A DIE FOR EXTRUDING HONEYCOMB STRUCTURAL BODIES

Abstract: An extrusion die for extruding ceramic honeycomb structural bodies has body discharge channels (4) having the desired honeycomb arrangement and body supply holes (2) communicating with the body discharge channels (4). In order to reduce variation of flow resistance of the supply holes (2), which is caused by differing roughnesses of the holes (2), the overall flow resistance of each hole (2) is by giving it at least two inner periphery surface zones (5,6) of different inner dimensions, which zones may be coaxially arranged.

Description: The present invention relates to an extrusion die and a process for producing such an extrusion die. More particularly, the present invention relates to a honeycomb-shaped extrusion die adapted to extrude ceramic honeycomb structural bodies comprising body discharge channels and a plurality of independent body supply holes communicating with the body discharge channels as well as a process for producing the same. Ceramic honeycomb structural bodies are used as catalyst carriers for purifying exhaust gases from internal combustion engines, as fine particle-­ capturing filters, as heat retainers, etc. Such ceramic honeycomb structural bodies are constituted by ceramic materials such as cordierite, alumina, silicon-carbide, mullite etc. There are known processes for producing the bodies by extruding the ceramic material with use of an extrusion die. For instance, conventional extrusion dies are shown in Figs. 3(A) and 3(B) attached (see U.S. Patent 4,373,895, U.S. Patent 3,790,654 and Japanese patent publication No. 57-61,592). The conventional example (extrusion die 1) shown in Fig. 3(A) comprises a plurality of body supply holes 2 through which a body fed under pressure by a body feeder (not shown) is passed, body stay zones 3 com­ municating with the body supply holes 2, and body discharge channels 4 having an arrangement corresponding to that of ceramic honeycomb structural bodies to be extruded (hereafter briefly referred to as "honeycomb structural bodies"). Fig. 3(B) is a partial sectional view of another conventional example. This example of Fig. 3(B) comprises a plurality of body supply holes 2 and body discharge channels 4 directly communicating with the body supply holes 2. As is the case with the conventional examples in Figs. 3(a) and 3(B), it is generally necessary to make uniform a flow rate of the body passing through the respective body supply holes 2 so that high quality honeycomb structural bodies may be extruded. For this purpose, there are also known extrusion dies (not shown) in which a plate of noodle holes(Japanese patent publication No. 59-53,844) or a rectifier plate (Japanese patent publication No. 59-46,763) is provided on the side of the body supply holes. In general, the body supply holes of the above conventional extrusion dies have a straight cylindrical shape. They are bored by drills. However, since hard metals such as die steel are used as the extrusion dies, such boring has poor workability. Further, there is a possibility that a chip produced in the boring enters between the drill and workpiece resulting in roughness of the inner peripheral surface of the body supply hole. Thus, the surface roughness differs among the inner peripheral surfaces of the respective body supply holes. As mentioned in the foregoing, uniformalized flow resistance of a plurality of the body supply holes is an important requirement for production of high quality honeycomb structural bodies. When the inner diameter and the depth of the body supply holes are made constant, the flow resistance depends upon the roughness of the inner peripheral surface of the body supply holes. In addition, when the body supply holes are straight as in the case of the above-mentioned extrusion dies, the surface roughness very much influences the flow resistance because the body supply holes are relatively small. Therefore, there arises large variations in flow resistance among the body supply holes of the conventional extrusion die. As a result, there exists an undesirable problem that it is difficult to manufacture honeycomb structural bodies of a high quality. In order to avoid the above-mentioned problem, the following countermeasures have conventionally been taken: For instance, the roughness of the inner peripheral surface is improved by honing or reaming after the body supply holes are bored. When the depth of the body supply holes is great, the surface roughness becomes more non-uniform. In order to make the surface roughness of the body supply holes uniform, a die is divided into two die units, and slits and supply holes are machined in one of the die units, while only supply holes are formed in the other die unit. Then, they are bonded together. However, there occurs a problem that a manufacturing cost of the extrusion dies rises due to increased working steps. The present invention aims to solve the above­ mentioned problems, and to provide extrusion dies in which the flow resistance of a plurality of body supply holes is made more uniform or substantially uniform by a simple measure. According to a first aspect of the present invention, there is provision of an extrusion die for extruding ceramic honeycomb structural bodies, said extrusion die comprising body discharge channels having a desired