A method and an apparatus for extruding ceramic bodies

Abstract: A method of extruding ceramic bodies in a downward direction is disclosed, which comprises the steps of supporting the extruded body (8) being downwardly extruded through an extruding die (4) to prevent deformation of the extruded body, and cutting the extruded body at a given length. An apparatus for effecting this extruding method is also disclosed, which comprises an extruding unit provided with a die (4) at a bottom thereof, a unit (10,24) for holding the extruded body being downwardly extruded, and a cutter (12) for cutting the extruded body in the given length. The upward force applied is controlled in dependence on the sensed length of the extruded body.

Description: The present invention relates to a method and an apparatus for extruding ceramic bodies with excellent dimensional accuracy. In order to extrude ceramic bodies, there has formerly been used a lateral extrusion method in which the ceramic bodies are horizontally extruded. This is illustrated in GB-A-1542599. Since the extruded bodies are expediently arranged for the succeeding steps in the case of the lateral extrusion method, this method is functional and effective in view of the whole extrusion process. Apparatus for extruding thin walled ceramic honeycomb structures is for example disclosed in US-A-4178145 and in DE-A-1579016. Small size ceramic honeycomb structural shaped bodies having a diameter of not more than about 150 mm or a height of not more than about 150 mm when being horizontally extruded have a large cell density with a wall pitch of from 1 to 3 mm, and are light in weight. Thus, they will not deform by their own weight. Accordingly, they can be horizontally extruded. However, if the above conventional lateral extrusion technique is employed for ceramic components, such as ceramic rotors for pressure wave type superchargers, which have a relatively heavy weight and a smaller cell density, the extruded bodies deform due to their own weight when they are extruded from a die in a given length. Consequently, ceramic extruded products having excellent dimensional accuracy could not be obtained. Also, since the weights of large size ceramic honeycomb structural bodies having a diameter of more than 150 mm or a height of more than 150 mm when being horizontally extruded exceed their strength in the case of the lateral extruding, the extruded bodies deform due to their own weight so that products having excellent dimensional accuracy cannot be obtained. On the other hand, there is also known a method of extruding ceramic bodies in an upward direction. In this upward extruding method, bending of extruded bodies is corrected by hand during extrusion, and the extruded bodies are taken out by using a jig. Thus, roundness is outside a range of ± 1.0 mm and bending amount is 1.0 mm or more. (Here, roundness means the largest value between measured differences from the maximum diameter to the minimum diameter at each of opposite end faces in the case of an extruded body having a round section and the bending amount is the maximum distance ℓ between a dried body 20 having a length of 150 mm and a base plate 21 carrying the body 20 thereon as shown in Fig. 7). Therefore, it is an object of the present invention to provide a method and an apparatus for extruding ceramic bodies, which solve the problems encountered by the conventional lateral extrusion method and upward extrusion method. The present invention provides a method as set out in claim 1. The extruded body is preferably held by the following holding method and holding unit. That is, the extruded body being extruded downwardly is held at its outer peripheral face in a radially inwards direction. The holding unit is designed to effect such a holding method. In the alternative, while the extruded length of the body being extruded or a value in proportion to the extruded length of the extruded body is measured, a reaction force consisting of a force in proportion to the extruded length and a holding force is upwardly applied to the lower end of the extruded body, and the holding unit is designed to effect such a method. These and other optional features and advantages of the present invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings, with the understanding that modifications, variations and changes of the same could be made by the skilled person in the art to which the invention pertains. For a better understanding of the invention, reference is made to the attached drawings, wherein: Fig. 1 is a schematic view of an embodiment of the ceramic material extruding apparatus according to the present invention; Fig. 2 is a view illustrating a method of generating a reaction force; Fig. 3 is a graph showing the relation between the reaction force and the extruded length of an extruded body in the case of Fig. 2; Fig. 4 is a view illustrating another method of generating a reaction force; Fig. 5 is a graph showing the reaction between the reaction force and an extruding time in the case of Fig. 4; Fig. 6 is a view illustrating still another method of generating a reaction force; Fig. 7 is a view illustrating definition of the bended amount of the extruded body; Fig. 8 is a graph showing changes in extruding pressure with the lapse of time; Fig. 9 is an example of ceramic extruded bodies for the production of ceramic rotors to be used in a pressure wave type supercharger; and Fig. 10 is a perspective view of the ceramic extruded body shown in Fig. 9. An example of the construction of the present invention will be explained based on a preferred embodiment shown in Fig. 1. Fig. 1 is a schematic view showing an embodiment of an apparatus for extruding ceramic bodies according to the present invention. A ceramic billet 5 is downwardly extruded through a die nozzle 4 by a push force of a plunger 6 from a cylinder 7 which is reduced in pressure by a vacuum pump 3. In this case, the push force of the plunger 6 is appropriately determined by a hydraulic pressure of a hydraulic unit 1 which is controlled by a controller unit 2. The extruded body 8 being downwardly extruded is received by a receiving tray 24 supported by a receiving table 10 at the die surface. At that time, a plunger 11 for the receiving table 10 is so controlled by a controller unit 9 that a reaction