WO2013159210A1

WO2013159210A1 - Shooting pot sensor for injection molding machine

Info

Publication number: WO2013159210A1
Application number: PCT/CA2013/000419
Authority: WO
Inventors: Robert Schad; Stephen Mracek
Original assignee: Athena Automation Ltd.
Priority date: 2012-04-27
Filing date: 2013-04-29
Publication date: 2013-10-31

Abstract

An injection cylinder for an injection molding machine includes a cylinder housing and an injection piston slidably disposed in the cylinder housing. The injection piston is translatable between a piston retracted position and a piston advanced position, the injection piston having a length and an internal cavity extending axially along at least a portion of the piston length. A sensor rod is fixed to the housing and extends axially through at least a portion of the internal bore of the piston. A first sensing element is slidably disposed within the internal bore of the piston, the first sensing element moveable relative to the piston between sensor advanced and sensor retracted positions, and the first sensing element moveable with the piston when translating between the piston advanced and piston retracted positions.

Description

TITLE: SHOOTING POT SENSOR FOR INJECTION MOLDING MACHINE FIELD

[0001] The teaching disclosed herein relates to injection molding machines, and to methods and apparatuses for filling a shooting pot with melt and injecting the melt from the shooting pot to a mold.

BACKGROUND

[0002] U.S. Pat. No. 7,670,537 (Ujma et al.) discloses active decompression to prevent melt drool from a mold (12, 14) or runner system (20) by the selective coupling and de-coupling of an injection piston (34) to a plunger (38). Following successive injection and hold phases of an injection molding process, the runner and channel system is partially de-compressed by drawing back together, over a short distance, the plunger (38) and the injection pressure (34) as one unit. The injection piston is then mechanically de-coupled from the plunger, with the injection piston (34) withdrawn to essentially its final shot position, but minus a customary packing distance. Plastic melt (100), extruded into a shooting pot (28) positioned in front of the plunger (38), is permitted to push the plunger backwards, but now with minimal work expenditure. When the plunger again contacts the injection piston (34), melt pressure now causes both units to be moved back to reach a final shot size volume. Finally, the injection piston (34) drives the plunger (38) forward to eject melt accumulated within the shooting pot (28).

SUMMARY

[0003] The following summary is intended to introduce the reader to this specification but not to define any invention. In general, this specification discusses one or more methods or apparatuses for filling a shooting pot with melt and injecting the melt from the shooting pot to a mold.

[0004] An injection unit for an injection molding machine, includes: a shooting pot disposed between an injection nozzle and a plunger, the shooting pot having an adjustable volume for receiving and dispensing melt, the plunger reciprocally slidable relative to the injection nozzle to increase and decrease the volume of the shooting pot; an injection cylinder adjacent the plunger, the injection cylinder comprising a piston slidably disposed within a cylinder housing, the injection cylinder including an injection chamber on one side of the piston for urging the piston towards the injection nozzle when pressurized; and a position sensor comprising a static sensor element fixed relative to the cylinder housing and at least a first dynamic sensor element coupled to the piston and axially movable relative to the static sensor element, the position sensor providing an output signal responsive to the axial position of the first dynamic sensor element relative to the static sensor element, the first dynamic sensor element movable relative to the piston between a sensor advanced position and a sensor retracted position, and the first dynamic sensor element moveable with the piston relative to the cylinder housing as the piston moves towards and away from the injection nozzle.

[0005] At least one of the static sensor element and dynamic sensor element may comprise a sensor rod extending axially within at least a portion of the cylinder housing. In some examples, the static sensor element comprises a sensor rod extending axially within at least a portion of the cylinder housing. In some examples, the sensor rod is fixed relative to the cylinder housing. In some examples, the first dynamic sensor element is fixed to a first carrier, the sensor rod extending through at least a portion of the first carrier.

[0006] The piston may comprise an internal cavity, and the first carrier may be disposed in the internal cavity, the carrier slidable between the sensor advanced and sensor retracted positions. The carrier may be biased to the sensor advanced position.

[0007] The piston may comprise a front face for engaging a rear face of the plunger, and the carrier may comprise a trigger surface proud of the front face when the sensor is in the sensor advanced position. The trigger surface may be generally flush with the front face when the carrier is in the sensor retracted position.

[0008] The piston may comprise an end wall with an axial thickness extending between the front face of piston and a bottom of the internal cavity, the carrier may comprise an actuating pin passing through a bore in the end wall, and the trigger surface may comprise an end of the actuating pin.

