Dies and method for molding cage out of synthetic resin

Abstract: There is disclosed a die used to mold a cage having a cylindrical body out of a synthetic resin such that the strength of the portion of the molded cage formed by two meeting streams of the injected material is increased. The die comprises die members having parting surfaces in abutment with each other. A mold cavity (1) conforming in shape with the cylindrical body of the cage is formed between the die members. When the die members are in abutment with each other, a gate (2) and a resin reservoir (3) are formed along one parting surface and outside the mold cavity. The gate and resin reservoir are in communication with the cavity.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a die used to mold a cage out of a synthetic resin and, more particularly, to a die used to mold a cage that holds balls of a ball bearing or sprags of a one-way clutch according to claims 1 and 2, and to a method of molding such a cage, using a die. according to claim 6. The GB-465,234 discloses "Improvements in the Molding of Organic Materials". It is pointed out that at the end of a stream of thermoplastic material, cloudy or brittle portions may occur in the molded articles. Therefore, according to the document, the leading end of the stream is received by an auxiliary cavity connective with the mold cavity and forms a projection of the molded article which may be removed after molding while the article is molded from a material which does not belong to the material of the leading end portion. The DE 37 18 693 A1 discloses a molding of a bearing cage. Description of the Related Art Generally, a cage made of a synthetic resin comprises a cylindrical body 10 and plural pockets 11 formed in the outer surface of the body, as shown in Fig. 4. The pockets 11 act to receive rolling balls or sprags and are circumferentially spaced from each other. The illustrated cage is a drilled cage. Besides, so-called crown-type cages (not shown) are available. Such a cage made of a synthetic resin is normally fabricated by injection molding. This is now described in detail by referring to Fig. 5. An annular cavity 12 conforming to the profile of a molded cage is formed in a die. A molten synthetic resin is poured into the cavity 12 from a gate 13. The poured synthetic resin separates into two streams flowing to the left and to the right, respectively, inside the cavity 12. These two streams meet at the opposite side of the gate 13 and recombine into one stream. If the resinous material poured from the gate 13 as described above moves a relatively long distance until the two streams meet at the opposite side of the gate 13, the molten resinous material cools down as the two streams go toward the meeting point. If the two cooled streams meet, a weld W weaker than other portions is formed at the meeting point. In the cage of the synthetic resin including this weld, it tends to incur breakage in use. Therefore, if the meeting point of the two streams of the poured resinous material is designed to form a thin-walled portion, the decreased cross section is combined with the weld, thus resulting in a still lower strength. To compensate for the decrease in strength as described above, reinforcing materials such as glass fibers, carbon fibers, or metal fibers may be added. However, the added fibers disturb the orientation of the fibers at the meeting portion W of the two streams in the cage of synthetic resin. Again, the strength tends to decrease at this meeting portion W. One method free of this drawback is to provide plural gates circumferentially spaced around the cavity. Streams of the synthetic resin poured from the gates move a short distance to the meeting points. Thus, the streams of the poured material meet such that the temperature is hardly lowered. This increases the strength of the meeting points. However, as the number of the gates increases, the number of welds increases accordingly. Furthermore, the structure of the molding die is complicated. This complicated die is cumbersome to machine. Especially, where the molding die is designed to offer numerous molded products, gates must be increased for each of numerous cavities. Consequently, it is difficult to design the molding die itself. In addition, it is very difficult to machine the die. This leads to a great increase in the cost. For this reason, it is difficult to increase the number of gates. Moreover, the weight of the sprue runner increases. Additionally, a limitation is imposed on the maximum number of gates. Where the cage has a large diameter, if the number of gates is increased, the gates may be more widely spaced from each other circumferentially. In this case, the poured resinous material flows a relatively long distance. Again, there arises the possibility that welds which might have insufficient strength are formed. SUMMARY OF THE INVENTION Accordingly, it is a main object of the present invention to provide a die capable of molding a cage out of a synthetic resin in such a way that welds formed by meeting streams of poured resinous material have improved strength. It is another object of the invention to provide a die capable of molding a cage out of a synthetic resin with a limited number of gates such that welds formed by meeting streams of poured resinous material have improved strength. It is a further object of the invention to provide a method of fabricating a cage, using the die described above. The above objects are achieved in accordance with the teachings of the invention by a die according to claims 1 and 2 used to mold a cage out of a synthetic resin, the cage having a cylindrical body. The die comprises die members having parting surfaces abutting against each other to form a mold cavity