Abstract:
A film stretcher utilizes a spring bias to clamp the film. The film stretcher may include a stretch head having one or more clamps positioned about the film in locations necessary to clamp the film for stretching in one or more axes. The clamp may utilize a spring to provide the clamping force. The stretch head may include structures that operate to counter the spring bias to move the clamp to one state and then to allow the spring bias to move the clamp to another state. Thus, the stretch head may effectively clamp and unclamp the film.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Application 60/974,835 filed on Sep. 24, 2007, which is incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    Embodiments provide for a device that stretches films such as those constructed of polymers or similar materials. 
       BACKGROUND 
       [0003]    Biaxial orientation resulting from biaxial stretching is a common way to achieve mechanical, optical, or transmissive properties in extruded or cast films ranging from potato chip bags to water bottles to nicotine patches. A laboratory grade instrument is used in the development of the production processes for a material, ongoing verification during production, and/or support of material scientists who are developing new materials and products. There are many instrumentation options for studying films; however, the primary data generated directly by these instruments are stress-strain curves. 
         [0004]    With laboratory biaxial film stretchers, the film sample is clamped on all four sides with the clamps forming a pantograph mechanism. Thus a uniform strain may be applied on opposite sides as the film is stretched in the pantograph mechanism. The pantograph mechanism may often hold the film sample within an oven where temperature can be increased to a desired point since temperature may be a factor whose impact is being determined by the laboratory experimentation. 
         [0005]    Conventional laboratory film stretchers utilize pneumatics to operate clamps of the pantograph mechanism. These pneumatics utilize small tubes that stretch from one clamp to the next so that the pneumatic pressure is communicated across all of the clamps for all four sides of the pantograph mechanism. The use of pneumatics for the clamps presents several issues. The tubes exert measurable forces on the clamps during movement of the stretching process, and this force introduces noise into the stress-strain curves being produced. Furthermore, the pneumatics include O-rings and other items that are made of materials that are affected by environmental factors. For example, the pneumatics employ a very dry nitrogen that degrades the O-rings. Furthermore, high temperatures that may be present within the oven during testing and such materials may break down over relatively short intervals. Failures of parts such as O-rings in the clamping system may result in poor clamping of the film sample and ultimately unreliable test results. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a perspective view of an embodiment of a film stretcher for a given axis. 
           [0007]      FIG. 2  shows a right side view of the embodiment of a film stretcher for a given axis. 
           [0008]      FIG. 3  shows a front view of the embodiment of a film stretcher for a given axis. 
           [0009]      FIG. 4  shows a left side view of the embodiment of a film stretcher for a given axis. 
           [0010]      FIG. 5  shows a top view of the embodiment of a film stretcher for a given axis. 
           [0011]      FIG. 6  shows a rear view of the embodiment of a film stretcher for a given axis. 
           [0012]      FIG. 7  shows a bottom view of the embodiment of a film stretcher for a given axis. 
           [0013]      FIG. 8  shows a perspective view of a clamp area of the embodiment of a film stretcher for a given axis. 
           [0014]      FIG. 9  shows a side view of an embodiment of a clamping finger for embodiments of a film stretcher. 
           [0015]      FIG. 10  shows a top view of the embodiment of a clamping finger for embodiments of a film stretcher. 
           [0016]      FIG. 11  shows a front view of the embodiment of a clamping finger for embodiments of a film stretcher. 
           [0017]      FIG. 12  shows a perspective view of the embodiment of a clamping finger for embodiments of a film stretcher. 
           [0018]      FIG. 13  shows a top view of the embodiment of the clamping finger when in a first position. 
           [0019]      FIG. 14  shows a side cross-sectional view of the embodiment of the clamping finger when in the first position. 
           [0020]      FIG. 15  shows a top view of the embodiment of the clamping finger when in a second position. 
           [0021]      FIG. 16  shows a side cross-sectional view of the embodiment of the clamping finger when in the second position. 
           [0022]      FIG. 17  shows a top view of the embodiment of the clamping finger when in a third position. 
           [0023]      FIG. 18  shows a side cross-sectional view of the embodiment of the clamping finger when in the third position. 
           [0024]      FIG. 19  shows a perspective view of an embodiment of a stretch head. 
           [0025]      FIG. 20  shows a perspective view of an alternative embodiment of a clamping finger for embodiments of a film stretcher. 
           [0026]      FIG. 21  shows a cross-sectional side view of the alternative embodiment of the clamping finger for embodiments of a film stretcher. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments provide for clamping fingers that use springs to bias clamps of the clamping fingers so that stretch head may employ such clamping fingers in place of pneumatic clamping fingers. Embodiments further provide mechanisms for controlling a state of spring bias of the clamping fingers where the state of spring bias is related to the clamps being in an open or clamped position. 
