Abstract:
The invention is directed to a bi-directional check ring for a two-stage injection unit. More specifically, a close fitting ring is inserted behind the plunger head of the melt accumulator so that normal running clearances between the plunger and barrel bore can be used. The outer diameter of this check ring has a very close fit with the inner diameter the injection barrel, but is not rigidly connected to the plunger. Accordingly, the plunger is free to “floats” while injection and plastication/filling take place. The open volume between the ring and plunger is small, but sufficient to permit the ring to “float” very close to the barrel inner diameter, without being influenced by the alignment of the plunger and injection drive motor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a two-stage injection unit for an injection molding machine and, more particularly, to a check ring assembly used in conjunction with the plunger of the melt accumulator in a two-stage injection unit. 
     2. Description of the Related Art 
     The injection unit of an injection molding machine provides essentially two functions during the course of a normal cycle of operation; namely, injection and extruder. In a standard reciprocating screw injection molding machine, the extruder function is accomplished when the screw is rotated, gradually moving plastic melt toward the forward end of the screw, thereby creating a pressure or force to move the screw rearward to its pre-injection position as the melt accumulates. When a sufficient amount of material is accumulated (“a shot”), the screw is moved rapidly forward (without rotation) to inject the melt straight into the mold, thus performing the injection function. 
     The injection unit of a molding machine can also be designed as a “two-stage” system where the extruder and injection functions are performed by separate machine elements. In a two-stage injection system, the extruder or plasticizing function is still performed by a feed screw in a heated barrel, but all or part of the plastic melt is diverted into an “accumulator” rather than being conveyed directly to the mold. The accumulator is subsequently operated to perform or, at least, assist in performing the injection function. The accumulator is essentially a variable volume reservoir comprising a tubular barrel and a reciprocating plunger. The relative size of the barrel and plunger, as well as the stroke of the plunger, will vary according to the quantity of melt required to fill the mold. The advantages of a two-stage injection unit include more uniform plastication of material, reduced wear on the screw and barrel, and the potential for higher injection pressures. 
     In the prior art, two stage injection plungers have operated at very small running clearances between the outer diameter of the plunger head and the inner diameter of the bore of the barrel. This type of construction minimized the amount of material that flowed over the plunger during injection. (Plastic melt flowing to the “back” side of the plunger causes a major housekeeping nuisance and becomes a source of costly material scrap.) The close running clearance also improved injection efficiency because a higher percentage of injected material went into the mold instead of over the plunger as injection pressure rose to a higher level, as is required to fill thin wall parts. Since plunger strokes were very short (usually less than three times the plunger diameter) in early prior art units, the alignment between the plunger shaft that transmitted injection force and the plunger head could be maintained without encountering significant problems of galling or pick-up between the barrel and the plunger head. 
     Recent developments in two-stage injection have recognized the value of significantly increasing the length of stroke of the plunger relative to the diameter of the plunger head. More specifically, since the diameter of the plunger head determines the load carrying requirements for the mechanism that drives the plunger, larger shot capacities and greater shot accuracy can be accomplished with the two-stage design by providing increased length of stroke at relatively small plunger diameters. However, with the longer strokes, the close running clearance between the plunger and barrel used in the prior art is not practical, since it renders the assembly prone to the galling and pick-up problems noted above. If the clearance is increased to avoid these problems, the leakage past the plunger during injection increases significantly, causing the housekeeping and waste problems mentioned previously. The increased clearance also increases the likelihood that material that previously flowed past the plunger will contaminate the new melt that enters the accumulator during the filling process. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved construction for the plunger of the melt accumulator in a two-stage unit that will enable relatively long plunger injection strokes and minimize leakage of melt past the plunger. It is a further object of the invention to provide a plunger construction where alignment between the plunger head, plunger shaft and drive mechanism is less critical that the prior art constructions. 
     In accordance with these objectives, the invention provides a close fitting ring inserted behind the plunger so that normal running clearances, for example, clearances in the range of those used for feed screws in reciprocating screw injection units having similar geometry, can be used. The outer diameter of this check ring has a very close fit with the inner diameter the injection barrel, but is not rigidly connected to the plunger. Accordingly, it is free to “float” while injection and plastication/filling take place. Since it is a specific objective that no plastic flows through the ring of the invention (unlike the “check rings” used with reciprocating screws), the open volume between the ring and plunger can be extremely small and still permit the ring to “float” very close to the barrel inner diameter, without being influenced by the alignment of the plunger and injection drive mechanism. This is an important advantage since such alignment can be difficult with the long plunger shafts desired for larger shot size and more accurate control. 
