HEATING COIL BOBBIN

Abstract: To obviate the need for expensive, hard to work ceramic materials as bobbins for dielectric heating coils used to heat the hot runner in an injection molding machine the bobbin body (80) is made from a low magnetic permeability metal or metal alloy. A slit (83) is formed in the body and extends partway or fully along the body length. Additional slits (84) may also be included.

Description: This invention relates to heating coil bobbins and in particular to such bobbins used for the heating coils used to heat the hot runners of injection molding machines. There is known a so-called cold runner molding system wherein a mass of resin is charged in a resin passage connecting a molding machine nozzle and a mold cavity; that is, a so-called runner is cooled and solidified together with another mass of resin charged in the mold cavity. The solidifed runner is thereafter removed together with the solidified product on mold opening. There is also known a hot runner type injection molding system wherein a mass of resin to be molded is cooled and solidified within the mold cavity whilst maintaining a runner molten. The runner in the molten state is re-used in the next molding cycle. In such a hot runner type injection molding system, a heating coil is used to heat and maintain the runner in its molten state, as disclosed in Japanese Laid-Open Patent Application JP-A-61-197216. The heating coil is normally mounted about a cylindrical bobbin and unitized by the use of a filler material such that the coil will not be deformed and can be easily positioned in place. The bobbin used herein must be made of any suitable material that will not be heated under the action of magnetic flux from the heating coil and can withstand raised temperatures. In the prior art, ceramic materials have been used to form the dielectric heating coil bobbins. Since such bobbins require an increased accuracy in size, specialty ceramics must be used. However, such ceramics are expensive and difficult to work. The present invention aims to provide a heating coil bobbin which may be made of an inexpensive and easily workable material without need for the expensive and hard to work specialty ceramics at present in use. The present invention, therefore provides a heating coil bobbin for a heating coil used in heating the hot runner in an injection molding machine, said bobbin being characterized in that it comprises a tubular body made of a metal or alloy having a low magnetic permeability, the body having a slit formed therein, the slit extending a given length along the body. In a preferred embodiment, the slit extends along the length of the bobbin body, which may, for example, be formed of stainless steel. In another preferred embodiment at least one further slit extends along a part of the body length. Since the heating coil bobbin has a main body formed of a metallic material having a low magnetic permeability and having at least one slit formed therein and extending along the length of the bobbin, eddy currents will flow along an increased circumferential path formed about the bobbin body by said slit as the magnetic flux from the heating coil passes through the bobbin body. This results in substantial increase of the resistance in the bobbin. Therefore, the magnetic flux will reach a member disposed within the bobbin body, that is, a tip wherein eddy current is created to heat the tip. Since very little eddy current passes through the bobbin body, heating of the bobbin body itself will be limited. As a result, only the hot runner intended to be heated can be heated effectively. Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic view of a hot runner type injection molding system incorporating one embodiment of a heating coil bobbin embodying the present invention; Figure 2 is a cross-sectional view illustrating the construction of a hot runner portion of the injection molding system into which one heating coil bobbin embodying the present invention is incorporated; Figure 3 is a perspective view exemplifying a bobbin usable in the heating coil embodying the present invention; and Figure 4 is a perspective view of a modified bobbin for the heating coil. Referring to Figure 1, there is shown a hot runner type injection molding system into which a heating coil bobbin constructed according to the present invention is incorporated. The injection molding system comprises a mold assembly 10 having four mold cavities 12a, 12b, 12c and 12d. The mold assembly 10 comprises a stationary half 14 rigidly mounted on a stationary die plate in an injection molding machine (not shown) and a moveable half 16 securely mounted on a movable die plate. When the movable half 16 is pressed against the stationary half 14, that is, when the mold assembly 10 is closed, the aforementioned four mold