Abstract:
A method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing-metering phase in which a predetermined amount of a molten material is accumulated forwardly of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder, a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material, and an injecting phase in which the molten material is injected. The method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency axially of the screw during the plasticizing-metering phase. By vibrating the molten material at a low frequency during the relatively long plasticizing-metering phase, the viscosity of the molten material can be reduced. Further, since vibrations are generated axially of the screw, sufficient vibrations can be applied to the tip of the molten material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for controlling a screw injecting apparatus such that vibrations are given to a molding material to thereby decrease its viscosity, as well as to such a screw injecting apparatus. 
     2. Description of the Related Art 
     A typical prior screw injecting apparatus is shown in section in FIG. 9 hereof. The screw injecting apparatus  100  comprises a heating cylinder  101 , a screw  102  received in the heating cylinder  101  rotatably and movably back and forth, an injecting cylinder  103  for moving the screw  102  back and forth, and a hydraulic motor  105  for rotating the screw  102  by means of a piston rod  104  of the injecting cylinder  103 . In the injecting apparatus thus arranged, plasticizing-metering phase, waiting phase and injecting phase as described are carried out: 
     Plasticizing-Metering Phase: Raw molding materials are fed from a hopper  107  into the heating cylinder  101  during. rotation of the screw  102  and heated by the heating cylinder  101  while being transferred toward a discharging nozzle  108  by rotation of the screw  102 . By friction heat arising from the transfer and heat transmitted from the heating cylinder  101 , the molding materials are plasticized and kneaded. The screw  102  is pushed back rightwardly by a counter force of a molten material accumulated around the tip of the nozzle  108 . The amount of the molten material is measured by metering the retreating stroke of the screw  102 . 
     Waiting Phase: After completion of the metering, the hydraulic motor  105  and the screw  102  are held inactive until the molten molding material becomes ripe for injection. 
     Injecting Phase: By the action of the injecting cylinder  103 , the screw  102  is advanced at one stroke to cause the molten molding material accumulated forwardly of the screw  102  to be injected through the nozzle  108  into a mold not shown. 
     It is important to fill up cavities of the mold before solidification of the molten molding material therein progresses. Thus, the faster the injection speed becomes, the better. To speed up the injection, one may propose (1) to make the injection cylinder more pressurized, (2) to make the injection cylinder have an increased diameter, (3) to melt the molding material at an increased temperature, or (4) to decrease the viscosity of the molten molding material without relying on temperature increase. 
     The proposals (1) and (2) lead to up-sizing of the apparatus, thereby increasing the cost of production. 
     In the case of the proposal (3), the viscosity decreases by increase of the melting temperature. However, as can be seen from p-v-T (pressure-volume-temperature interrelation) characteristics, the density decreases as the temperature increases. Thus, volume variation becomes large when the temperture is changed from a molding temperatue to a normal temperature. To compensate for the large volume variation, a large pressure becomes necessary. In a normal injection molding process, pressure control is effected through a gate, whereby the pressure distribution of the resin within the cavities of the mold becomes non-uniform. This non-uniformity becomes more significant when the pressure is increased. Thus, the proposal to increase the melting temperature of the molding material. Rather, the temperature of the molding material should be as low as possible so that the temperature difference between the molding material and the mold can be kept to a minimum. 
     Consequently, the proposal (4), that is, to decrease the viscosity of the molten molding material without relying on temperature increase, has been seriously noted by researchers. The present inventors have continuously researched to realize the proposal (4) by means of mechanical measures. 
     FIG. 10 hereof graphically shows the results of such inventors&#39; research. As can be readily appreciated from the figure, the inventors have found during the research that the viscosity of the resin material significantly changes when the material is vibrated by a vibration frequency in a given range. 
