Abstract:
An apparatus for injection molding two compatible polymeric materials, in which two substantially coaxial extrusion screws are used to plasticize the two materials into a common accumulation space. The charge comprising multiple layers of material is then injected into a closed mold by means of forward axial motion of the outer screw with respect to its enclosing barrel. Once inside the mold, the first material forms a skin layer totally or partially surrounding the other material. In this way a part having a plurality of material properties may be produced in a single operation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/230,546 filed Sep. 5, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The most common commercial form of injection molding machine is known as a “reciprocating screw”. In this type of machine, thermoplastic polymer is melted, mixed, and conveyed by means of a screw having one or more flights rotating within a heated pressure vessel. The screw is also permitted to translate axially to allow for the accumulation of melted material at the end of the screw. When sufficient melt has accumulated, the screw is stopped and translated forward to inject the melted material into a closed mold. In common practice a non-return valve is situated at the downstream end of the screw to prevent back flow into the screw flights during the injection portion of the cycle. 
     A variation of this process, known as co-injection or sandwich molding, has been commercially practiced for a number of years. In its most common embodiments shown in FIGS. 1 and 2, co-injection is achieved by means of a molding machine fitted with two or more plasticizing units, each one containing a reciprocating screw  12 ,  14  enclosed in a separate heated barrel  16 ,  18 . The output of these plasticizing units is brought together by a system of manifolds  20  which convey the several materials to the point of injection  22  into the mold  24 . According to well known principles of viscous flow, the first material to enter the mold remains substantially on the outside of the molded part, and material injected later remains substantially in the core of the part. The resulting “sandwich” construction yields a number of advantages, the principle ones being: (1) to make a part with a chemically foamed core, gaining the light weight, low pressure, and flat surfaces of a foam part without the characteristic streaky exterior, (2) to use low cost recycled, “off-spec” or uncolored material where it is not visible, and (3) to make a part with different properties on the inside and outside, as for instance the presence or absence of reinforcing fibers or other property-changing additives. 
     These advantages are offset by the high cost and complexity of a machine requiring two or more independent reciprocating screws, together with the associated controls for simultaneous and/or sequential injection. Numerous attempts have therefore been made to reduce this complexity by having at least the injection function be performed by a single element, building a composite shot containing a plurality of melted materials within a single accumulation space. Examples of these attempts include U.S. Pat. No. 4,978,493 to Kersemakers et al., U.S. Pat. No. 3,966,372 to Yasuike et al., and U.S. Pat. No. 5,443,378 to Jaroschek et al. In all of these examples, at least one secondary extrusion screw and barrel is caused to communicate with the primary barrel by means of a melt-carrying manifold structure through which the secondary portion of the shot is charged. Because of the multiple barrels and screw drives, machines of this type still have disadvantages involving the high initial cost of the required components and associated control capability. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a device and method whereby the sandwich molding process can be performed within the space of a single heated barrel, and furthermore by means of a machine having a single rotational screw drive. 
     Another object of the present invention is provide a device and method for making possible adaptation of a conventional single-material molding machine to perform the sandwich molding process with relatively minor and low-cost changes. 
     A further object of the present invention is to provide an injection unit having an externally heated pressure vessel hereafter called the “barrel” enclosing a primary, outer, screw having one or more flights. The screw being movable rotationally and axially to respectively plasticize and inject a metered quantity of melted thermoplastic material. The screw has a hollow cylindrical bore extending through its length which forms the enclosure for a second, inner, screw having a smaller diameter than the primary screw. The secondary screw remains substantially fixed in its axial orientation with respect to the primary screw, but can be rotated within the primary screw to plasticize material. The downstream end of the bore in the primary screw is provided with an opening to allow material from the secondary screw to be extruded into an accumulation space in front of the primary screw. The opening is preferably provided with a non-return valve to keep material from leaking back into the secondary screw. 
     Another object of the present invention is to provide a clutch mechanism between the primary and secondary screws to transmit rotation to the respective screw elements. Rotational motion is applied by means of a hydraulic or electric motor to the upstream end of the secondary screw. With the clutch closed, relative rotation occurs only between the primary screw and the outside barrel causing primary material to be extruded. With the clutch opened, relative rotation occurs between the two screws, causing secondary material to be extruded. 