conforming to the profile of the body of the cage. When the parting surfaces are in abutment with each other, at least one gate and a resin reservoir are formed along one parting surface outside the cavity. The gate and reservoir are in communication with the cavity. Preferably, at least one gate described above is one located around the cavity. The resin reservoir and the gate are positioned in a diametrically opposite relation. Preferably, the resin reservoir is smaller in volume than the cavity. Preferably, the resin reservoir is in communication with the cavity through a narrow passageway. Where a cage is fabricated, using the above-described molding die, if streams of the poured resinous material meet at the meeting point in the cavity of the molding die such that the temperature of the front-end portions of the streams drops, the meeting streams flow out of the cavity into the resin reservoir. In consequence, no weld forms at the meeting point in the cavity. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects as well as advantages of the invention will become clear by the following description of preferred embodiments of the invention with reference to the accompanying drawings wherein like parts in each of the several figures are identified by the same reference numerals and wherein: Fig. 1 is a plan view illustrating a die according to a first preferred embodiment of the invention; Fig. 2 is a plan view similar to Fig. 1, but illustrating a die according to a second preferred embodiment of the invention; Fig. 3 is a plan view similar to Fig. 1, but illustrating a die according to a third preferred embodiment of the invention; Fig. 4 is a perspective view of a conventional cage made of a synthetic resin; and Fig. 5 is a plan view of the prior art cage used to mold a cage out of a synthetic resin. DETAILED DESCRIPTION OF THE INVENTION In dies equipped with plural gates, these gates are normally circumferentially regularly spaced from each other around the cavity. The two streams of the resinous material injected into the cavity meet at the midway point of each region between the successive gates. Therefore, there exist plural meeting points of the streams in these regions. Every meeting point is provided with a resin reservoir similar to that of the first embodiment of the invention. In this type of multiple-gate die, the resin reservoirs are as many as the gates. Since the resinous masses formed in the resin reservoirs are discarded similarly to the sprue runner, if the number of the resin reservoirs is increased, the amount of wasted resinous material will increase. First through third embodiments of the present invention given below use dies each having plural gates. A die used to mold a cage out of a synthetic resin in accordance with the first preferred embodiment of the invention is now described by referring to Fig. 1. In this embodiment, two gates 2A and 2B are formed and circumferentially spaced from each other around a mold cavity 1 formed by the die. It is to be noted that these gates 2A and 2B are not uniformly circumferentially spaced around the cavity. The two gates 2A and 2B form a major circumferential region d 1 and a minor circumferential region d 2 . That is, d 1 > d 2 . The streams of the resinous material poured into the cavity 1 from the gates 2A and 2B meet at the midway points in the two regions d 1 and d 2 . In this first embodiment, one resin reservoir 3 is formed at the meeting point of the streams in the major circumferential region d 1 . In the molding die according to the first embodiment, the synthetic resin injected from the gates 2A and 2B separates into two opposite streams from the injection position. The streams of the material meet at the midway points in the circumferential regions d 1 and d 2 . In the minor circumferential region d 2 of these two regions d 1 and d 2 , the streams travel a short distance until they meet and so the two streams quickly meet. Therefore, little temperature drop takes place in this minor region d 2 . Hence, the bonding strength of this meeting point is increased. On the other hand, in the major circumferential region d 1 , the two streams of the injected material move a long distance until they meet. Therefore, the front-end portions of the streams tend to suffer from large temperature drops. However, the resin reservoir 3 is formed at the meeting point in this major circumferential region d 1 . Thus, the front-end portions of the streams meet and go out of the cavity 1 into the resin reservoir 3. In consequence, no weld forms unlike in the prior art technique. In this first embodiment of the invention described above, there exists only one resin reservoir 3. Therefore, only a limited amount of resin is wastefully consumed. Referring next to Fig. 2, there is shown a die used to mold a cage out of a synthetic resin in accordance with the second preferred embodiment. Three gates 2A, 2B, and 2C are formed on a mold cavity 1 in the die and circumferentially spaced from each other. A major circumferential region d1 is formed between the gates 2A and 2C. Minor circumferential regions d 2 and d 3 are formed between the gates 2A and 2B and between the gates 2B and 2C, respectively. (d 1 > d 2 , d 3 ) One resin reservoir 3 is formed at the meeting point located at the midway point in the major circumferential region d 1 . In the minor circumferential regions d 2 and d 3 , the streams flow only limited distance until they meet. Therefore, in these minor regions, the streams of the injected synthetic resin quickly meet with almost no temperature drop. Hence, the bonding strength of the meeting points is increased. In the major circumferential region d 1 , the streams of the injected material move a relatively long distance until they