WO2016039982A1

WO2016039982A1 - Gate closing control for injection molding system

Info

Publication number: WO2016039982A1
Application number: PCT/US2015/046843
Authority: WO
Inventors: Patrice Fabien Gaillard; Angelo Mier
Original assignee: Husky Injection Molding Systems Ltd.
Priority date: 2014-09-11
Filing date: 2015-08-26
Publication date: 2016-03-17

Abstract

In an injection molding system, a trigger time at which closing of a gate is to be triggered may be computed based on a determined closing duration for closing the gate. The computing may be performed so as to provide a desired interval between an end of a hold phase of a molding cycle and the gate achieving a fully closed position. The computing may be performed automatically, for example in response to detecting a change in the closing duration for closing the gate. At the computed trigger time, closing of the gate may be triggered. The gate may form part of a hot runner that may be used with a multi-cavity injection mold. The gate may include a valve stem that is movable according to a motion profile that may be dynamically changeable.

Description

GATE CLOSING CONTROL FOR INJECTION MOLDING SYSTEM TECHNICAL FIELD [0001] The present disclosure relates to injection molding systems, and more particularly to gate closing control for an injection molding system.

BACKGROUND

[0002] In an injection molding system, a molded article may be formed from molding material that is injected into a molding cavity of a mold. The molding material may be a resin or plastic material, such as polyethylene terephthalate (PET), which may be melted prior to its injection into the molding cavity. Injection molding can be used to form various types of molded articles. One example of a molded article is a so-called "preform" that is capable of being subsequently reheated and blow-molded into a beverage container, such as a bottle or the like.

[0003] In a multi-cavity injection molding system, melted molding material, which may be referred to as "melt," may be delivered to multiple molding cavities in the mold through a melt distribution network referred to as a "hot runner." At least some of the channels of the hot runner may be terminated by nozzles for introducing melt into respective molding cavities. Melt ejected from the hot runner may be controlled through a gate at the tip of each nozzle. Each gate may be selectively opened and closed through reciprocation of a valve stem for example. For instance, the valve stem may be retracted within the nozzle to open the gate and allow free flow of melt therethrough. The valve stem may alternatively be positioned so as to block or close the gate and thereby stop melt flow.

[0004] A valve stem may cycle between a "gate fully open" position and a "gate fully closed" position many times during molding system operation. The opening and closing of the gate may be controlled by an actuator based on signals from a controller that may also control other aspects of the injection molding system, such as the opening and closing of the mold. [0005] A signal triggering the closing of the gate may be associated with or tied to an end of an injection hold phase of a molding cycle. The injection hold phase may follow an injection fill phase of the molding cycle in which melt is injected through the gate into the molding cavity to form a molded article. In the injection hold phase, the gate is kept open so that melt may continue to flow into the molding cavity, as necessary, to fill any gaps that may form in the molding cavity due to shrinkage of the molded article within the molding cavity as the melt cools and hardens.

SUMMARY

[0006] According to an aspect, there is provided a method of closing a gate of an injection molding system, the method comprising: based on a determined closing duration (Tsc) for closing the gate, computing a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate becoming fully closed; and at the computed trigger time, triggering closing of the gate.

[0007] In some embodiments, the setting of the trigger time is performed

automatically in response to a detected change in the closing duration (Tsc) for closing the gate.

[0008] In some embodiments, the gate comprises a valve stem and wherein the detected change in the closing duration comprises a detected change in a motion profile of the valve stem.

[0009] In some embodiments, the above method further comprises dynamically determining the closing duration for closing the gate by measuring a time required for the gate to close.

[0010] In some embodiments, the above method further comprises receiving, via a user interface, user input specifying the desired interval between the end of the hold phase of the previous molding cycle and the gate becoming fully closed.

[0011] In some embodiments, the user interface constrains the user input to require the desired interval between the end of the hold phase of the previous molding cycle and the gate becoming fully closed to be at or above a predetermined threshold. [0012] In some embodiments, the gate comprises a valve stem and wherein the triggering comprises sending a signal for causing a valve stem actuator to actuate the valve stem.

[0013] In another aspect, there is provided a method of operating an injection molding system, the method comprising: detecting a change in a closing duration (Tsc) for closing a gate of the injection molding system; and automatically adjusting a time (Ttrigger) at which closing of the gate is triggered, the automatic adjusting maintaining a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate becoming fully closed despite the change in the closing duration.

[0014] In some embodiments, the gate comprises a valve stem and the detecting of the change in the closing duration for closing the gate comprises detecting a change in a motion profile of the valve stem.

