Apparatus for producing casting products molded in epoxy resin

CROSS REFERENCE TO RELATED APPLICATION

0001This application claims benefit of priority to Japanese Patent Application No. 11-220050 filed August 3, 1999, the entire content of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

0002This invention relates to an APG (Automatic Pressure Gelation) apparatus for producing casting products, molded in an epoxy resin, for high voltage apparatus that has a voltage of 6 kilo-volts or higher, which mass-produces the casting products in a short period of time, and an APG method for producing casting products. The casting product is, for example, a bushing, an insulation separator or the like, which are used for an electric power apparatus or a power reception and distribution apparatus of a gas insulated switchgear or the like, a resin molded coil or molded valve.

DISCRIPTION OF THE BACKGROUND

0003An APG (Automatic Pressure Gelation) method is a method of producing a casting product made of an epoxy resin in a short period of time by hardening and forming the epoxy resin. In general, an APG apparatus that uses the APG method includes a pair of molds, a resin mixing tank connected to the molds through a pipe with a vacuum chamber, and an opening and closing system for opening and closing the pair of molds.

0004An epoxy resin possessing high reactivity, that is a resin that hardens in a short time, which contains silica powder or alumina powder, is used for molding. The epoxy resin is stirred and mixed with a hardening agent in the resin mixing tank while being degassed in a vacuum.

0005A metal conductor or an insert, which are pre-heated and dried, are set into the mold beforehand. After closing the molds in a vacuum chamber by the opening and closing system and then evacuating the vacuum chamber, the epoxy resin is injected into the molds from an inlet disposed on the bottom of the molds by applying pressure to the resin mixing tank.

0006After finishing injecting the epoxy resin into the molds, the epoxy resin is heated and hardened in a state of keeping the pressure to the epoxy resin applied from the resin mixing tank side at about 0.1-0.5 [MPa].

0007As to the APG method, a temperature of the epoxy resin before injection is normally held at a low temperature of 40-60 degrees C in order to ensure a pot life (usable time of the epoxy resin), while a temperature of the molds is held at a high temperature of higher than 120 degrees C in order to obtain casting products in a short period of time.

0008According to the above APG method, a large casting product made of a resin weighing more than 10 [kg] may be produced and released from a mold in an exceedingly short time (e.g. 20-60 minutes). The casting product released from the mold is transferred into a secondary curing oven until reaction of the epoxy resin settles and is hardened completely.

0009A crack resistant property, dimensional precision and a partial discharge property of the casting product are affected by deviation from set conditions of a temperature of the epoxy resin to be injected, pressure applying to the epoxy resin and an injecting speed of the epoxy resin, and a producing condition such as a hardening time or temperature balance of the mold. Concerning the above APG method, since the epoxy resin has high reactivity and heats up at hardening, there is the possibility of raising fraction defective.

0010Further, if hardening balance of the epoxy resin in the mold is disturbed, remaining stress of the casting product increases and gives rise to a contraction mark such as a depression or the like. As a result, crack resistant property and dimensional precision worsen due to the contraction mark or the like, and further electric characteristics such as a partial discharge property strikingly deteriorate due to a remaining void or a separation of the epoxy resin and the metal conductor caused by the remaining stress.

0011There is one approach to use a mold having a plurality of molding cavities for holding the epoxy resin and to produce a plurality of casting products simultaneously in order to increase productivity. This approach is only adopted to produce relatively small insulating products such as an insulator. In such case, a forked pipe for supplying and injecting a resin to the molding cavities is generally disposed outside of the mold.

0012However, there is a problem that it is difficult to control a temperature and a coefficient of viscosity of the resin existing in the forked pipe. As a result, since injecting time needed for supplying the resin to the molding cavities varies widely, deviations from set conditions of molding may not be disregarded. Accordingly, it is difficult to apply such casting product for high voltage apparatus having a voltage of 6 KV or higher.