[0009] The first dynamic sensor element may comprise a first magnet.

[0010] The injection unit may comprise a second dynamic sensor element, the second dynamic sensor element axially moveable relative to the static sensor element and relative to the first dynamic sensor element. The second dynamic sensor element may be fixed within a second carrier. The second dynamic sensor element may be fixed relative to the piston. The second dynamic sensor element may comprise a second magnet.

[001 1] The injection cylinder may comprise a retraction actuator for pre- positioning the piston to a retracted position in spaced axial relation from the plunger. The retraction actuator may comprise a retraction chamber on a side of the piston opposite the injection chamber.

[0012] According to some aspects, a two-stage injection unit for an injection molding machine comprises: a plasticizing unit for delivering melt to a shooting pot, the shooting pot having an adjustable volume; an injection cylinder spaced apart from the shooting pot and comprising a piston slidably mounted in an injection cylinder housing, the injection cylinder comprising a retraction fluid chamber on one side of the piston for retracting the piston away from the shooting pot when pressurized, and an injection chamber on an opposed side of the piston for urging the piston towards the shooting pot when pressurized; a plunger slidably disposed between the shooting pot and the injection cylinder, the plunger slidable between a plunger advanced position and a plunger retracted position to adjust the volume of the shooting pot; and a position sensor disposed within the injection cylinder, the position sensor registering the position of the piston relative to the injection cylinder housing, and the position sensor registering approach and contact of the plunger with the piston when the plunger is pushed to the plunger retracted position by the melt.

[0013] The position sensor may comprise a static sensor element fixed relative to the injection cylinder housing and at least a first dynamic sensor element coupled to the piston and axially movable relative to the static sensor element, the sensor providing an output responsive to the axial position of the first dynamic sensor element relative to the static sensor element.

[0014] The first dynamic sensor element may be movable relative to the piston between a sensor advanced position and a sensor retracted position. The first dynamic sensor element may be moveable with the piston relative to the injection cylinder housing. The static sensor element may comprise a sensor rod extending axially within at least a portion of the injection cylinder housing. The dynamic sensor element may comprise a magnet fixed within a carrier.

[0015] The piston may comprise an internal cavity, and the carrier may be disposed in the internal cavity, and the carrier may be slidable between the sensor advanced and sensor retracted positions. The carrier may be biased to the sensor advanced position. The piston may comprise a front face for engaging a rear face of the plunger, and the carrier may comprise a trigger surface proud of the front face when in the sensor advanced position. The trigger surface may be generally flush with the front face when the carrier is in the sensor retracted position. The piston may comprise an end wall with an axial thickness extending between front face of piston and a bottom of the internal cavity, the carrier may comprise an actuating pin passing through a bore in the end wall, and the trigger surface may comprise an end of the actuating pin.

[0016] According to some aspects, an injection cylinder for an injection molding machine comprises: a cylinder housing; an injection piston slidably disposed in the cylinder housing, the injection piston translatable between a piston retracted position and a piston advanced position, the injection piston having a length and an internal cavity extending axially along at least a portion of the piston length; a sensor rod fixed to the housing and extending axially through at least a portion of the internal bore of the piston; a first sensing element slidably disposed within the internal bore of the piston, the first sensing element moveable relative to the piston between sensor advanced and sensor retracted positions, and the first sensing element moveable with the piston when translating between the piston advanced and piston retracted positions.

[0017] The piston may comprise an end wall with an axial thickness extending between a front face of the piston and a bottom of the internal cavity, the first sensing element may be mounted within a carrier, and the carrier may comprise an actuating pin passing through a bore in the end wall.

[0018] According to some aspects, a method of filling a shooting pot with melt for injection into a mold, comprises: pre-positioning an injection piston at a retracted position spaced apart from a plunger by a gap; transferring melt into a shooting pot from a plasticizing unit, the incoming melt pushing the plunger towards the injection piston; engaging an actuating pin with the plunger, the actuating pin protruding from the injection piston; in response to step (c), transitioning control of the melt transfer from speed control to pressure control; stopping the flow of melt into the shooting pot when a target pressure has been reached; and moving the injection piston to an advanced position to urge the melt from the shooting pot into the mold.

