Method for manufacturing a degreased structure and method for manufacturing a sintered body using a degreased structure

Abstract

To reduce acidic exhaust gas generated upon degreasing a resin structure molded using a mixed material containing a polyacetal resin and metal powder.SOLUTION: Degreasing is executed in the presence of formic acid produced from a resin structure without the intervention of additional acid to obtain a degreased structure.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This invention relates to the debinding of resin structures.

Background Art

[0002] Generally, a molded body is manufactured by a 3D printer. As one of them, a laminated structure (green body) laminated and molded by a 3D printer using a mixed material containing metal powder and thermoplastic resin is heated after molding to remove the thermoplastic resin and debound into a brown body, and this brown body is sintered to obtain a sintered body.

[0003] In debinding before sintering, it is required to remove the thermoplastic resin by an appropriate method according to the type of thermoplastic resin used as a binder. When polyacetal (polyoxymethylene) used as a resin material for 3D printers is used as the thermoplastic resin, a method of heating and removing polyacetal while performing acid decomposition treatment in an environment into which an acid such as nitric acid gas is introduced is known. For example, Patent Document 1 proposes a method of adding and heating at least one acid (Claim 11) selected from nitric acid, oxalic acid, formic acid, and acetic acid as a debinding catalyst for removing a binder from a molded product composed of metal powder and a binder.

[0004] In addition, Patent Document 2 proposes a method of debinding by mixing and introducing formic acid into a carrier gas from a support material containing a polyacetal resin. Furthermore, Patent Document 3 proposes a method of introducing formic acid gas to decompose the resin and operating a ventilation fan to continuously remove the binder from the molded body as a method for manufacturing an inorganic sintered molded body.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] However, in acid-added degreasing, which involves adding acids such as nitric acid gas, a large amount of acidic gas is generated along with the degreased polyacetal, making it necessary to introduce a carrier gas or continuously remove it using a fume hood. This resulted in the generation of large amounts of acidic gas, and the treatment of this exhaust gas became a problem.

[0007] Therefore, the purpose of this invention is to reduce the amount of acidic gas emitted when degreasing a green body to a brown body in order to obtain a sintered molded body, thereby reducing the workload and environmental burden. [Means for solving the problem]

[0008] This invention solves the above problem by obtaining a degreased structure by degreasing a resin structure, which is molded using a mixed material containing a resin that decomposes to produce a carboxylic acid gas and a metal powder, in the presence of the carboxylic acid generated from the resin structure, without the need for an additional acid to be added.

[0009] Specifically, the resin can be a polyacetal resin, and the carboxylic acid produced by decomposition can be formic acid.

[0010] "Without the addition of any other acid" means degreasing without the use of any acid other than the carboxylic acid generated from the resin structure. "Acids other than the carboxylic acid generated from the resin structure" refers not only to non-carboxylic acid acids such as sulfuric acid and nitric acid, but also to carboxylic acids other than those generated from the resin structure. Ideally, no such acids should be added, but even if a small amount is added, it should be kept to such an extent that it does not substantially contribute to degreasing, with only the carboxylic acid generated from the resin structure being involved. In other words, this invention involves heating the resin structure to decompose the resin itself, retaining the carboxylic acid generated by this decomposition around the resin structure without gas flow, thereby creating a high-concentration carboxylic acid gas environment around the resin structure. By maintaining this state, degreasing with carboxylic acid proceeds without the use of any additionally added acid. [Effects of the Invention]

[0011] The degreasing method for resin structures according to this invention results in acid-containing exhaust gas consisting solely of carboxylic acids derived from the resin that were present in the original resin structure during molding. Conventionally, acid-containing exhaust gas from separately added acids also had to be treated, but this treatment burden can be eliminated. Furthermore, conventional methods using separately added acids required gas flow equipment to prevent the acid concentration in the system from becoming too high, but such equipment is unnecessary, enabling degreasing with a more compact apparatus. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing an example of a heating container used in the method for manufacturing a degreased structure according to this invention. [Figure 2] Schematic diagram showing the equipment used in the verification example. [Figure 3] Graphs showing the rate of mass change with respect to continuous heating time in the examples and reference examples. [Figure 4] Photographs showing degreasing completed and incomplete samples in the examples and reference examples. [Figure 5] Graph showing the changes in formaldehyde and formic acid concentrations over time in the additional examples. [Figure 6] Graph showing the change in sample mass over time in the additional example. [Figure 7] Graph showing the relationship between formic acid concentration and mass loss rate in the additional examples. [Modes for carrying out the invention]