0013On the other hand, there is a mold having a forked pipe formed inside thereof. Such mold is used for an injection molding method which uses a thermoplastic resin. In this case, since the thermoplastic resin remaining in the forked pipe is removed at every injection, there is a merit that it is easy to control balance of the injecting time for molding cavities. However, with respect to the APG method that uses a thermosetting resin, since the thermosetting resin exisiting in the forked pipe hardens fast, injecting paths leading to the molding cavities are blockaded first. As a result, a liquid resin to be injected is prevented from filling molding cavities, especially it is difficult for the liquid resin to fill a space created by a contraction of the injected epoxy resin, thereby impeding a normal operation of the APG apparatus.

0014Further, in case of molding a casting product formed in a complicated shape that integrates a metal conductor, a resin injection and hardening balance of the injected resin are especially critical matter. If the resin hardens at a low speed, it is hard to ensure satisfactory hardness of the casting product at a time of releasing the casting product. On the other hand, if the resin hardens at a high speed, the resin contracts in a state of being bound with a mold formed in a complicated shape. Therefore, a permanent set may be caused to the casting product. The permanent set of the casting product may cause a separation of a metal conductor and the resin, or an insert and the resin, which deteriorates a partial discharging property and causes cracks.

0015Especially, the above problem becomes more problematic with the increase of a heat-resisting property of the epoxy resin. That is, since a coefficient of contraction increases as the heat-resisting property rises, a processing control for molding becomes more difficult. Therefore, many problems remain of the above conventional molding method, in case of increasing productivity of the APG method that uses a thermosetting resin possessing a glass-transition temperature of 100 degrees C or higher. The glass transition temperature represents a heat-resisting property.

SUMMARY OF THE INVENTION

0016Accordingly, one object of this invention is to provide an APG apparatus which may mold a plurality of casting products having a satisfactory crack resistant property, satisfactory dimensional precision and a satisfactory partial discharge property simultaneously.

0017The present invention provides an APG apparatus for producing a casting product made of a thermosetting resin by heating said thermosetting resin, including a pair of molds, at least one of said molds includes a plurality of molding cavities injected with the thermosetting resin, a forked path for leading the thermosetting resin to the molding cavities, and a cut configured to prevent heat from conducted to the forked path, and a heat plate configured to heat and mount the molds, whereby a plurality of casting products are produced simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

0018A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a front view showing a mold of an APG apparatus, mounted on one of a pair of heat plates;</li><li>FIG. 2 is a side view showing the pair of molds 2a and 2b mounted on the pair of heat plates 1a and 1b;</li><li>FIG. 3 is an enlarged sectional view of a metal conductor shown in FIG. 2;</li><li>FIG. 4 is a side view showing a first releasing device for releasing a casting product from a mold;</li><li>FIG. 5 is a a side view showing a second releasing device for releasing a casting product from a mold;</li><li>FIG. 6 is a front view showing a positioner having a tapered guide of a mold;</li><li>FIG. 7 is a front view showing two convex portions formed on the upper and lower sides of thin-walled portion of a flange of a casting product;</li><li>FIG. 8 is a side view showing a mechanical cylinder used for fixing magnet washers on a mold; and</li><li>FIG. 9 is a side view, partly in section, of the principle part of a fixing rod.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0019An APG apparatus that produces three T-shaped bushings for the C-GIS (Cubicle-type Gas Insulated Switch gear) used in apparatus having a voltage of 22-77KV simultaneously is hereinafter described as a first embodiment of the present invention referring to drawings.

(Basic structure of the APG apparatus)

0020Basic structure of the APG apparatus of the first embodiment is explained referring to FIG. 1. FIG. 1 is a front view showing a mold of the APG apparatus, mounted on one of a pair of heat plates.

0021In the APG apparatus, a pair of molds 2a and 2b mounted on the pair of heat plates 1a and 1b capable of adjusting the temperature thereof are left-right symmetrical (only mold 2a is shown in FIG. 1). The respective molds 2a and 2b are moved by a hydraulic opening and closing device, and closed in a vacuum chamber.

