Abstract:
According to embodiments, there is provided a method ( 300 ) of controlling a first melt accumulator ( 121 ). The first melt accumulator ( 121 ) is part of an injection unit ( 100 ), the injection unit ( 100 ) including an extruder ( 102 ) for preparing molding material and a second melt accumulator ( 123 ). The first melt accumulator ( 121 ) and the second melt accumulator ( 123 ) are configured to sequentially receive molding material from the extruder ( 102 ). The extruder ( 102 ) is being operable to produce molding material in at least near continuous manner, both the first melt accumulator ( 121 ) and the second melt accumulator ( 123 ) being associated with a respective shot size set-point. The method comprises detecting ( 310 ) an indication of an out-of-boundary condition associated with operation the first melt accumulator ( 121 ); responsive to said detecting ( 310 ), controlling ( 320 ) the first melt accumulator ( 121 ) to continue accepting molding material beyond its respective shot size set-point.

Description:
TECHNICAL FIELD 
       [0001]    The present invention generally relates to, but is not limited to molding of molded articles and more specifically, but not limited to, a method of controlling a melt accumulator. 
       BACKGROUND 
       [0002]    Molding is a process by virtue of which a molded article can be formed from molding material (such as Polyethylene Terephthalate (PET), Polypropylene (PP) and the like) by using a molding system. Molding process (such as injection molding process) is used to produce various molded articles. One example of a molded article that can be formed, for example, from PET material is a preform that is capable of being subsequently blown into a beverage container, such as, a bottle and the like. 
         [0003]    A typical injection molding system includes inter alia an injection unit, a clamp assembly and a mold assembly. The injection unit can be of a reciprocating screw type or of a two-stage type. Within the reciprocating screw type injection unit, raw material (such as PET pellets and the like) is fed through a hopper, which in turn feeds an inlet end of a plasticizing screw. The plasticizing screw is encapsulated in a barrel, which is heated by barrel heaters. Helical (or other) flights of the screw convey the raw material along an operational axis of the screw. Typically, a root diameter of the screw is progressively increased along the operational axis of the screw in a direction away from the inlet end. 
         [0004]    As the raw material is being conveyed along the screw, it is sheared between the flights of the screw, the screw root and the inner surface of the barrel. The raw material is also subjected to some heat emitted by the barrel heaters and conducted through the barrel. As the shear level increases in line with the increasing root diameter, the raw material, gradually, turns into substantially homogenous melt. When a desired amount of the melt is accumulated in a space at discharge end of the screw (which is an opposite extreme of the screw vis-à-vis the inlet end), the screw is then forced forward (in a direction away from the inlet end thereof), forcing the desired amount of the melt into one or more molding cavities. Accordingly, it can be said that the screw performs two functions in the reciprocating type injection unit, namely (i) plasticizing of the raw material into a substantially homogeneous melt and (ii) injecting the substantially homogeneous melt into one or more molding cavities. 
         [0005]    The two stage injection unit can be said to be substantially similar to the reciprocating type injection unit, other than the plasticizing and injection functions are separated. More specifically, an extruder screw, located in an extruder barrel, performs the plasticizing functions. Once a desired amount of the melt is accumulated, it is transferred into a melt accumulator, which is also sometimes referred in the industry as a “shooting pot”, the melt accumulator being equipped with an injection plunger, which performs the injection function. 
         [0006]    U.S. Pat. No. 6,241,932 issued to Choi et al. on Jun. 5, 2001 discloses a method and system of operating a two stage injection molding machine wherein movement of the injection plunger in the shooting pot is coordinated with movement of the plasticizing screw and melt flow into the shooting pot such that the plunger provides minimal resistance to the melt flow into the shooting pot while avoiding the production of voids or air inside the melt. The undesired shear forces to which the melt is exposed are thus reduced, correspondingly reducing the melt degradation products which would otherwise result. 
