Compression molding die assembly for rubber members and rubber member produced thereby

Abstract: A molding die assembly includes an upper die (20) and a lower die (10), wherein at least one of the upper and lower dies is provided with a cavity corresponding to the shape of a rubber member to be molded, wherein at least one of the upper and lower dies is provided with an annular projection (13) around a cavity. The distance (t) between the upper (20) and lower (10) dies at the annular projection (13) is made smaller than the distance between the opposed surface portions, other than the cavity, of the upper and lower dies. A rubber member produced by the above mentioned molding die assembly is also provided.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compression molding die assembly for a rubber member, such as a closure to be inserted in a mouth of a container, or a piston to be inserted into a syringe. The present invention also relates to a process for obtaining a rubber member. 2. Description of the Related Art The above mentioned rubber member, having a cylindrical shape, is conventionally formed by a compression molding process. Namely, a masticated rubber material is preformed into a flat sheet of rubber as a raw material for molding, and the rubber sheet is molded into a predetermined shape using a molding die assembly including an upper die and a lower die having a plurality of cavities. The shortest (closest) distance between the upper die and the lower die of the die assembly is determined so that an appropriate distance is provided between opposed surfaces of the die surface portions of the upper and lower dies, other than the cavities. Rubber members molded in the cavities are interconnected by a thin rubber sheet extending between the opposed surfaces of the upper and lower dies. The rubber members removed from the die assembly, which are interconnected by the thin rubber sheet, and are cut from the thin rubber sheet, so that individual rubber members can be obtained. Fig. 8 shows an example of a molding die assembly for a rubber piston 31. A lower die 10 is provided with a plurality of substantially cylindrical cavities 11 corresponding to the shape of the rubber piston 31. An upper die 20 is provided with projections with external threads (male screw portions) 25 which are inserted into the corresponding cavities 11 of the lower die 10, but do not contact the lower die 10. The masticated rubber material which has been preformed in a flat sheet is fed between the lower die 10 and the upper die 20. The die surface portion of the lower die 10 which is opposite to the upper die 20, other than the cavities 11, defines a flat surface 12. The closest distance "d" between the flat surface 12 and the upper die 20 upon molding is determined in accordance with the thickness of the masticated rubber material preformed in the form of a flat sheet. In general, the closest distance "d" is determined to be identical to the thickness of the masticated rubber material preformed in the form of a flat sheet. Fig. 9 shows a molded product (sheet) 30 produced by the molding die assembly shown in Fig. 8, having a plurality of rubber pistons 31 which are interconnected by a thin rubber sheet 32. The rubber pistons (cylindrical rubber pistons) 31 are cut from the connecting rubber sheet 32 and are separated from one another, as shown in Fig. 10, in which the cut surface 33 is exposed. The rubber piston 31 is provided on the center portion thereof with a threaded hole (female screw portion) 35 formed by the threaded projection 25 of the upper die 20, so that a plunger of a syringe can be screw-engaged into the threaded hole (internal thread) 35. Fig. 11 shows an example of a known molding die assembly for a rubber closure. The lower die 10 is provided with a plurality of cylindrical cavities 11 corresponding to the shape of the rubber closure. The upper die 20 is provided with a plurality of cavities 21 whose diameter is larger than the diameter of the cavities 11 and which are coaxial to the corresponding cavities 11. The die surface portion of the lower die 10 which is opposite to the upper die 20, other than the cavities 11, defines a flat surface 12. The die surface portion of the upper die 20 which is opposite to the lower die 10, other than the cavities 21, defines a flat surface 22. The closest distance "d" between the flat surfaces 12 and 22 upon molding is determined depending on the thickness of the masticated rubber material preformed in the form of a sheet. The molded product 40 thus obtained includes a plurality of rubber closures (molded members) 41 interconnected by a connecting rubber sheet 42. The individual rubber closures 41 are cut at cut surfaces 43. The cut surfaces 33 or 43 of the above described individual rubber pistons 31 and rubber closures 41 are exposed. However, the exposed cut surfaces 33 or 43 are adhesive, so that if the exposed surfaces 33 or 43 are brought into contact with each other, the cut surfaces may adhere to each other. Moreover, when a large number of rubber pistons 31 or rubber closures 41 are conveyed along a conveyance line, they may interfere with a part of the conveying apparatus, interrupting the conveyance operation. Furthermore, the cut surface 33 of the rubber piston 31 increases the