Patent Document

FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to an apparatus and method for thermoforming plastic articles having heavy undercuts, and in particular, to a magnetic articulated sliding undercut mechanism designed to form the undercut portions.  
       BACKGROUND OF THE INVENTION  
       [0002]     While thermoforming plastic articles from a heated sheet of plastic material may have production advantages over molding articles by the so-called injection molding process, one advantage of utilizing the injection molding process has been the ability to produce plastic articles in relatively complex shapes. For example, with injection molding, it is possible to produce relatively deep undercuts in various portions of a molded article. However, one of the practical limitations of the injection molding process relates to the minimum wall thickness which can be obtained in the molded article. The production of relatively thin walled articles by injection molding is difficult, if not impossible, to achieve. Also, the production rate of an injection molded process is far less than that of a thermoforming process.  
         [0003]     By utilizing a thermoforming process, it is feasible to produce plastic articles such as containers, lids, and other plastic articles having relatively thin walls. As compared to an injection molding process, a thermoforming process has the advantage of relatively high speed production and considerably more economical tooling costs. However, the thermoforming technique has had the disadvantage of not adapting itself to rapid molding of complex shapes and, in particular; it has been difficult to efficiently produce a thermoformed article having an undercut portion.  
         [0004]     It has been possible to produce undercuts in a plastic article by the thermoforming process by the use of expanding collets or split mold assemblies. However, the use of expanding collets or split molds greatly increase the complexity and tooling cost of the machinery. More importantly, expanding collets and split molds greatly increase the length of the production cycle of the plastic article.  
         [0005]     There have been several developments directed to producing an undercut portion in a thermoformed plastic article. One such development includes the use of cooled mold portions which impinge upon selected areas of a heated plastic sheet and rigidify the same so that the uncooled portions can be folded to form an undercut or reverse bend section. Such an approach is disclosed in more detail in U.S. Pat. No. 3,284,553.  
         [0006]     Another approach to producing undercut portions in thermoformed plastic articles is disclosed in U.S. Pat. No. 3,337,664. This patent discloses the use of differential air pressure across selected portions of the heated plastic sheet to cause an undercut portion to be formed in the plastic article.  
         [0007]     Still another approach to forming undercut portions in thermoformed plastic articles is disclosed in U.S. Pat. No. 3,126,582. In this patent, a sheet of plastic material is vacuum formed around a mold member carrying a piece of resilient material which projects beyond the edge of the adjacent mold section to form the undercut portion, and which flexes to facilitate stripping of the formed article from the mold without the need of having a movable mold member.  
         [0008]     A more recent approach to forming undercut sections has been the use of air spring activated sliding or pivoting undercut sections that are part of a stripper plate assembly that defines a lower portion of a forming cavity. The sliding undercut sections move along an inclined surface with the stripper plate member so that the plastic article can be formed and removed. The stripper plate is usually actuated with air cylinders in strategic positions. When these cylinders are fired after the initial thermoforming process is complete, and the mold starts to open, they move the stripper plate in a downward movement, such that the pressurized air from the cylinders causes the air springs to extend and move the sliding undercut sections downwardly and outwardly to a retracted position. Once the part is finally free from the cavity and sliding undercut mechanism, the stripper plate cylinders will relax and allow the stripper plate to come back into the down position. The return of the stripper plate to the down position also forces the sliding undercut mechanism downwardly by compression of the air springs. However, the sliding undercut mechanism doesn&#39;t always return to its starting or relaxed position before the next forming process occurs. When this occurs, the next part will be deformed and unusable. Even more, this condition could cause damage to the mold components.  
         [0009]     An apparatus for thermoforming plastic articles is needed wherein the sliding undercut mechanism is retracted to its starting or relaxed position by the use of means other than air springs. Such means would allow the sliding undercut mechanism to set itself in the same position every time the stripper plate started its way back to the relaxed position. Having the sliding undercut mechanism in its starting position before the next forming process occurred would ensure that all parts are formed correctly and that damage does not occur to partially retracted components.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention relates to a sliding undercut mechanism for use with an apparatus for forming a plastic article from a sheet of plastic material heated to a predetermined forming temperature. The apparatus comprises a first mold section having forming surfaces defining a forming cavity, a second mold section axially movable along a forming axis in one direction toward the first mold section for urging at least a portion of the heated plastic sheet into the forming cavity of the first mold section to form the plastic article and a first predetermined distance away from the first mold section along the forming axis in an opposite direction relative to both the forming cavity and the undercut forming element for removal of the formed plastic article from the forming cavity, and a stripper plate movable a second predetermined distance along the forming axis in the opposite direction for freeing the formed plastic article from the forming cavity by initially moving the formed plastic article the same predetermined distance in the opposite direction along the forming axis relative to the forming cavity.  
