Patent Publication Number: US-2022212387-A1

Title: Injection apparatus and related methods

Description:
This application is a continuation of International Application Ser. No. PCT/CA2020/051286, filed Sep. 25, 2020, which claims the benefit of Provisional Application Ser. No. 62/906,168, filed Sep. 26, 2019, the entire contents of which are hereby incorporated by reference. 
     FIELD 
     The specification relates generally to apparatuses and methods associated with plasticizing and injecting mold material into a mold of an injection molding machine. 
     BACKGROUND 
     U.S. Pat. No. 5,540,495 A (Pickel) relates to an extruder screw drive having a first and a second motor, a screw mechanism connected to the first motor and to the extruder screw for translating it in the extrusion cylinder, and a slide mechanism connected to the second motor and to the extruder screw for rotating it in the extrusion cylinder. The screw mechanism and the slide mechanism are coaxial and partially fit into one another to provide an axially compact arrangement. The motors can be hollow shaft electric motors axially aligned together, and pressure regulated hydraulic axial force can be added to the extruder screw during plasticating as it retracts due to increased volume of plastic at an output end of the extrusion cylinder. 
     U.S. Pat. App. Pub. No. 2004/0173925 A1 (Melkus) relates to a control method for controlling the back pressure in an injection molding machine which includes a first motor that axially displaces a screw and a second motor that turns the screw, whereby both motors act upon a common shaft. In order to control the back pressure, a speed value for controlling the second motor is furnished as a rotational speed input value to a control circuit for controlling the speed or rotation speed of the first motor. The back pressure is thus controlled in dependence on a pressure differential via the difference in rotation speeds of both motors. 
     SUMMARY 
     The following summary is intended to introduce the reader to various aspects of the applicant&#39;s teaching, but not to define any invention. 
     According to some aspects, an injection apparatus for an injection molding machine includes: (a) a barrel extending along an axis; (b) a nozzle at a front end of the barrel for discharging melt; (c) a screw in the barrel, the screw rotatable about and translatable along the axis; and (d) a drive assembly for driving translation and rotation of the screw. The drive assembly includes: (i) a housing having a front end coupled to the barrel and a rear end axially opposite the front end; (ii) a spindle in the housing, the spindle extending along the axis and fixed to the screw; (iii) a first motor in the housing, the first motor having a hollow first rotor through which the spindle passes, the first rotor rotationally fixed to the spindle for driving rotation of the screw about the axis, and the spindle axially translatable relative to the first rotor for accommodating translation of the screw along the axis; and (iv) a second motor in the housing axially rearward of the first motor toward the rear end of the housing, the second motor having a hollow second rotor through which the spindle passes, the second rotor coupled to the spindle and rotatable relative to the spindle in a rotational first direction for advancing the screw along the axis and in a rotational second direction opposite the first direction for retracting the screw along the axis. 
     In some examples, the spindle has an internal first conduit extending between a first conduit intake end open to a rear of the spindle for receiving lubricant and a first conduit discharge end open to a first interface between the spindle and the first rotor for discharging the lubricant into the first interface. 
     In some examples, the spindle has an internal second conduit extending between a second conduit intake end open to a rear of the spindle for receiving lubricant and a second conduit discharge end open to a second interface between the spindle and the second rotor for discharging the lubricant into the second interface. 
     In some examples, the spindle comprises a spline portion extending along the axis and the first rotor comprises a spline nut coupled to the spline portion of the spindle. In some examples, the spindle comprises a ball screw portion extending along the axis and the second rotor comprises a ball nut coupled to the ball screw portion. 
     In some examples, the drive assembly includes a bearing assembly mounted between the second rotor and the housing adjacent the rear end of the housing, the bearing assembly accommodating rotation of the second rotor relative to the housing and transferring at least a portion of a rearwardly directed axial force exerted on the second rotor to the housing. 
     In some examples, each of the first motor and the second motor is axially fixed relative to the housing. 
