Abstract:
A injection molding method involves measuring, using at least one strain gauge sensor, a change in strain in a mold side of a mold cavity, approximating a pressure within the mold cavity based on the change in strain, comparing the approximated pressure to a pre-set trigger point, and if the approximated pressure equals or exceeds the pre-set trigger point, activating a virtual cavity sensor having an optimal pre-defined pressure-time curve, wherein upon activation, the virtual cavity sensor tracks approximated pressures calculated from the change in strain measurements measured by the at least one strain gauge sensor over time and compares the results of the approximated pressure tracking to the optimal pre-defined pressure-time curve.

Description:
FIELD OF THE INVENTION 
       [0001]    This application relates generally to injection molding and, more specifically, to methods for using a strain gauge on the exterior of a mold surface adjacent to a parting line of the mold to approximate conditions, such as pressure, within an injection mold and, if an approximated condition hits or exceeds a pre-set trigger point, perform additional calculations and/or adjust the injection molding process, thereby using the strain gauge along with a virtual cavity sensor as part of a pressure sensing system. 
       BACKGROUND OF THE INVENTION 
       [0002]    During injection molding, molten thermoplastic material flows into a mold cavity of a mold via one or more gates. The pressure within the mold cavity is an important vector, as insufficient pressure may result in improperly formed parts while excessive pressure may result in damage to the mold. The pressure at the melt flow front provides information relevant to the injection molding process overall by, for example, enabling calculation of how fast a mold cavity is filling and how long cooling of the molded part within the cavity is likely to take. Some injection molding processes aim to have the melt flow front follow a particular pressure pattern over time in order to optimize the injection molding process. For example, some injection molding processes maintain a pressure balance between the air pressure in the cavity, which changes based on the mold cavity geometry as the molten thermoplastic material moves into the cavity of the mold, and the pressure at the melt flow front, in order to produce a final product that is fully relieved of internal stresses that would otherwise undesirably lead to shrink, sink and warpage. For such injection molding systems, determining the real-time pressure at the melt flow front is important in order to determine whether the desired pressure/time curve is properly being followed and, if deviations from the pressure/time curve are identified, to make adjustments to correct the pressure of the melt flow front. 
         [0003]    Ideally, sensors for measuring the pressure within a mold cavity and at a melt flow front would be indirect, easy to install, and inexpensive. Direct sensors, such as sensors placed within a mold cavity, leave undesirable marks on part surfaces. For example, while demand for injection molded parts with high gloss finishes has been increasing, direct sensors positioned in the mold cavity have a tendency to mar the high gloss finish of the parts. As a result, indirect sensors that are not located in the mold cavity are preferable. Some current indirect sensors include parting line sensors, ejector or static pin sensors, and ultrasonic sensors. Unfortunately, these indirect sensors cannot always be placed in optimal locations, sometimes require that a mold apparatus undergo a period of downtime in order to be machined so that the sensor can be mounted, and can be expensive. The same problems plague the current use of strain gauge sensors for indirectly measuring pressure within a mold cavity. For example, strain gauge sensors have been used in conjunction with molding apparatuses having ejector sleeves or long core pins, but not all injection molding apparatuses are configured to include an ejector sleeve or long core pin. 
       SUMMARY OF THE INVENTION 
       [0004]    Embodiments within the scope of the present invention are directed to the use of strain gauge sensors to approximate conditions, such as pressure, within a mold cavity or at a melt flow front during an injection molding process. The strain gauge sensor or sensors, along with a virtual cavity sensor, are used as part of a pressure sensing system to sense approximate pressure within the mold cavity, such as at the melt flow front, and adjust the injection molding process if the pressure deviates from a pre-set trigger point or range. In embodiments within the scope of the present invention, at least one strain gauge sensor is mounted on an exterior of a mold, such as on the exterior of a mold plate of a stacked mold, adjacent to a parting line located between two mold sides defining one or more mold cavities of an injection molding system. In some embodiments of the present invention, two strain gauge sensors are used, one adjacent to a parting line of the mold and near the nozzle to the mold cavity and the other adjacent to a parting line of the mold and at a location that is downstream of the nozzle when molten thermoplastic material is injected into the mold cavity. 
         [0005]    The strain gauge sensors work by measuring surface strain on a mold surface that occurs over the course of a standard injection molding process. In a typical injection molding apparatus, a mold cavity is formed between two mold sides, which are held together under pressure by a press or clamping unit. Thus, along the parting line of the mold, a closing force is exerted by the press or clamping unit. When molten thermoplastic material is injected into the mold cavity, the molten thermoplastic material exerts an opening force along the parting line of the mold. Ideally, the opening force exerted by the molten thermoplastic material is less than the closing force exerted by the clamping unit. If the opening force is greater than the closing force, the mold sides are forced apart and flashing, or leakage of the molten thermoplastic material, occurs. A strain gauge sensor placed on the exterior of the mold surface adjacent to a parting line of a mold is able to sense the surface strain changes on the mold surface that occur over time as a result of the closing and opening forces. 