honeycomb arrangement and independent body supply holes communicating with the body discharge channels, wherein the body supply holes are so designed that each of them may have a plurality of inner peripheral surface zones of different inner dimensions and said inner peripheral surface zones may be coaxially arranged. According to another aspect of the present invention, there is a provision of a process for producing an extrusion die adapted to extrude ceramic honeycomb structural bodies, said extrusion dies comprising body discharge channels having a desired honeycomb arrangement and a plurality of independent body supply holes communicating with the body discharge channels, said process comprising the steps: of boring said body supply holes such that each of the body supply holes may have a plurality of coaxial inner peripheral surface zones of different inner dimensions, and said body supply holes may an uniform flow resistance, and forming said body discharge channels which communicate with the body supply holes and have honeycomb arrangement corresponding to the ceramic honeycomb structural body to be extruded. Embodiments of the invention are described below by way of non-limitative example and with reference to the accompanying drawings, in which:- Fig. 1(A) is a plan view of an embodiment of the extrusion die according to the present invention; Fig. 1(B) is a sectional view of the embodiment in Fig. 1(A) taken along a line Ib-Ib; Figs. 2(A) through 2(C) are sectional views of other embodiments of the extrusion die according to the present invention; and Fig. 3(A) and Fig. 3(B) are views illustrating conventional extrusion dies. In more detail, Figs. 1(A) and 1(B) show the extrusion die 1, having body supply holes 2, body discharge channels 4. Each body supply hole 2 has a first inner peripheral surface portion 5 and a second inner peripheral surface portion 6. The fundamental constituent feature of the extrusion die according to the present invention is that each of the body supply holes is constituted by a plurality of coaxial inner peripheral surface zones having different inner dimensions, that is, in the embodiment of Figs. 1(A) and 1(B), a first inner peripheral surface 5 having an inner diameter Φ₁ and a second inner peripheral surface 6 having an inner diameter Φ₂. The body supply hole 2 is formed by first forming the first inner peripheral surface having the inner diameter of Φ₁ to a depth of d₁ by means of a drill and then forming the second inner peripheral surface 6 having the inner diameter of Φ₂ by means of another drill over a depth d₂ so as to make the supply hole 2 communicate with the body discharge channels 4. As mentioned in the foregoing, the extrusion die according to the present invention is provided with the body supply holes each having a plurality of inner peripheral surface zones of different inner dimensions. Therefore, as compared with conventional extrusion dies having straight-shaped body supply holes, the flow resistance of the body supply holes in the extrusion die according to the present invention is far larger. Accordingly, even when some difference exists in roughness among the inner peripheral surfaces of the body supply holes in the extrusion die according to the present invention, the influence of variations in the surface roughness upon the flow resistance can be almost ignored. That is, with the present invention, since the flow resistance of the body supply holes can be made substantially uniform, honeycomb structural bodies of a high quality can be manufactured. In addition, with the present invention, it is possible to omit machining steps such as honing or reaming of the inner peripheral surfaces of the body supply holes for improving the surface roughness. In the illustrated embodiment of Figs. 1(A) and 1(B), the first inner peripheral surface 5 and the second inner peripheral surface 6 constituting the body supply hole 2 are shown so formed that their depths d₁ and d₂ are substantially equal. But it is preferable that the depths d₁ and d₂ are appropriately selected depending upon the shape, the cell density and the outer size of the honeycomb structural body. For instance, when the honeycomb structural body has a high cell density and/or a large outer size, d₁ is preferably smaller than d₂ so as to assure the strength of the extrusion die. The process for producing the embodiment illustrated in Figs. 1(A) and 1(B) will be explained in comparison with processes for producing the conventional extrusion dies described above. The conventional extrusion dies are produced by boring a plurality of body supply holes in a die material of a desired shape from one working surface thereof by a drill, and forming the body discharge channels in a desired honeycomb arrangement from the other working surface to communicate with the body supply holes by a well-known discharge working method or a thin blade cutter. In such a conventional producing process, since the body supply holes are straight, a limitation is imposed upon the machining depth [(d₁+d₂) shown in Fig. 1(B)] in relation to the diameter of the drills used. If this limitation is exceeded, it becomes difficult to remove cut chips. Owing to this, the roughness of the inner peripheral surface of the body supply holes becomes coarse and non-uniform.When the machined holes curve, the