         [0028]      FIGS. 1-8  show an example of a film stretcher for one axis. This film stretcher  100  may be used in conjunction with a clamping mechanism on the opposite side that is either fixed, yet uses a same or similar clamping finger configuration, or one that is also mobile and uses the same or similar clamping finger configuration. As is also discussed below, this film stretcher  100  may additionally be used in conjunction with a clamping mechanism on each of the perpendicular sides where one side is mobile and the other is fixed or where both are mobile. The discussion below in relation to  FIG. 19  presents an example of a stretch head that may utilize the film stretcher  100 . 
         [0029]    This illustrative embodiment of a film stretcher  100  includes an electric motor  102  such as the SGMAH08AAF41-1 servomotor from Yaskawa America Electric, Inc. of Waukegan, Ill. This motor  102  provides rotary movement that is used to slide the clamping assembly forward to attach to the film sample and then to pull the clamping assembly backward to begin stretching the film sample after the film sample has been clamped. It will be appreciated that the forward and backward motion may be provided in other manners for other embodiments of the film stretcher. For example, pneumatics may be utilized to provide this forward and backward movement. 
         [0030]    The motor  102  of this embodiment is mounted to a plate  104  and has an output shaft coupling  106  to a drive screw  128 . Thus, as the motor  102  turns, the drive screw  128  also turns. The drive screw  128  extends to a guide block  134  where the drive screw  128  is supported but freely turns, such as via a bearing within the guide block  134 . The plate  104  is attached to a torque plate  108 . A support shaft  124  is affixed to and extends from the torque plate  108  to the guide block  134 . Although not shown, the plate  104 , torque plate  108 , and guide block  134  may be mounted to a beam that holds these parts in a fixed relationship relative to one another. The guide block  134  maintains a fixed position, abutting the oven within which the film sample is being stretched. 
         [0031]    In this embodiment a sub-assembly moves forward and backward as a unit as a result of activation of the motor  102  in order to provide the stretching of the film sample. This sub-assembly includes a draw plate  114 , pneumatic cylinders  110 ,  112 , a threaded coupling  118 , an actuator shaft  126 , and a primary D-ring (PDR)  136 . The actuator shaft  126  extends into the oven where the PDR  136  is located. 
         [0032]    As the drive screw  128  turns, the threaded coupling  118  is moved along the drive screw  128 , either toward the guide block  134  or away from it depending upon the direction of the rotation of the drive screw  128  as provided by the motor  102 . The threaded coupling  118  is rigidly attached to the draw plate  114  such that the draw plate  114  moves forward and backwards as well. The pneumatic cylinders  110 ,  112  and the actuator shaft  126  are also rigidly attached to the draw plate  114  and move accordingly. During this forward or backward movement, a supporting coupling  123  that is rigidly attached to the draw plate  114  slides along the support shaft  124 . 
         [0033]    Attached to the PDR  136  includes the clamping assembly that includes several individual clamping fingers  144  bounded by a fixed support  140  and a sliding support  142 . Thus, as the motor  102  is activated, the actuator shaft  126  moves the PDR  136  and the attached clamping fingers  144 . When the clamping fingers  144  are clamped down onto the film sample, then as the PDR  136  is pulled by the motor  102  toward the guide block  134 , the film sample is stretched so long as the opposite side of the film sample is restrained. 
         [0034]    In this embodiment another sub-assembly moves forward and backward as a unit together with the aforementioned sub-assembly in response to activation of the motor  102 . This sub-assembly includes a secondary D-ring (SDR) plate  116 , a floating guide block  148 , pneumatic cylinders  120 ,  122 , SDR shafts  130 ,  132  and an SDR  18 . While this sub-assembly may move forward and backward as a unit with the aforementioned sub-assembly during activation of the motor  102 , this sub-assembly may also move independently of the aforementioned sub-assembly in order to control whether the clamping fingers  144  are in the clamped or unclamped position. 
         [0035]    Thus, upon the motor  102  moving the sub-assemblies forward to reach the starting point where the film sample is inserted, this sub-assembly activates independently of the aforementioned sub-assembly to move to a first position to open the clamps if necessary and to move to a second position to close the clamps upon introduction of the film sample. As discussed below, this sub-assembly may further move to a third position where the clamps remain closed but the mechanism for opening and closing the clamps is isolated from these sub-assemblies. This isolation may be provided to avoid a sudden acceleration during stretching from causing the clamps to open. 
         [0036]    This sub-assembly maintains its movement as a unit with the previous sub-assembly by the pneumatic cylinders  110 ,  112  maintaining a fixed shaft position during the forward and backward movement of the draw plate  114 . These pneumatic cylinders  110 , 112  have shafts that are fixed to the SDR plate  116  such that movement of the draw plate  114  results in corresponding movement of the SDR plate  116  so long as the pneumatic cylinders  110 ,  112  maintain the fixed shaft position. 
         [0037]    One manner of maintaining the fixed shaft position of the pneumatic cylinders  110 ,  112  is by having the pneumatic cylinders  110 ,  112  bias into the most extended or most retracted shaft position (i.e., the internal piston is held by pneumatic pressure against its internal stop or against an external stop). As shown for this embodiment, the piston may reach its internal stop when the shaft position is fully extended. As is further shown in these figures, external stops  146  are provided to limit the retracted shaft position. As is discussed below, a state of spring bias of the clamping fingers  144  is controlled by the shaft position movement of the pneumatic cylinders  110 ,  112  to either open or close the clamps, and the external stops  146  may be used to limit the movement to match a range of movement that is accepted by the clamping fingers. 
         [0038]    The movement of the shaft position of the pneumatic cylinders  110 ,  112  serves to move the SDR plate  116  relative to the draw plate  114 . This also has the effect of moving the SDR shafts  130 ,  132  relative to the actuator shaft  126 , and hence moves the SDR  18  relative to the PDR  136 . The SDR shafts  130 ,  132  also extend into the oven where the SDR  18  is located. In this embodiment, the movement of the SDR  18  relative to the PDR  136  ultimately controls the state of spring bias of the clamping fingers  144  to either open or close the clamps. 
         [0039]    The clamping fingers  144  are each interconnected in this embodiment by a scissor-like mechanism  150 . The scissor-like mechanism  150  is anchored to the PDR  136  on one side by the fixed support  140  and is guided along the PDR  136  by the sliding support  142 . The sliding support  142  glides along an outer leading edge of the PDR  136  and also glides along a groove or slot  139  formed in the SDR  18 . As discussed below, the slot  139  is also pertinent to controlling the state of spring bias of the clamping fingers  144 . Each of the clamping fingers  144  is able to move laterally along the PDR  136  in a manner controlled by the scissor-like mechanism  150 . This movement is necessary when stretching is occurring in the axis that is perpendicular to the axis of stretch being provided by the film stretcher  100 . This movement occurs because the distance between each clamping finger  144  increases as the film sample is stretched in that perpendicular axis. 
         [0040]    Additional details of the clamping fingers of one illustrative embodiment are shown in  FIGS. 9-18 . In this particular embodiment, the clamp of the clamping finger  144  is provided by the force exerted by a clamp piston  5  onto a lower clamp finger  3  where an edge of a film sample is trapped between these two items. In this particular embodiment, the clamp piston  5 , and hence the clamp, is biased to a closed position by a collection of leaf springs  1  upon a clamp rocker  9  being released from a detained state referred to as position  1 . In this position  1  where a first spring bias state occurs, the SDR  18  has been pushed further from the guide block  134  by the pneumatic cylinders  110 ,  112 , where the SDR  18  pushes against a slider  6 . The slider  6  reaches a point where a precision washer  11  which acts as a roller rides along the slider  6  until entering a detent which coincides with the pneumatic cylinders  110 ,  112  reaching their internal piston stops. The slider  6  may glide along a track in the adjacent surface of the lower finger  3 . 
         [0041]    As shown, the rocker  9  serves as a lever about dowel pin  10  to push a dowel pin  7  via another precision washer  12  which in turn pushes the clamp piston  5 . This push forces clamp piston  5  against the bias of the leaf springs  1  to separate the clamp piston  5  from the lower clamp finger  3 . The film sample may be inserted to the opening between the clamp piston  5  and the lower finger  3 . Then, the pneumatic cylinders  110 ,  112  draw the SDR  18  backwards until the pneumatic cylinders  110 ,  112  reach the stop, such as the external stop  146 , which places the slider  6  in position  2  where a second spring bias state occurs. At this position, the slider  6  is out of contact with the rocker  9  which has been forced back into position by the bias of the leaf springs  1  against the clamp piston  5  and dowel pin  7 . The film sample is now firmly clamped between the clamp piston  5  and the lower finger  3 . 
         [0042]    At this point, this illustrative embodiment takes further action to isolate the clamping from the acceleration of the PDR  136  and hence the SDR  18  during the stretching movements. The pneumatic cylinders  120 ,  122  are activated until reaching their stops in order to push the SDR  18  forward to a point where it is no longer in contact with the slider  6 . Thus, violent acceleration of the SDR  18  during stretching movement avoids contact with the slider  6 , and thus prevents any movement of rocker  9 . Therefore, movement of clamp piston  5  is avoided so that the clamp does not introduce noise to the measurements and does not release the film sample. 
         [0043]    The clamping finger  144  is constructed by mounting the leaf springs  1  atop an upper clamp finger  2 . A retainer  4  may be placed above the leaf springs with screws  15  and dowel pins  16  holding the retainer  4  in place and with proper alignment. The preload of the leaf springs  1  may be set as desired by shimming the leaf springs  1  since the force applied by the leaf springs  1  is dependent upon displacement. The clamping finger  144  slides along the PDR  136  when stretching is occurring in the perpendicular axis so to facilitate that sliding, a D-ring roller  13  is included to roll along the adjacent surface of the PDR  136 . 
         [0044]    Furthermore, to mount to the scissor-like mechanism  150 , the clamping finger  144  includes a dowel pin  8  that serves as a fixed attached point for one end of the scissor while the groove  160  in the upper finger  2  and lower finger  3  serves as a moving attachment point for the other end of the scissor. As the clamping fingers  144  are stretched apart, the attachment points of the scissor to the clamping finger  144  moves closer together, so the groove  160  allows for that movement of the attachment point relative to the dowel pin  8 . Notches  162  in the upper finger  2  and lower finger  3  allow for clearance of scissor pins when the clamping fingers  144  are closest together. 
         [0045]    Various materials may be used in constructing the film stretcher  100  and the clamping fingers  144 . The components of the film stretcher  100  may include materials such as stainless steel, carbon steel, or aluminum. The components of the clamping fingers  144  may include materials capable of withstanding elevated temperatures such as stainless steel or carbon steel. In particular, the leaf springs  1  may be constructed of stainless steel (for example, grade 17-4) or Iconel (for example, grade 718). 
         [0046]      FIG. 19  shows a stretch head  200  for providing biaxial stretching using at least two film stretchers with at least one for each axis. In this example, it can be seen that the PDR  136  is on one side of the film sample while another PDR  136 ′ is on the opposite side. The PDR  136 ′ may maintain a fixed position so that the amount of stretching in this particular axis is provided entirely by the movement of the PDR  136 . However, even if fixed, this PDR  136 ′ still provides a set of clamping fingers  144 ′ that may operate to clamp and un-clamp in the same manner as the clamping fingers  144  and that may slide along the PDR  136 ′ in response to stretching in the perpendicular axis. A force transducer  202 , such as a piezoelectric force sensor, may also be present to make the force measurements during the stretching. 
         [0047]    As is also shown in  FIG. 19 , the perpendicular axis may mirror the present axis by providing a PDR  137  and another PDR  137 ′ on the opposite side of the film sample. The PDR  136  may be driven by a film stretcher like the film stretcher  100  discussed above. The PDR  137 ′ may be driven, or as shown may be fixed like PDR  136 ′. In either case, both the PDR  137  and PDR  137 ′ may utilize the same mechanism for clamping as discussed above. A force transducer  204  may be present to measure the force in this perpendicular axis. 
         [0048]    As can be seen, the PDRs  136 ,  136 ′ overlap with the PDRs  137 ,  137 ′ of the perpendicular axis. Thus, this overlap is accounted for by elevating the PDRs  137 ,  137 ′ relative to the PDRs  136 ,  136 ′. The geometry of the clamping fingers  145 ,  145 ′ differ from that of the clamping fingers  144 ,  144 ′ so as to account for the elevation difference of the PDRs while maintaining the clamp position at the same elevation for all clamping fingers  144 ,  144 ′,  145 , and  145 ′. Furthermore, the SDR shafts  130 ,  132  and SDR  18  are at different elevations for the PDR  137  to avoid collisions with the PDRs  136 ,  136 ′. As can also be seen, the mounting tab  206 ,  208  for the actuator shaft  126  is off-center of the PDR  136 ,  137  in this example since the opposite sides are fixed and the film sample grows outward from one of its corners rather than expanding from its center. 
         [0049]      FIG. 20  shows a portion of a clamping finger  300  with an alternative spring bias mechanism. In this example, either disk springs or coil springs  314  are being used to bias the clamp rather than leaf springs. In this example a clamp piston  308  is being biased toward a lower finger  306  by a collection of disk or coil springs  314  located within an upper finger  304  while the clamp piston  308  biases a dowel pin  316  toward a rocker (not shown). The disk or coil springs  314  may be pre-loaded with an adjustment screw  302  that threads down into the upper finger  304 . 
         [0050]    In this embodiment, other details may be the same as or similar to the previous embodiment  144 , such as including a D-ring roller  310  to roll along an adjacent surface of the PDR  136  and groove  312  to allow for movement of the adjoining end of the scissor. Furthermore, the dowel pin  316  and clap piston  308  may be moved by a rocker being positioned by movement of a slider as discussed above for the previous embodiment of the clamping finger  144 . 
         [0051]    While various embodiments of film stretchers and particularly clamping fingers have been disclosed, it will be appreciated that various changes in the details may be made while still falling within the scope of the invention.