meet. Therefore, the front-end portions of the streams tend to suffer from temperature drop. However, the resin reservoir 3 formed at the meeting point causes the front-end portions of the streams to meet and enter the reservoir 3 from the cavity 1. For this reason, no weld appears unlike in the prior art technique. Also, in this second embodiment, there exists only one resin reservoir in the same way as in the first embodiment. Consequently, only a limited amount of resin is wasted. Referring next to Fig. 3, there is shown a molding die according to the third preferred embodiment of the invention. In this embodiment, there exist three gates 2A, 2B, and 2C. Major circumferential regions d 1 and d 2 are formed between the gates 2A and 2C and between the gates 2B and 2C, respectively. A minor circumferential region d 3 is formed between the gates 2A and 2B. (d 1 , d 2 > d 3 ) Resin reservoirs 3B and 3A are formed at the meeting points in the two major circumferential regions d 1 and d 2 , respectively. In this third embodiment, the injected synthetic resin behaves in the major and minor circumferential regions in the same way as in the second embodiment. Therefore, description of the synthetic resin is omitted here. In this embodiment, a great drop in the strength of the meeting points of the streams of the molten material and generation of welds can be prevented. In this third embodiment, the two resin reservoirs 3A and 3B are provided and so the amount of resin wasted is somewhat larger than in the first through second embodiments. However, this embodiment is especially adapted for the fabrication of a large cage on which gates cannot be closely spaced from each other. As described in connection with the first through third embodiments of the invention, where the die has plural gates, one of plural circumferential regions formed between the gates is so short that a large strength drop is not induced. In longer circumferential regions, resin reservoirs are formed at meeting points of streams of the injected resinous material. Even in this case, the number of the resin reservoirs should be reduced to a minimum to decrease the amount of wasted resin.

Claims: A die consisting of a pair of die members (4,5) and adapted to mold a cage out of a synthetic resin, said cage having a cylindrical body (10), said die comprising: parting surfaces abutting against each other to form a mold cavity (1) conforming in shape with said body (10) of said cage, said mold cavity (1) providing a space in which said synthetic resin separates into two streams flowing in opposite directions when said synthetic resin is injected into said mold cavity from at least one gate (2); said at least one gate (2) being formed along one of said parting surfaces and located outside said mold cavity when said parting surfaces are in abutment with each other, said gate (2) being in communication with said mold cavity (1); and one resin reservoir (3) formed along one of said parting surfaces and located outside said mold cavity (1) when said parting surfaces are in abutment with each other, said reservoir (3) being located in a position where said two streams meet, wherein said at least one gate is two gates (2A,2B) circumferentially spaced from each other around said mold cavity (1), said two gates form a major circumferential region and a minor circumferential region therebetween, and said one resin reservoir (3) is located in said major circumferential region in a position where said two streams meet. A die consisting of a pair of die members (4,5) and adapted to mold a cage out of a synthetic resin, said cage having a cylindrical body (10), said die comprising: parting surfaces abutting against each other to form a mold cavity (1) conforming in shape with said body (10) of said cage, said mold cavity (1) providing a space in which said synthetic resin separates into two streams flowing in opposite directions when said synthetic resin is injected into said mold cavity from at least one gate (2); said at least one gate (2) being formed along one of said parting surface and located outside said mold cavity when said parting surfaces are in abutment with each other, said gate (2) being in communication with said mold cavity (1); and one resin reservoir (3) formed along one of said parting surfaces and located outside said mold cavity (1) when said parting surfaces are in abutment with each other, said reservoir (3) being located in a position where said two streams meet, wherein said at least one gate (2) is plural gates (2A,2B,2C) circumferentially spaced from each other around said mold cavity (1), said plural gates form a major circumferential region and minor circumferential regions therebetween, and said one resin reservoir (3) is located in said major circumferential region in a position where said two streams meet. The die as claimed in one of claims 1 or 2, wherein said resin reservoir (3) is narrower in volume than said mold cavity (1). The die as claimed in one of claims 1 or 2, wherein said resin reservoir is in communication with said mold cavity (1) via a narrow passageway. A method of molding a cage having a cylindrical body out of a synthetic resin, said method comprising steps of: preparing a die as claimed in one of claims 1 or 2; injecting said synthetic resin in a molten state into said mold cavity via said g ate so that said synthetic resin separates into two streams flowing in opposite directions inside said mold cavity; and storing said synthetic resin in said resin reservoir in a position where said two streams meet.

Inventors: ASHIDA SHIGETAKA, TERADA TADAHIRO, ASHIDA, SHIGETAKA, TERADA, TADAHIRO
Applicants: KOYO SEIKO CO, KOYO SEIKO CO., LTD.
Application year: 1998
Main classification: B29C45
IPC codes: B29C33, B29C45, F16C33