INSULATOR AND MANUFACTURING METHOD OF POLY¬

CARBONATE Technical Field [1] The present invention relates to a polycarbonate insulator and a manufacturing method thereof, and more particularly, to a polycarbonate insulator, wherein a main body for fixing an overhead line is formed through injection molding with a poly¬ carbonate resin and glass fibers on a portion of fittings for use in fixing the insulator to an electric pole or causing the insulator to be suspended from an object so that the insulator has a simplified structure and can be manufactured with ease, and a manu¬ facturing method thereof. [2] Background Art [3] In general, an insulator is mounted on an electric pole using bolts and constructed to make the flow of electricity safe by placing electric wires thereon while maintaining insulation from the electric pole. Insulators are largely divided into post insulators, insulators for cutout switches, suspension insulators, support insulators and the like according to the shape and usage thereof. In such an insulator, a portion thereof to be connected to an electric wire is made of Ethylene Propylene Diene Monomer (EPDM), silicon or porcelain, and a portion thereof to be connected to an electric pole is constructed of a metallic bolt. [4] Reference will be made to FIGS. 1 and 2 that are respectively a perspective view and a sectional view of a conventional insulator. FIGS. 1 and 2 show a post insulator (100) among conventional insulators. The post insulator (100) is largely composed of a housing (101), a fixing means (103), and fittings (104). As for their structure, the insulator includes the housing (101) made of silicone and having skirts (105), each of different sizes formed on an outer surface of the housing, a core made of FRP and formed through insert molding to have upper and lower protrusions of predetermined sizes at upper and lower ends in the housing (101), fixing means (103) molded with an epoxy resin and having an overhead line passing groove (102) formed at an upper portion thereof and a hemispherical annular groove formed at a lower periphery thereof so that the fixing means can be installed above the housing (101) and the core so as to support an overhead line thereon, and the fittings (104) formed below the housing (101) and the core so as to fix the insulator to an electric pole. [5] The fittings (104) are press-fitted and assembled in such a way that the lower periphery of the housing (101) and the lower protrusion are enclosed by a press-fit recess formed at an upper portion of the fittings. In addition, the fixing means is bonded and assembled using a conventional adhesive in such a way that an upper periphery of the housing (101) and the upper protrusion can be enclosed by the lower bonding groove of the fixing means. [6] However, there is a problem in that since the conventional post insulator is constructed by mechanically press-fitting the upper and lower protrusions into the fixing means and the fittings, respectively, an additional machine for performing the press-fitting is required. [7] There is another problem in that since the insulator requires many parts, pro¬ ductivity suffers and production costs increase due to the assembly of these parts. [8] There is still another problem in that since the housing and the core are fixedly bonded to each other by means of adhesive, the adhesive does not function properly when the insulator is used for a long time. [9] Disclosure of Invention Technical Problem [10] Accordingly, the present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a poly¬ carbonate insulator, wherein a main body for fixing an overhead line is formed without any additional assembly processes, which otherwise would be required in production as separate parts, by performing injection-molding on a portion of an outer periphery of fittings, which are used in fixing the insulator to an electric pole or causing the insulator to be suspended from an object, with a mixed resin containing a poly¬ carbonate resin and glass fibers so that the insulator has light weight and a simplified structure and can be manufactured with ease, and to provide a manufacturing method thereof. [H] Technical Solution [12] To achieve the object, the present invention provides an insulator comprising fittings for fixing the insulator to an electric pole or causing the insulator to be suspended from an object, and a main body for fixing an overhead line thereto, wherein the fittings and the main body are integrally injection-molded in such a manner that a portion of the fittings is embedded as an insert in the main body made of a polycarbonate resin composition containing 70-90 wt% of polycarbonate resin and 10-30 wt% of glass fibers. [13] In the insulator of the present invention, the polycarbonate resin may have a viscous molecular weight of 20,000-35,000. [14] The present invention provides a method of manufacturing an insulator, comprising the steps of: placing fittings for fixing the insulator to an electric pole inside a filling cavity of a mold including stationary and movable mold halves each of which has a lateral half of the filling cavity corresponding to the shape of the insulator; fixing the fittings by inserting fixing pins from outside into the mold to determine the relative position of the insert with respect to the filling cavity following the insert placing step; removing moisture contained in a polycarbonate resin and glass fibers to prevent quality deterioration upon completion of molding following the insert fixing step; raising the temperature of the mold to improve fluidity