Method of making a porous sintered body, a compound for making the porous sintered body, and the porous sintered body

Abstract

A thermal formation sintering compound containing a binder, a sinterable powder material and a pore formation material, for formation into a predetermined shape in a thermal formation step, removal of the binder in a degreasing step, and sintering of the powder material in a sintering step is provided. The binder contains a low-temperature draining component which melts in the thermal formation step, begins draining at a temperature lower than a draining temperature of the pore formation material, and drains at a temperature lower than a temperature at which the pore formation material drains; and a high-temperature draining component which melts in the thermal formation step, begins draining after the pore formation material begins draining, and drains at a temperature higher than does the pore formation material.

Claims

1. A thermal formation sintering composition comprising: a binder; a sinterable powder material having a first average particle size; and a pore formation material having a second average particle size, a beginning draining temperature and a complete draining temperature, wherein the pore formation material does not soften or melt in mixing step or injection molding step applied to the thermal formation sintering composition, wherein the binder comprises: a low-temperature draining component which melts during thermal formation of the thermal formation sintering composition, the low-temperature draining component beginning draining at a temperature lower than the beginning draining temperature of the pore formation material, and completely draining at a temperature lower than the complete draining temperature of the pore formation material; and a high-temperature draining component which melts during thermal formation of the thermal formation sintering composition, the high-temperature draining component beginning draining at a temperature higher than the beginning draining temperature of the pore formation material, and completely draining at a temperature higher than the complete draining temperature of the pore formation material, wherein a content of the pore formation material is 60 to 80 volume % in the thermal formation sintering composition.

2. The sintering composition according to claim 1, wherein the binder contains the low-temperature draining component at a rate of 40 volume percent through 70 volume percent.

3. The sintering composition according to claim 1, wherein the high-temperature draining component contains at least two binder components each draining at a draining temperature differing from that of the others, after the pore formation material has drained.

4. The sintering composition according to claim 1, wherein the pore formation material comprises polymethylmethacrylate resin, wherein the low-temperature draining component comprises a wax, and wherein the high-temperature draining component comprises polypropylene.

5. The sintering composition according to claim 1, wherein the pore formation material comprises polymethylmethacrylate resin, wherein the low-temperature draining component comprises a wax, and wherein the high-temperature draining component comprises polyacetal.

6. The sintering composition according to claim 4, wherein the high-temperature draining component further comprises polyacetal.

7. The sintering composition according to claim 1, wherein the binder is drained by 0.1 volume percent through 5.0 volume percent of an entire content thereof included in the thermal formation sintering composition before the pore formation material begins draining, and wherein the binder remains un-drained by 5 volume percent through 40 volume percent of an entire content thereof included in the thermal formation sintering composition upon complete drainage of the pore formation material.

8. The sintering composition according to claim 1, wherein the second average particle size of the pore formation material is larger than the first average particle size of the sinterable powder material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a conceptual diagram showing a cross sectional structure of a formed body according to an embodiment.

(2) FIG. 2 is a conceptual diagram showing a cross sectional structure of the formed body after a first degreasing step.

(3) FIG. 3 is a conceptual diagram showing a cross sectional structure of the formed body after a second degreasing step.

(4) FIG. 4 is a conceptual diagram showing a cross sectional structure of the formed body after a third degreasing step.

(5) FIG. 5 is a conceptual diagram showing a cross sectional structure of a sintered body.

(6) FIG. 6 is a flowchart showing steps of the present invention.

(7) FIG. 7 is a graph of a degreasing step, showing how a pore formation material and a binder decrease.

(8) FIG. 8 is a conceptual diagram showing a cross sectional structure of a sintered body according to a second embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(9) Hereinafter, description will cover embodiments which are application examples of the methods of making porous sintered bodies according to the present invention to metal powder injection molding.

