arm 46, the nozzle 48 can be inserted through the open top 30 of the mould 10 and the mould 10 thereby filled with slip 53 (Fig. 6). The level of slip 53 in the mould 10 is detected by conventional level sensors within the sensor unit 47 and at the completion of the filling process the arm 46 is raised so that the nozzle 48 clears the top of the mould 10. The two hoses 45 simultaneously fill two moulds 10. After filling the moulds 10 are moved by the conveyor 2 into the consolidation section 4.

The fill station 3 is mounted on wheels 44 and is therefore movable up and down along the conveyor 2. In this way the casting up time is adjustable either side of 44 minutes to suit the type of the article being cast.

Also illustrated in Fig. 5 is a preferment in the form of a stationary cover board 51 which is positioned over the open top 30 of the mould 10 at the cast consolidation station 4. The effect of the cover board 51 is to reduce evaporation of the slip within the mould 10 and hence reduce any skinning on the upper surface of the slip. Any appreciable skin which forms may create an undesirable blemish on the interior of the cast article during the slip draining procedure.

The consolidation of slip within the mould 10 is illustrated in Fig. 6 (the cradle 34 not being illustrated). Here it will be seen that each pallet 26 of the conveyor 2 slides on nylon bushes 49 located in side channels 50 of the conveyor 2.

It will also be seen in Fig. 6 that the outlet 42 of the mould 10 is closed by means of a valve plate 52. As a consequence, the interior of the mould 10 is filled with slip 53 and a layer of deposited slip 55 builds up on the porous interior surfaces of the mould 10. In view of the interior shape of the mould 10, the deposited slip 55 adopts the desired shape of the toilet pan 20.

After the moulds 10 have moved through the consolidation section 4, the valve plate 52 is slowly and progressively opened by means of the mechanism illustrated in Fig. 8 which comprises a pair of pivoted crank arms 56 operated by an hydraulic ram 57. As a consequence, the outlets 42 of a pair of moulds 10 are slowly opened and the slip 53 slowly drains from the moulds 10 into a slip drain 58 which leads back to a slip holding tank 59 (Fig. 2 only).

This gives rise to the situation illustrated in Fig. 7 where the outlet 42 is opened but some slip 53 remains in what will become the water seal area of the pan 20. In order to remove the remaining slip 53, it is necessary to tilt the cradle 34 and mould 10 as illustrated in Figs. 9 and 10. It will be seen that the bed 60 of the conveyor 2 is progressively tilted by means of a hydraulic cylinder 62. As a consequence, the remaining slip 53 within the green pan 20 formed by the deposited slip 55 drains slowly from the pan 20. In this way any marks caused by runs of undeposited slip over the deposited slip 55 are avoided.

The preferred sequence of events is to open the valve plates 52 by about 10% for I i minutes. Then the valve plates 52 are fully opened whilst the mould 10 is tilted by 60° in the remaining 30 seconds. The mould is then held tilted for a further 10 steps or 20 minutes. The procedures to be carried out at the end of the drain station 5 are illustrated in Fig. 11. Here the robot 7 which is provided with four pivoted fingers 65 each carrying a wedge-shaped scoop 66, is actuated so that the scoops 66 grab and remove a collar 67 of deposited slip formed in the open top 30 of the mould 10. Immediately following this operation the rim ring 31 is manually removed so as to leave a clean lower edge on the inside rim of the toilet pan 20 which at this stage is still in a very green and soft condition. The mould 10 is then returned to the horizontal position.

The next stage in the production process is the microwave setting (or accelerated drying) of the green deposited slip 55 within the mould 10. This is done by moving the mould 10 together with its green deposited slip 55 into the first microwave oven 8 as illustrated in Fig. 12. The mould 10 remains within the oven 8 and a waveguide 71 delivers microwave radiation (typically 20-30 kW at 922MHz) into the interior of the oven 8. This generates a substantial amount of moisture as the water within the deposited slip 55 (and to a lesser extent within the mould 10 itself) is heated and expelled. Within the oven 8 is a plastics cowling 72 which is connected to an exhaust 73 so that moisture generated within the oven 8 is extracted by means of the exhaust 73 and a fan (not illustrated). Within the oven 8 in order to provide a desired distribution of microwave energy, there are metallic screens (schematically illustrated at 75) which are positioned in order to reflect and deflect the microwave energy into the desired areas.

At the end of the consolidation section 4, the moisture content of the green deposited slip 55 is approximately 25% and by the end of the drain section 5 this moisture content has been reduced to 21 % . However, at the end of the first microwave oven 8, the moisture content of the deposited slip 55 has been reduced to approximately 18% and at this stage the deposited slip 55 is sufficiently strong for the pan 20 to be self supporting and to be demoulded. Any further drying at this stage would shrink the greenware over any male parts of the mould and make de-moulding difficult. The mould as it exits the oven 8 is as illustrated in Fig. 13.

