JP2011144419A - Sintering method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a product having a cavity or a through-hole therein is difficult to produce by powder injection molding. <P>SOLUTION: The sintering method comprises steps of: kneading a first composition containing a first powder of a thermoplastic resin; carrying out injection molding so that the kneaded first composition becomes a core; incorporating the core into an outer mold so as to assemble a mold; kneading a second composition comprising a powder chosen from the group consisting of a metal and a ceramic and a second powder of a binder for powder injection molding; injecting the kneaded second composition into the mold and carrying out injection molding to obtain a green compact; and sintering the green compact with the core incorporated therein. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a sintering method by a powder injection molding method such as metal powder injection molding (MIM) or ceramic powder injection molding (CIM).

  A technique of forming a green body by injection molding metal powder together with a suitable binder and subjecting it to sintering is called metal powder injection molding (MIM). MIM is suitable for the purpose of producing a precise sintered body, and its use is gradually expanding. Ceramic powder injection molding (CIM) in which metal powder is replaced with ceramic powder has also been put into practical use. Patent documents 1 and 2 disclose related technology.

  Unlike a casting method that uses a mold that does not have a fixed shape, such as foundry sand, an injection-molded mold is basically a fixed metal. Metal molds can be precisely processed, and MIM and CIM are suitable for precise molding of complex shapes. On the other hand, a problem arises when trying to manufacture an article having a cavity or a through hole inside. A core is required to form a cavity or a through-hole, but the core must be pulled out before sintering or crushed and removed after sintering. Since the green body is merely an aggregate of powder and binder, the number of cases where the core can be pulled out without affecting its shape is limited. Even if the core is to be crushed after sintering, the core should be strong enough to withstand the injection pressure, so it is difficult to crush and remove it. Therefore, when it is going to manufacture the product which has a cavity and a through-hole by MIM or CIM, the process by a drill etc. is additionally required. The shape that can be produced for the convenience of processing is remarkably restricted, and furthermore, the finish of the inner surface (surface roughness) is problematic, and of course the productivity is also reduced.

Japanese Patent No. 4240512 Japanese Patent No. 4317916

  An object of this invention is to provide the sintering method by the powder injection molding suitable for manufacturing the product which has a cavity and a through-hole inside.

  According to one aspect of the present invention, the first composition containing the first thermoplastic resin powder is kneaded, the kneaded first composition is injection-molded to become a core, and the core Assembling the mold into the outer mold, the mold is assembled, and the second composition containing any powder selected from the group consisting of metal and ceramic and the powder of the second powder injection molding binder is kneaded and kneaded. Injection of the prepared second composition into the mold to obtain a green body, and sintering the green body with the core incorporated therein.

  Preferably, the first thermoplastic resin is made of acrylic. Further preferably, the first composition further includes a first powder injection molding binder, and the first powder injection molding binder comprises polylactic acid, polyoxymethylene, polypropylene, and 150 ° C. An organic compound having a viscosity of 200 mPa · s or less and a second thermoplastic resin having a Vicat softening point of 130 ° C. or less are included. More preferably, the sintering step includes a step of raising the temperature from 70 to 150 ° C. at a rate of 0.3 to 2 ° C./min. More preferably, the sintering step includes a step of holding at 70 to 150 ° C. for 30 minutes or more.

  Even if the core is sintered with the molded body being sintered, the core disappears during the sintering process, and a powder injection molded product having cavities and through-holes inside without additional cavities and through-holes being processed. Can be manufactured.

FIG. 1 is a flowchart illustrating the steps of a sintering method by powder injection molding using a core according to an embodiment of the present invention. FIG. 2 is a cross-sectional view of an example of a sintered body suitable for the sintering method. FIG. 3 is a cross-sectional view of an example of a mold including a core adapted to sinter the sintered body. FIG. 4 is a cross-sectional view of an example of a mold for injection molding the core. FIG. 5 is a plan view of the green body taken out of the mold together with the core.

  Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

  Although this embodiment can be used for manufacturing various sintered bodies, an example of a suitable sintered body is a pipe joint as shown in FIG. That is, FIG. 2 shows a pipe joint 1 which is an example of a sintered body, and has a through hole 3 for allowing a fluid to flow inside.

