Method for producing molded article

Abstract

To provide a method for producing a molded body by which the intrusion of water into a vacuum packaged body can be prevented to a high degree when the vacuum packaged body is subjected to cold isostatic pressing by producing the vacuum packaged body having excellent liquid tightness of a spherical molded body of ceramic powder.SOLUTION: A vacuum package is manufactured by storing a spherical molded body of ceramic powder in a resin container composed of an approximately hemispherical recessed part and an edge part obtained by thermoforming a non-oriented film in a state that its upper part is projected from the edge part, and then fusing a biaxially oriented film as a lid material at the edge part of the resin container under vacuum, and then the vacuum package is subjected to cold isostatic pressing.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a novel method for producing spherical molded bodies of ceramic powder, and more specifically, to a method for producing a molded body in which a vacuum package having excellent liquid-tightness is produced from the spherical molded body, thereby making it possible to highly prevent water from entering the vacuum package when the spherical molded body is cold isostatically pressed. [Background technology]

[0002] Spherical compacts obtained by molding ceramic powder into a spherical shape are used as intermediates for spherical sintered products such as bearing balls, and are fired to produce spherical sintered bodies. In the firing process, in order to improve the strength of the spherical sintered bodies, it is considered effective to increase the apparent density of the spherical compacts of ceramic powder (hereinafter simply referred to as spherical compacts) as much as possible.

[0003] As a simple means for increasing the apparent density, a method has been proposed in which the spherical molded bodies are pressurized by cold isostatic pressing (hereinafter also referred to as "CIP"). At this time, to avoid contact with water, the spherical molded bodies are subjected to CIP while packaged in a water-impermeable substrate. Specifically, a method has been disclosed in which a nylon-polyethylene double-layer bag is used as a flexible synthetic resin storage bag, and the spherical molded bodies are placed one by one in this nylon-polyethylene double-layer bag. After creating a vacuum inside the bag, the opening is sealed, and the resulting vacuum-packaged body is subjected to CIP (see Patent Document 1).

[0004] The above-mentioned method for obtaining a vacuum packaged product has room for improvement in terms of workability due to the use of a bag.

[0005] Meanwhile, a known method for vacuum packaging a product is so-called "deep-draw vacuum packaging," in which a package is placed in a resin container with a thermoformed edge, and then a lid material film is heat-sealed to the edge of the resin container under vacuum (see Patent Document 2). This method does not require the preparation of a separate bag, and by supplying a film for thermoforming and a film for the lid material, the steps of thermoforming the film, placing the package in the container, and heat-sealing the lid material film under vacuum can be carried out continuously, and it is thought that automation is easy. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 3-73310 [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-115066 Summary of the Invention [Problem to be solved by the invention]

[0007] However, when the deep-draw vacuum packaging described in Patent Document 2 is applied to the spherical molded body, the unique spherical shape causes significant deformation of the resin container during vacuum packaging, making it difficult to ensure sufficient adhesion with the lid material and potentially affecting the subsequent CIP process. Specifically, as described in Patent Document 2, when a spherical molded body is completely placed in a resin container with a thermoformed edge, a lid material film is placed over the opening of the resin container, and a vacuum is applied, a large amount of film remains, particularly in the upper wall of the resin container. Furthermore, the film is easily wrinkled due to deformation to fit the spherical molded body when vacuumed. When a vacuum package is obtained by fusing a lid material film to the spherical molded body, water easily penetrates through the wrinkles present in the fused portion during CIP. Furthermore, the wrinkles in the film may cause deformation of the surface of the spherical molded body during CIP. Such water penetration into the vacuum package is particularly fatal for nitride-based ceramics and is an unavoidable issue in the CIP process of spherical molded bodies.