body supply holes deviate on the body discharge side to make the conformity between the body supply holes and the body discharge channels poorer. In contrast, the production process according to the present invention can avoid the above-mentioned problems. That is, as shown in Fig. 1(B), holes of an inner diameter of Φ₁ (the first inner peripheral surface 5) are bored at a specific depth of d₁ by a drill. Then, holes having an inner diameter of Φ₂ (Φ₂<Φ₁) (the second inner peripheral surface 6) are similarly drilled coaxially with the central axis of the first inner peripheral surface 5, thereby forming body supply holes 2. Thereafter, the desired extrusion die is produced by forming body discharge channels 4 having a desired honeycomb arrangement according to the discharge working process or a thin blade cutter to communicate with the body supply holes. In this production process of the present invention, since the body supply holes 2 are bored in two separate stages of forming the holes of the depth of d₁ and the depth of d₂, chips are easily removed. Thus, body supply holes 2 which are relatively free from occurrence of flaws at the inner peripheral surfaces due to the chips can be stably obtained. Further, since the body supply hole is constituted by the first and second inner peripheral surfaces 5 and 6 having the different inner dimensions, the intrinsic flow resistance becomes larger. Thus, the influence of the roughness of the inner peripheral surfaces (the first and second inner peripheral surfaces 5 and 6 in the embodiment shown in Figs. 1(A) and 1(B)) of the body supply holes upon the flow resistance can be ignored. In conclusion, a extrusion die which has uniformalized flow resistance of its body supply holes 2 and allows the extrusion of the honeycomb structural bodies of high quality can be produced. In order further to facilitate removal of chips produced in the boring of the body supply holes 2, the following production process may be used. That is, pre­ liminary holes smaller than the intended inner dimensions Φ₁ and Φ₂ are bored, and body discharge channels are machined to communicate with the preliminary holes. Then, the supply holes 2 are fully machined in the above-mentioned way. The body discharge channels 4 are not necessarily fully machined in a desired honey­ comb arrangement just subsequent to the boring of the preliminary body supply holes, but preliminary body discharge channels which communicate therewith may be formed first. Then, the body discharge channels 4 having the desired honeycomb arrangement are machined after the body supply holes 2 are fully bored. The embodiment in Figs. 1(A) and 1(B) includes body supply holes 2 each constituted by the first inner peripheral surface 5 and the second inner peripheral surface 6. The body supply holes may be designed to having three or more inner peripheral surface zones of different inner dimensions. In the embodiment of Figs. 1(A) and 1(B), the body supply holes are of a cylindrical shape, but they may be designed in a shape (for instance, a rectangular section) other than the cylindrical shape. As having been described in the foregoing, in the extrusion die of the present invention, the intrinsic flow resistance of the body supply holes is increased by providing a stepped portion or step portions in the inner peripheral surface of each of the body supply holes, so that the influences of the roughness of the inner peripheral surfaces of the body supply holes upon the flow resistance can be substantially ignored. In other words, the honeycomb structural bodies having a high quality can be extruded by making the flow resistance of the body supply holes formed in the extrusion die uniform. The similar effects in the embodiment of Figs. 1(A) and 1(B) can be exhibited by the embodiments illustrated in Figs. 2(A) through 2(C). In the embodiment of Fig. 2(A), a plurality of inner peripheral surface zones of body supply holes 2 are constituted by helical threads 7. In the embodiment of Fig. 2(B), a plurality of circumferential parallel grooves are formed in the inner peripheral surfaces of the body supply holes 2. In the embodiment of Fig. 2(C), a recess 9 is formed in the inner peripheral surface of each of the body supply holes 2. The extrusion dies illustrated in Figs. 2(B) and 2(C) may be made by bonding techniques (that is, for instance, an extrusion die is formed by bonding a die unit having first holes with another die unit having second holes such that the first and second holes may be axially arrayed) Although the embodiments illustrated in Figs. 1(A) and 1(B) and Figs. 2(A) through 2(C) have been explained, the present invention is not limited thereto. The extru­ sion die according to the present invention may be constituted by combining the techniques in these embodiments. As having been detailed in the foregoing, the present invention allows manufacturing of the ceramic honeycomb structural bodies of a high quality because the flow resistance of the body supply holes is made uniform while the influence of the roughness of the inner peripheral surfaces of the body supply holes is reduced or avoided. Besides, since a machining step for improving the roughness of the inner peripheral surface of the body supply holes can be omitted, the working steps are simplified and manufacturing cost can be reduced.

