Abstract:
A forming station for a thermoformer in which the lower platen is lowered and moved laterally on linear bearings on the machine frame after two sheets are formed in side by side mold-plug assist tool sets so as to align the respective molds and formed sheets. The formed sheets are squeezed together with flanges on the formed sheets thereby fused together by a draw mechanism including a series of hydraulic cylinders which are connected to coupled elements carried by the upper and lower platens so as create the squeezing movement when the cylinders are pressurized. A precision control is exercised over the operation of each of the cylinders using a position sensing device associated with each cylinder to insure equal movement of the output member of each cylinder. The lower platen vertical drive is activated in correspondence with the travel produced by the hydraulic cylinders to reduce the load on a brake holding the upper platen stationary.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the U.S. provisional patent application Ser. No. 60/398,266, filed Jul. 24, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns thermoforming forming stations and processes. Thermoforming is a process of forming preheated plastic sheet material by using differential fluid pressures to conform the sheet material to mold surfaces. Twin sheet thermoforming involves the thermoforming of two sheets which are then fused together to form a completed part. This has been done in manufacturing automotive fuel tanks for example, in which the top and bottom tank halves are formed from separate sheets which are then fused together along abutting flanges on each sheet. This process has particular advantages when used for manufacturing fuel tanks as described in U.S. published patent application 2002/0017745 A1. 
     Various apparatus and processes have been devised for carrying out such twin sheet thermoforming. 
     Copending application U.S. Ser. No. 10/036,316, filed on Dec. 26, 2001 describes a forming station for twin sheet thermoforming involving simultaneous forming of two sheets and subsequent fusing of the two formed sheets in a single forming station to produce a hollow part. Mold and plug assist tools on one platen are shifted simultaneously with respect to tooling on the other platen in order to align the molds containing two formed sheets in preparation for fusing flanges on the sheets by pressing the same together. 
     This arrangement is an improvement over an apparatus shown in U.S. Pat. No. 6,372,176 which shows transfer of tooling between separate forming stations since two separate forming presses are required. Another arrangement is shown in U.S. Pat. No. 5,658,523 which uses independent actuators for the plug assist and mold, with the plug assist moved away from the platen when the molds are aligned. In this case, a separate actuator for vertical motion of the plug assist is also required. 
     U.S. Pat. No. 5,814,185 assigned to the same assignee as the present application describes an improved arrangement for carrying out precision uniform fusing of two thermoformed sheets using hydraulic cylinders and adjustable locking shafts able to be coupled together at various relative positions of the platens. 
     If the plug assist and mold tools are mounted together for lateral movement on the platen by linear bearings, the high fusing pressure could damage these bearings, which already must bear the heavy weight of the tools, when the hydraulic cylinders are activated to fuse the sheets together. 
     This results since the hydraulic pressure acts to draw the platens together, which pressure is felt by the bearings supporting the tool set shiftable on the platen. Also, shifting of the tools on the platen requires a much larger platen, increasing its cost and mass which necessitates larger bearings, motors, etc. 
     The aforementioned copending application also describes shifting of the upper plug assist and mold tools. The upper mold retains one of the formed sheets, and the lateral shifting movement of the mold results in an increased possibility of the formed sheet dropping out of the mold during this shifting movement. 
     It is the object of the present invention to provide a forming station and process for twin sheet thermoforming which minimizes the number of actuators necessary to carry out the process. It is a further object to provide an improved forming station in which the bearings supporting movable tools are not loaded by the fusing pressure. 
     It is still another object to make such a twin sheet thermoforming process more trouble free. 
     SUMMARY OF THE INVENTION 
     The above recited objects as well as others which will become apparent upon a reading of the following specification and claims are accomplished by mounting the lower platen to be movable on the machine frame, to enable shifting of a lower mold and plug assist tool set to be offset with respect to an upper mold and plug assist tool set to align the upper mold and lower mold to enable fusing together of thermoformed sheets previously formed in the respective molds. 
