Abstract:
A thermoforming apparatus including a pallet for use with a thermoforming machine that thermoforms a plastic billet into a usable form, the pallet including a billet support surface for supporting thereon a billet, and a gripper mounted on the pallet and arranged for movement between a first position, wherein the gripper does not grip a billet placed on the billet support surface, and a second position, wherein the gripper grips the billet placed on the billet support surface.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to scrapless thermoforming of thermoplastic materials into containers and the like, and particularly to a platter for holding and transporting a thermoplastic billet in a thermoforming machine. 
   BACKGROUND OF THE INVENTION 
   A typical thermoforming machine includes a series of stages that transform thermoplastic material (e.g., polypropylene, polyvinyl chloride, polystyrene and others) into finished containers by means of heat and pressure. The finished containers are typically used to hold food, and may be formed into a variety of sizes and shapes depending on the thermoforming process. 
   Thermoplastic materials may be fed into a thermoforming machine in the form of a continuous sheet or as individually cut blanks or billets. When the input materials are individual thermoplastic billets, the process is called scrapless thermoforming because the finished containers need not be cut from the sheet after forming, reducing the amount of scrap material. In scrapless thermoforming, a billet feeding unit is typically used to load individual billets onto a conveying device in the machine. 
   Prior to being formed into containers, the billets must be heated to the desired temperature. The desired temperature depends on the structure of the machine being used as well as the desired properties of the end product. For example, containers may be formed while the thermoplastic material is below the crystalline melt point of the material, this being known as solid-phase pressure forming. Other methods involve heating the material to its melting point prior to forming, this being known as melt-phase thermoforming. 
   A conventional scrapless thermoforming machine has several stages used to create formed containers. First, the billets are loaded into the machine. Second, an oven is used to heat the billets to the desired temperature. Third, a form station or form press utilizes air pressure and vacuum to form the individual containers. After exiting the form press, the formed containers are removed from the machine at an unloading station. Other stations may be added to the thermoforming machine as desired, such as a pre-heating oven prior to entry into the main oven. A conveyor is typically used to transport the billets through the various stations. 
   Whether the containers are formed utilizing solid-phase forming or melt-phase forming, a problem that must be addressed involves the deformation of the thermoplastic material as it is heated. Because the billets are typically supported in a horizontal fashion with only peripheral supports beneath each billet due to the configuration of the form press, the billets tend to sag or droop between the supports when heated. In some cases, the billets may entirely fall out of their supports when heated due to the material deformation. 
   The thermoforming art has tried many methods of preventing material deformation from adversely impacting the thermoforming process. One method involves minimizing the size of the billets. This method prevents the formation of larger containers. Another method is to utilize round billets such that the resultant uniform deformation allows the support to retain the billet during heating. This method precludes the forming of containers having a non-uniform cross-section, such as rectangular or oval containers. Yet another method of addressing the deformation issue is to utilize a ring placed on top of the billet to strengthen the frictional grip on the billet during heating. This method presents additional manufacturing challenges with respect to how to efficiently place individual rings on top of each billet during the loading process. 
   An example of a prior art frame for holding a billet is U.S. Pat. No. 6,896,506 to Jordan, which describes a thermoforming machine that includes a loading station configured to load a thermoplastic billet onto a frame. The frame has an outer ring and a billet support blade defining a channel therebetween. The billet support blade engages the billet to maintain the billet on the frame during heating of the billet. A heating apparatus is configured to heat the billet and a forming station is configured to form the billet into a container. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to provide a novel platter (or frame, the terms being used interchangeably throughout) for holding and transporting a thermoplastic billet in a thermoforming machine, as is described more in detail hereinbelow. A novel thermoforming machine that uses the platters is also described. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
       FIG. 1A  is a simplified pictorial illustration of a thermoforming machine, constructed and operative in accordance with an embodiment of the present invention; 
       FIG. 1B  is a simplified pictorial illustration of a billet that may be formed with the thermoforming machine of  FIG. 1A , in accordance with an embodiment of the present invention; 
