Abstract:
A mold for injection molding of a plastic bottle cap has a cavity which defines the outside slope of the top and skirt of the cap. A core and a core insert fixed to the top of the core define the inside of the cap and constitute a single, non-moving part. A tapered stripper ring below the cavity engages the base of the cap skirt. After the cap has been injected through a hot tip in the cavity, the cavity is retracted. The stripper ring is then elevated relative to the core by means of ejector pins. As this is proceeding, air is released under the top of the cap to break vacuum tending to hold the cap on the cap insert. The cap is thus ejected from the mold. A reduced cycle time is achieved. The wall thickness of the cap may be reduced, thus reducing material cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a new and improved apparatus and method for forming a plastic cap of the type used with five gallon water bottles. More particularly the invention relates to the mold for forming such a cap and the method whereby the cap is fabricated. 
     2. Description of Related Art 
     Caps resembling the cap illustrated in the drawings and hereinafter described have been molded on injection molding equipment for many years. The particular cap referred to is similar to part of the disclosure in U.S. Pat. No. 5,232,125. By reason of the molding apparatus and method of the present invention, the cap may be made much more rapidly and with less use of material. 
     SUMMARY OF THE INVENTION 
     The cap shown in some of the drawings and hereinafter described is of a particular style known as “non-spill”. The well of the cap is closed with a plug and the cap is applied to a bottle such as a five gallon bottle used for spring water and the like. It will be understood that the present invention may be used in the fabrication of other caps wherein there is a top and a skirt depending from the periphery thereof. 
     The molds hereafter described may be installed in conventional plastic injection molding machines. The molds and the method of forming the cap make possible the use of thinner walls throughout the cap resulting in reduction in the weight of material. In addition, the cycle time for operation of the molding machine is substantially reduced. Reduction in cycle time and reduction in weight are great economic advantages. By way of example, the weight of a cap in accordance with the present invention may be reduced from approximately 13 grams to approximately 8 grams and the cycle time may be reduced from about 15 seconds to about 8 seconds by practice of the present invention. 
     One of the features of the invention is that the cap is ejected from the mold by pushing upward against the bottom edge of the cap skirt, whereas previously a push pin has pushed against the top of the cap. A push pin method of ejection requires the top wall thickness be much greater then actually necessary for cap function. Otherwise unacceptable top “doming” on ejection will result. A further disadvantage of using a push pin to push up against the underside of the top of the cap is that the top tends to be deformed upward resulting in the upper portion of the skirt being stressed to pull inwardly against the mold, thereby increasing the force necessary to eject the cap and thereby requiring thicker walls. In accordance with the present invention, by pushing upward on the bottom edge of the skirt, the stress on the cap top is considerably less and inward distortion of the skirt of the cap is avoided. This makes possible the thinning of the wall thickness of the cap skirt. Increased wall thickness results in substantial material requirements for prior art closures. 
     Another feature of the present invention is that in practical effect the core around which the cap is molded does not employ parts which slide relative to each other. In previous mold constructions, the interior of the skirt is defined by a core which is hollow and within the hollow is a push pin which slides relative to the core during a portion of the cycle of operation. Since in accordance with the present invention the parts do not move relative to each other, cooling of the mold is much improved and heat transfer through relatively moving surfaces is avoided. 
     Still another advantage of the invention is the fact that the parting line between the cavity portion of the mold and the core part of the mold coincides with the bottom edge of the cap skirt. Such a construction makes it possible to include ribbing or design material on the outside of the lower portion of the skirt. 
     Another feature of the invention is that the stripper ring which originally is in contact with the core and the core have mating upwardly inwardly tapering surfaces. Accordingly when the stripper ring moves upward relative to the core there is no potential for galling which might inhibit movement of the parts. One of the problems with molds heretofore used to fabricate caps of the type used with the present invention is that the parts of the mold tend to gall inasmuch as there are three separate parts in the “lower” part of the mold, all of which must move relative to each other. Further, because the parts slide relative to each other, heat transfer between the mating surfaces is poor because there must be sufficient clearance between the parts to permit such movement. These problems are solved in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description serve to explain the principles of the invention. 
     FIG. 1 is a schematic vertical sectional view through a mold used in the prior art. 
     FIG. 2 is a view similar to FIG. 1 showing somewhat schematically a mold in accordance with the present invention. 
     FIG. 3 is an enlarged vertical sectional view through a portion of the mold of FIG.  2 . 
     FIG. 4 is a view similar to FIG. 2 showing completion of a step in the operation of the mold. 
     FIG. 5 is a view similar to FIG. 4 of a still later step in the operation of the mold. 
     FIG. 6 is vertical sectional view of a cap according to the current invention positioned on the neck of a container. 
