Grommet and method of producing the same

Abstract: A grommet 1 includes a corniced part 2 in a shape of a bellows, and two engaging parts 3, 4 integrally formed on opposite ends of the corniced part and adapted to be engaged with respective bodies to be fitted such as a chassis panel 5 and a door panel 6. The grommet 1 is formed in a hollow tubular shape in such a manner that the corniced part 2 is smaller in diameter than the engaging parts 3, 4. The grommet 1 can be obtained by molding thermoplastic elastomer by injection molding so that the diameter of the engaging parts is less than five times as large as the diameter of the corniced part, opening molds 17, 18 for forming an outer face thereof, and thereafter, enlarging the corniced part thereby removing the grommet from a core 19 for forming an inner face thereof.

Description: The present invention relates to a method of producing a grommet, and more particularly a grommet including an intermediate part and engaging parts, and formed in such a manner that the intermediate part is smaller in diameter than large diameter tubular portions of the engaging parts. Recently, so called power window systems have been installed on vehicle doors so that window panels can be easily opened and closed up and down by operating an electric motor provided therefor. The vehicle doors are also provided with speakers, door locks, and so on. Therefore, the electric motor for the power window system and so on must be supplied with power, and wire harnesses are equipped across chassis and the doors. Usually, each of the wire harnesses is protected by an elastically deformable grommet from water and dust. A highly-expanded, dimensionally heat-recoverable grommet including a large diameter tubular portion continued from one end of a small diameter tubular portion in a shape of a bellows is described in WO-A-95 01256. The average inner diameter of at least part of the expanded ready-to-apply article is at least four to six times the average inner diameter of the same part(s) of the article in its unexpanded state. A suitable formulation of polymeric material which may include cross-linking agents, processing aids and reinforcing fillers is injection moulded and cross-linked to form the heat-expandable feed-through article (grommet). The grommet to be produced by the method of the present invention includes an intermediate part in a shape of a bellows and two engaging parts each including a large diameter tubular portion continued from the intermediate part. The intermediate part is formed to have a smaller diameter than the engaging parts which are integrally formed on opposite ends of the intermediate part. The engaging parts are adapted to be engaged with a chassis panel and a door panel respectively. The conventional grommet has been made of rubber (JP-A-10297389) which is heavy (its specific gravity is about 1.3), and has badly affected weight reduction of an automobile. Moreover, rubber must be subjected to vulcanizing process which requires lot of working time, and productivity of the grommet has been poor. Further, because rubber is unable to be recycled, effective use of resources has been impossible. The present invention has been made in view of the above described problems, and it is an object of the invention to provide a method of producing a grommet of a kind mentioned above which is light weight, able to be recycled, and excellent in productivity. According to the present invention, there is proposed a method of producing a grommet including an intermediate part in a shape of a bellows, and two engaging parts integrally formed on opposite ends of the intermediate part and each including a large diameter tubular portion continued from the intermediate part and adapted to be engaged with respective bodies to be fitted, the grommet being formed in a hollow tubular shape in such a manner that the intermediate part is smaller in its inner diameter than the inner diameters of the large diameter tubular portions of the engaging parts. The method comprises steps of molding thermoplastic elastomer into the grommet by injection molding so that the inner diameter of the large diameter tubular portions of the engaging parts is less than five times as large as the inner diameter of the intermediate part. Molds are used for forming the outer surface of the grommet and a fixed core part is used for forming the inner surfaces of the intermediate part and of the large diameter portions. After opening the molds, the intermediate part is enlarged in diameter, thereby removing the grommet from said fixed core part. Because the thermoplastic elastomer which is similar to rubber in characteristics is employed as material, the grommet thus obtained is light weight. Further, it has become possible not only to recycle the material but to produce the grommet by employing injection molding, resulting in a rapid molding. Still further, dimensional accuracy of the inner surface of the grommet will be stabilized, and, of