cavities 12a - 12d will be formed between the stationary and movable halves 14 and 16. The stationary mold half 14 comprises a mounting plate 18 attached to the stationary die plate, a manifold block 22 rigidly mounted on the mounting plate 18 to locate a heat insulation 20 therebetween, and a cavity plate 26 firmly mounted on the manifold block 22 to dispose a support block 24 therebetween. The cavity plate 26 is provided with four recesses 28a, 28b, 28c and 28d respectively opened at the side thereof facing to the movable mold half 16. Each of the four recesses 28a - 28d cooperates with the respective one of four cores 17a, 17b, 17c and 17d formed on the movable mold half 16 to form the respective one of the aforementioned four cavities 12a - 12d. The cavity plate 26 also comprises four recesses 30a, 30b, 30c and 30d formed therein at the side opposite to the aforementioned recesses 28a - 28d, that is, at the side facing to the manifold block 22. Each of the four recesses 28a - 30d is aligned with the respective one of the recesses 28a - 28d and opened at the side of the cavity plate 26 facing to the manifold block 22. The stationary mold half 14 further comprises resin passages each of which connects the injection molding machine nozzle (not shown) with the mold cavities 12a - 12d through gates 32a, 32b, 32c and 32d which are formed on the bottoms of the respective recesses 30a - 30d. The resin passage comprises a so-called sprue portion 34a which may be connected directly with the nozzle of the molding machine and a so-called runner portion 34b divided into four sub-runners within the manifold block 22. The end of each of the sub-runners in the runner portion 34b adjacent to the corresponding one of the gates 32a - 32d is formed by the respective one of tubular tips 36a, 36b, 36c and 36d. There is a heating coil units 38a, 38b, 38c or 38d which is disposed about each of the tips 36a - 36d. As will be described, each of the heating coil units 38a - 38d may be energized by the high-frequency current to heat the corresponding tip, as will be described later. The manifold block 22 also has any suitable heating means (not shown) for heating it up to a desired temperature. Molten resin injected from the molding machine nozzle is charged into the respective mold cavities 12a - 12d through the aforementioned resin passage. Normally, the cavity plate 26 and the movable mold half 16 are cooled. After the resin material charged in the mold cavities 12a - 12d has been cooled and solidified, the movable mold half 16 is moved backwardly to open the mold assembly 10. Thereafter, a product molded in each of the mold cavities 12a - 12d is separated from the stationary mold half 14 while being carried by the respective one of the cores 17a - 17d on the movable mold half 16. At this time, there may be produced various undesirable phenomena, for example, such as a stringing between a gate and the corresponding product if the temperature of resin at a position near the corresponding gate is too high, a leakage of molten resin through the gate or gates when the mold is in its open position, or a solidification of resin near the gate or gates to prevent the next injection molding cycle if said temperature is too low. Accordingly, the temperature of resin near the gates 32a - 32d must be maintained within a critical range of temperature between the undesirably high and low temperatures. The heating coil units 38a - 38d are connected in series with one another and also with a high-frequency power supply circuit 42 through connector means in a junction box 40. The power supply circuit 42 comprises a rectifying circuit 44 for rectifying AC current from a source to convert it into DC (pulsating) current, a switching element 46 for repeating its alternative opening and closing (ON and OFF) operation under the control of a temperature controlling circuit 52 which will be described, a transformer 48, a capacitor C connected in parallel to the primary side of the transformer 48 and a filter circuit 50. The four heating coil units 38a - 38d are connected in series with the secondary side of said transformer 48. The temperature controlling circuit 52 includes four temperature sensors 54a, 54b, 54c and 54d each of which is in contact with the respective one of said tips 36a - 36d to detect its temperature. The output of each of the temperature sensor 54a - 54d is applied sequentially to an amplifying circuit 58 through a switching circuit 56. After amplified, the output signal of the amplifying circuit 58 is supplied to an A/D converter 60 wherein the signal is converted into a digital signal which in turn is stored in a memory 64 under the control of a control circuit 62. The control circuit 62 also is connected with a temperature setting input circuit 66 and a temperature indicating circuit 