[0015] According to another aspect, there is provided a controller for an injection molding system, the controller comprising: at least one processor; and a memory storing instructions, that, when executed by the at least one processor, cause the controller to: based on a determined closing duration (Tsc) for closing a gate of the injection molding system, compute a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate achieving a fully closed position; and at the computed trigger time, trigger closing of the gate.

[0016] In some embodiments, the controller further comprises a timer, wherein the instructions further cause the controller to measure the closing duration (Tsc) using the timer.

[0017] In some embodiments, the computing of the trigger time is performed automatically in response to a detected change in the closing duration (Tsc).

[0018] According to another aspect, there is provided a non-transitory machine- readable medium storing instructions, that, when executed by a controller of an injection molding system, cause the controller to: based on a determined closing duration (Tsc) for closing a gate of the injection molding system, compute a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) of the injection molding system and the gate achieving a fully closed position; and at the computed trigger time, trigger closing of the gate.

[0019] In some embodiments, the computing of the trigger time is performed automatically in response to a detected change in the closing duration (Tsc) of the gate.

[0020] In some embodiments, the instructions further cause the controller to dynamically determine the closing duration for closing the gate by measuring a time required for the gate to close.

[0021] In some embodiments, the instructions further cause the controller to: receive, from a human-machine interface, an indication of the desired interval between the end of the hold phase of the previous molding cycle and the gate achieving the fully closed position.

[0022] Other features will become apparent from the drawings in conjunction with the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In the figures which illustrate example embodiments,

[0024] FIG. 1 is a schematic representations of an example embodiment of an injection molding system; [0025] FIG. 2 is a schematic representation of a portion of an example hot runner of the system of FIG. 1 in which an example nozzle is shown;

[0026] FIG. 3. is a graph illustrating a motion profile of a valve stem of the nozzle of FIG. 2;

[0027] FIG. 4 is a flowchart illustrating operation of the injection molding system of FIG. 1 ;

[0028] FIG. 5. is a graph illustrating a motion profile of the valve stem of FIG. 2 over multiple molding cycles; and

[0029] FIG. 6 illustrates a user interface displayed on a human-machine interface of the injection molding system of FIG. 1 . DETAILED DESCRIPTION

[0030] In this description, any use of the term "exemplary" should be understood to mean "an example of" and not necessarily to mean that the example is preferable or optimal in some way.

[0031] FIG. 1 illustrates a schematic representation of a non-limiting example injection molding system 10. The system 10 may be used for injection molding of articles from a molding material such as Polyethylene or Polyethylene-Terephthalate (PET), as a non-limiting example. The system 10 comprises an injection molding machine 20, a controller 30, and a user interface 40. The system 10 may include other components or subcomponents not expressly shown in FIG. 1 .

[0032] The injection molding machine 20 comprises a multi-cavity mold and a hot runner 100, neither of which is expressly illustrated in FIG. 1 .

[0033] The multi-cavity mold contains multiple molding cavities, each molding cavity for molding an article substantially simultaneously with other articles being molded in the other molding cavities of the mold, in a single injection molding cycle. The injection molding cycle (or simply "molding cycle") may include various phases. One phase is an injection fill phase (or simply "injection phase"), in which melt is injected through gates into respective molding cavities to form molded articles. Another phase is an injection hold phase (or simply "hold phase"), in which the gates are kept open so that melt may continue to flow into the molding cavities as necessary to occupy any gaps that may form within the molding cavities due to shrinkage of the molded articles as the melt cools and/or hardens. These phases will be described in more detail below.

[0034] The hot runner comprises multiple channels, at least some of the channels being terminated by nozzles for introducing melt into respective molding cavities via respective gates. A portion of the hot runner of injection molding machine 20 depicting a single nozzle is schematically illustrated in FIG. 2.

[0035] Referring to FIG. 2, the illustrated portion of the hot runner 100 includes a single nozzle assembly 1 10. It will be appreciated that the hot runner 100 may have many other similar nozzle assemblies that are not expressly illustrated in FIG. 2. It will further be appreciated that the hot runner 100 of FIG. 2 is merely an example.

