Abstract:
A pressure measuring device for an injection molding machine having a plasticizing screw operated by a rotary drive and an injection drive includes a load detector having a deformation zone which forms part of a transmission member of the injection drive and is of reduced cross section, and a sensor. The deformation zone elastically deforms in response to a driving force acting on the transmission member, with the sensor measuring a change in shape of the deformation zone.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the priority of German Patent Application, Serial No. 103 37 551.1, filed Aug. 14, 2003, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates, in general, to a pressure measuring device for an injection molding machine. 
   Nothing in the following discussion of the state of the art is to be construed as an admission of prior art. 
   A typical injection molding machine includes an injection mold and an injection unit for injecting a shot of plastic melt into the injection mold. The injection unit is provided with a rotary drive to rotate a plasticizing screw for advancing plastic in a barrel while being plasticized, and an injection drive or thrust generator for axially moving the screw to thereby inject the shot into the adjacent injection mold. In order to measure the thrust applied on the plasticizing screw by the injection drive for determining the melt pressure of the plastic melt in the barrel, the use of a load detector has been proposed. Examples include European Patent specification EP 0 350 872 B1, U.S. Pat. Nos. 5,206,034 and 6,247,913. The load detector is hereby positioned in the drive train between the injection drive and the plasticizing screw and includes a force transmitting element, which elastically deforms when being subjected to the thrust, and a pickup device for measuring the deformation. Conventional load detectors suffer shortcomings because the force transmitting element is realized in the form of a separate mechanical and fairly expensive precision structure and because the integration of the measuring device inside the injection unit so that a replacement is only difficult to implement and time-consuming. When the load detector is placed directly in the driveshaft, which is coupled with the plasticizing screw, in the shaft region between driveshaft bearing and the plasticizing screw, the supply of auxiliary energy and the transmission of the measuring value is difficult to implement between the part that conjointly rotates with the driveshaft and the stationary part of the load detector. 
   It would therefore be desirable and advantageous to provide an improved pressure measuring device for an injection molding machine to obviate prior art shortcomings and to attain high precision in measurement while being simple in structure and reliable in operation. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, a pressure measuring device for an injection molding machine having a plasticizing screw operated by a rotary drive and an injection drive, includes a load detector having a deformation zone which forms part of a transmission member of the injection drive and has a reduced cross section and which elastically deforms in response to a driving force acting on the transmission member, and a sensor for measuring a change in shape of the deformation zone. 
   The present invention resolves prior art problems by integrating the elastically deformable force transmitting element of the load detector in the transmission member of the injection drive so that the force transmitting element is axially upset or compressed when subjected to the thrust of the injection drive and thereby shortened and at the same time widened. The transmission member may hereby be a screw driveshaft having a cylindrical shaft portion constructed of reduced diameter to define the deformation zone and positioned between the plasticizing screw and a bearing for the driveshaft. As an alternative, the injection drive may be a spindle drive, with the transmission member being part of the spindle drive and having a cylindrical portion of reduced diameter to define the deformation zone. 
   The portion of the transmission member that is reduced in cross section is easy to make, highly unlikely to fail because the axial force is applied evenly, and is able to change its shape commensurate with an applied load (thrust). The measuring range of a measuring device according to the present invention can be easily expanded or reduced through change of the effective measuring distance. 
   According to another feature of the present invention, the sensor may be constructed to measure in a contactless manner a change in shape of the deformation zone, such as a change in length and/or thickness of the deformation zone. The sensor may hereby be a laser scanner or an inductive pickup device. This configuration is especially advantageous, when the deformation zone is provided in a rotating transmission member because of the absence of complicated rotating arrangements for transmitting auxiliary energy and measuring signals. 
