Abstract:
A knife-like die for cutting a thermoformed plastic article from a sheet of thermoformable plastic. The die includes a die build up plate that is mounted to a first platen and a striker plate that is mounted to a second platen. The die further includes a die board having a knife element mounted thereto. A heating element heats the knife to a temperature that allows for easy cutting of the thermoformable plastic. The die board also includes a die travel stop which prevents the die from being forced into the striker plate thus damaging the knife element and a die location pilot that engages a feature on the sheet of thermoformable plastic to align the article with the die.

Description:
FIELD OF THE INVENTION  
       [0001]     The present relates to cutting dies, more specifically, to a cutting die for use in cutting thermoformed plastic containers.  
       BACKGROUND  
       [0002]     The use of thermoformable plastic such as polyethylene terephthalate (PET) for packaging has risen sharply in recent years, replacing glass and aluminum in many applications. To manufacture the thermoformable plastic containers, plastic, in the form of sheets, is heated, formed and then trimmed from the sheet. To facilitate trimming, the plastic around the article being trimmed is often heated prior to trimming. One type of trimming system that is employed uses a steel-rule die in which a knife blade that is held in a wooden board is brought into contact with a heated striker plate to sever the plastic article from the sheet of plastic. While the steel-rule die is relatively inexpensive and provides satisfactory cutting characteristics, steel rule dies are less durable than more costly die alternatives such as machined tool steel dies and forged dies. This is because the steel-rule is susceptible to damage from the pressure between the cutting edge and the striker plate. A typical steel-rule die has a life of 5,000 strikes before it must be replaced or sharpened.  
       SUMMARY  
       [0003]     A die for a trim press that cuts thermoplastic articles from thermoformable plastic is made more durable by incorporating features that reduce the pressure put on the knife blade during cutting. A positive stop that limits die travel also limits the resulting pressure on the knife blade. A heating element that heats the knife blade softens the thermoformable plastic on contact to reduce the pressure necessary to cut the plastic. A material sensing circuit can be formed by sensing direct contact between conductive portions of the die and striker plates to indicate the absence of thermoplastic material and cause retraction of the die before damage is done to the knife. Die registration features can be incorporated into the positive stop to align the die and the article prior to cutting.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a perspective view of the cutting die according to the present invention;  
         [0005]      FIG. 2  is a top view of the cutting die shown in  FIG. 1 ; and  
         [0006]      FIG. 3  is a cross-sectional view of the cutting die of  FIG. 2  taken along ling  3 - 3 ; and  
         [0007]      FIG. 4  is a side view of a trim press with the cutting die of  FIG. 1  installed. 
     
    
     DETAILED DESCRIPTION  
       [0008]     Turning first to  FIG. 4 , a cutting die  10  is shown in a trim press assembly. The die  10  is secured to an upper platen  50  on the trim press via a series of build up plates  24 ,  26  that will be described in more detail later. The arrow in  FIG. 4  shows the direction of platen travel during a cutting action. The die  10  acts against a lower trim build up plate  56 , sometimes referred to as a striker plate, located on a lower platen  55  to cut a plastic article  53  from a sheet of thermoplastic material  11 . The trim build up plate  56  on the platen  55  provides a solid, flat surface against which the cutting die can work. A plurality of auxiliary build up plates  57  provide additional support for the trim build up plate  56 .  
         [0009]     Turning now to the figures,  FIG. 1  and  FIG. 2  show cutting die  10  having a cutting blade  12  mounted to a die board  20 . The cutting die  10  further includes two triangular cutting blades  16   a ,  16   b . Cutting blades  16   a ,  16   b  cut out a triangular shaped hole in the plastic packaging which can be used as a hanger for the packaging. Additional cutting blades may be incorporated as needed. The cutting blade  12  can take on any shape depending on the shape of the containers being made. In the disclosed embodiment, the cutting blade  12  takes on a generally rectangular shape having a central indentation on either side. The cutting blade  12  is normally made of steel. The hardness of the steel used is dependent on desired cutting characteristics. For cutting simple designs, a blade with a steel hardness of 50-55 Rockwell C may be employed. The harder steel blades tend to wear better than softer steel. However, softer dies on the order of 45-50 Rockwell C may have to be used to prevent breakage during die formation when complex shapes and sharp bends are used. The cutting blade  12  includes a cutting edge  13  that engages an article to be cut. A heating element  14  surrounds the cutting blade  12  such that it is in thermal communication with the cutting blade  12 . The heating element  14  is supplied power by two leads  15 . During operation, the heating element heats the cutting blade  12  to a predetermined temperature that assists the cutting blade  12  in cutting the plastic material  11 . The temperature to which the cutting blade  12  is heated is dependent on the material being cut. Preferably, the blade  12  is heated enough to allow easy cutting of the material but does not cause the material to become “stringy” or melt during the cutting operation. For instance, when cutting PET the blade  12  is heated to a temperature of about 220° to about 230° F. At this temperature, the PET material is easily cut without melting the PET or causing it to become “stringy.” 
         [0010]     Heating the cutting blade  12  minimizes the force needed to cut the plastic material  11  thereby decreasing the wear on the cutting surface  13 . However, the temperature of the cutting blade  12  is kept below a temperature at which plastic material  11  would stick to the cutting blade  12 . In order to monitor the temperature of the cutting blade  12 , a thermocouple TC is inserted between the heating element  14  and the cutting blade  12 . In a preferred embodiment, the thermocouple is connected to a monitor along with leads  15 . The monitor measures the temperature through the thermocouple TC and applies the appropriate amount of power to the heating element  14  through leads  15  to maintain a constant temperature of the cutting blade  12 .  