[0036] The example nozzle assembly 1 10 of FIG. 2 includes a nozzle body 1 12, a nozzle tip 1 18 and a valve stem 1 16 that is used to selectively open and close a gate (orifice) 1 14. The nozzle assembly 1 10 regulates the flow of melt from a melt channel 1 19 throughout the injection molding cycle. In the present embodiment, opening and closing of the gate 1 14 is achieved through reciprocation of the valve stem 1 16. The valve stem 1 16 moves between a first position (as illustrated in FIG. 2), in which the tip of the valve stem closes the gate 1 14 so as to prevent melt from flowing from the gate 1 14, and a second position (not expressly illustrated), in which the tip of the valve stem 1 16 is withdrawn into the nozzle tip 1 18 (rightwardly in FIG. 2) to allow melt to flow from the gate 1 14. The first and second positions of the valve stem 1 16 may be considered to correspond with fully closed and fully open conditions of the gate 1 14, respectively. When the example gate 1 14 is in the fully closed condition, a distal portion of the valve stem 1 16 of FIG. 2 may protrude through the gate orifice 1 14, as shown in FIG. 2, but this is not necessarily true of all embodiments.

[0037] In the present embodiment, reciprocation of the valve stem 1 16 is driven by an actuator assembly 125 responsive to control signals received from the controller 30 (FIG. 1 ). The actuator assembly 125 comprises an actuator 126 that may for example be a servo-motor or a servo-hydraulic actuator.

[0038] The illustrated nozzle assembly 1 10 further includes a nozzle heater 122 and a manifold bushing 140. The nozzle heater 122 may be configured to maintain a target temperature for the melt flowing through the nozzle tip 1 18. The manifold bushing 140 provides a branch from an internal runner network and also houses a portion of the valve stem 1 16. These elements are not necessarily present or may take other forms in alternative embodiments.

[0039] Referring back to FIG. 1 , the injection molding system 10 further includes a controller 30 for controlling the operation of the injection molding machine 20, including the opening and closing of the gate 1 14. The controller 30 may comprise one or more processors (not expressly illustrated) and memory (also not expressly illustrated) storing instructions for execution by the one or more processors for effecting gate closing control as described herein. The controller 30 is in communication with, and sends commands to, the injection molding machine 20. The commands may include commands sent to, e.g., the actuator assembly 125 (including actuator 126), for triggering a closing of the gate 1 14. The controller 30 may for example be a

programmable logic controller (PLC), such as Allen-Bradley® SLC™ 5/05, computer or other form of computing device.

[0040] In some embodiments, operation of the controller 30 as described herein may be governed, in whole or in part, by instructions that may be loaded from a non- transitory machine-readable medium 32, such as an optical disk or magnetic storage medium for example, into a volatile or non-volatile memory associated with the controller 30 for interpretation by and/or execution by one or more processors of the controller 30. [0041] The injection molding system 10 further includes a human machine interface (HMI) 40 (FIG. 1 ) that may be used by a human operator to control aspects of the operation of the injection molding system 10, possibly including the inputting of operating parameters governing the operation of the injection molding system 10. The HMI 40 may include, but is not necessarily limited to, a keyboard, a pointing device such as a mouse or trackball, a touchscreen, one or more physical controls such as buttons, or the like.

[0042] FIG. 3 illustrates a graph 300 showing a position of the valve stem 1 16 over time during a hypothetical closing of gate 1 14 of FIG. 2, wherein the triggering event for initiating closing of the gate is an end of a hold phase of a molding cycle of the injection molding system 10. The graph 300 illustrates how use of the end of hold phase as the triggering event may result in undesirable consequences in the event of a dynamic change in a motion profile of the closing valve stem (or, more generally, in the event of a dynamic change in the gate closing duration). [0043] Referring to FIG. 3, the vertical axis of graph 300 represents valve stem position in generic units of measurement or distance, and the horizontal axis of graph 300 represents time in generic time units. As shown on the vertical axis of graph 300, the valve stem 1 16 is movable between a first position in which the gate 1 14 is fully closed and a second position in which the gate is fully open. Initially (at time t=0 units), the valve stem 1 16 is in the latter position. Different phases of a molding cycle of the injection molding system 10 are indicated at the top of the graph 300, including injection fill, injection hold, cooling, mold opening, ejector forward, part ejection, ejector back, and mold closing. [0044] The graph 300 shows thee different motion profiles 310, 320 and 330, each representing a different way of closing the gate 1 14.

[0045] The first motion profile 310, which appears in FIG. 3 as a solid line spanning points A, B, C, D E, F, G and H of the graph 300, illustrates a closing of the gate 1 14 wherein the valve stem 1 16 moves at a steady rate between a "gate fully open" position (point C on profile 310) and a "gate fully closed" position (point E on the profile). Starting at approximately time t=1 units of graph 300, an injection fill phase (or simply "injection phase") of a molding cycle is commenced. During that phase, melt is injected through the open gate 1 14 into an associated molding cavity of the mold. In some embodiments, the injection may be performed by a shooting pot, which may be considered essentially as a plunger within a vessel that receives melt from a

plasticizing (melting) unit. In other embodiments, injection may be performed by a reciprocating screw that is also used to melt the molding material.