     In some examples, the drive assembly includes a rotary first encoder having a first encoder disc mounted coaxially to the first rotor for measuring rotational displacement of the first rotor relative to the housing. 
     In some examples, the drive assembly includes a rotary second encoder having a second encoder disc mounted coaxially to the second rotor for measuring rotational displacement of the second rotor relative to the housing. 
     In some examples, the housing has a generally sealed interior, and the spindle, the first motor, and the second motor are generally enclosed in the sealed interior. 
     According to some aspects, an injection apparatus for an injection molding machine includes: (a) a barrel extending along an axis; (b) a nozzle at a front end of the barrel for discharging melt; (c) a screw in the barrel, the screw rotatable about and translatable along the axis; (d) a shot chamber in the barrel axially intermediate the screw and the nozzle for holding melt; (e) a drive assembly for driving translation and rotation of the screw. The drive assembly includes: (i) a housing having a front end coupled to the barrel; (ii) a spindle in the housing, the spindle extending along the axis and fixed to the screw; (iii) a first motor in the housing, the first motor having a hollow first rotor through which the spindle passes, the first rotor rotationally fixed to the spindle for driving rotation of the screw about the axis, and the spindle axially translatable relative to the first rotor for accommodating translation of the screw along the axis; (iv) a second motor in the housing, the second motor having a hollow second rotor through which the spindle passes, the second rotor coupled to the spindle and rotatable relative to the spindle in a rotational first direction for advancing the screw along the axis and in a rotational second direction opposite the first direction for retracting the screw along the axis. The injection apparatus further includes: (f) a controller configured to operate the drive assembly to, for each injection cycle: (i) apply a holding torque to the first rotor for inhibiting rotation of the screw about the axis relative to the housing; (ii) during (i), apply an injection torque to the second rotor in the first direction to exert an axial force on the spindle for urging the screw to advance toward the nozzle; (iii) during (ii), determine an injection pressure value based on the holding torque, the injection pressure value corresponding to a reactionary pressure of melt in the shot chamber during application of the holding and injection torque; and (iv) adjust the injection torque to bring the injection pressure value toward a target pressure value, the target pressure value corresponding to a target pressure for the melt in the shot chamber during injection of the melt into a mold. 
     In some examples, the controller is operable to determine the holding torque based on an electrical current being drawn by the first motor to apply the holding torque. 
     In some examples, the controller is further operable to, prior to (iv), compare the injection pressure value to the target pressure value, and in response to determining that the injection pressure value corresponds to the target pressure value, maintain the holding and injection torques and repeat (ii) and (iii), and in response to determining that the injection pressure value does not correspond to the target pressure value, proceed to (iv). 
     In some examples, the controller is further operable to determine the target pressure value based on an axial position of the screw. 
     In some examples, the controller is further operable to determine the axial position of the screw based on output from a first encoder for measuring rotational displacement of the first rotor relative to the housing and a second encoder for measuring rotational displacement of the second rotor relative to the housing. 
     In some examples, the controller is operable to, after (iv), repeat (ii) to (iv) until detection of one or more termination conditions. 
     According to some aspects, a method of operating an injection apparatus drive assembly to regulate melt injection pressure includes: (a) applying a holding torque to a hollow first rotor to inhibit rotation of a spindle about an axis, the spindle passing through the first rotor along the axis and fixed to an injection screw; (b) during (a), applying an injection torque to a hollow second rotor to exert an axial force on the spindle, the spindle passing through the second rotor along the axis and the axial force urging the injection screw to advance toward a nozzle; (c) during (b), determining an injection pressure value based on the holding torque, the injection pressure value corresponding to a reactionary pressure of melt in a shot chamber axially intermediate the screw and the nozzle during application of the holding and injection torque; and (d) adjusting the injection torque to bring the injection pressure value toward a target pressure value, the target pressure value corresponding to a target pressure for the melt in the shot chamber during injection of the melt into a mold. 
     In some examples, (c) includes determining the holding torque based on an electrical current being drawn to apply the holding torque. 
