Abstract:
A method and apparatus for thermoforming deep drawn parts such as cups in which a vacuum is momentarily applied to a preheated sheet material when overlying a clamping plate prior to clamping and advance of the plug assists to delay contact with a lip molding feature projecting from each mold cavity. This avoids a molding defect due to premature cooling of a localized region of the sheet material when the sheet material comes into contact with the lip molding features.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The application claims the benefit of U.S. provisional Ser. No. 60/432,175, filed Dec. 11, 2002, now abandoned. 

   BACKGROUND OF THE INVENTION 
   This invention concerns thermoforming in which plastic sheet material is preheated and then advanced over a mold cavity plate, and then drawn into the mold cavities by the application of differential fluid pressure to the sheet material, as by development of a vacuum within the cavities and/or a positive pressure on the sheet material on the opposite side from the cavities. Proper stretching of the sheet material is critical to producing a properly formed part, as too much stretching in localized areas can reduce the wall thickness to an unacceptable degree. An unsightly appearance of the finished product can result from nonuniform stretching of the sheet material. Sophisticated tooling has been developed to insure proper stretching of the sheet material including clamping rings tightly engaging the sheet material surrounding each mold cavity, which are forced against the sheet material prior to thermoforming the parts. This limits the stretching of sheet material to the section secured within the clamping ring. 
   Another widely practiced thermoforming technique, particularly when making deep drawn parts such as disposable cups, involves the used of “plug assists”, which are plug shaped members advanced into the sheet material to force it into the mold cavity prestretch the material by movement into the mold cavity prior to forming the part so as to prevent localized over stretching. 
   Another critical aspect of thermoforming is preventing localized cooling of the material as by contact with meal surfaces which could chill the sheet material at the point of contact, preventing proper stretching of the material around the point of contact. 
   The plug assists are typically constructed of syntactic foam to prevent chilling by contact of the sheet material therewith. 
   When thermoforming disposable drink cups, a lip is formed on the upper edge to produce a finished look. The lip is partially formed during molding by a projecting ring on the mold cavity with the sheet wrapped around the rim during thermoforming. The lip is rolled under in a subsequent lip rolling operation. Early contact with the lip ring is apt to occur during plunging of the plug assists, and this contact can cause a slight chilling of the sheet material at the point of contact, forming an irregular ridge in the sidewall as the chilled portion is drawn down into the cavity during the plug assist plunge. This is particularly apt to occur when thermoforming from thin gauge plastic fed off a roll at high speed. 
   Such a ridge is unsightly, particularly in clear plastic cups, and can make the release of the cups from a stack more difficult. The ridge also can interfere with printing on the cup side wall. 
   In an effort to avoid this problem, the projecting edge has been recessed into a “moat” feature, but this has led to difficulties in trimming, since the trim tooling must reach into the moat. Furthermore, additional sheet material is used up in the molding operation since a larger clamped circle is necessitated. 
   It is an object of the present invention to provide a thermoforming method and apparatus which avoids contact of the heated sheet material with lip ring edge of the mold cavity during a plug assist plunge but does not involve the use of “moats” to recess the lip ring edge. 
   SUMMARY OF THE INVENTION 
   The above object, as well as other objects which will become apparent upon a reading of the following specification and claims, are achieved by applying a momentary pulse of vacuum to the sheet in a region on the sheet side opposite the mold cavities surrounding each plug assist, so as to draw that sheet material region away from the mold lip rim and against the top of the retracted plug assists, forming a shallow dimple aligned with the mold cavity and the associated plug assist. 
   This shallow dimple is formed just before the plunge of the plug assists so as to delay contact of the sheet material with the lip forming mold cavity ring edge until after the plug assist plunge. This avoids formation of the irregular ridge caused by such contact with the lip forming insert rim described above. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a tooling assembly for a thermoforming machine. 
       FIG. 2  is a partial vertical sectional view of a portion of the thermoforming tooling together with associated press structure and other components depicted diagrammatically, together comprising an apparatus according to the present invention. 
   

   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. 
   The present invention concerns a thermoforming apparatus and process in which sheet material (either fed off a roll, extruded or precut) is advanced into a preheating station to be heated to the proper temperature, then transferred into a forming station whereat the articles are formed. The sheet material with the parts formed therein is then transferred to a trim station whereat the cups or other parts are cut free from the rest of the sheet material, using a separate press and tooling. The cups may be stacked and transferred to other operations such as lip rolling, printing, packaging, etc. 
   Trimming is also sometimes done in the forming station itself. 
   Such trimming apparatus is generally well known in the art and will not be described herein. 
   The forming tooling includes a lower tooling assembly  10  shown separately in  FIG. 2  and is preferably of the type described in copending application U.S. Ser. No. 09/996,062, filed on Nov. 28, 2001, assigned to the same assignee as the present application, and incorporated by reference herein. An upper tooling assembly  14  ( FIG. 1 ) includes a mold plate  20  mounted in an upper platen  16 . 
