JP2012062221A - Method for producing sintered compact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a sintered compact, enabling to effectively prevent a compact as a precursor thereof from being cracked or chipped while it is transferred after being degreased.SOLUTION: The method for producing a sintered compact includes: a step of kneading a raw material powder with a resin material under heating to obtain a molding raw material; a step of injection-molding the molding raw material to obtain a compact; a first degreasing step of removing part of the resin material from the compact; a second degreasing step of removing the remainder of the resin material therefrom; and a firing step, wherein the resin material contains wax, a first binder, and a second binder. The heat decomposition temperature of the wax is lower than those of the first binder and the second binder, and the heat decomposition temperature of the first binder is lower than that of the second binder. In the first degreasing step, the compact is heated to a temperature ranging from the flow start temperature of the first binder to below the heat decomposition temperature of the second binder and then cooled to a temperature below the flow start temperature of the first binder. The second degreasing step and the firing step are successively performed.

Description

  The present invention relates to a method for manufacturing a sintered body, and more particularly, to a method for manufacturing a sintered body that can effectively prevent the molded body from being damaged when the molded body after degreasing is conveyed.

  A sintered body made of an inorganic material is manufactured by firing a molded body formed by, for example, a press molding method, an extrusion molding method, a cast molding method, a tape molding method, an injection molding method, or the like.

  The injection molding method has an advantage that a molded body having a complicated shape can be obtained and the time required for the molding process is short. However, the amount of the resin component contained in the molded body is relatively large, and if these are rapidly removed in the degreasing process, defects such as swelling of the molded body may occur.

  Patent Document 1 discloses that the step of degreasing a molded body obtained by injection molding is divided into a degreasing preliminary step and a heat degreasing step. However, in such a degreasing process, resin components (binder, wax, plasticizer, etc.) contained in the molded body are excessively removed, and the strength of the degreased body (the molded body after the degreasing process) is increased before the degreasing process. It will fall rather than the intensity | strength of a molded object.

  As a result, when the degreased body is transported from the heating furnace used for degreasing to another heating furnace (firing furnace), there has been a problem that the degreased body is cracked or chipped.

Japanese Examined Patent Publication No. 6-47684

  This invention is made | formed in view of such an actual condition, and it aims at providing the manufacturing method of the sintered compact which can prevent effectively a crack and a chip | tip of a molded object at the time of conveyance of the molded object after degreasing | defatting.

In order to achieve the above object, a method for producing a sintered body according to the present invention comprises:
A step of heating and kneading the raw material powder and the resin material to obtain a raw material for molding;
Injection molding the raw material for molding to obtain a molded body;
A first degreasing step of removing a part of the resin material from the molded body;
A second degreasing step for removing the remaining resin material;
A firing step of firing the molded body after the first degreasing step and the second degreasing step,
The resin material contains at least a wax, a first binder, and a second binder, and a thermal decomposition temperature of the wax is lower than a thermal decomposition temperature of the first binder and the second binder. The thermal decomposition temperature is lower than the thermal decomposition temperature of the second binder,
In the first degreasing step, after the temperature range of not less than the flow start temperature of the first binder and less than the thermal decomposition temperature of the second binder, it is cooled to a temperature less than the flow start temperature of the first binder,
The second degreasing step and the firing step are performed continuously.

  In the present invention, in the first degreasing step, first, the wax having a low thermal decomposition temperature is removed from the molded body. At this time, fine pores are formed in the portion where the wax is removed. On the other hand, in the first degreasing step, the first binder starts to flow, penetrates the entire molded body, and penetrates around the pores in a fragile state. At this time, the second binder exhibits sufficient shape retention as a binder without being decomposed, so that the shape of the molded body can be maintained.

  Then, when it cools to the temperature below the flow start temperature of a 1st binder, a 1st binder solidifies and can exhibit sufficient shape retention as a binder. At this time, since the first binder penetrates the entire holes formed by removing the wax, the holes are reinforced and will not be deformed. Therefore, the strength of the molded body after the first degreasing step can be improved in a state where the shape is maintained. That is, the strength of the molded body after the first degreasing step can be increased more than the strength of the molded body before the first degreasing step.