     More specifically, vibration frequencies are shown in herz (Hz) along the horizontal axis while the material viscosity is shown in poise along the vertical axis. PMMA (poly(methyl methacrylate)), a typical resin material, was imparted various vibrations while maintaining it at 240° C. While it exhibits the viscosity of 126×10 3  poise at the frequency of 0 Hz, the PMMA exhibited the viscosity of 65×10 3  poise at 5 Hz, 14×10 3  at 30 Hz, and 9×10 3  poise at 55 Hz. 
     While it exhibits the viscosity of 63×10 3  at the frequency of 0 Hz, PC (polycarbonate) exhibited 26×10 3  poise at 15 Hz, and 15×10 3  poise at 40 Hz. 
     The research has thus revealed that it becomes possible to satisfactorily decrease the viscosity of the resin material by vibrating the material at the frequency ranging from 5 Hz to 40 Hz, desirably at 15 Hz or more. 
     A technique for applying vibrations to a molding material during injection thereof is known from, for example, Japanese Utility Model Laid-Open Publication No. SHO-63-197113 entitled “SHAPING INJECTION MOLDING APPARATUS WITH SHAKING CAPABILITY”. The known apparatus includes an ultrasonic oscillator mounted in a heating cylinder thereof and an ultrasonic wave generator disposed remotely from the heating cylinder for ultrasonically vibrating the ultrasonic oscillator. In response to a high frequency signal, the ultrasonic wave generator generates a ultrasonic wave for vibrating the ultrasonic oscillator. The resulted ultrasonic vibrations are applied to a molten resin material being injected, thereby fully filling up finely-patterned portions and complex-shaped portions of a molded product. 
     Ultrasonic waves have a frequency above about 20 kHz which is too high for humans to hear. The ultrasonic wave generator is designed to produce such waves. This and the above-described arrangement bring about the following problems: 
     (a) it is likely that fine bubbles be formed in the molten material, because an extremely high vibration frequency is applied to the molten material, thereby suddenly changing the pressure of the molten material; 
     (b) there remains a fear that the effect of the vibrations or shaking may not reach deepest portions of the cavities of the mold, because the injecting phase is finished too quickly; and 
     (c) being disposed within the heating cylinder, the ultrasonic oscillator will present a bar to the flow of the molten material. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method for controlling a screw injecting apparatus, which does not give rise to the problems (a)-(c). 
     Another object of the present invention is to provide a screw injecting apparatus in which the inventive method can be performed. 
     According to one aspect of the present invention, there is provided a method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing-metering phase in which a predetermined amount of a molten material is accumulated forwardly of a tip of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder, a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material, and an injecting phase in which the molten material is injected, which method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency axially of the screw during the plasticizing-metering phase. 
     By vibrating the molten material at a low frequency during the relatively long plasticizing-metering phase, the viscosity of the molten material can be reduced. Further, since vibrations are generated axially of the screw, sufficient vibrations can be applied to the tip of the molten material. 
     According to a second aspect of the present invention, there is provided a method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing-metering phase in which a predetermined amount of a molten material is accumulated forwardly of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder, a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material, and an injecting phase in which the molten material is injected, which method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency axially of the screw during the waiting phase. 
     By vibrating the molten material at a low frequency during the waiting phase, the viscosity of the molten material can be reduced. Further, since vibrations are generated axially of the screw, sufficient vibrations can be applied to the tip of the molten material. 
     The vibration may desirably be produced by means of an injection cylinder mechanically connected to the screw. 
     By minutely moving the piston rod of the injection cylinder back and forth, the screw can be vibrated at a predetermined frequency. Use of the injection cylinder for vibration generation avoids the possible cost increase. 
     According to a third aspect of the present invention, there is provided a method for controlling a screw injecting apparatus having a heating cylinder and a screw slidably and rotatably received in the heating cylinder and designed to carry out a plasticizing-metering phase in which a predetermined amount of a molten material is accummulated forwardly of the screw by plasticizing and kneading a raw molding material via the screw and the heating cylinder, a waiting phase in which the screw is held inactive for a period between completion of the metering and subsequent injection of the molten material, and an injecting phase in which the molten material is injected, which method comprises the step of causing the apparatus to vibrate the molten material at a predetermined low frequency in a direction of flow of the material during the injecting phase. 