  First, it is necessary to form the green body from which the shape of the pipe joint 1 should be obtained after sintering by injection molding. Referring to FIG. 1, first, a mold 10 for injection molding is manufactured.

  FIG. 3 shows an example of the mold 10. Since the volume shrinkage of about 20% occurs in the process of the green body being sintered to become a sintered body, the mold 10 is designed in consideration of the volume shrinkage. The mold 10 includes outer molds 11, 13, and 15 and a core 20. The outer mold can be divided into a plurality of appropriate parts so that the green body can be taken out after injection molding. In this example, the outer mold includes a first mold 11, a second mold 13, and a nest 15. The first mold 11 can be further divided in the front-rear direction with respect to the paper surface. The first mold 11 and the second mold 13 may be provided with appropriate positioning means such as a pair of protrusions and engagement holes in order to enable accurate mutual positioning. The outer surface of the insert 15 is slightly tapered and is positioned relative to each other by fitting into the corresponding inner surface of the hole of the first mold 11. Alternatively, other appropriate positioning means may be provided as a cylindrical surface without being tapered. The fitting surfaces between the molds are preferably formed so as to closely contact each other, but a slight gap may be provided partially in order to degas.

  The first mold 11 is provided with a runner 17 and allows introduction of a mixture of a metal or ceramic powder and a binder. A space surrounded by the outer molds 11, 13, 15 and the core 20 is a space 19 to be a green body mold except for the runner 17.

  The first mold 11, the second mold 13, and the insert 15 constituting the outer mold are made of an appropriate metal such as SKD11 (JIS G 4404) and are manufactured by a known processing method.

  The core 20 is formed by injection molding. FIG. 4 is an example of a mold 30 for injection molding the core 20. The mold 30 includes a runner 31 and a cavity 33 communicating with the runner 31. The cavity 33 corresponds to the outer shape of the core 20. The mold 30 can also be divided into a plurality of pieces as appropriate.

  The composition to be used for injection has a property of disappearing in the sintering process described later. An example of such a composition is a mixture of a thermoplastic resin and appropriate additives. The thermoplastic resin typically disappears by melting, decomposing and evaporating above 200 ° C.

  Examples of the thermoplastic resin include styrene-based, acrylic-based, cellulose-based, polyethylene-based, vinyl-based, nylon-based, and fluorocarbon-based resins. In particular, the acrylic resin has strength enough to withstand the injection pressure, has an appropriate viscosity during kneading, and has good melting, decomposition, and evaporation in the sintering process.

  A higher mixing ratio of the thermoplastic resin is advantageous in terms of shape stability of the green body, but a lower mixing ratio is advantageous in terms of fluidity in the kneading and sintering processes. Therefore, the mixing ratio of the thermoplastic resin is preferably 50% by weight or more and less than 100% by weight, more preferably 70% by weight or more and less than 90% by weight.

  The additive can be appropriately selected for various purposes such as the purpose of adjusting the viscosity and fluidity and the purpose of adjusting the shape stability of the composition. Examples of the additive include polyoxymethylene, polypropylene, an appropriate organic compound, and a binder for powder injection molding.

  Since polyoxymethylene increases the strength of the composition, the addition of polyoxymethylene is advantageous in terms of increasing the shape stability of the core. The green body is softened and shrunk at the initial stage of sintering. Particularly, polyoxymethylene having a Vicat softening point of 150 ° C. or more is advantageous in that it can easily prevent the green body from being deformed in the process of softening and shrinking. Polyoxymethylene has good decomposition and evaporation in the sintering process, and does not leave a residue in the sintered body.

  Since polypropylene increases the toughness of the composition, the addition of polypropylene is advantageous in terms of preventing damage to the core. In particular, those having a Vicat softening point of 130 ° C. or more are advantageous in that the green body can be easily prevented from being deformed in the process of softening and shrinking. Polypropylene has good decomposition and evaporation in the sintering process, and does not leave a residue in the sintered body.