  Therefore, even if the molded body after the first degreasing step is conveyed, the strength is sufficiently ensured, so that breakage such as cracks and chips can be effectively prevented. In addition, cracks and chips during processing of the molded body can be effectively prevented. Therefore, the yield as a product can be improved.

  Subsequently, since the second degreasing process is performed in a state where the shape of the holes is maintained, the resin material contained in the molded body is smoothly removed through the holes. At this time, since the second binder has a higher thermal decomposition temperature than the first binder, the second binder exists inside the molded body even after the first binder is vaporized and removed, and the shape of the molded body can be maintained. . As a result, the shape of the formed body can be maintained until the end of the degreasing step, and deformation of the sintered body in the firing step performed subsequent to the second degreasing step can be effectively prevented.

  Preferably, the resin material further has a plasticizer. Since the plasticizer is contained, the flexibility to the molded body is given and the crystallization speed of the resin is slowed down, so that the solidification of the first binder proceeds evenly throughout the molded body. .

  Preferably, the wax is substantially removed in the first degreasing step. By doing in this way, the intensity | strength of the molded object after a 1st degreasing process can be raised reliably.

  Preferably, when the content of the resin material contained in the molded body before the first degreasing step is 100%, the resin material contained in the molded body after the first degreasing step The amount is 10-60%.

  By making content of the resin material contained in the molding after the 1st degreasing process into the above-mentioned range, while the above-mentioned effect is acquired and the resin component is removed moderately, the 2nd degreasing process and Even in the firing step, the molded body does not swell, and deformation of the obtained sintered body can be effectively prevented.

FIG. 1 is a schematic cross-sectional view of an apparatus for manufacturing a ferrite magnet according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the amount of the resin component remaining in the molded body before and after the first degreasing step and the breaking load of the molded body.

  Hereinafter, the present invention will be described based on embodiments shown in the drawings.

  The sintered body in the present invention is not particularly limited, and examples thereof include ceramics and metal materials. In this embodiment, ceramics are preferable, and ferrite magnets are particularly preferable. As the ferrite, hexagonal ferrites such as magnetoplumbite type M phase and W phase are preferably used.

Such ferrite is particularly preferably MO.nFe ₂ O ₃ (M is preferably one or more of Sr and Ba, n = 4.5 to 6.5). Such ferrite further contains rare earth elements, Ca, Pb, Si, Al, Ga, Sn, Zn, In, Co, Ni, Ti, Cr, Mn, Cu, Ge, Nb, Zr, and the like. May be.

In particular, ferrite having hexagonal magnetoplumbite type (M type) ferrite containing A, R, Fe and M as constituent elements shown below as a main phase is preferable. However, A is at least one element selected from Sr, Ba, Ca and Pb, R is at least one element selected from rare earth elements (including Y) and Bi, and M is , Co and / or Zn. The total composition ratio of these metal elements of A, R, Fe, and M is based on the total amount of metal elements.
A: 1 to 13 atomic%,
R: 0.05 to 10 atomic%,
Fe: 80 to 95 atomic%,
M: 0.1 to 5 atomic%.

  In this ferrite, the composition formula of ferrite when R is present at the A site and M is present at the Fe site can be expressed as shown in the following formula 1. Note that x, y, and z are values calculated from the above amounts.

_{A 1-x R x (Fe} 12-y M y) z O 19 ... Formula 1

  In the method for manufacturing a sintered body according to the present embodiment, first, the ferrite raw material powder is prepared. As the raw material powder, an oxide or a compound that becomes an oxide by firing may be used. Next, the prepared raw material powders may be mixed and calcined as necessary. The calcination is carried out in the atmosphere at, for example, 1000 to 1350 ° C. for 1 second to 10 hours, particularly when obtaining finely calcined powder of M-type Sr ferrite at 1000 to 1200 ° C. for about 1 second to 3 hours. Good.

  Such a calcined powder is substantially composed of granular particles having a magnetoplumbite type ferrite structure, and the average particle size of the primary particles is 0.1 to 1 μm, particularly 0.1 to 0.5 μm. Preferably there is. The average particle diameter may be measured by, for example, a scanning electron microscope (SEM).

  The raw material powder or calcined powder may be pulverized as necessary. The pulverization may be dry, wet, or a combination thereof.