     In some examples, the method further includes, prior to (d), comparing the injection pressure value to the target pressure value, and in response to determining that the injection pressure value corresponds to the target pressure value, maintaining the holding and injection torques and repeating (b) and (c), and in response to determining that the injection pressure value does not correspond to the target pressure value, proceeding to (d). 
     In some examples, the method further includes determining the target pressure value based on an axial position of the screw. 
     In some examples, the method further includes determining the axial position of the screw based on output from a first encoder for measuring rotational displacement of the first rotor relative to the housing and a second encoder for measuring rotational displacement of the second rotor relative to the housing. 
     In some examples, the method further includes, after (d), repeating (b) to (d) until detection of one or more termination conditions. 
     According to some aspects, a method of operating an injection apparatus drive assembly to regulate melt plasticization pressure includes: (a) applying a plasticization torque to a hollow first rotor to drive rotation of a spindle about an axis for filling a shot chamber with melt, the spindle passing through the first rotor along the axis and fixed to an injection screw; (b) during (a), applying a retraction torque to a hollow second rotor to control retraction of the spindle along the axis during application of the plasticization torque, the spindle passing through the second rotor along the axis; and (c) monitoring and adjusting the plasticization torque and the retraction torque to maintain a target back pressure of melt in the shot chamber during rotation and retraction of the spindle. 
     According to some aspects, an injection apparatus for an injection molding machine includes: (a) a barrel extending along an axis; (b) a nozzle at a front end of the barrel for discharging melt; (c) a screw in the barrel, the screw rotatable about and translatable along the axis; and (d) a drive assembly for driving translation and rotation of the screw. The drive assembly includes: (i) a housing having a front end coupled to the barrel; (ii) a spindle in the housing, the spindle extending along the axis and fixed to the screw; (iii) a first motor in the housing, the first motor having a hollow first rotor through which the spindle passes, the first rotor rotationally fixed to the spindle for driving rotation of the screw about the axis, and the spindle axially translatable relative to the first rotor for accommodating translation of the screw along the axis; (iv) a rotary first encoder having a first encoder disc mounted coaxially to the first rotor for measuring rotational displacement of the first rotor relative to the housing; (v) a second motor in the housing, the second motor having a hollow second rotor through which the spindle passes, the second rotor coupled to the spindle and rotatable relative to the spindle in a rotational first direction for advancing the screw along the axis and in a rotational second direction opposite the first direction for retracting the screw along the axis; and (vi) a rotary second encoder having a second encoder disc mounted coaxially to the second rotor for measuring rotational displacement of the second rotor relative to the housing. 
     In some examples, at least one of the first encoder disc and the second encoder disc is generally annular, and the spindle passes through the at least one of the first encoder disc and the second encoder disc. 
     In some examples, the injection apparatus further includes a controller configured to, during each injection cycle: (i) determine a first rotor rotational displacement of the first rotor over a time interval based on output from the first encoder; (ii) determine a second rotor rotational displacement over the time period based on output from the second encoder; and (iii) determine an axial displacement of the screw based on a differential between the first rotor rotational displacement and the second rotor rotational displacement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic elevation view of an example injection molding machine; 
         FIG. 2  is a perspective view of an injection apparatus of the machine of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the injection apparatus of  FIG. 2 , taken along line  3 - 3  of  FIG. 2  and showing a screw of the injection apparatus in an advanced configuration; 
         FIG. 4  is a cross-sectional view like that of  FIG. 3 , and showing the screw in a retracted configuration; 
         FIG. 5  is an enlarged view of a portion of  FIG. 3 ; 
         FIG. 6  is a flow chart showing an example method of regulating melt injection pressure; 
         FIG. 7  is a flow chart showing an example method of regulating melt plasticization pressure; and 
         FIG. 8  is another example of an injection apparatus suitable for use with the machine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Referring to  FIG. 1 , an example injection molding machine  100  is shown. The machine  100  includes a machine base  102 , a first platen  106  supported by the machine base  102  for carrying a first mold section  106   a  of a mold, and a second platen  108  supported by the machine base  102  for carrying a second mold section  108   a  of the mold. The second platen  108  is translatable toward and away from the first platen  106  to close and open the mold. In the example illustrated, a plurality of tie bars  110  extend between the first and second platens  106 ,  108  for coupling the platens  106 ,  108  together and exerting a clamp load across the mold when stretched. 