         [0006]    In response to surface strain changes, the strain gauge sensor emits an electrical signal, typically in the range of −10 to 10 Volts. The signal emitted by the strain gauge sensor is received and used by a controller to approximate one or more conditions within the mold, such as the pressure within the mold cavity. In certain molds in which the ratio of the length of the flow channel to the thickness of the molded part is great, i.e. molds having a high L/t ratio, the pressure at the melt flow front may be approximated based on the signals emitted by the strain gauge sensor(s). These approximations may be useful for adjusting the injection molding process. For example, the amount of pressure within the mold cavity may be approximated and compared to a maximum permissible mold cavity pressure in an effort to ensure that the mold cavity is not damaged by excessive mold cavity pressure. 
         [0007]    If a condition calculated by the controller exceeds a pre-set trigger point, a pressure sensing system of which the strain gauge sensor is a part may modify operation of an injection molding system in an effort to bring or return the sensed parameter, such as approximated pressure at the melt flow front, to a level within the pre-set range (e.g., below the pre-set trigger point). In addition to one or more strain gauge sensors, a the pressure sensing system of the present disclosure includes a virtual cavity sensor, which can be implemented as a set of instructions (e.g., a software program) stored on a non-transitory machine-readable medium and executed by one or more general-purpose or special-purpose processors. The virtual cavity sensor can read data from, and in some cases write data to, a machine-readable memory storing data representative of an optimal pre-defined pressure-time curve corresponding to the part being manufactured by the injection molding process and the material forming the part. The pre-defined pressure time curve is defined by an equation or relationship in which time is the independent variable and pressure is the dependent variable. In some embodiments, the pre-defined pressure-time curve is a parabolic curve that approaches an asymptote at the maximum pressure setting the mold cavity is set to achieve. The virtual sensor compares the real-time pressure approximated from the strain gauge reading over time to the optimal pre-defined pressure-time curve to determine whether the injection molding process is operating as expected. In conjunction with other information available from the injection molding apparatus, such as how far the screw that -injects the molten thermoplastic material into the mold cavity has traveled, the virtual sensor can determine the viscosity of the molten thermoplastic material and the percentage of the mold cavity that has been filled. In the event that the real-time data approximated from the strain gauge sensor reading indicates that the optimal pre-defined pressure-time curve is not being followed, the virtual sensor system can direct the injection molding apparatus to take corrective action, such as by altering the rate and/or force of advancement of the screw -injects additional molten thermoplastic material in order to increase the pressure at the melt flow front or within the mold cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter that is regarded as the present invention, it is believed that the invention will be more fully understood from the following description taken in conjunction with the accompanying drawings. Some of the figures may have been simplified by the omission of selected elements for the purpose of more clearly showing other elements. Such omissions of elements in some figures are not necessarily indicative of the presence or absence of particular elements in any of the exemplary embodiments, except as may be explicitly delineated in the corresponding written description. None of the drawings are necessarily to scale. 
           [0009]      FIG. 1  illustrates, semi-schematically, an injection molding apparatus constructed according to the present disclosure, wherein two strain gauge sensors are located on the exterior surfaces of mold sides adjacent to a parting line between the mold sides, the first strain sensor gauge being on the first mold side in the vicinity of the nozzle to the mold cavity, and the second strain sensor gauge is on the second mold side downstream of the nozzle; 
           [0010]      FIG. 2  illustrates a stacked mold for a multi-cavity injection molding system, including a pair of plates defining two mold sides, at least one of the plates having a strain gauge sensor located on an exterior surface thereof adjacent to the parting line between the mold sides, shown at a time prior to any forces being exerted on the mold cavity; 
           [0011]      FIG. 3  illustrates the stacked mold for a multi-cavity injection molding system illustrated in  FIG. 2 , when a closing force is being exerted by a clamping unit (not shown) just prior to, or at the initiation of, injection of molten thermoplastic material into the mold cavities; 
           [0012]      FIG. 4  illustrates the mold cavities, mold cores, and first and second mold sides present in the stacked mold for a multi-cavity injection molding system illustrated in  FIG. 3 . 
           [0013]      FIG. 5  illustrates the plate surrounding the mold cavities and first mold sides in the stacked mold for a multi-cavity injection molding system illustrated in  FIGS. 3-5 . 
           [0014]      FIG. 6A-B  illustrates both the plate surrounding the mold cavities and the plate surrounding the mold cores in the stacked mold for a multi-cavity injection molding system illustrated in  FIGS. 3-5 . 