       FIGS. 2A-2E  are simplified illustrations of a pallet, which may be used in the thermoforming machine of  FIG. 1A , constructed and operative in accordance with an embodiment of the present invention, wherein  FIGS. 2A and 2B  are respective pictorial and top-view illustrations of the pallet,  FIG. 2C  is a sectional view of a gripper of the pallet that is used to grip a billet and  FIGS. 2D and 2E  are details of parts of the gripper; 
       FIGS. 3A-3D  are simplified illustrations of a pallet, which may be used in the thermoforming machine of  FIG. 1A , constructed and operative in accordance with another embodiment of the present invention, wherein  FIGS. 3A and 3B  are respective pictorial and top-view illustrations of the pallet,  FIG. 3C  is a detailed pictorial illustration of a gripper of the pallet that is used to grip a billet, and  FIG. 3D  is an exploded illustration of the pallet; 
       FIG. 4  is a simplified exploded illustration of a pallet, constructed and operative in accordance with yet another embodiment of the present invention; 
       FIGS. 5A-5C  are simplified illustrations of the pallet of  FIG. 4 , with a billet placed thereupon and not yet gripped by the grippers, wherein  FIGS. 5A and 5B  are respective pictorial and top-view illustrations of the pallet ( FIG. 5B  showing a cam frame in a first position where the grippers do not grip the billet), and  FIG. 5C  is a sectional illustration of the gripper and actuator system for moving the gripper; 
       FIGS. 6A-6C  are simplified illustrations of the pallet of  FIG. 4 , with the billet gripped by the grippers, wherein  FIGS. 6A and 6B  are respective pictorial and top-view illustrations of the pallet ( FIG. 6B  showing a cam frame in a second position where the grippers grip the billet), and  FIG. 6C  is a sectional illustration of the gripper and actuator system showing the gripper gripping the billet (on the top side of the billet); 
       FIGS. 7A-7D  are simplified illustrations of the pallet of  FIG. 4 , with the billet gripped by the grippers and moved to one of the heating stations (pre-heating or main heating stations) of the thermoforming machine, wherein  FIGS. 7A and 7B  are respective pictorial and top-view illustrations of the pallet and heating element,  FIG. 7C  is a sectional illustration of the pallet showing the billet softened by the heating with a middle section of the billet sagging down, and  FIG. 7D  is a sectional illustration of the gripper and an undersheet gripper being moved to grip the billet from its underside; 
       FIGS. 8A-8D  are simplified illustrations of the pallet of  FIG. 4 , with the billet gripped by the grippers and moved to the pressure vacuum station of the thermoforming machine, wherein  FIGS. 8A and 8B  are respective pictorial and top-view illustrations of the pallet and upper pressure unit,  FIG. 8C  is a sectional illustration of the pallet showing the upper pressure unit and a lower vacuum molding unit prior to forming the billet into a container, and  FIG. 8D  is a sectional illustration of the gripper and the undersheet gripper gripping the billet during forming thereof; 
       FIGS. 9A-9D  are simplified illustrations of the pallet of  FIG. 4 , wherein  FIGS. 9A and 9B  are respective pictorial and top-view illustrations of the pallet and upper pressure unit (the latter omitted in  FIG. 9B ),  FIG. 9C  is a sectional illustration of the pallet showing the billet being formed into a container by the upper pressure unit and lower vacuum molding unit, and  FIG. 9D  is a sectional illustration of the gripper and the undersheet gripper, which do not grip the billet but rather are moved outwards by the upper pressure unit, and wherein the billet is held by the upper pressure unit; 
       FIGS. 10A-10D  are simplified illustrations of the pallet of  FIG. 4 , wherein  FIGS. 10A and 10B  are respective pictorial and top-view illustrations of the pallet and upper pressure unit (the latter omitted in  FIG. 10B ),  FIG. 10C  is a sectional illustration of the pallet showing the lower vacuum molding unit removed downwards from the billet (now container), the container being by the upper pressure unit, and  FIG. 10D  is a sectional illustration of the upper pressure unit holding the edge of the container; and 
       FIGS. 11A-11C  are simplified illustrations of a pallet, constructed and operative in accordance with still another embodiment of the present invention, wherein  FIG. 11A  is a pictorial illustration of the pallet with shape-changing grippers, such as piezoelectric or bimetallic grippers,  FIG. 11B  is a pictorial illustration of the pallet with gripper covers omitted to expose the shape-changing devices, and  FIG. 11C  is a sectional illustration of the shape-changing gripper. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Reference is made to  FIG. 1A , which illustrates a thermoforming machine  10 , constructed and operative in accordance with a non-limiting embodiment of the present invention. 
   Thermoforming machine  10  may include a plurality of stations for storing, transporting and scraplessly transforming billets (thermoplastic blanks) into containers or other items. The direction of the work flow is indicated by arrows  12 . The billets are held and transported in pallets  8 , embodiments of which are described in detail further below. 