     Another object of the invention is to provide a secondary feed throat to convey granular raw material to the inner screw through openings cut through the wall of the hollow outer screw. 
     A further object of the invention is to provide a machine for forming objects having: (1) a first rotatable screw; (2) a second rotatable screw; and (3) a single screw drive for selectively causing rotation of the first rotatable screw and the second rotatable screw. This machine can also include: (a) a single barrel containing the first rotatable screw and the second rotatable screw; and/or (b) a clutch for selectively connecting the single screw drive to one of: (1) the first rotatable screw; and (2) both the first rotatable screw and the second rotatable screw. Furthermore, the first rotatable screw and the second rotatable screw can be substantially coaxial. Also, the first rotatable screw can operate on a first material; and the second rotatable screw can operate on a second material such that each of the objects is formed from a combination of the first material and the second material. 
     An additional object of the invention is to provide a machine for forming objects having: (1) a barrel; (2) a first rotatable screw; (3) a second rotatable screw; and (4) an accumulation space for forming a shot, such that the barrel surrounds at least a portion of each of the first rotatable screw, the second rotatable screw, and the accumulation space. Also, the first rotatable screw can operate on a first material; the second rotatable screw can operate on a second material; and the shot formed in the accumulation space can contain first material and second material. Further, the first material can be a skin material; the second material can be a core material; and at least one of the first rotatable screw and the second rotatable screw can inject the shot into a mold such that skin material encapsulates core material. In addition, the accumulation space can enlarge when at least one of the first rotatable screw and the second rotatable screw move axially. 
     Another object of the invention is to provide a screw assembly for a machine for forming multi-material objects having: (1) an outer screw for operating on a first material, the outer screw having a hollow inner section; and (2) an inner screw for operating on a second material positioned substantially within the hollow inner section of the outer screw; wherein at least one of the outer screw and the inner screw includes a non-return valve to prevent backflow of material into at least one of the outer screw and the inner screw. In addition, each of the outer screw and the inner screw can include a respective non-return valve. Further, the non-return valve can be selected from the group consisting of: a ball type valve, a ring type valve, and a poppet type valve. 
     A further object of the present invention is to provide a machine for forming objects having: (1) a hollow barrel defining a longitudinal direction; (2) a screw for operating on a material in the barrel and having an elongated tip, the screw movable in the longitudinal direction; and (3) a nozzle having an orifice for passage of material from the barrel, and a tip portion for receiving the elongated tip of the screw. In addition, the screw can have a hollow bore which continues into a passageway through the elongated tip to an outlet in the tip. Also, a second screw can be positioned within the hollow bore for operating on a second material in the hollow bore and/or the nozzle can have a storage area for retaining material until the elongated tip substantially reaches an inner face of the orifice. 
     An additional object of the invention is to provide a method of forming multi-material objects having the steps of: (1) engaging a screw drive to cause a first screw to extrude a first material into an accumulation space; (2) engaging the screw drive to cause a second screw to extrude a second material into the accumulation space forming a composite shot; and (3) injecting the composite shot into a mold by axial movement of at least one of the first screw and the second screw. Also, the step of engaging the screw drive to cause the first screw to extrude can include the step of increasing pressure on a clutch disc; and/or the step of engaging the screw drive to cause the second screw to extrude can include the step of decreasing pressure on the clutch disc. 
     Another object of the invention is to provide a method of forming multi-material objects having the steps of: (1) rotating a first screw to cause a first material to be extruded; (2) forming by axial movement of at least one of the first screw and a second screw an accumulation space having a forward side, a rearward side, and a periphery, such that the first material accumulates toward the forward side of the accumulation space; (3) rotating the second screw to cause a second material to be extruded into the accumulation space; (4) enlarging the accumulation space by additional axial movement of at least one of the first screw and the second screw, such that the second material accumulates toward the rearward side and outer periphery of the accumulation space; and (5) forcing the first and second material in the accumulation space into a mold. In addition, the axial movement can be caused by the first material exiting the first screw; and/or the additional axial movement can be caused by the second material exiting the second screw. Also, the first material can be a skin material and the second material can be a core material. 
