Method for pressing ceramic powders, and equipment for its implementation

Abstract: In a method for pressing ceramic powders comprising a powder compaction stage within a mould, after exerting the powder compacting pressure the powder is subjected to close-together pressure pulses while maintaining the compacting pressure applied. The press for implementing the method is comprising a punch (5) supported by the movable cross-member (4) and the mould (101). Below the punch (5) is spread an elastic membrane (53) with means for feeding a hydraulic liquid to the rear of the membrane (53). The hydraulic liquid feed means are comprising a hydraulic liquid source (16) connected to the rear of the membrane (53) and a distributor valve (15) arranged to connect said rear alternately to the source (16) and to discharge. The distributor valve (15) generates close-together pressure pulses while the pressing force exerted by the press applied.

Description: Ceramic tiles are commonly formed by pressing material in powder form, of between 1% and 10% moisture content, within a mould. This forming method is commonly known as dry forming. The soft material is loaded into the mould by known means. After the mould has been closed by punches operated by the pressing members, the powder undergoes initial light pressing, with consequent volume reduction, to facilitate powder deaeration. Said initial light pressing, also known as the first pressing, is followed by the deaeration stage, during which pressing is interrupted and the mould is sometimes reopened to allow the air to escape. This is followed by the main pressing to a pressure of about 400 kg/cm 2 , which ensures perfect powder compaction. The main pressing generally takes place in several successive steps at increasing pressure up to the maximum pressure. The pressing force exerted by the upper cross-member of the press is distributed over the total surface of the tiles pressed during each cycle. It should be noted that each time reference is made to the term "pressure" in the text, this unless otherwise specified means the compacting pressure to which the powder is subjected within the forming mould. The largest currently available presses have a capacity (pressing force) of 4000 tonnes, and during each cycle are able to press a surface area of not exceeding 10,000 cm 2 , for example they can operate a die having three impressions of 54 cm x 54 cm. Achieving powder densification sufficient to ensure good quality of the finished product as the tile size increases requires an ever increasing press pressing force, implying presses of ever greater dimensions. The object of this patent is to increase the powder densifying effect within the tile forming mould without increasing the press pressing force. A method for pressing ceramic powder tending to achieve the aforesaid result is known, comprising subjecting to repeated blows the press cross-member with the upper part of the mould which exerts the pressing force on the powder. This method, described in the document in the name of the present applicant DE 19642437, has the advantage over conventional static pressing that for equal pressing force exerted by the press a greater powder densification is obtained, ie a higher powder density. The known method has however a certain number of drawbacks which have prevented its implementation on an industrial scale. In this respect the effects produced by the blows cannot in practice be kept under valid control because secondary factors intervene such as friction resistance, play between the moving parts, and inertia and other phenomena, which modify the system parameters. The impossibility of maintaining the system parameters under control has resulted in vibration being partly transmitted to the machine structure, with problems of excessive noise, loosening of members or breakages occurring. Moreover to be able to apply the method successfully and with easily available means, the punch and the press cross-member must be of relatively small mass and hence dimensions. The document JP 63199099 discloses a compaction system in which the compaction pressure is given by a pump, and is maintained while ultrasonic vibrations are generated into the liquid fed by the pump. 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, but wherein the pulsation generation means are different and separated from the pressure generation means. 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 DE 4320203 discloses a mould of substantially constant volume, parallelepiped shaped, having two opposite walls subjected to vibrations to improve the compaction of chalky sandstone material without substantial reduction of its volume. The object of the invention is therefore to eliminate said drawbacks of the known method. The object is attained, according to the invention, by subjecting the powder mass to be compacted both to the press pressing force and simultaneously to vibrations which are limited substantially to the powder mass without involving the pressing members. This is in accordance with the pressing method and device defined in the claims. The merits and the constructional and operational characteristics of the invention will be more apparent from the description given hereinafter with reference to the accompanying drawings, which show a preferred embodiment thereof. Figure 1 is a schematic view of a ceramic press with its hydraulic operating means at the commencement of the pressing cycle according to the invention. Figures 2 to 6 show the press of Figure 1 in successive operating positions. Figure 7 shows the diagrams of the pressure to which the powder is subjected within the press of Figures 1 to 6. Figures 1 to 6 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 oil source and to the outside 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 at least one punch 5, in the interior of which there are provided channels 51 connected to a conduit which opens externally. An elastic membrane 53 provided with support feet 54 is spread below the block 5 and is held in position by a perimetral frame 55, the support feet maintaining the membrane slightly raised from the block 5. The conduit 52 is connected via a distributor valve 15 to a pressurized oil source 16 via a bidirectional shut-off valve 17 governed by the upstream and downstream pressure, and a maximum pressure valve 18. The distributor valve 15 is controlled to feed pressurized oil pulses between the membrane 53 and the punch 5. Below the punch 5 there is a mould 10 comprising a die 101 and a movable base 102, both supported by the press bed 11. The mould shown is of the movable die type, with the die descending under the thrust of the punch, but could also be of any other known type. The die 101 is supported by the pneumatic pistons 103, which act as deformable elastic means. The initial volume of the forming cavity is defined by the level of the die 101 and by the rest position of the movable base 102 of the mould 10. The method will now be described with reference to Figures 1 to 6. After the soft material has been loaded into the cavity of the mould 10 the press cross-member is lowered until the punch 5 rests on the die to close the mould, a first light pressing then being carried out to expel air from the material to be pressed. During the first pressing the punch and the die move into the position shown in Figure 2, the distributor valve 15 is in the configuration shown in Figures 1 and 2, and the space to the rear of the membrane 53 is full of oil which cannot flow out. Having carried out the first deaeration pressing, the punch is slightly raised from the die into the position shown in Figure 3. The second pressing is then carried out, in which the press assumes the configuration shown in Figure 4. In this configuration the die 101 rests on the bed 11, outflow of the oil contained behind the membrane 53 still being prevented. Then, maintaining the piston 2 descended with the distributor valve 12 positioned as in Figure 4, a powder pulsation pressing stage is commenced during which, by suitably operating the distributor valve 15, the pressure of the oil behind the membrane 53 is made to pulsate at a frequency and amplitude regulated by the press control system. During this stage the pressing force exerted by the piston 2 is maintained constant, such that the piston 2 remains stationary together with the cross-member 4 and with the punch 5 resting on the bed 11 via the die 101. The pressure pulses transmitted to the oil behind the membrane 53 have a minimum value greater than zero, and a maximum value which cannot exceed the compacting pressure corresponding to the press pressing force divided by the surface area of the mould punch or punches. In this respect, if this value is exceeded the maximum pressure valve operates. Usual removal from the mould follows as shown in Figure 6, in which the configurations of the hydraulic control circuits can also be seen. The number of pulsations required to achieve the result is between ten and fifty pulsations per cycle, after which the densifying effect deriving from the pulsations does not substantially increase because of saturation. The densifying effect of the pulsations increases with increasing pressing force applied to the punch and increasing liquid pressure on the rear of the membrane. The densifying effect is greater as the maximum pulsation pressure increases. Considering for example a pulsating pressing cycle at 200 bar, a considerable density increase is noted. With 8 pulsations the same density as a standard cycle at 300 bar is obtained (50% increase in the equivalent static pressing force), and with 16 pulsations the same effect is obtained as a standard 350 bar cycle (75% increase in the equivalent static pressing force). An increased number of pulsations obviously results in a longer press cycle time, with reduced productivity. In contrast, achieving high densities with lower pressures allows production to be increased (in terms of maximum pressable surface area) for the same press. The described example of the method of the invention is subject to numerous modifications. The pulsating cycle can vary in terms of pulse frequency, number, intensity and pressure waveform, which can assume one of the forms shown in Figure 7. Moreover, instead of applying the pulsation only during the last pressing stage, a pressure increasing towards the maximum value can be applied from the start of pressing, while at the same time gradually increasing the thrust on the movable cross-member until the maximum value is reached. Finally, ultrasound can be applied to the oil behind the membrane.

