Patent Publication Number: US-2011052746-A1

Title: Die head for forming a multi-layer resin and an extrusion-forming machine having the same

Description:
TECHNICAL FIELD 
     This invention relates to a die head for forming a multi-layer resin by covering the surrounding of a core layer of a molten resin flowing out from a core layer-forming flow passage with a main layer flowing out from a main layer-forming flow passage, and an extrusion-forming machine having the same. 
     BACKGROUND ART 
     As is widely known among people skilled in the art, a composite molten resin material comprising an outer molten resin layer (main layer) and at least an inner molten resin layer (core layer) wrapped in the outer molten resin layer is quite often used as a synthetic resin material for forming a preformed article (usually called preform) that is to be formed (usually, blow-formed) into a container for beverages or for forming a container lid or a cup by cutting a molten resin extruded from an extrusion-forming machine into a drop (mass) followed by compression-forming. As the outer molten resin, a synthetic resin having excellent mechanical properties and sanitary properties is, usually, selected. As the inner molten resin layer, recycled material is selected for reusing the resources, or a functional synthetic resin is selected which is excellent concerning either or both the oxygen-absorbing property and gas-barrier property. 
     By using an extrusion-forming machine having a plurality of die heads, on the other hand, the molten resin for forming the main layer flows through an outer molten resin flow passage, the molten resin for forming the core layer flows through an inner molten resin flow passage, and the molten resins are, thereafter, ejected from ejection ports of the die heads. When the extrusion-forming machine is provided with the plurality of die heads as described above, it is desired that the molten resins are uniformly distributed and fed to the die heads and are ejected to maintain uniformity in the forming or in the formed articles. A further improved uniformity is required when the molten resins comprise a plurality of kinds of layers or masses, or when a composite molten resin of a mixture thereof is used. 
     To maintain uniformity, the molten resin must be fed in an equal amount to the die heads. If the plurality of die heads are the same ones, the uniformity can be maintained by setting equal the length of branched flow passages from the ejection ports of the extrusion-forming machine to the die heads. In practice, however, even if the distance is set to be equal from the ejection ports to the die heads, it is difficult to equally distribute and feed the molten resin to the die heads so as to be equally ejected due to dispersed machining precision of the flow passages and die heads and due to dispersion in the temperature. Besides, the lengths of the flow passages cannot often be set to be equal. 
     To cope with this, a patent document 1 discloses a forming machine for forming a composite resin material in which a plunger and a measuring chamber are arranged in the die head to suitably eject a molten resin from an ejection port . A patent document 2 discloses an injection-forming machine by using a servo mechanism for a plunger. 
     Patent document 1: JP-A-7-68631 
     Patent document 2: JP-A-6-79771 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention: 
     According to the above patent document 1 and patent document 2, however, the composite molten resin is so formed that the surface of fault thereof extends in the direction of discharge (axial direction), which is not to forma composite resin material in which one of the molten resin is a core layer of the form of a dumpling and the whole surface of the core layer is wrapped with a main layer as contemplated by this application. 
     According to the technology disclosed by the patent document 1, further, the molten resin is fed, via a rotary mechanism, to an accumulator (measuring chamber) arranged in a die head and, therefore, the structure of the die head becomes complex. According to the technology disclosed by the patent document 2, the molten resin is once fed to a resin passage (measuring chamber) and is injected by using a ring plunger. If the plunger operation control of this technology is employed, however, the air is trapped by the molten resin or vacuum bubbles generate therein due to a negative pressure (with respect to the atmospheric pressure) in the measuring chamber caused by the plunger that moves back at the time of feeding the molten resin into the measuring chamber. When there are provided a plurality of die heads, therefore, the extrusion-forming cannot be uniformly effected maintaining good quality. 
     The present invention was accomplished in view of the above circumstances and has an object of providing a die head for forming a multi-layer resin, which is equipped with a plunger and a measuring chamber, and is capable of suitably ejecting a required amount of a molten resin from the die head of an extrusion-forming machine, and an extrusion-forming machine having the same. 
     Means for Solving the Problems: 
     In order to solve the above problems, the present invention is concerned with a die head for forming a multi-layer resin comprising at least a main layer-forming flow passage for flowing a molten resin into a nozzle of the die head of an extrusion-forming machine passing through the die head, and a core layer-forming flow passage for intermittently flowing a molten resin into the nozzle of the die head passing through the die head, so that the surrounding of a core layer flowing out from the core layer-forming flow passage is covered with a main layer flowing out from the main layer-forming flow passage, wherein at least either the main layer-forming flow passage or the core layer-forming flow passage is provided with a measuring chamber into which the molten resin is pressure-introduced from the upstream side of the flow passage due to the pressure-feeding force of the molten resin, the measuring chamber is provided with a plunger which measures the molten resin while moving back receiving the pressure-feeding force of the molten resin and moves forward so as to eject the measured molten resin to the downstream side of the flow passage, and at least the amount of ejecting the molten resin measured by the plunger and the timing for starting the ejection are controlled by a control unit that controls the back-and-forth motion of the plunger. 
     In the die head for forming a multi-layer resin, the plunger is driven by a servo motor, the servo motor is controlled by the control unit at a moment when the molten resin is pressure-fed into the measuring chamber, the plunger moves back in a state where the plunger is receiving a backward-moving pressure of the molten resin that is pressure-introduced into the measuring chamber, and a position to where the plunger moves back is limited. 
     In the die head for forming a multi-layer resin, it is desired that the measuring chamber and the plunger are incorporated in the die head. 
     In the die head for forming a multi-layer resin, one or more molten resin flow passages are provided on the outer side of the core layer-forming flow passage, and at least any one of the flow passages, the main layer-forming flow passage or the core layer-forming flow passage is provided with the measuring chamber and the plunger. 
     The die head for forming a multi-layer resin is provided with an inner moving block, and the inner moving block works both as an opening/closing valve of the main layer-forming flow passage and as the core layer measuring chamber. 