sliding resistance with respect to the inner barrel of the syringe. In addition to the foregoing, if the cut surface is large, i.e., if the axial length of the cut surface is large, the thickness of the connecting rubber sheet 32 or 42 is also large, thus resulting in an increased amount of wasted rubber material. Obviously, the size of the cut surface can be, in theory, reduced by decreasing the closest distance "d" between the lower die 10 and the upper die 20. However, since the size of the cut surface is determined depending on the thickness of the masticated rubber material as mentioned above, it is very difficult in practice to make the cut surface smaller. In particular, if the masticated rubber material preformed in the form of a sheet is too thin, mold defects occur. To prevent this, in general, the thickness of the masticated rubber material to be fed between the upper and lower dies is set relatively large. Under these circumstances, a reduction in the size of the cut surfaces is restricted in the conventional molding die assembly. Molding die assemblies comprising an upper die, a lower die and a cavity provided to at least one of said dies are known from US-A-5 693 164, JP-A-03039218, DE 801479 C. Further prior art is disclosed in WO 9524301A, US-A-4036675 and US-A-4635807. SUMMARY OF THE INVENTION It is an object of the present invention to provide a compression molding die assembly in which the size of the cut surfaces of simultaneously molded rubber members, at which the rubber members are cut from a connecting rubber sheet, can be made small. Another object of the present invention is to provide a process for obtaining a rubber member having a small cut surface. In order to achieve the above mentioned objects, a compression molding die assembly according to claim 1 is provided. Preferably, the cavity is in the form of a cylinder substantially corresponding to the shape of a cylindrical rubber piston to be molded. Preferably, the cavity is in the form of a cylinder substantially corresponding to the shape of a rubber closure to be molded. In the case where the maximum diameter of the cavity is not greater than 10mm, the closest distance between the upper and lower dies at the annular projections around the cavity is not greater than 0.5mm. In the case where the maximum diameter of the cavity is in the range of 10mm to 20mm, the closest distance is not greater than 0.8mm. In the case where the maximum diameter of the cavity is greater than 20mm, the closest distance is not greater than 1.0mm. Preferably, the molding die assembly is made of stainless steel. A rubber member can be produced using the above described molding die assembly structures. According to another aspect of the present invention, there is provided a process for obtaining a rubber member by cutting the rubber member from a connecting rubber sheet formed by a compression molding process and including a plurality of rubbers member which are formed on the connecting rubber sheet, wherein each of said rubber members includes an annular recess which is formed around each of said rubber members on the connection rubber sheet, and wherein said rubber member is cut from the connecting rubber sheet at the annular recess to constitute a cut surface of said rubber member. In the case where the maximum diameter of the rubber member is not greater than 10mm, the thickness of the cut surface is not greater than 0.5mm. In the case where the maximum diameter is in the range of 10mm to 20mm, the thickness of the cut surface is not greater than 0.8mm. In the case where the maximum diameter is greater than 20mm, the thickness of the cut surface is not greater than 1.0mm. The connecting rubber sheet including the rubber member is formed by a compression molding process. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be discussed below in detail with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a first embodiment of a molding die assembly for molding rubber pistons according to the present invention; Figure 2 is a perspective view of a mold sheet produced by a molding die assembly shown in Fig. 1; Figure 3 is a sectional view of a second embodiment of a molding die assembly for molding rubber pistons according to the present invention; Figure 4 is a sectional view of a third embodiment of a molding die assembly for molding rubber pistons according to the present invention; Figure 5 is a sectional view of a first embodiment of a molding die assembly for molding rubber closures according to the present invention; Figure 6 is a sectional view of a second embodiment of a molding die assembly for molding rubber closures according to the present invention; Figure 7 is a sectional view of a third embodiment of a molding die assembly for molding rubber closures according to the present invention; Figure 8 is a sectional view of a known molding die assembly for molding rubber pistons; Figure 9 is a perspective view of a mold sheet produced by a known molding die assembly shown in Fig. 8; Figure 10 is a perspective view of a single rubber piston separated from a mold sheet shown in Fig. 8; and Figure 11 is a sectional view of a known molding die assembly for molding rubber closures. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows an embodiment of the present invention applied to a molding die assembly for a rubber piston. The die assembly in the illustrated embodiment is different from the known die assembly shown in Fig. 8, in that the lower die 10 is provided with annular projections 13 projecting toward the upper die 20 around the substantially cylindrical cavities 11. The remaining structure of the illustrated embodiment is the same as that shown in Fig. 8. Furthermore the upper die 10 and the lower die 20 are made of stainless steel. With this structure, even if a masticated rubber material having a thickness the same as that used in the prior art, the minimum distance "t" between the projections 13 of the lower die 10 and the upper die 20 is made smaller than the distance "d" in the prior art. Fig. 2 shows a mold sheet 30 produced by the molding die assembly shown in Fig. 1. In Fig. 2, an annular recess 34, the depth thereof corresponding to the minimum distance "t", is formed around the rubber piston (cylindrical rubber piston) 31, so that the thickness of the connecting rubber sheet 32 is reduced at the recess 34. Consequently, the cut surface 33 at which the rubber piston 31 is cut and separated from the rubber sheet 31 can be reduced in comparison with the prior art shown in Figs. 8 through 10. Thus, the above-mentioned problem with the cut surface can be substantially eliminated. Fig. 3 shows another embodiment of the present invention in which the annular projections 23 are formed on the upper die 20, in place of the annular projections 13 formed on the lower die 10, to surround the cavities 11 when the dies are closed. Fig. 4 shows yet another embodiment of the present invention, in which the annular projections 13 and 23 oppose each other, and are formed on the lower die 10 and the upper die 20, respectively, to surround the cavities 11 when the dies are closed. Accordingly, the closest distance "t" between the annular projections 23 of the upper die 20 and the lower die 10, or the closest distance "t" between the annular projections 23 of the upper die 20 and the corresponding annular projections 13 of the lower die 10 can be made smaller than the distance "d" of the prior art. Figs. 5 through 7 show an embodiment of the present invention applied to a molding die assembly for a rubber closure. In Figs. 5 through 7, the elements corresponding to those in Fig. 11 are designated with like reference designators. In an embodiment shown in Fig. 5, the annular projections 24 which project toward the lower die 10 are formed around the cavities 21 of the upper die 20. In an embodiment illustrated in Fig. 6, the annular projections 14 which project toward the upper die 20 are formed around the cavities 11 of the lower die 10. The inner diameter of the annular projection 14 corresponds to the inner diameter of the cavity 21. In an embodiment shown in Fig. 7, both the lower die 10 and the upper die 20 are provided with annular projections 14 and 24, respectively. In the embodiments illustrated in Figs. 5 through 7, even if a masticated rubber material having the same thickness as that of the prior art is used, the closest distance "s" between the annular projections 24 of the upper die 20 and the lower die 10, the closest distance "s" between the annular projections 14 of the lower die 10 and the upper die 20, or the closest distance "s" between the annular projections 24 of the upper die 20 and the annular projections 14 of the lower die 10, can be made smaller than the distance "d" of the prior art. Accordingly, an annular recess 44, the depth thereof corresponding to the minimum distance "s", is formed around the rubber closure (molded closure) 41, so that the thickness of the connecting rubber sheet 42 of the mold sheet 40 is reduced at the annular recess 44. Consequently, the cut surface 43, at which the rubber closure 41 is cut and separated from the connecting rubber sheet 42, can be reduced in comparison with the prior art shown in Fig. 11. Thus, the above-mentioned problem with the cut surface can be substantially eliminated. Likewise with the prior art, the molding die assemblies in the above described embodiments can be utilized in a conventual compression molding process. The following rubber molds (pistons 31 or closures 41) were obtained using the molding die assembly described above, according to the present invention: 1) In the case where the maximum diameter D of the mold 31 or 41 (i.e., the maximum diameter of the cavities 21) is constructed not greater than 10mm, the closest distance "t" or "s" is not greater than 0.5mm; 2) In the case where the maximum diameter D is in the range of 10 to 20mm, the closest distance "t" or "s" is not greater than 0.8mm; 3) In the case where the maximum diameter D is greater than 20mm, the closest distance "t" or "s" is not greater than 1.0mm. The masticated rubber material can be selected from IIR, NBR, BR, or EPDM, etc., and is not limited to a specific material. As can be understood from the above discussion, the cut surfaces of the simultaneously molded rubber members at which the rubber members are cut from the mold rubber sheet can be made small. Consequently, the rubber member is substantially free from the problems that occur with a rubber member having a large cut surface. Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.