     In the example illustrated, the machine  100  includes an injection apparatus  116  supported by the base  102  for plasticizing and injecting resin or other mold material (also referred to as “melt”) into the mold. Referring to  FIG. 2 , in the example illustrated, the injection apparatus  116  includes a barrel  118  extending along a barrel axis  120  and a nozzle  122  at a front end of the barrel  118  for discharging melt from the barrel  118 . 
     Referring to  FIG. 3 , in the example illustrated, the injection apparatus  116  further includes a screw  124  in the barrel  118  and extending along the axis  120 , and a shot chamber  126  ( FIG. 4 ) in the barrel  118  axially intermediate the screw  124  and the nozzle  122  for holding melt. The screw  124  is rotatable about the axis  120  for plasticizing resin or other mold material (supplied, for example, from a hopper  127 ) and filling the shot chamber  126  with melt. The screw  124  is translatable along the axis  120  toward and away from the nozzle  122  to alternately load the shot chamber  126  with melt and to inject the melt into the mold. 
     In the example illustrated, the injection apparatus  116  includes a drive assembly  128  for driving rotation and translation of the screw  124 . The drive assembly  128  includes a housing  130  having a front end  130   a  coupled to the barrel  118  and a rear end  130   b  axially opposite the front end  130   a.  The drive assembly  128  further includes a spindle  132  in the housing  130 . The spindle  132  extends along the axis  120  and is fixed to the screw  124 . 
     In the example illustrated, the drive assembly  128  includes a first motor  134  in the housing  130 . In the example illustrated, the first motor  134  comprises a hollow shaft motor having a first stator  135  fixed relative to the housing  130  and a hollow first rotor  136  through which the spindle  132  passes. The first rotor  136  is rotationally fixed to the spindle  132  for driving rotation of the screw  124  about the axis  120 , and the spindle  132  is axially translatable relative to the first rotor  136  for accommodating translation of the screw  124  along the axis  120 . In the example illustrated, the spindle  132  comprises a spline portion  138  extending along the axis  120  and the first rotor  136  comprises a spline nut  139  coupled to the spline portion  138  of the spindle  132 . In the example illustrated, the spline portion  138  comprises an external spline profile on an exterior of the spindle  132 , and the spline nut  139  comprises a complementary internal spline profile on an interior of the spline nut  139 . 
     In the example illustrated, the drive assembly  128  further includes a second motor  144  in the housing  130 . In the example illustrated, the second motor  144  comprises a hollow shaft motor having a second stator  145  fixed relative to the housing  130  and a hollow second rotor  146  through which the spindle  132  passes. The second rotor  146  is coupled to the spindle  132  and rotatable relative to the spindle  132  in a rotational first direction for advancing the screw  124  along the axis  120  and in a rotational second direction opposite the first direction for retracting the screw  124  along the axis  120 . In the example illustrated, the spindle  132  comprises a ball screw portion  148  extending along the axis  120  and the second rotor  146  comprises a ball nut  149  coupled to the ball screw portion  148 . 