     A further object of the invention is to provide a method of plugging-off a multi-material object having a core material encapsulated by a skin material having the steps of: (1) filling a storage area in at least one of a barrel and a nozzle with a portion of a skin material, the storage area positioned adjacent a path of an elongated tip of an injector; (2) moving the injector axially such that the elongated tip of the injector is received into a tip portion of the nozzle thereby injecting skin material and core material into a mold; and (3) further moving the injector such that another portion of the injector moves into the storage area thereby causing injection of the portion of skin material into the mold to plug off the object. 
     An additional object of the invention is to provide a method of plugging-off a multi-material object having a core material encapsulated by a skin material having the steps of: (1) filling a storage area in an elongated tip of an injector with a portion of skin material; (2) moving the injector axially such that the elongated tip of the injector is received into a tip portion of the nozzle thereby injecting skin material and core material into a mold; and (3) extruding the portion of skin material from the storage area into the mold to plug off the object. 
     Another object of the invention is to provide a machine for forming multi-material objects having a first screw for operating on a first material and a second screw for operating on a second material within the same barrel, each screw having a non-return value preventing backflow into either screw; and an accumulation space formed by the axial retraction of one or both screws. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a device used for traditional co-injection molding which includes two injection barrels and a manifold to join the melt streams as the streams are being injected; 
     FIG. 2 illustrates another device used for traditional co-injection molding which includes two injection barrels sharing a single accumulator space; 
     FIG. 3 is a schematic view of one embodiment of the present invention; 
     FIG. 4 is a schematic view of a ring-type non-return valve according to one embodiment of the present invention; 
     FIG. 5 is a schematic view of a ball-type non-return valve according to one embodiment of the present invention; 
     FIG. 6 is a schematic view of a poppet-type non-return valve according one embodiment of the present invention; 
     FIG. 7 is a schematic view of an outer screw providing an elongated protrusion outlet according to one embodiment of the present invention; 
     FIG. 8 is a schematic view of one embodiment of the present invention employing a clutch mechanism for a retrofit situation which uses conventional hydraulic back pressure controls of a traditional single barrel molding device to actuate the clutch and brake; and 
     FIG. 9 is a schematic view of another embodiment of the present invention utilizing a gearing mechanism instead of a clutch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 3, one embodiment 100 of the present invention utilizes components from a traditional standard injection molding machine. These components include a heated cylindrical pressure vessel or barrel  102 . The barrel  102  has a cylindrical bore  104  in which plastic material  106  is melted and accumulated. The barrel  102  also has an opening or feed throat  108  through which the raw material  106  is introduced into the cylindrical bore  104 . The end of the bore  104  opposite to the feed throat  108  is provided with a nozzle including a substantially conical section  110  in which the bore  104  diameter is reduced to terminate in a small-diameter channel  112  which communicates with the mold (not shown). 
     Like a traditional standard injection molding machine, this embodiment of the present invention is further provided with an injection platen  114  mounted on guide ways  116  such that the injection platen  114  can move in a linear direction by means of hydraulic cylinders  118 . In a traditional embodiment, the hydraulic cylinders  118  pull-in the platen  114  to which a single screw would be attached to provide injection force on the melt. The platen  114  further includes a mounting point  120  for a rotational drive  122  to rotate the traditional single screw. The rotational drive  122  is typically a hydraulic or electric motor capable of high torque at low rotational speeds. 
     The present invention departs from the form of a traditional standard injection molding machine at least in that the single plasticizing screw member is replaced by two coaxial screw members  130 ,  132 . The outer or outside screw  132  has at least one external thread  134  to convey material forward inside the barrel  102 . A means, such as an open space, is provided to allow melted material  106  to exit the end of the threaded portion of the outer screw  132  and accumulate in front of the outer screw. Further means, such as a check valve, is provided to prevent the reverse from happening, that is to keep material so accumulated from reentering the threaded portion of the outer screw  132 . A ring-type check valve  136  is one traditional means for providing both these functions. The outer screw  132  is further provided with a cylindrical bore  138  which forms the enclosure for a secondary, inner or inside screw  130  of smaller diameter than the primary outer screw  132 . The inner screw  130  also has at least one external thread  140  to convey and melt plastic material  142  which in turn is caused to flow through an opening  144  in the tip of the outer screw  132 , into a common accumulator space  146 . The opening  144  is also provided with a non-return valve  148 . As melt  106 ,  142  is accumulated, the volume of the accumulator space  146  is allowed to increase by means of backward motion of the injection platen  114  and the screws  130   e ,  132  attached thereto. 