   At one end of thermoforming machine  10  may be a front platter lift station  14 . Pallets  8  may enter station  14  from a lower conveyor  16 . The pallets  8  may be lifted in station  14  to an upper level, where they exit to an upper conveyor  18 . The first station on the upper conveyor  18  is a loading station  20  that loads pre-cut foils  22 , herein referred to as billets  22 , onto pallets  8 . Billets  22  may be constructed of any thermoplastic material capable of being formed into containers by means of heat and pressure, such as but not limited to, polypropylene. 
   After one of the billets  22  is loaded into one of the pallets  8 , the pallet  8  may move to a preheating station  24 , which performs initial heating of the billet  22 . Afterwards, the billet  22  may pass to a main heating station  26  for additional heating. The billet  22  may be heated to a temperature permitting solid-phase thermoforming, for example, at the next station, which is a pressure vacuum station  28 . 
   The pressure vacuum station  28  may include a plug assist, which is well known in the art and available from many manufacturers, for forming thermoplastic containers from individual billets  22 . As is known in the art, the plug assist lowers a forming tool into the heated billet  22 , pushing billet  22  down into a form cavity that is of the final desired container shape. Air pressure and/or vacuum may be used to help pull or push billet  22  into the form cavity. The form cavity may then be removed and the finished container may be ejected to a tray stacking station  30 . The pallets  8  may be conveyed by a rear platter lift station  32  downwards to the lower conveyor  16 , which conveys the pallets  8  back to the front platter lift station  14  to start the next cycle. 
   Reference is now made to  FIG. 1B , which illustrates one kind of billet  22  that may be formed with the thermoforming machine of  FIG. 1A , in accordance with an embodiment of the present invention. The billet  22  shown in  FIG. 1B  is a generally rectangular sheet which may have rounded corners. The billet  22  includes a feeder element  19 . One non-limiting example of feeder element  19  is a depression  21  formed at the edge of one of the sides of the billet  22 . The feeder element  19  allows one individual billet to be picked from the stack of billets and fed into the pallet, as is described below with reference to  FIGS. 2A and 2B  (and may be included in the other embodiments of the invention). 
   Reference is now made to  FIGS. 2A-2E , which illustrate a pallet  108 , which may be used in the thermoforming machine  10 , constructed and operative in accordance with an embodiment of the present invention. 
   Pallet  108  may include a tray  34  with a peripheral billet support surface  36 . The billet support surface  36  may support a peripheral underside of billet  22  (and thus not touch the portions of the finished container that are readily visible to a consumer or end user of the container). A frame  38  may extend from tray  34 , which may be sized to the size of the billet  22 . At different points on the frame  38  (such as on each of its four sides, not all of which are shown in  FIG. 2A  for the sake of simplicity), may be positioned grippers  40 . In this embodiment, gripper  40  (seen best in  FIG. 2D ) may include a billet-gripping surface  42  (which may be roughened as shown in  FIG. 2D ), designed to pass over and press against the billet  22 . Gripper  40  may be formed with an oval aperture  44 , through which passes a pin  46  ( FIG. 2C ). Gripper  40  may have a curved abutment  48  that extends slightly into a channel  50 . A leaf spring  51  ( FIG. 2B ) or other suitable biasing device may be disposed in channel  50 . A tightening cam  52  ( FIG. 2E ) is rotatingly mounted in a hole  53  ( FIG. 2C ) of tray  34  and has a cam head  54  that is positioned to turn against curved abutment  48 . Cam head  54  may be fashioned with a screwdriver slot for facilitating turning thereof. Thermoforming machine  10  may include a screwdriver-blade-type actuator at loading station  20  for closing cam head  54  against curved abutment  48  and another actuator at station  28  for opening the cam head  54  to release the billet  22 . 
   Pallet  108  may include a billet-picking member  99 , which is arranged to pick one individual billet from the stack of billets and feed that billet into pallet  108 . Billet-picking member  99  may include a bar or other suitable protrusion on one of the long sides of frame  38 . At loading station  20 , each individual billet must be fed from the stack of billets to an individual pallet. In the present invention, unlike the prior art, there is no active device at loading station  20  that pushes a billet into the pallet. Instead, in the present invention, this may be accomplished by moving pallet  108  past the bottom billet of the stack of billets, whereby billet-picking member  99  moves into the depression  21  of the bottom billet and “sweeps” the bottom billet away from the rest of the stack into the pallet  108 . Thus, the pallet is self-loading—it itself loads the billet therein. 