0022In the APG apparatus shown in FIG. 1, a forked pipe 50 is formed inside of the mold 2a. The forked pipe 50 includes a main pipe 3 extending in a horizontal direction and having an inlet 4 at the center thereof, and three branch pipes 7 extending in a vertical direction and connecting the main pipe 3 with three molding cavities 5a of the mold 2a. After closing the molds 2a and 2b in the vacuum chamber and then evacuating the vacuum chamber, the epoxy resin is injected into the main pipe 3 through the inlet 4 disposed on the bottom of the molds 2a and 2b by applying pressure to a resin mixing tank. The epoxy resin is then divided into three and distributed to each of the molding cavities 5a by the branch pipes 7, thereby hardening the injected epoxy resin and producing three casting products simultaneously.

0023A bisphenol type of liquid epoxy resin, which possesses a glass-transition temperature of over 110 degrees C after hardening, is preheated and agitated in a vacuum with an acid anhydride hardening agent containing about 70 weight percentages of silica powder, and then injected.

0024According to the above APG apparatus, three T-shaped bushings for the C-GIS may be produced simultaneously. Therefore, the T-shaped bushing may be produced three times as fast as a conventional APG apparatus, and forty eight times as fast as a conventional molding apparatus.

(Structure for preventing heat from conducted to the forked pipe 50)

0025Structure for preventing heat from conducted to the forked pipe 50 is explained referring to FIGs. 1 and 2. FIG. 2 is a side view showing the pair of molds 2a and 2b mounted on the pair of heat plates 1a and 1b.

0026The molds 2a and 2b respectively include rectangular cooling grooves 6a and 6b that prevent heat transmitted from the heat plates 1a and 1b from conducted to the main pipe 3, and cooling pipes 8a and 8b along the branch pipes 7 that prevents heat transmitted from the heat plates 1a and 1b.

0027Although a satisfactory cooling effect may be achieved by merely forming the rectangular cooling grooves 6a and 6b and the cooling pipes 8a and 8b, cooling nozzles 9a and 9b running air are attached to the rectangular cooling grooves 6a and 6b and the cooling pipes 8a and 8b in order to increase cooling effect in this embodiment.

0028Further, cooling water may be used as a cooling material instead of air in order to increase cooling effect. In order to circulate the cooling water, for example, spiral pipes running cooling water are disposed in the rectangular cooling grooves 6a and 6b and the cooling pipes 8a and 8b.

0029As described above, since the main pipe 3 and the branch pipes 7 integrated in the mold 2a are refrigerated, hardening progress of the epoxy resin existing in the forked pipe 50 may be delayed. Accordingly, a system for supplying the epoxy resin existing in the forked pipe so to a space created by a contraction of the injected epoxy resin in the molds 2a and 2b may surely operate. Further, dimensional precision, mechanical property and electrical property of the casting product may be improved.

(Structure of the forked pipe)

0030Structure of the forked pipe 50 is explained referring to FIG. 2.

0031The forked pipe 50 composed of the main pipe 3 and the branch pipes 7 is formed only in the mold 2a. A cross section of the forked pipe 50 is formed in a semicircle. The three branch pipes 7 have the same length to each other so that a distance between the main pipe 3 and the molding cavities 5a is made equal regardless of the number of molding cavities.

0032The number of processing steps for making the molds 2a and 2b is reduced, because the forked pipe 50 is formed only in the mold 2a. Further, the epoxy resin may be injected into the molding cavities 5a equally at the same time, since the three branch pipes 7 have the same length to each other.

0033As described above, since the structure of the forked pipe 50 is made ideal, injection balance may be improved and fraction defective may be reduced.

(Degassing structure of molds)

0034Degassing structure disposed at the upper side of the molding cavities 5a is explained referring to FIG. 1.