of a molten resin of a poly¬ carbonate resin composition, thereby preventing defects in a molded product following the pre-drying step; causing a portion of the fittings to be embedded inside the poly¬ carbonate resin composition while filling the filling cavity with the polycarbonate resin composition containing 70-90 wt% of polycarbonate resin and 10-30 wt% of glass fibers following the mold preheating step; withdrawing the fixing pins outside the mold and applying pressure to the polycarbonate resin composition to compensate volume reduction due to shrinkage during cooling of the polycarbonate resin composition following the polycarbonate resin composition filling step; solidifying the polycarbonate resin composition by cooling the mold following the polycarbonate resin composition pressurizing step; releasing a unitary molded insulator from the mold using an ejector while separating the movable and stationary mold halves from each other following the polycarbonate resin composition plasticizing step; and removing internal stress of the molded product to enhance close contact of the insert with the polycarbonate resin composition folllowing the molded product releasing step. [15] In the method of the present invention, the polycarbonate resin has a viscous molecular weight of 20,000-35,000 in the polycarbonate resin composition filling step. [16] Advantageous Effects [17] As described above, since the insulator of the present invention is produced as a finished product in a factory, there is an advantage in that an additional machine for press-fitting upper and lower protrusions into a fixing means and fittings, respectively, is not required. [18] Further, since the insulator of the present invention is produced as a finished product comprising a main body and fittings, there are advantages in that productivity is improved and production costs decrease. [19] Moreover, according to the present invention, since other portions of the insulator except fittings are made of a polycarbonate resin composition, there are advantages in that the total weight of the product decreases and the strength thereof is improved. [20] Furthermore, according to the present invention, since an adhesive is not used, there is an advantage in that any problem rising from use of an adhesive does not occur. [21] In addition, according to the present invention, since injection molding is performed with a polycarbonate resin composition by using fittings as an insert, the fittings can be firmly coupled to the polycarbonate resin composition and thus are not loosened or separated during a long-term use. Therefore, there is an advantage in that the coupling of these parts is not loosened, thereby improving durability. [22] Brief Description of the Drawings [23] FIG. 1 is a perspective view of a conventional insulator. [24] FIG. 2 is a sectional view of the conventional insulator. [25] FIG. 3 is a sectional view of an insulator according to the present invention. [26] FIG. 4 illustrates a process of the present invention. [27] Best Mode for Carrying Out the Invention [28] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. [29] An insulator of the present invention has the same constitutionally shape as that of a conventional insulator. In other words, a housing, a fixing means, fittings and an overhead line passing groove of the insulator of the present invention are the same as those of a conventional post insulator in view of their constitutions. Other portions of the insulator of the present invention except the fittings are made of a polycarbonate resin and glass fibers. Therefore, since a perspective view of the insulator of the present invention is the same as that of the conventional insulator shown in FIG. 1, it will be omitted. [30] Referring to a sectional view of the insulator (10) of the present invention shown in FIG. 3, the insulator (10) of the present invention includes a main body (11) and fittings (20). As for their constitutions, skirts (15) of different sizes are radially formed at a predetermined interval on an outer periphery of the cylindrical main body (11); a hemispherical, overhead line passing groove (12) is formed in one direction in an upper surface (13) of the main body (11); a hemispherical annular groove (14) is formed in the outer periphery of the main body (11) below the overhead line passing groove (12); and the fittings (20) for use in fixing the insulator to an electric pole or causing the insulator to be suspended from an object are formed at a lower portion of the main body (11). [31] A polycarbonate resin has superior mechanical strength, particularly, in terms of impact strength, electrical properties, and transparency. Therefore, the polycarbonate resin is widely used as engineering plastic in a variety of fields such as those of electric/electronic devices and automobiles. [32] The present invention is based on the fact that combination of a polycarbonate resin with glass fibers at a predetermined ratio results in a glass fiber reinforced poly¬ carbonate resin with superior rigidity, dimensional stability and impact resistance. [33] In the present invention, the skirts (15) are integrally formed with the main body (11) to form a unitary main body. The material of the main body is a polycarbonate resin composition containing 70-90 wt% of polycarbonate resin of which viscous molecular weight is 20,000-35,000, and 10-30 wt% of glass fibers. [34] In the present invention, the glass fibers are added to the polycarbonate resin to provide mechanical strength thereto. The reason the composition ratio is