[0008] Therefore, the object of the present invention is to provide a method for producing spherical molded bodies from ceramic powder by CIP, which enables the production of spherical molded bodies without the problems caused by water intrusion by producing a vacuum package with excellent liquid-tightness and subjecting this to CIP. [Means for solving the problem]

[0009] As a result of extensive research into solving the above-mentioned problems, the inventors have found that in the deep-draw vacuum packaging, the resin container is made up of a roughly hemispherical recess and edge, the spherical molded body is placed inside with its upper part protruding beyond the edge, and the protruding part is covered with a lid material before vacuum packaging. In addition, by using specific films as the films constituting the resin container and the lid material, it is possible to highly prevent a decrease in liquid-tightness due to the occurrence of wrinkles in the vacuum packaged spherical molded body, thereby solving the above-mentioned problems and thereby completing the present invention.

[0010] That is, according to the present invention, there is provided a method for producing spherical molded articles, which comprises placing spherical molded articles of ceramic powder in a resin container having a substantially hemispherical recess and a rim obtained by thermoforming a non-oriented film, with the spherical molded articles protruding at their upper portions beyond the rim, and then fusing the rim of the resin container with a biaxially oriented film as a lid under vacuum to produce a vacuum package, and then cold isostatically pressing the vacuum package.

[0011] In the present invention, the height of the spherical molded body protruding from the edge of the resin container is preferably 10 to 50% of the diameter of the spherical molded body in order to further suppress deformation of the container during vacuum packaging.

[0012] In addition, it is also effective to set the ratio (V0 / V) of the volume (V) of the spherical molded body to the virtual spherical volume (V0) estimated from the approximately hemispherical recess of the resin container to be 1.1 to 2.5 in order to further suppress deformation of the container during vacuum packaging.

[0013] The present invention is particularly effective when the ceramic powder is a nitride-based ceramic powder, which is susceptible to contact with water.

[0014] Furthermore, the present invention is preferable because the resin container is a multi-cavity container having a plurality of recesses formed along the edge, which allows for mass production. In this case, by performing vacuum fusion of the biaxially oriented film as a lid material only on the edge located on the outer periphery of the resin container, it is possible to efficiently remove the spherical molded bodies from the vacuum package after CIP treatment. [Effects of the Invention]

[0015] According to the manufacturing method of the present invention, in the manufacturing method of spherical molded bodies of ceramic powder by CIP, it is possible to manufacture vacuum-packaged bodies while highly preventing the occurrence of wrinkles that affect liquid-tightness, and by subjecting this to CIP, it is possible to improve the apparent density of the spherical molded bodies without causing problems such as water penetration into the vacuum-packaged body or deformation of the spherical molded body surface due to wrinkles. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram showing one embodiment of a resin container of the present invention. [Figure 2] 1 is a schematic diagram showing a state in which a spherical molded body is contained in a resin container of the present invention. [Figure 3] 1 is a schematic diagram showing one embodiment of a vacuum package of the present invention; [Figure 4] 1 is a schematic diagram showing another embodiment of the vacuum package of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] In the present invention, the spherical ceramic powder compacts subjected to CIP are formed using known ceramic powders by known forming methods and have sufficient strength to withstand handling in vacuum packaging. Examples of ceramic powders include oxide-based ceramics such as aluminum oxide and zirconium oxide, nitride-based ceramics such as silicon nitride, aluminum nitride, and boron nitride, and mixtures thereof. The present invention is particularly effective for compacts containing nitride-based ceramics, for which water penetration during CIP is a major problem. The ceramic powder may contain known sintering aids. Furthermore, the ceramic powder may be molded by adding a known resin binder to the ceramic powder. Common molding methods include die pressing and rubber pressing.

[0018] In the present invention, the spherical molded body preferably has a diameter of 5 mm to 10 cm, particularly preferably about 5 mm to 5 cm.

[0019] The manufacturing method of the present invention will be described below with reference to the accompanying drawings.

[0020] In the present invention, the container for accommodating the spherical molded bodies is obtained by thermoforming a non-stretched film, and is a resin container 1, as shown in Fig. 1, consisting of a substantially hemispherical recess 1a forming the container portion and a rim 1b. By forming the container portion into a substantially hemispherical recess 1a, it is possible to effectively prevent the occurrence of wrinkles when the film tightly contacts the spherical molded bodies during vacuum packaging. Furthermore, the rim 1b functions as an allowance for fusion with the film of the lid material in vacuum packaging, as described below, and also as a partition between each container in the embodiment of a multi-cavity container, as described below.