[0013] The embodiments of this invention will be described in detail below. This invention is a method for manufacturing a degreased structure (brown body) obtained by molding a resin structure (e.g., a green body) using a 3D printer or the like with a mixed material containing metal powder and a resin that decomposes to produce carboxylic acid gas, and then heating it after molding to remove the thermoplastic resin, and further a method for manufacturing a sintered body obtained by sintering this brown body.

[0014] The metal powder is preferably one that is resistant to corrosion in the carboxylic acid environment created in the invention. Examples include SUS316L and SUS630, but is not limited to these. Furthermore, the metal powder is preferably of a particle size that can be output by a 3D printer or the like in order to obtain the resin structure.

[0015] Examples of resins that decompose to produce carboxylic acids include polyacetal resins that produce formic acid gas. Other resins that produce acetic acid gas may also be used. Among these, polyacetal resins are preferred because the formic acid produced is easy to handle. While the degree of polymerization of the resin is not particularly limited, it is desirable that it have a softening point sufficient for layer molding by a 3D printer.

[0016] The mixed material is obtained by mixing a material containing the metal powder and the resin. The mixing ratio of the metal powder and the resin is preferably 95:5 to 80:20 by mass ratio, and more preferably 93:7 to 85:15. If there is too much of the metal powder, there will be too little of the resin serving as a binder, making it difficult to perform laminated molding. On the other hand, if there is too little of the metal powder, there is a risk that the shape cannot be maintained during debinding or the strength becomes insufficient during sintering.

[0017] In addition to the metal powder and the resin, the mixed material may contain other additives as long as they do not inhibit the implementation of the present invention. Examples of such additives include waxes for smoothing the fluidity and formability of the resin structure.

[0018] To obtain the debound structure according to this invention, a debinding step is performed to obtain a debound structure by debinding in the presence of carboxylic acid generated from the resin structure without the intervention of an acid added separately. Here, without the intervention of an acid added separately means not only acids other than carboxylic acids such as nitric acid and oxalic acid, but also a situation where the separately added carboxylic acid does not substantially participate in debinding. By continuing the situation where the carboxylic acid generated by decomposing the resin of the resin structure exists at a high concentration around the resin structure, the resin is decomposed to generate more carboxylic acid, increasing the carboxylic acid concentration in the environment and efficiently proceeding with debinding.

[0019] In the initial stage of this debinding step, since there is no carboxylic acid around the resin structure, the decomposition of the resin by oxygen occurring slightly in an oxygen - present environment is gradually advanced to generate carboxylic acid. Here, the oxygen - present environment may be normal air. It is advisable to suppress the release of the generated carboxylic acid as much as possible and keep it around the resin structure to gradually increase the carboxylic acid concentration in the vicinity.

[0020] When the resin structure is undergoing this degreasing process, the temperature is preferably 150°C or higher, and more preferably 160°C or higher, if the resin is polyacetal. Below 150°C, the decomposition of polyacetal by oxygen does not begin, and the reaction does not proceed. Above 160°C, the rate of progress becomes practical. On the other hand, it is preferably 190°C or lower, more preferably 180°C or lower, and even more preferably 175°C or lower. Above 180°C, the softening of the resin becomes significant, and above 190°C, the resin softens too much, and there is a risk that the molded resin structure will not be able to maintain its shape.

[0021] In this degreasing process, it is preferable to raise the carboxylic acid concentration around the resin structure to 50 ppm or higher during the intermediate stage of increasing carboxylic acid concentration. When the carboxylic acid concentration reaches 50 ppm or higher, the rate of resin decomposition and degreasing tends to increase rapidly thereafter, making it easier to continue the decomposition. It is preferable that the concentration reaches 100 ppm or higher in the final stage, and more preferably 500 ppm or higher. Below 100 ppm, depending on the structure of the resin structure, sufficient decomposition by carboxylic acid may not proceed to the interior. However, in reality, if the temperature is above 50 ppm, the carboxylic acid concentration will increase sufficiently as long as the volume inside the apparatus used in the degreasing process is not unnecessarily large. Furthermore, by accumulating a substance capable of releasing carboxylic acid in the apparatus used in the degreasing process, it is possible to further increase the surrounding carboxylic acid concentration to 500 ppm or higher.