  When pulverization is performed, the average particle size of the pulverized powder particles is preferably in the range of 0.03 to 0.7 μm, more preferably in the range of 0.1 to 0.5 μm.

  The obtained raw material powder may be molded by heating and kneading in the injection molding machine together with the resin material, but in this embodiment, the dried raw material powder and the resin material are kneaded and molded into pellets with a pelletizer or the like. To do. The kneading is performed with, for example, a kneader. As the pelletizer, for example, a twin-screw single-screw extruder is used.

  The resin material contains a wax, a first binder, and a second binder. In these, the thermal decomposition temperature and the flow start temperature are in a specific relationship described later. Here, the thermal decomposition temperature means a temperature at which the resin material starts to be removed from the molded body, for example, a temperature at which the resin material starts to evaporate and vaporize.

  The flow start temperature is a temperature at which the resin material softens and can easily move between the raw material powders constituting the molded body. In this embodiment, when the flow start temperature is reached, the resin material is sufficiently distributed (penetrated) in the raw material powder or the molded body. The flow start temperature is substantially the same concept as the melting point of the resin material, but may be a glass transition point or the like.

  In addition to natural waxes such as carnauba wax, montan wax, and beeswax, synthetic waxes such as paraffin wax, urethanized wax, and polyethylene glycol are used as the wax.

  The wax mainly improves the fluidity of the raw material at the time of molding, but if it is too much, the molded product tends to become brittle. Therefore, the content of the wax is preferably 2 to 20% by weight, more preferably 4 to 10% by weight with respect to 100% by weight of the raw material powder.

  Further, since the wax is easily vaporized (easy to be thermally decomposed) in the resin material, most of the wax tends to be removed from the molded body in the initial stage of the degreasing process (first degreasing process described later). The thermal decomposition temperature of the wax is preferably 110 to 350 ° C, more preferably 200 to 330 ° C.

  As the first binder and the second binder, a polymer compound such as a thermoplastic resin is used.

  The first binder improves the fluidity of the raw material during molding and maintains the shape of the molded body (holds the shape).

  The first binder is not particularly limited as long as it has a thermal decomposition temperature higher than that of the wax, but is preferably a crystalline resin and has a relatively low flow initiation temperature. This is because the first degreasing step can be performed at a relatively low temperature. The thermal decomposition temperature of the first binder is preferably 180 to 450 ° C, more preferably 250 to 450 ° C. Moreover, the flow start temperature of the first binder is preferably 100 to 300 ° C, more preferably 130 to 200 ° C.

  In the degreasing step (particularly, the second degreasing step), the first binder is liquefied and then thermally decomposed (vaporized) and removed from the molded body. Specific examples of the first binder include polyethylene, polypropylene, polyacetal, polyamide, polymethylpentene, and polyethylene terephthalate.

  If the content of the first binder is too large, the resin removed at the time of degreasing increases, so that cracks are likely to occur in the molded body. If it is too small, the moldability of injection molding deteriorates and the shape of the molded body It tends to be difficult to maintain. Therefore, the content of the first binder is preferably 2 to 20% by weight and more preferably 4 to 10% by weight with respect to 100% by weight of the raw material powder.

  Even after the first binder is thermally decomposed and removed from the molded body, the second binder is present and retained in the molded body, and can effectively prevent deformation of the molded body during firing.

  The second binder is not particularly limited as long as it has a thermal decomposition temperature higher than that of the wax and the first binder, but is preferably an amorphous resin having a viscosity higher than that of the first binder at the same temperature.

  The thermal decomposition temperature of the second binder is preferably 200 to 500 ° C, more preferably 300 to 460 ° C.

  In the second degreasing step, the second binder is not rapidly liquefied, or is thermally decomposed after being present in a state of high viscosity even when liquefied, and is removed from the molded body. By exhibiting such behavior, the performance as a binder can be sufficiently exerted until immediately before thermal decomposition, and the shape of the molded body can be maintained. Specific examples of the second binder include acrylic resins typified by polymethyl methacrylate, polystyrene, ethylene vinyl acetate copolymer, and polycarbonate.