limited is to guarantee dimensional stability. If the composition ratio of the polycarbonate resin is greater than 90 wt% and that of the glass fibers is less than 10 wt%, the polycarbonate resin composition has higher viscosity in a molten state, i.e., lower fluidity, whereby it is difficult to injection-mold thin portions of the insulator such as skirts and thus the di¬ mensional stability is degraded. [35] Meanwhile, if the composition ratio of the polycarbonate resin is less than 70 wt% and that of glass fibers is greater than 30 wt%, impact resistance can be maintained to a certain degree but it is difficult to perform mixing and kneading. [36] In addition, the viscous molecular weight of the polycarbonate resin is limited because stress is significantly lowered if the viscous molecular weight is less than 20,000. If the viscous molecular weight is not less than 35,000, impact strength increases but there is a risk of brittle fracture at a corner portion in a case where the radius of the corner is small. [37] As for the glass fibers of the present invention, it is possible to use any one of non- alkali glass, low-alkali glass and alkali glass. Specifically, as for the glass fibers of the present invention, fibrous structures including a fiber with an average diameter of 2-30D, preferably 5-15D, for example, a chopped strand, a milled fiber and roving, are useful. Such glass fibers may be used alone or in combination. [38] The glass fibers should be dispersed in consideration of appropriate balance of strain resistance and reinforcement. Preferably, the average length (mean length by weight) of the fiber is preferably 50-1, 000D. More preferably, it is 100~500Din a molded product. [39] The average diameter is measured by using a centrifugal settling type particle size distribution measuring instrument manufactured by Shimazu Co., in which average particle distribution is measured according to a centrifugal precipitation method. [40] While the polycarbonate resin composition of the present invention is basically composed of a polycarbonate resin and glass fibers, a variety of additives may be added to the composition, if necessary, so far as the addition does not impede the object of the present invention. That is, an ultraviolet stabilizer, a flame retardant, a colorant, a release agent, or a plasticizer may be added to the composition. [41] In the polycarbonate resin composition of the present invention, the respective components can be combined, mixed and kneaded using conventional mixing and kneading means such as co-kneaders, ribbon blenders, and drum tumblers. [42] The fittings (20) are made of a metallic material and have female threads or an anchor bolt at a portion that is not connected to the main body according to a counterpart object which the insulator will be fixed to or suspended from so that the insulator can be fixed to an electric pole or suspended from an object. [43] The fittings (20) are constructed such that when the main body (11) is injection- molded with the polycarbonate resin and the glass fibers, a portion of the fittings is inserted into the main body (11) or inserted into the main body (11) while enclosing a portion of the outer periphery of the main body (11). [44] The main body (11) and the fittings (20) are integrally manufactured by placing the fittings (20) inside a filling cavity of a mold defining stationary and movable mold halves, each having a lateral half of the filling cavity in the form of the insulator (10) and by performing injection-molding with the mixed resin composition composed of the polycarbonate resin and the glass fibers. [45] Mode for the Invention [46] A manufacturing method of the insulator according to the present invention will be described. As shown in FlG. 4 illustrating a process of the present invention, the manu¬ facturing method of the present invention includes the following sequential steps: a fittings manufacturing step (step 31), an insert placing step (step 32), an insert fixing step (step 33), a mold pre-drying step (step 34), a mold pre-heating step (step 35), a polycarbonate resin composition filling step (step 36), a polycarbonate resin comp osition pressurizing step (step 37), a polycarbonate resin composition plasticizing step (step 38), a molded product releasing step (step 39) and a molded product heat treatment step (step 40). [47] First, the step of manufacturing the metallic fittings (step 31) is performed. This step is a step of processing the fittings (20) such that the fittings are formed at the lower portion of the insulator main body (11) for use in fixing the insulator to an electric pole or causing the insulator to be suspended from an object and have female threads or an anchor bolt according to a counterpart object which the insulator will be fixed to or suspended from. [48] The fittings (20) are constructed such that when the insulator main body (11) is injection-molded with the polycarbonate resin composition, a portion of the fittings is inserted into the main body (11) or inserted into the main body (11) while enclosing a portion of the outer periphery of the main body (11). [49] After the fittings manufacturing step (step 31), the insert placing step (step 32) is performed to place the fittings (20) inside the filling cavity of the stationary mold half. The mold used in the present invention is largely composed of the movable mold half, an intermediate mold part and the stationary mold half. A split core or a slide core, or both are separately manufactured from the mold such that the core(s) can be placed in the mold. The