[0046] During the injection hold phase (or simply "hold phase") of the molding cycle, which starts at approximately time t=4 units of graph 300, the position of the valve stem 1 16 remains unchanged, i.e. gate 1 14 remains open. The open gate 1 14 permits melt to continue to flow into the molding cavity of the mold, as necessary, to occupy any gaps that may form within the molding cavity due to shrinkage of the cooling molded article within the molding cavity. [0047] Referring still to the motion profile 310 of FIG. 3, at approximately time t=7 units, closing of the gate 1 14 commences, i.e. the valve stem 1 16 begins to move along the trajectory defined by the line spanning points C and E on the motion profile 310. In particular, triggering of gate closure occurs at the end of the hold phase, e.g. by an end-of-hold signal generated by the controller 30, on or about time t=7 units.

[0048] Movement of the valve stem 1 16 (i.e. closing of the gate 1 14) continues at a steady rate according to motion profile 310 until time t=10 units, at which point the gate becomes fully closed and movement of the valve stem 1 16 ceases (see point E on the motion profile 310). The closing duration for closing the gate 1 14 in this example is accordingly 3 units of time (10 units - 7 units).

[0049] The gate 1 14 remains closed during the subsequent cooling, mold opening, ejector forward, part ejection, ejector back and mold closing phases, which occur between points F and G of motion profile 310.

[0050] Thereafter, opening of the gate commences at approximately time t=38 units (see point G on motion profile 310) in preparation for a subsequent molding cycle.

[0051] In some applications or embodiments, it may be considered that the closing of the gate 1 14 at a steady rate, e.g. according to motion profile 310 of FIG. 3, is less than optimal in certain respects. For example, if the speed of the valve stem has been set to minimize gate closing duration, the speed of the moving valve stem 1 16 may be fairly high. In that case, there may be a risk of damage to either or both of the valve stem 1 16 and the gate orifice 1 14 when these components come into contact with one another. For this reason or for other reasons, it may be desired to decelerate the valve stem 1 16 upon approaching the fully closed condition of the gate 1 14. This is illustrated in the second motion profile 320. [0052] Referring still to FIG. 3, the second motion profile 320 is illustrated as a dashed line spanning points A, B, C, D, F, G and H of graph 300. As was the case for motion profile 310, the triggering event for closing the gate 1 14 is the end of the hold phase (e.g. end-of-hold signal). The end-of-hold signal triggering event may be operator-configurable, and the end-of-hold signal may be machine-generated. [0053] The second motion profile 320 substantially overlaps the first motion profile 310, except between points D and F. As such, the dashed-line portions of motion profile 320 between points A and D and between points F and H may not be visible against the overlapping, solid line portions of motion profile 310 in FIG. 3. [0054] It will be appreciated that, between points D and F of the motion profile 320 of FIG. 3, the speed of the valve stem 1 16 is reduced relative to the speed of the valve stem 1 16 between points C and D of the same motion profile 320. The area between points D and F of the motion profile 320 may accordingly be referred to as a

"deceleration curve". In other words, in motion profile 320, the valve stem

progressively slows down as it approaches the fully closed condition of gate 1 14. The slowdown may for example be intended improve gate quality, e.g. to reduce a risk of misalignment of the valve stem 1 16 in the gate 1 14 upon closing, or to reduce a risk of damaging the valve stem 1 16 or gate 1 14 in comparison to what might occur if the valve stem 1 16 is closed according to the first motion profile 310. Alternatively, or in conjunction, a slower speed may beneficially reduce crown flash. The slowing might for example be achieved by sending appropriate control signals from controller 30 to the actuator 126 of the actuator assembly 125 (see FIG. 2) for causing a desired deceleration of the valve stem 1 16.

[0055] Because the speed of the valve stem 1 16 between points C and D of motion profile 320 is slower than the speed of the valve stem 1 16 in the corresponding portion of motion profile 310, closing of the gate 1 14 takes longer than it did according to motion profile 310. More specifically, by virtue of the change from motion profile 310 to motion profile 320, the gate 1 14 does not fully close until time t=12 units, which is later than the time t=10 units at which the gate became fully closed in motion profile 310. A change from motion profile 310 to motion profile 320 accordingly results in an increase in the closing duration for closing the gate 1 14, from 3 time units (10 units - 7 units = 3 units of time) to 5 time units (12 units - 7 units = 5 units of time).