         [0007]    U.S. Pat. No. 6,514,440 to Kazmer, et al. issued on Feb. 4, 2003 discloses an injection molding apparatus, system and method in which the rate of material flow during the injection cycle is controlled. According to one preferred embodiment, a method of open-mold purging is provided in an injection molding system including a manifold to receive material injected from an injection molding machine. The method includes the steps of selecting a target purge pressure; injecting material from the injection molding machine into the manifold; and controlling the purge pressure to substantially track the target purge pressure, wherein the purge pressure is controllable independently from the injection molding machine pressure. 
         [0008]    U.S. Pat. No. 4,311,446 to Hold et al. issued on Jan. 19, 1982; U.S. Pat. No. 4,094,940 to Hold on Jun. 13, 1978; U.S. Pat. No. 3,937,776 to Hold et al. on Feb. 10, 1976; and U.S. Pat. No. 3,870,445 to Hold et al. on Mar. 11, 1975 each teaches a method and apparatus for controlling the parameters of injection molding processes in a machine having a barrel with a plasticating chamber and a screw, rotatably and slidably disposed in said chamber, hopper means adjacent one end of said chamber communicating therewith and nozzle means disposed in the other end of said chamber communicating with a mold. Control of the injection molding process is achieved through an event recognition philosophy by sensing screw position, screw injection velocity, melt temperature, comparing the values at certain instances during the work cycle with known or desired values and using these values, changes of values and differences of values to monitor and initiate changes in the process parameters. 
       SUMMARY 
       [0009]    According to a first broad aspect of the present invention, there is provided a method of controlling a first melt accumulator. The first melt accumulator is part of an injection unit, the injection unit including an extruder for preparing molding material and a second melt accumulator. The first melt accumulator and the second melt accumulator are configured to sequentially receive molding material from the extruder, the extruder being operable to produce molding material in at least near continuous manner. Both of the first melt accumulator and the second melt accumulator being associated with a respective shot size set-point. The method comprises detecting an indication of an out-of-boundary condition associated with operation the first melt accumulator; responsive to said detecting, controlling the first melt accumulator to continue accepting molding material beyond its respective shot size set-point. 
         [0010]    According to a second broad aspect of the present invention, there is provided a method of controlling a first melt accumulator. The first melt accumulator is part of an injection unit, the injection unit including an extruder for preparing molding material. The first melt accumulator is configured to receive molding material from the extruder and to inject the molding material into a molding cavity. The extruder is operable to produce molding material in at least near continuous manner. The first melt accumulator is associated with a respective shot size set-point. The method comprises detecting an indication of an out-of-boundary condition associated with operation the first melt accumulator; responsive to said detecting, controlling the first melt accumulator to continue accepting molding material beyond its respective shot size set-point. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    A better understanding of the embodiments of the present invention (including alternatives and/or variations thereof) may be obtained with reference to the detailed description of the embodiments along with the following drawings, in which: 
           [0012]      FIG. 1  depicts a partially sectioned frontal view of an injection unit implemented according to a non-limited embodiment of the present invention. 
           [0013]      FIG. 2  depicts a partially sectioned top view of the injection unit of  FIG. 1 . 
           [0014]      FIG. 3  depicts a flow chart showing steps of a non-limiting embodiment of a method for controlling a melt accumulator, the method being implemented in accordance with a non-limiting embodiment of the present invention. 
       
    
    
       [0015]    The drawings are not necessarily to scale and may be illustrated by phantom lines, diagrammatic representations and fragmentary views. In certain instances, details that are not necessary for an understanding of the embodiments or that render other details difficult to perceive may have been omitted. 
       DETAILED DESCRIPTION OF EMBODIMENTS 
       [0016]    With reference to  FIG. 1  and  FIG. 2 , an injection unit  100  implemented in accordance with non-limiting embodiments of the present invention, will now be described in greater detail, in which figures,  FIG. 1  depicts a partially sectioned frontal view of the injection unit  100  and  FIG. 2  depicts a partially sectioned top view of the injection unit  100 . 