     In the example illustrated, the machine  100  includes a controller  150  ( FIG. 1 ) configured to control operation of the drive assembly  128 . In the example illustrated, the controller  150  is operable to control energization of the first and second motors  134 ,  144  to translate the screw  124  along the axis and/or rotate the screw  124  about the axis  120 . The controller  150  is operable to control the axial and rotational velocity of the screw  124  based on the rotational velocity of the first and second rotors  136 ,  146  relative to each other and the housing  130 . In the example illustrated, the controller  150  is operable to control energization of the first and second motors  134 ,  144  to rotate the second rotor  146  in the rotational first direction relative to the first rotor  136  for advancing the screw toward the nozzle  122 ; rotate the second rotor  146  in the rotational second direction relative to the first rotor  136  for retracting the screw from the nozzle  122 ; and rotate the first rotor  136  relative to the housing  130  in the first or second direction for rotating the screw  124  in the first or second direction, respectively, relative to the housing  130 . 
     In the example illustrated, the controller  150  is operable to monitor the rotational (angular) and axial (linear) displacement and/or velocity of the screw  124  during an injection cycle based on the rotational displacement of the first and second rotors  136 ,  146 . Referring to  FIG. 5 , in the example illustrated, the drive assembly  128  includes a rotary first encoder  152  having a first encoder disc  153  mounted coaxially to the first rotor  136  for measuring rotational displacement of the first rotor  136  relative to the housing  130 , and a rotary second encoder  154  having a second encoder disc  155  mounted coaxially to the second rotor  146  for measuring rotational displacement of the second rotor  146  relative to the housing  130 . In the example illustrated, each of the first and second encoder discs  153 ,  155  is generally annular and coaxial with the axis  120 . In the example illustrated, the spindle  132  passes through at least the first encoder disc  153 . 
     In the example illustrated, the controller  150  is operable to determine a first rotor rotational displacement of the first rotor  136  over a time interval based on output from the first encoder  152  and to determine a second rotor rotational displacement over the time interval based on output from the second encoder  154 . In the example illustrated, the controller  150  is operable to determine the rotational displacement and/or velocity of the screw  124  over the time interval based on the first rotor rotational displacement, and to determine the axial displacement and/or velocity of the screw  124  over the time interval based on a differential between the first rotor rotational displacement and the second rotor rotational displacement (and, for example, a pitch of the ball screw portion  148 ). 
     In the example illustrated, each of the first motor  134  and the second motor  144  is axially fixed relative to the housing  130 , and the second motor  144  is axially rearward of the first motor  134  toward the rear end  130   b  of the housing  130 . Referring to  FIG. 5 , in the example illustrated, the drive assembly  128  includes a bearing assembly  156  mounted between the second rotor  146  and the housing  130  adjacent the rear end  130   b.  The bearing assembly  156  accommodates rotation of the second rotor  146  relative to the housing  130  and can transfer at least a portion of a rearwardly directed axial force exerted on the second rotor  146  to the housing  130  (e.g. a rearwardly directed force exerted by the spindle  132  on the second rotor  146  when urging the screw  124  forward against melt). In the example illustrated, the bearing assembly  156  comprises a thrust bearing  158  mounted axially intermediate a rearwardly directed rotor surface  160  fixed relative to the second rotor  146  and a housing surface  162  fixed relative to the housing  130  and directed toward the rotor surface  160 . 
     Still referring to  FIG. 5 , in the example illustrated, the housing  130  has a generally sealed interior  164 , and each of the spindle  132 , the first motor  134 , and the second motor  144  are generally enclosed in the sealed interior  164 . This can help to, for example, contain contaminants that may otherwise be emitted from the drive assembly  128  due to rotation and/or translation of drive assembly components, which may be useful in, for example, clean room applications. 
     In the example illustrated, the spindle  132  has an internal first conduit  166  extending between a first conduit intake end  168  at a rear of the spindle  132  for receiving lubricant, and a first conduit discharge end  170  open to a first interface  172  between the spindle  132  and the first rotor  136  for discharging the lubricant into the first interface  172 . In the example illustrated, the spindle  132  further has an internal second conduit  174  extending between a second conduit intake end  176  at the rear of the spindle  132  for receiving lubricant and a second conduit discharge end  178  open to a second interface  180  between the spindle  132  and the second rotor  146  for discharging the lubricant into the second interface  180 . 