     The upper and lower platens are each driven vertically to carry out sheet forming by a servo motor driving pinion gears, each engaging a gear rack on a respective one of a set of gear rack posts arranged about each platen. The molds and retained formed sheets are moved into alignment after the two sheets have been formed by respective aligned mold and plug assist tools. 
     One of the platens, preferably the lower platen, is mounted on linear bearings and advanced and retracted thereon by pinion gears engaging respective fixed gear racks extending alongside each side of the lower platen, a motor provided to drive the pinion gears. This enables horizontal shifting of the lower platen and mold and plug assist carried thereon to bring the respective molds into alignment. 
     After the molds are aligned by shifting of the lower platen, and the flanges of the formed sheets aligned, the lower platen is drawn up to the upper platen, being held in position by brake devices, to fuse the flanges on the formed sheets together by a series of end to end aligned and selectively coupled elements secured to the upper and lower platens respectively. Activation of a series of hydraulic cylinders in the general manner described in U.S. Pat. No. 5,814,185 acting through the coupled elements draws the lower platen upwardly to squeeze the abutting flanges on the formed sheets. This same pressure fuses the formed sheets together in a manner which does not load the linear bearings supporting the lower platen for horizontal shifting movement. 
     Thus, only a single press and forming station is required to form both sheets simultaneously and to subsequently fuse the two formed sheets together to complete the part. This minimizes the number of actuators required and reduces the complexity and cost of the apparatus. In addition, by not mounting the mold and plug assist for shifting movement on one of the platens, the size and weight of that platen is reduced over that which would be required if this shifting movement was carried out on the platen. 
     An improvement to the arrangement of the &#39;185 patent referred to above is incorporated in that the hydraulic cylinders each have a linear transducer associated therewith to create electrical signals corresponding to the extent of movement of each of the cylinder rods. 
     These signals are used to control the flow of pressurized hydraulic fluid to each cylinder. 
     This enables a more precise control over the stroking of each cylinder rod to insure that uniform squeezing around the part flange occurs. 
     In addition, a load cell is mounted to each cylinder rod to generate signals indicating when all of the clearances in the elements and couplings have been eliminated and fusing compression has begun. The vertical platen drive is only then made to respond to signals from a linear transducer to drive the lower platen by its vertical drive to the position produced by the hydraulic cylinders. This unloads the weight of the lower platen from the upper platen brakes. This insures that flange compression is carried out by the hydraulic cylinder pressures and not attempted to be done by the platen drive. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A–1E  are diagrammatic views of a forming station according to the present invention, depicting various stages of the forming process. 
         FIG. 2  is a perspective view of the main components of the forming station according to the invention shown in  FIG. 1 . 
         FIGS. 3A and 3B  are respective plan views of the lower and upper platen tool receiving surfaces. 
         FIG. 3C  is a view of the section  1 — 1  in  FIG. 3A . 
         FIG. 3D  is a view of the section  2 — 2  in  FIG. 3A . 
         FIG. 4  is a perspective view of the lower platen assembly with the machine frame shown in phantom lines. 
         FIG. 5  is a plan view of the lower platen assembly with the machine frame shown in phantom lines. 
         FIG. 6  is a side elevational view of the forming station shown in  FIG. 2 . 
         FIG. 7  is a plan view of the forming station showing a top view of the upper platen. 
         FIG. 8  is a partially sectional view through the lower platen assembly to show certain details of the hydraulic draw system. 
     
    
    
     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  FIGS. 1A–1E , the basic forming station states are diagrammatically represented. An upper platen  10  has a female mold  12  and male plug assist  14  mounted side by side on its undersurface. 
     A lower platen  16  has a female mold  18  mounted thereon aligned beneath the plug assist  14  and adjacent thereto a male plug assist  20  aligned beneath the upper mold  12 . 
     As shown in  FIG. 1A , a sheet feed system advances preheated “A” and “B” sheets each releasably clamped in a respective cavity in a clamping frame  22  into the space between the upper and lower tool sets, each sheet aligned with a respective mold-plug assist tool set. Such sheet feed systems are generally well known and may be designed for the requirements of the particular application. 