force consisting of a force in proportion to the extruded length of the extruded body 8 and a holding force may be applied to the receiving table 10. The term "holding force" is used in this case to mean a force by which the extruded body is fixed to the receiving tray. This holding force is a given pressure upwardly applied to the extruded body. When the shaped body is supported by holding the outer peripheral face thereof, the holding force is a pressure to hold the outer peripheral face. The receiving table 10 descends while the extruded body 8 is supported on the receiving tray 24 on the receiving table 10, thereby shaping the ceramic body in a given length. As mention above, a receiving table unit is constituted by the receiving tray 24 for receiving the extruded body 8 from the underside and fixing it, the receiving table 10 supporting the receiving tray 24 from the underside, and the receiving table plunger 11 for ascending and descending the receiving table 10. Next, the extruded body 8 is cut by a cutter 12 to a given length, and then the receiving table plunger 11 descends to take the extruded body 8 away as a green product. Particular methods of applying the reaction force to the extruded body 8 by the receiving table 10 and the receiving tray 24 via the receiving table plunger 11 will be mentioned later. Fundamentally speaking, the extruded length of the extruded body 8 is measured by an extruded body length-measuring means 13 (which is a magnet scale as a displacement meter in Fig. 1), and the push pressure of the receiving table plunger 11 is appropriately controlled by the controller unit 9 based on the measured length. In this case, the extruded body is well prevented from deforming by applying the reaction force to the receiving table 10 in synchronization with the moving speed of the receiving table 10, that is, the descending speed of the receiving table plunger 11, with the extruding speed of the extruded body 8. It is preferable that a projection is formed on the receiving tray 24 so that the extruded body may be held on the receiving tray to prevent slipping between the receiving tray 24 and the extruded body 8 during the extruding, thereby preventing deformation of the extruded body. It is preferable to set the force in proportion to the length of the extruded body and the holding force in a range not exceeding 5 times the weight of the extruded body having said length. The phenomenon that the extruded body deforms due to variation in the extruding pressure as shown in Fig. 8 can be prevented by applying the holding force and the force proportional to the length of the extruded body to the extruded body as the reaction force. Further, the phenomenon that the body being extruded deforms through being pulled near the extruding die surface due to its own weight can simultaneously be prevented. When the extruded body 8 is to be cut to a given length by the cutter 12, perpendicularity and flatness at the end face of the extruded body product relative to the extruding direction can be improved by synchronizing the cutter 12 with the extruding speed of the extruded body 8. Consequently, the lower end face of the extruded body can fully be supported by the receiving tray 24 in the succeeding step, thereby permitting continuous production of extruded bodies so that they do not fall down. In this case, it is not needed to improve the perpendicularity and the flatness by machining the end face again. Therefore, this processing step can be omitted and wasted portions of the extruded body can be reduced. The cutting may be effected by the cutter 12 at such a location that the length of the ceramic body 5 extruded downwardly may correspond to that of two extruded bodies 8. The extruded bodies may be continuously produced by alternatively and repeatedly extruding the ceramic body and cutting it to a given length. Since the ceramic green products having excellent dimensional accuracy may be obtained according to the present invention as mentioned above even if the extruded bodies 8 have a relatively heavy weight and small cell density, the shaped bodies may preferably be used as ceramic rotors for use in pressure wave type superchargers, etc. The present invention may also be employed for extruding large size ceramic honeycomb structural bodies having a diameter of more than 150 mm or large size ceramic honeycomb structural bodies having a minor axis of more than 150 mm. In Figs. 9 and 10 are shown an embodiment of a ceramic honeycomb structural body to be used to produce a ceramic rotor for a pressure wave type supercharger. In the illustrated embodiment, the ceramic extruded body 8 for the rotor has a cylindrical shape, as a whole, with through holes 25 arranged in concentrically inner and outer annular rows. The through holes 25 are opened at axially opposed end faces. A reference numeral 26 indicates a cell wall separating the through holes 25. Next, the method of applying the reaction force to the extruded body will be explained. In a first method, the reaction force is generated while the length of the extruded body is being measured. As shown in Fig. 2, the location of the receiving table 10 is measured by the displacement meter 13 (for instance, a magnet scale), and a reaction force in proportion to the location of the receiving table 10 (the length of the shaped body) is generated by the pressure of a fluid inside the cylinder of the receiving table plunger 11 based on a control signal from the controller unit 9. A reference numeral 14 is a D/A converter which is to convert digital signals to analog signals. The D/A converter converts dimensional data (digital signals) from the magnet scale 13 to analog signals, and feeds the latter signals to the controller unit 9 to control the pressure of the receiving table plunger 11. Fig. 3 shows the relation between the reaction force and the length of the extruded body. As shown in Fig. 3, the holding force may preliminarily be given as the reaction force. That is, a line "a" in Fig. 3 corresponds to a case where the holding force is preliminarily given and the reaction force is given in proportion to the extruded length of the extruded body. A line "b" corresponds to a case where no holding force is preliminarily given. A line "c" is a case where the extruded body is formed while a constant