Method and plant for dry-forming crockery in general, particularly plates

Abstract: In a method for dry-forming crockery by means of isostatic moulds, comprising compacting the powder within an isostatic mould, the powder is subjected to close-together pressure pulses during exertion of the compacting pressure. The press is comprising an isostatic mould (6), a punch (5) and a base (6) with a membrane (67). Means are provided for feeding a hydraulic liquid onto the rear of the membrane (67) for isostatically pressing the powder in the mould. The pressurized hydraulic liquid feed means comprise a source of hydraulic liquid connected to the rear of the membrane (67) via a distributor valve (15) arranged to move and to connect the rear of the membrane (67) alternately to the source of liquid and to the discharge. The distributor valve (15) oscillates between these two positions and generates pressure pulses to the rear of the membrane.

Description: This invention relates to the manufacture of plates and crockery of fine ceramic, such as porcelain, and in particular to so-called isostatic methods and plants. The isostatic method for dry-compacting powders uses so-called isostatic moulds in which a base has the shape of one side of the plate, against which there rests an elastomeric membrane of substantially constant thickness, sealedly fixed perimetrally. The base of the mould is positioned in front of a movable punch of the same shape as the other side of the plate. The various stages in forming the crude plate, ie ready for firing in a suitable kiln, comprise arranging a layer of soft ceramic powder on the base, moving the punch towards the base until it closes the compartment formed between the base and punch without exerting substantial pressure on the powder, and feeding a pressurized liquid onto the rear of the elastomeric membrane which covers the base, with consequent raising of the membrane from the base on which it rests, and compression of the material to achieve the compaction required to form the plate. Subsequent withdrawal of the punch makes the plate accessible for its removal, after which the cycle is repeated. In the known art the aforedescribed basic process can undergo various modifications in terms of the method of feeding the powder, the method of removing the plate, the positioning of the mould with its axis horizontal or vertical, the method of fixing the membrane, etc. The aforedescribed cycle stages are however common to all known isostatic methods, and will hence be taken as the starting point in describing this invention. The document DD139109 discloses a method and apparatus for dry forming crockery by means of isostatic moulds having a punch shaped as one of the two sides of the crockery, a base shaped as the other side of the crockery, and a membrane having the same shape as the punch, which rests thereon and is sealedly fixed perimetrally thereto, and means for moving the punch towards the base and means for feeding a hydraulic liquid onto the rear of the membrane when the punch is positioned against the base to define a forming chamber, with consequent rising of the membrane from the punch on which it rests, and compression of the material to achieve the compaction required to form the crockery, wherein the base is subjected to vibrations. The document JP 63030199 discloses a isotropic pressure pressing method in which the compaction pressure is given by a pressure generator, and is maintained while pulsation is generated by actuating a cylinder via an electric motor. The method according to JP 63030199 achieves an improved material compaction, wherein the pulsation generation means are different and separated from the pressure generation means. With the improvements in material technology and the ongoing increase in the dimensions of objects to be formed, increasingly greater forming pressures are required, with consequent increase in press dimensions and capacity. The object of this invention is to achieve material compaction suitable for modern technological requirements while maintaining the pressure of the membrane operating liquid relatively low, and hence maintaining low stressing forces on the press structure and a low required capacity of the pressurized hydraulic liquid generator. This object is attained according to the features of claims 1 or 3. It has been found that the pressure pulses to which the membrane is subjected increase the powder densifying effect compared with the densification obtained at the same pressure by the traditional method. The number of pressure pulses to be imposed on the membrane is conveniently less than 50 per cycle, it having been found that after a certain number of pulses a saturation effect occurs so that compaction is not further substantially increased. The number of pressure pulses can also be uncontrolled, if the pulses are generated by a vibrator or by an ultrasound generator in contact with the hydraulic liquid. It has also been fund that the effect of pressure pulses increases as a function of the base pressure applied, in the sense that whereas for low pressure, of the order of 100 bar, compaction increases by a certain amount, for higher base pressures the increase in compaction is comparatively much greater. The merits and the characteristics of the invention will be more apparent from the description given hereinafter with reference to the figures of the accompanying drawings, which show a preferred embodiment thereof by way of non-limiting example. Figure 1 is a schematic view of a plant for forming plates from dry material in powder form. Figure 2 shows the plant of Figure 1 in a subsequent stage of the cycle. Figure 3 shows the plant of Figure 1 in a subsequent stage of the cycle. Figure 4 shows the plant of Figure 1 in a subsequent stage of the cycle. Figure 5 shows diagrams of the compacting pressure to which the powder is subjected within the mould of the press of Figures 1 to 4. The figures show the main hydraulic press cylinder 1 within which there slides a piston 2, to the rod 3 of which the movable cross-member 4 is fixed. The hydraulic cylinder 1 is connected above and below the piston 2 to a pressurized hydraulic liquid source and to discharge respectively, and vice versa, by the distributor valve 12 and the pipes 121 and 122. Between the pressurized oil source 13 and the distributor valve 12 there is a maximum pressure valve 14. The movable cross-member 4 lowerly carries a steel punch 5 covered with a rubber membrane 51. Below the punch 5 there is positioned the isostatic mould 6 rigid with the press bed 7. The mould 6 comprises a lower part 61 having a depressed central region 62 from which there extends a conduit 63 opening externally for the hydraulic operating liquid for the mould. To the lower part 61 there is rigidly connected the upper part 64 provided with through conduits 65 which open into the depressed central region 62. An outer frame 66 sealedly locks a membrane 67 of an elastomer such as rubber, which rests on the upper surface of the mould upper part 64. The conduit 63 is connected to a source 16 of pressurized hydraulic liquid. Downstream of the source 16 there are a maximum pressure valve 18 of adjustable setting, and a servovalve 15 which can operate either as a distributor valve or as a hydraulic pulsation generator, in this latter case by oscillating between the feed position and the discharge position. The aforedescribed plant operates as follows. When the plant is in the stand-by position between one cycle and the next, it assumes the configuration shown in Figure 1 and is ready to commence the cycle. Having received the powder the mould is closed to assume the configuration shown in Figure 2, in which the powder is subjected only to the very light pressure due to mould closure. The powder is compressed in the stage shown in Figure 3, in which the distributor valve 12 is fed as in Figure 2, the distributor valve 8 allowing pressurized hydraulic liquid to occupy the space below the membrane, to exert a base pressure on the membrane and through this onto the powder. On attaining the base pressure the pulse generator 10 operates to cyclically discharge the liquid feed pipe and reclose it immediately afterwards, with repeated action, to subject the membrane 67 and with it the powder contained in the mould to a cycle of close-together pulses, similar to vibrations. Alternatively the programmed and magnetically controlled slider of the distributor valve 12 can operate. The pressure of the liquid fed by the source is between 100 and 400 bar, the pulse frequency being between 1 and 50 Hz. On termination of the powder densification stage the plant is put into the configuration of Figure 4, in which the object, such as a plate, formed by the compressed powder is extracted. The hydraulic pulse generator can take the form of other devices, such as a rotary distributor valve suitably operated by an hydraulic actuator or an electric or hydraulic motor of adjustable speed and position, which connects the feed conduit 63 for the chamber behind the membrane to a hydraulic pressure source and to discharge alternately. The pulsation cycle can vary in terms of frequency, number and intensity of pulses and the form of the pressure wave, which can assume one of the forms shown in Figure 5.