     In  FIG. 1A , the upper and lower platen vertical drives  24 ,  26  cause the platens  10 ,  16  to be advanced to form the respective sheets with the tools by a conventional thermoforming process, using vacuum, and/or fluid pressure, etc. in the well known manner. 
     The lower platen  16  is thereafter retracted, the clamping frame  22  first releasing the now formed B sheet so that the formed B sheet remains in the mold  18  as the lower platen  16  is retracted. 
     As indicated by  FIG. 1C , the lower platen  16  is horizontally shiftable, and a horizontal drive  24  is activated to shift the lower platen  16  horizontally to a position whereat lower mold  18  is aligned beneath upper mold  12 , each retaining their respective formed B and A sheets. A robot  26  or other loader device can emplace an insert  28  within the formed bottom or B sheet. 
     The upper and lower platens  10 ,  16  are drawn together by a hydraulic draw mechanism  30  as indicated in  FIG. 1D  to fuse the abutting flanges on the formed A and B sheets together. This mechanism  30  is generally similar to that described in U.S. Pat. No. 5,814,185 here incorporated by reference, with certain improvements described below. The upper platen  10  is held in position by brakes as described below and in the &#39;185 patent, the lower platen  16  and upper platen  10  are coupled together by mechanical elements connected to hydraulic cylinders. The cylinders when pressurized draw the lower platen  16  up towards the stationary upper platen  10  as described below. A transducer senses this movement, and the servo motor drive is controlled so as to vertically move the lower platen  16  so as to follow this squeezing movement to support the weight of the lower platen  16 , to avoid overloading the brake holding the upper platen  10 , as described below in further detail. 
     The clamping frame  22  releases the upper A sheet, so that when the lower platen  16  is again lowered, as indicated in  FIG. 1E  the completed part P will be in the lower mold to be accessible by an unloader  32  to be removed. 
       FIG. 2  shows the basic components of the forming station according to the invention, various conventional details and components not shown in the interests of clarity. 
     A machine frame  34  supports an upper platen assembly  36  including the upper platen  10  suspended over a lower platen assembly  38  including the lower platen  16 . 
     Both the upper platen  10  and lower platen  16  are mounted for vertical motion as described above by respective vertical drive systems. 
     The upper platen  10  is driven by an electric servo motor  40  mounted on the top of the upper platen  10  which drives cross shafts  42  ( FIG. 7 ) via transmissions  46  and various couplings shown. Four pinion gears  48  are respectively driven by the cross shafts  42  and engage vertical gear rack posts  50  so that a vertical motion of the platen  10  results when the motor  40  is operated. 
     The posts  50  are supported by an upper platen framework  37 . 
     A disc brake mechanism  52  is associated with each shaft  42  to allow the upper platen  10  to be held in any position along the path of its vertical travel. 
     In accordance with the arrangement of U.S. Pat. No. 5,814,185 referenced above, four vertical coupling elements, here comprising shafts  54 , are carried by the upper platen assembly  36 , each having a key feature  56  on their lower ends. The shafts  54  are aligned with four coupling receptacles  58  each affixed to an output rod of a respective short stroke large diameter hydraulic cylinder  60  mounted to the lower platen  16 , the output rods comprising the other matable coupling elements. 
     When the lower platens  16  move towards the upper platen  16  with the molds aligned, the key ends  56  enter the receptacles  58  to be mated therewith. When coupling is to be carried out, the shafts  54  are rotated by power cylinders  55  to lock the key ends  56  of the shafts  54  to the receptacles  58 . The vertical position of each of the shafts  54  may be individually manually adjusted for the particular tooling by rotation in a threaded member (not shown). Alternatively, the simultaneous chain driven adjustment mechanisms shown in U.S. Pat. No. 5,814,185 may be employed if tools of significantly varying size are to be used. 
       FIGS. 3A and 3B  show the tool mounting surfaces of the lower platen  16  and upper platen  10 . 
     A main platen section  16 A has a region  16 B where the mold  18  is mounted using a series of clamps  62 . Locating pins  64  are also used extended with cylinders  66  ( FIG. 3C ). 