  If the content of the second binder is too large, it takes a long time for degreasing and induces cracks and deformation. If it is too small, the shape retention of the molded product is insufficient in the second degreasing step. Thus, the sintered body tends to be deformed. Therefore, the content of the second binder is preferably 0.1 to 10% by weight, more preferably 0.3 to 5% by weight with respect to 100% by weight of the raw material powder.

  The resin material may further include a plasticizer, a lubricant, other sublimable compounds, etc. in addition to the wax and binder described above.

  As the plasticizer, for example, phthalate ester is used, and diisodecyl phthalate, dilauryl phthalate, butyl lauryl phthalate, di-normal octyl phthalate, DOP and the like are preferable. Other examples include diisodecyl adipate, dioctyl adipate, phosphorus Preference is given to tricresyl acid and the like.

  The above-described pellets are put into the injection molding apparatus 2 shown in FIG. 1 and molded. The injection molding apparatus 2 includes an extruder 6 having a hopper 4 into which pellets 10 are charged, and a mold apparatus 8 for molding a melt of the pellets 10 extruded from the extruder 6 in a cavity 12. In this embodiment, since a ferrite magnet is manufactured, a magnetic field is applied to the mold apparatus 8 to perform CIM (ceramic injection molding) molding.

  The pellets 10 put into the hopper 4 are heated and melted and kneaded at, for example, 160 to 230 ° C. inside the extruder 6 to become a forming raw material, and are injected into the cavity 12 of the mold apparatus 8 by a screw. The temperature of the mold apparatus 8 is 20 to 80 ° C. What is necessary is just to determine the magnetic field applied to the metal mold apparatus 8 suitably.

  The obtained molded body is subjected to a degreasing step to remove the resin component. First, in the first degreasing step, the temperature is at least equal to or higher than the flow start temperature of the first binder and lower than the thermal decomposition temperature of the second binder. Specifically, it is preferably maintained for about 10 to 100 hours in a temperature range of 150 to 300 ° C, more preferably 180 to 280 ° C. By setting it as such a temperature range, most waxes contained in a molded object are removed from a molded object. In particular, by setting the temperature in the range of 180 to 280 ° C., almost the entire amount of the wax is vaporized and removed. At this time, fine pores are formed in the molded body due to the vaporization of the wax.

  On the other hand, the first binder starts to flow in the first degreasing step. Then, it penetrates into the molded body and also penetrates around the pores formed by the vaporization of the wax. Accordingly, the degree of dispersion of the first binder in the molded body is better than after the molding. When the first binder starts to flow, the performance as the binder is degraded. However, the molded article has a second binder, and the second binder is in a solidified state without being decomposed in the first degreasing step. Therefore, even if the shape retention by the first binder is slightly lowered, the molded body is not deformed by the solidified second binder.

  Then, if it cools to the temperature below the flow start temperature of a 1st binder, a 1st binder will solidify. At this time, the brittle holes are reinforced as a whole by the solidified first binder, and the shape thereof is reliably maintained. Moreover, since the wax that is the main cause of the brittleness of the molded body is vaporized, the molded body after the first degreasing step is retained by the first binder and the second binder, and the strength thereof is the first degreasing step. It is larger than the strength of the previous molded body.

  In order to make the strength of the molded body after the first degreasing step greater than the strength of the molded body before the first degreasing step, the content of the resin component before and after the first degreasing step may be in a specific relationship. Specifically, when the resin component contained in the molded body before the first degreasing step is 100% by weight, the resin component contained in the molded body after the first degreasing step is preferably 10 to 60%, more preferably Is 20 to 50%, and the wax may be almost removed.

  In this way, since the wax that adversely affects the shape retention of the molded body is almost removed, the molded body after the first degreasing process is removed from the heating furnace, and the second degreasing process and the firing process are performed. Even when transported to the furnace, the molded body is not damaged such as cracks or chips. Moreover, efficiency improvement of a process is realizable by dividing the heating furnace used for a 1st degreasing process, and the heating furnace used for a 2nd degreasing process and a baking process.

  In addition, cracking and chipping can be effectively prevented even during processing of the molded body. In particular, when the molded body is a ferrite magnet, there is almost no residual magnetic field due to the thermal history in the first degreasing step, so that it is possible to prevent adhesion of processing waste and the like.