Next (as illustrated in Fig. 14) a pneumatic ram 77 and pivoted crank 78 co- operate to release the latches 40 which maintain the sides 36 and 37 of the cradle in the clamping position. This is done just as a hydraulic ram 79 is momentarily moved up so as to take the load off the latch 40. Then the hydraulic ram 79 is operated downwardly so as to pivot the sides 36,37 outwardly as seen in Fig. 14 thereby releasing the clamping effect of the cradle 34. This leads to the position illustrated in Fig. 15 where the sides 36,37 have moved away (as indicated by broken lines) and thus the side walls 28,29 and top 30 of the mould 10 are resting on the foot 27 of the mould.

As illustrated in Fig. 16, the second robot 12 then operates a demoulding jaws 80 which consists of a set of pneumatically operated pivoted linkages and including two inclined plates 82,83. These enable the side walls 28,29 and the open top 30 together with the included deposited slip or pan 20, to be removed from the foot 27. These are then carried by the robot 12 and deposited on the dis-assembly and re-assembly station 13 (Fig. 1). Next the demoulding jaws 80 are opened and raised slightly as illustrated in Fig. 17 so as to permit a pair of opposed bosses 85 to grasp the opposite edges of the open top 30 and lift same from the side walls 28,29. The top 30 is then placed at the station 13 so as to be ready for re-use.

Located at the dis-assembly and re-assembly station 13 are side jaws 87 which, as seen in Figs. 18 and 19 again consist of opposed pairs of pneumatically powered

5 linkages. The side jaws 87 are operated to grasp opposite ends of each of the side walls 28 and 29. With the side walls 28 and 29 thereby held, a cylindrical post (not illustrated) is inserted upwardly from below into the downwardly directed, and open, outlet 42 of the pan 20. This locates the pan on a small conveyor which leads to a fettling table 14. In addition the chock 69 is removed from the inlet 43 of the pan. ι o Since the pan 20 is located by the cylindrical post, the jaws 87 are moved outwardly as seen in Fig. 19 to de-mould the pan 20. The cylindrical post is removed and the pan 20 conveyed to the fettling table 14. Meanwhile the jaws 87 are moved inwardly to re-unite the side walls 28 and 29. The chock 69 is then replaced. The robot 12 replaces the top 30 (onto which another rim ring 31 has been previously

15 manually installed), the jaws 87 are retracted, and the robot uses jaws 80 to lift the reassembled side walls 28,29, top 30 and chock 69 and return same to the re-loading station 15 where the foot 27 awaits. The foot 27 has been spayed with a 1 :6 mixture of talc and water as a release agent prior to the side walls 28,29 and top 30 being re-united with the foot 27. Then the hydraulic ram 79 is reactivated as illustrated in Fig. 20 so

20 as to re-clamp the re-assembled mould 10 in the cradle 34. This leads to the position illustrated in Fig. 20. That is to say, all the demoulding steps have been reversed except that the mould 10 is now empty. Also the demoulded pan stands freely on the fettling table 14 where it receives attention from an operator who, with a moist cloth, is able to brush over any minor blemish, or the like.

25 As indicated in Figs. 1 and 2, the clamped re-assembled mould is then moved from the re-loading station 15 into the drying oven 16. It is at this position on the conveyor 2 that the moulds are initially supplied and, if required, moulds can be removed for replacement and/or repair. Continued operation of the conveyor 2 then introduces the re-assembled clamped mould 10 into the drying oven 16 where it is again subjected to microwave energy from a waveguide 71. Again metallic screen(s) 75 are used to deflect the heating effect of the microwave energy. The screen 75 is mounted in a split tube 76 of TEFLON to prevent arcing. A cowling 72 and exhaust 73 constitute the steam extraction apparatus 19 to permit extraction of steam generated from water which is evaporated by heating of the water within the mould 10.

As indicated in Figs. 1 and 2, after exiting from the drying oven 16, a weighing station 17 is provided where each mould 10 can be weighed to ensure that a sufficient amount of water has been extracted from the mould 10 to permit its re-use. Also any new mould which is heavy can be identified and not filled at the fill station. After a single cycle when empty such a new mould will be sufficiently dry to commence production. After leaving the weighing station 17, the moulds 10 continue through a mould conditioning section 18 where further steam extraction is applied by means of the steam extraction apparatus 19 which is essentially similar to the cowling 72 and exhaust 73 illustrated within the microwave oven but instead are located above the conveyor 2. In this connection, the open outlet 42 and open top 30 permit a continuous stream of air to pass through the mould whilst drying. A similar situation applies during the draining and first oven stages. Finally an outlet closing station 22 is reached which is essentially identical to the outlet opening station 23 of the drain station 5, but instead operates so as to close the valve plate 52 of Fig. 8.