  Furthermore, an appropriate organic compound lowers the viscosity in the sintering step, and therefore adding this is advantageous in terms of facilitating melting of the core. For example, polylactic acid such as a polymer of L-lactic acid or D-lactic acid is preferable because it has good decomposition and evaporation in the sintering process and does not leave a residue in the sintered body. In particular, when polyoxymethylene is added, an organic compound having a viscosity at the Vicat softening point temperature of 200 mPa · s or less facilitates melting and outflow of the core in the process of shrinking the green body. Such organic compounds include fatty acid esters, fatty acid amides, phthalic acid esters, paraffin wax, microcrystalline wax, polyethylene wax, polypropylene wax, carnauba wax, montan wax, urethanized wax, maleic anhydride modified wax, polyglycol. Examples of such compounds are listed. Only one selected from such organic compounds may be added, or two or more may be added.

  A known powder injection molding binder may be used instead of or in addition to the above-described additives. Examples of the powder injection molding binder include polylactic acid, polyoxymethylene, polypropylene, an organic compound having a viscosity at 150 ° C. of 200 mPa · s or less, and a second thermoplastic resin having a Vicat softening point of 130 ° C. or less. Can be suitably used. Such a powder injection molding binder is generally available under the name MRM-1 (trade name of IHI Turbo).

  The above-mentioned thermoplastic resin and additives are used in a finely crushed form for the convenience of kneading. The finely crushed form may be any form convenient for kneading, such as powder, granule, strip, etc. In the specification and claims, the term “powder” includes all these aspects. Define and use. The mixture of the thermoplastic resin powder and the additive powder is kneaded by heating to 100 to 150 ° C. to obtain an appropriate viscosity. After kneading, the air-cooled solid is pulverized and subjected to injection molding.

  The above-mentioned kneaded mixture is heated to 160 to 200 ° C. to give sufficient fluidity, and injected into the mold 30 through the runner 31 with a pressure of about 100 MPa. The mixture is molded by filling the cavity 33 of the mold 30 without a gap, and is taken out from the mold 30 and air-cooled, whereby the core 20 is obtained.

  The mold 10 is assembled in combination with the core 20. First, the core 20 has one end fitted to the second mold 13 and the other end fitted to the insert 15. Then, the first mold 11 is fitted to the second mold 13 and the insert 15 so as to sandwich them, and the mold 10 having a form as shown in FIG. 3 is assembled. The mold 10 assembled in this way is introduced into an injection molding machine. As the injection molding machine, a general machine for MIM can be used.

  In parallel with the above-described steps, the composition to be subjected to powder injection molding is kneaded. For such a composition, a mixture of metal powder or ceramic powder and a binder is suitable.

  As the metal powder or ceramic powder, powders of various materials can be used according to required characteristics. As a material of the pipe joint 1, for example, SUS316 (JIS G 4303 to 4305) is suitable in consideration of corrosion resistance, and powder of such material is used.

  As the binder, a general powder injection molding binder can be used. Examples of the powder injection molding binder include polylactic acid, polyoxymethylene, polypropylene, an organic compound having a viscosity at 150 ° C. of 200 mPa · s or less, and a second thermoplastic resin having a Vicat softening point of 130 ° C. or less. Can be suitably used. Such a powder injection molding binder is generally available under the name MRM-1 (trade name of IHI Turbo). Such a binder may be the same as or different from the binder used for forming the core 20.

  The mixing ratio of the binder can be adjusted as appropriate. A larger binder mixing ratio is advantageous for maintaining the shape of the molded body. On the other hand, a smaller binder mixing ratio is advantageous in that it suppresses volume shrinkage due to sintering. Therefore, the mixing ratio of the binder can be, for example, 30 to 60% by volume, and more preferably 35 to 50% by volume with respect to the metal powder or ceramic powder.

  The mixture of the metal powder or ceramic powder and the binder is heated to 100 to 150 ° C. and kneaded to give an appropriate viscosity. The above-mentioned kneaded mixture is heated to 160 to 200 ° C. to give sufficient fluidity, and injected into the mold 10 through the runner 17 together with a pressure of about 100 MPa. The mixture is molded by filling the cavity 19 in the mold 10 without any gap. Thereafter, the green body 40 is taken out together with the core 20 by a procedure almost opposite to that for assembling the mold 10.