     Rotation of the inside screw  130  is caused by a substantially solid connection to said rotational drive  122  on the injection platen  114 . Similar to the single screw of a traditional injection molding machine, splines  150  are machined into the end of the inner screw  130  and are fitted into matching splines  152  of a spline bushing in the spline shaft  154  of the rotational drive  122  which is mounted to the platen  114 . A thrust bearing (not shown) is provided to protect the screw motor of the rotational drive  122  from the full injection force. Rotation of the outer screw  132  is caused by selective actuation of a clutch mechanism  160  connected on one side to splines  162  in the outer screw  132  and on the other side to the rotational drive  122 . When the clutch mechanism  160  is engaged, there is no relative rotation between the inner screw  130  and the outer screw  132 . When the clutch mechanism  160  is disengaged, only the inner screw  130  is powered. Optionally, means to provide braking force, such as the Belleville spring discussed below, can be provided for the outer screw  132  while the clutch mechanism  160  is disengaged. 
     Raw skin material  142  is introduced to the threaded portion of the inside screw  130  by means of openings  170  in the wall  172  of the hollow outside screw  132 . A secondary feed throat  174  is provided to contain and direct the skin material  142  into those openings  170 . 
     One important aspect of the present invention that is the system of check valves  136 ,  148 . Although a primary function of a check valve is to prevent back-flow into the screw threads, development of the present invention has shown that the shape and configuration of the check valve  148  of the inner screw  130  has a substantial influence on the resulting product. Experimentation has shown that a simple hole  180  (FIG. 4) or a ball-check  182  (FIG. 5) produce a spherical front to the material in the inner screw  130 . However, one preferred embodiment of the check valve has been shown to be a poppet type  184  (FIG. 6) with an annular exit path  186 . One advantage of the poppet type valve  184  is the total isolation of inner screw  130  which allows the operation of the machine in a single-material mode, that is both screws  130 , 132  extruding the same material, with no chance of material contamination or degradation which may be created by unavoidable dead spots in the flow of materials caused by traditional multilayer mold machine designs. 
     Another important aspect of the present invention is that a later and more abrupt transition from skin material  142  to core material  106  during injection has been found to be desirable. This later and more abrupt transition has been achieved by extruding the charge of core material  106  in a torus or doughnut shape rather than a more spherical shape. Thus, if the core material  106  is extruded toward the periphery of the accumulation space and/or rearwardly, that is on the screw assembly side of the accumulation space  146 , virtually all contamination of the skin material  142  by the core material  106  on the surface of the finished product can be avoided. 
     In embodiments of the present invention where the core material  106  is extruded from the outer screw  132 , the torus shape is achieved by building the shot in reverse, that is, the skin material  142  is first extruded from the inner screw  132  and the core material  106  is then extruded from the outer screw  130  behind the extruded skin material  142  thereby forming the torus shape around the tip of the outer screw  132 . 
     However, in other embodiments of the present invention the core material  106  is extruded from the inner screw  132  and the skin material  142  is extruded from the outer screw  132 . This embodiment occurs, for example, when the inner screw  130  is fed core material  106  from feed throat  174  while the outer screw  132  is fed skin material  142  from feed throat  108 . In these types of embodiments, to achieve the later and more abrupt transition, the poppet type valve  184  is desirably employed both to reduce the minimum amount of metal in contact with both melt streams thereby allowing for a more rapid switchover during injection, and to extrude the core material  106  toward the periphery of the accumulation space  146 . 