Claims: A method for pressing ceramic powder by means of a punch (5) supported by the movable cross-member (4) of a press and a forming cavity opposed to said punch (5), wherein a membrane (53) rests on the punch (5) and is sealedly fixed only perimetrally thereto, means (1,2) for pressing the punch against the powder contained in the forming cavity, and means for feeding (51,52,16) a hydraulic liquid onto the rear of the membrane (53) when the punch (5) is positioned against the powder to exert the compacting pressure being provided, characterised by arranging a layer of soft ceramic within the cavity; exert controlled pressure on the powder by the punch (5) supported by the movable cross member (4); subjecting the powder to controlled close together pressure pulses via a distributor valve (15) arranged to connect the rear of the membrane (53) alternately to source and to discharge while maintaining the controlled pressure and without modifying the position of the movable cross member (4). A method as claimed in claim 1, characterised in that the pressure pulses are of a discrete number. A method as claimed in claim 1, characterised in that the pressure pulses have a maximum value equal to the powder compacting pressure deriving from the static pressing force exerted by the press, and a minimum value greater than zero. Equipment for dry-forming powders, comprising a punch (5) supported by the movable cross-member (4) of a press, and a mould (10) opposing said punch (5) and provided with a forming cavity, characterised in that below the punch (5) there is spread an elastic membrane (53) sealedly fixed peripherally, means (51,52,16) being provided for feeding a hydraulic liquid to the rear of the membrane (53) when the punch (5) interacts with the forming cavity to compress the powder contained within it, said pressurized hydraulic liquid feed means comprising a hydraulic liquid source (16) connected to the rear of the membrane (53) via a distributor valve (15) arranged to connect said rear alternately to the source and to discharge, and pressure pulse generator means positioned between said distributor valve (15) and said membrane rear (53). A plant as claimed in claim 4, characterised in that said pulse generator means consist of the actual distributor valve (15) itself, the slider of which is controlled by electromagnetic programmed means. A plant as claimed in claim 4, characterised in that said pulse generator means consist of a spring-controlled valve of adjustable setting which when the set pressure is exceeded connects the hydraulic liquid feed conduit to discharge, and below said pressure connects said conduit to the membrane rear. A plant as claimed in claim 4, characterised in that said pulse generator means are a vibration source positioned in contact with the hydraulic liquid. A plant as claimed in claim 4, characterised in that said pulse generator means are 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, B30B11