holding force is given. It is preferable to set the maximum value of the reaction force at not more than 5 times the weight of the extruded body of a given length. In a second method, a reaction force is generated, while a displacement of the plunger is being detected. As shown in Fig. 4, the reaction force is generated by utilizing the fact that the displacement of the plunger 6 is proportional to the extruded length of the extruded body. The displacement of the plunger 6 is detected by measuring the flow amount of a working oil in a hydraulic unit 1 to drive the plunger 6. Therefore, the displacement of the plunger is determined by measuring the flow amount of the working oil, and accordingly the extruded length of the extruded body is detected. Based on the measured extruded length, a reaction force can be generated in proportional to the length of the extruded body. Fig. 5 shows the relation between the reaction force and the accumulated flow amount of the working oil. In a third method, a force applied to the receiving table 10 is detected, and a force in proportion to the extruded length of the extruded body and a holding force are generated. In this case, the reaction force is applied by a servomotor 16 through a load cell 15 and a lead screw 17 instead of the magnet scale as means for measuring the extruded length of the extruded body and the receiving table plunger 11 for applying the reaction force in Fig. 2. This method as a matter of course falls inside the scope of the present invention. As shown in Fig. 6, according to this method, a force applied to the receiving table 10 is detected by the load cell 15, and a force proportional to the extruded length of the extruded body and a holding force are generated based on that force. The reaction force is controlled by the servomotor 16 through the screw. In the following, the present invention will be explained in more detail with reference to specific examples thereof. As a matter of course, it is evident that the present invention will not be limited to these examples. Example This example is a case where the given length corresponds to that of two extruded bodies. Six kg of methyl cellulose powder as a binder and 24 kg of water were fully kneaded by a kneader into 100 kg of a raw material consisting of 100 parts by weight of silicon nitride powder having the average particle diameter of 10 µm, and 8 parts by weight of magnesium oxide and 5 pars by weight of cerium oxide as a sintering aid, and a vacuum deaired billet of 180 mm () × 1,000 mm (length) was prepared from the mixture by using a vacuum pugmill. The body was fed to a cylinder of a 200 ton vertical-plunger type downwardly extruding machine. An extruding die having the minimum thickness of 0.7 mm and the maximum thickness of 12 mm was set at the lower side of the cylinder. A shaped body was extruded through the extruding die by pressing the billet with a piston of the plunger extruding machine from the upper side, and had a form of a rotor having an outer diameter of 150 mm () for use in a pressure wave type supercharger. The 150 mm () body extruded through the die was received at the die surface by a receiving tray supported by a receiving table, and a force proportional to the extruded length of the extruded body and a holding force were applied to the receiving table by a device which was driven in synchronization with the extruding speed or the extruded body. While the extruded body was supported by the receiving tray, the extruded body having a length of 360 mm was formed. Then, the piston was stopped to interrupt the extruding operation. Next, the extruded body was cut to a length of 180 mm by a thin wire cutter and taken out. Then, the receiving tray was brought up again into contract with the body. While the body was supported again by the receiving tray, another extruded body was formed in a length of 360 mm. Then, the piston was stopped to interrupt the shaping operation. The above-mentioned shaping operation was repeated, thereby obtaining numerous shaped bodies. The succeeding heating and firing steps were effected under the same conditions as employed in Example 1, thereby obtaining ceramic rotors for use in pressure wave type superchargers. The obtained ceramic rotors for use in the pressure wave type superchargers had excellent dimensional accuracy, that is, the bending amount of not more than 1.0 mm and the roundness inside the range of ±1.0 mm. As mentioned above, according to the method for extruding ceramic materials in the present invention, excellent dimensional accuracy and reduced unbalance of ceramic extruded products can be attained even in the case that the products have a relatively heavy weight and a smaller cell density, such as ceramic rotors for use in pressure wave type superchargers. Consequently, production cost of products can advantageously be reduced. Also, extruded bodies having excellent dimensional accuracy can be obtained even in the case of large size ceramic honeycomb structural bodies having a diameter of more than 150 mm and large size ceramic honeycomb structural bodies having a minor axis of more than 150 mm. Thus, production cost of the products can advantageously be reduced.

Claims: A method of extruding a ceramic body (8) in a downward direction through an extruding die (4), wherein the body (8) being extruded is supported on a receiving member (10) during its extrusion to prevent its deformation, characterized by the steps of (a) extruding the body (8) to a length corresponding to two times the desired length of the cut product, (b) stopping extrusion and cutting the body (8) at a position selected to give a cut product having said desired length, and (c) repeating said steps (a) and (b) to give a plurality of cut products of said desired length. An extruding method according to claim 1 or claim 2, wherein the extruded body (8) is a ceramic honeycomb structural body. An extruding method according to any one of the preceding claims, wherein the extruded body (8) is a ceramic rotor for use in a pressure wave type supercharger.

Inventors: KATO KIMINARI - TOWN DENJIYAMA, WATANABE KEIICHIRO, YOKOI MASUMI, KATO, KIMINARI 1-302, TOWN DENJIYAMA, WATANABE, KEIICHIRO, YOKOI, MASUMI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1988
Main classification: B28B3
IPC codes: B28B3, B28B11