Abstract:
A platen drive system and method of leveling platens in a forming station of a twin sheet thermoforming machine in which each platen is driven vertically by four independently operated drive units, each having an electric servo motor, the drive units arranged in two sub sets disposed on opposite sides of an associated platen. The platen is leveled during set up by individual operation of the servo motors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 61/212,816 filed on Apr. 16, 2009. 
    
    
     REFERENCE TO A COMPUTER PROGRAM LISTING APPENDIX 
     Incorporated by reference are program listings contained on an accompanying CD-ROM (with copy filed herewith), containing two files, LLC-HMI and LLC-PLC created on Mar. 29, 2010. 
     BACKGROUND OF THE INVENTION 
     This invention concerns twin sheet thermoforming machines and more particularly platen drives for platens included in a forming station. In the forming station, upper and lower molds are carried by upper and lower platens respectively for forming separate part halves in a well known manner. After the part halves are formed, the upper and lower platens are moved together to fuse the two part halves together. This process and such thermoforming machines are described in U.S. Pat. Nos. 5,814,185 and 6,969,246 assigned to the same assignee as the present application, and are incorporated herein by reference. 
     It is critical that the molds mate perfectly in order to produce a uniform seam thickness for such a twin sheet molding process when the platens are advanced together, and this requirement in turn requires both platens to be perfectly level to be parallel when fully advanced, partially with large platens. 
     However, it is difficult to achieve level platens with large area platens and requires extremely time consuming manual adjustments requiring disassembling of gearing. The meshing of drive gearing with fixed gear rack guide posts typically used to guide the platen motion makes this very slow, difficult and time tedious since variations of the degree of engagement of the drive unit gearing engagement with the four gear racks affects the level condition of such a platen, i.e. a greater degree of engagement slightly elevates the associated platen and a lesser degree of engagement slightly lowers the platen changing the degree of gear engagement with one post may affect another gear-post engagement. 
     In addition, the perfectly plumbed guide gear rack posts engage guide bushings on the platens during travel of the platens, and even a slight out of level condition would create significant binding in the post/sleeve engagement surfaces, particularly in large area platens and result in rapid wear of the guide bushings to significantly increase maintenance costs. 
     Conventional platen drives involve one or two motors driving respective sets of gearing which engage associated fixed gear posts to drive the platens. 
     Machining a large platen is costly as very large machining centers are required to accommodate such platens and align bores at either end of the platen for the respective gearing. 
     Accordingly, it is an object of the present invention to provide a method for the leveling platens in a thermoforming machine station which reduces the time and difficulty of setting the platens level. 
     It is a further object to provide a platen drive for thermoforming machine forming station platen which allows simplified leveling of the platens and also lower a cost manufacture of the platens. 
     SUMMARY OF THE INVENTION 
     The above recited objects and other objects which will become apparent by those skilled in the art upon a reading of the following specification and claims are achieved by providing thermoformer platen drive comprised of four independently operated platen drive units arranged in two unit sub sets on opposite sides of an associated platen which each drive units include an electric servo motor with encoder and gearing which are normally simultaneously operated to vertically drive each leveled platen up or down. The drive unit controls allow the practice of a method of platen leveling in which each of servo motor may be operated individually in a leveling mode to enable bringing the platen into a level condition quickly and easily by operating each servo motor as needed to level the platen. Differences in the degree of meshing engagement of the various drive pinions with respect to a respective gear rack in this arrangement can be accommodated by the individual servo motor operation and thus does not affect the ability to level the platens. 
     Thereafter, the individual drive units are operated simultaneously to vertically move each engaged part of the platen precisely equally to maintain a level condition of the platen. 
     The independent operation of each drive unit allows each gear box/motor drive unit mount to be individually machined and assembled to locating surfaces on the platen machined level in machining centers. Since bore alignment between the mounts are not necessary because of the independently operated drive units, lower cost machining of the platens and drive unit mounts is made possible. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front elevational view of a lower platen and support structure incorporating a platen drive system according to the present invention. 
         FIG. 1A  is an end view of one of the drive units shown in  FIG. 1 . 
         FIG. 2  is a plan view of the lower platen structure shown in  FIG. 1  with a diagrammatic depiction of the servo motor control operating the four electric servo motors. 
         FIG. 2A  is a top view of one of the drive units shown in  FIG. 2 . 
         FIG. 3  is a side elevational view of the lower platen structure shown in  FIGS. 1 and 2 . 
         FIG. 4  is an enlarged view of the section  4 - 4  taken in  FIG. 2 . 
         FIG. 4A  is an enlarged fragmentary sectional view of a portion of the gear post and guide box shown in  FIG. 4 . 
         FIG. 4B  is an enlarged fragmentary sectional view of a one end of the gear post attachment shown in  FIG. 4 . 
         FIG. 5  is an enlarged view of the section  5 - 5  in  FIG. 2 . 
         FIG. 6  is a pictorial view of the lower platen structure shown in  FIGS. 1-3   
         FIG. 7  is an inverted pictorial view of a lower platens shown in  FIGS. 1-3 . 
         FIG. 8  is a pictorial view from one side of a drive unit mount. 
         FIG. 9  is a pictorial view of the drive mount shown in  FIG. 8  from the reverse side. 
         FIG. 10  is a plan view of the upper platen structure with a diagrammatic representation of the servo motor control operating the four electric servo motors. 
         FIG. 11  is a front elevational view of the upper platen structure shown in  FIG. 10 . 
         FIG. 12  is an enlarged view of the section  12 - 12  taken in  FIG. 10 . 
         FIG. 13  is an enlarged view of the section  13 - 13  taken in  FIG. 10 . 
         FIG. 14  as a pictorial view from the bottom of the upper platen structure shown in  FIGS. 10 and 11 . 
         FIG. 15  is a view of a screen shot of one made of the servo motor controller seen in a platen leveling set up mode of the controller. 
     
    
    
     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,  FIGS. 1-9  show a lower platen structure  10  and details thereof, which include a lower platen assembly  12  movably mounted vertically on a support framework  14 . 
     Such platen assemblies are used in the forming station of twin sheet thermoforming machines as mentioned above and described in the molds (not shown) are normally mounted on the platens above referenced patents, incorporated herein by reference. 
     There are a set of four electric servo motor/gear box drive units provided, in subsets of two drive units  16 A,  16 B mount on opposite sides of the platen  12 , here the front and rear side of the lower platen  12  assembly movable mounted on the lower platen support framework  14 . 
     The drive units  16 A are more closely spaced to allow clearance for feeding sheets between the upper and lower platens on a rotary transfer machine in the well known manner. The sides of the platens are normally left clear of support posts in order to allow sheet material to be loaded onto the molds in the well known manner. 
     The drive units  16 A,  16 B include an electric servo motor and a right angle gear box  22  having a hollow shaft output  24 . Suitable drive units are of a type commercially available under the trade name SEW EURO DRIVE. These motors include a built in encoder which allow precise control over the extent of rotation of the tubular output shaft  24  in the manner well known in the art. 
     A respective pinion shaft  26  (FIGS.  4 , 5 ) is received in each of the hollow gear box output shaft  24  and clamped thereto with a clamping collar  25  to be rotatably connected thereto. 
     A pinion gear  28  is keyed to the pinion shaft  26 , which has its outer end passed through a bore  42  in plate  33  of a drive unit mount  30 , and supported in a bearing  40 . 
     A gear rack  50  machined into a support-guide post  44  adjacent each drive unit  16 A,  16 B is engaged by the associated pinion gear  28  as seen in  FIG. 4 . Each guide post  44  is fixedly mounted to the lower platen structure frame  14  at the top and bottom ends, the ends adjusted at assembly to plumb the posts  44 . When the drive units  16 A,  16 B are operated, the lower platen  12  is moved up or down on the lower platen support frame  14 . 
     Each support guide post  44  is received in a guide sleeve  46  and is slidable in guide bushings  48  at the top and bottom of the sleeve  46  ( FIG. 4A ) included in each of the mounts  30 . Each sleeve  46  has a cut out to allow the pinion gear  28  to be positioned to engage a gear rack  50  machined into one side of the guide post  44 . 
     The guide post  44  is slidable within the bushings  48 . In practice, one of the posts  44  is initially plumbed with respect to the frame structure  14  by end plates  52  fastened to slotted blocks  54  fixed to frame  14  by screws passing through slots  56  allowing adjustment in one direction. A shimming space  58  allows adjustment in the other direction. Once plumbing of one of the posts  44  is achieved, the plates  54 ,  52  are welded together. 
     The other three posts  44  are then plumbed by making any adjustments necessary to release any binding that occurs when the platen  12  is elevated up and down on the posts  44 . 
     The independent operation of the platen drive units  16 A,  16 B eliminates the need to accurately align bores in the mounts  30  with each other on the platen, greatly easing the difficulty of the machining requirements. 
     A series of hydraulic bayonet couplings  58  are provided for enabling twin sheet thermoforming operations where two part halves are fused together as described in U.S. Pat. Nos. 5,814,185 and 6,969,246 thermoforming machines assigned to the same assignee as the present application. The other features and details of such twin sheet thermoforming machines are not set forth herein, not forming a part of the present invention. 
     Initially, the platen  12  is leveled using a machinists&#39; level. In this mode, the drive units  106 A,  16 B are set for independent operation in a set up mode to carry the platen out leveling process. 
       FIG. 15  shows the operator&#39;s control screen in that mode. 
     In this mode, each servo motor  17  is briefly operated (or “bumped”) individually as necessary until the platen  12  is brought to a level. The encoder condition for each motors  17  included in the drive units is then zeroed. 
     During normal operation, the motors  17  are all operated simultaneously to move exactly the same distance using the encoder outputs to achieve such operation moving the platen  12  vertically while maintaining a level condition, in the well known manner. A suitable software program for carrying out the leveling control is contained in the accompanying Appendix CD&#39;s. 
       FIGS. 10-14  show an upper platen structure  60  also included in a twin sheet thermoformer forming station which is similar to the lower platen structure  10 , also including two sets of independent electric servo motors/gear box drive units  62 A,  62 B mounted to an upper platen  64 . These units power vertical movement of the upper platen  64  on an upper platen support frame  66 . 
     Each drive unit has an output hollow tube  68  receiving a pinion shaft  70  clamped thereto at one end with a clamp  71  ( FIG. 13 ). A pinion gear  74  is keyed to the shaft  70  which is supported in a mount  72  located on horizontal machined surfaces  75  on the upper platen  64 . Each pinion gear  74  engages teeth on a gear rack/support guide post  76  received in a guide sleeve  78  included in the mount  72 . The gear rack/support guide post  76  is adjusted to be plumb by caps  80  and slotted blocks  82  so that the upper platen  64  can move freely without binding, as described above in connection with the lower platen  12 . 
     Rods  84  have ends  88  adapted to be locked to the hydraulic bayonet couplings  58  on the lower platen  12  to carry out squeeze fusing of twin parts in the manner described in the U.S. patents referenced above. 
     Thus, a much simplified platen leveling process is enabled as well as reduced machining costs for large platen forming presses by the independent four motor drive arrangement shown.