     During injection, the ring restricts material “back-flowing” over the plunger and coming out the back of the accumulator barrel, in a manner similar to the tight running clearance of earlier plunger designs. In addition, the ring provides protection against melt contamination as the plunger retracts during plastication/filling by preventing small amounts of material which may have passed over the ring (during injection) from re-entering the melt stream by passing back over the plunger and mixing with the accumulated melt for the next shot. In fact, the ring is even more efficient in the retraction direction because there is only minimal pressure to force the material over the ring. The clear benefit is in keeping the freshly plasticized material free from contamination by other materials or colors processed previously which may reside on the plunger shaft behind the ring. Such material may have thermally decomposed over time and would be particularly objectionable if passed through to the mold cavity. The invention makes it possible to process certain engineering materials that were previously not compatible with two stage injection units. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a two stage injection unit incorporating a plunger assembly according to the present invention. 
     FIG. 2 is a side view of the two stage injection unit as shown in FIG.  1 . 
     FIG. 3 is a sectional view of the melt accumulator of the two-stage electric injection unit, taken along the line  3 — 3  of FIG.  1 . 
     FIG. 4 is an enlarged view of a portion of the plunger assembly illustrated in FIG. 3, focusing on the elements of the present invention that restrict the flow of plastic material past the plunger head. 
     FIG. 5 is an isometric view of the check ring as taught by the present invention. 
     FIG. 6 is an isometric view of the plunger seat as taught by the present invention. 
     FIG. 7 is an isometric view of the plunger spacer as taught by the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is particularly well suited for use in a two-stage electric injection unit  10  of an injection molding machine (not shown). Since the general structure and operation of injection molding machines are well known, only those aspects of the apparatus that are different or take on a new use with respect to two-stage electric injection will be described. 
     The apparatus of the present invention is used in conjunction with a two-stage electric injection unit  10  which includes components that are specifically designed to implement electric motor drive technology in a two-stage injection unit. Preferably, the primary elements are an electrically driven extruder  12  and a melt accumulator  14 . The extruder  12  is intended only for plasticizing and, therefore, has a non-reciprocating feed screw (not shown) within the barrel  16 . If desired, however, the concepts of the present invention can also be applied to a two-stage injection system that incorporates a reciprocating feed screw. 
     As is generally known in the art, material is supplied to the extruder  12  in any convenient manner, such as by a hopper  20 . The rotational power for the feed screw is also provided in a conventional manner, as by an electric motor  22 , connected to a speed reduction gearbox  24  that drives the feed screw. Since the movement of the feed screw is rotational only, the drive system is greatly simplified over the injection units having a screw, which must also reciprocate. 
     The accumulator  14  is essentially a variable volume reservoir by virtue of a cylindrical barrel  26  and a plunger  28  that moves linearly within the barrel  26 . The plunger  28  preferably has a flighted head  30  and an elongated shaft  32 . The relative size of the bore  27  of the barrel  26  and plunger head  30 , as well as the stroke of the plunger  28 , will vary according to the quantity of melt required to fill the mold. In the constriction of melt accumulator  14 , it is desirable to configure the end-shape of the barrel  26  and plunger  28  in a way that minimizes the amount of resin remaining in the barrel  26  when the plunger  28  is fully extended, as will be more fully discussed later. 
     The outlet of the extruder  12  connects to accumulator  14  via a suitable conduit  34 . At a convenient point between the extruder  12  and the inlet to the accumulator  14 , a ball check valve or other suitable non-return device  36  is provided to control the direction of the flow through conduit  34 . When the accumulator  14  is activated to inject plastic into a mold cavity and maintain pressure during pack and hold, the check valve  36  prevents a back-flow of melt into the extruder  12  due to the pressure differential during the injection phase. The outlet of the accumulator  14  is connected to the injection mold (not shown) via a suitable nozzle  38 . 
     By optimizing the length of stroke and diameter of the plunger head  30 , important advantages of an all-electric machine design can be realized. The diameter of the plunger head  30  dictates the load carrying requirements for the screw mechanism that converts the rotary motion of the motor  54  into linear motion for the plunger  28 . However, the larger shot capacities can be easily accomplished with the two-stage design by providing increased length of stroke at relatively small plunger diameters. The disadvantage is that the longer the plunger shaft  32 , the more difficult it is to align the plunger  28  with the drive mechanism. Accordingly, alignment problems are overcome by increasing the clearances between the plunger head  30  and plunger shaft  32  with the bore  27  of the accumulator barrel  26 . 
     Preferably, the plunger head  30  is designed to have a “normal” running clearance with the bore  27  of the barrel  26 ; that is, clearances in the range used for feed screws in reciprocating screw injection units having similar geometry, about 0.006 in. for a 3.000 in. diameter screw. This ensures good injection control with minimal metal-to-metal contact between the head  30  and bore  27 . In fact, the clearance between the outer diameter of the plunger head  30  and the bore  27  allows space for plastic melt to flow between these two metal surfaces, so that the melt effectively serves as a lubricant. The plunger shaft  32  is preferably slightly smaller in diameter than the head  30  to provide additional clearance, but is as large as practical to carry the force for injection. 