(10) TABLE-US-00001 TABLE 1 Binder Drain Mixing Ratio Component Starting Drain Particle to All Ratio Temp. Temp. Composition Component Size (Vol %) (Vol %) (° C.) (° C.) Pore Formation PMMA 50 μm 60 100 240 400 Material Metal Powder SUS316L 10 μm 20 Binder A Wax 12.8 64 100 210 340 (Low-temp.) Binder B POM 2.1 10.5 352 488 (High-temp.) PP 5.1 25.5 255 497

(11) In the present embodiment example, materials and conditions as shown in Table 1 are used: Metal powder is provided by SUS 316L which has an average particle size of 10 μm; pore formation material is provided by PMMA (polymethylmethacrylate resin) which has an average particle size of 50 μm. In addition, binders are provided by three kinds of binder components; i.e. wax (a compound wax made of natural wax and synthetic was), POM (polyacetal) and PP (polypropylene). These components are mixed uniformly at a ratio shown in Table 1 to make an injection compound.

(12) There is no specific limitation to the kind of metal powder to be used as the sintering compound. Other metal powders, ceramic powders or a mixture of a plurality of materials selected from these may be used as long as they are sinterable.

(13) There is no specific limitation to an average particle size of the metal powder. So called submicron particles which have a particle size of not greater than 1 μm may be used, or large powder particles of about 100 μm may also be used. Preferably however, the particle size should be selected from a range of 1 μm through 30 μm for the sake of increased sintering performance. It should be noted here that when manufacturing a sintered body which has a porosity not smaller than 50%, the metal powder should preferably have an average particle size smaller than that of the pore formation material.

(14) The amount of metal powder to be mixed, which varies depending on the porosity targeted, should preferably be selected from a range of 15 through 30 volume percent of the total compound mix. If the amount is smaller than 15 volume percent, the amount of binder is relatively large, leading to increased shrinkage or deformation in the degreasing step and sintering step. On the other hand, the amount exceeding 30 volume percent reduces fluidity, leading to poor operability.

(15) In the present example, the pore formation material is provided by PMMA particles having an average particle size of 50 μm. The PMMA particles have a draining-start temperature of 240° C. On the other hand, their draining temperature at which they are removed completely from the formed body is 400° C. In the present example, the draining-start temperature is a decomposition starting temperature of the relevant component. The pore formation material does not soften or melt in the mixing step or injection molding step. Further, in the present example, the pore formation material is granular, having a spherical particle shape; however, the pore formation material may be of other types such as fibriform, baculiform, and so on.

(16) In the present example, two kinds of binders are used: Binder A is a component which has a draining temperature lower than that of the pore formation material, and Binder B which has a draining temperature higher than that of the pore formation material.

(17) The binder A is a wax component: Its draining-start temperature is 210° C. approximately whereas its draining temperature is 340° C.

(18) The binder B contains POM (polyacetal) and PP (polypropylene). The POM has a draining-start temperature of 352° C. and a draining temperature of 488° C. The PP, on the other hand, has a draining-start temperature of 255° C., and a draining temperature of 497° C. The draining temperatures assume that the degreasing step is performed at a temperature rising rate of 20° C./hour.

(19) As is clear from these draining-start temperatures, the binder A starts draining at a draining-start temperature lower than that of the pore formation material. On the other hand, the binder B has a draining-start temperature higher than that of the pore formation material. In the present example, removal of the binder and removal of the pore formation material are defined by decomposition starting temperature and decomposition completing temperature. It must be understood, however, that a characteristic of the present invention is to keep part of the binder un-drained at the end of draining of the pore formation material. If the binder drains in a different mode, the definition criterion should preferably be a temperature or a period of time for the binder to complete draining actually.

(20) The compound of the above-described component composition is mixed at a temperature (200° C. approx.) which is low enough not to cause softening of the pore formation material made into the form of pellet. Then, the compound is molded into a predetermined shape using an injector. The injection molding step is performed also at a temperature low enough not to deform the shape of the pore formation material.

