Method of injection-molding coil spool of ignition coil device

This application relates to and incorporates herein by reference Japanese Patent Application No. 10-365962 filed on Dec. 24, 1998.

1. Field of the Invention

The present invention relates to a method of injection-molding coil spools for ignition coil devices, and particularly to a method of injection-molding a spool of a primary coil of a stick-type ignition coil device.

2. Related Art

Generally, a stick-type ignition coil device for engines includes a column-shaped central core, a primary spool disposed coaxially with the core, a primary coil wound around the primary spool, a secondary spool disposed coaxially with the core and disposed radially inside the primary spool, and a secondary coil wound around the secondary spool.

Since the stick-type ignition coil device is installed in a plug hole of each cylinder of an internal combustion engine, an outer diameter of the stick-type ignition coil device should not be greater than an inner diameter of the plug hole. Therefore, the spools are preferably thinned and elongated in order to ensure winding space of the coils.

The spools are formed by injection-molding a resin. However, this necessitates an insulation tape around the thinned winding portion of the primary spool in order to ensure the dielectric withstanding voltage, because the primary spool is located between the primary coil and the secondary coil.

The present invention has an object to provide a method of injection-molding a coil spool of an ignition coil device, which can ensure an adequate dielectric withstanding voltage without requiring an insulation tape around the coil spool thereby to reduce production costs.

According to the present invention, a coil spool of an ignition coil device is formed by molding a resin in a cavity of a molding die. A gate of the molding die is arranged at a part where a portion other than a coil winding portion is to be formed. An insulating resin is injected through the gate to form the coil spool. The coil spool and the coil are for a primary unit of the ignition coil device, and disposed radially outside of a secondary unit of the ignition coil device.

Preferably, the gate is a side-type and arranged at a part where each flange is to be formed on the coil spool. Alternatively or in combination, the gate is arranged to open in a circumferential direction in a ring shape at a part where a cylindrical part of the coil spool which is outside the flanges in an axial direction is to be formed. Further, alternatively or in combination, the gate is arranged at an axial end of the coil spool and extends in an axial direction.

FIG. 1 is a sectional view showing an ignition coil device to which the present invention is applied; and

FIG. 2 is a sectional view showing gates of a molding die to mold a primary spool according to an embodiment of the present invention.

An embodiment of the present invention will be described hereinbelow with references to the drawings.

Referring first to FIG. 1, an ignition coil device 10 has a cylindrical coil casing 11 which is made of an insulating resin and has an upper casing 12 on its top. A connector housing 14 is fit into the upper casing 12 and a terminal pin 13 is press-fit into the connector housing 14 . An igniter 16 is mounted on bases 15 integrally formed in the connector housing 14 . An ignition signal outputted from an engine control computer (not shown) is inputted to the igniter 16 through the terminal pin 13 to control energization and deenergization of a primary coil 20 .

A cylindrical central core 18 is disposed in a radial center of the coil casing 11 , and a cylindrical outer core 17 is coaxially disposed in an inner peripheral side of the coil casing 11 . The primary coil 20 is wound around a cylindrical primary spool 19 made of an insulating resin, and is disposed in an inner peripheral side of the cylindrical outer core 17 . Further, a secondary coil 22 is wound around a cylindrical secondary spool 21 made of an insulating resin, and is disposed in an inner peripheral side of the primary spool 19 . The secondary coil 22 is wound to extend generally the same length in the axial direction as that of the primary coil 20 . That is, both coils 20 and 22 are positioned at generally the same position in the axial direction. A conductive terminal plate 25 is fixed to a bottom of the secondary spool 21 and connected with an end of the secondary coil 22 .

Further, the central core 18 is disposed in an inner peripheral side of the secondary spool 21 which is in a bottomed cylinder-shape. At the top side and the bottom side of the central core 18 , cushions 23 are positioned to protect the central core 18 from excessive stress. The cushions 23 are made of heat resisting elastic material such as anti-magnetostrictive sponge or elastomer. Moreover, the coil casing 11 and upper casing 12 are vacuously filled with an insulating potting resin such as an epoxy thermosetting resin.

A high voltage tower 26 is attached in contact with the bottom of the coil casing 11 . A terminal cup 28 which is integral with a high voltage terminal 27 is inserted or press-fit into a central top of the high voltage tower 26 . The high voltage terminal 27 is press-fit into the terminal plate 25 and electrically connected. By inserting the high voltage tower 26 into a plug hole (not shown) and pressing into the top of a spark plug (not shown), a conductive spring 29 accommodated within the terminal cup 28 is pressed against the terminal of the spark plug. Thus, an end of the secondary coil 22 is electrically connected to the terminal of the spark plug through the terminal plate 25 , high voltage terminal 27 , terminal cup 28 and spring 29 .

Since the stick-type ignition coil device 10 is mounted in the plug hole, the outer diameter of the coil casing 11 is not greater than the inner diameter of the plug hole. Thus, in order to ensure a winding space for the primary coil 20 , for example, the primary spool 19 preferably has a thickness between 0.4 millimeter and 1.0 millimeter, and a length between 50.0 millimeters and 150.0 millimeters.