   In the position shown in  FIG. 2C , tightening cam  52  has been turned against curved abutment  48  so that gripper  40  slides over billet  22  (to the right in the sense of  FIG. 2C ) and presses it against billet support surface  36  of tray  34 . In order to release the billet  22 , tightening cam  52  should be turned away from curved abutment  48  so that leaf spring  51  pushes gripper  40  off billet  22  (to the left in the sense of  FIG. 2C ). Generally, the grippers  40  hold the periphery of billet  22  during thermoforming at station  28  ( FIG. 1A ) and are released to allow removal of the finished container. 
   Reference is now made to  FIGS. 3A-3D , which illustrate a pallet  208 , which may be used in the thermoforming machine  10 , constructed and operative in accordance with another embodiment of the present invention. 
   Pallet  208  may include a tray  60  with a peripheral billet support surface  62 . Billet boundary members  64  may extend from an inner frame  61  (four are shown in  FIG. 3D ) affixed to tray  60 , members  64  being arranged and spaced to the size of the billet  22 . At different points on one side of the tray  60  (such as at each of the four corners), may be positioned grippers  66 . In this embodiment, gripper  66  (seen best in  FIG. 3C ) may include a curved cam head  68  mounted by means of a post  69  on an arm  70 . Gripper  66  may have a billet-gripping surface  72  (which may be spiral or otherwise roughened as shown in  FIG. 3C ), designed to pass over and press against the billet  22 . Gripper  66  is on the upper side of tray  60  (so that it can swing over and hold a billet), whereas post  69  may pivot by means of a pin (not shown) that goes through a hole  74  formed in frame  61  ( FIG. 3D ) and connects to arm  70  which is on the underside of tray  60 . 
   A cam frame  76  may be positioned on the lower side of tray  60 . Cam frame  76  may be formed with an aperture  78  at the middle of each of its four sides and an elongate cam aperture  80  at two opposing corners thereof. A guide pin  81  extending from tray  60  is received in cam aperture  80  to guide movement of cam frame  76 . A plurality of actuator arms  82  (four are used in the illustrated embodiment) may be in operative communication with cam frame  76  by means of pegs  84  that fit into apertures  78 . Each actuator arm  82  has a hole  86  formed at one end thereof to which arm  70  is pivotally attached. Each actuator arm  82  also has an elongate aperture  88  formed at another end thereof and a guide member  90  depending from tray  60  slides in aperture  88 . 
   The operation (closing and opening) of pallet  208  is now described with reference to  FIG. 3B . One of the actuator arms  82  may be moved in the direction of an arrow  92 . The actuator arm  82  may be moved by any suitable moving means (not shown), such as but not limited to, a linear actuator, piston, step motor, etc., positioned at stations  20  (for closing pallet  208 ) and  28  (for opening pallet  208 ). Only one of the actuator arms  82  need be linked to the moving means because the other actuator arms move together with that one arm; however, other arms may be connected to moving means as well. 
   Moving the actuator arms  82  in the direction of arrow  92  causes cam frame  76  to pivot about the center of tray  60 , as indicated by arrow  94 . The motion of arms  82  is transferred to cam frame  76  by pegs  84  that fit into apertures  78 . The motion is guided by means of guide pin  81  that slides in cam aperture  80  and by means of guide member  90  (not seen in  FIG. 3B ) that slides in aperture  88 . The motion of arms  82  also turns grippers  66 , by means of arms  70  that are connected to hole  86 , so that grippers  66  swing over and grab billet  22  (this position being indicated by the broken lines  65  in  FIG. 3A ). When grippers  66  have been turned to hold billet  22 , guide pin  81  has traveled to the opposite end of cam aperture  80  and is geometrically locked thereat to hold and lock grippers  66  in place. Releasing the grippers  66  is done by reversing the process. Accordingly, moving the actuator arms  82  in the direction of arrow  96  (opposite to  92 ) causes cam frame  76  to pivot about the center of tray  60 , as indicated by arrow  98  (opposite to  94 ). As before, generally the grippers  66  hold the periphery of billet  22  during thermoforming at station  28  ( FIG. 1A ) and are released to allow removal of the finished container. 
   In a non-limiting summary so far, the gripper of the pallet of  FIGS. 2A-2E  is arranged for linear movement between a first position, wherein the gripper does not grip a billet placed in the pallet, and a second position, wherein the gripper grips the billet placed in the pallet. The gripper grips the billet by pressing the billet against a billet support surface of the pallet. The gripper of the pallet of  FIGS. 3A-3D  is arranged for rotational movement between a first position, wherein the gripper does not grip a billet placed in the pallet, and a second position, wherein the gripper grips the billet placed in the pallet. The gripper grips the billet by pressing the billet against a billet support surface of the pallet. 