68. The temperature setting input circuit 66 is adapted to apply a tip temperature setting selected by a setting dial or the like to the control circuit 62. The selected temperature at the tops is then stored in the memory 64 under the control of the control circuit 62. The control circuit 62 causes a calculatory circuit 70 to obtain an average value in the date of temperature from the respective temperature sensors 54a - 54d stored in the memory 64, that is, the temperatures of the respective tips 36a - 36d and to compare the obtained average value with the aforementioned temperature setting value. Depending on a difference between the average value and the temperature setting, the control circuit 62 controls an oscillator 72 to vary the output signal therefrom. The power supply circuit 42 is such arranged that if the frequency is lower within a predetermined range of frequency, a higher power is supplied to the heating coil units 38a - 38d. The control circuit 62 controls the oscillator 72 such that if the difference between the temperature setting value and the average value in the temperatures of the tops 36a - 36d is larger, the oscillator 72 oscillates correspondingly at a lower frequency. The oscillator 72 is set to oscillate at a frequency ranged between 20 KHz and 50 KHz. The output signal of the oscillator 72 is amplified in current by a drive circuit 74 to form a drive signal for driving the switching element 46. When the switching element 46 is repeatedly opened and closed depending on the oscillating frequency in the oscillator 72, a current flows in the primary side of the transformer 48 while a high-frequency current is induced at the secondary side of the transformer 48. That high-frequency current is then supplied to the four heating coil units 38a - 38d which are connected in series with the secondary side of the transformer 48. As each of the heating coil units 38a - 38d receives the high-frequency current, the respective one of the tips 36a - 36d in which that heating coil unit is mounted is heated by the action of electromagnetic induction. It is of course that the tips 36a - 36d are formed of any one of suitable materials capable of being heated by the electromagnetic induction. Such materials are well-known in the art, but should be selected under the consideration in that the tips 36a - 36d must withstand raised temperature and pressure. It is particularly desirable that one selects a material having a large mechanical strength even if the tips are heated to a raised temperature and a high magnetic permeability not varied depending on the variations of temperature. Such materials include hot molding materials SKD-61, SKD-62 and others. The temperature controlling circuit 52 performs the repetitive comparison of the average in the actual temperatures at the tips 36a - 36d from the temperature sensors 54a - 54d with the temperature setting. If the average value is lower than the temperature setting, the oscillator circuit 72 will have its oscillating frequency increased as the difference between the average and the temperature setting decreases. As that oscillating frequency increases, the frequency in the current flowing the primary side of the transformer 48 also increases. As a result, the frequency in the current supplied to the heating coil units 38a - 38d is also increased so as to reduce the power applied to the heating coil units 38a - 38d. In other words, the temperature controlling circuit 52 is adapted to supply the heating coil units 38a - 38d with an increased electric power if the actual temperatures at the tops 36a - 36d are lower than the temperature setting and when the difference therebetween is relatively high. If the actual temperatures approaches the temperature setting, the electric power to be supplied is increased. In such a manner, the actual temperatures at the tops 36a - 36d may be converged on the temperature setting. On the contrary, if the actual temperatures exceed the temperature setting, the electric power to be supplied is decreased at the rate depending on the magnitude of the difference between the averaged temperature and the temperature setting. Similarly, the actual temperatures at the tips may approach the temperature setting. In this connection, the temperature indicating circuit 68 is adapted to indicate the actual temperatures at the tops 36a - 36d, the temperature setting and other factors. Since the tips 36a - 36d themselves generate heat in the illustrated embodiment, the thermal response thereof may be improved in comparison with the prior art wherein similar tips were heated by separate heater means under the heat transfer. This enables the tips to control in temperature with an improved accuracy and without any time delay due to the ringing or heat transfer. Since the heating coil units 38a - 38d are