The entire procedure as described above is then repeated with re-filling of the moulds 10 at the fill station 3. With one or two minor exceptions where manual intervention is required, the entire process is controlled by control gear located within cabinets 89 (Fig. 1).

The production of the mould 10 is made possible by means of the mixing apparatus 90 illustrated in Fig. 22. This apparatus consists of a hopper 91 having a sealed lid 92 which carries an electric motor 93 which drives an impeller 94. An outlet 95 leads to a vacuum pump (not illustrated). In order to make the necessary slurry of plaster-of-paris from which the parts of the mould 10 are themselves moulded, a slurry of plaster-of-paris is poured into the hopper 91. The strands of long filaments selected from the group consisting of fibreglass, acrylic fibres, polypropylene or carbon fibres are then also placed into the

5 hopper 91 and the lid 92 sealed. Then the electric motor 93 is energised and the vacuum simultaneously applied to the outlet 95. As a consequence, the impeller 94 chops the long strands into short lengths but without the entrainment of air into the plaster of paris slurry. Further, the continued application of the vacuum to the plaster of paris slurry has the effect of de-gassing the slurry and thereby substantially reducing ι o the number of small bubbles or voids formed in the resultant mould. The preferred diameter for the fibres is preferably 11 microns within a range of from 10 to 20 microns and the preferred lengths of the chopped fibres is 6mm within a range of from 2 mm to 12 mm. Typically the percentage of fibre by dry plaster weight in the slurry is approximately 0.5 % .

15 In another embodiment, the mould 10 is fabricated, preferably simultaneously, from two layers which preferably form into an integral whole. This is as illustrated in Fig. 23 where the inner layer 100 is free of any fibrous or other reinforcement and the outer layer 101 is reinforced. This arrangement provides the advantage that the mould 10 can wear on its interior surface during the course of its mould life without any of the

20 fibres reaching the interior mould surface. Experience with fibre reinforced single layer moulds indicates that towards the end of the mould life such fibres do become exposed resulting in a pattern of small markings appearing on the moulded article. Whilst these can be easily sponged away with a damp cloth, it is advantageous not to have to perform this manual finishing step.

25 Other embodiments include reinforcing the plaster of paris mould 10 with scrim (such as woven glass fibres, woven polypropylene fibres, and the like) and reinforcing only the outer layer 101 with scrim or glass fibres. In addition, in place of the regular plaster of paris, micro porous plastic moulding materials (known per se) can also be used. The moulds 10 fabricated in accordance with the above described arrangements are able to withstand the application of microwave energy and increase the typical prior art (non-microwave) moulding life from approximately 60 mouldings to approximately 80-120 mouldings with the same raw material for the moulds. This increase in mould life, coupled with the very substantial reduction in cycle time from 12 hours to 2 or 3 hours results in a substantial increase in productivity as well as substantial savings in floor space.

Furthermore, the limited experimental experience to date indicates that the exterior surface of the moulds 10 do not become covered with the brown gum-like deposits of the prior art moulds. This suggests that the water is expelled from the mould vigorously in the form of small droplets of water rather than water vapour (or steam). As a consequence the salts present in the clay and slip are thought to remain in solution and are carried from the mould by these water droplets, rather than being left behind as a product of evaporation. In any event, the major determining factor regarding mould life is now thought to be the gradual increase in the size of the mould interior with repeated use.

The foregoing describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention.

Claims

CLAIMS :

1. A method of continuous production of at least partially dry green ware articles from slip, said method comprising the steps of:

(1) preparing a plurality of multi-part moulds from plaster of paris or equivalent material which is at least partially reinforced with chopped fibres; 5 (2) placing the moulds on a closed loop conveyor system having a plurality of stations and first and second microwave ovens through which said conveyor passes;

(3) filling the moulds with slip at a filling station;

(4) conveying the slip filled moulds to a draining station whilst allowing ι o the moulds to absorb water from the slip and thereby cast a green ware article;

(5) draining excess slip from the moulds at the draining station;

(6) at least partially drying the moulds and included green ware articles in said first microwave oven;

(7) dis-assembling the moulds to release the at least partially dried green 15 ware article therefrom;

(8) re-assembling the moulds;

(9) at least partially drying the assembled moulds in said second microwave oven;

(10) conveying the at least partially dried moulds to said filing station; and 20 (11) repeating steps (3) - (10).