[0019] Other aspects and features of the present specification will become apparent, to those ordinarily skilled in the art, upon review of the following description of the specific examples of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawings included herewith are for illustrating various examples of articles, methods, and apparatuses of the present specification and are not intended to limit the scope of what is taught in any way. In the drawings: [0021] Figure 1 is a cross section taken through an exemplary two- stage injection unit of an injection molding machine;

[0022] Figure 2 is an enlarged view of a portion of a plunger injection apparatus of the injection unit of Figure 1 , showing a plunger between a plunger advanced position and a plunger retracted position, and a piston in first piston retracted position;

[0023] Figure 3 is an enlarged view of a portion of the plunger injection apparatus of the injection unit of Figure 1 , showing the plunger in a plunger retracted position and a piston in second piston retracted position;

[0024] Figures 4A to 4E are enlarged views a portion of the plunger injection apparatus of the injection unit of Figure 1 , sequentially showing the positions of the plunger and piston, as well as a position sensor, as the injection unit goes through an injection cycle;

[0025] Figure 5A is a perspective view of a carrier of an alternate position sensor;

[0026] Figure 5B is a cross-section taken along line 5B-5B in Figure 5A;

[0027] Figure 6 is an enlarged cross-sectional view of an injection cylinder and an alternate exemplary position sensor;

[0028] Figure 7 is an enlarged cross-sectional view of an injection cylinder and another alternate exemplary position sensor shown in a first position; and

[0029] Figure 7A is an enlarged cross-sectional view of the structure of

Figure 7, showing the sensor in a second position.

DETAILED DESCRIPTION [0030] Various apparatuses or processes will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover processes or apparatuses that differ from those described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or process described below is not an embodiment of any claimed invention. Any invention disclosed in an apparatus or process described below that is not claimed in this document may be the subject matter of another protective instrument, for example, a continuing patent application, and the applicants, inventors or owners do not intend to abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.

[0031] Referring to Figure 1 , an exemplary two-stage injection unit 100 for injecting melt into the mold of an injection molding machine is shown. The injection unit 100 includes a plasticizing apparatus 102 which plasticizes resin (also referred to as "melt"), a plunger injection apparatus 104 which forces the plasticized resin into a mold, and an injection nozzle 106 through which the plasticized resin passes into the mold.

[0032] A valve 108 is provided between the plasticizing apparatus 102, plunger injection apparatus 104, and injection nozzle 106. The valve 108 is moveable between a first (transfer) position, in which the valve 108 provides fluid communication between the plunger injection apparatus 104 and the plasticizing apparatus 102 (in isolation from the nozzle), and a second (injection) position, in which the valve provides fluid communication between the plunger injection apparatus 104 and the injection nozzle 106 (in isolation from the plasticizing apparatus 102). In Figure 1 , the valve 108 is shown in the injection position. A valve actuator 109 can be energized to rotate the valve 108 counter clockwise (by about 45 degrees) in the example illustrated, to move the valve to the transfer position and provide fluid communication between the plasticizing unit 102 and a shooting pot 18 of the plunger injection apparatus 104.

[0033] The plasticizing apparatus 102 includes a plasticizing screw 10 housed within a barrel 1 12 and rotationally and translationally driven by a plasticizing drive 1 14. In the example illustrated, the plasticizing drive 114 includes a transfer piston 115a that can be urged forward by pressurizing a transfer fluid chamber 115b. The barrel 112 is in communication with a transfer channel 1 16, which is selectively in communication with the shooting pot 1 18 of the plunger injection apparatus 104 via the valve 108.

[0034] Resin can be transferred from a hopper 1 19 into the barrel 1 12, where it is plasticized by rotation of the plasticizing screw 110. During plasticization, the valve 108 is in the second position, and the transfer channel 1 16 is closed to (or isolated from) the shooting pot 1 18. As the plasticizing screw 1 10 is rotated and the resin is plasticized, the plasticized resin (or melt) is urged towards the transfer channel 1 16, filling the barrel 1 12 in front of the screw 110, and the plasticizing screw 1 10 is forced rearwards by the accumulating melt to a retracted position.

[0035] Referring still to Figure 1 , the shooting pot 1 18 has an adjustable volume, and receives and dispenses melt. The shooting pot 118 is disposed between the injection nozzle 106 and an injection plunger 120 of the plunger injection apparatus 104. The injection plunger 120 is reciprocally slidable relative to the injection nozzle 106, between a plunger advanced position (shown in Figure 1 ) and a plunger retracted position (shown in Figure 3), to increase and decrease the volume of the shooting pot 118. The plunger 120 is, in the example illustrated, pushed to the plunger retracted position as the shooting pot 1 18 receives melt, and is pushed to the plunger advanced position to dispense the melt.