core and the intermediate mold part may be eliminated according to the design of a mold. [50] The stationary mold half is fixed to a main body of an injection molding machine, and the intermediate mold part and the movable mold half are normally separated from the main body of the injection molding machine but are brought toward and in close contact with the stationary mold half during an injection molding process. [51] Each of the stationary and movable mold halves is formed with the recessed lateral half of the filling cavity corresponding to the shape of the insulator (10). The movable mold half is installed to be brought toward or spaced apart from the stationary mold half. When the movable mold half is brought toward and in close contact with the stationary mold half, a complete filling space is defined. In the insert placing step (step 32), the fittings (20) are placed in the filling cavity in advance. [52] The mold is provided at a proper position with a circular, a semicircular or a trapezoidal runner for leading the molten resin of the polycarbonate resin composition, which has been injected from a nozzle of the injection molding machine, to the filling cavity. [53] The position of the runner can be determined in consideration of a cooling rate of the polycarbonate resin composition according to the length of the fittings and the filling amount of the polycarbonate resin composition. Therefore, the position of the runner is not an unchangeable factor. [54] After the insert placing step 32, the insert fixing step (step 33) of fixing the fittings (20) inside the filling cavity of the mold is performed in such a manner that fixing pins are inserted from the outside into the movable mold half to determine the relative position of the fittings (20) with respect to the filling cavity. In this step, in order to determine the relative position of the fittings (20) with respect to the filling cavity after the movable mold half is brought toward and in close contact with the stationary mold half, the fixing pins pneumatically operated by air cylinders be inserted from the outside into the mold between the movable mold half and the stationary mold half to fix the fittings (20). [55] By doing so, the fittings (20) are fixedly placed at the center of the space of the filling cavity to maintain a predetermined gap with respect to a wall defining the filling cavity. The fixing pins are withdrawn after the polycarbonate resin composition is injected in the polycarbonate resin composition filling step (step 36. described later). Spaces in the polycarbonate resin composition produced by the presence of the fixing pins are filled with the polycarbonate resin composition in the polycarbonate resin composition pressurizing step (step 37. described later). [56] The positions of the fixing pins are not limited to the positions between the movable mold half and the stationary mold half but may be limited only to the movable mold half or the stationary mold half. The reason the fixing pins are used is that the fittings (20) are positioned at the center of the filling cavity so that molten resin of the poly¬ carbonate resin composition can be uniformly distributed around the fittings (20). [57] After the insert fixing step (step 33), pre-drying step (step 34) is performed to remove moisture contained in the polycarbonate resin and the glass fibers. Even a little amount of moisture in the hydrophilic polycarbonate resin may attack binding portions of polymeric polycarbonate and break a chain at high temperatures, thereby deceasing its molecular weight and degrading its physical properties. Thus, this step is for drying the resin for 3-4 hours at a temperature of about 120°C until the content of the moisture drops below 0.01-0.02% resin, thereby preventing the aforementioned quality degradation after molding. [58] If a pulverized or recycled polycarbonate resin is used, drying time should be extended. Re-drying should be carried out for a dried resin if the resin is exposed to the air for more than 15 minutes. As for a drying method, a dehumidifying dryer is preferably used. In case of a hot air type hopper drier, it should be carefully used to achieve sufficient drying. [59] After the pre-drying step (step 34), a mold preheating step (step 35) for raising the temperature of the mold is performed to improve fluidity of the molten resin of the polycarbonate resin composition, thereby preventing the occurrence of an inferior molded product. This step is for preheating the mold because injection of the poly¬ carbonate resin composition, i.e., filling of the polycarbonate resin composition, in a state where the temperature of the mold is low, causes a crack, a defective shrinkage or the like in a molded product or produces an incomplete molded product, thereby having influence on the reliability of a product. [60] As for a configuration for heating the mold, it is possible to employ a direct heating method of automatically raising the temperatures of the exterior and interior of the mold by interposing a heating plate such as a heater between the mold halves of the mold and using an external power source, or an indirect heating method of passing the mold through the interior of a heating apparatus in the form of a continuous passage or a heating tunnel in order to heat the mold. During this step, the high temperature in the mold enables a molded product to be produced with superior fluidity, a highly glossy surface and low deformation, although it still has a problem of a longer molding cycle. In contrast, low temperature becomes a cause