(21) After the injection molding, the formed body 1 has a structure as shown in a conceptual image given in FIG. 1. As shown in FIG. 1, large particles of the pore formation material 2 are surrounded by a mixture of metal powder 3, the binder A and the binder B, and distribution is substantially uniform. For easier understanding, FIG. 1 illustrates the binders A, B in the form of particle; however, the binders A and B melt during the molding step, and distribute to fill spaces between the particles. Of the binders, small black circles represent the binder A while large black circles represent the binder B.

(22) Next, a degreasing step is performed. Hereinafter, reference will be made to FIG. 2 through FIG. 4, and FIG. 6 along with the description.

(23) In the present example, temperature in the degreasing furnace is raised at a rate of 20° C./hour. As the temperature in the furnace reaches and goes beyond 210° C., the binder A begins draining. This is a first degreasing step S103 which continues until the pore formation material begins draining. In the first degreasing step S103, predetermined gaps are formed between the metal particles by the time when the pore formation material 2 begins to drain. In the present example, approximately 2 volume percent of the binder A drains in the first degreasing step.

(24) When the degreasing temperature reaches and exceeds the draining-start temperature (240° C.) of the pore formation material, the binder A drains, the pore formation material 2 begins to drain, and further, the binder B begins to drain, i.e. a second degreasing step takes place. In the present example, the pore formation material 2 begins draining at a temperature not lower than 240° C. while the binder B begins draining at a temperature not lower than 255° C. In other words, the three components drain simultaneously in the second degreasing step. FIG. 2 shows a state where the binder A has drained completely. For easier understanding, FIG. 2 illustrates as if no part of the pore formation material 2 or binder B has been drained; however, part of them has already been draining. As is clear from FIG. 2, draining of the binder A leaves continuous gap spaces 4 among the metal particles. Therefore, it is possible to drain the pore formation material 2 smoothly via these gap spaces. On the other hand, since the binder B is present and provides bonding between the metal particles, a sufficient level of strength is maintained while the pore formation material drains. Further, as the draining speed of the pore formation material 2 increases, so does the amount of the binder draining, providing sufficient gap space for the pore formation material to drain through. Therefore, it is possible to let the pore formation material drain while keeping the shape of the pores 5.

(25) As the degreasing temperature reaches 400° C., the binder A and the pore formation material 2 have completely drained as shown in FIG. 3, leaving the pores 5. After the pore formation material 2 has drained, the binder B is slightly softer, and provides bridging between the metal particles 3 thereby providing increased shape retentionability. Further, the binder B prevents the metal particles from dropping into the pores 5 which are left by the pore formation material. The metal particles do not change their relative positions, providing an intermediate degreased body of highly accurate shape and dimensions.

(26) After the pore formation material is drained, the temperature is further increased to perform a third degreasing step (S105), to remove the binder B. The third degreasing step leaves a degreased body as shown in FIG. 4, formed by the metal particle 3. For easier understanding, FIG. 4 illustrates as if the particles are separated from each other; however, there is only little shrinkage in the degreased body because the particles distribute in a three-dimensional manner in up-and-down directions in the drawing.

(27) Next, the temperature of the furnace is increased beyond the sintering temperature of the metal, to perform a sintering step thereby sintering the metal particles. The sintering step causes metal particles to be bonded with adjacent metal particles, shrinks gap spaces between the adjacent particles, and yields a sintered body 6 as shown in FIG. 5. The sintered body is shrunken from the formed body approximately by the amount of the binder mixed thereto.

(28) FIG. 7 shows how the pore formation material 2 and the binder (the entire body of A and B) decrease in the present example. The graph has its vertical axis representing the rate of decrease, and the horizontal axis representing the degreasing temperature. As shown in the figure, first, part of the binder A begins to drain in the present example. This provides a channel in the formed body, for the pore formation material 2 to drain through. Next, the pore formation material 2 begins to drain: The rate of decrease in the pore formation material 2 exceeds that of the binder at a certain temperature, and this situation continues until the pore formation material drains completely, i.e. the pore formation material 2 completes draining before the binder does. In other words, the degreasing step is performed while securing drainage paths for the pore formation material 2 and while providing reinforcement by the binder, with part of the binder remaining un-drained even at a point when the pore formation material has completely drained. This enables to allow the degreasing step to proceed, with increased shape retentionability of the formed body.