[0056] In view of the longer gate closing duration in motion profile 320, the in-mold cooling time (e.g. the time between either the "stem fully closed" position (point F) of the motion profile 320, or the end of hold time, and the mold opening time) decreases to maintain the same overall molding cycle time. In some embodiments, to avoid a possibly undesired effect upon molded article quality from a shortened in-mold cooling time (e.g. increased susceptibility to deformation upon ejection), it may be desired to deviate from the motion profile 320 in favor of an extended in-mold cooling time of the same length as in motion profile 310. However, such a deviated profile (not shown in FIG. 3) would have the result of extending the overall cycle time. A longer cycle time may be considered undesirable for other reasons, e.g. because an overall throughput of the injection molding system 10, in terms of total molded articles manufactured over a particular operating interval, may decrease, or because capital expended per manufactured article may accordingly increase.

[0057] By moving the valve stem 1 16 according to the third motion profile 330 of FIG. 3, it may be possible to attain the above-described benefit(s) of the deceleration curve while avoiding the above-mentioned, possibly undesirable effect(s) of either a shortened in-mold cooling cycle or of a longer overall molding cycle. [0058] Referring to FIG. 3, it can be seen that the third motion profile 330 is depicted as a dotted line spanning points A, B, E, F, G and H of graph 300. Because the third motion profile 330 substantially overlaps the first motion profile 310 except between points B and E, the dotted line of motion profile 330 is only visible between points B and E. In contrast, the dotted line is not visible between points A and B or between points F and H, where it substantially overlays the solid line of the first motion profile 310.

[0059] The third motion profile 330 differs from the first and second motion profiles 310 and 320 in that closing of the gate 1 14 is triggered before the end of the hold phase, at a time of approximately t=5 units in the illustrated example, rather than being triggered at (or by) the end of the hold phase, at time t=7 units in the illustrated example. Because closing of the gate commences earlier, the valve stem 1 16 can be moved along a similar trajectory as that of the second motion profile 320— including the deceleration curve— while achieving or maintaining full closure of the gate 1 14 at the same time (t=10 units) as in the first motion profile 310 that lacked the deceleration curve. [0060] Example operation 400 of the injection molding system 10 for achieving a motion profile similar or identical to the motion profile 330 of FIG. 3 is illustrated in FIG. 4. In particular, FIG. 4 shows a flowchart of operations that may be used to

dynamically compute, set or adjust the time at which closing of the gate 1 14 is commenced so as to achieve or maintain a desired interval between closing of the gate and the end of the hold phase of the molding cycle despite a change in a motion profile of a valve stem (or other gate component) resulting in an increased closing duration of the gate. The operation 400 may be effected or governed by the controller 30 of FIG. 1 , possibly based on instructions loaded from non-transitory machine- readable medium 32.

[0061] The operation 400 of FIG. 4 will be described in conjunction with graph 500 of FIG. 5. Graph 500 illustrates a motion profile of the valve stem 1 16 over multiple molding cycles of the injection molding machine 20. In particular, graph 500 shows how dynamic adjustment of a trigger time at which closing of the gate is triggered, relative to the end of the hold phase of the molding cycle, may be performed without increasing an overall molding cycle time. Operation 400 may occur in conjunction with a "learning phase 502" during which aspects of operation of the injection molding system 10 are measured and used for computing, setting or adjusting the time at which the gate closing is triggered. For example, operation 400 may be based on measurements made during the learning phase 502, e.g. using a timer forming part of the controller 30, with the purpose of operation 400 being to determine a trigger time Ttrigger for a current molding cycle 504. It is noted that whatever learning phase may be used need not necessarily immediately precede the molding cycle in which the gate closing trigger time is being set or adjusted. [0062] Referring to operation 402 of FIG. 4, initially an indication of a desired interval (denoted herein as Tgc) between an end of the hold phase of each molding cycle and complete closure of the gate 1 14 (i.e. the gate 1 14 achieving the fully closed condition) is received. In some embodiments, the indication may take the form of user input from a human-machine interface 40. [0063] For example, referring to FIG. 6, a graphical user interface (GUI) 600, may be presented on a display screen of the human-machine interface 40 of the injection molding system 10 (FIG 1 ). As shown in FIG. 6, the example GUI 600 may include a prompt 602 instructing a human operator to specify a desired interval between the end of the injection hold phase and complete closure of the gate 1 14. The GUI 600 may also include a text box 604 and/or other form of GUI control 606 (e.g. a slider) for facilitating provision of user input specifying the desired interval. The desired interval may for example be entered in the form of time units, e.g. as seconds, milliseconds, or some other unit of measure. The entered value may be based on a desired part or gate quality for example.