     To restrict back-flow over the plunger head  30  due to the clearance with the bore  27 , a check ring  40  is provided between the plunger head  30  and plunger shaft  32 . To simplify machining and assembly of the elements comprising the plunger  28 , a spacer  42  and seat  44  provide the bearing surfaces for the check ring  40 . In particular, the plunger  28  is assembled by placing the spacer  42  on a reduced diameter, threaded extension  46  of the plunger shaft  32 . The check ring  40  is then placed over the spacer  42  and the seat  44  is placed on the extension  46  against the spacer  42 . The plunger head  30  has a threaded bore  48  to engage the threads on the extension  46 , and is tightened so that the spacer  42  and seat  44  are held tightly between the plunger head  30  and plunger shaft  32 , as shown in FIGS. 3 and 4 of the drawings. 
     The check ring  40  is designed to have minimal clearance with the bore  27 : for example, about 0.001 in. for a bore diameter of 3.000 in. More generous clearance, e.g. about 0.010 in., is allowed between the end surfaces  62  of the check ring  40  and the bearing surface  64  of spacer  42  on one end and the bearing surface  66  of seat  44  at the other end. Substantially greater clearance, e.g. about 0.030 in., is allowed between the inner diameter  58  of the check ring  40  and the extended shoulder  60  of the spacer  42 . This construction essentially allows the check ring  40  to “float” relative to the rest of the plunger  28 . This is an important advantage since it makes alignment between the long plunger shaft  32  and the elements of the drive system less critical. 
     The plunger  28  of accumulator  14  is preferably actuated by an electromechanical drive assembly  50 , see FIGS. 1 and 2. The drive assembly  60  preferably includes a screw mechanism  52 , such as a roller screw or ball screw, a variable speed electric motor  54  and coupling, such as a drive belt  56 , between the screw mechanism  52  and motor  54 . The driven end of the screw mechanism  52  connects to the motor  54 ; the opposite end of the screw mechanism  52  connects to the plunger shaft  32  by means of a suitable coupling (not specifically shown). Preferably, the coupling includes a one-way clutch that allows the screw mechanism  52  to rotate freely with respect to the plunger  28  during injection to transmit efficiently linear (horizontal) force from the screw mechanism  52  to the plunger  28  without adversely affecting the melt contained in the accumulator  14 . However, reverse rotation of the screw mechanism  52  (during refill of the accumulator) engages the one-way clutch, causing the plunger  28  to rotate within the barrel  26 . 
     Although it is desirable for the check ring  40  to slide relatively easily in the bore  27  during translational movement of the plunger  28 , it is preferable that the ring  40  not rotate with the plunger  28 . This can be accomplished by designing the elements to control frictional forces. For example, the end surfaces  62  of the check ring  40  are made so that they not parallel to the opposing bearing surfaces  64 ,  66  of the spacer  42  and seat  44 , respectively. More specifically, they are at a slight angle (about 1°) relative to each other to create a circular “line” of contact for an efficient seal with minimal friction. In contrast, the outer diameter of the check ring  40  has only a small clearance with the bore  27 , and has a relatively large peripheral surface area. Accordingly, there is greater friction on the outer diameter of the check ring  40  than there is on the end surfaces  62 , so the ring  40  does not rotate with the plunger  28 . 
     A cycle of operation of an injection molding machine, incorporating the two-stage injection unit  14  of the present invention will now be described. The feed screw is rotated within the barrel  16  by the extruder motor  22  to begin plastication of the material that will be supplied as plastic melt to the accumulator  14 . The rotation of the feed screw builds pressure at the end of the screw, opening the check valve  36  and causing material to flow through the conduit  34  and into the accumulator  14 . 
     The inlet to the accumulator  14  is positioned so that melt flowing into the barrel  26  will pass over the flighted plunger head  30 . The incoming melt will flow along the flights, cleaning out melt carried over from the previous shot and moving it toward the outlet end of the barrel  26 , causing the pressure in the accumulator  14  to build. When the pressure of the plastic melt reaches a certain level, it will begin to force the plunger  28  rearwardly, thereby moving the screw of the mechanism  52  and motor  54  toward the rear of injection unit  10 . The rearward movement of plunger  28  applies a force to the screw mechanism  52  through the coupling, causing the screw to move likewise to the rear; as the screw moves through the associated nut, it rotates in a reverse direction. This reverse rotation of the screw is imparted to the plunger  28  via engagement of the one-way clutch, as described above. The rotation of plunger  28  further aids in cleaning carry-over material from the flighted head  30  by enhancing the wiping action of the inflow of new melt. 