           [0015]      FIG. 7A-B  illustrates the stacked mold for a multi-cavity injection molding system illustrated in  FIGS. 3-6 , when both a closing force is being exerted by a clamping unit (not shown) and an opening force is being exerted by molten thermoplastic material as it is injected in the mold cavity; 
           [0016]      FIG. 8  illustrates graphically a plot of a parabolic pre-defined pressure-time curve that may be utilized by a virtual sensor system of the present disclosure as a desired pressure-time curve to which an injection molding system may be controlled to achieve based on feedback from the strain gauge(s); 
           [0017]      FIG. 9  illustrates a two layer stack injection molding apparatus utilizing strain gauge sensors in conjunction with multiple mold cavities; and 
           [0018]      FIG. 10  illustrates a flow diagram of an example method for adjusting the internal melt pressure (IMP) using readings from a strain gauge sensor, which can be implemented in the controller of  FIG. 1 , for example. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to the figures in detail,  FIG. 1  illustrates an exemplary injection molding apparatus  10  for producing thermoplastic parts in high volumes (e.g., a class  101  injection mold, or an “ultra-high productivity mold”, a class  102  (medium-to-high productivity mold), or class  103  (a medium productivity mold)). The injection molding apparatus  10  generally includes an injection system  12  and a clamping system  14 . A thermoplastic material may be introduced to the injection system  12  in the form of thermoplastic pellets  16 . The thermoplastic pellets  16  may be placed into a hopper  18 , which feeds the thermoplastic pellets  16  into a heated barrel  20  of the injection system  12 . The thermoplastic pellets  16 , after being fed into the heated barrel  20 , may be driven to the end of the heated barrel  20  by a ram, such as a reciprocating screw  22 . The heating of the heated barrel  20  and the compression of the thermoplastic pellets  16  by the reciprocating screw  22  causes the thermoplastic pellets  16  to melt, forming a molten thermoplastic material  24 . The molten thermoplastic material is typically processed at a temperature of about 130° C. to about 410° C. 
         [0020]    The reciprocating screw  22  forces the molten thermoplastic material  24  toward a nozzle  26  to form a shot of thermoplastic material, which will be injected into a mold cavity  32  of a mold  28  via one or more gates. The molten thermoplastic material  24  may be injected through a gate  30 , which directs the flow of the molten thermoplastic material  24  to the mold cavity  32 . The mold cavity  32  is formed between first and second mold sides  25 ,  27  of the mold  28  and the first and second mold sides  25 ,  27  are held together under pressure by a press or clamping unit  34 . The press or clamping unit  34  applies a clamping force during the molding process that is greater than the force exerted by the injection pressure acting to separate the two mold halves  25 ,  27 , thereby holding the first and second mold sides  25 ,  27  together while the molten thermoplastic material  24  is injected into the mold cavity  32 . In a typical high variable pressure injection molding machine, the press typically exerts 30,000 psi or more because the clamping force is directly related to injection pressure. To support these clamping forces, the clamping system  14  may include a mold frame and a mold base. 
         [0021]    Once the shot of molten thermoplastic material  24  is injected into the mold cavity  32 , the reciprocating screw  22  stops traveling forward. The molten thermoplastic material  24  takes the form of the mold cavity  32  and the molten thermoplastic material  24  cools inside the mold  28  until the thermoplastic material  24  solidifies. Once the thermoplastic material  24  has solidified, the press  34  releases the first and second mold sides  25 ,  27 , the first and second mold sides  25 ,  27  are separated from one another, and the finished part may be ejected from the mold  28 . The mold  28  may include a plurality of mold cavities  32  to increase overall production rates. The shapes of the cavities of the plurality of mold cavities may be identical, similar or different from each other. (The latter may be considered a family of mold cavities). 
         [0022]    A controller  50  is communicatively connected with a first strain gauge sensor  52  and a screw control  36 . The first strain gauge  52  is located on the exterior surface of first mold side  25  near the parting line between the first mold side  25  and the second mold side  27  and in the vicinity of the nozzle  26 . The controller  50  may include a microprocessor (or another suitable processing unit, or several such units), a non-transitory memory, and one or more communication links. The controller  50  may also be optionally connected to a second strain gauge sensor  53  located on the exterior surface of the second mold side  27  near the parting line between the first mold side  25  and the second mold side  27  and downstream of the nozzle  26 . Although two strain gauge sensors are utilized in the embodiment depicted in  FIG. 1 , other embodiments within the scope of the present invention may employ only one strain gauge sensor or may use more than two strain gauge sensors. Moreover, embodiments within the scope of the present disclosure may use strain gauge sensors located at other locations on a mold side. 
         [0023]    The strain gauge sensors  52  and  53  sense surface strain of the mold, as discussed in greater depth with respect to  FIGS. 2-4 . Data from the strain gauge sensors  52  and  53  may be communicated to a processor that calculates the change in the surface strain of the mold. Electric signals  54  from the strain gauge sensors  52 ,  53  may travel along one or more electrical paths, such as wires  54 , depicted in  FIG. 1  in solid lines, ranging in strength from 0 to 10 Volts. The controller  50  may be connected to the screw control  36  via wired connection  56 . In other embodiments, the controller  50  may be connected to the screw control  36  via a wireless connection, a mechanical connection, a hydraulic connection, a pneumatic connection, or any other type of communication connection known to those having ordinary skill in the art that will allow the controller  50  to communicate with the screw control  36 . 