[0036] Referring to Figures 2 and 3, the plunger injection apparatus 104 further includes an injection cylinder 122, which is adjacent the plunger 120 and spaced apart from the shooting pot 118, so that the plunger 120 is disposed between the shooting pot 1 18 and the injection cylinder 122. The injection cylinder 122 urges the plunger 120 to the plunger advanced position to dispense the melt.

[0037] The injection cylinder includes a piston 124 slidably disposed within a cylinder housing 136. The piston 124 includes a front face 130 for engaging a rear face 132 of the plunger 120. The piston 124 is slidable between a piston advanced position (shown in Figure 4E), and at least a first piston retracted position (shown in Figures 3 and 4C) spaced axially away from the piston advanced position. The piston 124 may optionally be moveable to a second piston retracted position (shown in Figures 2 and 4A) disposed axially forward of the first retracted position, towards the piston advanced position.

[0038] The first piston retracted position generally corresponds to a "full shot" position in which the volume of the shooting pot 1 18 is equal to that required to fill the mold. The optional second piston retracted position can correspond to a "short-shot" position in which the volume of the shooting pot 118 is less than that required to fill the mold.

[0039] When the plunger is in the advanced position and the piston 124 is in either the first or second retracted position, the piston 124 and plunger 120 are generally spaced apart from each other by an axial gap. For example, when the plunger is in the advanced position and the piston 124 is in the second retracted position, the rear face 132 of the plunger 120 can be spaced apart from the front face 130 of the piston 24 by a first gap 21 (see Figure 4A).

[0040] The piston can be moved to the first retracted position by a force exerted by the plunger 120 as it is urged rearwards when the shooting pot is filled with melt. Alternatively or additionally, a retraction fluid chamber 126 adjacent one face of the piston may be pressurized to move the piston 124 to the first retracted position. When in the first retracted position, the rear face of the plunger may push against the front face 130 of the piston 124 as a result of pressure in the transfer fluid chamber 115b. This force may be resisted by holding the injection chamber 128 full of fluid, and thereby holding the piston 124 in the first piston retracted position.

[0041] The injection cylinder 122 includes an injection chamber 128 on one side of the piston 124, for urging the piston 124 towards the injection nozzle 106 and towards the piston advanced position when pressurized. The optional retraction fluid chamber 126 may be disposed on an opposite side of the piston 124, for moving the piston 124 away from the injection nozzle 106 towards the first piston retracted position when pressurized.

[0042] Referring now to Figures 4A to 4E, in operation, at the beginning of an injection cycle (having just completed a previous cycle), the plunger 120 is in the plunger advanced position, and the piston 124 has been moved to a plunger retracted position. In the example illustrated, the plunger is moved to the optional second retracted position (Figure 4A). In other examples, the plunger can be moved to the first retracted position (Figure 4C). Pre-positioning the piston to one of the first or second retracted positions can be effected by supplying pressurized fluid to the retraction fluid chamber 126.

[0043] When plasticization is complete, the valve 108 is moved to the first position, and the plasticization screw 110 is advanced to urge the melt from the barrel 1 12 to the shooting pot 1 18. As the shooting pot 118 is filled with melt, the incoming melt pushes the plunger 120 rearward, as shown in Figure 4B, towards a plunger retracted position and towards the piston 124.

[0044] As the plunger 120 retracts, the rear face 132 of the plunger 120 will contact the front face 130 of the piston 124, as shown in Figure 4C. In the example illustrated where the piston 124 has been pre-positioned in the second retracted position, the plunger may, after initial contact with the piston 124, continue to move rearwards to the plunger retracted position, pushing the piston from the second retracted position to the first retracted position. In examples where the piston 124 has been pre-positioned in the first retracted position, the plunger will cease to translate rearwards upon contact with the piston 124. The rate at which the melt is transferred from the barrel 1 12 to the shooting pot is, in the example illustrated, controlled by a flow control valve that regulates the rate of flow of oil into the transfer chamber 1 15b. The rate can be confirmed by monitoring the speed at which the transfer piston 115a translates, for example via a transfer displacement sensor 115c mounted in the transfer cylinder.