     The inner moving block, further, works as an opening/closing valve of the core layer-forming flow passage, and switches the opening and closing of the main layer flow passage and the core layer flow passage. 
     The plunger can work as a flow passage opening/closing valve to the measuring chamber in the main layer flow passage and/or the core layer flow passage. 
     The die head for forming a multi-layer resin can be used for an extrusion-forming machine equipped with a plurality of the die heads for forming the multi-layer resin that have the measuring chamber and the plunger. 
     A multi-layer drop-forming machine is realized by additionally providing the die head for forming a multi-layer resin with a cutter which periodically cuts the multi-layer resin ejected from the ejection port into a multi-layer drop, by periodically moving the plunger back and forth, and by bringing the period for moving the plunger back and forth into agreement with the period for cutting the multi-layer resin by the cutter. 
     The operation of the plunger can be brought into synchronism with the operation of the cutter. 
     Effect of the Invention: 
     By using the control unit, the die head for forming the multi-layer resin of the invention controls the measurement and the amount of ejecting the molten resin based on the back-and-forth movement of the plunger, measures the timing for forward movement (start of ejection of the molten resin) , ejects the core layer molten resin or ejects the main layer molten resin to form a preferred composite molten resin. At the time of measuring the molten resin, further, the plunger is moved back while applying a pressure of the pressure-feeding force of molten resin to the plunger, so that the molten resin is contained in the measuring chamber effectively preventing the air or vacuum bubbles from being trapped by the measured molten resin, and correctly measures the molten resin in the measuring chamber without gap and ejects it. 
     More desirably, the plunger is more correctly driven and controlled by a servo motor to more preferably effect the measurement. 
     With the plunger being incorporated in the die head, the measurement and ejection can be effected near the ejection port decreasing loss and dispersion caused by loss of pressure in the pipe and more effectively preventing the trapping of air and vacuum bubbles. 
     By providing one or more molten resin passages on the outer side of the core layer-forming flow passage, it is made possible to form a multi-layer resin having the core layer arranged near the center of the ejection port. Here, when the plunger is provided on the core layer side, the correctly measured core layer molten resin can be extruded maintaining good accuracy of weight preventing the trapping of air and vacuum bubbles. When the plunger is provided on the other layer side, further, a cleaning shot can be effected for pushing out the core layer by utilizing the resin extruded from the plunger. 
     Upon providing the inner moving block in the die head for forming the multi-layer resin, the inner moving block working as the opening/closing valve of the main layer-forming flow passage and as the core layer measuring chamber, it is allowed to construct the die head in a simple structure in a small size. 
     With the inner moving block working as the opening/closing valve of the core layer-forming flow passage and switching the opening and closing of the main layer flow passage and the core layer flow passage, the die head can be more simply constructed in a small size. 
     With the plunger working as a flow passage opening/closing valve to the measuring chamber in the main layer flow passage and/or the core layer flow passage, the die head can be simply constructed in a small size. 
     When the extrusion-forming machine is provided with a plurality of the die heads of the invention, a preferred composite molten resin can be nearly uniformly ejected from the die heads by suitably controlling the molten resin pressure-feeding force from the upstream of the flow passage or by suitably controlling the mechanism/device (e.g., servo motor) that moves the plunger up and down. 
     Upon bringing the period of back-and-forth movement of the plunger of the die head for forming the multi-layer resin into agreement with the cutting period of the cutter, a forming machine is realized which is capable of successively obtaining the multi-layer resin drops in a predetermined amount in a state where the core layer resin is arranged at a predetermined position. 
     Upon bringing the operation of the plunger into synchronism with the operation of the cutter, the position of the core layer in the multi-layer drop does not deviate in successively forming the multi-layer resin drops. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS: 
       [ FIG. 1 ] is a plan view schematically illustrating a forming system  1  for executing the compression-forming by using an extrusion-forming machine according to a first embodiment of the invention (same as a second embodiment except an increased number of die heads and flow passages) . 
       [ FIG. 2 ] is a plan view illustrating a portion of the compression-forming portion of the forming system of  FIG. 1  on an enlarged scale. 
       [ FIG. 3 ] is a sectional view illustrating a die head of the extrusion-forming machine of  FIG. 1  on an enlarged scale. 
       [ FIG. 4 ] is a view for illustrating the operation of the die head of  FIG. 3 , wherein A is a sectional view of the initial position, B is a sectional view in a state of measuring the molten resins of the main layer and the core layer, and C is a sectional view in a state where a flow passage (opening/closing valve) of the core layer molten resin is opened. 
       [ FIG. 5 ] is a view (continued from  FIG. 4 ) for illustrating the operation of the die head of  FIG. 3 , wherein A is a sectional view in a state where the molten resin in a core layer measuring chamber is extruded, B is a sectional view in a state where a flow passage (opening/closing valve) of the core layer molten resin is closed, and C is a sectional view in a state where the molten resin of the main layer is extruded. 
       [ FIG. 6 ] is a sectional view of the die head of the extrusion-forming machine according to a second embodiment of the invention on an enlarged scale. 
       [ FIG. 7 ] is a view illustrating the operation of the die head of  FIG. 6 , wherein A is a sectional view of the initial position, B is a sectional view in a state of measuring the molten resin of the main layer and extruding the molten resin of the core layer, and C is a sectional view in a state where the flow passages of the core layer molten resin and the main layer molten resin are changed over (core layer molten resin flow passage is closed, main layer molten resin flow passage is opened) . 
       [ FIG. 8 ] is a view (continued from  FIG. 7 ) for illustrating the operation of the die head of  FIG. 6 , wherein A is a sectional view in a state of extruding the main layer and measuring the molten resin of the core layer, and B is a sectional view in a state where the flow passages of the core layer molten resin and the main layer molten resin are changed over (core layer molten resin flow passage is opened, main layer molten resin flow passage is closed). 