     By vibrating the molten material during the injecting phase, the molten material is rendered to have reduced viscosity until the injection is completed. 
     The vibration in the injecting phase may desirably be produced by means of a needle valve mechanism provided for closing an injecting nozzle of the apparatus. 
     Desirably, the predetermined frequency falls in a range of 5 to 40 Hz. A frequency lower than 5 Hz can not reduce the viscosity of the molding material sufficiently. A frequency higher than 40 Hz makes no much difference in viscosity reduction rate. Therefore, a vibration frequency should be chosen from a range between 5 to 40 Hz depending on resins used. 
     According to a fourth aspect of the present invention, there is provided a screw injecting apparatus, which comprises a heating cylinder for heating a molding material being transferred therethrough, a nozzle disposed at a tip of the heating cylinder for serving as an injection port, a needle valve mechanism having a needle for closing the nozzle, a screw received in the heating cylinder for transferring the molding material toward the nozzle while being rotated, the screw being moved, when the nozzle is closed by the needle, in a direction away from the nozzle by the molding material heat-molten in the heating cylinder and accumulated forwardly of a tip of the screw, a hydraulic motor for rotating the screw, a cylinder actuator connected to a proximal end of the screw for axially vibrating the screw back and forth, and a hydraulic pressure controller for driving the cylinder actuator to minutely vibrate the screw back and forth so as to apply vibrations to the molten molding material located around the screw tip when the screw is moved in the direction away from the nozzle and to make the screw advance toward the nozzle for injecting the molten molding material from the nozzle when the needle is opening the nozzle. 
     Preferably, the needle valve mechanism further comprises a primary cylinder having a primary piston slidable therein, a secondary cylinder formed integrally with the primary cylinder and having a secondary piston slidable therein, and a piston rod extending axially from the secondary piston, the needle for closing/opening the nozzle being connected to a distal end of the piston rod. The hydraulic pressure controller drives the primary cylinder so that the primary piston advances along with the secondary piston of the secondary cylinder to move the needle toward the nozzle to thereby close the latter, and, as the needle is opening the nozzle, drives the secondary cylinder separately from the primary cylinder so that the secondary piston moves back and forth to move the needle back and forth to thereby apply vibrations to the molten molding material passing through the nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view illustrating a screw injecting apparatus according to the present invention; 
     FIG. 2 is an enlarged sectional view illustrating a needle valve mechanism of the screw injecting apparatus; 
     FIGS. 3A and 3B illustrate the plasticizing-metering phase and waiting phase carried out in the screw injecting apparatus; 
     FIGS. 4A to  4 C are timecharts of the plasticizing-metering phase; 
     FIGS. 5A and 5B are timecharts of the waiting phase; 
     FIG. 6 illustrates the injecting phase of the screw injecting apparatus; 
     FIGS. 7A to  7 C are timecharts of the injecting phase; 
     FIG. 8 is a general timechart illustrating the relevant phases of the screw injecting apparatus; 
     FIG. 9 is a cross-sectional view illustrating a typical conventional screw injecting apparatus; and 
     FIG. 10 is a graph showing relations between vibration frequencies and resin material viscosity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description is merely exemplary in nature and is in no way intended to limit the invention or its application or uses. 
     Referring initially to FIG. 1, a screw injecting apparatus  10  according to the present invention comprises a heating cylinder  11 , a screw  12  accommodated in the heating cylinder  11  rotatably and movably back and forth (slidably in a right-and-left direction in the figure), an injection cylinder  13  connected to a proximal end of the screw  12  for moving the screw  12  back and forth, a hydraulic motor  15  for rotating the screw  12  via a piston rod  14   a  of the injection cylinder  13 , a hopper  16  as an entry port for raw molding materials, a metering scale  17  for metering the amount of a molten molding material, a needle valve mechanism  30  to be discussed in detail later, and a hydraulic pressure circuit and a control section  28  explained below. Within the injection cylinder  13 , there are defined a front oil chamber  20   a  and a rear oil chamber  20   b . In place of the hydraulic motor  15 , an electric motor may also be used. 