         [0011]     The sliding undercut mechanism is located at an angle θ between the stripper plate and the first mold section and is pulled upwards from a forming position to a retracted position along the angle by the stripper plate when the stripper plate moves the second predetermined distance. The angle θ of the sliding undercut mechanism is between about 10° to about 30° relative to an axis that is perpendicular to the first mold section. The sliding undercut mechanism comprises undercut forming elements for forming an undercut portion in the plastic article when the second mold section urges the heated plastic sheet into the forming cavity of the first mold section. The apparatus further comprises magnets coupled to the bottom of the first mold section and located below the sliding lock mechanism. The magnets, along with the stripper plate mechanism, help move the sliding lock mechanism from the retracted position to the forming position after the plastic article is removed from the apparatus.  
         [0012]     In operation, the second mold section is moved along a forming axis in one direction toward the first mold section for urging at least a portion of the heated plastic sheet into the forming cavity of the first mold section to form the plastic article while the undercut forming element is in the forming position. The second mold section moves the plastic article a first predetermined distance away from the first mold section along the forming axis in an opposite direction relative to both the forming cavity and the undercut forming element for removal of the formed plastic article from the forming cavity. The stripper plate is then moved a second predetermined distance along the forming axis in the opposite direction for freeing the formed plastic article from the forming cavity by initially moving the formed plastic article the same second predetermined distance in the opposite direction along the forming axis relative to the forming cavity. The stripper plate pulls the sliding undercut mechanism upwards from the forming position to a retracted position along the angle when the stripper plate moves the same second predetermined distance such that the stripper plate, sliding undercut mechanism, and plastic article are all simultaneously moved in an opposite direction along the forming axis for the first predetermined distance.  
         [0013]     After removing the plastic article, the stripper plate and the sliding undercut mechanism are returned to the forming position. The use of magnets coupled to the bottom of the first mold section and located below the sliding lock mechanism, along with the stripper plate mechanism, help return the sliding undercut mechanism to its starting position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an exploded perspective side view of the first mold section showing the forming cavity, sliding undercut mechanism, and magnets.  
         [0015]      FIG. 2  is a cross sectional view of the thermoforming apparatus.  
         [0016]      FIG. 3  is a cross sectional view of the first mold section, sliding undercut mechanism, stripper plate, and the formed, untrimmed thermoplastic part with scrap still intact.  
         [0017]      FIG. 4  is a perspective view of the single formed thermoplastic part showing the undercut portions.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]      FIG. 1  is an exploded perspective side view of the first mold section  10 . The first mold section  10  comprises the forming cavity  11  having a forming surface  12 , the sliding undercut mechanisms  13 , a side retainer  14  that is located in front of the sliding undercut mechanisms  13  and coupled to the forming cavity  11 , and a bottom retainer  15  that is located underneath the sliding undercut mechanisms  13  and the side retainer  14 , is coupled to the forming cavity  11 , and has pockets  30  on its underside for the housing of magnets  16 . The magnets  16  are inserted into the pockets  30  and have their undersides coupled to the bottom retainer  15 . The forming cavity  11  and side and bottom retainers  14 ,  15  are of a metal material, preferably aluminum. The sliding undercut mechanisms  13  are preferably stainless steel, but could be comprised of another metal material. For the purposes of this invention, the magnets  16  used were rare earth magnets. However, other magnets, such as Alnico magnets, could also be used. The side and bottom retainers  14 ,  15  are coupled to the forming cavity  11  and the magnets  16  are coupled to the bottom retainer  15  via coupling means  17  such as clamps, screws, fasteners, or a nut and bolt combination. The coupling means  17  preferably are aluminum or steel material, however the coupling means  17  may be comprised of any metal, metal alloy, or nonmetal that would provide rigid structural support. The sliding undercut mechanisms  13  contain the undercut forming elements  18  required to form the undercut portions  29 .  
         [0019]      FIG. 2  is a cross sectional view of the thermoforming apparatus  20 . In addition to the elements of the first mold section  10 , as shown in  FIG. 1 ,  FIG. 2  includes the second mold section  21  and the stripper plate mechanism  22 . The first and second mold sections  10 ,  21  are shown in an advanced or forming position. The second mold section  21  is moved along a forming axis, designated in  FIG. 2  as A, in one direction toward the first mold section  10  for urging at least a portion of the heated plastic sheet  26  into the forming cavity  11  of the first mold section  10  to form the plastic article  25  while the undercut forming element  18  is in the forming position. The heated sheet  26  of thermoplastic material may be polyethylene, polystyrene, or other well known thermoplastic materials. The thermoplastic material is initially extruded into sheet form and is then fed into a heating station (not shown) where it is heated to the desired forming temperature. From the heating station, the heated sheet  26  is fed into the thermoforming apparatus  20  such that, as shown in  FIG. 3 , the sheet  26  is positioned between the first and second mold sections  10 ,  21 . While not shown in the drawings, conventional means are provided for moving the first mold section  10  and the second mold section  21  toward and away from one another along an axial path designated in  FIG. 2  as forming axis A.  