     Further, as shown in FIG. 7, one embodiment of the present invention eliminates contamination of the outer surface of molded parts by traces of core material by use of an elongated tip  202  on the outer screw  132 . At the beginning of an extrusion cycle, when the screw assembly  205  is fully forward, that is, the hydraulic cylinders  118  have pulled-in platen  114 , the end of the elongated tip  202  is nearly in contact with the rear face  204  of the machine nozzle  206 . Consequently, as skin material  142  is extruded out of the elongated tip  202 , any traces of core material  106  remaining within the barrel  102  and nozzle  206  are forced back toward the rear of the accumulation space  146 . Subsequent extrusion of core material  106  from the outer screw  132  will produce the desired toroidal shape to the interface between the two materials  106 ,  142 . The elongated tip  202  can contain a check valve  208  of the ball, poppet, or ring type as discussed above. 
     In the embodiment of FIG. 7, heat to melt the skin material  142  in the inside screw  130  is conducted through the wall  172  of the outside screw  132 . For this reason the process requires that the primary and secondary materials  106 ,  142  be processable at similar temperatures. Further heat is generated in the materials as the rotation of the inner screw imparts shear energy to the material. In fact, this shear source usually provides the greater part of the total heat absorbed by the melt. 
     According to the present invention, the inner screw  130  has a smaller diameter than the outer screw  132 . The diameter may be limited by the strength of the hollowed out outer screw  132  in the feed section where the flights are the deepest. For example, the strength of a hollow screw has been defined by the following equation from  Injection Molding Handbook,  Rosato &amp; Rosato, 2 nd  edition, 1995:          D   3     +     KT   S     +     d   3                            
     where: 
     D=the root diameter of the feed section of the outside screw (inches/millimeters) 
     d=the maximum diameter of the hollow core (inches/millimeters) 
     T=the maximum torque applied by the drive motor (foot-pounds/newton-meters) 
     S=the tensile strength of the screw steel (pounds per square inch/bars) 
     K=a constant—102 for English system units, and 9.6 for metric units. 
     Thus, for example, a typical screw on a 68 ounce (1925 gram) machine has a diameter of 3.54 inches (90 millimeters). With a flight depth of 0.44 inches (11 millimeters) in the feed zone, the root of the screw is 2.66 inches (68 millimeters) in diameter. Such a screw is powered by a hydraulic motor capable of 4000 ft-lb (5440 newton-meters), and is composed of 4140 steel with a tensile strength of 100,000 pounds per square inch (6895 bars). Solving for the diameter of the inside hollow core in the equation yields a maximum diameter for the inner screw  130  of 2.45 inches (62 millimeters), which is 69% of the outer screw  132  diameter. The inside bore  138  could be even larger if the outer screw  132  were fabricated from one of the common high-strength screw alloys such as H13 or CPM-9V (tool steel alloys available from Crucible Materials Corporation of Syracuse, N.Y., the latter alloy containing chromium, molybdenum, and 9% vanadium) which has a tensile strength of 285,000 pounds per square inch (19,655 bars). H13 is a generic tool steel designation of the American Iron and Steel Institute (AISI). 
     The throughput rate of an extrusion screw is proportional to the square of its diameter, so a screw having a diameter ⅔ smaller than another screw will have only 44% of the throughput, other factors being equal. This deficiency is partially offset by the fact that the inside screw  130  will necessarily have a higher length/diameter (“L/D”) ratio, which increases throughput. The typical outside screw will have an L/D ratio of 18-20 and the respective inside screw with have of a L/D ratio of 28-32. The L/D ratio of the outside screw  132  will decrease by as much as four diameters as the shot in the accumulation space  146  builds and the screw assembly  205  moves back further reducing its output rate. On the other hand, the inner screw  130  is always engaged to its full length. The smaller inner screw  130  can also be run at higher rotational speeds, further increasing its throughput. In general, extruder throughput is not a limiting factor for sandwich molded parts since these parts tend to have heavy wall sections, in which case cycle time is limited by mold cooling time, not extruder throughput. 