     First servo valve  21  is of the four-port, oil-pressure-selectable type and hence has ports A and B connected to the injection cylinder  13 , a port P connected to a source of hydraulic pressure  24 , and a port T connected to an oil tank  25 . The first servo valve  21  can be switched by the control section  28  such that a high pressure oil is supplied from the hydraulic pressure source  24  to the front and rear oil chambers  20   a ,  20   b  and discharged from the latter to the oil tank  25 . By minutely regulating the amount of oil supply to and discharge from the chambers  20   a ,  20   b  through the first servo valve  21 , the hydraulic pressure within the front and rear oil chambers  20   a ,  20   b  can be regulated as desired. 
     Selector valve  22  has ports A and B connected to a primary cylinder (discussed later) of the needle valve mechanism  30 . The valve  22  moves the needle valve mechanism  30  largely by supplying oil from the hydraulic pressure source  24  to the primary cylinder and discharging the oil from the primary cylinder to the oil tank  25 . 
     Second servo valve  23  has ports A and B connected to a secondary cylinder (discussed later) of the needle valve mechanism  30  and moves (vibrates) a needle  31  of the valve mechanism  30  back and forth minutely by supplying oil from the hydraulic pressure source  24  to the secondary cylinder and discharging the oil therefrom to the oil tank  25 . 
     Reference numerals  26  and  27  designate pressure sensors for detecting the internal pressure of the front and rear oil chambers  20   a ,  20   b . On the basis of outputs from the pressure sensors  26 ,  27 , the control section  28  controls the oil pressure selection of the first and second servo valves  21 ,  23  and the selector valve  22 . 
     Turning now to FIG. 2, the needle valve mechanism  30  comprises a guide  32  engaged in a block  19  connecting the heating cylinder  11  and the nozzle  18 , the primary cylinder  34  connected to the guide  32  via a bracket  33 , a primary piston  35  slidably disposed internally of the primary cylinder  34 , the secondary cylinder  37  formed integrally with the primary piston  35 , a secondary piston  38  slidably disposed internally of the secondary piston  37 , front and rear piston rods  41 ,  42  extending from the secondary piston  38  in a forward-and-rearward direction, an elongated rod  44  received in the piston rods  41 ,  42  and fixed to the latter via bushes  43 ,  43 , the needle  31  disposed at a distal end of the elongated rod  44  and capable of making advancing and retreating movements, and an amplitude adjusting nut  46  mounted to a proximal end of the elongated rod  44 . 
     In FIG. 2, the needle  31  is placed in an open position in which the nozzle  18  of the heating cylinder  11  is opened. When the primary piston  35  is advanced or moved in the direction of arrow a relative to the primary cylinder  34  via a hydraulic pressure generated by switching the selector valve  22 , the needle  31  moves to a close position in which the nozzle  18  is closed. During a plasticizing-metering phase, the nozzle  18  is closed by the needle  31 . By thus moving the primary piston  35  back and forth relative to the primary cylinder  34 , the nozzle  18  can be opened and closed by the needle  31 . 
     Upon injection of a molten material, the needle  31  is positioned to open the nozzle  18 , as shown in FIG. 2, while the secondary piston  38  is moved reciprocally (back and forth) in the direction of arrow b relative to the secondary cylinder  37 . In this instance, the needle  31  never closes a flow path of the molten material in its advance limit. In its half-open position, the needle  31  thus vibrates the molten material, thereby decreasing the viscosity of the material. 