[0064] In the present example, user input may be entered specifying an interval of three (3) time units between the end of the hold phase and complete closure of the gate 1 14, as in the third motion profile 330 of FIG. 3 (between time t=10 units to time t=13 units). This may involve the user entering a "3" in the edit box 604 or manipulating a movable indicator 608 along a slider 606 to specify the desired interval of 3 units. Various alternative mechanisms for specifying the interval may be used in alternative embodiments. In some cases, an interval Tgc of zero could be specified.

[0065] Some embodiments of GUI 600 may employ a GUI construct 610 for limiting a value of Tgc that can be specified using the GUI 600. For example, referring to FIG. 6, the dashed line 610 may be displayed to represent a limit beyond which the movable indicator 608 may not be slid leftwardly along slider 606. This may prevent the user from specifying a Tgc that is too short, which may result in too early of a gate closure during the hold phase for example. The user interface may thus limit the user input that can be entered to ensure that the desired interval is not below a

predetermined threshold. In other words, the user GUI 600 may effectively set a lower limit for Tgc values that will be accepted, e.g. based on a desired part quality or gate quality.

[0066] In operation 404 of FIG. 4, an end of hold phase time is determined. This may be performed in various ways. In one approach, the end time of the hold phase of a previous molding cycle (denoted herein as TpreviousEOH) may be determined. For example, in FIG. 5, at some point on or after time t=48 units, TpreviousEOH may be determined to have occurred at time t=48 units. In another approach, the anticipated end time of the hold phase of a current molding cycle (denoted herein as T_EOH) may be determined. For example, in FIG. 5, based on a knowledge of a previous trigger time TpreviousEOH of the immediately preceding cycle or an earlier cycle and a

presumption that an injection molding cycle duration (denoted Tc), which may be measured in a past molding cycle, is consistent between molding cycles, then T_EOH may be determined by adding Tc (or a multiple of Tc) to TpreviousEOH. For example, in FIG. 5, if Tc has been measured at 41 time units and TpreviousEOH of the immediately preceding molding cycle occurs at time t=48 units, then T_EOH may be determined (projected) to occur at time t=89 units (i.e. 48 + 41 = 89 units).

[0067] It will be appreciated that, to the extent that determining the injection molding cycle duration (Tc) is performed as part of operation 404, it may be done in various ways. For example, an elapsed time between consecutive end-of-hold signals may be measured. In FIG. 5 for example, the cycle time of the first complete cycle of graph 500 may be measured as the elapsed time between the first end-of-hold signal at time t=7 and the second end-of-hold signal at time t=48, or 41 time units. Alternatively, the cycle time may be determined in another way, e.g. by measuring elapsed time between other points in consecutive molding cycles, by reading a cycle time system parameter from a memory of controller 30, or otherwise. It will be appreciated that the determined time for triggering gate closure for the current cycle may be based, at least in part, on a presumption that Tc for the current molding cycle will be the same as Tc in the previous molding cycle.

[0068] Referring to operation 406 of FIG. 4, a closing duration for closing the gate (denoted Tsc) is determined. The closing duration refers to the duration required to move the valve stem 1 16 from the "gate fully open" position to the "gate fully closed" position, according to a desired motion profile— which, in the present example, includes the desired deceleration curve. The closing duration may for example be determined by measuring an elapsed time between a signal that triggered the closing of the gate, e.g. an end-of-hold signal, and a detection of full closure of the gate, e.g. as by a sensor forming part of the injection molding machine 20. In some embodiments, the measuring may be performed by a timer forming part of controller 30. In the example of FIG. 5, such measuring over time t=7 to time t=12 reveals that the closing duration Tsc is 5 time units (12 -7 = 5). As will be appreciated, in the present embodiment, the time of triggering gate closure for the current cycle will be based, at least in part, on a presumption that Tsc for the current molding cycle will be the same as Tsc in the previous molding cycle.

[0069] The order of operations 402, 404 and 406 of FIG. 4 may differ in alternative embodiments. [0070] Referring to operation 408 of FIG. 4, a trigger time at which closing of the gate is to be triggered (denoted Ttrigger) is dynamically set or adjusted so as to provide the desired interval (Tgc) between the end of the hold phase (T_EOH) and complete gate closure. The operation 408 may be performed automatically, e.g. in response to a detected change in the closing duration (Tsc) for closing the gate, so that the time at which the gate becomes completely closed relative to end-of-hold is unchanged, possibly with a view to keeping the molding cycle duration from increasing. Performing trigger time adjustment automatically may be advantageous over, e.g., alternative approaches in which gate closure is always triggered at a predetermined time (e.g. at the end of the hold phase) or is manually adjusted every time that a change in a motion profile occurs involving a changed closing duration is detected. The latter approaches may risk a throughput decrease and/or impact upon a quality of the molded article, as mentioned above, upon occurrence of a change in the gate closure (e.g. valve stem) motion profile.