       [ FIG. 9 ] is a plan view schematically illustrating the extrusion-forming machine equipped with a plurality of die heads according to a modified embodiment of the invention. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
       2  extrusion-forming machine 
       7  die head 
       20  ejection port 
       31 ,  55  outer blocks 
       32  inner block 
       31   c,    55   c  discharge holes 
       33  lift valve 
       34 ,  58  main layer measuring chambers 
       35 ,  41 ,  59 ,  65  annular plungers 
       36 ,  42 ,  60 ,  66 ,  80  lift mechanisms/devices (servo motors) 
       37 ,  61  main layer flow passages 
       39 ,  63  core layer measuring chambers 
       43 ,  67  core layer flow passages 
       56  inner moving block 
       57  shaft-like valve 
     W opening/closing valve 
     X first opening/closing valve 
     Y second opening/closing valve 
     Z third opening/closing valve 
     Best Mode for Carrying Out the Invention 
     A die head for forming a multi-layer resin according to a first embodiment of the invention and an extrusion-forming machine having the same will now be described with reference to the drawings. 
       FIG. 1  is a plan view schematically illustrating a forming system  1  for executing the compression-forming by using an extrusion-forming machine having a die head for forming a multi-layer resin of the invention, and  FIG. 2  is a plan view illustrating a portion of the compression-forming portion of the forming system of  FIG. 1  on an enlarged scale. 
     The forming system  1  includes an extrusion-forming machine  2  according to the invention, a synthetic resin conveyer device  3 , a compression-forming device  4 , and a discharge device  5 . 
     The extrusion-forming machine  2  is equipped with a plurality of molten resin feed means  45 ,  50  and  83 , i.e., main layer molten resin feed means  45 , core layer molten resin feed means  50  and outer main layer molten resin feed means  83  (here, the molten resin feed means  83  may be omitted in this embodiment, but is used in a second embodiment described later) that use different molten resins as starting materials. The molten resin feed means  45 ,  50  and  83  form molten resin by heating, melting and kneading synthetic resin materials such as PP and PET as well as a functional synthetic resin material. The extrusion-forming machine  2  has, on the front end side thereof, a die head  7  to which the molten resins are fed from the molten resin feed means  45 ,  50  and  83 . The die head  7  has resin flow passages formed therein extending up to an ejection port  20  (see  FIG. 3 ) formed in the lower surface at a front end portion thereof . That is, the molten resin feed means  45 ,  50  and  83  send the molten resins to the die head  7 ; i.e., the molten resins sent from the molten resin feed means  45 ,  50  and  83  are extruded from the ejection port  20  as a composite molten resin that will be described later. 
     If described with reference to  FIG. 1  and  FIG. 2 , the synthetic resin conveyer device  3  includes a rotary disk  11  that rotates in a direction indicated by an arrow e. A plurality of cutting/holding units  14  are arranged on the circumferential edge of the rotary disk  11  maintaining an equal distance in the circumferential direction. Accompanying the rotation of the rotary disk  11 , the cutting/holding units  14  are conveyed through a circular conveyer passage that extends along the circumferential edge of the rotary disk  11 , and are conveyed through a receiving zone  18  positioned just under the ejection port  20  of the die head  7  and facing thereto and through a resin feed zone  21  positioned over the compression-forming device  4  and facing a predetermined portion thereof. The cutting/holding unit  14  cuts the composite molten resin ejected from the die head  7  into a mass of the composite molten resin (multi-layer drop)  8  which is, then, fed to a metal mold  30  of the compression-forming device  4 . The metal mold  30  compression-forms the multi-layer drop  8  into a preform, a container or the like, which is, thereafter, handed over to the discharge device  5  so as to be conveyed to the next step. 
       FIG. 3  is a sectional view illustrating the die head  7  of the extrusion-forming machine  2  (for easy comprehension of the structure of the die head  7  and its operation, depth lines are omitted). 
     The die head  7  has an outer periphery of a circular shape in horizontal cross section, and includes an outer block  31  arranged on the outer circumferential side, the outer block  31  being nearly of an annular shape forming a large-diameter hole  31   a  at an intermediate position thereof in the up-and-down direction, forming an intermediate-diameter hole  31   b  on the upper side of the large-diameter hole  31   a,  i.e., on the upper side of the outer block  31 , and forming a nozzle  31   c  on the lower side of the large-diameter hole  31   a,  i.e., on the lower side of the outer block  31 . The lower side of the large-diameter hole  31   a  is formed in the shape of an inverse circular truncated cone. 
     An inner block  32  is arranged in the intermediate-diameter hole  31   b  and in the large-diameter hole  31   a.  In the inner block  32 , a cylindrical inner hole  32   a  is formed having the same diameter. The lower portion of the inner hole  32   a  is formed in the shape of an inverse conical truncated cone. 
     In the center of the inner hole  32   a,  a shaft-like lift valve  33  is arranged with its axis being oriented in the up-and-down direction. 
     Being positioned in the large-diameter hole  31   a,  a main layer measuring chamber  34  (see also  FIG. 4B ) is formed in an annular region between the outer block  31  and the inner block  32 , and a main layer flow passage  37  is formed on the lower side of the main layer measuring chamber  34 . The downstream side of the main layer flow passage  37  [when a certain portion from the molten resin feed device (e.g. , main layer molten resin feed means  45  or core layer molten resin feed device  50  in this embodiment) to the ejection port  20  of the die head  7  is seen, the molten resin feed device side is hereinafter called upstream side, and the ejection port side is called downstream side] is communicated with the nozzle  31   c  at all times and is, further, communicated with the ejection port  20  of the die head  7  on the downstream side thereof. 
     In the main layer measuring chamber  34 , an annular plunger  35  is arranged to move back and forth in the up-and-down direction in the main layer measuring chamber  34 , i.e., being connected to a device/mechanism that is capable of moving up and down. As the device/mechanism for moving the plunger  35  up and down, there can be exemplified an air cylinder, a hydraulic cylinder, a mechanical cam, a crank mechanism and a link mechanism, which may, further, be linked to a known mechanism/device such as a spring or a dumper. In this embodiment, a servo motor  36  is linked to the annular plunger  35  enabling the annular plunger  35  to move up and down in the main layer measuring chamber  34  in the up-and-down direction. 