0035Each of the molding cavities 5a has a first slit 10 disposed at the upper part thereof away from the inlet 4 and forming a first gap of 0.1-1.0 mm between the pair of molds 2a and 2b, for directing an outflow of the epoxy resin, a trap 11 disposed at the upper part of the first slit 10 and containing the outflow from the first slit 10, and a second slit 12 disposed at the upper part of the trap 11 and forming a second gap of 20-100 µm between the pair of molds 2a and 2b.

0036According to the above structure, efficiency in degassing the epoxy resin at a molding process and a sealing effect for the epoxy resin are ensured, thereby producing casting products having few void defects.

0037Further, as shown in FIGs. 1 and 2, a spot heater 13 or an air seal 14 capable of expanding by air pressure is disposed adjacent to the second slit 12. It is enough that only one of the spot heater 13 and the air seal 14 is used, although both of them are shown in FIG. 2.

0038In case of installing the spot heater 13, the spot heater 13 prevents an outflow of the epoxy resin from the second slit 12 at a time of pressing and hardening process by assisting to harden the epoxy resin in the second slit 12. In case of installing the air seal 14, the air seal 14 prevents an outflow of the epoxy resin from the second slit 12 at a time of pressing and hardening process by sealing a gap of the second slit 12.

0039Furthermore, as shown in FIGs. 1 and 2, a slope 15 is formed in each of the molding cavities 5a so that any surfaces of the molding cavity 5a contacting with the first slit 10 incline toward the first slit 10 at an angle of more than 10 degrees.

0040Accordingly, bubbles in the epoxy resin are collected by the slopes 15. As a result, efficiency in degassing the bubbles through the first slit 10 increases.

0041Moreover, the trap 11, disposed between the first slit 10 and the second slit 12, has a volume of 4-8% of the volume of a casting product produced by the molding cavities 5a and 5b.

0042Voids in the epoxy resin are surely removed by the above ideal structure of the trap 11 for holding the bubbles.

0043According to the above degassing structure, casting products having no void defects, no contraction marks (i.e. depressions) and no weld marks (i.e. traces of the flow of the epoxy resin) may be produced. As a result, casting products possessing a satisfactory partial discharge property may be produced.

0044Sealing structure for preventing the epoxy resin from sticking to electrical contact points of a built-in metal conductor of a casting product is explained referring to FIGs. 1, 2 and 3.

0045As shown in FIGs. 1 and 2, a metal conductor 16 composing the T-shaped bushing is set to each of the molding cavities 5a. An enlarged sectional view of the metal conductor 16 is shown in FIG. 3.

0046In FIG. 3, the metal conductor 16 has electrical contact points 17 connecting external cables. Pumped ring seals 20 are placed to the electrical contact points 17 in a way such that the contact points 17 are covered with the ring seals 20 in order to keep the epoxy resin 19 from intervening between the electrical contact points 17 and a first inner mold 18a or a second inner mold 18b.

0047A ring seal made of TEFLON (trade-mark of Du Pont) may be used instead of the pumped ring seal 20. In this case, the TEFLON ring seal prevents the epoxy resin 19 from intervening between the electrical contact points 17 and the inner molds 18a or 18b by a thermal expansion of the TEFLON ring seal and a clamping force of the opening and closing device.

0048As described above, the ring seals keep the epoxy resin from sticking to the electrical contact points 17, thereby preventing contact failure in the electrical contact points 17. Further, a necessary work to remove burrs (i.e. leakage of the epoxy resin) at a finishing step may be reduced.

0049With the reduction of the hardening process of the epoxy resin, handling works to release casting products from the molds 2a come to account for a lot of all processing steps. However, works needed for a finishing step and product deficiencies may be reduced by using the above sealing structure.

(Releasing device)

0050Releasing device for releasing the casting product from the mold is explained referring to FIGs. 2 to 5.