[0071] Two example approaches for determining the trigger time Trigger are discussed below. These approaches are exemplary and not exhaustive.

[0072] In a first approach, Ttrigger is determined relative to the TpreviousEOH by determining a time offset (denoted Toffset, a duration) from the end of hold phase time of the previous molding cycle (TpreviousEOH) at which closing of the gate should be triggered. The Toffset value is then added to an end-of-hold time for a previous molding cycle to determine Ttrigger for a subsequent molding cycle.

[0073] The time offset Toffset may be determined by subtracting, from cycle time Tc (which is presumed to be the same for consecutive molding cycles in this example), the closing duration Tsc, and then adding the desired interval Tgc between the end of the hold phase and complete closure of the gate 1 14. Such computation of Toffset may be expressed in the form of equation (1 ) below.

[0074] Toffset = Tc - Tsc + Tgc (1 )

[0075] The order of operations of equation (1 ) is not important. [0076] In the example of FIG. 5, equation (1 ) may be used to calculate a Toffset of 39 units, as follows:

[0077] Toffset = 41 units - 5 units + 3 units [0078] = 39 units

[0079] Then Ttrigger may be determined by adding the Toffset value to the time of the end of hold phase for the preceding molding cycle. In the example of FIG. 5, this may yield a trigger time of time t=87 units, as per equation (2) below.

[0080] Ttrigger = TpreviousEOH + Toffset (2)

[0081 ] = 48 units + 39 units

[0082] = 87 units, i.e. at trigger gate closing at time t=87 [0083] In a second approach, Ttrigger may be determined relative to the anticipated end of hold time for the current molding cycle (T_EOH), which may be known due to a periodicity of end of hold phase events. Such computation of Toffset may be expressed in the form of equation (3) below.

[0084] Ttrigger = T_EOH - Tsc + Tgc (3) [0085] = 89 units - 5 units + 3 units

[0086] = 87 units, i.e. at trigger gate closing at time t=87 [0087] Regardless of the manner in which Ttrigger is determined, when gate closing is triggered at the determined trigger time of t=87 units, the closing of the gate occurs over the course of the next Tsc time units (5 units in the example), with the gate becoming fully closed at time t=92 units, i.e. at the desired duration Tgc (3 units) after the current molding cycle's end of hold (t=89 units).

[0088] As should be apparent from the above, the periodic nature of injection molding cycles over the course of multiple molding cycles may permit the controller 30 to "learn" the schedule of end-of-hold events. Based in part on this information and an expectation that the periodicity will not change, a trigger time Ttrigger may be

determined for initiating a closing of a gate for each molding cycle so as to provide a consistent interval (in the steady state) between an anticipated end of the hold phase time and gate closure, regardless of the currently operative valve stem motion profile. That is, the controller 30 triggers closing of the gate at an appropriate time, which may be before (or, in some embodiments, possibly after) an end-of-hold event, so that the gate becomes fully closed at the desired time relative to the end-of-hold event, regardless of the currently operative gate closing motion profile. This may prevent the molding cycle time from undesirably increasing from a desired duration, while still allowing for adjustment of the profiled gate closing motion that may, e.g., improve a quality of gate closure or of the molded article. [0089] It will be appreciated that operations 402-408 above may, in some

embodiments, be executed in a start-up mode of operation of the injection molding system 10 that occurs before steady-state injection molding system operation. During the start-up mode, the injection molding system 10 may perform a number of molding cycles to permit heaters to heat up to their desired operation temperature, to permit the melt to achieve a homogeneous state, and so forth.

[0090] In operation 410, if it is determined that recalibration should be performed, then control returns to operation 406. In some embodiments, recalibration may for example be triggered when the injection molding system 10 returns to a start-up mode of operation. In some embodiments, recalibration may occur periodically or on demand. To the extent that that the Tsc determined in operation 406 during recalibration differs from an earlier Tsc, this will be detected, and the trigger time Ttrigger that is set in operation 408 will account for the new value of Tsc. The Ttrigger value will be

determined, based on the new Tsc so as to preserve the desired Tgc. Depending upon the implementation, one or more molding cycles may need to be executed before any adjustment in the trigger time Ttrigger is performed and takes effect.