  As shown in FIG. 5, the green body 40 taken out from the mold 10 is in a state in which the core 20 is incorporated therein. In this state, the green body 40 is introduced into a sintering furnace capable of controlling the atmosphere. While purging the inside of the furnace with a non-oxidizing gas such as nitrogen under an appropriate reduced pressure, the green body 40 is raised at a temperature rising rate of about 0.3 to 2 ° C./min by an appropriate heating means such as a carbon heater. Warm up. In order to increase the efficiency of temperature increase, the temperature increase rate may be increased up to about 70 to 150 ° C., and the temperature increase rate may be decreased beyond that.

  As the pre-sintering progresses during the temperature raising process, the green body 40 softens and contracts. However, since the core 20 also has the above-described composition, the green body 40 similarly softens and contracts. . Since the softening and shrinkage of both are harmonized, the green body 40 appropriately retains its shape and structure by the core 20 in this process. In order to coordinate both softening and shrinkage, a step of maintaining the temperature at about 70 to 150 ° C. for 30 minutes or more may be provided. Further, the outside of the green body 40 may be supported by an appropriate jig in order to maintain the outer shape.

  Above 200 ° C., the temperature raising conditions generally employed by those skilled in the art in MIM and CIM can be applied. For example, the above-described temperature increase may be continued up to about 500 ° C., or a step of appropriately maintaining the temperature at 400 to 500 ° C. may be provided. In such a process, the binder and the core 20 in the green body 40 are melted, decomposed, evaporated and converted into a gas, which is discharged out of the furnace together with the purge gas.

  Then, it is good also as a temperature increase rate of about 2-20 degree-C / min to sintering temperature. The sintering temperature can be appropriately determined depending on the metal powder or ceramic powder, and is, for example, 1200 to 1400 ° C. As is generally done, the sintering temperature may be held for 1 hour or more. In addition, the purge may be switched to an inert gas such as argon before or at the stage of maintaining the sintering temperature.

  When the sintering is completed, the furnace is cooled while continuing the purge. The cooling rate should be appropriately controlled so as not to give an excessive thermal shock to the sintered body. After sufficient cooling, nitrogen or the like is introduced to bring the inside of the furnace to atmospheric pressure, and the sintered body is taken out.

  In addition, although the batch type example was demonstrated for sintering, in view of efficiency, you may utilize a continuous furnace suitably.

  Although it can be used as a product as it is sintered, its inner and outer surfaces are covered with a lightly fixed powder-like film. Accordingly, barrel processing is preferably performed. That is, the sintered body is inserted into a barrel processing apparatus together with a crushed stone, a metal needle and an appropriate amount of water, and stirred. Then, the coating covering the inner and outer surfaces of the sintered body is removed, and smooth inner and outer surfaces appear. That is, smooth and glossy inner and outer surfaces can be obtained without machining or polishing, and the roughness is typically about Ra = 1.6.

  According to the present embodiment, the green body is sintered with the core incorporated therein, but the core disappears during the sintering process, so it is not necessary to remove it after sintering. Further, the shape of the through-hole is appropriately maintained by the core even in the powder injection molding process and the initial stage of sintering. Therefore, the pipe joint 1 obtained by the above-described process has a desired shape and smooth inner and outer surfaces without performing additional machining. Unlike the through hole formed by machining, the through hole of the pipe joint 1 can have a smooth shape and a smooth inner surface, so that the fluid can flow smoothly and does not form a liquid pool. In a chemical plant, a painting line, etc., it is particularly suitable for application to piping systems, pump systems, control equipment, etc. that require high controllability and response. A smooth inner surface is advantageous in that it suppresses turbulent flow in the through-hole and reduces flow velocity resistance, and is advantageous in that it can suppress adverse effects such as noise and vibration in piping systems, pump systems, and control equipment. is there.

  The preferred embodiment has been described above by taking a pipe joint having a bent through hole as an example. This embodiment can be applied to the sintering of various products. For example, the through hole is not limited to a bent shape as illustrated, but may be a simpler shape or a more complicated shape. For example, as illustrated in FIG. 2, the present embodiment can be applied not only to the elbow type but also to a pipe joint having a more complicated bent shape. Moreover, the target product does not need to have openings at both ends, and may have openings at only one side. Furthermore, although the example which forms a structure like a through-hole or a cavity inside a sintered compact was demonstrated, it is applicable also when forming a part of external structure. If the outer structure is supported by an equivalent of the above-described core, the outer structure can be prevented from being deformed in the preliminary sintering stage.