  Although the temperature range of a 2nd degreasing process should just be determined according to the resin material to be used, Preferably it is 150-600 degreeC, More preferably, it is 200-500 degreeC. The holding time is preferably 1 to 50 hours, more preferably 3 to 30 hours.

  In the second degreasing step, first, the first binder is vaporized and removed from the molded body. At this time, since the liquefaction of the second binder does not proceed rapidly and has sufficient shape retention, the holes are not deformed. Therefore, the first binder is vaporized through the holes and removed smoothly.

  Even after the first binder is removed, since the second binder exists in the molded body in a solidified state or in a high viscosity state, the shape retention of the molded body can be sufficiently maintained. . In the second degreasing step, the molded body is vaporized and removed while being prevented from being deformed, and subsequently subjected to the firing step. Therefore, the molded body is fired without deformation, and a sintered body without deformation is obtained. .

  Thus, by performing a 2nd degreasing process and a baking process continuously, deformation | transformation of the sintered compact (ferrite magnet) obtained can be prevented, aiming at the efficiency of a process.

  The sintered body obtained through the above steps is processed as necessary to obtain a ferrite magnet.

  The present invention is not limited to the above-described embodiment, and can be variously modified within the scope of the present invention. For example, the raw material powder is not limited to ferrite, and may be other ceramic powders or metal materials.

  Hereinafter, although this invention is demonstrated based on a more detailed Example, this invention is not limited to these Examples.

Example 1
As a sintered body, a ferrite magnet having a composition represented by La _0.4 Ca _0.2 Sr _0.4 Co _0.3 Fe _11.3 O ₁₉ was produced. The following were used as starting materials. Fe ₂ O ₃ powder (containing Mn, Cr, Si, Cl as impurities), SrCO ₃ powder (containing Ba and Ca as impurities), La (OH) ₃ powder, CaCO ₃ powder, Co ₃ O ₄ powder Prepared. The above starting materials and additives were pulverized with a wet attritor, dried and sized, and calcined in air at 1230 ° C. for 3 hours to obtain granular calcined powder.

  The calcined powder was subjected to dry coarse pulverization / wet pulverization. After the wet pulverization, the calcined powder (raw material powder) was dried at 100 ° C. for 10 hours. When the average particle size of the dried raw material powder was examined by SEM, it was 0.3 μm.

  10% by weight of paraffin wax as wax, 8% by weight of polypropylene as first binder, 2% by weight of acrylic resin as second binder, 100% by weight of raw material powder after drying, lubricant, plasticizer, Along with a sublimation compound, the mixture was kneaded with a kneader and formed into pellets with a pelletizer. The kneading was performed under conditions of 150 ° C. and 2 hours.

  The thermal decomposition temperature of paraffin wax was 200 ° C., the flow initiation temperature of polypropylene was 165 ° C., the thermal decomposition temperature was 295 ° C., and the thermal decomposition temperature of the acrylic resin was 340 ° C.

  Next, the pellet 10 was injection-molded in the mold apparatus 8 using the injection molding apparatus 2 using the magnetic field shown in FIG. Before injection into the mold apparatus 8, the mold apparatus 8 is closed, a cavity 12 is formed inside, and a magnetic field is applied to the mold apparatus 8. The pellet 10 was heated and melted and kneaded at, for example, 160 ° C. inside the extruder 6 to be a forming raw material, and injected into the cavity 12 of the mold apparatus 8 by a screw. The temperature of the mold apparatus 8 was 40 ° C. The thickness of the molded body after the magnetic field injection molding process was 2 mm, and an arc-shaped flat plate was molded.

  About the obtained molded object, the fracture | rupture load shown below was measured and the intensity | strength of the molded object was evaluated.

  The breaking load was calculated based on the load at which both ends of the molded body were supported, the concentrated load was gradually increased from 0 N at the center of the molded body, and the molded body was broken. The breaking load of the molded body immediately after molding was 60N.

  Moreover, as a result of measuring content of the resin material which remains in a molded object by TG (thermogravimetric) analysis, the amount of residual resin materials (weight reduction amount) was 15.6%. That is, the resin component accounted for 15.6% of the weight of the molded body of 100%.