of deformation, crazing or a crack. Therefore, the temperature of the mold is in a range of 85-1100°C. The temperature of a cylinder is in a range of 260~300°Csince the polycarbonate resin has a melting point of about 220°C and is not softened below 150°C [61] A core feeding step may be implemented between the insert fixing step (step 33) and the mold pre-heating step (step 35). This step is for feeding a split core or a side core, or both after the movable mold half is brought into contact with the stationary mold half. The split core or the side core, or both are configured within the mold to form features such as the skirts (15). At this time, an upper core part is fixedly assembled to the bottom of the stationary mold half and a lower core part is fixedly assembled to the top of the movable mold half. In this step, the core can be moved vertically and laterally. This step is not an inevitable but optional. [62] After the mold preheating step (step 35), the polycarbonate resin composition filling step (step 36) is conducted in such a manner that the filling cavity is filled with the polycarbonate resin composition so that the polycarbonate resin composition can be placed around an outer periphery of the fittings (20). This step is for causing the fittings (20) to be embedded in the polycarbonate resin composition while the filling cavity of the mold is filled with the molten resin of polycarbonate resin composition. The molten resin of the polycarbonate resin composition injected from a conical nozzle of an injection molding machine flows through a sprue, which is in contact with the nozzle, and a runner, which is a short trough or passage with a circular or trapezoidal cross section, into the filling cavity via a circular gate installed at an invisible place on a molded product, a place where the molten resin of the polycarbonate resin composition can rapidly flow from an inlet of the filling cavity to an opposite side, a place where distortion of the molded product does not occur. Preferably, the runner and the gate are large in size, and the sprue runner and the cavity is ground well to have small flow resistance. In addition, the gate is possibly positioned at a thick portion of a molded product. [63] The polycarbonate resin composition includes 70-90 wt% of polycarbonate resin and 10-30 wt% of glass fibers, wherein the viscous molecular weight of the poly¬ carbonate resin is 20,000-35,000. [64] The reason the composition ratios of the polycarbonate resin composition and the viscous molecular weight of the polycarbonate resin composition are limited has been explained in connection with the insulator of the present invention. In addition, the glass fibers and the additives used in the present invention have also been explained in connection with the insulator of the present invention. Therefore, descriptions thereof will be omitted. [65] Since the method of combining, mixing and kneading the polycarbonate resin composition in the manufacturing method of the present invention has been described in connection with the insulator of the present invention, a description thereof will be omitted. [66] The molten resin of the polycarbonate resin composition is forced to flow into the filling cavity through the nozzle, the sprue and the gate by means of a pressing device of the injection molding machine, such as a heating cylinder defining a screw or the like. The injection molding machine should be operated such that it is operated at a low injection rate at an initial filling stage and the injection rate is then increased to a higher injection rate at a switching stage. However, if the gate is too small, the poly¬ carbonate resin composition may be deteriorated due to the higher injection rate. Thus, the number of revolutions of the screw is set at 40~80rpm. Although back pressure should be maintained as low as possible so that the resin can be plasticized, a uniform molten polymer is generated and exhaust can be smoothly made, it is preferably 3~30kg/cm2. The temperature of the mold is 70~120°C and injection pressure is l,000~l,500kg/cm2 so that the mold can be filled as quickly as possible and stress in the mold can be suppressed as much as possible. The upper limit of the molding temperature is 320°C but it may be 330~340°C in case of high cycle molding. [67] Preferably, the injecting amount is set at 20-75% of the full capacity and an optimum value thereof is about 40-60%. Although the length-to-diameter ratio of the screw is ideally 20:1, it should be maintained at at least 15:1. A compression ratio is preferably 1.5:1-3.0:1. [68] Following the polycarbonate resin composition filling step (step 36), the poly¬ carbonate resin composition pressurizing step (step 37) of applying pressure to the molten resin of the polycarbonate resin composition is effected. In this step, the fixing pins are withdrawn outside the mold and volume reduction due to shrinkage during cooling of the polycarbonate resin composition is compensated. This step is for applying pressure by further advancing the pressing device after the filling cavity has been filled with the polycarbonate resin composition and the fixing pins have been withdrawn outside the mold. As for a shrinkage generated when the molten resin of the polycarbonate resin composition is changed from a molten state to a solid state, the polycarbonate resin composition has a molding shrinkage rate of 0.005~0.007cm/cm, which is cooling shrinkage. Therefore, in this step, volume reduction due to the shrinkage is compensated and voids generated by the presence of the fixing pins