     In use, the controller  150  operates the drive assembly  128  to advance the screw  124  axially forward toward the nozzle  122  for injection of a shot of melt from the shot chamber  126  into the mold. In the example illustrated, the controller  150  is operable to control the drive assembly  128  during injection to regulate the melt injection pressure in the shot chamber  126 . The melt injection pressure can be regulated by the controller  150  according to, for example, the process  300  shown in  FIG. 6 . 
     Referring to  FIG. 6 , in the example illustrated, at  310  of the process  300 , the controller  150  operates the drive assembly  128  to apply a holding torque to the first rotor  136  for inhibiting rotation of the spindle  132  and the screw  124  about the axis  120 . The controller  150  further operates the drive assembly  128  to apply an injection torque to the second rotor  146  for exerting an axial force on the spindle  132  to urge the screw  124  to advance toward the nozzle  122 . In the example illustrated, the holding torque inhibits undesired rotation of the screw  124  while the injection torque is applied by the second rotor  146 . The magnitude of the holding torque is, in the example illustrated, monitored over time and used as an input to facilitate desired control of the injection process, as further explained hereinafter. 
     The holding torque generally provides a rotationally resistive force on the screw  124  in a direction opposite the direction in which the screw  124  is inclined to rotate as a result of, for example, forces exerted by the second motor  144  and by the melt pressure during injection. In the example illustrated, the screw  124  is inclined to rotate in the rotational first direction as the second motor  144  urges the second rotor  146  to rotate in the rotational first direction. The holding torque is, at least when the screw begins translating from the retracted toward the advanced position, applied in the rotational second direction to resist the rotational force exerted on the screw as a result of rotation of the second motor  144 . 
     In the example illustrated, the holding torque holds the first rotor  136  generally stationary relative to the housing  130  (e.g. brings the rotation of the first rotor  136  to zero or near zero relative to the housing  130 ), and the injection torque rotates the second rotor  146  in the rotational first direction relative to the first rotor  136  to advance the screw toward the nozzle  122 . The holding torque can be selected to limit rotation of the first rotor  136  to a relatively low rotational speed so that the first rotor  136  is generally stationary relative to the housing  130 . The low rotational speed can be, for example, under 2 revolutions per minute, or in some examples, under 1 revolution per minute. In some examples, the holding torque can hold the first rotor  136  completely stationary relative to the housing  130  during application of the injection torque (e.g. the first rotor  136  can be rotationally locked relative to the housing  130 ). 
     At  320 , during application of the holding and injection torques, the controller  150  operates to determine an injection pressure value based on the holding torque. The injection pressure value corresponds to the reactionary pressure of melt in the shot chamber  126  during application of the holding and injection torques. Determining the injection pressure value based on the holding torque may reduce the need for, for example, designated melt-pressure sensors, and may help allow for more precise pressure readings over a wider range of injection pressures relative to some melt-pressure sensors. The magnitude of the holding torque may be made available to the controller  150  directly from one or more sensors, or may be calculated based on parameters indicative of, and/or proportional to, the holding torque. 
     In the example illustrated, the controller  150  energizes the first motor  134  to apply the holding torque via the first rotor  136 , and energizes the second motor  144  to apply the injection torque. In the example illustrated, the controller  150  determines the magnitude of the holding torque based on the amount of electrical current being drawn by the first motor  134  as the first motor  134  applies the holding torque. Holding the first rotor  136  generally stationary during  320  may facilitate a more accurate measurement of the reactionary pressure by, for example, reducing dynamic effects (e.g. acceleration or deceleration torque) on the first rotor  136  that may otherwise affect the amount of electrical current drawn by the first motor  134 , independent of the reactionary pressure of the melt. 