         [0024]    The controller  50  may use the change in surface strain calculated from the information provided by the first and second strain gauge sensors  52  and  53  to approximate conditions, such as pressure, within the mold cavity or, in certain molds, particularly in which the ratio of the length of the flow channel to the thickness of the molded part is large, at the melt flow front of the molten thermoplastic material  24 . These approximations may then be used to adjust the injection molding process. For example, the amount of pressure within the mold cavity  32  may be approximated and compared to a maximum permissible mold cavity pressure in order to ensure that the mold cavity  32  is not damaged by excessive mold cavity pressure. If the amount of pressure within the mold cavity  32  is approximated to be dangerously close to the maximum permissible mold cavity pressure, or even in excess of a safety threshold well below the maximum permissible mold cavity pressure, the controller  50  may direct the screw control  36  to cease -injecting molten thermoplastic material. Other approximations may be used to control the molding process, such that variations in material viscosity, mold temperatures, melt temperatures, and other variations influencing filling rate, can be used to trigger adjustments to the injection molding system by the controller  50 . These adjustments may be made immediately during the molding cycle, or corrections can be made in subsequent cycles. Furthermore, the approximations may be averaged over a number of cycles and then used to make adjustments to the molding process by the controller  50 . 
         [0025]    Additionally, the controller  50  is in communication with a virtual cavity sensor  51 , which is implemented as a program, or a set of software instructions. More generally, however, a virtual cavity sensor can be implemented in hardware (e.g., as an application-specific integrated circuit (ASIC)), firmware, software, or any suitable combination thereof. In this disclosure, the term “virtual cavity sensor” can refer to a module that determines the value of a process variable, such as pressure, without directly measuring this process variable. The strain gauge sensors  52  and  53  and the virtual cavity sensor  51  together form a pressure sensing system that generates data indicative of pressure within the mold cavity  32 . As used herein, the term “pressure sensing system” refers to any suitable group of components, which can include both software-implemented and hardware-implemented components, that generate a signal indicative of pressure based on a signal indicative of another process variable. 
         [0026]    If based on the surface strain information provided by the first and second strain gauge sensors  52  and  53 , a processor associated with the controller  50  determines that pressure within the mold cavity exceeds a pre-set trigger point (or is outside of a predetermined acceptable range), the virtual cavity sensor  51  signals the controller to adjust the screw control  36  to bring the pressure within the mold cavity back to a level below the trigger point (or within the predetermined acceptable range). 
         [0027]      FIGS. 2-7  illustrate a stacked multi-cavity injection molding system utilizing a strain gauge sensor  154 . As shown in  FIG. 3 , the stacked multi-cavity injection molding system has mold cavities  132  in first mold sides  125  and complementary mold cores  135  in second mold sides  127 . The stacked multi-cavity injection molding system depicted in  FIGS. 2-7  has a total of four mold cavities  132  ( 132   a,    132   b,    132   c,    132   d ) in four first mold sides  125  ( 125   a,    125   b ,  125   c,    125   d ) and four mold cores  135  ( 135   a,    135   b,    135   c , 135   d ) in four second mold sides  127  ( 127   a,    127   b,    127   c,    127   d ), although one or more of the mold cavities  132  in first mold sides  125  or mold cores  135  in second mold sides  127  are omitted from  FIGS. 3-5  to allow other elements to be visible in the drawing. Stacked multi-cavity injection molding systems within the scope of the present invention may have more than four or less than four mold cavities  132  in first mold sides  125  and mold cores  135  in second mold sides  127 . 
         [0028]    As shown in  FIG. 4 , the mold cavities  132   a,    132   b,    132   c,  and  132   d  are surrounded by plate  133 . As shown in  FIG. 5 , the mold cores  135  in second mold sides  127  are surrounded by plate  137 . In the embodiments depicted in  FIGS. 2-7 , a strain gauge sensor  154  is located on the exterior of plate  133  adjacent to the parting line of the mold between plate  133  and plate  137 . In other embodiments within the scope of the invention, more than one strain gauge sensor  154  may be used, and one or more strain gauge sensor may be placed on plate  133 , plate  137 , on any of first mold sides  125 , or on any of second mold sides  127 . In  FIGS. 2-7 , the strain gauge sensor  154  is located near a corner of plate  133 . However, in some embodiments, the strain gauge sensor  154  may be placed near the middle of plate  133 , which experiences a greater change in strain as a result of the forces acting on the mold cavities  132  than the corners of the plate that may be supported by guide pins or leader pins. In some embodiments, a determination of whether to place strain gauge sensor  154  or plate  133  or  137  is made on the basis of which plate experiences more strain, which is influenced by which plate is made of a less stiff material, is thinner, or has more cuts through it that reduce its stiffness. In some embodiments, the plate  137  experiences more strain and is therefore a prime location for a strain gauge sensor  154 . Interfering components or features, such as connectors for cooling channels, may influence the location of strain gauge sensor  154  in some embodiments. In some embodiments in which a mold has more than one parting line, the strain gauge sensor  154  may be placed closest to the parting line that more directly experiences the opening force caused by the injection of molten thermoplastic material, which in some embodiments is the parting line from which molded parts are ejected. 
         [0000]    The strain gauge sensors described in the present application, including strain gauge sensors  52 ,  53 , and  54  depicted in  FIGS. 1-4 , could be chosen from a variety of commercially available strain gauge sensors. For example, the Kistler “Surface Strain Sensor” Type 9232A or Rocktest Telemac&#39;s “Surface Mount Strain Gauges” Models SM-5A or SM-5B would work in embodiments within the scope of the present invention. Such strain gauge sensors are designed for quick and easy installation, as they may have anchor blocks that can be welded to the injection molding apparatus or may be screwed into the surface of the injection molding apparatus. 