(29) By retaining the binder B when draining the pore formation material, walls which define individual pores remain intact. Thus, it is possible to form accurate porous bodies based on the rate of addition of the pore formation material.

(30) Further, in the present example, the binder B contains two binder components each having a different draining temperature from the other. This prevents premature draining of the remaining binder which remains after the pore formation material 2 has drained. Therefore, it is possible to prevent the formed body from being subjected to excessive stresses in the process when the binder drains. As a result, even after the pore formation material 2 has drained, the method provides good shape retentionability, enabling to prevent the formed body from deforming.

(31) TABLE-US-00002 TABLE 2 Comparative Example Mixing Drain Ratio to Starting Drain Particle All Temperature Tempera- Composition Component Size (Vol %) (° C.) ture Pore PMMA 50 μm 60 240 400 Formation Material Metal SUS316L 10 μm 20 Powder Binder A Wax 10 210 340 Binder B PS 10 280 360 (polystyrene)

(32) Table 2 shows a combination ratio in a comparative example. In the comparative example, the binder B which drains at a temperature higher than that of the pore formation material 2 is replaced by a binder C (polystyrene) which drains at a temperature lower than that of the pore formation material 2. All of the others in the combination ratio are identical with those in the embodiment example 1, so no more description will be given here. Further, there is no difference from the embodiment example in the degreasing step or the sintering step; however, it should be noted that the third degreasing step is not performed in the comparative example since the binder B is not mixed thereto.

(33) Measurements were made to sintered bodies which were made in the embodiment example 1 and the comparative example. In the embodiment example, the porous body had an approximately 60% porosity, matching to the combination ratio of the pore formation material 2. In the comparative example on the other hand, the porous body had an approximately 40% porosity despite the fact that the same combination ratio of the pore formation material was used. Further, microscopic observation of the structure of the sintered body according to the embodiment example 1 revealed that the example 1 had substantially uniform, spherical pores. In the comparative example on the other hand, pore shapes were irregularly deformed and the sizes were smaller. From these observations, it became clear that a porous sintered body which has a high level of accuracy can be formed by using the binder B thereby retaining the shape which is formed with the metal particles, during the time when draining the pore formation material.

(34) As shown in FIG. 6, the second degreasing step S104 may be followed by an interim work step S106. Since it is possible, according to the present embodiment, to retain the binder after the pore formation material has drained, a sufficient level of strength is retained in the porous formed body, and therefore, a variety of works can be performed to the sintered body even after the body became porous.

(35) The interim work step may include e.g. machining like in convention. Further, it is now possible to fill the pores and continuous pore spaces in the formed body with a variety of functional substances.

(36) The second degreasing step shrinks the formed body very little, so the pores have a large size. Thus, it is now possible to load the intermediate body with functional substances which have not been possible to load according to conventional methods. There is no specific limitation to the method for the loading; for example, a high level of porosity allows use of fluid as a way to fill the spaces with functional substance, while it is also possible to use a mechanical method of filling a functional substance only in the surface.

(37) A substance which acts as a catalyst may be filled in the pores before sintering is performed in the sintering step. According to this process, the catalytic substance is fixed in inner walls of the pores, so it is now possible to fix expensive substances such as platinum inside of the pores.

(38) Further, as shown in FIG. 8, it also becomes possible to have these continuous pores 5 hold particles 8 which are not sintered together with the porous sintered body, through an additional step performed later.

(39) In the present embodiment, a porous formed body containing pores which communicate with each other are formed, then the particles 8 are filled, and thereafter, the third degreasing step S107 and the sintering step S108 are performed. Since the sintering step shrinks the pores 5, it is also possible to make the particles movable but inescapable from the pores.

(40) The fillers may be selected from metal oxides such as titanium oxide and functional ceramics such as apatite.