     The vibration range of the needle  31  is determined by an amplitude h which can be varied by turning an amplitude adjusting nut  46  rightwardly or leftwardly. As a front end surface of the amplitude adjusting nut  46  hits a rear end surface of the secondary cylinder  37 , the needle  31  can not advance any further. This is the advance limit of the needle  31 . Reference numeral  47  designates a nut for preventing loosening of the amplitude adjusting nut  46 . 
     Discussion will now be made as to the operation of the described screw injecting apparatus with reference to FIGS.  3 A and  3 B. 
     FIG. 3A shows an initial stage of the plasticizing-metering phase. By turning the screw  12  via the hydraulic motor  15 , a molding material is transferred from the hopper  16  to around a tip of the screw  12 . During this transfer, the molding material is turned into a molten state. Then, the screw  12  is vibrated axially via the injection cylinder  13  at a given low frequency (e.g, 5 to 40 Hz). 
     FIG. 3B shows the last stage of the plasticizing-metering phase and a waiting phase. By minutely vibrating a piston  14   b  of the injection cylinder  13  via the first servo valve  21 , the tip of the screw  12  vibrates in synchronism. As a result, the molten material  51  vibrates as shown by reference character c. More details will be given later in relation to FIGS. 4A to  4 C. 
     Reference is next made to the timecharts of FIGS. 4A to  4 C. In the plasticizing-metering phase, with the needle  31  closing the nozzle  18 , the molten material is carried forward by the screw  12  and accumulated in front thereof. The pressure of the accumulated molten material pushes back the screw  12 . At this time, the piston  14   b  in the injection cylinder is also pushed back, causing a hydraulic pressure producing oil in the rear oil chamber  20   b  to be discharged to the oil tank  25 . The pressure of that oil is regulated to thereby control the back pressure arising therein. For example, when the screw  12  and the piston  14   b  are being pushed back, a small amount of pressurizing oil is supplied to the rear oil chamber  20   b  and at the same time a small amount of oil is discharged from the front oil chamber  20   a . This causes the screw  12  and the piston  14   b  to slightly advance (in a leftward direction in the figure). Immediately thereafter, the first servo valve  21  is switched over to supply a small amount of oil to the front oil chamber  20   a  and to discharge a small amount of oil from the rear oil chamber  20   b , thereby causing the screw  12  and the piston  14  to retreat or move backward (in a rightward direction in the figure) again. By repeating such a hydraulic pressure switch-over operation, the screw  12  moves back and forth (vibrates minutely). 
     FIG. 4A shows the back pressure in the injection cylinder. The pressure of the rear oil chamber  20   b  is increased by a vibration pressure resulted from adding a predetermined pressure to the back pressure, thereby causing the screw  12  and the piston  14   b  to advance. Then, the predetermined pressure is subtracted from the vibration pressure to cause the screw  12  and the piston  14   b  to retreat. The back pressure and the vibration pressure occur alternately. In the illustrated example, the vibration pressure occurs at the frequency of ½ per cycle. 
     As a result of the foregoing back pressure control (regulation of the pressure of the rear oil chamber), the metering speed changes as shown in FIG.  4 B. 
     FIG. 4C shows a mode of displacement of the screw. The screw retreats with time. However, since the speed of retreat of the screw is not constant, as can be appreciated from FIG. 4B, the screw displacement becomes wavy. This means that the screw moves back and forth as it retreats. By such back and forth movement of the screw, the molten material can be vibrated. 
     Reference is made next to FIGS. 5A and 5B showing the timecharts of the waiting phase. In normal molding, no particular control is performed in the waiting phase. But in the waiting phase of the present invention, pressure control, like that done in the injecting phase (for high pressure injecting advance and low pressure injecting advance), is carried out in the waiting phase by supplying a pressurizing oil from the hydraulic pressure source to the injection cylinder. 
     FIG. 5A shows the back pressure arising in the injection cylinder. By pressure control, high pressure and low pressure occur alternately as shown in the figure. In the illustrated example, the occurrence of the high pressure and the low pressure is 1:1 per cycle. 