[0021] The approximately hemispherical recess 1a preferably has a bottom surface that extends 30 to 50%, preferably 40 to 50%, from the bottom of a perfect hemisphere, assuming that the bottom is 50% of the way down from the bottom of the sphere. The bottom surface may be formed directly on the edge 1b, or may be formed via a wall surface extending downward from the edge 1b. The wall surface may be a vertical cylinder or a cone-like surface that widens upward. The length of the wall surface is preferably 30% or less, particularly 25% or less, of the depth of the recess 1a.

[0022] By making the recess 1a of the resin container approximately hemispherical in this way, it becomes easier to mold by thermoforming, and the occurrence of wrinkles in the vacuum packaged body obtained by vacuum packaging the spherical molded body, particularly in the resin container portion, can be effectively suppressed.

[0023] In the present invention, the resin container 1 can be obtained by thermoforming a non-stretched film. The non-stretched film is made of a known thermoplastic resin, preferably one with low water permeability. Examples of the thermoplastic resin include polyolefins such as polypropylene and polyethylene, and nylon. The non-stretched film may be a single-layer film or a laminate film provided with a heat-sealing layer or the like. The thickness of the non-stretched film may be determined appropriately depending on the size of the spherical molded product, in other words, the size of the resin container, taking into account moldability and conformability during vacuum packaging. The thickness is preferably in the range of, for example, 50 to 250 μm, and preferably 70 to 200 μm.

[0024] The thermoforming may be performed by any known method in which the unstretched film is heated to a temperature below its melting point but above its softening point and then molded using a mold material. By such thermoforming, the film is stretched at the portions that will become the recesses 1a of the unstretched film, resulting in a substantially hemispherical shape.

[0025] One of the features of the present invention is that the resin container 1 contains spherical molded bodies with their upper portions protruding from the edge 1b, as shown in FIG. 2 . Conventionally, when deep-draw resin containers are used to vacuum-package products, the products are completely contained within the resin container. However, when attempting to apply the above-mentioned method to vacuum-packaging spherical molded bodies, problems arise, such as the generation of large wrinkles in the film in close contact with the upper half of the spherical molded bodies and the significant deformation of the resin container itself, as described above. In contrast, the present invention uses the resin container described above, which allows the lower portion of the spherical molded bodies to approximate the shape of the container, thereby reducing the shrinkage rate during vacuum packaging. Furthermore, the upper portion of the spherical molded bodies protrudes from the edge 1b of the container, which is the cause of the wrinkles, and the protruding portion is covered by deforming a lid film, as described below, thereby effectively preventing the generation of wrinkles during vacuum molding in a container containing spherical molded bodies.

[0026] The protruding height (h) of the spherical molded body 2 from the edge 1b of the resin container is preferably 10 to 50%, particularly 10 to 30%, of the diameter of the spherical molded body. That is, if it is less than 10%, the above-mentioned problem of wrinkles occurring due to thermoforming of the resin container 1 becomes significant, and if it is more than 50%, deformation of the film of the lid material becomes large, which tends to cause large wrinkles.

[0027] Furthermore, it is preferable that the ratio (V0 / V) of the volume (V) of the spherical molded body to the virtual spherical volume (V0) estimated from the hemispherical container is 1.1 to 2.5, where the virtual spherical volume (V0) is the volume of a whole sphere assumed to include the curved surface of the approximately hemispherical recess 1a of the resin container as a part of the volume.

[0028] By setting V0 / V within the above range, excess film can be reduced when the film in the recess 1a of the resin container comes into close contact with the spherical molded body during vacuum packaging, thereby effectively suppressing the occurrence of wrinkles in the vacuum molded body and also preventing deformation of the edge 1b.

[0029] In the present invention, a vacuum package is produced by using a biaxially stretched film as a lid material 3 in a resin container containing the spherical molded body 2 with its upper part protruding beyond the edge 1b, and fusing the biaxially stretched film at the edge of the resin container under vacuum.