Claims: A method for dry forming crockery by means of isostatic mould having a punch (5) shaped as one of the two sides of the crockery, a base (64) shaped as the other side of the crockery, and a membrane (67) having the same shape as the base (64), which rests on the base (64) and is sealedly fixed perimetrally thereto, means for moving the punch (5) towards the base and means (16,63,65) for feeding a hydraulic liquid onto the rear of the membrane (67) when the punch (5) is positioned against the base (64) to define a forming chamber being provided, characterised by arranging a layer of soft ceramic powder on the base (64), moving the punch (5) toward the base (64) until it closes the compartment formed between the base (64) and punch (5) without exerting substantial pressure on the powder, and feeding a pressurised liquid onto the rear of the elastomeric membrane (67) which covers the base (64) with consequent rising of the membrane (67) from the base on which it rests, and compression of the material to achieve the compaction required to form the crockery, during exertion of the compacting pressure the powder being subjected to close together pressure pulses via a distributor valve (12) arranged to connect the rear of the membrane alternatively to the source and to discharge. A method as claimed in claim 1, characterised in that the pressure pulses are of a discrete number. A plant for dry-forming crockery, comprising an isostatic mould having a punch (5) shaped as one of the two sides of the crockery, a base (64) shaped as the other side of the crockery, wherein a forming chamber is completely defined by said punch positioned against said base and a membrane (67) having the same shape as the base (64), which rests on the base (64) and is sealedly fixed perimetrally thereto, means being provided moving the punch (5) towards the base (64) and means (13,63,65) for feeding a hydraulic liquid onto the rear of the membrane (67) when the punch (5) is positioned against the base (64) to define the forming chamber characterised in that the pressurized hydraulic liquid feed means (13,63,65) comprise a source of hydraulic liquid connected to the rear of the membrane (67) via a distributor valve (12) arranged to connect said rear alternately to the source and to discharge, oscillating between these positions such as to generate pressure pulses applied to the rear of the membrane (67) in the manner of pulsation. A plant as claimed in claim 3, characterised in that said distributor valve (12) is a servo valve of slide type controlled by suitable programmed means. A plant as claimed in claim 3, characterised in that said distributor valve (12) is a rotary distributor valve operated at adjustable speed and position. A plant as claimed in claim 3, characterised by comprising a vibration source positioned in contact with the hydraulic liquid so as to generate pressure pulses applied to the rear of the membrane in the manner of pulsation. A plant as claimed in claim 3, characterised by comprising an ultrasound source positioned in contact with the hydraulic liquid.

Inventors: CASSANI GIUSEPPE, CASSANI, GIUSEPPE
Applicants: SACMI, SACMI COOPERATIVA MECCANICI IMOLA SOC. COOP. A RESP. LIM.
Application year: 1998
Main classification: B28B3
IPC codes: B30B15, B28B3