     One goal of the present invention is to provide a retrofit or rebuild option on a standard single-material injection molding machine. The only controlled event in the cycle of the present invention that does not happen in the molding cycle of a conventional single-material molding machine is the engaging and disengaging of the clutch mechanism  160 . However, most conventional molding machines have the capability to apply selective back pressure with the injection cylinders  118  during various portions of the extrusion cycle. Back pressure is used to aid compaction and mixing of the melt. The inside screw  130 , being axially fixed with relation to the outside screw  132 , has no need for externally applied back pressure. Therefore, the selective application of back pressure can be the signal and force for closing the clutch mechanism  160 . 
     FIG. 8 shows a preferred embodiment of such a retrofit clutch mechanism assembly  210 . A disc-shaped member  212  is fixed to the end of the outer screw  132  by means of a splined or keyed connection  162 . The disc  212  is provided with friction material on both faces  214 ,  216 . The disc  212  is enclosed between the spline shaft  154  of the rotational drive motor  122  and the rear side  226  of the secondary feed hopper  174 , with just enough clearance so that disc  212  may rotate freely. Surfaces adjacent to the disc  212  are also provided with friction material  218 ,  220 . When back pressure is applied by the hydraulic cylinders  118  to the platen  114 , the spline shaft  154  presses against disc  212  which is connected by splines  162  to the outer screw  132 . In this way, the retrofit clutch mechanism  160  is closed and the outer screw  132  rotates at the same speed as the spline shaft  154 . The clutch mechanism  160  remains closed while the composite melt accumulates in front of the screw assembly  205  so long as sufficient back pressure from the hydraulic cylinders  118  and the accumulating melt is maintained. When the back pressure is relieved by reducing the oil pressure in the hydraulic cylinders  118 , a Belleville spring  222  and roller bearing  224  between the spline shaft  154  and the disc  212  presses the disc  212  against the rear side  226  of the feed hopper  174 , thereby providing braking action. A Belleville spring  222  is a disc spring with a conical steel washer capable of holding high compressible forces. As an example, the 68 ounce (1925 gram) machine described above has a total back pressure adjustable from 0 to 29,000 pounds (0 to 129,000 newtons). In that example the Belleville spring  222  would be pre-loaded to about 2000 pounds (8900 newtons), or approximately the lowest commonly used back pressure. 
     However, in one preferred embodiment of the present invention, the spring  222  and roller bearing  224  were not needed since the pumping action of the inner screw  130  relative to the outer screw  132  and the resistance of an inner check ring  188  provides sufficiently strong forward pressure to hold the clutch disc  212  tightly against the secondary feed hopper  174  thereby providing sufficient braking action to prevent the outer screw  132  from rotating. 
     Conventional software provided with molding machines can be used to control this retrofit clutch mechanism assembly  210 . Typically, such software provides at least two steps of screw rotation (revolutions per minute) and back pressure control. For example, the single motor rotational drive  122  can turn a 100 revolutions per minute with no pressure for 75% of the recovery stroke to cause the inner screw  130  to extrude skin material  142 . Then, the single motor rotational drive  122  can turn at 50 revolutions per minute and 125 pounds per square inch (8.6 bars) of hydraulic oil back pressure which engages (closes) the clutch mechanism to cause the outer screw  132  to extrude core material  106 . Additionally, timing holes (not shown) can be provided on the circumference of the disc  212  to determine if the clutch mechanism  160  is properly engaged. 
     An alternative embodiment of the present invention is shown in FIG. 9 for applications in which injection force is limited and/or the diameter of the injection screw is desirably minimized such as on an electric molding machine  300  where the injection pressure is transmitted by a ball screw rather than a hydraulic cylinder. 
     In this embodiment, outer screw  302  is selectively rotated by independent drive motor  316 . The outer screw  302  remains in one position axially with respect to the feed housing  306  and is allowed to rotate on bearings  308 . Openings  310  through the outer screw  302  allow communication between the inner screw  312  and the secondary feed hopper  314 . 
     During screw recovery, drive motor  304  causes inner screw  312  to rotate, drawing material from feed hopper  314 . Material thus accumulating at the downstream end of inner screw  312  causes this inner screw to be forced back with respect to the outer screw  302 . Independently, drive motor  316  causes outer screw  302  to rotate, drawing material from feed hopper  318 . This further material also accumulates in the common space  320  downstream of inner screw  312 . 