The primary spool 19 is formed with an upper flange 19 b and a lower flange 19 c to define a winding portion 19 a therebetween so that the primary coil 20 is wound between the flanges 19 b and 19 c to face the secondary coil 22 in the radial direction. The primary spool 19 is made by using a molding die 100 shown in FIG. 2, in which only an outer die is shown.

The flowability of a molten resin decreases in accordance with the passage area in the molding die 100 , that is, in accordance with the thinness and elongation of the primary spool 19 . If an axially-extending side gate 100 a is formed in the molding die 100 , at a part where the winding portion 19 a of the primary spool 19 is made and the primary spool 19 is made by injecting the molten resin from the long side gate 100 a into a cavity in the molding die, the resin deterioration (molecular orientation) occurs around the long side gate due to shearing force of the molten resin. Further, the small cracks occur in the winding portion 19 a , when the molding-resulting part corresponding to the long side gate 100 a is cut off after the injection molding. As a result, the dielectric withstanding voltage of the winding portion 19 a is reduced. Thus, it is necessary to wind an insulation tape around the winding portion 19 a in order to ensure the dielectric withstanding voltage against the high voltage generated from the secondary coil 22 .

According to this embodiment, a molding gate 100 b , 100 c , 100 d or 100 e of the molding die 100 to mold the primary spool 19 is positioned at a part where the winding portion 19 a is not formed. That is, the gate is positioned to face a part of the primary spool 19 which does not face a main part of the secondary coil 22 when the resin is injection-molded. Specifically, the gate is positioned to be outside the winding portion 19 a in the axial direction.

For instance, the gate 100 b is arranged at a position to form the upper flange 19 b of the primary spool 19 and opens in the radial direction. The gate 100 c is arranged at a position to form a lower flange 19 c and opens in the radial direction. The gate 100 d is arranged at a position to form an outer peripheral face of a cylindrical portion 19 d which extends downwardly from the winding portion 19 a . The gate 100 d opens in the circumferential direction to surround the cylindrical portion 19 d . The gate 100 e is arranged at a position to form the bottom face of the cylindrical portion 19 d and opens in the axial direction.

Since there is the irregularity on the outer peripheral faces of the upper flange 19 b and the lower flange 19 c , the gates 100 b and 100 c are in a side-type which does not open over an entire circumference. In the case of the side-type gate, considering the flowability of resin, it is preferred to use a plurality of side gates 100 b , 100 c so that the flowability of the molten resin can be uniformed.

On the other hand, since an outer peripheral face of the cylindrical portion 19 d is in a cylindrical shape, it is preferred that the gate 100 d is in a ring-type which opens over an entire circumference. Moreover, since the bottom face of the cylindrical portion 19 d is in a ring shape, it is also preferred to use the ring-type for the gate 100 e . From the ring gates 100 d and 100 e , a molten resin can equally fill the cavity in the molding die 100 to form the thin winding portion 19 a of the primary spool 19 , and a moldability is improved than in the case of a side gate. However, it is also possible to arrange a plurality of side gates at the outer peripheral face and the bottom face of the cylindrical portion 19 d.

In order to form the thin and elongated primary spool 19 by arranging the gates except around the winding portion 19 a , it is necessary to use an insulating resin having a high flowability. Here, such a resin is, for example, {circle around (1+L )} polybutylene terephthalate (PBT) or polyethylene terephthalate (PET), {circle around (2+L )} polyphenylene oxide (PPO)/polystylene (PS), {circle around (3+L )} liquid crystal polymer and the like. These resins are suitable for the primary spool 19 because of high dielectric withstanding voltage, flowability and adhesiveness with the epoxy type filler 24 .

Moreover, by arranging the gate on the primary spool 19 except on the winding portion 19 a , resin deterioration and cracks occur only on a part which dose not oppose the main part of the secondary coil 22 , that is, where a voltage applied from the secondary coil 22 is relatively low. Therefore, the adequate dielectric withstanding voltage can be ensured without using an insulation tape, and the process of winding insulation tape becomes unnecessary. Consequently, the productivity is improved in accordance with the reduction in the number of processes.

In this case, the high voltage generated by the secondary coil 22 becomes lower towards the upper side and higher towards the bottom side in the axial direction. Therefore, if the gate 100 b is arranged to correspond to the upper flange 19 b where the applied voltage is the lowest in the primary spool 19 , disadvantage due to resin deterioration or cracks around the gate becomes the least.

Further, if the ring gate 100 d is arranged to correspond to the cylindrical portion 19 d the outer peripheral face of which is cylindrical, the molten resin can be equally filled into all around the thin winding portion 19 a of the primary spool 19 so that the moldability is improved.

The gate is not limited to the side-type or ring-type, but it may be a pin gate or the like. Further, the secondary spool may also be formed by the same method of injection molding.