Patent Publication Number: US-2006006587-A1

Title: Formation of punched, blind holes through one wall of a double wall blow molded structure

Description:
RELATED APPLICATIONS  
      This patent application is a divisional of currently pending U.S. patent application Ser. No. 10/166,840, filed Jun. 11, 2002, which claims the benefit of, under Title 35, United States Code, Section 119(e), U.S. Provisional Patent Application Ser. No. 60/334,382, filed Nov. 30, 2001. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to an apparatus and method for forming holes in a plastic article, and more particularly to an apparatus and method for forming punched, blind holes through one wall of a double wall blow molded structure.  
     BACKGROUND OF THE INVENTION  
      In the blow molding of double wall articles, it is often desirable to form a hole through one or both of the walls. Where such a hole is to be formed through both walls, which are usually separated by an air space, close dimensional control is obtainable, as shown in  FIG. 1 . Referring to this Figure, it is possible to mold sidewalls  10  and to pinch off a slug  11  in the center, and to subsequently remove slug  11  by punching it out. The resulting hole is precisely controlled as to cross-sectional and longitudinal dimensions by proper dimensioning of mold components. Likewise, enlarged cross sections  12  at the entry end of the main hole portion can also be precisely sized and shaped to form counterbores, or countersinks. Similarly, reliefs  13  for threaded nuts or snap-fitting barbs can be formed at the other end of the hole.  
       FIG. 2  shows in section the same molded hole configuration as does  FIG. 1  with barbed attachment means  14  on a threaded insert  15  permanently installed into the molded hole. Note that the holes of  FIGS. 1 and 2  are not necessarily restricted to round cross-sections. Alternately, they may be square or oval or hexagonal or any other shape appropriate to the application at hand.  
      The molding technique for hole forming shown in  FIGS. 1 and 2  permits the economical production of precisely sized holes, with strong and accurately dimensioned sidewalls, in design situations where a hole is to be formed through both inner and outer walls of a double wall structure. However, a problem arises because it is often not desirable or even possible to have the hole extend through both walls. Sometimes styling considerations demand a smooth and unbroken exterior wall surface on either the inside or outside wall of a double wall structure. And in other instances the distance between the two walls is too great to permit forming a molded through hole of relatively small cross-section. In such situations, any hole in either wall, for the attachment of some sort of insert, must be a blind hole extending through one wall only of the double wall structure.  
       FIG. 3  illustrates a typical blind hole  16  formed in accordance with known methods. It should be noted that this hole  16  also can be controlled by mold dimensions as to cross-section and length and configuration of an enlarged area at the entry end. However, a problem often remains. The hole  16  of  FIG. 3  cannot be enlarged at its inner end, because any significant enlargement of the mold cross-section in this area would fatally impede removal of the part from the mold. And yet, it is often essential to anchor an insert in the hole. Self-tapping screws may be used for attachment, but they generally do not hold well, because the preferred materials for most double wall molding, the olefin plastics, are both slippery and relatively soft, meaning that any thread will tend to loosen or strip. Therefore, the preferred attachment method, where one wall must remain unbroken, is by means of barbed engagement of the insert against the inner surface of a single wall which is drilled or punched to form a hole open to the space between the walls.  
      A hole formed in such fashion is shown in  FIG. 4 . Referring to this Figure, the wall of plastic as initially molded  17  (shown in phantom) drapes over a projection in the mold during the molding process to form a recess  18 , or lies against a flat mold surface (not shown). After molding, a drill, boring tool, punch or the like  19  is activated, by conventional means, to remove the material in the cross-section  20  of what then becomes the hole. Although a hole may be formed in this fashion, this method suffers from a number of disadvantages of its own.  
      The wall thickness of the plastic in the punched cross-section is not always precisely controllable. Neither does the plane  21  of the punch always extend through the wall at a point of maximum thickness. Therefore, while the dimension of the molded sidewall portion  24  of the hole is precisely controllable, the portion  22  which is cut away or punched is not. And if the total side wall thickness  23  varies, then a barbed insert can not be made to seat properly on a consistent basis. If the thickness  23  is not sufficient, the barb will seat but with objectionable play; if the thickness is too large, the barb will not seat at all.  
      What is needed, therefore, is a method of forming a hole of a precise effective depth through a wall of plastic material, where the hole must be formed as a blind hole with forming tool access from one side only.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide an apparatus and method for forming holes in a plastic article which can be used to form blind holes through one wall of a double wall blow molded structure.  