   In the gripper of the pallet of  FIGS. 2A-2E , each individual gripper is actuated to move between the first and second positions. In the gripper of the pallet of  FIGS. 3A-3D , a plurality of grippers are actuated together (by the action of the cam frame and actuator arms) to move between the first and second positions. 
   In the gripper of the pallet of  FIGS. 2A-2E , the gripper is held in place in the second positions by means of a biasing device (e.g., leaf spring  51 ). In the gripper of the pallet of  FIGS. 3A-3D , the grippers are held in place in the second positions by means of geometric locking (e.g., guide pin  81  being geometrically locked at one end of cam aperture  80 ). 
   Reference is now made to  FIG. 4 , which illustrates a pallet  308 , constructed and operative in accordance with yet another embodiment of the present invention. As will be explained further below, this embodiment combines some of the features of the embodiments described above and adds some of its own. 
   Pallet  308  may include a tray  110  together which is assembled an inner frame  112  that has a peripheral billet support surface  114 . Grippers  116  (four are shown in  FIG. 4 ) may be mounted on frame  112  and arranged for linear motion inwards towards and outwards from the center of tray  110 . A guide member  118  may be assembled with gripper  116 . Guide member  118  may be biased by a biasing device, such as a leaf spring  120 , and may have its linear movement limited by a pin  121  that slides in a slot  119  of gripper  116 . Gripper  116  is arranged to pass over and press against a billet, as described below. 
   A cam frame  122  may be positioned on the lower side of tray  110 . Cam frame  122  may be formed with an aperture  124  at the middle of each of its four sides and a L-shaped cam aperture  126  at corners thereof. Gripper  116  may be in operative communication with cam frame  122  by means of a holding tooth  130  that fits into aperture  124 . Holding tooth  130  has a shaft  109  that can enter a groove  111  (not seen in  FIG. 4 , but seen in  FIG. 5A ) in gripper  116  so as to hold gripper  116  in place (as will be described further below). The holding tooth  130  may be biased by a biasing device, such as a leaf spring  132 . The swivel movement of cam frame  122  may be guided by a swivel pin  134  that is received in an elongate oval aperture  136  formed in cam frame  122 . 
   Pallet  308  may further include an under-billet gripper  138 , which may be a rod with a gear teeth  140  formed at ends thereof. Gear teeth  140  mesh with a cogwheel rail  142  which is actuated (moved back and forth) by movement of cam frame  122 . This may be accomplished by means of a pin  128  which depends from cogwheel rail  142  and is received in cam aperture  126 . The cogwheel rail may be supported by a support guide  144 . The linear motion of cogwheel rail  142  imparts rotary motion to under-billet gripper  138 . Under-billet gripper  138  may include upper and lower hinges  146  and  148 , respectively. 
   The operation of pallet  308  is now described with reference to  FIGS. 5A-11D . 
   Reference is now made to  FIGS. 5A-5C , which illustrate billet  22  placed upon pallet  308 . The billet  22  is supported on surface  114  of frame  112  and is not yet gripped by grippers  116 . Pin  128  is in an “idle” position in cam aperture  126 . 
   Reference is now made to  FIGS. 6A-6C . The cam frame  122  may be swiveled to pivot about the center of tray  110 , as indicated by arrows  137 . The swiveling motion may be imparted to cam frame  122  by any suitable moving means (not shown), such as but not limited to, a linear actuator, piston, step motor, etc., positioned at stations  20  (for closing pallet  308 ) and  28  (for opening pallet  308 ). The swivel motion of cam frame  122  causes holding teeth  130  to move grippers  116  linearly towards the center of tray  110  and over billet  22 , as indicated by arrows  135 . The movement of grippers  116  is guided by guide members  118 . In this manner, billet  22  is now gripped by grippers  116 . It is noted that the grippers  116  are locked in place by the action of the biasing device  120 . This will permit further movement of cam frame  122  to actuate the under-billet grippers  138 . Pin  128  is now at one of the ends of cam aperture  126 . 