course, vulcanizing process is not required. By molding the grommet by injection molding in such a manner that a ratio in inner diameters between the large diameter tubular protions of the engaging parts and the intermediate part is less than five, the core portion for forming the engaging parts can be passed through the intermediate part after enlarging thereof without difficulty, after the molding. Fig. 1 is a sectional view showing one embodiment of a grommet produced according to the present invention; Fig. 2 is a sectional view of essential parts of an engaging part and an intermediate part to be engaged with a chassis panel; Fig. 3 is a sectional view of essential parts of an engaging part and an intermediate part to be engaged with a door panel; Fig. 4 is a schematic view of a metal mold for explaining molding process of the grommet. Now, one embodiment of the present invention will be described referring to the drawings. In Fig. 1, a grommet 1 for a vehicle door is formed of elastomer which has characteristics similar to rubber in flexibility and strength, etc. , and is in a hollow tubular shape having an intermediate part 2 and two engaging parts 3, 4 at opposite ends of the intermediate part 2. The grommet 1 is attached to a chassis panel 5 and a door panel 6 enabling a wire harness 7 arranged therebetween to be protected from water and dust. In short, the wire harness 7 can be protected by the grommet 1. The chassis panel 5 and the door panel 6 constitute bodies to be fitted mentioned in the claim. The intermediate part 2 is formed in a hollow tubular shape, and the wire harness 7 is adapted to pass through the intermediate part 2. Because the intermediate part 2 is formed in a bellows shape, the grommet can easily follow opening and closing movements of the vehicle door by expansion and contraction of the intermediate part. Opposite end portions of the intermediate part 2 are formed in an annular shape having a same inner diameter as threads projecting inwardly in a middle part of the intermediate part 2. The engaging parts 3, 4 are integrally continued from the opposite end portions. These opposite end portions are formed rectilinearly. In the present embodiment, the inner diameter of the opposite end portions of the intermediate part 2 is D1, as shown in Figs. 2 and 3. Preferably, the wire harness 7 is held in tight fit inside the intermediate part 2. Referring to Figs. 1 and 2, the engaging part 3 includes a large diameter tubular portion 8 which is continued from the intermediate part 2 and adapted to be engaged with a hole in the chassis panel 5 which is not shown, and a funnel shaped tubular portion 9 which is in a substantially funnel shape in cross section and continued from the large diameter tubular portion 8 inside of the chassis panel 5. The engaging part 3 is so designed as to guide out the wire harness 7 to the chassis side. The large diameter tubular portion 8 has an inner diameter D2 which is larger than the inner diameter D1 of the intermediate part 2 (D2>D1) . The large diameter tubular portion 8 is provided with an annular groove 10 around its outer circumference to be engaged with the above mentioned hole in the chassis panel 5. The funnel shaped tubular portion 9 is provided with a slit 11. Referring to Figs. 1 and 3, the engaging part 4 includes a large diameter tubular portion 12 which is continued from the intermediate part 2 and adapted to be engaged with a hole in the door panel 6 which is not shown, and a small diameter tubular portion 13 which is smaller in diameter than the large diameter tubular portion 12 and extends rectilinearly. The large diameter tubular portion 12 has an inner diameter D3 which is larger than the inner diameter D1 of the intermediate part 2 (D3>D1), and is provided with an annular groove 14 around its outer circumference to be engaged with the above mentioned hole in the door panel 6. It is to be noted that the large diameter tubular portion 12 is formed in such a manner that the inner diameter D3 is less than five times as large as the inner diameter D1 of the intermediate part 2 for convenience of molding. In the present embodiment, the inner diameter D3 is equal to the inner diameter D2. An inner diameter D4 of the small diameter tubular portion 13 is set to be slightly larger than the inner diameter D1 of the intermediate part 2 and adequately smaller than the inner diameter D3. The grommet 1 having the above described structure is formed by injection molding by means of a metal mold 15 as shown in Fig. 4. The metal mold 15 has at least one cavity part 16 which is composed of molds 17, 18 and a core 19. The molds 17, 18 are adapted to form an outer surface of the grommet 1, while the core 19 is adapted to form an inner surface of the grommet 1. The core 19 consists of a fixed part 20 and a movable part 21. When the metal mold 15 is tightened, a molding cavity 22 is formed in the cavity part 16, into which molten elastomer (thermoplastic elastomer) is injected. After the