Claims: A compression molding die assembly for compression molding a rubber sheet (30) having a predetermined thickness into a plurality of rubber elements (31), comprising an upper die (20) and a lower die (10), wherein at least one of said upper (20) and lower dies (10) is provided with a plurality of cavities (11, 21) corresponding in their shapes to said rubber members (31) to be compression molded, and wherein the distance between the opposed surface portions (12), other than said cavities (11, 21) of said upper (20) and lower dies (10), upon compression molding is determined in accordance with the thickness of said rubbber sheet (30), characterized in that at least one of said upper (20) and lower dies (10) is provided with a plurality of annular projections (13, 14, 23, 24), each around one of said plurality of cavities (11, 21), respectively, wherein the distance between said upper (20) and lower dies (10) at said annular projections (13, 14, 23, 24) upon compression molding is made smaller than the distance between the opposed surface portions (12), other than said cavities (11, 21), of said upper (20) and lower dies (10) upon compression molding. A compression molding die assembly according to claim 1, wherein each cavity (11, 21) is in the form of a cylinder substantially corresponding to the shape of a cylindrical rubber piston (31) to be molded. A compression molding die assembly according to claim 1, wherein each cavity (11, 21) is in the form of a cylinder substantially corresponding to the shape of a rubber closure (31) to be molded. A compression molding die assembly according to claim 1, 2 or 3, wherein: in the case where the maximum diameter of each cavity (11, 21) is not greater than 10mm, the closest distance between said upper and lower dies at said annular projection around each cavity (11, 21) is not greater than 0.5mm; in the case where the maximum diameter of each cavity (11, 21) is in the range of 10mm to 20mm, the closest distance is not greater than 0.8mm; and in the case where the maximum diameter of each cavity is greater than 20mm, the closest distance is not greater than 1.0mm. A compression molding die assembly according to anyone of the foregoing claims wherein said molding die assembly is made of stainless steel. A rubber member (31), characterized in that said rubber member (31) is produced using a compression molding die assembly according to claim 1, 2, 3, 4 or 5. Process for obtaining a rubber member (31), characterized by cutting said rubber member (31) from a connecting rubber sheet (32) formed by a compression molding process and comprising a plurality of rubber members (31) which are formed on said connecting rubber sheet (32), wherein each of said rubber members (31) comprises an annular recess (34) which is formed around each of said rubber members (31) on said connection rubber sheet (32), and wherein said rubber member is cut from said connecting rubber sheet (32) at said annular recess (34) to constitute a cut surface (33) of said rubber member (31); wherein: in the case where the maximum diameter of said rubber member (31) is not greater than 10mm, the thickness of said cut surface (33) is not greater than 0.5mm; in the case where the maximum diameter is in the range of 10mm to 20mm, the thickness of said cut surface (33) is not greater than 0.8mm; and in the case where the maximum diameter is greater than 20mm, the thickness of the cut surface (33) is not greater than 1.0mm.

Inventors: ASAI KOUICHI, SUDO MASAMICHI, SUDO MORIHIRO, ASAI, KOUICHI, SUDO, MASAMICHI, SUDO, MORIHIRO, Asai, Kouichi, c/o Daikyo Seiko Ltd., Sudo, Masamichi, c/o Daikyo Seiko Ltd., Sudo, Morihiro, c/o Daikyo Seiko Ltd.
Applicants: DAIKYO SEIKO LTD, DAIKYO SEIKO, LTD.
Application year: 2000
Main classification: B29C33
IPC codes: B29C33, B29C43, B29L31, B29K21