         [0029]      FIG. 2  illustrates a stacked multi-cavity injection molding system at a time prior to any forces being exerted on the mold cavities  132 , as the injection molding system is in an open condition. With no external forces acting on it, the plate  133  has a baseline thickness X 1 . Likewise, the plate  133  has a baseline strain, which may be zero, inherent in its construction. The strain gauge sensor  154  detects a strain, for example με 1 , based on the sensed baseline strain of the plate  133 . 
         [0030]      FIG. 6A  illustrates the mold cavities  132  under the closing force exerted by a press or clamping unit (not depicted). Under the closing force, the plate  133  changes shape slightly. For example, the thickness of the plate  133 , which was previously X 1 , is changed by an amount AX to have a new thickness X 2 . Likewise, the strain in the plate  133  changes. The strain gauge sensor  54  detects a strain, for example με 2 , that corresponds with this change. In general, the closing force results in a compressive strain. However, this may not always be the case depending on the particular injection molding apparatus in use and the location of the strain gauge sensor  154 . As a result, the change in strain detected by the strain gauge sensor  154  over time may be either positive or negative, or the system of the present invention may operate based on an absolute value of change in strain over time.  FIG. 6B  is an enlarged view of the portion of  FIG. 6A  having the strain gauge sensor. 
         [0031]      FIG. 7A  illustrates the mold cavities  132  under both the closing force exerted by a press or clamping unit (not depicted) and an opening force exerted by thermoplastic molten material as it is injected into the mold cavities  132 . Plate  133  responds to the combination of the forces. For example, the thickness of plate  133 , which was X 1  when no forces were applied to the mold cavities  32  and X 2  when only a closing force was applied to the mold cavities  132 , is changed by an amount ΔX to have a new thickness X 3  The displacement caused by the addition of the opening force to the closing force also results in a change in the surface strain, and the strain gauge sensor  154  detects a strain, for example με 3 , that corresponds with this change. In general, the opening force results in a relaxing tensile strain. However, this may not always be the case depending on the particular injection molding apparatus in use and the location of the strain gauge sensor  154 . As a result, the change in strain detected by the strain gauge sensor  154  may be either positive or negative. Depending on where the strain gauge sensor  154  is located, the strain gauge sensor  154  may detect a change in tensile strain or a change in compressive strain. The pre-set trigger point that activates a virtual cavity sensor, such as virtual cavity sensor  51  depicted in  FIG. 1 , will generally occur when the mold cavities  132  are experiencing both an opening force and a closing force as depicted in  FIG. 7A  as this is the period of time during the injection molding process when molten thermoplastic material is actually being injected into the mold cavities  132  to form a part.  FIG. 7B  is an enlarged view of the portion of  FIG. 7A  having the strain gauge sensor. 
         [0032]      FIG. 8  depicts an optimal pre-defined pressure-time curve that can be used by a virtual cavity sensor  51 . The independent (horizontal) axis represents time, and the dependent (vertical) axis represents pressure. The origin is a pre-set trigger point, which generally occurs at or near a time when molten thermoplastic material begins to enter a mold cavity  32 ,  132 , or  232  and exert an opening force detectable by a strain gauge sensor. In some embodiments, the optimal pre-defined pressure-time curve is parabolic with an asymptote located a maximum pressure setting that occurs when a part is fully formed. In some embodiments, the pressure-time curve is defined by two variables: 1) the time for a mold cavity to fill to 75%, and 2) the maximum pressure setting. The virtual cavity sensor  51  compares the real-time pressure approximated from readings by strain gauge sensors over time to the optimal pre-defined pressure-time curve to determine whether the injection molding process is operating as expected. In conjunction with other information available from the injection molding apparatus  10  or  210 , such has how far the reciprocating screw  22  has traveled as measured by a mechanism connected to the screw  22 , the virtual cavity sensor  51  can determine the viscosity of the molten thermoplastic material  24  or  224  and the percentage of the mold cavity  32 ,  132 , or  232  that has been filled. In the event that the real-time data approximated from the readings from strain gauge sensors  52 ,  53 ,  154 , or  256  indicates that the optimal pre-defined pressure-time curve is not being followed, the virtual cavity sensor  51  can direct the injection molding apparatus  10  or  210  to take corrective action, such as by -injecting additional molten thermoplastic material  24  or  224  in order to increase the pressure at the melt flow front or within the mold cavity  32 ,  132 , or  232 . An example method of taking such corrective action is discussed below with reference to  FIG. 10 . 
         [0033]    In the examples discussed above, in addition to determining approximate pressure, the virtual cavity sensor  51  causes the injection molding apparatus  10  or  210  to take corrective action. In another embodiment, the virtual cavity sensor  51  only generates a signal indicative of the pressure inside the mold cavity, and another component determines whether to direct the injection molding apparatus  10  or  210  to take corrective action in view of the output of the virtual cavity sensor  51  and possibly other signals or pre-set values. This component can be implemented as part of the controller  50  of  FIG. 1 , for example. More generally, functionality related to approximating pressure using strain gauge sensor readings, comparing the readings to a pre-defined curve, and determining whether corrective action should be taken, may be distributed among the components  50 ,  51 , etc. in any suitable manner. 