     An auxiliary lower platen section  16 C is attached to the main section  16 A and has a region  16 D to which the plug assist tool  20  is mounted using clamps  68  and locating pins  70 . 
     The upper platen  10  likewise has a main section  10 A having a region  100 B which receives the mold  12 , mounted by an array of clamps  72 . Fixed locating pins  74  are also used ( FIG. 3D ). 
     An auxiliary section  10 C is attached to the main section  10 A and has a region  10 D receiving the plug assist tool  14 . 
     Clamps  76  and locating pins  78  are used to mount the plug assist tool  14  in these regions. 
     The lower platen assembly  38  is mounted on a carriage frame  80  supported on the machine frame  34  for movement by a pair of linear bearings  82  each attached to one of a pair of frame members  84 . Upper mating bearing/housings  86  are attached to carriage frame members  88 . An electric motor  90  and right angle drive  92  drive a pair of shafts  94  each having a pinion gear  96  to one end ( FIG. 5 ). Each gear  96  is in engagement with a respective gear rack  98  supported alongside each frame member  84 . The motor  90  thus advances or retracts the carriage frame  80  to either of a first or second position of the lower platen assembly  38  shown in  FIGS. 1A and 1C , to enable positioning of the mold  18  either below the plug assist  14  or the upper platen mold  12 . 
     The lower platen assembly  38  is moved vertically by a vertical drive including an electric servo motor  100  driving cross shafts  102  each having a pinion gear  104  fixed at one end. A series of four vertical gear rack posts  106  is engaged by one of each pinion gears  104 . The posts  106  are secured by carriage frame members  108 ,  110 . 
     The upper platen assembly  36  also includes four guide posts  35  which are received in guide bores  37  ( FIGS. 6 and 4 ) in standoffs  124  and plate  122  ( FIG. 3 ) to properly align the platens  10 ,  16  during the sheet fusing step. 
     The lower platen assembly  38  is accurately located horizontally in each of its first or advanced position and second or retracted position by wedge locator pins  112  driven by power cylinders  114  to engage locator spaces  116  on the outside of each carriage frame member  88 . Cushioning shocks  118  are located on the frame  34  so as to engage the carriage frame  80  as it moves into each position. 
     The basic hydraulic draw mechanism used in U.S. Pat. No. 5,814,185 is here also used but with certain improvements. 
     A built in individual position signal generator is incorporated into each hydraulic cylinder  60 , which transmits signals corresponding to the position of its output rod to a system controller  128  which in turn controls a proportioning valve  130  for each cylinder so as to insure precisely uniform travel of each cylinder rod to accurately achieve uniform compression of the fused flanges on the completed part P. Such cylinders equipped with built in position signal generators are commercially available. Also included is an auxiliary cylinder combined with a linear transducer  120  which generates electrical signals corresponding to the travel of the output rods of hydraulic cylinders  60  by having an output shaft connected to antirotation plate  122  connected to each pair of receptacles  58  and the pair of stand offs  124 . 
     A load cell  126  is attached to each hydraulic cylinder output rod between the receptacle  58  and plate  122 . Each load cell  126  generates an electrical signal after the clearances are taken up when the cylinders  60  are activated. Built in clearances are required between the receptacles  58  and end keys  56  in order to allow the rotation necessary to lock these components together axially. 
     The auxiliary cylinder-transducer  120  creates an output signal corresponding to the extent of travel of the lower platen  16 . This signal is transmitted to the industrial controller  128  along with the load cell signals. 
     The industrial controller  128  causes the vertical drive motor  60  of the lower platen to be driven up to follow the motion caused by hydraulic cylinders  60 , as described in the &#39;185 patent. 
     However, this action is delayed until the load cell signals indicate that the clearances have been eliminated and compression of the formed sheet flanges has begun. 
     This is necessary to insure that the motor  100  does not itself attempt to drive the lower platen  16  to compress the sheet flanges, but rather all compression is done by the much larger force cylinders  60 , and thus the drive motor  100  acts only to counteract the weight of the lower platen  16  and relieve the brake  52  of the upper platen  10  of this weight.