         [0011]     The cutting die  10  further includes stoppers  18   a  and  18   b  which are located at either end of the die board adjacent to the cutting blade  12  but not located in the area defined by the blade  12 . The stoppers  18   a ,  18   b  extend through the die board  20  to a distance just below the top of the cutting blade  12 . The distance from the top of the of the stoppers  18   a ,  18   b  to the top of the cutting blade  12  is preferably equal to the thickness of the plastic to be cut. During operation, the cutting die  10  presses down on the plastic material  11  to begin the cutting process. Pressure is applied until the travel of the die is prevented due to the stoppers  18   a ,  18   b  contacting the plastic material  11  and the striker plate  56 . During this procedure, the cutting blade  12  cuts the plastic article  11  in a direction towards the striker plate  56 . Use of the stoppers  18   a ,  18   b  prevents excessive pressure on the cutting blade  12  thus preventing damage. The stoppers only allow the blade  12  to cut to a predetermined depth, based on the height difference between the top of the stoppers  18   a ,  18   b  and the top of the cutting edge  13 , taking into account the thickness of the plastic material  11 , in turn reducing wear to the cutting edge  13  associated with excessive pressures in the cutting process.  
         [0012]     The internal surface of the cutting blade  12  has affixed thereto ejectors  17  which are used to aid in removal of the cut article from the inside of the cutting die  10  upon completion of the cut. As the cut is being performed, the ejectors  17  are compressed by the article as pressure is being applied to the cutting die  10 . Once the cut is made, the die is then retracted away from the cut article and the ejectors  17  begin to decompress, expelling the cut portion of the article from the inside periphery of the cutting blade  12 . The ejectors  17  can be constructed from any material as apparent to one of ordinary skill in the art in view of this disclosure. In the preferred embodiment, the ejectors  17  are constructed from a rubber compound.  
         [0013]     The die board  20  is loosely coupled to a metal trim die buildup plate  24  by bolts  22 . The trim die buildup plate  24  is preferably metal and acts to prevent deflection of the die during the cutting cycle. An additional second build up plate  26  further adds to the stability of the cutting die  10  during operation. The second buildup plate  26  is preferably wood but can be constructed from other materials to provide more or less weight if needed.  
         [0014]     Turning now to  FIG. 3 , a cross-sectional view of the cutting die is illustrated showing the stopper  18   b  and the connection of the die board  20  to the buildup plates  24 ,  26 . The die board  20  is loosely mounted to the buildup plate  24  via the threaded bolt  22 . The threaded bolt  22  extends through the second buildup plate  26 , metal buildup plate  24 , and the die board  20 . The die board  20  includes a bore for housing the threaded bolt  22 . The bore is larger in diameter than the diameter of the threaded bolt  22 . This allows the die board  20  to move on the build up plate  24  in both a longitudinal and latitudinal direction. The threaded bolt  22  is held loosely in place by a nut  54 . A washer  52  is placed over the threaded bolt  22  between the nut  54  and the die board  20  so that the nut  54  cannot fall back through the bore in the die board  20 . This creates a loose connection fastening the die board  20  with the buildup plates  24 ,  26  yet allows the die board  20  freedom of movement for adjustments during the cutting cycle.  
         [0015]     The stoppers  18   a ,  18   b  ( 18   b  shown) include a conical internal surface  40 . This surface mates with a protrusion on the plastic material  11  such that the cutting die is properly aligned to make a cut in an exact location. During the operation of the cutting cycle, the plastic material  11  is moved on the striker plate under the cutting die  10 . The cutting die  10  is then lowered onto the sheet to perform the cut. As the die is lowered, the conical protrusion on the sheet aligns with the conical internal wall  40 . As described, the die board  20  is loosely connected to the buildup plates, therefore, the die board  10  can easily move into alignment with the protrusion on the plastic material  11  by aligning with the internal conical wall  40  of the stopper  18   b . In an alternative embodiment, the alignment can take place by having the die board securely mounted to the buildup plates while the plastic material  11  is moved into alignment with the die board.  
         [0016]     In the present embodiment, the stopper  18   b  extends from the buildup plate  24  to a point just below the top of the cutting blade  12 . The bottom of the die board  20  includes a chamfer which allows the bottom of the stopper  18   b  to sit flush on the buildup plate  24 . The stopper is prevented from dislodging from the die board  20  by a retaining ring  42 . The retaining ring is larger in diameter than the bore in the die board  24  that houses the stopper  18   b  thus preventing the stopper from dislodging. The stopper  18   b  contacts the buildup plate at a joining surface area  44 . By contacting the buildup plate directly, minimal damage from cutting pressure is imparted to the die board  20  because the pressure is at least in part absorbed by the buildup plate  24 .  
         [0017]     During operation, it is helpful to determine if the sheet of preformed plastic material  11  is present under the cutting die and ready for cutting. If no material is present and the press is activated, damage could be done to the cutting die. Therefore, it is necessary to determine if the material is present for the cutting operation. One way to do this is to apply and electrical current to the cutting blade itself or to the metal build up plate. As the pressed is lowered, if no material is present, the blade will make contact with the striker plate and complete a circuit through the blade and the striker plate. Completion of the circuit signals the die drive to retract thereby minimizing damage to the blade. In an alternative, current can be applied to the die build up plate  24 . Current is then transferred to the cutting blade due to the blades contact with the buildup plate  24 . In  FIG. 3 , the cross-section shows the blade making contact with the buildup plate  24 , however, the blade does not contact the buildup plate in all regions but rather bridges the board in these bridging regions to prevent the section of the die board within the blade from being severed. Contact between the blade and buildup plate occurs in several sections around the perimeter of the blade thereby connecting the cutting blade  12  to the die board  20 .  
         [0018]     In the foregoing description, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit or scope of the present invention as defined in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather that a restrictive sense.