     Upon moving the annular plunger  35  up and down, a main layer measuring portion  34   a  formed under the annular plunger  35  in the main layer measuring chamber  34  undergoes the expansion and contraction in the up-and-down direction to increase and decrease the volume. The servo motor  36  is connected to control means that is not shown and is electrically controlled. 
     Being positioned in the inner hole  32   a  of the inner block  32 , a core layer measuring chamber  39  (see also  FIG. 4B ) is formed in an annular region between the inner block  32  and the lift valve  33 , and a core layer flow passage  43  is formed on the lower side of the core layer measuring chamber  39 . A valve hole  48  is provided on the downstream side of the core layer flow passage  43 . The valve hole  48 , together with the lift valve  33 , opens and closes the core layer flow passage  43 . At the descended position shown in  FIG. 3 , the opening/closing valve W (lift valve  33  and valve hole  48  together are referred to as opening/closing valve W) assumes the closed state, and assumes the opened state if the lift valve  33  moves up. With the opening/closing valve W in the opened state, the core layer flow passage  43  is communicated with the nozzle  31   c  and is, further, communicated with the ejection port  20  of the die head  7  on the downstream side thereof. The lift valve  33  is electrically controlled by control means that is not shown. 
     In the core layer measuring chamber  39 , an annular plunger  41  is arranged to move back and forth in the up-and-down direction in the core layer measuring chamber  39 , i.e., being connected to a device/mechanism that is capable of moving up and down. In this embodiment, a servo motor  42  is linked to the annular plunger  41  like the annular plunger  35 , and the annular plunger  41  is allowed to move up and down in the core layer measuring chamber  39  in the up-and-down direction. Upon moving the annular plunger  41  up and down, a core layer measuring portion  39   a  formed under the annular plunger  41  in the core layer measuring chamber  39  undergoes the expansion and contraction in the up-and-down direction to increase and decrease the volume. The servo motor  42  is connected to control means that is not shown and is electrically controlled. 
     A main layer feed port  44  on the upstream side of the main layer flow passage  37  arranged on the outer side of the die head  7  is connected to main layer molten resin feed means  45 . The molten resin feed means  45  includes an extruder  46  and a gear pump  47  connected to the downstream side thereof. The main layer molten resin in the molten state extruded from the extruder  46  is fed to the main layer flow passage  37  through the gear pump  47 . 
     A core layer feed port  49  on the upstream side of the core layer flow passage  43  arranged on the inner side of the die head  7  is connected to core layer molten resin feed means  50 . The molten resin feed means  50  includes an extruder  51  and a gear pump  52  connected to the downstream side thereof. The core layer molten resin in the molten state extruded from the extruder  51  is fed to the core layer flow passage  43  through the gear pump  52 . 
     Next, described below is the operation of the extrusion-forming machine according to the first embodiment of the present invention. 
     Owing to the above-mentioned constitution, the extrusion-forming machine  2  shown in  FIG. 1  heats, melts and kneads the synthetic resin materials such as polyethylene terephthalate and the like, and conveys the multi-layer drop (molten resin)  8  to the die head  7 . 
     In the initial state as shown in  FIG. 3  and  FIG. 4A , the lift valve  33  is arranged at the descended position to place the opening/closing valve W in the closed state. Therefore, the core layer flow passage  43  is not communicated with the ejection port  20 . Further, the annular plunger  35  in the main layer measuring chamber  34  and the annular plunger  41  in the core layer measuring chamber  39  are arranged at the descended positions, respectively. The annular plungers  35  and  41  are imparted with upward pushing forces (loads) due to the flow of the molten resins being controlled by the servo motors  36  and  42  to which they are connected. Upon receiving the pushing forces, the annular plungers  35  and  41  ascend to set their positions or to set their ascending speeds relative to the measuring chamber  39 . Here, the main layer molten resin is pressure-fed from the main layer molten resin feed means  45  to the main layer feed port  44 , and is nearly continuously pressure-fed to the nozzle  31   c  on the downstream side passing through the main layer flow passage  37 . 
     Thus, the main layer molten resin is nearly continuously pressure-fed from the main layer molten resin feed means  45  and the core layer molten resin is continuously pressure-fed from the core layer molten resin feed means  50 . In the main layer flow passage  37 , the front end (lower end) portion of the annular plunger  35  is arranged at the descended position where it faces the main layer flow passage  37 , and the pressure-feeding force of the molten resin exerts a force on the front end portion of the annular plunger  35  so as to push it up. As described above, the annular plunger  35  is operated by the servo motor  36  and is resisting against the pushing force of the main layer molten resin. Here, the servo motor  36  gradually lifts the annular plunger  35  while it is receiving the pushing force up to a predetermined height in the main layer measuring chamber  34  as shown in  FIG. 4B  so that the main layer molten resin is contained in the main layer measuring portion  34   a  and is measured. Due to this action, the main layer molten resin is contained in the main layer measuring portion  34   a  and desirably fills the main layer measuring portion  34   a  without gap in a state of receiving a pressure and, desirably, a predetermined pressure. 
     Due to the pressure-feeding force of the main layer molten resin as described above, part of the main layer molten resin fills the main layer measuring portion  34   a;  i.e., the molten resin is contained in the main layer measuring portion  34   a  so that it is filled with the main layer molten resin of a predetermined amount while the remaining molten resin nearly continuously flows into the nozzle  31   c  on the downstream side. 
     Near the valve hole  48  (see  FIG. 3 ) of the core layer flow passage  43 , on the other hand, the front end portion of the annular plunger  41  is arranged at the descended position where it faces the core layer flow passage  43 . In this state, the opening/closing valve W comprising the lift valve  33  and the valve hole  48  is closed and, therefore, the core layer flow passage  43  is closed, whereby a force acts on the annular plunger  41  to push it up due to the pressure-feeding force of the core layer molten resin. The annular plunger  41  is operated by the servo motor  42  and is resisting against the pushing force of the core layer molten resin. Here, the servo motor  42  gradually lifts the annular plunger  41  while it is receiving the pushing force up to a predetermined height in the core layer measuring chamber  39  as shown in  FIG. 4B  so that the core layer molten resin is contained in the core layer measuring portion  39   a  and is measured. Due to this action, the core layer molten resin is contained in the core layer measuring portion  39   a  and desirably fills the core layer measuring portion  39   a  without gap in a state of receiving a predetermined pressure; i.e., the core layer molten resin of a predetermined amount is filled therein (measured). 
