TILE MANUFACTURE

Abstract: Apparatus for preparing a pressed green product for kilnfiring in tile manufacture, in which a mixture (10) of clay and inert material such as basalt are fed to an air-entraining device (12) for filling a die (20) while the die cavity is in part defined by a false die member (26) having an exaggerated contour, the false die member being replaced prior to placing the die in a press (32) by a die member (31) contoured in accordance with the pressed product.

Description: TILE MANUFACTURE TECHNICAL FIELD This invention relates to the manufacture of articles such as roof tiles and other kiln-fired products manufactured using clay-based materials comprised of clay and inert material such as basalt. BACKGROUND ART The manufacture of roof tiles from such materials has been proposed in CSIRO Australian Patent 428,006, but until now no technique for the manufacture of a satisfactory commercial product has been found. SUMMARY OF THE INVENTION In accordance with the present invention, a method of manufacturing kiln-fired articles wherein a mixture of dried and milled clay and particulate substantially inert material which exhibits substantially no quartz inversion is presssed in a die and subsequently fired, is characterised in that the mixture, with a free water content chosen to provide correct plasticity of the mix, is introduced into the cavity of the die by air-entrained blowing. The invention finds particular application in the manufacture of articles having a non-uniform thickness in the direction of pressing, typically roof tiles, where the method is further characterised in that the shape of the article prior to pressing is defined by mould means into which the mixture is blown, the mould means providing at those points where the pressed product is to have greater thickness than the minimum thickness of the product, a thickness which exceeds the final thickness of the product by an amount which increases in proportion to the extent by which the thickness of the pressed product at that point is to exceed that minimum thickness. Such a technique overcomes the problems experienced in the prior art, particularly in the manufacture of roof tiles, where the thickness may vary significantly across the width of the tile. While it is less difficult to manufacture a tile of regular cross section by pressing and then firing, it has in the past been impossible in a production environment to produce by this method contoured tiles having a non-constant thickness in the direction of pressing without damage or weakness. This problem has arisen, it is believed, because of the lack of uniform density in the pressed tile, due to the relative lack of compression of the material in the thicker parts of the tile. In the preferred method of performance of the invention, the mould means comprises a die provided with a false die member, and the method of the invention further comprises the steps of removing the false member after the die has been filled, replacing the false member with a die member conforming to the contour of the pressed product, and pressing the mixture to form the pressed product. The combination of air-entrained filling of the die cavity with the use of a false die member enables the achievement of substantially constant density throughout the tile. The false member is contoured such that the thickness of the material in the filled die prior to pressing is proportionately greater in the direction of compression, in those areas which are to have greater thickness in the pressed tile. The use of air-entrained filling enables this to be achieved without any redistribution of the mixture such as may lead to lamination of the product. With the correct free water content as discussed below, it is found that the filling of the die by the air- entrained technique described enables the false die member to be removed and the die transported for engagement with the pressing die member, without slumping or other loss of shape of the unpressed product. This represents a significant advance in the production of satisfactory pressed and fired articles from the subject materials. Preferably, the inert material and clay are mixed in a ratio of between 70-30 and 60-40, and it is preferred that the clay content not exceed 50%, to avoid excessive plasticity, to maintain minimum shrinkage, and to enable rapid firing without excessive cracking or warping. In the practice of the present invention the free moisture content of the clay/inert material mixture is adjusted to provide correct plasticity, and is preferably the minimum content which will give satisfactory pressing of the product. As the free moisture content is increased, the tendency of the mixture to flow during pressing will increase, and the practical upper limit of free moisture content is that at which, for a given design of die and pressure of operation, the material exudes from the die. Normally of course, the moisture content is maintained well below that level. As the free moisture is reduced, the plasticity of the mixture is reduced, and the practical lower limit of free moisture content is that below which, after pressing, satisfactorily constant density cannot be obtained in the pressed product, or that below which the integrity of the presssed product is lost so that it cannot be removed from the die or is too weak for subsequent handling prior to firing. In general, the free water content will best be that which is substantially, or only slightly greater than, the critical water content, that is to say, the minimum water content at which the mixture will compact. In practice it has been found that 4-6% added water has given satisfactory results with a basalt/clay mixture. Preferably, the pressure employed is less than that which will cause fracturing of the inert material, and should be less than that which causes unacceptable lamination in the article. The lower limit of satisfactory pressure is believed to be in the region of 14 mPa, and typical working pressures are around 32 mPa. While basalt is the preferred inert material used in the practice of the invention, other