[0045] When the plunger 120 is in the plunger retracted position and the piston 124 is in the first plunger retracted position (Figure 4C), the shooting pot holds the desired volume of melt. Flow of oil into the transfer chamber 115b is no longer necessary for translation of the transfer piston (since rearward motion of the plunger will have ceased), but maintaining a certain pressure in the transfer chamber 1 15b is desirable for "packing" the melt into the shooting pot, which can improve accuracy of the shot size. The pressure can be controlled by a pressure control valve.

[0046] Once the desired packing pressure has been achieved, the valve 108 can be moved to the second position. The injection chamber 128 is then pressurized to advance the piston 124 and the plunger 120, as shown in Figure 4D. As the piston 124 and plunger 120 are advanced, melt is forced out of the shooting pot 18 and through the nozzle 106 into the mold. The piston 124 and the plunger 120 may be advanced until they are in the piston advanced position and the plunger advanced position, respectively, as shown in Figure 4E.

[0047] Referring back to Figures 2 and 3, the plunger injection apparatus 104 further includes a position sensor 134 that is substantially housed within the injection cylinder 134. The position sensor 134 can facilitate accurate filling of the shooting pot by facilitating the transition from speed (flow) control to pressure control. For example, the position sensor 134 can register the approach and contact of the plunger 120 with the piston 124 when the plunger 120 is pushed towards the plunger retracted position and the gap 121 is closed. The sensor 134 can optionally register the position of the piston 124 relative to the injection cylinder housing 136. [0048] In the example shown, the position sensor 134 is generally disposed within the injection cylinder 122, and includes a static sensor element 138 fixed relative to the cylinder housing 136, and at least a first dynamic sensor element 140 coupled to the piston 124 and axially movable relative to the static sensor element 138. In the example illustrated, the first dynamic sensor element 140 is movable relative to the piston 124 between a sensor advanced position (shown in Figure 2) and a sensor retracted position (shown in Figure 3), and is also moveable with the piston 124 relative to the cylinder housing 136 as the piston 124 moves towards and away from the nozzle 106. The position sensor 134 provides an output responsive to the axial position of the first dynamic sensor element 140 relative to the static sensor element 138.

[0049] Referring still to Figures 2 and 3, in the example shown, the first dynamic sensor element 40 includes a magnet 144 fixed within a carrier 146. The carrier 146 includes a generally tubular body 149 having a hollow interior 150, and which has a front end 154 and a rear end 155. The magnet 144 is generally annular and is fixed within the hollow interior 150 adjacent the rear end 155 of the tubular body 149.

[0050] The piston 124 includes, in the example illustrated, a generally cylindrical and axially extending internal cavity 148, and the carrier 146 is disposed within the internal cavity 148. The carrier 146 is slidable within the cavity 148 between the sensor advanced position and the sensor retracted position. The carrier 146 is biased to the sensor advanced position by a spring 151. In the example shown, the spring is external to the carrier 146, and is provided in the internal cavity 148 between a carrier end face at the rear end 155 of the tubular body and a bearing surface 147 of the piston 124. In the example illustrated, the bearing surface 147 comprises an inner surface of an end plug 157, the end plug 157 closing off one end of the cavity and having an aperture through which the sensor rod passes in sealed sliding fit.

[0051] Referring still to Figures 2 and 3, in the example shown, an actuating pin 152 extends from a front end 154 of the tubular body 149. The pin may have a diameter less than that of the tubular body 149. The piston 124 includes an end wall 156 that has an axial thickness extending between the front face 130 of the piston 124 and a bottom 160 of the cavity 148 (shown in Figure 3). A bore passes through the end wall 156, and the actuating pin 152 passes through the bore. The actuating pin has a front end 158, which provides a trigger surface 162. When the sensor 134 is in the sensor advanced position, the trigger surface 162 extends proud of the front face 130 of the piston 124. When the sensor 34 is in the sensor retracted position, the trigger surface 162 is generally flush with the front face 130.

[0052] Referring still to Figures 2 and 3, in the example shown, the static sensor element 138 includes a sensor rod 142. The sensor rod 142 is fixed to the cylinder housing 36 and extends axially within at least a portion of the cylinder housing 136. In the example shown, the sensor rod 142 extends into the internal cavity 148, into the bore 150 of the carrier 146, and through the magnet 144. The position sensor operates by registering the axial position of the magnet 144 with respect to the sensor rod 142. A connector 142a is, in the example illustrated, mounted to an exterior surface of the rear wall of the cylinder for communicating an output signal from the sensor 134 to the electrical system of the machine.