     After the core layer molten resin of the predetermined amount is filled in the core layer measuring chamber  39 , the lift valve  33  is lifted up as shown in  FIG. 4C  to open the opening/closing valve W so that the core layer flow passage  43  is communicated with the nozzle  31   c.  Next, as shown in  FIG. 5A , the servo motor  42  is operated by a control unit that is not shown to descend the annular plunger  41  on the core side to extrude the core layer molten resin in the core layer measuring portion  39   a  into the nozzle  31   c  through the valve hole  48 . The main layer molten resin has already been pressure-fed into the nozzle  31   c,  and the core layer molten resin is ejected as a dumpling into the main layer molten resin layer b (a sign b is attached to the main layer molten resin that has flown into the nozzle  31   c ) in the nozzle  31   c.    
     Referring to  FIG. 5B , after the core layer molten resin a (a sign a is attached to the core layer molten resin of the shape of a dumpling ejected into the nozzle  31   c ) is ejected, the lift valve  33  is descended down to the valve hole  48  while maintaining the annular plunger  41  on the core side at the descended position to thereby close the opening/closing valve W. 
     Referring, next, to  FIG. 5C , the annular plunger  35  on the main layer side is descended by the control device to eject the molten resin in the main layer measuring portion  34   a  into the nozzle  31   c  (hereinafter called cleaning shot, and a sign c is attached to the molten resin ejected by the cleaning shot). The molten resin c ejected by the cleaning shot plays the role of flowing away the core layer molten resin adhered to the front end of the lift valve  33  with the main layer resin. 
     Due to the above cleaning shot and the flow of the main layer molten resin from the main layer flow passage  37  to the nozzle  31   c  when the state of  FIG. 4A  is resumed, the core layer molten resin a ejected like a dumpling is wrapped at its upper portion, too, with the main layer molten resin b to form a composite molten resin. 
     Thus, the core layer molten resin a is wrapped in the main layer molten resin b, and the composite molten resin flowing through the nozzle  31   c  is discharged from the ejection port  20  of the die head  7 , and is cut at a desired position into a multi-layer molten resin mass (multi-layer drop) (usually, the composite molten resin is so cut that the core layer molten resin a is arranged nearly in the center of the drop). 
     If described from forming the multi-layer drop through to forming a preform by way of the forming system  1  of  FIGS. 1 and 2 , the composite molten resin including the core layer extruded from the ejection port  20  of the die head  7  is cut between the core layers by the cutter  17  shown in  FIG. 2 , and is separated away from the ejection port  20  to form a multi-layer drop  8 . When the multi-layer drop  8  is formed from the composite molten resin and is cut away from the ejection port  20 , the first and second holding members  15  and  16  are closed to hold the multi-layer drop  8 . The multi-layer drop  8  held by the cutting/holding unit  14  of the closed state is moved to a position over a metal mold (female mold)  30  of the compression-forming device  4 . The composite molten resin is fed to the metal mold  30 , and a preform is formed by compression-forming. The preform is cooled and is, thereafter, handed over to the discharge device  5  (see  FIG. 1 ). 
     According to this embodiment as described above, the servo motors  36  and  42  are driven by the control devices that are not shown to control the positions of the annular plungers  35  and  41  or their ascending/descending speeds, to measure the ascending/descending timing, to eject the core layer molten resin, and to clean-shoot the main layer molten resin to thereby form a desirable composite molten resin. In measuring the molten resin, the annular plungers  35  and  41  ascend being controlled by their corresponding servo motors  36  and  42  while receiving loads against the pressure-feeding forces of the molten resins so that the molten resins are contained in the measuring portions  34   a  and  39   a,  effectively preventing the air from being trapped in the measured molten resins or preventing the formation of vacuum bubbles, correctly measuring the amounts of molten resins in the measuring portions  34   a  and  39   a , and ejecting them. In particular, the core layer molten resin a is ejected by the measured amount (does not drip down from the main layer resin passage like the main layer molten resin b) and is ejected in a correct amount compounded by the valve opening/closing operation by the lift valve  33 . Therefore, even if the core layer molten resin feed means  50  is not provided with the gear pump  52 , the core layer molten resin a can be ejected in a correct amount owing to the controlled ascending/descending motion of the annular plunger  41  and the lift valve  33 . 
     The rates of feeding the molten resins of the main layer molten resin feed means  45  and of the core layer molten resin feed means  50  are set constant so that the masses (multi-layer drops of the multi-layer molten resin are successively obtained in constant amounts and in a state where the core layer resin is arranged nearly at the predetermined position. Further, the annular plungers  35 ,  41  and the lift valve  33  are reciprocally moved at a constant period and, besides, the frequency the cutter  17  passes over the ejection port  20  (period of cutting the composite molten resin by the cutter  17 ) is set constant. Desirably, further, the period of reciprocal operation of the annular plungers  35 ,  41  and the lift valve  33  is brought into agreement with the period of cutting the composite molten resin by the cutter  17 . Here, it is desired to maintain synchronism so that the motions of the annular plunger  35 , annular plunger  41 , lift valve  33  and cutter  17  will not gradually undergo deviation. For instance, based on a moment the cutter  17  has passed over the ejection port  20 , the operation timings of the annular plunger  35 , annular plunger  41  and lift valve  33  are maintained, i.e., the ejection start timings of the annular plunger  35 , annular plunger  41  and lift valve  33  may be so instructed as to be deviated by desired periods from the above moment that is based upon in order to obtain a desired multi-layer drop. Or, based on a given operation timing of the annular plunger  35 , annular plunger  41  or lift valve  33 , the operation timings of the other plunger and cutter  17  may be instructed. Or, the annular plungers  35 ,  41 , lift valve  33  and cutter  17  may be electrically controlled so as to be successively operated being linked to each other, or their drive systems may be mechanically linked together. 