         [0017]    Within the instantly illustrated embodiment, the injection unit  100  is of a two-stage type and to that extent, the injection unit  100  comprises an extruder  102  and a melt accumulator  122 . The extruder  102  houses a screw (not depicted) for plasticizing raw material, as will be described in greater detail herein below. In some embodiments of the present invention, the extruder  102  can be implemented as a twin screw extruder and, to that end, the extruder  102  can house a set of two screws (not depicted). The extruder  102  (or to be more precise, the screw within the extruder  102 ) is actuated by a screw actuator  108 . In the specific non-limiting embodiment of the present invention, the screw actuator  108  comprises an electric motor coupled to the extruder  102  via a gear box (not separately numbered); however, this need not be so in every embodiment of the present invention. As such, it should be appreciated that the screw actuator  108  can be implemented differently, such as a hydraulic actuator, a mechanical actuator or a combination thereof. It should be noted that in alternative non-limiting embodiments, the injection unit  100  can be implemented as a single-stage injection unit with a reciprocating screw. 
         [0018]    In some embodiments of the present invention, the extruder  102  can operate in a continuous plasticizing manner (i.e. extruder  102  can be implemented as a continuous extruder). In other embodiments, the extruder  102  can operate in a near continuous plasticizing manner. 
         [0019]    In the specific non-limiting embodiment depicted herein, the screw actuator  108  imparts a rotational movement onto the screw of the extruder  102  and it is this rotational movement that performs a dual function: (a) plasticizing of the raw material and (b) transfer of the raw material into the melt accumulator  122 , as will be described in greater detail herein below. As such, within this implementation, the screw of the extruder  102  is not associated with a reciprocal movement. In alternative embodiments, however, which are particularly applicable but not limited to scenarios where a single screw is employed in the extruder  102 , the screw of the extruder  102  can be associated with the reciprocal movement, which can be imparted by the screw actuator  108  or by separate means (not depicted). 
         [0020]    The injection unit  100  further includes a material feeder  110 . The material feeder  110  is configured to supply raw material to the extruder  102 . The material feeder  110  can be configured as a controlled (or metered) feeder or as a continuous feeder. 
         [0021]    In a specific non-limiting embodiment of the present invention, the raw material is PET. In alternative embodiments, other materials or a mix of materials can be used. In a particular implementation of the embodiments of the present invention, the raw material includes a combination of virgin raw material and recycled raw material, in a particular proportion. The virgin raw material (which can come in a form of pellets, for example) and the recycled raw material (which can come in a form of flakes, for example) can be mixed at the material feeder  110  or at another upstream device (not depicted), such as a drier (not depicted), for example. 
         [0022]    In a particular scenario, the raw material fed through the material feeder  110  may include 25% of the recycled raw material and 75% of the virgin raw material. In another particular scenario, the raw material may include 50% of the recycled raw material and 50% of the virgin raw material. In yet another particular scenario, the raw material may include 75% of the recycled raw material and 25% of the virgin raw material. Naturally, the exact combination of the raw material used can be different. It should be further noted that embodiments of the present invention can be applied to the injection unit  100  that processes only virgin raw material or only recycled raw material. 
         [0023]    In addition to the material feeder  110 , in some embodiments of the present invention, there may be provided an additive feeder (not depicted) for adding additional substances, such as for example colorants, acetaldehyde (AA) blockers and the like, to the extruder  102 . Such additive feeders are well known in the art and, as such, will not be described here at any length. 
         [0024]    There is also provided a filter  112 , located fluidly in-between the extruder  102  and the melt accumulator  122 . The purpose of the filter  112  is to filter impurities and other foreign matters from the plasticized material being transferred from the extruder  102  to the melt accumulator  122 . It should be noted that in some embodiments of the present invention, which include but are not limited to scenarios where only virgin raw material is used, the filter  112  can be omitted. The specific implementation for the filter  112  is not specifically limited and, as an example, an off-the-shelf filter from Gneuss Inc. of Matthews, N.C. (www.gneuss.com) can be used to implement the filter  112 . 
         [0025]    Within the specific non-limiting embodiment being depicted herein, the melt accumulator  122  is implemented as a dual melt accumulator and to that extent the melt accumulator  122  can include two instances of the melt accumulator  122 —a first melt accumulator  121  and a second melt accumulator  123 , selectively fluidly coupled to the extruder  102 , as will be described in greater detail herein below. In alternative non-limiting embodiments of the present invention, the melt accumulator  122  can include only a single instance of the melt accumulator  122 . 