   Upper tooling assembly  14  includes an array of cavity inserts  18  secured in bores  12  in a mold, each insert defining a mold cavity  22 . As described above, each insert  18  is formed with a lip  25  projecting below the lower surface of the mold plate  20  to mold a partially formed lip on the molded cup (not shown). 
   The upper press platen  16  is able to be lowered and raised by an actuator  24  in the well known manner. 
   The mold cavity  22  defines the shape of the article to be thermoformed such as a disposable plastic cup from preheated sheet material S advanced over the lower tooling assembly  10 . The lower tooling assembly is mounted on a lower platen  26  able to be raised with an actuator  28 . 
   The lower tooling assembly  10  comprises an open ended box  68  constructed of metal plates and includes an array of plug assists  32  each aligned with a respective mold cavity  20 . The plug assists  32  (only three shown) are normally retracted in wells  30  defined by each one of a series of bores machined into a thick aluminum clamping plate  34  included in the lower tooling assembly  10 . 
   The clamping plate  34  is supported on a spacer plate  35  and an assembly of one or more steel support plates  36 ,  38  fastened together face to face. Porting flow passages  27  machined into the abutting faces of plates  36 ,  38  provide manifolding for communicating air pressure or vacuum from a compressed air source  40  or vacuum source  42  into each well  30  via control valves  44 ,  46  depicted diagrammatically. The support plates  36 ,  38  are mounted by a series of stand off plates  37  facing in different directions, and secured to a bolster plate  47  forming the bottom of the box  68  and attached to the top of the lower platen  26  and the underside of the lower support plate  38 . 
   Each plug assist  32  is secured to one end of a rod  50  received in a bottom piece  52  fixed to the lower end of each plug assist  32 . The plug assists  32  are commonly constructed of syntactic foam material, such that a metal bottom piece  52  formed with a socket is required for a secure connection to the rods  50 . 
   Each rod  50  passes through seals (not shown) to seal the wells  30 , preventing leakage of air in or out of the wells  30 . 
   The lower ends of the rods  50  are connected to a common spider plate  54 , located beneath the support plates  36 ,  38 , and also the clamp plate  34 , having spaces to accommodate the standoff plates  28 . 
   The spider plate  54  is itself connected to a set of shafts  56  extending down through bores  58  in the lower platen  26 , and connected to a third platen  60  lying below the lower platen  26 . 
   An actuator  62  is connected to the third platen  60  for vertically stroking the same, to cause the spider plate  54  to move the rods  50  and plug assists  32  up into the respective mold cavities  22  during a forming cycle to control the stretching of the preheated sheet material. 
   A series of annular clamping rings  64  are provided each fixed surrounding the top of a respective well  30  and each having an annular clamping edge. The clamping rings  64  are moved to clamp a region of the heated sheet material S against the lower surface of the upper tooling assembly  14  when the upper platen  24  and lower platen  26  are moved together so as to the lip engage a respective section of the sheet material S extending across the wells  30  to control the stretching of the preheated sheet material S during thermoforming, in the manner well known in the art. 
   In the method according to the present invention, a vacuum is momentarily applied in the sealed wells  30  after the preheated sheet material S is in position overlying the lower tooling assembly  10 , by programming causing the machine control  65  to operate momentarily the valve  46  placing the wells  30  in communication with the vacuum source  42 . 
   This causes the regions of sheet material S lying over each retracted clamping ring  64  to be drawn away from the upper mold cavity tooling  14  into said wells  30 , and against the tops of the plug assists  32 , as shown in  FIG. 1 , forming a dimpled shape within each of the clamping rings  64 . Thus, as the upper platen  16  and lower platen  26  are moved together to engage the clamping rings  64 , contact between the insert lip  25  and the sheet material S is delayed until after the plug assists have been extended. 
   The development of a vacuum in each well  30  and the dimpling of the sheet material S is quite rapid such that only a momentary operation of the valve  46  is typically necessary, on the order of a fraction of a second. This dimpling is controlled as the local areas of the sheet material S is drawn against the top of each retracted plug assist  32 . The plug assists  32  do not chill the sheet material since they are constructed of syntactic foam, as mentioned above. 
   Furthermore, such contact is deployed as the plug assists  32  are advanced into the mold cavities  22 . The machine control  65  thereafter causes the valves  44 ,  46  to be operated so as to communicate the vacuum source  42  with the mold cavities  22  and the air pressure source  40  to the wells  30  to form the parts in the mold cavities  22 . 
   The plug assist  32  and the platens  24 ,  26  are subsequently retracted, and the sheet material S with the formed parts stripped from the mold cavities  22  as by using stripper plate  66 , the formed sheet S then transferred to a trim station (not shown) for cutting the formed parts free, all in the conventional fashion. 
   This improved method and apparatus can be provided by a relatively simple and inexpensive modification of existing equipment and has been found to avoid the problems described above over extended production runs.