   According to another aspect of the present invention, an injection molding machine includes a plasticizing screw, a rotary drive for rotating the screw, an injection drive having a force transmission member for moving the screw in an axial direction and a load detector having a deformation zone which forms part of a transmission member of the injection drive and has a reduced cross section and which elastically deforms in response to a driving force acting on the transmission member, and a sensor for measuring a change in shape of the deformation zone. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
       FIG. 1  is a longitudinal section of a main portion of an injection unit of an injection molding machine provided with a pressure measuring device in accordance with a first embodiment of the present invention; and 
       FIG. 2  is a longitudinal section of a main portion of an injection unit of an injection molding machine provided with a pressure measuring device in accordance with a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Throughout all the Figures, same or corresponding elements are generally indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the drawings are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
   Turning now to the drawing, and in particular to  FIG. 1 , there is shown a longitudinal section of a main portion of an injection unit forming part of an otherwise not shown injection molding machine. The injection unit includes a plasticizing barrel  4  which houses a plasticizing screw  5  which is connected to a driveshaft  3 . The screw  5  is rotated by a rotary drive  2  and moved in axial direction by an injection drive or thrust generator  1 . The combination of injection drive  1  and rotary drive  2  is received in a housing  6  and acts on the driveshaft  3  to thereby transmit the rotational and axial movements to the screw  5 . 
   The injection drive  1  is constructed as spindle drive comprised of a rotatable but axially immobile threaded nut  8 , which is operated e.g. by a belt drive  7 , and a threaded shaft  9  which carries the nut  8  and is mounted to the housing  6  in such a manner as to be movable in axial direction but constraint from rotating. Thrust is applied by the threaded shaft  9  via a pivot bearing  10  onto the driveshaft  3  and from there is transmitted to the plasticizing screw  5 . 
   The rotary drive  2  includes a journal  12  which is rotatably mounted in the housing  6  and driven by a belt pulley  11 . The driveshaft  3  is linked in fixed rotative engagement to the journal  12  via a sliding fit  13  which allows an axial displacement driveshaft  3  in correspondence to the thrust of the injection drive  1 . In the filling and metering phase, the plasticizing screw  5  is caused to rotate by the rotary drive  2  and, independently thereform, is moved translationally in axial direction in the injection phase by the injection drive  1 . The axial force applied upon the plasticizing screw  5  is hereby ascertained by a load detector, generally designated by reference numeral  14 , as a measure for the melt pressure of plastic melt in the barrel  4 . 
   As shown in  FIG. 1 , the load detector  14  includes a deformation zone  15  which is formed integral with the driveshaft  3  in the form of a cylindrical shaft portion  16  which is reduced in diameter and positioned between the plasticizing screw  5  and the pivot bearing  10  for the driveshaft  3 . In other words, the cylindrical shaft portion  16  is positioned in a region of the driveshaft  3  which is free of any bearings in order to effectively eliminate errors in measurement as a consequence of bearing friction or other interferences. The load detector  14  further includes a pickup device for measuring elastic changes in shape of the shaft portion  16  (deformation zone  15 ), i.e. changes in length and/or thickness of the shaft portion  16  when subjected to the thrust force. The pickup device operates in a contactless manner and is implemented in the form of a laser scanner which includes a laser transmitter  18  and a laser receiver  19 . Both the laser transmitter  18  and the laser receiver  19  are mounted to a sleeve-like attachment  20  of the threaded shaft  9  and operatively connected to an evaluation circuit  22  via auxiliary energy and measuring signal lines  21 . The laser scanner  18 ,  19  operates at high cycle rate of up to 400 Hz and its measuring range can easily be adjusted by means of the evaluation circuit  22 . The attachment is closed by a lid  23  to protect the load detector  14  and the deformation zone  15  of the pressure measuring device against external influences while allowing easy access. Suitably, the attachment  20  is detachably secured to the threaded shaft  9 . 
   Referring now to  FIG. 2 , there is shown a longitudinal section of a main portion of an injection unit of an injection molding machine provided with a pressure measuring device in accordance with a second embodiment of the present invention. Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. The description below will center on the differences between the embodiments. In this embodiment, provision is made for a load detector having a deformation zone  15 , which forms an integral part of the threaded shaft  9  and is defined by a cylindrical portion of reduced diameter. The load detector further includes a contactless sensor in the form of an inductive pickup device  24  which detects length changes of a plunger  25  in relation to a measuring coil  26  and converts the measured quantity into corresponding measuring signals for input in the evaluation circuit  22 . The plunger  25  is hereby secured in the deformation zone  15  to an inside wall of the cylindrical portion of the threaded shaft  9  at a distance to the measuring coil  26  which is secured to an opposite inside wall of the cylindrical portion. 
   While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. The embodiments were chosen and described in order to best explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. 
   What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims and includes equivalents of the elements recited therein.