[0053] Referring again to Figures 4A to 4E, when the plunger is in the advanced position and the piston has been pre-positioned to a retracted position, the plunger and the piston are generally separated by an axial gap (e.g. gap 121 ). The carrier 146 is in the sensor advanced position, and the trigger surface 162 extends proud of the front face 130.

[0054] As the shooting pot 1 18 is filled with melt, the plunger 120 is moved towards the plunger retracted position, and the rear face 132 of the plunger 120 approaches the front face 130 of the piston 124. Prior to contacting the front face 130 of the piston 124, the rear face 32 of the plunger 120 will contact the trigger surface 162, as shown for example in Figure 4B. [0055] Further rearward movement of the plunger (associated with further filling of the shooting pot) will urge the carrier 146 rearward towards the sensor retracted position. This initial movement of the carrier 146 with respect to the sensor rod 142 provides a signal that the plunger 120 is approaching the piston 124. In the example illustrated, the spring biasing force is easily overcome by the force urging the plunger rearward, and movement of the carrier 146 towards the retracted position takes place without any corresponding translation of the piston 124. Upon detecting this approach signal, control of the transfer actuator can switch from speed control (flow rate control) to force control (pressure control). In the example illustrated, this switch or transition from flow control to pressure control causes fluid flow into the transfer chamber 115b to be reduced (eventually reaching a generally zero flow, pressure maintaining condition). Accordingly the rate at which melt is urged into the shooting pot 118 is reduced, and the speed of the plunger 120 is reduced.

[0056] As the plunger 120 further retracts, the carrier 146 will be urged further rearward until the rear face of the plunger abuts the front face 130 of the piston 124. The movement of the carrier 146 relative to the piston is a known axial dimension, generally equal to the amount by which the trigger surface protrudes proud of the front face 130 when the carrier is in the advanced position. Since the piston 124 does not move during displacement of the sensor to the retracted position, the position of the piston

[0057] As the plunger 120 further retracts, it urges the piston 124 rearward from the first piston retracted position towards the second piston retracted position, shown in Figure 4C. The carrier 146 moves rearward with the piston 124, and this movement can be monitored by the position sensor 134. Once the sensor indicates that the piston 124 has reached the second piston retracted position, transferring melt to the shooting pot can be stopped, for example, by moving the valve 108 to the second (injection) position. The injection chamber 128 may be pressurized to advance the piston 124 and the plunger 120, and to urge the melt from the shooting pot 118 into the mold. [0058] By reducing the speed of the plunger 120 (and piston 124) when moving the piston 124 to the second piston retracted position, the positioning of the piston 124 may be better controlled, facilitating more precise positioning of the piston 124 and greater accuracy of the volume of melt injected into the mold each cycle.

[0059] Referring now to Figures 5A and 5B, wherein like reference numerals are used to refer to like features of Figures 1 to 4, incremented by 400, an alternate example of a position sensor is shown. In this example, the rear end 555 of the tubular element 549 of the carrier 546 extends axially through the interior of the helical compression spring 551. The spring 551 is compressed axially between a collar fixed to, and extending radially outward of, the body 549 and the bearing surface of the piston. Further, in this example, a bushing 564 is provided around the tubular element 549. The bushing 564 is mounted forwardly of the rear end 555, and forwardly of the magnet 544. The body 549 is hollow and has apertures in the body sidewall providing fluid communication between the interior of the body and the injection chamber of the injection cylinder 522.

[0060] Referring now to Figure 6, wherein like reference numerals are used to refer to like features of Figures 1 to 4, incremented by 500, an alternate example is shown, wherein the position sensor 634 includes a first dynamic sensor element 640 and a second dynamic sensor element 656. In the example illustrated, the first dynamic sensor element 640 includes a first magnet 644 fixed within a first carrier 646. The first carrier 646 includes a tubular element 649, and the first magnet 644 is generally annular and is fixed within the hollow interior 650 of the tubular element 649 at the rear end 655 of the tubular element 649.

[0061] The second dynamic element 656 comprises a second magnet fixed to the piston and having a central aperture through which the sensor rod 642 passes. In the example illustrated, the second magnet is fixed to the end plug 657, which serves as a second carrier for the second magnet. By providing the second magnet 645, the position sensor 634 is operable to independently sense the position of the piston 624 and the approach and contact of the plunger 620. The position sensor 634 can more positively distinguish between movement of the first carrier and movement of the piston.