[0030] In the production of the vacuum-formed body, it is important to use a biaxially stretched film as the lid material 3. Specifically, the present inventors have confirmed that when a non-stretched film is used as the lid material in vacuum packaging, even if water penetration due to wrinkles in the fused portions of the vacuum-packaged body obtained by vacuum packaging can be prevented, the spherically-formed body contained therein becomes wet when subjected to CIP. In contrast, it has been confirmed that when a biaxially stretched film is used as the lid material, water penetration through the film surface can be effectively prevented when subjected to CIP. This, together with the use of the resin container, can highly improve the liquid-tightness of the vacuum-packaged body when subjected to CIP.

[0031] Although the mechanism of action of using a biaxially stretched film as the above-mentioned lid material is not clear, it is presumed that the biaxially stretched film stretches uniformly during vacuum packaging, and therefore the phenomenon of localized overstretching that occurs with unstretched film and water penetration through that area is less likely to occur.

[0032] The biaxially stretched film constituting the lid material may be made of the same material as the non-stretched film, or may be made of the same material as the non-stretched film. The stretched film may be a single-layer film or a laminated film further provided with a heat seal layer or the like.

[0033] The stretching ratio of the biaxially stretched film is not particularly limited as long as it is stretched to a degree that exhibits the above-mentioned properties. Biaxially stretched films have lower elongation than the non-stretched films, and when covering spherical molded bodies protruding from a resin container in vacuum packaging, they can be tightly attached without being partially excessively stretched. The elongation (longitudinal and transverse) of the biaxially stretched film varies depending on the material and thickness and cannot be generally specified, but films with an elongation of, for example, about 30 to 200% (generally, non-stretched films are often 350% or more) are preferably used. The above values ​​are measured according to JIS K 7127. Furthermore, the thickness of the biaxially stretched film is preferably in the range of 40 to 200 μm, preferably 50 to 100 μm, taking into account the ease of tight attachment to the spherical molded bodies and strength.

[0034] In the present invention, a biaxially stretched film is used as a lid material and is fused under vacuum at the edge of the resin container to produce a vacuum package as shown in Fig. 3. That is, a vacuum package of spherical molded bodies 2 is produced by using a biaxially stretched film as a lid material 3 and fusion-bonding the edge of the resin container 1 under vacuum. At this time, fusion-bonding at the edge 1b of the resin container is carried out so that the fused portion 4 surrounds the recess of the resin container.

[0035] The vacuum package can be produced using a known chamber-type vacuum packaging device. The pressure in the vacuum packaging is preferably as low as possible, preferably −90 kPa or less, and more preferably −100 kPa or less, in order to sufficiently compress the spherical molded bodies during CIP.

[0036] In the present invention, the vacuum-molded body may be manufactured using a resin container having one recess for each spherical molded body, as shown in Figure 3 above. However, for mass production, a multi-cavity container is recommended in which the resin container has multiple recesses 1a formed via the edge 1b, as shown in Figure 4.

[0037] 4 shows a plan view and an AA' cross-sectional view of a vacuum package in which spherical molded bodies 2 are accommodated in the multi-cavity resin container 1 and a lid member 3 is fused thereto.

[0038] In this case, it is preferable to perform fusion bonding under vacuum using a biaxially stretched film as a lid material, forming a continuous fused portion 4 only on the edge located on the outer periphery of the resin container, as shown in Figure 4, since this reduces the number of fused portions around each spherical molded body after CIP treatment, allowing the spherical molded bodies to be removed efficiently. It is preferable to remove the film covering the vacuum package before degreasing, as this has a significant effect in reducing the amount of work involved.

[0039] In the present invention, the vacuum package produced by the above method is subjected to CIP, whereby it is compressed and its apparent density is increased. The CIP can be carried out using known equipment and known conditions. For example, the pressure conditions are typically 50 to 200 MPa.