Next, this molded body was heated to 210 ° C., a part of the resin material was removed, and then cooled to room temperature (first degreasing step). The first degreasing step was 96 hours. Note that the atmosphere in the first degreasing step was a humidified N ₂ atmosphere (oxygen concentration of 0.01% or less).

  For the molded body after the first degreasing step, the strength and the content of the resin material remaining in the molded body were measured in the same manner as described above. Further, the amount of wax remaining in the molded body was calculated from the measurement result of the amount of the residual resin material, and the ratio with respect to 100% of the amount of wax contained in the molded body immediately after molding was calculated. The results are shown in Table 1.

  The molded body after the first degreasing step was conveyed to a firing furnace, and the second degreasing step and the firing step were continuously performed. At the time of conveyance, no cracking or chipping of the molded body occurred.

  In the 2nd degreasing process, it heated up to 600 degreeC over 15 hours, and removed the resin material which remained in the molded object. Note that the atmosphere in the second degreasing step was an air atmosphere. Then, the temperature was further raised to 1200 ° C. and held at that temperature for 1 hour to obtain a ferrite magnet sintered body (firing step).

  About the obtained sintered body of the ferrite magnet, the presence or absence of deformation and cracks was visually evaluated. The results are shown in Table 1.

Example 2
A ferrite magnet was produced in the same manner as in Example 1 except that the atmosphere in the first degreasing step was a humidified N ₂ / air atmosphere (oxygen concentration 5%), and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

Example 3
A ferrite magnet was prepared in the same manner as in Example 1 except that the atmosphere in the first degreasing step was a humidified N ₂ / air atmosphere (oxygen concentration 5%) and the temperature was raised to 250 ° C. Evaluation was performed. The results are shown in Table 1.

Comparative Example 1
A ferrite magnet was prepared in the same manner as in Example 1 except that the atmosphere in the first degreasing step was a humidified N ₂ / air atmosphere (oxygen concentration 5%) and the temperature was raised to 300 ° C. Evaluation was performed. The results are shown in Table 1.

Comparative Example 2
A ferrite magnet was produced in the same manner as in Example 1 except that the temperature was raised to 600 ° C. in the first degreasing step, and the same evaluation as in Example 1 was performed. The results are shown in Table 1.

Comparative Example 3
The ferrite magnet was prepared in the same manner as in Example 1 except that the atmosphere in the first degreasing step was a dry N ₂ atmosphere (oxygen concentration 0.01% or less) and the temperature was raised to 160 ° C. Was evaluated. The results are shown in Table 1.

  The graph showing the relationship between the residual resin amount and the breaking load in Table 1 is shown in FIG. From Table 1 and FIG. 2, the strength of the molded body after the first degreasing step is such that the strength of the molded body after the first degreasing step is higher than the flow start temperature of the first binder in the first degreasing step. It can be confirmed that it is higher than that. Moreover, since the intensity | strength intensity | strength of the molded object after a 1st degreasing process is high, the crack at the time of conveyance and after baking, a deformation | transformation, etc. were not seen.

Claims (4)

A step of heating and kneading the raw material powder and the resin material to obtain a raw material for molding;
Injection molding the raw material for molding to obtain a molded body;
A first degreasing step of removing a part of the resin material from the molded body;
A second degreasing step for removing the remaining resin material;
A firing step of firing the molded body after the first degreasing step and the second degreasing step,
The resin material contains at least a wax, a first binder, and a second binder, and a thermal decomposition temperature of the wax is lower than a thermal decomposition temperature of the first binder and the second binder. The thermal decomposition temperature is lower than the thermal decomposition temperature of the second binder,
In the first degreasing step, after the temperature range of not less than the flow start temperature of the first binder and less than the thermal decomposition temperature of the second binder, it is cooled to a temperature less than the flow start temperature of the first binder,
The method for producing a sintered body, wherein the second degreasing step and the firing step are continuously performed.   The method for producing a sintered body according to claim 1, wherein the resin material further includes a plasticizer.   The method for producing a sintered body according to claim 1 or 2, wherein the wax is substantially removed in the first degreasing step.   When the content of the resin material contained in the molded body before the first degreasing step is 100%, the content of the resin material contained in the molded body after the first degreasing step is 10 It is -60%. The manufacturing method of the sintered compact in any one of Claims 1-3.

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