     FIG. 5B shows the corresponding positions of the screw. The screw moves back and forth between a metering stopped position and a pressure-advanced position, whereby the molten resin material is vibrated. 
     Referring to FIG. 6, discussion will be made next as to the injecting phase of the screw injecting apparatus according to the present invention. Firstly, the primary piston  35  is made to retreat. At the same time, the back and forth movement of the secondary piston  38  is initiated to minutely vibrate the needle  31 . This vibration continues until termination of the injection. Next, the screw  12  is moved forward quickly (at a stroke) through the injection cylinder to cause the molten material to be injected from the nozzle  18 . 
     Shown in FIGS. 7A to  7 C are timecharts of the injecting phase of the inventive screw injecting apparatus. For moving the screw back and forth during the injection phase, difficulty is experienced in controlling the position, speed and pressure of the screw. Thus, in the present invention, vibrations are given by means of a shaking rod, keeping the filling speed control and filling pressure control as they are done in normal molding. 
     As shown in FIG. 7A, the shaking rod (needle  31 ) is moved back and forth. By moving the shaking rod back and forth, the volume of the resin passage within the nozzle  18  varies and the speed of flow of the molten resin through the nozzle varies as shown in FIG.  7 B. By the back and forth movement of the shaking rod, the resin pressure within the nozzle varies as shown in FIG.  7 C. The added pressure corresponds to a resin pressure inside the nozzle as the shaking rod advanced while the subtracted pressure corresponds to a resin pressure inside the nozzle as the shaking rod retreated. By thus moving the shaking rod back and forth, the volume and the pressure inside the nozzle are varied to thereby apply minute vibrations to the molten resin inside the nozzle. 
     Reference is now made to FIG. 8 illustrating a general timechart of the various phases involved in the screw injecting apparatus according to the present invention. Time is given along the horizontal axis. Given along the vertical axis are a screw position, a needle valve position, needle valve vibration, screw vibration and back pressure within the injection cylinder. 
     As can be seen from the general timechart, vibrations are applied to the molten resin during the plasticizing-metering phase and the waiting phase by vibrating the screw. During the injecting phase, vibrations are applied to the molten resin by vibrating the needle valve. 
     In the plasticizing-metering phase, the screw is caused to gradually retreat by rotating it, whereupon the needle valve is placed in a non-vibrating close postion with the screw vibrated in its stead. The resin being metered is supplied with the back pressure via the injection cylinder. 
     In the waiting phase, the screw is again vibrated while it is stopped rotating, whereupon the needle valve is placed in the non-vibrating close position. The molten resin positioned forwardly of the tip of the nozzle is supplied with the maintaining back pressure via the injection cylinder. 
     In the injecting phase, the screw is quickly advanced, whereupon the needle valve is opened and vibrated to apply continuous vibrations to the molten resin. Vibration of the screw is stopped, and the maintaining back pressure in the injection cylinder is cancelled. 
     More accurately, the back pressure in the injection cylinder should be graphed to show partially waved but this has been omitted for the sake of simplicity. 
     A technique for axially vibrating the screw at a predetermined frequency may be selected arbitrarily. For example, the conventional servo valve as employed in the present embodiment may also be used to produce the required vibrations. Alternatively, the back pressure in the injection cylinder may be controlled by means of a booster connected with a back pressure chamber of the injection cylinder. A small pistion for exclusively for screw vibration may be mechanically connected to a shaft of the screw. 
     It will also be readily appreciated by those skilled in the art that a technique for vibrating the molten material in a flow direction thereof may also be selected arbitrarily. 
     In place of the primary and secondary cylinders, the described needle valve mechanism may have only the primary cylinder capable of valve opening/closing and shaking operations. 
     Obviously, various minor changes and modifications of the present invention are possible in the light of the above teaching. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described.