0051As shown in FIGs. 3 and 4, in a first embodiment of the releasing device, knock-pins (rods) 22 are disposed in the mold 2a, thereby pressing out the inner mold 18a supporting the metal conductor 16 and a base mold 21 fixed on the bottom of the metal conductor 16 by a pitch of 3-6mm with the same stroke. A releasing rod pushes out the knock-pins 22 by a lever, or by hitting the releasing rod with a hammer. For example, as shown in FIG. 4, the knock-pins 22 are pressed out to the right by pushing down a releasing rod 23. Or, as shown in FIG. 5, the releasing rod 23 is pressed down by a hammer action generated in a way of dropping a weight 40 coupled to the top of the releasing rod 23 after lifting the weight 40 up, thereby pushing out the knock-pins 22 to the right.

0052As shown in FIGs. 2 and 3, in a second embodiment of the releasing device, the knock-pins 22 operate together with the opening and closing device. That is, as shown in FIG. 2, as the molds 2a and 2b are moved with the hydraulic opening and closing device, knock-pins 22 hit against a stopper 24 disposed at the back of the heat plate 1a, thereby pushing out the inner mold 18a and the base mold 21 by a pitch of 3-6mm with the same stroke.

0053According to the first and second embodiments of the releasing device, casting products may be released in a short time without operating a force on a relatively soft portion of the casting product, that is, a portion of the epoxy resin.

0054Accordingly, product deficiencies that may happen at a time of releasing casting products may reduce by using the above releasing device. As a result, electrical property of the casting products is improved.

(Positioning structure for a pair of molds)

0055A positioning structure for the pair of molds 2a and 2b is explained referring to FIGs. 2, 6 and 8.

0056As shown in FIG. 2, in a first embodiment of the positioning structure, the pair of molds 2a and 2b are respectively graduated on their side surfaces with lines 25, thereby determining a vertical relationship of the molds 2a and 2b.

0057Although it is generally hard to position a mold to a heat plate, the molds 2a and 2b are easily positioned in appropriate places according to the first embodiment of the positioning structure, thereby facilitating a replacement work of the molds 2a and 2b.

0058As shown in FIG. 6, in a second embodiment of the positioning structure, a positioner 27 having a tapered guide 26 is fixed on each of the heat plates 1a and 1b in order to locate the molds 2a and 2b in an appropriate place.

0059Although it is generally hard to position a mold to a heat plate, the molds 2a and 2b are easily positioned in an appropriate place according to the second embodiment of the positioning structure, thereby facilitating a replacement work of the molds 2a and 2b.

0060According to the first and second embodiments of the positioning structure, necessary steps for positioning the molds 2a and 2b may be reduced, thereby increasing productivity of the casting products.

(A configuration of thin-walled portion of the casting product)

0061A configuration of thin-walled portion of the casting product such as the T-shaped bushing is explained referring to FIG. 7.

0062As shown in FIG. 7, two convex portions 30 are formed on the upper and lower sides of thin-walled portion of a flange 29 of the casting product 28 so that thick nesses X of the thin-walled portions of the flange 29 are 15 mm in thickness. That is, each of the molding cavities 5a and 5b has a supplement space located in a position corresponding to the thin-walled portions of the casting product 28 and supplied with the epoxy resin.

0063The reinforcement effect by the convex portions 30 of the casting product 28 prevents a deformation of the flange 29 caused by a contraction force generating in a direction of a large volume of the epoxy resin at hardening. Accordingly, casting products having a satisfactory dimensional precision can be obtained.

(Fixing structure for an insert of a casting product)

0064A fixing structure for an insert such as a female screw integrated in a casting product is explained referring to FIGs. 7 and 8.

0065As shown in FIG. 7, inserts 31 used for fixing the casting product 28 are integrated in the casting product 28. The inserts 31 are fixed on the mold 2b via magnet washers 32 attached to the mold 2b before molding. That is, after screwing the inserts 31 to the magnet washers 32, the magnet washers 32 are fixed on the bottom of the mold 2b magnetically. With the contraction of the epoxy resin at hardening, the magnet washers 32 are off the mold 2b.