         [0034]      FIG. 9  illustrates an alternative arrangement of injection molding apparatus  10  depicted in  FIG. 1 . In  FIG. 6A , the injection molding apparatus  210  is has a two layer stacked molding configuration. Multiple mold cavities  232  are present and may be stacked on one another. To accommodate the stacked configuration, molten thermoplastic material  224  flows through two gates  230  into the mold cavities  232 . A plurality of strain sensors  256  may be utilized at or near the parting lines of the injection molding apparatus  10 . A benefit of having a plurality of strain sensors  256  in the multiple mold cavities  232 , as opposed to only having a single mold cavity  232  outfitted with a strain sensor, is that the virtual sensor  51  (pictured in  FIG. 1 ) can determine the percentage fill of each of the mold cavities  232 . This is important because in certain molding configurations, some mold cavities  232  may fill sooner or later than other mold cavities  232 , such that the percentage fill of a single mold cavity  232  is not necessarily representative of the fill of all of the plurality of mold cavities  232 . In addition, traditional cavity sensors  258  may be present in mold cavities  232 . These traditional cavity sensors  258  may provide to the controller  50  (pictured in  FIG. 1 ) information that may be used to activate certain changes to the injection molding process that are not controlled by the virtual cavity sensor  51 . One benefit to having a plurality of strain gauge sensors  256  in an injection molding apparatus  210  in addition to traditional cavity sensors  258  is that traditional cavity sensors  258  do not provide any measurements for a given mold cavity  232  in the event that particular mold cavity  232  is not being filled with molten thermoplastic material  224  due to the mold cavity  232  being down for maintenance. However, the strain gauge sensors  256  located on that mold cavity  232  still provide strain change information that can be used to determine, for example, whether the down mold cavity  232  is experiencing a level of pressure that could result in damage to the mold cavity  232 . 
         [0035]      FIG. 10  illustrates an example method  300  for adjusting an injection melt pressure (IMP) at which thermoplastic material is injected into the mold cavity  32  (see  FIG. 1 ). The controller  50  can implement this method to dynamically adjust the signal supplied to the screw control  36  via the wired connection  56 , for example. 
         [0036]    The method  300  begins at block  302 , where the initial value of the IMP is obtained. At block  304 , a measurement from a strain gauge sensor (e.g., the strain gauge sensor  52 ) is obtained. In some embodiments, measurements from multiple strain gauge sensors are received at block  304 . An approximate cavity pressure is determined using the measurement(s) from the one or more strain gauge sensors (block  306 ). The virtual cavity sensor  51 , for example, can implement blocks  304  and  306 . 
         [0037]    Next, at block  308 , an adjustment value for the IMP is determined using the approximate cavity pressure determined at block  306 . To this end, the controller  50  and/or the virtual cavity sensor  51  can receive an indication of the appropriate mapping from an operator (e.g., “adjust the IMP by N units in response to each M-unit drop detected in the cavity”), from a pre-defined lookup table, according to a certain formula, or in any other suitable matter. At block  310 , the adjustment value is applied as feedback to the current value of the IMP to execute a feedback loop control scheme. 
         [0038]    If it is determined at block  314  that a certain predefined condition has been satisfied, such as the strain gauge measurement(s) providing data indicative of the melt flow-front having reached end-of-fill or another predetermined location within the mold cavity, the method  300  completes. Otherwise, the flow returns to block  304  to await a new reading from the strain gauge sensor. The timing of re-executing block  304  can be pre-configured or operator-controlled, for example. By way of example only, a new reading from the gauge sensor can be received continuously or nearly continuously, which, as those terms are used herein, include increments of every millisecond, every two milliseconds, every three milliseconds, every four milliseconds, every five milliseconds, or any other subdivision of time that may be required from initiation of fill to end-of-fill which is suitable for taking strain gauge measurements, comparing those measurements to data points consistent with predetermined or desired pressure-versus-time values, and adjusting IMP in an effort to counteract any discrepancies. The readings may be taken at equal or unequal increments of time, throughout fill or during particular intervals while filling the mold cavity. 
         [0039]    Part, parts, or all of any of the embodiments disclosed herein can be combined with part, parts, or all of other injection molding embodiments known in the art, including those described below. 
         [0040]    Embodiments of the present disclosure can be used with embodiments for injection molding at low constant pressure, as disclosed in U.S. patent application Ser. No. 13/476,045 filed May 21, 2012, entitled “Apparatus and Method for Injection Molding at Low Constant Pressure” (applicant&#39;s case 12127) and published as US 2012-0294963, which is hereby incorporated by reference. 
         [0041]    Embodiments of the present disclosure can be used with embodiments for pressure control, as disclosed in U.S. patent application Ser. No. 13/476,047 filed May 21, 2012, entitled “Alternative Pressure Control for a Low Constant Pressure Injection Molding Apparatus” (applicant&#39;s case 12128), now U.S. Pat. No. 8,757,999, which is hereby incorporated by reference. 