[0040] The spherical molded body having an increased apparent density by the method of the present invention can be degreased and fired by a known method to obtain a spherical sintered body. The spherical sintered body obtained by firing can be polished as necessary and used for applications such as bearing balls. [Example]

[0041] EXAMPLES In the following, examples are shown to explain the present invention more specifically, but the present invention is not limited to these examples.

[0042] Example 1 Silicon nitride powder was pressed into a mold to produce a spherical compact with a diameter of 19 mm.

[0043] Separately, a resin container was prepared by thermoforming a 100 μm-thick unstretched nylon film (elongation: approximately 400%) having a heat-seal layer on its surface. The container had a rim 1b and a recess 1a formed by a hemispherical bottom with a diameter of 21 mm and a conical wall extending 2 mm upward from the rim. The spherical molded body was placed in this resin container. The ratio (V0 / V) of the volume (V) of the spherical molded body to the virtual spherical volume (V0) estimated from the approximately hemispherical recess of the resin container was 1.35. The height of the spherical molded body protruding from the rim of the resin container was 34% of the diameter of the spherical molded body. After placing the spherical molded body in a resin container, a 75 μm thick biaxially oriented nylon film (elongation: approximately 200%) with a heat seal layer on the surface was set as a lid material in a vacuum chamber equipped with a heat sealing device so that the heat seal surfaces of the resin container were in contact with each other, and the lid material was fused to the edge of the resin container under a vacuum of -110 kPa to seal it, thereby producing a vacuum package.

[0044] The resulting vacuum package was treated in a CIP device at a pressure of 150 MPa.

[0045] After the CIP treatment, the film was removed, and it was found that the spherical molded bodies remained dry and their surfaces were clean with no dents or other marks caused by the film.

[0046] Comparative Example 1 A vacuum molded body was obtained in the same manner as in Example 1, except that the biaxially stretched film used in the resin container was replaced with the unstretched film, and the same CIP treatment was performed. After the CIP treatment, the film was removed and the spherical molded body was observed to be wet with water. Furthermore, when the film of the lid material was observed after peeling, it was found to be locally thin.

[0047] Comparative Example 2 In Example 1, a vacuum package was obtained in the same manner as in Example 1, except that the protrusion height of the spherical molded body from the resin container was 0%, and the same CIP treatment was performed. After the CIP treatment, the film was removed and the spherical molded body was observed. It was found to be wet with water. In addition, depressions that appeared to be wrinkles in the film were observed on the surface. Furthermore, when the fused portion of the film of the lid material after peeling was observed, several large wrinkles were observed. [Explanation of symbols]

[0048] 1 resin container 1a Approximately hemispherical recess 1b Edge 2 Spherical molded body 3 Lid material 4 Fusion part V0 virtual sphere

Claims

1. A method for producing a spherical molded product, comprising: placing a spherical molded product of ceramic powder in a resin container having a substantially hemispherical recess and a rim obtained by thermoforming a non-oriented film, with the spherical molded product's upper portion protruding beyond the rim; then fusing a biaxially oriented film as a lid material to the rim of the resin container under vacuum to produce a vacuum package; and then cold isostatically pressing the vacuum package.

2. 2. The method for producing spherical molded articles according to claim 1, wherein the height of the spherical molded articles protruding from the edge of the resin container is 10 to 50% of the diameter of the spherical molded articles.

3. The ratio of the volume (V) of the spherical molded body to the virtual spherical volume (V) estimated from the approximately hemispherical recess of the resin container ０ ) and the ratio (V ０ 2. The method for producing spherical molded articles according to claim 1, wherein the ratio of molten silicate to ... is 1.1 to 2.

5.

4. 2. The method for producing spherical molded articles according to claim 1, wherein the ceramic powder is a nitride ceramic powder.

5. 2. The method for producing spherical molded articles according to claim 1, wherein the resin container is a multi-cavity container having a plurality of recesses formed along the edge thereof.

6. 6. The method for producing a spherical molded article according to claim 5, wherein the biaxially stretched film is used as a lid material and the fusion bonding under vacuum is carried out only on the rim portion located on the outer periphery of the resin container.

Images