     Next, described below is the die head for forming the multi-layer resin according to a second embodiment of the invention and the extrusion-forming machine having the same. 
     In the above first embodiment, a two-kind-three-layer composite molten resin was formed through the die head by using the main layer and the core layer of molten resins of different kinds. Now, this embodiment forms a two-kind-three-layer or a three-kind-three layer composite molten resin. 
     Referring to  FIG. 6 , the die head  7  has an outer periphery of a circular shape in transverse cross section, and includes an outer block  55  arranged on the outer circumferential side. The outer block  55  has nearly an annular shape forming a large-diameter hole  55   a  at an intermediate position thereof in the up-and-down direction, forming an intermediate-diameter hole  55   b  on the upper side of the large-diameter hole  55   a,  i.e., on the upper side of the outer block  55 , and forming a nozzle  55   c  on the lower side of the large-diameter hole  55   a,  i.e. , on the lower side of the outer block  55 . The lower side of the large-diameter hole  55   a  is formed in the shape of an inverse circular truncated cone. 
     An inner moving block  56  is arranged in the intermediate-diameter hole  55   b  and in the large-diameter hole  55   a.  In the inner moving block  56 , a cylindrical inner hole  56   a  is formed having the same diameter on the upper side thereof. The lower portion of the inner hole  56   a  is formed in the shape of an inverse conical truncated cone. 
     In the center of the inner hole  56   a,  a shaft-like valve  57  is arranged in the up-and-down direction. The shaft-like valve  57  is forming a valve body  57   a  at its front end portion, and is provided with a core layer flow passage  67  that forms a transverse flow passage  67   a  in the upper part of the valve body and extending radially or in one or in two directions, and a longitudinal flow passage  67   b  in the center in the axial direction thereof on the upper side of the valve body  57   a.    
     Being positioned in the large-diameter hole  55   a,  a main layer measuring chamber  58  is formed in an annular region between the outer block  55  and the inner moving block  56 , and a main layer flow passage  61  is formed on the lower side of the main layer measuring chamber  58 . In the main layer measuring chamber  58 , an annular plunger  59  is arranged to move back and forth in the up-and-down direction in the main layer measuring chamber  58 , i.e., being connected to a device/mechanism that is capable of moving up and down. A servo motor  60  is linked to the annular plunger  59  enabling the main layer measuring chamber  58  to move up and down. Upon moving the annular plunger  59  up and down, a main layer measuring portion  58   a  formed under the annular plunger  59  in the main layer measuring chamber  58  undergoes the expansion and contraction in the up-and-down direction to increase and decrease the volume. The servomotor  60  is connected to control means that is not shown and is electrically controlled. 
     The inner moving block  56  can be ascended and descended by a lift mechanism, preferably, by a servo motor  80 , forms a valve body  56   b  at the lower end portion thereof, closes a valve hole  55   e  formed on the downstream side of the main layer flow passage  61  in the outer block  55 , and forms an opening/closing valve Z relying upon the valve hole  55   e  and the valve body  56   b  (valve hole  55   e  and valve body  56   b  are generally referred to as third opening/closing valve Z) to open and close the main layer flow passage  61 . In a state where the third opening/closing valve Z is opened, the main layer flow passage  61  is communicated with the nozzle  55   c  on the downstream side at all times and is communicated with the ejection port  20  of the die head  7  which is further on the downstream side. 
     Being positioned in the inner hole  56   a  of the inner moving block  56 , a core layer measuring chamber  63  is formed in an annular region between the inner moving block  56  and the shaft-like valve  57 , and a valve hole  72  is formed on the lower side of the core layer measuring chamber  63 . The valve hole  72 , together with the valve body  57   a  of the shaft-like valve  57 , opens and closes the core layer flow passage  67 . In a state shown in  FIG. 6 , the opening/closing valve X (valve body  57   a  and valve hole  72  together are referred to as first opening/closing valve X) assumes the opened state. The shaft-like valve  57  may be rendered to move up and down to open and close the first opening/closing valve X. In this embodiment, however, the inner moving block  56  moves up and down to open and close the first opening/closing valve X. Therefore, the shaft-like valve  57  does not move up and down. 
     In the core layer measuring chamber  63 , an annular plunger  65  is arranged to move back and forth in the up-and-down direction in the core layer measuring chamber  63 , i.e., being connected to a device/mechanism that is capable of moving up and down. As the device/mechanism for moving the plunger  65  up and down, there can be exemplified an air cylinder or a hydraulic cylinder. In this embodiment, a servo motor  66  is linked to the annular plunger  65  enabling the core layer measuring chamber  63  to be ascended and descended in the up-and-down direction. Upon moving the annular plunger  65  up and down, the core layer measuring portion  63   a  formed under the annular plunger  65  in the core layer measuring chamber  63  undergoes the expansion and contraction in the up-and-down direction to increase and decrease the volume. The servo motor  66  is connected to control means that is not shown and is electrically controlled. 
     In a state where the annular plunger  65  is lowered, the inner circumferential surface of the annular plunger  65  closes the transverse flow passage  67   a  formed in the shaft-like valve  57 , and closes the core layer flow passage  67  (see  FIG. 7B ). Therefore, the core layer flow passage  67  is forming the opening/closing valves at portions in a number of the transverse flow passages  67   a  formed substantially radially or in one or two directions or, for example, in  FIG. 6 , at the right and left two places (the opening/closing valve formed by the annular plunger  65  and the transverse flow passage  67   a  is referred to as second opening/closing valve Y) . Upon opening the first opening/closing valve X and the second opening/closing valve Y, the core layer flow passage  67  is communicated with the nozzle  55   c  and is, further communicated with the ejection port  20  of the die head  7  on the downstream side. 