(Example)
In order to search for a composition of a mixture suitable for producing a core suitable for powder injection molding, the following test was conducted.

  40% by volume of MRM-1 was mixed and kneaded with a metal powder having a composition corresponding to SUS316. A short tubular simulated molded body was injection molded from such a mixture. The simulated molded body has linear through holes by molding using a metal core. The core was withdrawn and a hollow through hole was submitted for testing.

The 12 mixtures listed in Table 1 were kneaded at 150 ° C. The mixing ratio is expressed in weight percent. And it filled with the through-hole of the above-mentioned simulation molded object, respectively. After the mixture solidified, the simulated compact was introduced into a sintering furnace and sintered. Sintering was carried out under a reduced pressure of 13.3 kPa while purging with nitrogen at a flow rate of 3 l / min. The temperature was raised to 800 ° C. at 67 ° C./min, and the temperature was raised from 800 ° C. to 1350 ° C. at 5 ° C./min. At 800 ° C. or higher, purging was performed with argon at a flow rate of 5 l / min. The temperature was maintained at 1350 ° C. for 3 hours and then cooled at 5.83 ° C./min. The sintered body was taken out from the furnace and the appearance was observed.

  Sample 2 had too high fluidity during the kneading process, and even if it was filled in the simulated molded body, it formed a nest. Samples 3 and 8 were too fluid. Therefore, it was judged that these could not be kneaded. Samples 1, 4 to 7 and 9 to 12 had no problem in kneading, but sample 12 was slightly lacking in fluidity than the others, and sufficient kneading took time. In Samples 1 to 4 and Samples 8 to 10, deformation or damage was observed in the appearance after sintering. Only Samples 5 to 7 and Samples 11 and 12 were normal sintered bodies. In particular, Samples 11 and 12 had beautiful and smooth inner and outer surfaces.

  Since Samples 5 to 7 and Samples 11 and 12 have no problem in the kneading process and have no problem in the shape of the sintered body, the mixing ratio of acrylic is preferably 50% by weight or more, more preferably 70% by weight. % Or more is estimated to be suitable. Further, from the viewpoint of kneadability, it is preferably less than 100% by weight, more preferably less than 90% by weight.

  Although the present invention has been described with reference to preferred embodiments, the present invention is not limited to the above-described embodiments. Based on the above disclosure, a person having ordinary skill in the art can implement the present invention by modifying or modifying the embodiment.

  Provided are a manufacturing method of a core for powder injection molding suitable for manufacturing a product having a cavity and a through hole therein, and a powder injection molding method using the core.

1 Pipe joint (an example of a sintered body)
3 Through-hole 10 Mold 11, 13, 15 Outer mold 20 Core 30 Mold 40 Green body

Claims (5)

Kneading the first composition containing the powder of the first thermoplastic resin,
Injection-molding the kneaded first composition to be a core,
Assemble the mold by incorporating the core into the outer mold,
Kneading a second composition containing any powder selected from the group consisting of metals and ceramics and a powder of a second powder injection molding binder,
Injection molding to obtain a green body by injecting the kneaded second composition into the mold,
Sintering the green body while incorporating the core,
A sintering method.   The sintering method according to claim 1, wherein the first thermoplastic resin is made of acrylic.   The first composition further includes a first powder injection molding binder, and the first powder injection molding binder has polylactic acid, polyoxymethylene, polypropylene, and a viscosity at 150 ° C. of 200 mPa · The sintering method of Claim 1 or 2 containing the organic compound which is s or less, and the 2nd thermoplastic resin which is Vicat softening point 130 degrees C or less.   The sintering method according to claim 1, wherein the sintering step includes a step of raising the temperature from 70 to 150 ° C. at a rate of 0.3 to 2 ° C./min.   The sintering method according to claim 1, wherein the sintering step includes a step of holding at 70 to 150 ° C. for 30 minutes or more.

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