[0062] Referring now to Figure 7, wherein like reference numerals are used to refer to like features of Figures 1 to 4, incremented by 600, another alternate example of a position sensor 734 is shown. Similarly to the position sensor 634, the position sensor 734 includes a first dynamic sensor element 740 and a second dynamic sensor element 756. The spring 751 is mounted around a rear end 755 of the tubular element 749 of the carrier 746 that passes through the center of the helical spring 751.

[0063] In the example illustrated, the axial position of the second magnet relative to the sensor rod (which is fixed relative to the cylinder housing) provides an accurate indication of the position of the piston relative to the cylinder. Furthermore, the first and second magnets are spaced axially apart by a dynamic element spacing that varies as the sensor moves between the sensor advanced and retracted positions. In particular, the spacing corresponds to a first dynamic element spacing 761 a when the sensor is in the sensor advanced position, corresponding to non-contact between the plunger and piston (Figure 7). Upon contact between the piston 724 and the plunger 720, the sensor is moved to the sensor advanced position and the spacing changes to a second dynamic element spacing 761b that is less than the first dynamic element spacing 761 a (see Figure 7A). Monitoring the dynamic element spacing and the position of the second dynamic element relative to the sensor rod can facilitate accurate determination of the components of the injection unit, during normal operation of the machine and at start-up.

[0064] While the above description provides examples of one or more processes or apparatuses, it will be appreciated that other processes or apparatuses may be within the scope of the accompanying claims.

Claims

1. An injection unit for an injection molding machine, comprising:

a) a shooting pot disposed between an injection nozzle and a plunger, the shooting pot having an adjustable volume for receiving and dispensing melt, the plunger reciprocally slidable relative to the injection nozzle to increase and decrease the volume of the shooting pot;

b) an injection cylinder adjacent the plunger, the injection cylinder comprising a piston slidably disposed within a cylinder housing, the injection cylinder including an injection chamber on one side of the piston for urging the piston towards the injection nozzle when pressurized; and

c) a position sensor comprising a static sensor element fixed relative to the cylinder housing and at least a first dynamic sensor element coupled to the piston and axially movable relative to the static sensor element, the position sensor providing an output signal responsive to the axial position of the first dynamic sensor element relative to the static sensor element, the first dynamic sensor element movable relative to the piston between a sensor advanced position and a sensor retracted position, and the first dynamic sensor element moveable with the piston relative to the cylinder housing as the piston moves towards and away from the injection nozzle.

2. The injection unit of claim 1 , wherein at least one of the static sensor element and dynamic sensor element comprises a sensor rod extending axially within at least a portion of the cylinder housing.

3. The injection unit of claim 1 , wherein the static sensor element comprises a sensor rod extending axially within at least a portion of the cylinder housing.

4. The injection unit of claim 3, wherein the sensor rod is fixed relative to the cylinder housing.

5. The injection unit of claim 3, wherein the first dynamic sensor element is fixed to a first carrier, the sensor rod extending through at least a portion of the first carrier.

6. The injection unit of claim 5, wherein the piston comprises an internal cavity, the first carrier is disposed in the internal cavity, and the carrier is slidable between the sensor advanced and sensor retracted positions.

7. The injection unit of claim 6, wherein the carrier is biased to the sensor advanced position.

8. The injection unit of any one of claims 5-7, wherein the piston comprises a front face for engaging a rear face of the plunger, and the carrier comprises a trigger surface proud of the front face when the sensor is in the sensor advanced position.

9. The injection unit of claim 8, wherein the trigger surface is generally flush with the front face when the carrier is in the sensor retracted position.

10. The injection unit of any one of claims 8-9, wherein the piston comprises an end wall with an axial thickness extending between the front face of piston and a bottom of the internal cavity, the carrier comprises an actuating pin passing through a bore in the end wall, and the trigger surface comprises an end of the actuating pin.

11. The injection unit of any one of claims 1-10, wherein the first dynamic sensor element comprises a first magnet.

12. The injection unit of any one of claims 1 -11 , further comprising a second dynamic sensor element, the second dynamic sensor element axially moveable relative to the static sensor element and relative to the first dynamic sensor element.

13. The injection unit of claim 12, wherein the second dynamic sensor element is fixed within a second carrier.

14. The injection unit of any one of claims 12-13, wherein the second dynamic sensor element is fixed relative to the piston.