are filled with the resin composition through the pressurization, wherein applied pressure and pressurizing time are maintained as low as possible. [69] After the polycarbonate resin composition pressurizing step (step 37), the poly- carbonate resin composition plasticizing step (step 38) of solidifying the molten resin of the polycarbonate resin composition is performed. This step is for solidifying the molten resin of the polycarbonate resin composition by cooling the mold, wherein cooling water or a refrigerant is forced to flow through a cooling bore or cooling passage formed in the injection molding machine or the mold to remove heat from the molten resin of the polycarbonate resin composition through the mold and the molten resin of the polycarbonate resin composition receives plasticity that is a property by which a deformation generated under a stress beyond an elastic limit is not restored to an initial state even though the stress is removed. [70] It is possible to employ a direct cooling method in which the mold is directly cooled as described above, or an indirect cooling method in which the mold is cooled by passing the mold through the interior of an additional external cooling device such as a cooling tunnel. [71] After the polycarbonate resin composition plasticizing step (step 38), the molded product releasing step is performed in such a manner that the movable mold half and the stationary mold half are separated from each other and a unitary molded product is released from the mold by using an ejector. This step is for releasing the unitary molded product from the mold by using the ejector while separating the movable mold half and the stationary mold half from each other. The molded product is released from the filling cavity by using ejector pins that are installed within the mold and fixed to an ejector plate so that they can advance together with the ejector plate when the mold is opened. [72] The ejector formed with an ejector rod, the ejector plate, the ejector pins and the like is installed within the mold in order to ensure that a finished product will be easily taken out upon completion of the molding. The ejector is constructed such that it can be retracted by means of an additional spring or the weight of the mold when the mold is closed and the filling cavity is filled with the resin composition. [73] When the core feeding step is performed between the polycarbonate resin composition plasticizing step (step 38) and the molded product releasing step (step 39), a step of returning the split core or the side core, or both is performed. In this step, when guide pins are retracted by means of springs, the split core or the side core, or both within the mold, which have been fed in the core feeding step, return to their original positions. Since the cores return to their original positions, it is possible to exclude interference thereof with the release of a molded product. [74] After the molded product releasing step (step 39), the molded product heat treatment step (step 40) of removing internal stress of the molded product is performed. In order to enhance the close contact of the insert with the polycarbonate resin composition, the molded product is placed in a heat-treating furnace and an atmosphere of the furnace is heated to 125~135°C and then slowly cooled to room temperature. This step is to remove internal cracks and stress in the polycarbonate resin composition, which are produced during the pressurizing and plasticizing steps and to make grains of molecules smaller, thereby improving ductility. [75] After the molded product heat treatment step (step 41), the step of inspecting the molded product is performed. In this step, acceptable molded products are selected while rejecting inferior products, including those produced since the polycarbonate resin composition did not sufficiently flow throughout the mold and was cooled and solidified so that a portion of a molded product is lost due to lack of the resin composition, those with a flash phenomenon in which an excess molten resin portion of the polycarbonate resin composition is formed on a molded product, those with a silver streak phenomenon in which a bundle of fine streaks are formed on or near the surface of a molded product in a flow direction of the resin composition, those with a sink mark phenomenon in which a depression appears on the surface of a molded product, those with a jetting phenomenon in which a serpentine portion is formed on the surface of a molded product, and those with a weld mark phenomenon in which fine streaks are created at a portion where the molten resin of the polycarbonate resin composition was split into two streams and the streams then joined together in the filling cavity. [76] Test results for polycarbonate resin compositions of examples of the present invention according to the ASTM standard method are shown in Table 1 below. [77] [78] Table 1

[79] Industrial Applicability [80] As apparent from the foregoing, since the insulator of the present invention is produced as a finished product in a factory, there is an advantage in that an additional machine for press-fitting upper and lower protrusions into a fixing means and fittings, respectively, is not required. Further, since the insulator of the present invention is produced as a finished product including a main body and fittings, there are advantages in that productivity can be improved and production costs can be decreased. Still better, since an adhesive is not used, there is an advantage in that no problem involving adhesive occurs. [81]