mafic rocks may be employed, and more generally, any material may be used which is capable of being provided in a particulate form which will compact with the milled clay, and which does not exhibit quartz inversion to reduce the strength of the product in the short heating and cooling regime which the invention allows and which is therefore preferred. In order to maintain the advantage of the short firing time which may be achieved in the practice of the present invention, arising from the nature of the materials used and the low free water content of the green product, the glaze employed preferably has a high solids content, and may be in the region of 70% solids. More preferably, a dry glaze may be employed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows in schematic form the stages of die filling and pressing of a green product in accordance with an exemplary embodiment of the present invention; and Fig. 2 shows a typical tile cross section and schematically illustrates the contour of the false head of the die for filling of the die in accordance with the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT In the practice of the invention according to a present embodiment in the manufacture of roof tiles now described by way of example only, a mixture of clay (30%) which has been dried and then milled to approximately 200 mesh, and basalt (70%), in the form of quarry scalpings, is prepared with added moisture of 4-5%. The prepared mixture is delivered by means of a conveyor 10 and a hopper 11 to an air-entrainment blower 12. A gate 13 with pneumatic actuator 14 controls the supply of material introduced in each successive charge, from the hopper to the blower 12. The blower 12 includes a central perforated container 15 surrounded by an air space 16 into which compressed air may be introduced in a short blast by means not illustrated here. Between the chamber 15 and the outlet tube 17 of the blower 12, a further gate 18 and pneumatic controller 19, allow the escape of entrained material, in synchronism with the air blast. In this way, material is fed continuously from the conveyor 10 and is introduced entrained with a blast of high speed air, in metered quantity to the die when this is located at the filling station below the blower 12. The die assembly consists of a die frame 20 surrounding a lower die 21, the frame 20 being mounted as shown somewhat schematically at 22 on rollers which engage rail portions 23 mounted on the bed 24 of a die frame lift unit 25. At this stage of the process, as foreshadowed above, the upper portion of the die assembly is provided by a false die head 26 which may be mounted at the lower end of the outlet tube 17, and which is provided with apertures by which the tube 17 communicates with the die cavity. The lower die 21 is shaped accurately to the contour of the lower surface of the pressed tile, while the false head 26 has, as described above, a contour in the vertical direction (that is to say in the direction of pressing) which is exaggerated so that the separation of the lower die and the false head is greater than the final thickness of the tile in those areas of the tile where its final thickness is to be greater than its minimum thickness. Fig. 2 shows schematically, the relationship between tile and false head contour, although it must be emphasised that the precise contour for the false head must be determined by experiment and experience for a particular tile shape. Fig. 2 shows a cross section through part of a tile 27, the line 28 showing a typical profile for the false head 26, in such a case. In the apparatus illustrated in Fig. 1, with the die assembly lifted by the die frame lift unit 25 into engagement with the head 26, the gate 13 is opened and then closed to introduce a metered quantity of the raw materials to the perforated chamber 15, an air blast is introduced into the chamber 16 and the gate 18 opened momentarily to allow the entrained material to be blown into and fill the die cavity. The lift unit 25 is then lowered to remove the lower die 21 and frame 20 from the head 17, bringing the rail sections 23 into alignment with transport rails 29. The die assembly is then moved to the central position illustrated in Fig. 1, below the upper die support and extraction unit 30 which, at this stage of the process, is supporting the upper die member 31. The unit 30 lowers the upper die 31 into the frame 20 and applies light pressure to the die head, to provide an initial packing of the material, this pre-compaction functioning also to remove some trapped air from the packed material. The support unit 30 releases the upper die 31 and retracts upwardly, and the die assembly is then moved to enter a press 32 shown only schematically here. The pressed green tile is formed in the press 32 at a pressure in the region of 32 x 10 6 Pascal. Upon completion of pressing, the die assembly is returned to the central position shown in Fig. 1, where the head 31 is lifted away by the unit 30, and, if necessary with the assistance of a tile punchout unit 33, the tile is removed manually or by vacuum lift, and transferred to a kiln car on which it is supported during transit through the firing kiln (not shown). The die assembly is then returned to the filling station, below the blower 12, and raised on the bed 24 to the position illustrated, from which the cycle of filling, pre-compacting and pressing, is repeated. Preferably, a number of dies are arranged in a circular configuration and indexed between stations at which the preceding steps are carried out. After the application of suitable glaze, the pressed green tiles are fired one-high in a tunnel kiln to a temperature of approximately 1050°C, with a total firing time of approximately 3 hours. It will be appreciated that the material composition and process parameters described herein in connection with embodiments of the invention may be varied to suit the materials employed and the characteristics required in the final product, and the present invention should not be regarded as limited by the data provided in the description of the embodiment.