     Although the check ring  40  does not rotate with the plunger  28  as it retracts, the ring  40  does create an effective seal at its outer periphery and from the contact between the end surface  62  and the bearing surface  66  of the seat  44 . This prevents any plastic residue remaining in the barrel  26  (behind the plunger head  30 ) from mixing into the flow of melt from the extruder  12 . In fact, the ring  40  will act to push any residue back to the rear of the barrel  26  where a drain passageway  68  is provided to permit accumulated residue material to flow from the barrel  26 . 
     If desired, the rate of rearward movement of the plunger  28  can be controlled by the motor  54 . In particular, the motor  54  can be used as a brake to impede the rotation of screw mechanism  52 , which slows the rearward movement of the plunger  28 , thereby increasing the back pressure of the plastic melt. Alternatively, the motor  54  can be used to speed up the rotation and rearward movement of the screw mechanism  52 , which increases the rate at which the plunger  28  moves back, thereby decreasing the back pressure of the melt. In either case, the rotational speed of the screw is imparted to the plunger  28  by the one-way clutch. 
     The extrusion function is complete and rotation of the feed screw is stopped when a sufficient charge of plastic melt is accumulated in front of the plunger  28  in the accumulator  14 , as required to fill the cavity of the mold. Concurrently with the extrusion function, the injection molding machine clamp unit has been operated to close and build pressure on the mold that will receive the plastic melt. 
     To initiate the injection function, the motor  54  is activated to cause the screw mechanism  52  to rotate and advance. The translational (linear) movement of the screw is imparted to the plunger  28  housed in the accumulator  14 . However, the rotation of the screw mechanism  52  is not imparted to the plunger  28  since the one-way clutch is disengaged when the screw rotates in the forward direction. The check ring  40  shifts slightly so that its end surface  62  comes into contact with the bearing surface  64  of the spacer  42 . The contact between these surfaces and the small clearance between the outer diameter of the ring  40  and the bore  27  creates an effective seal that substantially prevents melt from leaking past the plunger head  30  and remaining in the barrel  26  during injection. 
     The forward movement of the plunger  28  causes the accumulated plastic melt to be forced through the nozzle  38  and into the mold cavity. The injection pressure generated by movement of the plunger  28  moves the check valve  36  to a position that prevents transfer of the melted resin into the extruder  12 . After the bulk of material is transferred into the mold cavity, the injection accumulator  14  initiates pack, then “hold”, to maintain the proper pressure on the material until the molded part is properly formed. When the injection accumulator  14  reaches the “hold” portion of the cycle, it has emptied itself of material. In other words, the injection of plastic melt is accomplished by applying sufficient force to move the plunger  28  rapidly forward in the barrel 26 , forcing the melt to flow through the outlet of the injection accumulator  14 , on through the nozzle  38 , then into the mold. This approximate point in the cycle can be identified by the configuration shown in FIG. 3; the plunger head  30  in the accumulator  14  is fully forward in the barrel  26 , having completed the injection function. 
     As part of the injection process, it is highly desirable to avoid “dead” spots in the material flow path where plastic melt can remain stationary through repeated cycles, allowing it to degrade, possibly later mixing with good material and injected to form a poor quality part. Accordingly, a mating configuration between the head  30  of the plunger  28  and the outlet of the barrel  26  will minimize the amount of material remaining in the accumulator  14  after the shot is completed. The only significant carry-over material is in the flight of the plunger head  30  which is “wiped” clean by the inflow of new melt and rotation of the plunger  28  as the subsequent shot accumulates (and is injected into the mold during the next cycle of operation). 
     After sufficient hold/cool time, the pressure held by the injection accumulator  14  is released during mold decompress, which may include a slight retraction of the plunger  28 . The clamp unit can then operate to open the mold, eject the part(s), then re-close to begin a subsequent cycle, as required for a particular application. After the injection hold time is completed, at some point during the cooling time, the extruder  12  starts rotation of the feed screw to initiate the extrusion function as described previously and begin another cycle of operation. 
     While the invention has been illustrated in some detail according to the preferred embodiment shown in the accompanying drawings, and while the preferred embodiment has been described in some detail, there is no intention to thus limit the invention to such detail. On contrary, it is intended to cover all modifications, alterations, and equivalents falling within the scope of the appended claims. For example, although the drive couplings are generally described as belts and pulleys, other mechanical couplings, such as suitable gearing, can be used to perform the same function. In addition, other systems or mechanisms can be used to impart linear motion to the accumulator plunger  28 ; such as, a rack and pinion, a roller screw and nut in place, or a ball screw and ball nut as described. Alternatively, a suitably configured linear (electric) motor can be used to actuate the plunger  28  directly.