common in construction, only the heating coil unit 38a typical of the remaining coil units 38b, 38c and 38d will be described in construction with reference to Figure 2. Figure 2 shows a tip having its gate that is connected directly with one mold cavity in the mold assembly. The heating coil unit 38a is of a cylindrical configuration and comprises a bobbin body 80 disposed in the coil unit 38a at its center, a heating coil 81 wound about the bobbin body 80 and a coil casing 82 for housing the heating coil 81. These components are unitized by the use of any suitable filler material. In such an arrangement, the heating coil unit may be maintained at its desired configuration and easily positioned relative to the corresponding tip. The tip 36a is disposed in the interior of the heating coil unit 38a at its center, as shown. The heating coil unit 38a is firmly mounted within the mold assembly 10 by the use of any suitable means such that the positional relationship between the gate of the tip 36a and the heating coil 81 can be always maintained. In the illustrated embodiment, the tip 36a is in the form of a pipe-like member which is provided with a through-hole 90 defining a resin passageway adjacent to the gate opening. In the light of workability and manufacturing cost, the bobbin body 80 is preferably formed of a metal or alloy material having its high resistance and its low magnetic permeability, such as stainless steel. Merely by forming the bobbin body 80 of any one of such metal or alloy materials into a cylindrical configuration, there is created a problem in that the amount of eddy current generated in the tip 36a is reduced, resulting in undesirable heat generation in the tip 36a. As seen from Figure 3, the present invention provides a slit 83 formed axially in the bobbin body 80 and extending along the length thereof. This slit 83 functions to increase the amount of eddy current in the tip 36a such that the latter can be more efficiently heated by the eddy current. It is believed that this is because the circumferential eddy current path about the external surface of the bobbin body 80 is increased in its substantial length by the presence of the slit 83 to increase the resistance in the bobbin body 80. The heating coil 81 consists of a core formed of any metal having its good conductivity and its high corrosion resistance such as silver alloy, copper and the like and an insulating cover about the core. Depending on the size and other factors of the tip, the coil includes several turns to several tens turns about the tip. Since heat is transferred from the manifold block 22 to the rearward end of the tip 36a while heat is taken from the forward end of the same tip to the cavity plate 26, the heating coil 81 is desirably located as near the tip 36a as possible, such that the magnetic flux from the heating coil 81 will be concentrated into the forward end of the tip 36a. For the same reason, it is desirable that the forward end of the bobbin body 80 includes a plurality of partial slits 84 formed therein and extending from the forward end of the bobbin body toward the rearward end thereof, in addition to the complete slit 83, as shown in Figure 4. Such additional slit sections 84 further increase the substantial length of the eddy current path at the forward end of the bobbin body 80 to provide a further increased resistance. As a result, the thermal loss due to the eddy current in the bobbin can be further reduced to provide a further improved efficiency of heating at the tip itself. Although the aforementioned embodiments have been described as to the complete slit extending along the entire length of the bobbin body, the complete slit may be replaced by a partial slit extending only to the midway of the length of the bobbin body 80. It is also desirable that the wall thickness of the bobbin body is formed as thin as possible to increase the resistance in the bobbin body 80 and thus decrease the magnetic permeability therein.

Claims: A heating coil bobbin for a heating coil used in heating the hot runner in an injection molding machine, characterised in that the bobbin comprises a tubular body (80) made of a metal or metal alloy and having a low magnetic permeability, the body having a slit (83) formed therein, the slit extending a given length along the body. A heating coil bobbin as defined in claim 1 wherein the slit extends along the entire length of the bobbin body. A heating coil bobbin as defined in claim 2 wherein at least one further slit (84) extends along a part of the length of the body (80). A heating coil bobbin as defined in claim 1, 2 or 3 wherein said bobbin body (80) is formed of stainless steel.

Inventors: FUKAHORI, OSAMU, SHIBATA, ITSUO
Applicants: JU-OH INC.
Application year: 1989
Main classification: H05B6
IPC codes: B29C45, H05B6, H01F5