[0022] The apparatus used in this degreasing process is preferably airtight. However, it is advisable to provide an outlet in the apparatus surrounding the resin structure so that the concentration of carboxylic acid increases while adjusting the pressure. It is advisable to provide a valve in the outlet so that it can be opened or closed at any time. This is because if it is completely sealed and the concentration of carboxylic acid or other gases increases too much, the high pressure may prevent the resin structure from maintaining its shape or damage the apparatus.

[0023] Figure 1 shows an example of the structure of an apparatus for realizing such a degreasing process. A resin structure 11 formed by layer molding with a 3D printer is placed inside a heat-resistant container 12. The heat-resistant container 12 is preferably made of a material that can withstand the temperature required for the degreasing process, and is preferably made of metal as it heats up easily. The top of the heat-resistant container 12 is open and is provided with an openable and closable lid 13. The lid 13 is provided with a gas vent hole 14 that is large enough to allow gas to escape to the outside when the internal pressure of the heat-resistant container 12 rises. One gas vent hole 14 is sufficient, but if there is only one, it may become blocked for some reason, which could cause the internal pressure to rise too high. For safety reasons, it is preferable to have multiple gas vent holes 14. The diameter of the gas vent hole 14 should be between 1 mm and 1 cm. If it is too small, it may become blocked, and if it is too large, too much carboxylic acid gas will escape, and it will take too long for the carboxylic acid concentration inside the heat-resistant container 12 to rise.

[0024] Furthermore, it is desirable not to provide any mechanism for actively drawing gas from inside the heat-resistant container 12 or for releasing gas from inside, or if such a mechanism is provided, to not activate it. This is to maintain a high concentration of carboxylic acid around the resin structure 11 by actively preventing the carboxylic acid from leaking out of the heat-resistant container 12 as much as possible.

[0025] The carboxylic acid produced during the degreasing process is part of the resin originally contained in the resin structure. Compared to conventional methods that involve adding acid separately for degreasing, the amount of acid that needs to be treated as exhaust gas is significantly reduced.

[0026] The degreased structure (brown body) obtained by the manufacturing method according to this invention preferably has 40% or more of the resin contained in the original resin structure (green body) removed, and more preferably 60% or more. It is desirable that as much of the resin as possible be removed, and it is desirable that all of it be removed. However, if wax is contained as an additive, it is difficult to degrease the wax with this manufacturing method and some will remain.

[0027] Therefore, after degreasing using the manufacturing method according to this invention, a second degreasing may be performed to thoroughly remove any remaining resin and wax other than the resin. The second degreasing is preferably performed in an inert gas atmosphere such as nitrogen gas, and the degreasing temperature is preferably between 500°C and 800°C.

[0028] The degreased structure obtained by the manufacturing method according to this invention can be further sintered to remove the remaining resin and bond the contained metal powder to obtain a sintered body. [Examples]

[0029] The following provides specific examples of the method for manufacturing the degreased structure according to this invention. Using a 3D printer material (BASF: SUS316L) made by mixing polyacetal resin and steel powder as the mixed material, a cylindrical resin structure sample with an outer diameter of 3 cm, an inner diameter of 2 cm, and a height of 3 cm was molded using a 3D printer.

[0030] <Verification Example> First, using a conventional method, the resin structure was degreased in the presence of nitric acid. The experimental environment is described using Figure 2. The resin structure 21 is placed on aluminum foil 20, and a petri dish 24 containing nitric acid is placed on the same aluminum foil 20. The aluminum foil 20 can be heated on a hot plate 25. A glass lid 23 is placed over the resin structure 21 and the petri dish 24. An exhaust pipe 26 for exhausting nitric acid gas to a fume hood is attached to the center of the glass lid 23, and the exhaust is continuously performed through the fume hood.