[0091] It will be appreciated that, to the extent that a new Tgc is specified (e.g. by a user via GUI 600), re-execution of at least operations 402 and 408 may be required in order to adjust the Ttrigger as necessary to provide the newly specified Tgc.

[0092] Various alternative embodiments are possible. [0093] In some embodiments, the indication of Tgc is not received via a GUI 600 or an HMI 40 but rather is otherwise determined. For example, Tgc may be read from a data record stored on a machine-readable medium.

[0094] Although the above example describes a multi-cavity mold, the described approach can be used in molds having only a single cavity. [0095] It will be appreciated that the hot runner 1 10 and nozzle assembly 1 10 of FIG. 2 are only examples. In other embodiments, different forms of hot runner, nozzle assembly, nozzles, gates and means for opening and closing gates, may be used.

[0096] It will be appreciated that any TpreviousEOH value that may be computed need not necessarily represent an end of hold time of an immediately preceding molding cycle. The TprevoiusEOH value could represent an end of hold time of earlier molding cycle. In such cases, computation of Ttrigger may entail adding a multiple N (an integer) of Tc representing N intervening cycles between Tprevious EOH and the current molding cycle.

[0097] Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.

Claims

WHAT IS CLAIMED IS:

1 . A method of closing a gate of an injection molding system, the method comprising: based on a determined closing duration (Tsc) for closing the gate, computing a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate becoming fully closed; and at the computed trigger time, triggering closing of the gate.

2. The method of claim 1 wherein the setting of the trigger time is performed automatically in response to a detected change in the closing duration (Tsc) for closing the gate.

3. The method of claim 2 wherein the gate comprises a valve stem and wherein the detected change in the closing duration comprises a detected change in a motion profile of the valve stem.

4. The method of any one of claims 1 to 3 further comprising dynamically determining the closing duration for closing the gate by measuring a time required for the gate to close.

5. The method of any one of claims 1 to 4 further comprising:

receiving, via a user interface, user input specifying the desired interval

between the end of the hold phase of the previous molding cycle and the gate becoming fully closed.

6. The method of claim 5 wherein the user interface constrains the user input to require the desired interval between the end of the hold phase of the previous molding cycle and the gate becoming fully closed to be at or above a predetermined threshold.

7. The method of any one of claims 1 to 6 wherein the gate comprises a valve stem and wherein the triggering comprises sending a signal for causing a valve stem actuator to actuate the valve stem.

8. A method of operating an injection molding system, the method comprising:

detecting a change in a closing duration (Tsc) for closing a gate (1 14) of the injection molding system; and

automatically adjusting a time (Ttrigger) at which closing of the gate is triggered, the automatic adjusting maintaining a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate becoming fully closed despite the change in the closing duration.

9. The method of claim 8 wherein the gate comprises a valve stem and wherein the detecting of the change in the closing duration for closing the gate comprises detecting a change in a motion profile of the valve stem.

10. A controller for an injection molding system, the controller comprising:

at least one processor; and

a memory storing instructions, that, when executed by the at least one processor, cause the controller to:

based on a determined closing duration (Tsc) for closing a gate of the injection molding system, compute a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) and the gate achieving a fully closed position; and

at the computed trigger time, trigger closing of the gate.

1 1 . The controller of claim 10 further comprising a timer, wherein the instructions further cause the controller to measure the closing duration (Tsc) using the timer.

12. The controller of claim 1 1 wherein the computing of the trigger time is performed automatically in response to a detected change in the closing duration (Tsc).

13. A non-transitory machine-readable medium storing instructions, that, when executed by a controller of an injection molding system, cause the controller to:

based on a determined closing duration (Tsc) for closing a gate (1 14) of the injection molding system, compute a trigger time (Ttrigger) at which closing of the gate is to be triggered so as to provide a desired interval (Tgc) between an end of a hold phase of a molding cycle (T_EOH) of the injection molding system and the gate achieving a fully closed position; and

at the computed trigger time, trigger closing of the gate.

14. The machine-readable medium of claim 13, wherein the computing of the trigger time is performed automatically in response to a detected change in the closing duration (Tsc) of the gate.

15. The machine-readable medium of claim 13 or claim 14 wherein the instructions further cause the controller to dynamically determine the closing duration for closing the gate by measuring a time required for the gate to close.

16. The machine-readable medium of any one of claims 13 to 15 wherein the instructions further cause the controller to:

receive, from a human-machine interface, an indication of the desired interval between the end of the hold phase of the previous molding cycle and the gate achieving the fully closed position.