         [0020]     The second mold section  21 , which is made from metal, preferably aluminum, includes a main body portion  23  having a downwardly extending assist plug  24  securely attached thereto. The assist plug is utilized to initially force a portion of the sheet  26  of the thermoplastic material downwardly into the forming cavity  11  of the first mold section  10 . At this time, a vacuum source (not shown) can be applied to various spaced apart vacuum ports that are present in the first mold section  10  and open into the upper portion of the forming cavity  11 . The vacuum is applied to the ports to ensure that the heated plastic is in intimate contact with the side wall of the forming cavity  11 . In addition, the heated plastic is also brought into contact with the forming cavity  11  by the simultaneous application of pressurized air through the second mold section  21 . The pressurized air is introduced into the second mold section  21  under the assist plug  24 . The second mold section  21  is encased in an airtight box commonly referred to as the plug base.  
         [0021]     The stripper plate mechanism  22 , which is comprised of metal, preferably aluminum, is mounted for selective vertical movement toward and away from the first mold section  10 . The stripper plate  22  works with the sliding undercut mechanism  12  to pull the formed plastic article  25  from the forming cavity  11 . While not shown in the drawings, conventional means, such as air cylinders, are provided for controlling the vertical movement of the stripper plate mechanism  22 .  
         [0022]      FIG. 3  is a cross sectional view of the forming cavity  11 , sliding undercut mechanism  13 , stripper plate mechanism  22 , and the formed, untrimmed thermoplastic part  25  with scrap  26  still intact. After the part  25  is formed, the second mold section  21  moves a first predetermined distance away from the first mold section  10  along the forming axis in an opposite direction relative to both the forming cavity  11  and the undercut forming elements  18  for removal of the formed plastic article  25  from the forming cavity  11 . Removal of the article  25  begins by firing the stripper plate air cylinders and having the stripper plate mechanism  22  gradually pull the sliding undercut mechanism  13  upwards along a given angle a second predetermined distance in the same direction as the second mold section  21 . As the stripper plate  22  moves upwards, the undercut mechanism  13  is moving in both the upwards direction and sideways. The amount of stripper movement, along with the degree of total undercut for any particular design, aids in determining the angle that the undercut mechanism  13  must be positioned. For the purposes of this invention, the angle θ is between about 10 degrees and about 30 degrees relative to an axis B that is perpendicular to the first mold section  10 . The depth of the undercut  18  is also shown more clearly in  FIG. 3 . The undercut  18  can be of any depth, but for the purposes of this invention the depth ranges from between about one-eighth inch to about three-sixteenth inch.  
         [0023]     When the part  25  is finally free from the forming cavity  11  and sliding undercut mechanism  13 , the stripper plate air cylinders will relax and allow the stripper plate mechanism  22  to return to the relaxed or starting position. While the stipper plate mechanism  22  is returning to the relaxed position, the sliding undercut mechanism  13  will also start to move back down to the rest or relaxed position. When the stripper plate mechanism  22  returns to the full down rest or relaxed position, the magnets  16  will further pull the sliding undercut mechanism  13  back to its starting or relaxed position. The usage of the magnets  16  also allows the sliding undercut mechanism  13  to set itself in the same position every time the stripper plate mechanism  22  starts its way back to the molding or relaxed position. The untrimmed thermoplastic part  25  can be transferred to a cutting station where the individual plastic article  25  can be cut from the plastic sheet  26 .  
         [0024]      FIG. 4  is a perspective view of the single formed thermoplastic part  25 . The part  25  includes a generally rectangular base  27  surrounded by tapered walls  28  extending downwardly and slightly outwardly from the base  27 . The walls  28  are provided with undercut portions  29 .  
         [0025]     While the apparatus  20  illustrated in the drawings is specifically designed to form the undercut portion  29  in the article  25 , it will be appreciated that the apparatus  20  could readily be adapted to form other types of articles having different undercut configurations. For example, the article  25  could be of a cylindrical design and/or the undercut portion could be located at a different location along the wall  28  of the article  25 . Further, as will be appreciated, means can be provided for forming a single or a plurality of undercut portions  29  along the sidewall  28  of the plastic article  25 . Also, while the drawings illustrate one portion of the forming apparatus utilized for producing a single plastic article, the forming apparatus  20  can include a plurality of singular forming mechanisms for simultaneously forming a plurality of plastic articles.  
         [0026]     Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Technology Category: 7