injection molding, the molds 17, 18 are opened in directions of arrows P, and the movable part 21 of the core 19 slides in a direction of an arrow Q. On the fixed part 20 of the core 19, there remains the grommet 1 which has been thus molded. Then, the grommet 1 is air blown from inside of the intermediate part 2 (the intermediate part 2 is inf lated by means of an air inflator) to enlarge the inner diameter of the intermediate part 2. Thus, the grommet 1 as shown in Fig. 1 can be obtained by removing it from the fixed part 20 in the direction of the arrow Q. In Fig. 4, an area 23 contributes to forming of the intermediate part 2, and areas 24, 25 contribute to forming of the engaging parts 3, 4 respectively. The grommet 1 which has been formed through the above described molding process has been subjected to following tests according to a permanent extension test prescribed in JIS K 6301. Rabaron (black) (trade name) was employed as the thermoplastic elastomer. (1) Initial dimensions of the grommet have been measured at the following four points A-D. A: outer diameter of the large diameter tubular portion 8 of the engaging part 3 (See Fig. 2). outer diameter of the end portion of the intermediate part 2 adjacent to the engaging part 3 (See Fig. 2). C: outer diameter of the large diameter tubular portion 12 of the engaging part 4 (See Fig. 3). D : outer diameter of the end portion of the intermediate part 2 adjacent to the engaging part 4 (See Fig . 3 ) . The value at the point A was 50.1 mm, the value at the point B was 19.9 mm, the value at the point C is 50.0 mm, and the value at the point D is 20.1 mm (averages of n = 4) Thickness of the intermediate part 2 is 1.50 mm on the drawings. (2) The grommet 1 is mounted on an opening/closing device (not shown), and opened at respective opening/closing rates to be kept open for 10 minutes. (3) The grommet 1 is detached from the opening/closing device and left to stand for 10 minutes. (4) The grommet 1 is measured again at the following four points A-D after it has been left to stand. (5) After the tests, a sample is inserted into a simulation panel (not shown) to confirm that there is no problem. After the above tests, a crack has been created around a circular part in the depth of the slit 11 but not in the intermediate part 2, in case where the opening/closing rate was 500%. Insertion into the simulation panel can be conducted without problem. As the results, it has been found that if the inner diameters D2 and D3 of the large diameter tubular portion 8 and the large diameter tubular portion 12 are set to be less than five times as large as the inner diameter D1 of the bellow part 2, the grommet 1 free from any problem can be obtained. As described above, the grommet 1 according to the present invention has such effects that it is light weight and can be recycled. Further, since the vulcanizing process is not required, productivity will be enhanced Still further, because the grommet is produced by injection molding, the molding process can be conducted much faster and the productivity can be enhanced the more. Needless to say, dimensional accuracy of the inner surface of the grommet 1 can be stabilized thereby contributing to improvement of the productivity. As the thermoplastic elastomer, olefin elastomer can be also employed besides the above mentioned rabaron (black) which is stylene elastomer.

Claims: A method of producing a grommet (1) including an intermediate part (2) in a shape of a bellows, and two engaging parts (3, 4) being integrally formed on opposite ends of said intermediate part and each including a large diameter tubular portion (8, 12) continued from the intermediate part and adapted to be engaged with respective bodies to be fitted, said grommet being formed in a hollow tubular shape in such a manner that said intermediate part (2) is smaller in its inner diameter than the inner diameters of the large diameter tublar portions (8, 12) of said engaging parts (3, 4), comprising steps of: molding thermoplastic elastomer into said grommet by injection molding so that the inner diameter (D2, D3) of the large diameter tubular portions (8, 12) of said engaging parts (3, 4) is less than five times as large as the inner diameter (D1) of said intermediate part (2), wherein molds (17, 18) are used for forming the outer surfaces of the grommet (1) and a fixed core part (20) is used for forming the inner surfaces of the intermediate part (2) and of the large diameter tubular portions (8, 12) of the engaging parts (3, 4),and, after opening the molds, said intermediate part is enlarged in diameter thereby removing said grommet from said fixed core part (20).

Inventors: ARAI HIROAKI, ONO SHUJI, OOSUMI FUSAO, SAEKI SHINICHI, TAKEDA IKUO, ARAI, HIROAKI, ONO, SHUJI, OOSUMI, FUSAO, SAEKI, SHINICHI, TAKEDA, IKUO
Applicants: SEIEN GUM IND CO LTD, YAZAKI CORP, SEIEN GUM INDUSTRIAL CO., LTD., YAZAKI CORPORATION
Application year: 2001
Main classification: H02G3
IPC codes: H01B19, H02G3, H01B17, B60R16