     In the example illustrated, at  330 , the controller  150  operates to compare the injection pressure value (as determined in relation to the holding torque) to a target pressure value. The target pressure value corresponds to a target pressure for the melt in the shot chamber  126  during injection. The target pressure value can vary depending on the axial position of the screw  124 , for example, to provide a relatively higher target pressure for the melt as the screw  124  nears the end of the injection stroke. In such examples, step  330  includes operating the controller  150  to determine the axial position of the screw  124  relative to the housing  130 , and to determine the target pressure value based on the axial position of the screw  124 . In the example illustrated, the controller  150  is operable to determine the axial position of the screw  124  based on output from the first and second encoders  152 ,  154 . 
     Upon determining that the injection pressure value corresponds to the target pressure value, the controller  150  then maintains the holding and injection torques, and optionally proceeds to repeat steps  320  and  330 . 
     If the controller  150  determines that the injection pressure value does not correspond to the target pressure value, then at  340 , the controller  150  operates to adjust the injection torque to bring the injection pressure value toward the target pressure value. The holding torque is also adjusted to continue holding the first rotor  136  generally stationary during adjustment of the injection torque. 
     After adjustment, the controller optionally repeats steps  320  to  340  to continue regulating melt injection pressure during injection. The controller  150  optionally terminates the process  300  in response to detecting one or more termination conditions. The termination conditions can include, for example, the screw  124  completing the injection stroke and/or the reactionary pressure (or its rate of change) falling below a threshold indicating that injection of the melt is complete. 
     After injection is complete, the controller  150  operates the drive assembly  128  to plasticize melt by rotating the screw  124  while accommodating retraction of the screw  124  to re-fill the shot chamber  126  with melt for a subsequent injection cycle. In the example illustrated, the controller  150  is operable to control the drive assembly  128  during plasticization to regulate the melt plasticization pressure in the shot chamber  126 . The melt plasticization pressure can be regulated by the controller  150  according to, for example, the process  400  shown in  FIG. 7 . 
     Referring to  FIG. 7 , in the example illustrated, at  410  of the process  400 , the controller  150  energizes the first and second motors  134 ,  144  to apply a plasticization torque to the first rotor  136  for driving rotation of the spindle  132  (and the screw  124 ) about the axis  120  to fill the shot chamber with melt, and to apply a retraction torque to the second rotor  146  to control retraction of the spindle  132  (and the screw  124 ) along the axis  120  during application of the plasticization torque. At  420  of the process  400 , the controller  150  operates to monitor and adjust the plasticization torque and the retraction torque to maintain a target back pressure for the melt in the shot chamber  126  during rotation and retraction of the spindle  132  (and the screw  124 ). 
     Referring now to  FIG. 8 , another example of an injection apparatus  1116  is illustrated. The injection apparatus  1116  is similar to the injection apparatus  116 , and like features are identified by like reference characters, incremented by  1000 . 
     In the example illustrated, the injection apparatus  1116  includes a barrel extending along a barrel axis  1120  between a nozzle at a front end of the barrel and a drive assembly  1128  at a rear end of the barrel. The injection apparatus  1116  further includes a screw  1124  in the barrel and extending along the axis  1120 , and a shot chamber axially intermediate the screw  1124  and the nozzle. 
     The drive assembly  1128  includes a housing  1130 , a spindle  1132  in the housing  1130  and fixed to the screw  1124 , a first motor  1134  in the housing  1130  and having a hollow first rotor  1136 , and a second motor  1144  in the housing  1130  and having a hollow second rotor  1146 . In the example illustrated, the spindle  1132  comprises a spline portion  1138  extending along the axis  1120  and the first rotor  1136  comprises a spline nut  1139  coupled to the spline portion  1138 . In the example illustrated, the spindle  1132  further comprises a ball screw portion  1148  extending along the axis  1120  and the second rotor  1146  comprises a ball nut  1149  coupled to the ball screw portion  1148 . The drive assembly  1128  can be operated similar to the drive assembly  128  (for example, via the controller  150  according to the process  300  and/or  400 ). 
     In the example illustrated, the drive assembly  1128  includes a generally sealed internal chamber  1184  within the housing  1130  for containing lubricant. The internal chamber  1184  includes, in the example illustrated, an axially central portion that extends axially along the spline portion  1138  of the spindle  1132 . Lubricant in the central portion of the internal chamber  1184  can help lubricate the connection between the spindle  1132  and the spline nut  1139  along the spline portion  1138 . 