         [0042]    Embodiments of the present disclosure can be used with embodiments for non-naturally balanced feed systems, as disclosed in U.S. patent application Ser. No. 13/476,073 filed May 21, 2012, entitled “Non-Naturally Balanced Feed System for an Injection Molding Apparatus” (applicant&#39;s case 12130), now U.S. Pat. No. 8,911,228, which is hereby incorporated by reference. 
         [0043]    Embodiments of the present disclosure can be used with embodiments for injection molding at low, substantially constant pressure, as disclosed in U.S. patent application Ser. No. 13/476,197 filed May 21, 2012, entitled “Method for Injection Molding at Low, Substantially Constant Pressure” (applicant&#39;s case 12131Q) and published as US 2012-0295050, which is hereby incorporated by reference. 
         [0044]    Embodiments of the present disclosure can be used with embodiments for injection molding at low, substantially constant pressure, as disclosed in U.S. patent application Ser. No. 13/476,178 filed May 21, 2012, entitled “Method for Injection Molding at Low, Substantially Constant Pressure” (applicant&#39;s case 12132Q) and published as US 2012-0295049, which is hereby incorporated by reference. 
         [0045]    Embodiments of the present disclosure can be used with embodiments for co-injection processes, as disclosed in U.S. patent application Ser. No. 13/774,692 filed Feb. 22, 2013, entitled “High Thermal Conductivity Co-Injection Molding System” (applicant&#39;s case 12361) and published as US 2013-0221572, which is hereby incorporated by reference. 
         [0046]    Embodiments of the present disclosure can be used with embodiments for molding with simplified cooling systems, as disclosed in U.S. patent application Ser. No. 13/765,428 filed Feb. 12, 2013, entitled “Injection Mold Having a Simplified Evaporative Cooling System or a Simplified Cooling System with Exotic Cooling Fluids” (applicant&#39;s case 12453M), now U.S. Pat. No. 8,591,219, which is hereby incorporated by reference. 
         [0047]    Embodiments of the present disclosure can be used with embodiments for molding thinwall parts, as disclosed in U.S. patent application Ser. No. 13/601,514 filed Aug. 31, 2012, entitled “Method and Apparatus for Substantially Constant Pressure Injection Molding of Thinwall Parts” (applicant&#39;s case 12487D), now U.S. Pat. No. 8,828,291, which is hereby incorporated by reference. 
         [0048]    Embodiments of the present disclosure can be used with embodiments for molding with a failsafe mechanism, as disclosed in U.S. patent application Ser. No. 13/672,246 filed Nov. 8, 2012, entitled “Injection Mold With Fail Safe Pressure Mechanism” (applicant&#39;s case 12657) and published as US 2014-0127338, which is hereby incorporated by reference. 
         [0049]    Embodiments of the present disclosure can be used with embodiments for high-productivity molding, as disclosed in U.S. patent application Ser. No. 13/682,456 filed Nov. 20, 2012, entitled “Method for Operating a High Productivity Injection Molding Machine” (applicant&#39;s case 12673R) and published as US 2013-0221575, which is hereby incorporated by reference. 
         [0050]    Embodiments of the present disclosure can be used with embodiments for molding certain thermoplastics, as disclosed in U.S. patent application Ser. No. 14/085,515 filed Nov. 20, 2013, entitled “Methods of Molding Compositions of Thermoplastic Polymer and Hydrogenated Castor Oil” (applicant&#39;s case 12674M) and published as US 2014-0145374, which is hereby incorporated by reference. 
         [0051]    Embodiments of the present disclosure can be used with embodiments for runner systems, as disclosed in U.S. patent application Ser. No. 14/085,515 filed Nov. 21, 2013, entitled “Reduced Size Runner for an Injection Mold System” (applicant&#39;s case 12677M) and published as US 2014-0141117, which is hereby incorporated by reference. 
         [0052]    Embodiments of the present disclosure can be used with embodiments for moving molding systems, as disclosed in U.S. patent application Ser. No. 14/275,944 filed May 13, 2014, entitled “Low Constant Pressure Injection Molding System with Variable Position Molding Cavities:” (applicant&#39;s case 12896) and published as US 2014-0335219, which is hereby incorporated by reference. 
         [0053]    Embodiments of the present disclosure can be used with embodiments for injection mold control systems, as disclosed in U.S. patent application Ser. No. 14/176,505 filed Feb. 10, 2014, entitled “Injection Molding Machines and Methods for Accounting for Changes in Material Properties During Injection Molding Runs” (applicant&#39;s case 13020), now U.S. Pat. No. 8,980,146, which is hereby incorporated by reference. 
         [0054]    Embodiments of the present disclosure can be used with embodiments for injection mold control systems, as disclosed in U.S. patent application Ser. No. 14/448,648 filed Jul. 31, 2014, entitled “Injection Molding Machines and Methods for Accounting for Changes in Material Properties During Injection Molding Runs” (applicant&#39;s case 13021M) and published as US 2015-003518, which is hereby incorporated by reference. 
         [0055]    Embodiments of the present disclosure can be used with embodiments for injection mold control systems, as disclosed in U.S. patent application Ser. No. 14/448,726 filed Jul. 31, 2014, entitled “Injection Molding Machines and Methods for Accounting for Changes in Material Properties During Injection Molding Runs” (applicant&#39;s case 13022) and published as US 2015-0115491, which is hereby incorporated by reference. 