      Another object of the present invention is to provide an apparatus and method for forming holes in a plastic article having the above characteristics and which can be used to form holes enlarged at their inner ends in order to accommodate barbed inserts.  
      A further object of the present invention is to provide an apparatus and method for forming holes in a plastic article having the above characteristics and which can be used to form holes open to the space between the walls.  
      Still another object of the present invention is to provide an apparatus and method for forming holes in a plastic article having the above characteristics and which can be used to form holes in which the wall thickness of the plastic in the punched cross-section is precisely controllable.  
      These and other objects of the present invention are achieved by provision of an apparatus and method for forming holes in a plastic article. An apparatus for forming a hole in a blow molded structure includes a mold at least partially defining a cavity for creating the blow-molded structure. The mold has a mold surface against which a wall of the blow molded structure is blown during blow molding. The mold also has a hole formed through the mold surface. A punch is disposed within the hole in the mold surface, a portion of the punch extending out beyond the mold surface in an initial position such that the wall of the blow molded structure is initially formed therearound. The punch is movable to an activated position wherein the punch extends out beyond the mold to an extent greater than in the initial position. The punch includes a cutting edge along the portion thereof which extends beyond the mold surface.  
      The punch is moved from the initial position to the activated position at a time interval after blow molding selected so that at least a portion of, and preferably a majority by volume of, the material forming the wall of the blow molded structure is in a molten state such that the cutting edge of the punch forms a hole through the wall of the blow molded structure along a line substantially perpendicular to a direction of travel of the punch from the initial position to the activated position.  
      Preferably, the blow-molded structure comprises a double wall blow molded structure. Also preferably, a shelf is formed by the punch surrounding the hole created thereby. Most preferably, the shape of the shelf is variable by varying a parameter selected from the group consisting of the shape of the cutting edge of the punch, the temperature of punch, the temperature of the mold surface, the speed of movement of the punch, the timing of movement of the punch, and combinations of these. Preferably, the time interval after blow molding when said punch is moved from the initial position to the activated position falls within a range from about 5 seconds to about 45 seconds, and most preferably within a range from about 20 seconds to about 30 seconds.  
      In another aspect, a method for forming a hole in a blow molded structure is provided. A mold at least partially defining a cavity for creating the blow molded structure is provided, the mold having a mold surface against which a wall of the blow molded structure is blown during blow molding. The mold also has a hole formed through the mold surface. A punch is disposed within the hole in the mold surface of the mold, the punch having a cutting edge along the portion thereof which extends beyond the mold surface. A portion of the punch is extended out beyond the mold surface in an initial position such that the wall of the blow molded structure is initially formed therearound during blow molding, and the structure is then blow molded. After waiting for a time interval after blow molding, the time interval selected so that at least a portion of, and preferably a majority by volume of, the material forming the wall of the blow molded structure remains in a molten state, the punch is moved to an activated position wherein the punch extends out beyond the mold to an extent greater than in the initial position. A hole is thus formed through the wall of the blow molded structure with the cutting edge of the punch along a line substantially perpendicular to a direction of travel of the punch from the initial position to the activated position.  
      Preferably, the blow-molded structure comprises a double wall blow molded structure. Also preferably, a shelf is formed by the punch surrounding the hole created thereby during the step where the hole is formed. Most preferably, a shape of the shelf is variable by varying a parameter selected from the group consisting of the shape of the cutting edge of the punch, the temperature of punch, the temperature of the mold surface, the speed of movement of the punch, the timing of movement of the punch, and combinations of these. Preferably, the time interval after blow molding before moving the punch to the activated position falls within a range from about 5 seconds to about 45 seconds, and most preferably within a range from about 20 seconds to about 30 seconds.  
      The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a partially cross-sectional side view of a known hole configuration in which the hole extends through both walls of a double wall blow molded structure;  
       FIG. 2  is a partially cross-sectional side view of the known hole configuration shown in  FIG. 1  with a barbed insert member inserted therein;  
       FIG. 3  is a partially cross-sectional side view of a known hole configuration in which the hole is formed in only one wall of a double wall blow molded structure;  
       FIG. 4  is a partially cross-sectional side view illustrating a known method for forming a hole in which the hole extends through only one wall of a double wall blow molded structure;  
       FIG. 5  is a partially cross-sectional side view illustrating a method for forming a hole in which the hole extends through only one wall of a double wall blow molded structure in accordance with one embodiment of the present invention; and  
       FIG. 6  is a partially cross-sectional side view an embodiment of a hole in which the hole extends through only one wall of a double wall blow molded structure formed in accordance with the method illustrated in  FIG. 5 . 