   Reference is now made to  FIGS. 7A-7D , which illustrate billet  22  gripped by grippers  116  and moved to one of the heating stations (pre-heating or main heating stations  24  or  26 , respectively) of the thermoforming machine  10  of  FIG. 1 . Cam frame  122  has been swiveled in a direction indicated by arrow  139  and this motion causes the under-billet grippers  138  to rotate, together with upper and lower hinges  146  and  148 , so as to grip the peripheral edges of the underside of billet  22 . Billet  22  has now sagged in the middle on account of the heating. 
   Reference is now made to  FIGS. 8A-8D , which illustrate billet  22  gripped by grippers  116  and moved to pressure vacuum station  28  of thermoforming machine  10 . An upper pressure unit  150  is poised to move down on billet  22 , and a lower vacuum molding unit  152  that has been moved up to and against billet  22  from the underside thereof. The pressure of the lower vacuum molding unit  152  against billet  22  and the gripping of the peripheral edges by grippers  116  form a shoulder  154  on billet  22 . 
   Reference is now made to  FIGS. 9A-9D , which illustrate upper pressure unit  150  having been moved down on billet  22 . The upper pressure unit  150  displaces grippers  116  outwards (overcoming leaf springs  120 ). The top of billet  22  is now held by the upper pressure unit  150  and not by grippers  116 . By the application of pressure and vacuum, the molding units form a container out of billet  22 . Cam frame  122  may then be further swiveled in the direction indicated by arrow  139  and this motion causes the under-billet grippers  138  to rotate, together with upper and lower hinges  146  and  148 , so as to release their grip on the billet  22  (which has been formed into a container). The top peripheral edges of billet/container  22  are still held by the upper pressure unit  150 . The motion of cam frame  122  moves each holding tooth  130  upwards and shaft  109  of the holding tooth  130  re-enters groove  111  in gripper  116  so as to hold gripper  116  in place. 
   Reference is now made to  FIGS. 10A-10D , which illustrate removal of the lower vacuum molding unit downwards from the billet/container  22 . Again, the billet/container  22  is still held by the upper pressure unit  150 . 
   Reference is now made to  FIGS. 11A-11C , which illustrate a pallet  408 , constructed and operative in accordance with still another embodiment of the present invention. In this embodiment, pallet  408  includes shape-changing grippers  410 , such as piezoelectric or bimetallic grippers. Such grippers are known in the art, and are described, for example, in U.S. Pat. No. 5,788,453. Basically, in a piezoelectric gripper, application of a source voltage sets up stresses and strains in a piezoelectric material, thereby causing the piezoelectric material and gripper to move to a position that grips or holds the billet. Combination piezoelectric actuators are also known in the art which operate by orienting the piezoelectric material within a non-piezoelectric material in such a way that, upon movement of the piezoelectric material due to the application of the source voltage, the piezoelectric material creates stresses in the non-piezoelectric material that cause the non-piezoelectric material to move over a greater range than that possible by the piezoelectric material by itself. 
   In a bimetallic gripper, a bimetallic material has two metallic layers separated by an electrical and thermal insulator. The first layer of the bimetal may be heated to a greater degree than the second layer, as for example by applying a voltage to the first layer, so that thermal expansion causes the first layer to elongate more than the second layer. This causes an uneven thermal expansion of the two layers and thereby causes the gripper to move to a position that grips or holds the billet. 
   Shape-changing grippers may also be made of a shape-memory alloy, such as nitinol. 
   Pallet  408  may include a tray with a peripheral billet support surface, as in the other embodiments of the invention. Shape-changing grippers  410  (four are shown) may be mounted on a frame and arranged for linear motion inwards towards (as indicated by arrows  411 ) and outwards from the center of tray. 
   Pallet  408  operates similarly to pallet  308 , except for the grippers which are shape-changing grippers  410 . Instead of the holding tooth  130  of pallet  308 , pallet  408  may include a solenoid  412  that selectively moves a holding pin  414  up and down to either hold gripper  410  in place or release gripper  410 . (Holding in  414  may move in and out of a groove similarly to the groove  111  in  FIG. 9D . The solenoids  412  are thus examples of electrically actuated devices that can hold the grippers in place in the gripping position.) The solenoids  412  may receive electrical current from a power source (not shown) via electrical wiring  416  and electrical brushes  418 . Each shape-changing gripper  410  may include a shape-changing element  420 , such as a piezoelectric element, bimetallic element or shape-memory element. Upon energization of the shape-changing elements  420 , the shape-changing grippers  410  move (as indicated by arrows  411 ) to a gripping position for gripping the billet  22 . The shape-changing grippers  410  may be released by de-energization or different energization of the shape-changing elements  420 . 
   The scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.