     When sufficient material has accumulated to fill the desired mold cavity, linear actuator  326  which may be, for example, a hydraulic cylinder or ball screw, causes inner screw  312  to travel in an axial direction, injecting the accumulated material into the mold. Non-return valves  322 ,  324  at the downstream end of both screws  302 ,  312  prevent back flow of material during injection. 
     One aspect of multilayered parts is to plug-off, that is, encapsulate the core material  106  within the skin material  142  of the finished part at the end of a shot. The present invention largely achieves plug-off from the skin material  142  lodged in the dead area  240  at the forward periphery of the barrel  102 . This dead area skin material does not flow until the outer screw  132  displaces the skin material  142  in the dead area  240  at the end of the injection of the shot into the mold. The angle of the taper and the viscosity of the resin melt (plastic) affects how this dead area skin material flows. 
     One option to more explicit control of the extent of plug-off flow can be accomplished by making a well  242  in the face of the nozzle end cap  242  that would not be in the flow path until physically displaced by the incoming outer screw  132 . Another option is to telescope the end of the elongated tip  202  such that the elongated tip would contact the inner face  204  of the nozzle and pump skin material  142  from the space  244  between the inner check valve and the elongated tip  202 . Yet another option for plug-off is to fix the inner check valve  208  to the inner screw  130  and provide an additional stroke of the inner screw after the outer screw  132  reaches the forward stop. In all these plug-off options, the extent of the plug flow is the difference in the screw position between the onset of the fixed physical switch-over point and the final variable stopped position of the screw assembly  205  when the mold is full. This so-called cushion position is dictated by the difference between the physical mold volume and the shot size. 
     While the clutch mechanism  160  has been described in terms of a back pressure controlled disc  212 , any externally powered and controlled clutch could be used. Thus, the clutch mechanism  160  could operate pneumatically, with eddy currents, magnetically, or hydraulically. Furthermore, control of the clutch could be as simple as using a limit switch on a slider next to the platen  114 , or an external computer determining a screw position through a linear encoder. 
     Also, each screw could be controlled by a respective independent rotation source or system of gearing, as shown in FIG.  9 . If so provided, the rotation times of the screws can be overlapped which would save cycle time if the parts cool more quickly than the extrusion cycle time. Such overlapping could also benefit by reversing the flight direction of the inner screw so that the rotation of the outer screw will cause the inner screw to simultaneously extrude while the inner screw is held fixed by its own rotation source. 
     Alternatively, the outer screw could be axially fixed so that only the inner screw travels back during recovery, that is, for extrusion. While such an operation would require a separate thrust bearing for the outer screw and require a more complicated clutch mechanism because of the constantly changing distance between the barrel and the screw motor, higher injection pressure could be obtained since the smaller diameter inner screw would perform the injection. 
     Also, within the scope of this invention, more than two co-axial screws could be provided for additional layers given sufficient space, adequate steel strength for the screws, and room for plastic flow between the additional co-axial screws. Although the molding industry practice typically provides for a 20:1 L/D ratio for screws with a common range of 17:1 to 24:1, the extrusion industry has employed screws having L/D ratios ranging from 4:1 to 30:1. Thus, sufficient space can be provided for more than two co-axial screws. Further, all the inner screws need not extend to the front of the outer screw so long as the length of the barrel of each screw is sufficient, typically 18-20 diameters, to heat the melt. 
     While the embodiments of the present invention provided above utilize a single heater (primarily at start-up) from the non-flighted outside barrel to heat the melt, the inner screw could be independently heated along its axial bore. However, since most of the melting heat is produced by mechanical shear within the melt, some amount of independent heat control is possible by varying the shear rate of the two materials. However, use of such additional heat sources is not required by the present invention. 
     Also, the present invention has been primarily described in terms of injection molding. However, since reciprocating screws are used in other applications, such as for example, injection blow molding and extrusion blow molding, the present invention is applicable to such applications as well. 