         [0056]    Embodiments of the present disclosure can be used with embodiments for using injection molding to form overmolded articles, as disclosed in U.S. patent application Ser. No. 14/577,310 filed Dec. 19, 2014, entitled “Methods of Forming Overmolded Articles” (applicant&#39;s case 13190) and published as US 2015-0174803, which is hereby incorporated by reference. 
         [0057]    Embodiments of the present disclosure can be used with embodiments for controlling molding processes, as disclosed in U.S. Pat. No. 5,728,329 issued Mar. 17, 1998, entitled “Method and Apparatus for Injecting a Molten Material into a Mold Cavity” (applicant&#39;s case 12467CC), which is hereby incorporated by reference. 
         [0058]    Embodiments of the present disclosure can be used with embodiments for controlling molding processes, as disclosed in U.S. Pat. No. 5,716,561 issued Feb. 10, 1998, entitled “Injection Control System” (applicant&#39;s case 12467CR), which is hereby incorporated by reference. Embodiments of the present disclosure can be used with embodiments for molding preforms, as disclosed in U.S. patent application Ser. No. 61/952281, entitled “Plastic Article Forming Apparatus and Methods for Using the Same” (applicant&#39;s case 13242P), which is hereby incorporated by reference. 
         [0059]    Embodiments of the present disclosure can be used with embodiments for molding preforms, as disclosed in U.S. patent application Ser. No. 61/952283, entitled “Plastic Article Forming Apparatus and Methods for Using the Same” (applicant&#39;s case 13243P), which is hereby incorporated by reference. 
         [0060]    Embodiments of the present disclosure can be used with embodiments for brazed feed systems, as disclosed in U.S. patent application Ser. No. 62/032,071, entitled “Feed System for an Injection Molding Machine” (applicant&#39;s case 13488P), which is hereby incorporated by reference. 
         [0061]    Embodiments of the present disclosure can be used with embodiments for unbrazed feed systems, as disclosed in U.S. patent application Ser. No. 62/042,577, entitled “Feed System for an Injection Molding Machine” (applicant&#39;s case 13498P), which is hereby incorporated by reference. 
         [0062]    Embodiments of the present disclosure can be used with embodiments of heated molds for injection molding machines, as disclosed in U.S. patent application Ser. No. 62/045,373, entitled “Injection Molding with Localized Heating in Flow Challenge Regions” (applicant&#39;s case 13509P), which is hereby incorporated by reference. 
         [0063]    Embodiments of the present disclosure can be used with embodiments for retrofitting injection molding machines, as disclosed in U.S. patent application Ser. No. 62/053,499, entitled “Retrofitted Injection Molding Machines” (applicant&#39;s case 13553P), which is hereby incorporated by reference. 
         [0064]    Embodiments of the present disclosure can be used with embodiments for continuous injection molding, as disclosed in U.S. patent application Ser. No. 62/084,778, entitled “System and Method for Continuous Injection Molding” (applicant&#39;s case 13638P), which is hereby incorporated by reference. 
         [0065]    Embodiments of the present disclosure can be used with embodiments for continuous co-injection molding, as disclosed in U.S. patent application Ser. No. 62/084,787, entitled “Co-Injection with Continuous Injection Molding” (applicant&#39;s case 13639P), which is hereby incorporated by reference. 
         [0066]    Embodiments of the present disclosure can be used with embodiments for injection molding with coining, as disclosed in U.S. patent application Ser. No. 62/186,722, entitled “Sequential Coining” (applicant&#39;s case 13935P), which is hereby incorporated by reference. 
         [0067]    Embodiments of the present disclosure can be used with embodiments for injection molding control, as disclosed in U.S. patent application Ser. No. 62/186,739, entitled “Method of Injection Molding with Constant-Velocity Flow Front Control” (applicant&#39;s case 13936P), which is hereby incorporated by reference. 
         [0068]    Embodiments of the present disclosure can be used with embodiments of injection molding under certain equipment conditions, as disclosed in U.S. patent application Ser. No. 62/192,616, entitled “Injection Molding with a Leaking Check Ring” (applicant&#39;s case 13957P), which is hereby incorporated by reference. 
         [0069]    While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention. 
         [0070]    The dimensions and values disclosed herein are not to be understood as being strictly limited to the exact numerical values recited. Instead, unless otherwise specified, each such dimension is intended to mean both the recited value and a functionally equivalent range surrounding that value. For example, a dimension disclosed as “40 mm” is intended to mean “about 40 mm.” 
         [0071]    Every document cited herein, including any cross referenced or related patent or application and any patent application or patent to which this application claims priority or benefit thereof, is hereby incorporated herein by reference in its entirety unless expressly excluded or otherwise limited. The citation of any document is not an admission that it is prior art with respect to any invention disclosed or claimed herein or that it alone, or in any combination with any other reference or references, teaches, suggests or discloses any such invention. Further, to the extent that any meaning or definition of a term in this document conflicts with any meaning or definition of the same term in a document incorporated by reference, the meaning or definition assigned to that term in this document shall govern. 
         [0072]    While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.