     In the example illustrated, the internal chamber  1184  further includes a rear portion that extends rearward of, and is in fluid communication with, the axially central portion of the internal chamber  1184 . The rear portion includes, in the example illustrated, an inner rearward portion that extends axially along the ball screw portion  1148  of the spindle  1132 , radially between an outer surface of the ball screw portion  1148  of the spindle and an inner surface of the ball nut  1149 . The rear portion of the internal chamber  1184  further includes, in the example illustrated, an outer rearward portion that extends radially outward of an outer surface of the ball nut  1149 . 
     In the example illustrated, the internal chamber  1184  further includes a front portion that is adjacent a font end of the spindle  1132 . In the example illustrated, the front portion of the internal chamber  1184  is forward of the spline nut  1139  and the central portion of the internal chamber is rearward of the spline nut  1139 . The central portion and front portion of the intern chamber are in fluid communication via longitudinal channels disposed radially between the inner surface of the spline nut  1139  and the outer surface of the spindle  1132 . 
     The drive assembly  1128  includes, in the example illustrated, one or more conduits to provide access to the internal chamber  1184  from outside the housing  1128 . In the example illustrated, the drive assembly  1128  includes a first internal chamber conduit  1166  extending from a first conduit outer end at a back of the housing  1130  to a first conduit inner end that is open to the space between the inner surface of the spline nut  1139  and the outer surface of the spindle  1132 . In the example illustrated, the drive assembly further includes a second internal chamber conduit  1174  extending from a second conduit outer end at the back of the housing  1130  to a second conduit inner end that is open to the space between the inner surface of the ball nut  1149  and the outer surface of the spindle  1132   
     In the example illustrated, the drive assembly  1128  further includes a containment seal assembly  1186  to help enclose the internal chamber  1184 . This can help prevent lubricant, as well as other materials such as dust or particulate, from egressing from the internal chamber  1184  and from the housing  1128 . 
     In the example illustrated, the containment seal assembly  1186  comprises an inter-rotor seal  1188  adjacent the central portion of the internal chamber  1184 , between the first and second rotors  1136 ,  1146 . In the example illustrated, the inter-rotor seal  1188  inhibits dispersion of lubricant radially outwardly from between the first and second rotors  1136 ,  1146  while accommodating relative rotation therebetween. Providing a seal between the first and second rotors  1136 ,  1146  (rather than, for example, between each rotor  1136 ,  1146  and the housing  1130 ) may help to, for example, reduce the number of parts and/or space required for providing a suitable seal. In the example illustrated, the inter-rotor seal  1188  comprises a first O-ring  1190  held radially between an inter-rotor first seal surface  1192  fixed to the first rotor  1136 , and an inter-rotor second seal surface  1194  fixed to the second rotor  1146  and facing the inter-rotor first seal surface  1192 . In the example illustrated, the first O-ring  1190  is rotatable relative to at least one of the inter-rotor first seal surface  1192  and the inter-rotor second seal surface  1194  during normal operation. 
     In the example illustrated, the containment seal assembly  1186  further includes a spindle seal  1196  adjacent the front portion of the internal chamber  1184 , and between the spindle  1132  and the first rotor  1136 . In the example illustrated, the spindle seal  1196  inhibits dispersion of lubricant from between the spindle  1132  and the first rotor  1136  while accommodating relative axial translation therebetween. In the example illustrated, the spindle seal  1196  comprises a second O-ring  1198  held radially between a spindle first seal surface  1200  axially forward of the spline portion  1138  and fixed to the spindle  1132 , and a spindle second seal surface  1202  fixed to the first rotor  1136  and facing the spindle first seal surface  1200 . In the example illustrated, the second O-ring  1198  is generally axially fixed relative to the first rotor  1136  during normal operation.