         [0026]    Each of the first melt accumulator  121  and the second melt accumulator  123  includes an injection plunger  128  operatively disposed within the respective one of the first melt accumulator  121  and the second melt accumulator  123 . The injection plunger  128  is actuated by a respective one of an injection plunger actuator  130 , which in this particular embodiment of the present invention is implemented as a piston which actuates the injection plunger  128  via hydraulic means. However, in alternative non-limiting embodiments of the present invention, the injection plunger  128  can be actuated by a different type of an actuator (not depicted), such as mechanical actuator, electrical actuator and the like. 
         [0027]    There is also provided a distribution assembly  124 , located fluidly-in-between the extruder  102  and the melt accumulator  122 , downstream from the filter  112 . The distribution assembly  124  is implemented as a distribution valve and is configured to selectively fluidly connect:
       (a) the extruder  102  to the first melt accumulator  121  while connecting the second melt accumulator  123  to a nozzle  127 , which provides for fluid communication with a molding cavity (not depicted) either directly or via a melt distribution system (not depicted), such as a hot runner (not depicted) for enabling for melt transfer from the extruder  102  to the first melt accumulator  121  and melt injection from the second melt accumulator  123  into the molding cavity (not depicted) via the nozzle  127 ;   (b) the extruder  102  to the second melt accumulator  123  while connecting the first melt accumulator  121  to the nozzle  127 , for enabling for melt transfer from the extruder  102  to the second melt accumulator  123  and melt injection from the first melt accumulator  121  into the molding cavity (not depicted) via the nozzle  127 .       
 
         [0030]    Each of the first melt accumulator  121  and the second melt accumulator  123  is associated with a respective shot size set-point. The shot size set-point is generally set by keeping two variables in consideration: (a) the amount of molding material required for injection and filling; and (b) amount of time required to fill the given one of the first melt accumulator  121  and the second melt accumulator  123  given the throughput and the allocated transfer time. Within a typical prior art implementation, when the shot size set-point is reached for the given one of the first melt accumulator  121  and the second melt accumulator  123 , but the transfer is still not completed (for example, when the other one of the first melt accumulator  121  and the second melt accumulator  123  is still performing injection or holding operation) an alarm would be triggered and the operation of the injection unit  100  would generally be interrupted. This of course, would result in lost productivity. 
         [0031]    Also, provided within the architecture of  FIG. 1  and  FIG. 2  is a controller  126  (only depicted in  FIG. 1  for the sake of simplicity). Controller  126  can be implemented as a general-purpose or purpose-specific computing apparatus that is configured to control one or more operations of the injection unit  100 . It is also noted that the controller  126  can be a shared controller that controls operation of an injection molding machine (not depicted) that houses the injection unit  100  and/or other auxiliary equipment (not depicted) associated therewith. 
         [0032]    Amongst numerous functions that can be controlled by the controller  126 , some include (but are not limited to):
       Controlling the screw actuator  108  and more specifically the speed of rotation of the screw (not depicted) of the extruder  102 ;   Controlling the distribution assembly  124  for selectively implementing the melt transfer and melt injection switching between the two instances of the melt accumulator  122 , as has been discussed above;   Controlling the material feeder  110 , where the material feeder  110  is implemented as controlled feeder, also referred to sometimes by those of skill in the art as a volumetric feeder;   Controlling the above-mentioned additive feeder (not depicted) in those embodiments where such additive feeder is provided;   Controlling other auxiliary equipment (not depicted), such as a dryer and the like;   Performing a cycle optimization routine configured to analyze and optimize different parameters of the molding cycle.       
 
         [0039]    The controller  126  can comprise internal memory  140  configured to store one or more instructions for executing one or more routines. The internal memory  140  can also store and/or update various parameters, such as but not limited to:
       (i) Indication of a target throughput for the transfer of molding material between the extruder  102  and the melt accumulator  122 .   (ii) Set up parameters associated with the injection unit  100  or components thereof.       