Claims: 1. An extrusion die for extruding ceramic honey­ comb structural bodies, having body discharge channels (4) with a desired honeycomb arrangement and body supply holes (2) communicating with the body discharge channels (4), characterized in that the body supply holes (2) each have, along their length, at least two inner peripheral surface zones (5,6;7;8;9) of different internal dimensions. 2. An extrusion die according to claim 1 wherein said inner peripheral surface zones are coaxial. 3. An extrusion die according to claim 1 or claim 2 wherein said inner peripheral surface zones have different internal dimensions in the sense that, as seen in longitudinal section of the body supply hole (2) the radial spacing of the surface of the hole from the centre line of the hole is different at the different zones. 4. An extrusion die according to any one of claims 1 to 3, wherein each of said body supply holes (2) has two inner peripheral surface zones (5,6) of different inner dimensions, and the inner dimension of the inner peripheral surface zone (6) on the body discharge side is smaller than that of the inner peripheral surface zone (5) on the body supply side. 5. An extrusion die according to any one of claims 1 to 4, wherein the cross-sectional shape of the body supply hole in a direction orthogonal to the centre line thereof is circular or polygonal. 6. An extrusion die according to any one of claims 1 to 5, wherein the shape of the body supply hole (2) varies continuously or regularly. 7. An extrusion die according to any one of claims 1 to 6 wherein the body supply hole has a surface which is an internal helical thread shape (7). 8. An extrusion die according to any one of claims 1 to 6 wherein the body supply hole (2) has at least two inner surface zones of smaller cross-­ section separated longitudinally by inner surface zones (8;9) of larger cross-section. 9. A process for producing an extrusion due adapted to extrude ceramic honeycomb structural bodies, which die comprises body discharge channels (4) having a desired honeycomb arrangement and a plurality of body supply holes (2) communicating with the body discharge channels (4), characterized in that the process comprises the step of boring said body supply holes such that each of the body supply holes has at least two coaxial inner peripheral surface zones (5,6;7;8;9) of different internal dimensions along its length. 10. A process according to claim 9, wherein the body supply hole is made by boring in sequence at least two holes having respectively smaller inner dimensions in the sequence of boring. 11. A process according to claim 9 or claim 10, wherein machining the body discharge channels (4) is effected after the commencement and before the completion of boring the body supply holes. 12. A process according to claim 9, further comprising a step of bonding together extrusion die portions to form the extrusion die. 13. A process according to claim 9, wherein the body supply hole having the inner peripheral surface zones of the different inner dimensions are continuously worked.

Inventors: FUTAMURA, SHOJI C/O INSTITUTE OF TECHN. PRECISION, INOUE, SATORU, OZAKI, SEI
Applicants: INSTITUTE OF TECHNOLOGY PRECISION ELECTRICAL DISCHARGE WORKS, NGK INSULATORS, LTD.
Application year: 1986
Main classification: B28B3
IPC codes: B28B3