2. The method as claimed in claim 1 including the step of extracting released moisture from said microwave ovens.

3. The method as claimed in claim 1 or 2 wherein said draining is carried out by tilting the moulds.

25 4. The method as claimed in claim 1 , 2 or 3 wherein the fill station is movable relative to the conveyor to permit the time of water absorption by the moulds to be adjusted.

5. The method as claimed in any one of claims 1-4 including the step of providing a continuous air flow path through the moulds.

6. The method as claimed in any one of claims 1-5 including the step of not reinforcing the interior surface of said mould with said chopped fibres.

7. A method of fabricating moulds for use in the continuous production of at least partially dry green ware articles from slip utilizing microwave drying of the

5 moulds and/or moulds and cast green article, said method comprising the steps of:

(a) forming a slurry of plaster-of-paris or equivalent material;

(b) combining the slurry with a plurality of fibres formed into strands;

(c) agitating the slurry to disintegrate the strands and disperse the fibres whilst maintaining the mixture under reduced pressure to reduce the entrapment of gas ι o within the agitated slurry, and

8. The method as claimed in claim 7 wherein the interior surface of said mould is formed from plaster of paris or equivalent material which is not reinforced

15 with said fibres.

9. A method as claimed in claim 7 or 8 wherein the strands of fibres are immersed in a pre-determined volume of water prior to combining with the slurry.

10. A method as claimed in claim 7 8 or 9 wherein the fibres are selected from acrylic, fibreglass, polypropylene or carbon fibres.

20 11. A method as claimed in claim 7, 8, 9 or 10 wherein the wetting agent

DURASAR (a calcium salt of napthalene sulfonate polymer) is included in the slurry.

12. A self centring clamping cradle for holding slip ware moulds, said cradle comprising a base, a pair of sides each pivoted to said base in opposed fashion to define a set of jaws formed from said pair of sides, a like plurality of contact

25 projections extending from each said side and facing the corresponding contact projection of the other side, and a releasable latch means interconnecting said sides, whereby with said latch means released a mould having at least two parts can be placed on, or lifted from, said base, and with said latch latched said sides simultaneously clamp said mould parts together by clamping forces applied by said contact protrusions and centre said clamped mould between said sides.

13. A cradle as claimed in claim 12 wherein said base is mounted for movement in a first direction as part of a conveyor system, and is tiltable in a second

5 direction substantially transverse to said first direction to permit slip to be drained from said mould.

14. A cradle as claimed in claim 13 and including a staged drain cover release and closure mechanism.

15. A method for continuous production of at least partially dry green o ware articles from slip, said method being substantially as described with reference to

Figs. 1-21 or Figs. 1-21 and 23 of the drawings.

16. A method of fabricating moulds for use in the continuous production of at least partially dry green ware articles from slip, said method being substantially as described with reference to Fig. 22 or 23 of the drawings. 5

17. An at least partially dry green slip ware article produced by the method claimed in any one of claims 1-6 and 15.

18. A mould produced by the method claimed in any one of claims 7-11 and 16.

19. A method of clamping and self centring a mould utilizing the o apparatus claimed in any one of claims 12-14.

20. Apparatus for the continuous production of at least partially dry green ware articles from slip, said apparatus comprising:

(a) a closed loop conveyor system having a plurality of stations,

(b) first and second microwave drying ovens through which said conveyor 5 passes and each including steam extraction apparatus,

(c) a slip filling station located alongside said conveyor between said ovens and downstream of said second oven ,

(d) an excess slip draining station located in said conveyor system downstream of said slip filling station and upstream of said first oven, and (e) a mould dis-assembly and re-assembly station located in said conveyor system located between said ovens and downstream of said first oven, whereby empty re-assembled moulds pass through said second oven to be dried prior to being filled with slip at said slip filling station, slip within said moulds consolidates to form a green ware article before each mould reaches said excess slip draining station, said green ware article and mould are at least partially dried in said first oven, and said at least partially dried green ware article is released from said mould at said mould dis-assembly and re-assembly station.

21. The apparatus as claimed in claim 20 wherein said slip filling station includes adjustment means to alter the conveyed distance between said slip filling station and said excess slip draining station.

22. The apparatus as claimed in claim 20 or 21 wherein said excess slip draining station includes means to progressively open a drain outlet and means to tilt said mould.

23. The apparatus as claimed in claim 20, 21 or 23 wherein a mould weighing station is located downstream of said second oven and upstream of said slip filling station.

24. Apparatus for the continuous production of at least partially dry green ware articles from slip said apparatus being substantially as described with reference to Figs. 1-21 of the drawings.