     An outer main layer flow passage  81  is formed under the main layer flow passage  61  of the die head  7 . The outer main layer flow passage  81  is communicated with the nozzle  55   c  at all times and is, further, communicated with the ejection port  20  of the die head on the downstream side. 
     A main layer feed port  68  on the upstream side of the main layer flow passage  61  arranged on the outer side of the die head  7  is connected to main layer molten resin feed means  69 . The molten resin feed means  69  includes an extruder  70  and a gear pump  71  connected to the downstream side thereof. The main layer molten resin in the molten state extruded from the extruder  70  is fed to the main layer flow passage  61  through the gear pump  71 . 
     A core layer feed port  73  on the upstream side of the core layer flow passage  67  arranged on the inner side of the die head  7  is connected to core layer molten resin feed means  74 . The molten resin feed means  74  includes an extruder  75  and a gear pump  76  connected to the downstream side thereof. The core layer molten resin in the molten state extruded from the extruder  75  is fed to the core layer flow passage  67  through the gear pump  76 . 
     A main layer feed port  82  on the upstream side of the outer main layer flow passage  81  arranged on the lower side of the die head  7  is connected to outer main layer molten resin feed means  83 . The molten resin feed means  83  includes an extruder  84  and a gear pump  85  connected to the downstream side thereof. The main layer molten resin in the molten state extruded from the extruder  84  is fed to the outer main layer flow passage  81  through the gear pump  85 . 
     Next, described below is the operation of the extrusion-forming machine according to the second embodiment of the present invention. 
     In the initial state as shown in  FIG. 6  and  FIG. 7A , the inner moving block  56  is arranged at the descended position to open the first opening/closing valve X. The annular plunger  65  on the core side is arranged at the ascended position in a state of having measured the amount of the core layer molten resin in advance or through the above step of ejection through steps of  FIG. 7C  to  FIG. 8A  that will be described below, and the second opening/closing valve Y is in the opened state. Therefore, the core layer flow passage  67  is communicated with the ejection port  20 . Thereafter, the inner moving block  56  is arranged at the descended position to close the third opening/closing valve Z, and the annular plunger  59  in the main layer measuring chamber  58  is arranged at the descended position. In this embodiment, the molten resins of the same kind are layer pressure-fed from the main layer molten resin feed means  69  and from the outer main layer molten resin feed means  83 , and the molten resin of a different kind is pressure-fed from the core layer molten resin feed means  74 . 
     In this state, the outer main layer molten resin is pressure-fed from the outer main layer molten resin feed means  83  to the main layer feed port  82 , and is nearly continuously pressure-fed to flow into the nozzle  55   c  and the ejection port  20  passing through the outer main layer flow passage  81 . Further, the main layer molten resin is nearly continuously pressure-fed from the main layer molten resin feed means  69  to the main layer feed port  68 . Similarly the core layer molten resin is nearly continuously (or intermittently in case the core layer flow passage  67  is completely closed by the second opening/closing valve Y) pressure-fed from the core layer molten resin feed means  74  to the core layer feed port  73 . 
     In the main layer flow passage  61 , the front end portion of the annular plunger  59  is arranged at the descended position where it faces the main layer flow passage  61 , and the pressure-feeding force of the molten resin exerts a force on the annular plunger  59  so as to push it up. The annular plunger  59  is operated by the servo motor  60  and is resisting against the pushing force of the molten resin. Here, the servo motor  60  gradually lifts up the annular plunger  59  while it is receiving the pushing force up to a predetermined height in the main layer measuring chamber  58  as shown in  FIG. 7B  so that the main layer molten resin is contained in the main layer, measuring portion  58   a  and is measured. Due to this action, the main layer molten resin is contained in the main layer measuring portion  58   a  and desirably fills the main layer measuring portion  58   a  without gap in a state of receiving a predetermined pressure. 
     Due to the pressure-feeding force of the main layer molten resin as described above, the main layer molten resin fills the main layer measuring portion  58   a;  i.e., the molten resin is contained in the main layer measuring portion  58   a  so that it is filled with the main layer molten resin of a predetermined amount. 
     Near the transverse flow passage  67   a  of the core layer flow passage  67 , on the other hand, the annular plunger  65  on the core layer side is lowered as shown in  FIG. 7B  to extrude the core layer molten resin filled in the core layer measuring portion  63   a  in the preceding step into the nozzle  55   c.  The outer main layer molten resin (hereinafter, a sign d is attached to the outer main layer molten resin that flows into the nozzle  55   c ) has already been pressure-fed into the nozzle  55   c,  and the core layer molten resin a is ejected as a dumpling into the outer main layer molten resin layer d in the nozzle  55   c.  At this moment, the annular plunger  65  closes the transverse passage  67   a  of the shaft-like valve  57  to thereby close the second opening/closing valve Y, and interrupts the core layer molten resin from flowing into the core layer measuring portion  63   a.    
     Next, as shown in  FIG. 7C , the inner moving block  56  is arranged at the ascended position nearly simultaneously with the annular plunger  65  on the core layer side and at the same speed to close the first opening/closing valve X, to interrupt the flow of the core layer molten resin through the core layer flow passage  67 , and to open the third opening/closing valve Z so that the main layer flow passage  61  is communicated with the nozzle  55   c.    
     Next, as shown in  FIG. 8A , the core layer measuring portion  63   a  starts measuring the core layer molten resin. That is, in the transverse flow passage  67   a  of the core layer flow passage  67 , the front end portion of the annular plunger  65  is arranged at a position where it faces the core layer flow passage  67 . In this state, the first opening/closing valve X is closed and whereby a force acts on the annular plunger  65  on the core layer side to push it up due to the pressure-feeding force of the core layer molten resin. 