     EXAMPLE 1 
     A Milacron injection molding machine (Model Vista V85 initially equipped with a with an 85 ton toggle clamp and a 45 millimeter diameter barrel having a volume of 9.6 ounces (272 grams), and a screw, available from Milacron Inc., Cincinnati, Ohio) was converted in accordance with the present invention. A 25 millimeter inner screw  130  with a L/D ratio 40:1 was positioned within a hollow 50 millimeter outer screw  132  having a L/D ratio of 18:1 which in turn were positioned within a barrel  102  having a bore  104  of 50 millimeters. The nozzle&#39;s restricted orifice  280  has 0.125 inch (3.2 millimeters) diameter and the wider nozzle section  282  has a 0.50 inch (12.7 millimeters) diameter. When the screw assembly  205  is fully forward, the tip of the elongated section  202  is about 0.06 inches (1.5 millimeters) from the back face  204  of the orifice  280 . This close proximity causes the break between the liquid and solid state of the plastic to be near the restricted orifice  280  thereby causing virtually all of the accumulated melt to be removed with the sprue when the mold opens. This process leaves the nozzle  206  clean for the next accumulation to begin. A shorter, but wider bore of 0.625 inch (15.8 millimeters) was also tried with good results. 
     A two cavity mold produced identical parts each having an average wall thickness of 3.8 millimeters and a weight of 45 grams. Impact copolymer polypropylene mixed with 1% white color concentrate was introduced through the inner screw  130  as skin material  142 . Regrind copolymer polypropylene mixed with 1% blue color concentrate for identification purposes was introduced through the outer screw  132  as core material  106 . Barrel temperature was 4000 Fahrenheit (2050 Celsius) Skin material  142  was extruded into the common accumulation space  146  at 100 revolutions per minute (“rpm”) and zero back pressure, up to 55% of the finished shot length. Core material  106  was extruded at 40 rpm and 100 pounds per square inch (6.9 bars) back pressure for the balance of the shot. Parts were produced that contained 45% blue core material totally enclosed by white skin material. 
     EXAMPLE 2 
     The demonstration of Example 1 was repeated except that a mold producing two parts weighing 102 grams apiece was used. Using the same combination of materials, parts containing 30% regrind core material by weight were produced. 
     EXAMPLE 3 
     The machine and mold described in Example 1 were used to produce a part containing 30% polypropylene homopolymer core covered with skin of Dupont Elvax ethylene/vinyl acetate elastomer (available from Dupont Corp., Wilmington, Del.) Barrel temperatures were reduced to 3750 Fahrenheit (1900 Celsius). 
     EXAMPLE 4 
     The machine and mold described in Example 1 were used to produce a part containing 30% polypropylene homopolymer core covered with a skin of AES Santoprene thermoplastic rubber (available from Advanced Elastomer Systems, Akron, Ohio). Barrel temperatures of 3850-4000 Fahrenheit (1950-2050 Celsius) were used. 
     EXAMPLE 5 
     The machine of Example 1 was used to sequentially fabricate large 8 ounce (226 gram) polypropylene parts. Each of these parts require a 5.05 inch (128 millimeter) stroke (horizontal movement) of the 50 millimeter screw. Because each part requires 60 seconds to cool in the mold, the accumulation of each shot can occur while the previous shot is cooling. 
     More specifically, in this Example 5, injection of a previous shot takes approximately 3 seconds after which the injection pressure is held for an additional 5 seconds to allow the sprue portion to solidify under pressure. Then the pressure is relieved and the screw motor turned on without back pressure for approximately 30 seconds during which time the inner screw  130  extrudes the skin material  142  for the next shot through the elongated tip  202  into the accumulation space  146 . When the screw assembly  205  is 3.79 inches (96 millimeters) (about 75% of stroke length) from the rear of the restricted orifice  204 , back pressure is applied at 125 pounds per square inch (8.6 bars). This back pressure causes the spline bushing  154  to contact the clutch plate (disc)  212  of the outer screw to rotate and extrude core material  106  through the outer check ring  136  for approximately 10 seconds. When the full 5.05 inch (128 millimeter) stroke is complete, the rotational drive motor  122  turns off, stopping rotation, the back pressure is relieved, the mold opened to eject the previous part, and the new shot injected. 
     While the present invention has been described in terms of specific embodiments, this invention encompasses all variations and modification, including expedients by those skilled in the art, which come within the spirit of the specification and the scope of the appended claims.