[0031] In this environment, the mass of four resin structure samples prepared under the above conditions was measured before heating. Verification examples 1-4 were then conducted by varying the heating temperature and heating time, and the mass after heating was measured to calculate the amount of degreased polyacetal resin. The results are shown in Table 1 below.

[0032] [Table 1]

[0033] This demonstrated that approximately 60% by mass of the polyacetal resin contained in the resin structure can be degreased.

[0034] <Examples> The above resin structure sample was placed in the heat-resistant container shown in Figure 1, and the entire structure was heated in an electric furnace to perform a degreasing process.

[0035] (Reference example 1) The mass of one sample was measured, placed in a heat-resistant container, and the lid was closed. The entire apparatus was then heated in an electric furnace to 160°C for 12 hours. After that, it was removed from the electric furnace, the lid was opened, and the sample was removed from the heat-resistant container, and its mass was measured again. After the mass measurement, it was returned to the heat-resistant container, the lid was closed, and it was returned to the degreasing unit in the electric furnace and heated for 6 hours. After that, it was removed and its mass was measured again. This was repeated every 6 hours. The rate of change in mass from before heating was calculated and is shown in the graph in Figure 3.

[0036] (Example 1) The mass of one sample was measured, placed in a heat-resistant container, and the lid was closed. The entire apparatus was then heated in an electric furnace to 160°C for 10 hours. After that, it was removed from the electric furnace, the lid was opened, and the sample was removed from the heat-resistant container, and its mass was measured again. After the mass measurement, it was returned to the heat-resistant container, the lid was closed, and it was returned to the degreasing apparatus in the electric furnace and heated for 10 hours. After that, it was removed and its mass was measured again. This process was repeated every 10 hours. The rate of change in mass from before heating was calculated and is shown in the graph in Figure 3.

[0037] (Example 2) The mass of one sample was measured, placed in a heat-resistant container, and covered. The entire apparatus was then heated in an electric furnace to 160°C for 10 hours. After removing it from the electric furnace, opening the lid, and removing it from the heat-resistant container, the mass was measured and the mass change rate was calculated. Next, using another sample, the heating process was carried out similarly, except that the heating time was changed to 20 hours. The mass after heating was measured and the mass change rate was calculated. Similarly, using different samples, the heating times were changed to 25 hours, 30 hours, 40 hours, and 60 hours, and the mass change rates before and after heating were calculated. These mass change rates are shown in the graph in Figure 3.

[0038] (Consideration) Comparing Reference Example 1 and Example 1, the rate of mass change, i.e., the progress of degreasing, was more advanced in Reference Example 1 after 12 hours than after 10 hours. However, because Reference Example 1 opened the heat-resistant container every 6 hours to release the formic acid accumulated inside, the rate of mass change in Reference Example 1 after 24 hours was inferior to that of Example 1 after 20 hours, which had one fewer opening. In Example 1, the rate of mass change decreased significantly from 20 hours to 30 hours, suggesting that the degreasing process accelerates once conditions for formic acid generation are favorable. Furthermore, comparing the values ​​of each sample with those of Example 2, where the heat-resistant container was never opened until the final measurement of the rate of mass change, the samples in Example 2, which were not opened, showed a superior rate of mass change from 25 hours to 30 hours compared to those in Example 1, which was opened midway through the process. In the samples after 40 and 60 hours, most of the polyacetal resin contained in the original resin structure was degreased. It was confirmed that efficient degreasing can be achieved by maintaining a high-concentration environment due to the generated formic acid without opening the device.

[0039] <Verification of effective concentration> Similar to Example 1 above, the sample was placed in a heat-resistant container, covered, and the entire apparatus was heated at a predetermined temperature for a predetermined time to perform degreasing. After the predetermined time had elapsed, a hose was inserted into the gas vent hole, and the other end of the hose was connected to a syringe. The valve was opened, and 100 ml of gas from inside was drawn into the syringe. Then the valve was closed and the hose was removed. 20 ml of the gas from inside the syringe was introduced into a 100 ml gas testing and measuring instrument (Tube No. 216S, manufactured by Komei Rikagaku Kogyo Co., Ltd.). That is, the amount drawn in was adjusted to 20 ml, which is 1 / 5 of the instrument's default value of 100 ml, and the detection limit was increased by 5 times for measurement. After that, the container was opened and the mass of the sample that had been attempted to be degreased was measured to confirm the progress of degreasing.