Claims: CLAIMS 1. A method of manufacturing kiln-fired products wherein a mixture of dried and milled clay and a particulate substantially inert material which exhibits substantially no quartz inversion is pressed in a die and subsequently fired, characterised in that said mixture, with a free water content chosen to provide corect plasticity of the mix, is introduced into the cavity of a die by air- entrained blowing. 2. A method according to claim 1 for the manufacture of articles having a non-uniform thickness in the direction of pressing, further characterized in that the shape of said article prior to pressing is defined by mould means into which said mixture is blown, said mould means providing at those points where the pressed product is to have greater thickness than the minimum thickness of the product, a thickness which exceeds the final thickness of the product by an amount which increases in proportion to the extent by which the thickness of the pressed product at that point is to exceed that minimum thickness. 3. A method according to claim 2 wherein said mould means comprises said die provided with a false die member, said method further comprising the steps of removing said false member after the die has been filled, replacing said false member with a die member, and pressing said mixture to form the pressed product. 4. A method according to claim 2 wherein said inert material is crushed mafic rock. 5. A method according to claim 4 wherein said rock is basalt. 6. Apparatus for the preparation from product-forming pressed materials of a pressed green product for firing in the manufacture of kiln-fired articles having a non- uniform thickness in the direction of pressing, said product being pressed in a die between first and second die members to form said product, characterised in that said first die member is removable from said die, said apparatus further comprising a false die member, means engaging said false member with said die in place of said first member during filling of said die, means disengaging said false head after filling, and means engaging said first die member with said die in place of said false member prior to pressing. 7. Apparatus according to claim 6 wherein said false member is so contoured and dimensioned as to define when in engagement with said die, a die cavity the thickness of which, at those points where the pressed product is to have greater thickness than the minimum thickness of the product, exceeds the final thickness of the product by an amount which increases in proportion to the extent by which the thickness of the pressed product at that point is to exceed that minimum thickness. 8. Apparatus according to claim 6 further comprising air entraining apparatus, means engaging said die with said air entraining apparatus when said false member is engaged with said die, means introducing a metered quantity of said materials to said die, and means releasing air- entrained material from said air entraining apparatus thereby to fill the cavity of said die between said false member and said second member. 9. Apparatus according to claim 6 wherein said product is pressed at a pressure of between 14 mPa and that pressure which will produce fracturing of particles of said inert material.

Inventors: COLLETT GORDON WALLACE, SMALL BRUCE GORDON, THOMPSON JOHN CHARLES, TOMLINSON ROBERT KEITH, WYNANDS HENRY, COLLETT, GORDON, WALLACE, SMALL, BRUCE, GORDON, THOMPSON, JOHN, CHARLES, TOMLINSON, ROBERT, KEITH, WYNANDS, HENRY
Applicants: MONIER LTD, MONIER LIMITED
Application year: 1986
Main classification: B28B3
IPC codes: B28B3, B28B5, B28B13