0066In case of fixing the inserts 31 on the mold 2b, as shown in FIG. 8, a mechanical cylinder 33 may be used for fixing the magnet washers 32 on the mold 2b. After the epoxy resin gels while hardening, the mechanical cylinder 33 is removed.

0067Accordingly, the inserts 31 follow the contraction of the epoxy resin, thereby reducing a stress and preventing a separation of the epoxy resin and the inserts 31. Since an adhesive performance of the epoxy resin and the inserts 31 increases, mechanical and electrical property of the casting product 28 may be improved.

0068Further, each of the inserts 31 is fixed on the mold 2b by using a fixing rod 34 protecting surfaces of the insert 31.

0069FIG. 9 is a side view, partly in section, of the principle part of the fixing rod 34.

0070As shown in FIG. 9, the top of the fixing rod 34 is made of an elastic resin and has a concave for inserting the insert 31.

0071The fixing rod 34 prevents a mold release agent spread on the mold 2b from sticking to the inserts 31 at a time of fixing the inserts 31 on the mold 2b, thereby preventing a separation of the epoxy resin and the inserts 31 and increasing efficiency of work to fix the inserts 31.

0072Accordingly, since an adhesive performance of the epoxy resin and the inserts 31 increases, the casting product 28 having satisfactory mechanical and electrical properties may be obtained.

(An inner mold for supporting a metal conductor)

0073An inner mold for supporting a metal conductor integrated in a casting product is explained referring to FIG. 3.

0074As shown in FIG. 3, an inner mold 41 is integrated with each of the molds 2a and 2b without joints.

0075Since the integrated inner molds 41 assist in heating up the conductor 16 by heat conduction from the heat plates 1a and 1b, the epoxy resin hardens around the conductor 16 first, whereby an adhesive performance of the epoxy resin and the conductor 16 increases. As a result, a partial discharge property of the casting product may be improved by prevention of a separation of the epoxy resin and the conductor 16.

(Structure for leaving a casting product to a predetermined mold)

0076Structure for leaving a casting product to the mold 2a having the releasing device is described referring to FIG. 3.

0077The inner mold 18b forcibly secures the conductor 16 so as to always leave the casting product to the mold 2a having the releasing device at a time of separating the pair of molds 2a and 2b.

0078According to the above structure, since the casting product is surely left to the mold 2a having the releasing device, product deficiencies that happen at a time of releasing casting products may reduce.

(Other embodiments)

0079In the above APG apparatus, the molds 2a and 2b are mounted on the heat plates 1a and 1b. However, the molds 2a and 2b may integrate heaters instead of the heat plates 1a and 1b, in case of producing casting products in large quantities and using the APG apparatus exclusively. In this case, temperature distribution of the molds 2a and 2b may be controlled accurately. Especially, an adiabatic design for the forked pipe may be simplified.

0080Further, in the above description, temperature control for the forked pipe 50, a control for the second slit 12 and a control for the mechanical cylinder 33 are explained. It is required to execute these controls accurately according to a shape of the casting product. In such case, a dielectric measuring instrument that includes a dielectric sensor made of ceramic is disposed on either one of the molds 2a and 2b, thereby monitoring a hardening state of the epoxy resin and executing the above controls. Concerning the above control, since control timings may be adjusted by monitoring the hardening state of the epoxy resin existing in the second slit 12 and the forked pipe 50, the casting products may be produced with precision.

0081Although the above APG apparatus produces three T-shaped bushings simultaneously, different casting products from each other, for example, a bushing, an insulator and a rod for a circuit breaker may be produced simultaneously. In such case, a forked pipe and a degassing structure may be designed in the same way as the above described APG apparatus.

0082According to the APG apparatus of the present invention, a plurality of casting products having a satisfactory crack resistant property, satisfactory dimensional precision and a satisfactory partial discharge property may be produced simultaneously, thereby increasing the productivity of the casting products.

0083Various modifications and variations are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.