15. The injection unit of any one of claims 12-14, wherein the second dynamic sensor element comprises a second magnet.

16. The injection unit of any of claims 1-15, wherein the injection cylinder comprises a retraction actuator for pre-positioning the piston to a retracted position in spaced axial relation from the plunger.

17. The injection unit of claim 16, wherein the retraction actuator comprises a retraction chamber on a side of the piston opposite the injection chamber.

18. A two-stage injection unit for an injection molding machine, comprising:

a) a plasticizing unit for delivering melt to a shooting pot, the shooting pot having an adjustable volume;

b) an injection cylinder spaced apart from the shooting pot and comprising a piston slidably mounted in an injection cylinder housing, the injection cylinder comprising a retraction fluid chamber on one side of the piston for retracting the piston away from the shooting pot when pressurized, and an injection chamber on an opposed side of the piston for urging the piston towards the shooting pot when pressurized;

c) a plunger slidably disposed between the shooting pot and the injection cylinder, the plunger slidable between a plunger advanced position and a plunger retracted position to adjust the volume of the shooting pot; and

d) a position sensor disposed within the injection cylinder, the position sensor registering the position of the piston relative to the injection cylinder housing, and the position sensor registering approach and contact of the plunger with the piston when the plunger is pushed to the plunger retracted position by the melt.

19. The two-stage injection unit of claim 18, wherein, the position sensor comprises a static sensor element fixed relative to the injection cylinder housing and at least a first dynamic sensor element coupled to the piston and axially movable relative to the static sensor element, the sensor providing an output responsive to the axial position of the first dynamic sensor element relative to the static sensor element.

20. The two-stage injection unit of claim 19, wherein the first dynamic sensor element is movable relative to the piston between a sensor advanced position and a sensor retracted position.

21. The two-stage injection unit of claim 20, wherein the first dynamic sensor element is moveable with the piston relative to the injection cylinder housing.

22. The two-stage injection unit of claim 21 , wherein the static sensor element comprises a sensor rod extending axially within at least a portion of the injection cylinder housing.

23. The two-stage injection unit of claim 22, wherein the dynamic sensor element comprises a magnet fixed within a carrier.

24. The two-stage injection unit of claim 23, wherein the piston comprises an internal cavity, the carrier is disposed in the internal cavity, and the carrier is slidable between the sensor advanced and sensor retracted positions.

25. The two-stage injection unit of claim 24, wherein the carrier is biased to the sensor advanced position.

26. The two-stage injection unit of claim 23, wherein the piston comprises a front face for engaging a rear face of the plunger, and the carrier comprises a trigger surface proud of the front face when in the sensor advanced position.

27. The two-stage injection unit of claim 26, wherein the trigger surface is generally flush with the front face when the carrier is in the sensor retracted position.

28. The two-stage injection unit of claim 27, wherein the piston comprises an end wall with an axial thickness extending between front face of piston and a bottom of the internal cavity, the carrier comprises an actuating pin passing through a bore in the end wall, and the trigger surface comprises an end of the actuating pin.

29. An injection cylinder for an injection molding machine, the injection cylinder comprising:

a) a cylinder housing;

b) an injection piston slidably disposed in the cylinder housing, the injection piston translatable between a piston retracted position and a piston advanced position, the injection piston having a length and an internal cavity extending axially along at least a portion of the piston length;

c) a sensor rod fixed to the housing and extending axially through at least a portion of the internal bore of the piston; and

d) a first sensing element slidably disposed within the internal bore of the piston, the first sensing element moveable relative to the piston between sensor advanced and sensor retracted positions, and the first sensing element moveable with the piston when translating between the piston advanced and piston retracted positions.

30. The injection cylinder of claim 29, wherein the piston comprises an end wall with an axial thickness extending between a front face of the piston and a bottom of the internal cavity, the first sensing element mounted within a carrier, and the carrier comprising an actuating pin passing through a bore in the end wall.

31. A method of filling a shooting pot with melt for injection into a mold, comprising:

a) pre-positioning an injection piston at a retracted position spaced apart from a plunger by a gap;

b) transferring melt into a shooting pot from a plasticizing unit, the incoming melt pushing the plunger towards the injection piston; c) engaging an actuating pin with the plunger, the actuating pin protruding from the injection piston;

d) in response to step (c), transitioning control of the melt transfer from speed control to pressure control;

e) stopping the flow of melt into the shooting pot when a target pressure has been reached; and

f) moving the injection piston to an advanced position to urge the melt from the shooting pot into the mold.