 
         [0042]    Given the architecture described with reference to  FIG. 1  and  FIG. 2 , it is possible to execute a method for controlling a melt accumulator. 
         [0043]    Within embodiments of the present invention, the controller  126  can execute the method for controlling the melt accumulator. With reference to  FIG. 3 , which depicts a flow chart demonstrating a non-limiting embodiment of a method  300  executed in accordance with a non-limiting embodiment of the present invention, the method  300  will now be described in greater detail. 
       Step  310   
       [0044]    The method  300  starts at step  310 , where the controller  126  detects an indication of an out-of-boundary condition associated with operation of one of the first melt accumulator  121  and the second melt accumulator  123 . 
         [0045]    For example, in those embodiments of the present invention, there the melt accumulator  122  is implemented as the first melt accumulator  121  and the second melt accumulator  123 , this indication of the out-of-boundary condition can be triggered by the one of the first melt accumulator  121  and the second melt accumulator  123  having reached the shot size set-point during a given transfer cycle, while the other one of the first melt accumulator  121  and the second melt accumulator  123  is not yet ready to start accepting molding material. This can occur, for example, when the other one of the first melt accumulator  121  and the second melt accumulator  123  has not yet completed its respective filling or holding operations. 
         [0046]    In embodiments of the present invention where the melt accumulator  122  comprises only a single instance thereof, the indication of the out-of-boundary condition can be triggered responsive to the cycle time variations, bulk density variation of the raw material and the like. 
         [0047]    For the purposes of the description to be provided herein below, it shall be assumed that the indication of the out-of-boundary condition is association with the first melt accumulator  121 . 
       Step  320   
       [0048]    Responsive to the detection of the indication received as part of step  310 , the controller  126  proceeds to execution of step  320 , during which the controller  126  executes control of the first melt accumulator  121 . More specifically, within embodiments of the present invention, the controller  126  effectively overrides the shot size set-point and causes the filling operation of the first melt accumulator  121  to continue beyond the over-ridden shot size set-point. 
         [0049]    Execution of step  320  continues until the second melt accumulator  123  is ready to accept molding material. When the second melt accumulator  123  is ready to accept molding material, the controller  126  causes the distribution assembly  124  to shuttle to a position where it operatively fluidly connects the extruder  102  to the second melt accumulator  123 . 
       Step  330   
       [0050]    The controller  126  then proceeds to execution of step  330 , where the controller  126  adjusts the fill-to-hold switch-over position associated with the first melt accumulator  121  for the purposes of executing the next injecting and holding cycles of the first melt accumulator  121 . This adjustment is performed in order to compensate for the over-filling executed as part of step  320 . It is noted that step  330  is optional in some embodiments of the present invention. For example, in those embodiments where the transition between filling and holding is executed based on hydraulic pressure or time, step  330  can be modified or omitted altogether. 
       Step  340   
       [0051]    The controller  126  then proceeds to executing step  340 , where the controller  126  adjusts the feed rate, i.e. the rate at which molding material is transferred from the extruder  102  to the first melt accumulator  121  to eventually bring the shot size set-point associated with the first melt accumulator  121  to the pre-adjusted value that it was at prior to the adjustment executed as part of step  320 . This eventual adjustment to the original shot size set-point can be achieved in one cycle or over two or more cycles depending on specific implementation and magnitude of the adjustment executed as part of step  320 . 
         [0052]    A technical effect of embodiments of the present invention includes ability to address disruptions to the melt buffering process that are transient in nature. It is noted that execution of the method  300  can be activated during the totality of operation of the injection unit  100  or, alternatively, it can be triggered on-demand. This on-demand triggering can be executed using, for example, a human-machine interface (not depicted) or any other suitable means. 
         [0053]    The description of the embodiments provides examples of the present invention, and these examples do not limit the scope of the present invention. The concepts described above may be adapted for specific conditions and/or functions, and may be further extended to a variety of other applications that are within the scope of the present invention. Having thus described the embodiments, it will be apparent that modifications and enhancements are possible without departing from the concepts as described. Therefore, what is to be protected by way of letters patent are limited only by the scope of the following claims: