Abstract:
Preheated plastic sheets are squared and flattened to be engageable by clamping fingers of clamps on a clamping frame by an array of paddles having vertical and horizontal portions moved to engage any upturned or skewed sheet edges to flatten and square the same as said paddles are simultaneously moved longitudinally and vertically by respective actuators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional Ser. No. 60/407,323, filed Aug. 30, 2002. 

   BACKGROUND OF THE INVENTION 
   This invention concerns the handling of plastic sheets in preparation for thermoforming of the sheets into molded articles. In the well known process of thermoforming, plastic sheets are preheated in an oven prior to being shaped by the use of forming molds and fluid pressure. 
   In one known form of the process, individual rectangular sheets are each clamped in a rectangular clamping frame to be transferred into an oven and thence into a forming station. 
   In published U.S. Application US 2002/0017745 A1, a slow preheating of the sheets is described with a second rapid heating in a radiant heating station just prior to being formed in a forming station. 
   Such preheating has been done in a convection oven with pairs of sheets slowly moved through the oven on an indexing vertical conveyor including paddles on which the sheets rest. The pairs of preheated sheets are conveyed out of the convection oven to be clamped into a clamping frame for further processing in a radiant heating station where final heating is done, and thereafter to the forming station. 
   The preheated sheets are moved into respective clamping frames where clamps grip the edges of the sheets to secure the same in their respective frame. 
   Problems have been encountered in achieving proper clamping due to bowing and curling of the sheets due to the prior preheating step. 
   In addition, extensive handling of the sheets are involved in feeding the sheets into the convection oven, conveying the same through the oven, and transferring the preheated sheets out of the oven. Such extensive handling sometimes results in the sheets being skewed when moved into the clamping frame to be slightly misaligned with the frame. 
   This misalignment can interfere with proper clamping of the sheets in their respective clamping frames. 
   While sheet squaring devices have heretofore been developed to correct this problem, a sheet flattening capability has not been provided in such devices. 
   It is the object of the present invention to provide a method and apparatus for providing sheet squaring and flattening insuring proper clamping of preheated sheets within the clamping frames of a thermoformer machine. 
   SUMMARY OF THE INVENTION 
   The above object and other objects which will become apparent upon a reading of the following specification and claims, are achieved by a mechanism for flattening and squaring the rectangular sheets prior to clamping the sheets in a rectangular clamping frame. This is accomplished by an array of paddles positioned to engage the perimeter of a sheet supported on a lift table so as to have its edges overhanging the table perimeter. Each paddle has a vertical lower portion and a connected horizontal upper portion. 
   Each set of one or more paddles are arranged along one side of the sheet, mounted on a bar, opposite pairs thereof driven by an associated actuator for in and out movement towards and away from each other. The paddles are initially positioned in a retracted away position in which the vertical portions of the paddles all clear the sheet edges even when a sheet is presented skewed on the table. 
   The sets of paddles are all supported on a framework driven vertically by a set of vertically driven shafts so as to be movable down to bring the horizontal paddle portions into contact with any upturned sheet edges and to move the same down to flatten the edges, the horizontal paddles not lowered so far as to restrain the sheet on the table. 
   The opposite paddle sets are then simultaneously moved towards each other, contacting any skew sheet edges to square the same. 
   The sheets are positioned within a clamping frame during the flattening and squaring process so that the sheet may be clamped with clamps located between the paddles while being held in a flattened and squared condition. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of a part of a thermoforming sheet handling apparatus and a sheet flattening and squaring apparatus according to the present invention. 
       FIG. 2  is a perspective view of a flattening and squaring apparatus according to the present invention designed to act on two sheets at once. 
       FIG. 3  is a plan view of the apparatus shown in  FIG. 2 . 
       FIG. 4  is an end view of the apparatus shown in  FIGS. 2 and 3 . 
       FIG. 5  is an enlarged perspective view of a sheet engaging portion of the apparatus shown in  FIGS. 2-4 . 
       FIG. 6  is a plan view of the portion of the apparatus shown in  FIG. 5 . 
       FIG. 7  is a reversed side elevational view of the portion of the apparatus shown in  FIGS. 5 and 6 . 
       FIG. 8  is an end view of the portion of the apparatus shown in  FIGS. 5-7 . 
   

   DETAILED DESCRIPTION 
   In the following detailed description, certain specific terminology will be employed for the sake of clarity and a particular embodiment described in accordance with the requirements of 35 USC 112, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
   Referring to the drawings, and particularly  FIG. 1 , the present invention may be advantageously used with a thermoformer apparatus as described in copending application Ser. No. 10/218,982 filed on Aug. 14, 2002, and published application U.S. 2002/0017745 A1 in which two rectangular sheets are simultaneously processed to form a single part, but can also be used with single sheet thermoforming. 
   A preheating of pairs of the sheets S is slowly carried out prior to clamping in clamping frame  18 , in a convection oven  10 . 
   A conveyor  12  advances each sheet of each pair of sheets S onto a respective lift table  14  which is able to be raised and lowered by an actuator  16 . Each sheet S is centered on its lift table so that its edges all overhang the respective sides of the lift table  14 . 
   A rectangular clamping frame  18  can be positioned over each of two sheets S on a respective side by side table  14 , so that when the table  14  located therebelow is elevated, the sheet S is positioned within the rectangular opening within the clamping frame  18 . The clamping frames  18  are of a well known construction in which a series of clamping cylinders  20  are mounted along inside of the rails defining the frame, having gripping jaws or fingers  22  which are held open when a sheet S is being loaded therein. The sheet S thus clears the open gripping fingers  22  when moved up into the opening within the frame  18  by elevation of the lift table  14 . 
   The clamping frame  18  and clamping cylinders  20  are well known in the art and hence will not be described in further detail. The clamping cylinders  20  may be of a commercially available type in which the clamping fingers  82  are held open by application of fluid pressure to the cylinders  20  and are closed to be applied by a spring force when the cylinder fluid pressure is relieved. This allows transport of the clamping frames  18  through a final heating station and forming station with the sheet clamped therein without the need for applying fluid pressure to the clamping cylinders  20 . 
   The clamping frames  18  may be held on a support structure  24  for upward vertical movement to a transfer system (not shown) which system would move a series of frames  18  horizontally along support rails (not shown) to the thermoforming final heat and forming machine station respectively. 
   The sheet squaring and flattening apparatus  26  according to the present invention is supported above the clamping frames  18  and may be mounted on the same support structure  24  as indicated diagrammatically in  FIG. 1 , so as to be movable therewith. 
     FIGS. 2-4  show an apparatus  26  designed to square and flatten two side by side sheets S simultaneously. 
   An upper framework  28  includes a pair of end structures  30  (which could be attached to the support structure  24 ) having longitudinal steel beams  32  attached thereto as shown. A drive motor  34  and gear unit  36  on the beams  32  drive two right angle drives  38  which rotate respective drive shafts  40 . Each drive shaft  40  drives respective pairs of drive pinion gears  42 , each engaged with respective vertical gear racks  44  attached at their lower ends to plates  46  attached to the ends of longitudinal rectangular steel tube members  48 . The tube members  48  in turn support two squaring and flattening mechanisms  50 . Activation of the motor  34  causes each gear rack  44  to be moved up or down while remaining level and in perfect synchronism with each other in order to position each mechanism for a purpose as will be described below in reference to  FIGS. 5-8 . 
     FIGS. 5-8  show details of each squaring and flattening mechanism  50 , which includes a rectangular arrangement of pairs of elongated U channel end bars  52  and longitudinal bars  54 . 
   A rectangular array of paddles  56  are attached to be suspended below the bars  52 ,  54 . The paddles  56  are located to be disposed in spaces between the clamping cylinder  20  so as to be able to engage the overhanging edges of a sheet S disposed on a lift table  14 . 
   The paddles  56  each comprise a right angle plate having a lower depending vertical portion  58  integral with an upper horizontal portion  60 . A support bracket  62  is attached to the upper portion  60  of each paddle  56  and to one of the bars  52  or  54 . 
   The two end bars  52  are attached to one end of one of two gear rack shafts  64 ,  66  by means of bracket plates  68 ,  70  affixed to the top of a respective bar  52 . 
   A main mounting bracket  72  mounts a power cylinder  74  which can drive the gear rack shaft  64  in a lengthwise direction via a connector plate  76 . 
   The gear rack shaft  64  has bottom gear teeth drivingly engage a first gear  78  which rotates a second gear  80  drivingly engaging gear teeth on the top of the gear rack shaft  66  so cause lengthwise motion of the shaft  66  in the same direction, moving bars  52  towards or away from each other to bring the attached oppositely located vertical paddle portions  58  closer together or further apart in synchronism with each other. 
   The gear shafts  64 ,  66  are each supported in linear bearings  80  on the main bracket  72 . 
   An auxiliary support plate  84  is attached to one of the rails  48  and guides the gear shaft  66  with guide shaft  86  and plate  88 . This provides intermediate support to reduce vibration of the gear shaft  66 . 
   Similarly, the lengthwise bars  54  have a series of paddles  56  attached along their length. 
   A second pair of gear rack shafts  90 ,  92  each have one end attached to one of the bars  54  by means of a bracket plate  94 ,  96 . The opposite end of each shaft  90 ,  92  is guided in a linear bearing  98 ,  100  mounted to a respective bracket  94 ,  96 . 
   A power cylinder  106  drives gear rack shaft  90  by means of a plate connection  100 . A gear  102  is driven by engagement with the gear rack teeth on top of shaft  90  to rotate a connected gear  104  engaged with gear rack teeth on the bottom of shaft  92 . Thus, both shafts  90 ,  92  are driven longitudinally in synchronism by operation of the power cylinder  106  to move bars  54  and oppositely located sets of paddles  56  mounted simultaneously thereon towards or away from each other. 
   Each main bracket  72  has a top plate  108  is attached to a cross plate  46  so that motion of the vertical gear posts  44  raise and lower both mechanisms  50 . 
   The power cylinders  74 ,  106  and motor  34  are controlled by a suitable programmed industrial controller as is typically used to control other components of a thermoformer with which the apparatus of the present invention is used so that the bars  52 ,  54  are initially retracted apart. Thus, even if the sheets S are skewed somewhat, the depending portions  60  of the paddles  56  will clear the edges of the sheet S when the mechanisms  50  are lowered by operation of the motor  34  after the sheets S are elevated to be positioned within the clamping frames  18 . 
   The mechanisms  50  are lowered to bring the horizontal portion  58  of the horizontal paddles  56  into engagement with an upwardly curled or bowed overhanging edges of the sheets S and force the same down into the main plane of the sheet S. 
   Subsequently, the power cylinders  74  and  106  are operated to bring the vertical paddle portions  60  towards each other, engaging any sheet skewed side edges to square up the sheet S. 
   The clamps  20  are then operated to reliably clamp the sheet edges with clamping fingers  22 . 
   The movement of the paddles  56  is controlled to flatten any curled edges of the sheets S without undue pressure. 
   Similarly, the inward motion of the vertical paddle portions  60  is controlled to just square the sheets S.