[0040] (Reference example 2) After cleaning the electric furnace and the entire apparatus, the sample was removed after 30 hours at a furnace temperature of 140°C. The sample was found to be insufficiently degreased, with a mottled surface.

[0041] (Example 3) The furnace temperature was set to 145°C, and the sample was heated for 22 hours until the formic acid concentration inside the furnace reached 150 ppm. This resulted in almost complete degreasing, with almost all of the resin being removed. Photographs of this sample are shown as the two leftmost images in Figure 4.

[0042] (Reference example 3) When the furnace temperature was set to 110°C and the formic acid concentration inside the furnace was heated to 60 ppm after 22 hours, the layered structure on the sample surface remained intact, indicating that degreasing had not progressed. Photographs of this sample are shown as the two rightmost images in Figure 4.

[0043] (Example 4) In the electric furnace used in Example 3 and Reference Example 2, the furnace temperature was increased to 140°C on a different day, and the furnace was heated for 30 hours until the formic acid concentration inside the furnace reached 200 ppm. As a result, almost all of the resin was degreased, and complete degreasing was achieved. The reason why the formic acid concentration increased despite the furnace temperature being lower than in Example 3 is thought to be because polyacetal resin decomposition products remained in the furnace, contributing to the increase in formic acid concentration.

[0044] (Additional Example 1) The furnace temperature was set to 174°C (temperature measured internally; the electric furnace's device setting was 190°C), and the formaldehyde and formic acid concentrations inside the furnace were checked over time. The measurement results for formic acid and formaldehyde concentrations are shown in Table 2 and Figure 5 below. In the figure, the left vertical axis represents formaldehyde concentration, and the right vertical axis represents formic acid concentration. The unit is ppm. At the start of the experiment, both formaldehyde and formic acid were below the detection limit. It is thought that the formaldehyde result saturated at some point.

[0045] [Table 2]

[0046] It was observed that the formaldehyde concentration gradually increased in the initial stage, and then, after 20 hours, the formic acid concentration increased exponentially. Therefore, it is inferred that the polyacetal resin decomposes thermally, producing formaldehyde, and then the formaldehyde oxidizes in the air to produce formic acid.

[0047] Furthermore, the mass change of multiple samples over time was measured under similar conditions (Figure 6, left vertical axis), and the rate of mass change and the rate of mass reduction from the original mass were calculated from these values ​​(Figure 6, right vertical axis). The results are shown in the graph in Figure 6. The initial increase in the rate of mass reduction was small, but it was confirmed that the rate of mass reduction increased significantly after 20 hours and reached almost its end by 30 hours.

[0048] Furthermore, Figure 7 shows a graph plotting the formic acid concentration and the mass loss rate on the same processing time scale. In the figure, the left vertical axis represents the formic acid concentration, and the right vertical axis represents the mass loss rate. It was confirmed that the mass loss of the sample, i.e., degreasing, progressed significantly after about 20 hours, when the formic acid concentration in the furnace exceeded 50 ppm. [Explanation of Symbols]

[0049] 11, 21 Resin structure 12 Heat-resistant containers 13 Lid 14 gas vent holes 20 aluminum foil 23 Glass lid 24 Petri dishes 25 Hot Plate 26 Exhaust pipe

Claims

1. A method for manufacturing a degreased structure used to obtain a sintered molded body, A method for producing a degreased structure, comprising molding a resin structure using a mixed material containing a resin capable of decomposing to produce a carboxylic acid gas and steel powder, and then degreasing the resin structure in the presence of a carboxylic acid generated from the resin structure, without the interposition of a separately added acid, to obtain the degreased structure.

2. The method for producing a degreased structure according to claim 1, wherein the resin is a polyacetal resin and the carboxylic acid is formic acid.

3. A method for manufacturing a sintered molded body, comprising sintering a degreased structure obtained by the method for manufacturing a degreased structure according to claim 1 or 2 to obtain the sintered molded body.

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