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION  
      Referring to  FIGS. 5 and 6 , a hole  98  and an apparatus and method for forming hole  98  in a double wall blow molded structure  99  in accordance with the present invention are shown.  FIG. 5  shows a shaped punch  100  in position to pierce a molded wall  102  of plastic material. As shown in the Figure, while the object is being blow-molded, a portion of punch  100  extends out beyond the mold surfaces  101 , and wall  102  is initially molded therearound. At the instant that wall  102  is blown against punch  100  and its surrounding mold surfaces  101 , the material is hot and molten. The skin  104  of the material, perhaps 0.002 in. to 0.004 in. thick, freezes almost instantly against the cooler mold  101  and punch  100 . In so doing, this skin  104  forms a heat insulator between the mold  101  and punch  100  and the molten interior plastic  106  adjoining skin  104 . This remaining interior plastic material  106  continues to cool, but progressively more slowly as the thickening skin  104  insulates it from the cool mold  101  and punch  100 .  
      Punch  100  includes a cutting edge  108  surrounding a periphery thereof. While the precise configuration of the cutting edge  108  may be varied, it has been found that a punch  100  having a concave cutting edge  108  provides desirable results.  
      To form hole  98  of the present invention, punch  100  is activated early in the molding cycle, when most of the plastic wall  102  remains molten or almost so. Doing so shears the frozen plastic skin  104  immediately adjacent to cutting edge  108  of punch  100 . However, this break does not then continue in a direction parallel to the stroke of punch  100 . Rather, the break travels outwardly, generally parallel to the face of punch  100 , generally along line  110 , thereby creating a precisely controllable hole sidewall dimension consisting of molded sidewall  112  plus an increment equal to the thickness of the sheared skin  104 . Furthermore, as best seen in  FIG. 6 , the back edge  114  of hole  98  is almost perpendicular to the hole sidewall  1   12 , thereby providing a useful shelf  116  behind which a barb or other interference configuration can seat itself.  
      It should be understood by those skilled in the art that the precise time during the molding process which punch  100  is activated will vary depending upon a number of considerations, including the material used for creating sidewall  102 , the temperature of the material used for creating sidewall  102 , the temperature of mold  101  and punch  100 , the thickness of sidewall  102 , etc. However, it is to be understood that punch  100  is activated while at least some, and preferably a majority by volume, of the material forming sidewall  102  is in a molten state. For purposes of illustration, and not limitation, it has been found that in a typical situation wherein a hole having an approximate diameter of 0.375 in. is to be formed in an olefin plastic wall having an approximate thickness of 0.090 in. and a temperature of 350° F. using a convex punch having a configuration similar to that shown in  FIG. 5  and operating at normal room temperature, the punch should preferably be activated approximately 5 to 45 seconds after molding. During this time period, it has been found that the thickness of the skin of frozen material is approximately 0.008 in. to 0.010 in. Even more optimal results have been found to result when the punch is activated approximately 20 to 30 seconds after molding, when the thickness of the skin of frozen material is approximately 0.004 in. to 0.007 in.  
      In addition, it should be understood that the geometry of shelf  1   16  can be varied by changing the geometry of the face of punch  100 , by varying the temperature of punch  100  and its surrounding mold surfaces  101 , and by altering the speed or timing of punch travel.  
      The present invention, therefore, provides an apparatus and method for forming holes in a plastic article which can be used to form blind holes through one wall of a double wall blow molded structure, which can be used to form holes enlarged at their inner ends in order to accommodate barbed inserts, which can be used to form holes open to the space between the walls, and which can be used to form holes in which the wall thickness of the plastic in the punched cross-section is precisely controllable.  
      Although the present invention has been described with reference to a particular arrangement of parts, features and the like, it should be understood that the invention is in no way limited to the precise embodiments illustrated, and the particular arrangement of parts and features herein described are not intended to exhaust all possible arrangements of parts and features. For example, various ratios of depth to cross-section are obtainable, interference surfaces other than a barb can be utilized on the insert to be installed, and other types of inserts, such as “pop” rivets, may be used. Indeed many other modifications and variations will be ascertainable to those of skill in the art.