     The annular plunger  65  is operated by the servo motor  66  and is resisting against the pushing force of the core layer molten resin. Here, the servo motor  66  gradually lifts up the annular plunger  65  while it is receiving the pushing force up to a predetermined height in the core layer measuring chamber  63  as shown in  FIG. 8A  so that the core layer molten resin is contained in the core layer measuring portion  63   a.  Due to this action, the molten resin is contained in the core layer measuring portion  63   a  and fills the core layer measuring portion  63   a  without gap in a state of receiving a pressure, preferably, a predetermined pressure; i.e., the core layer molten resin is contained in the core layer measuring portion  63   a  so that it is filled with the core layer molten resin of a predetermined amount (measured). 
     In the main layer measuring chamber  58 , on the other hand, the annular plunger  59  on the main layer side is lowered by the control device to eject the molten resin in the main layer measuring portion  58   a,  and the molten resin c is ejected by cleaning shot to the front end portion of the lift valve  57  to flow away the core layer molten resin adhered to the end of the lift valve  53  with the main layer resin. The main layer molten resin c is extruded into the inside of the outer main layer molten resin d following the core layer molten resin a. 
     Next, as shown in  FIG. 8B , the inner moving block  56  is moved to a descended position to open the first opening/closing valve X and to close the third opening/closing valve Z. Thus, the die head  7  assumes the initial state shown in  FIG. 6  and  FIG. 7A  to complete a cycle of the step of extruding the molten resin. 
     The composite molten resin  8  including the core layer extruded from the ejection port  20  of the die head  7  is separated away from the ejection port  20  like in the above first embodiment, and is conveyed into the metal mold (female mold)  30  of the compression-forming device  4 ; i.e., the multi-layer drop  8  is fed into the metal mold  30  in which a preform is compression-formed. This embodiment, too, is capable of forming a composite molten resin like the above first embodiment. It is, further, possible to pressure-feed the main layer molten resin (molten resin of a different kind) different from the outer main layer molten resin and the core layer molten resin from the main layer molten resin feed means  69  (means  69  for feeding the molten resin of a different kind) to thereby form a three-kind-three-layer composite molten resin in which the molten resin of the different kind is arranged on the core layer of the form of a dumpling. 
     Like the first embodiment, the second embodiment, too, is provided with a cutter for cutting the composite molten resin, and the cutting period is brought into agreement or synchronism with the operation period of the annular plungers  59 ,  65  and the inner moving block  56  in order to successively obtain multi-layer drops in a constant amount and in a state where the core layer resin is arranged nearly at a predetermined position, which is desirable. 
     Though the embodiments of the invention were described above, the invention can be variously modified or altered without departing from the technical spirit of the invention, as a matter of course. 
     For example, the above first embodiment forms the two-kind-three-layer composite molten resin and the second embodiment forms the two-kind-three-layer or three-kind-three-layer composite molten resin. However, it is also possible to form many other many-kind-many-layer composite molten resin. For instance, the kinds and number of the layers can be increased by, further, providing a new measuring chamber, a plunger, an opening/closing valve and a flow passage on the inside (center side) of the core layer molten resin measuring chamber, or by, further, arranging another resin flow passage on the outer circumference of the main layer flow passage and the outer main layer resin flow passage. In the above embodiments, further, at the time of measuring the molten resins, loads were applied by the servo motors  36 ,  42 ,  60  and  66  to the annular plungers  35 ,  41 ,  59  and  65  to take measurements. However, it is also allowable to combine the servo motors with a known mechanism, or to use the above devices/mechanisms that move up and down instead of using the servo motors. It is, further, allowable to omit the servo motors but to control the molten resin ejection pressures of the extruders and gear pumps of the molten resin feed means in order to measure the molten resins relying only upon the loads of the molten resin pushing forces in compliance with the lift up (moving back) of the plungers. 
     The gear pump in the molten resin feed means may be omitted when the molten resin is measured by exerting a load by controlling the annular plunger that moves up and down. 
     The compression-forming machine that uses the multi-layer drops is not limited to the rotary type (Carousel) compression-forming machine only of this embodiment but may be a drop-fed compression-forming machine or a batchwise many-drop-fed compression-forming machine. Further, the multi-layer drops are not limited to be for being compression-formed but may also be used, for example, as pellets. Moreover, the composite molten resin may be cut into multi-layer drops containing the core layer by halves or may be cut into an elongated form containing a plurality of core layers instead of being cut into drops each containing a dumpling-like core layer. To obtain the multi-layer drops containing the core layer by halves, the cutting period of the cutter may be set to be twice as long as the operation period of the plungers and the valve mechanisms. To obtain the multi-layer drops in an elongated form containing a plurality of core layers, the operation period of the plungers and the valve mechanisms may be set to be an integer of times (an integer of  2  or larger) of the cutting period of the cutter. Further, the position of the core layer may be deviated to be higher or lower than the center of the drops by adjusting the operation timing of the plungers and the valve mechanisms and the cutting timing of the cutter. Or, the core layer may be divided to be arranged to the upper end and the lower end of the multi-layer drop so that the main layer only is arranged in the center. 
     Though not particularly referred to, the extrusion-forming machine  2  may be provided with a plurality of die heads  7  as shown in  FIG. 9 . This embodiment uses the die head  7  of the form based on the above second embodiment, the die head  7  having two die head portions  7   a  and  7   b  which are connected to the main layer molten resin feed means  45  (or  69 ), core layer molten resin feed means  50  (or  74 ) and outer main layer molten resin feed means  83  through flow passages. The flow passage of the main layer molten resin feed means  45  is represented by a solid line, the flow passage of the core layer molten resin feed means  50  is represented by a dotted line and the flow passage of the outer main layer molten resin feed means  83  is represented by a dot-dash chain line. In the case of the die head  7  of the form of the first embodiment, the outer main layer molten resin feed means  83  may be suitably omitted or its operation may be stopped. 
     When three or more die heads are used, they may be suitably arranged, e.g., linearly, like a lattice, annularly or radially. 
     When the plurality of die heads  7  are provided as described above, the force for pressure-feeding the molten resin from the upstream of the flow passage may be suitably controlled by the molten resin feed means or the mechanisms/devices (e.g., servo motors) for moving the plungers up and down may be suitably controlled so that a desired composite molten resin is nearly uniformly ejected from the die heads  7 .