     FIG. 7 is a side view of one form of cap corresponding to the current invention. 
     FIG. 8 is a vertical sectional view of a different cap embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     A typical cap formed by the molds and method hereinafter described and a complementary bottle finish is best seen in FIG.  6 . Cap  10  has a top  11 , here shown as being annular, with a well  12  formed centrally thereof. The well  12  is closed with a plug (not shown), all as described in U.S. Pat. No. 5,232,125. The present invention may, however, be used with other caps. For example, the cap  110  shown in vertical section in FIG. 8 is similar to that shown un FIG. 6 with the exception that well  12  is absent in the cap shown in FIG.  8 . Referring again to FIG. 6, depending from the periphery of top  11  is a skirt  13  having a bottom edge  14  and an internal locking bead  16  which cooperates with complementary retention structure on a large water bottle (neck finish  121 ), as well understood in the art. At at least one location on skirt  13  is a tear line (not shown but well understood in the art). One construction to provide for such a tearline is hereafter described. As shown in FIG. 7, depending from the bottom edge  14  is a tear tab  18  located in proximity to the tear line which the user may grip to tear the skirt  13  in order to remove the cap  10  from the bottle neck. 
     Locking bead  16  fits snugly below the upper “crown” portion  120  of the bottle finish  121 . A lower reduced diameter portion  122  of the bottle finish is spanned by the vertically extending lower skirt  13  of the closure  10 . At the base of the lower closure skirt the skirt cross-section is caused to be slightly thicker by incorporating an inward projection  124 . This inward projection reduces the internal diameter of the cap skirt at its lower terminus resulting in a snug fit between cap and bottle to prevent dirt and other contaminants from gaining access to the upper portion of the finish  121 . The thickened section at the base of the cap skirt also facilitates molding of the cap by preventing buckling as the stripper ring pushes up on the base of the skirt during ejection from the mold. 
     A line of weakness or tear line is incorporated in at least one location on skirt  13 . The tearline allows manual removal of the cap when desired. One construction to provide for such a tearline is illustrated in FIG.  7 . In FIG. 7, there is shown a side elevational view of the closure showing essentially vertical score lines  130 ,  131  positioned in the cap skirt  13  on either side of tear tab  18 . At least one of these dual weakened lines extends vertically through locking bead  16 . One method of extending the line of weakness through locking bead  16  is shown in FIG. 3, where steel mold projections  96  and  97  combine to extend the line of weakness through bead  16 . Only one of the dual score lines  130  extends close to the top of the cap, while the other  131  terminates at a location somewhat below the top. This is done to facilitate removal of the cap from the bottle. It has been found that if both tear lines extend to the top of the cap, continued pulling on the tab actually causes the locking ring  16  to increase its grip on the bottle, making the cap very difficult to remove. When only one tear line extends to the top of the cap, cap removal is greatly improved. 
     Heretofore caps  10  have been molded in accordance with the mold shown in FIG.  1 . The axes of such a mold, as well as that of the mold of the present invention are usually horizontal in use, but are illustrated and described in the specification and claims as vertical. What may be considered the upper part of the mold is commonly called a cavity  21  within which is a hot tip  22  through which molten plastic material (such as low density polyethylene) is injected into the space between the cavity  21  and the lower portion of the mold. In prior mold structure the parting line  23  between the two mold halves is located near, but substantially below, the top of the cap and considerably above the bottom edge thereof. 
     The lower portion of the mold comprises a core cavity  26 . Within core cavity  26  is a sleeve-like core  27  and within the sleeve  27  is a push pin  28 . In order to provide for water circulation, a push pin insert  29  is fixed to the upper end of the push pin. 
     In operation of the prior art mold shown in FIG. 1, with the parts closed as shown in FIG. 1 molten plastic is injected through the hot tip  22  into the space between the two mold halves. Thereupon the upper half or cavity  21  is separated from the lower portion of the mold. Push pin  28  and core  27  move upward relative to core cavity  26 , freeing the cap  10  from core cavity  26 . When the cap has cleared the core cavity  26 , core  27  ceases moving and push pin  28  continues moving, thereby stripping the cap from the core. 
     As heretofore discussed, the present invention provides considerable advantages over the prior art mold illustrated and hereinabove described. 
     FIG. 2 shows the two mold halves of the present invention closed. Cavity  36  includes a hot tip  37  here shown at an angle to the vertical axis. It will be understood that the hot tip  37  may be oriented in other positions. The parting line  38  between cavity  36  and core  41  is located at the bottom edge  14  of the cap skirt  13 . Fixed to the upper end of core  41  is a core insert  42 . On the exterior of core  41  is stripper ring  43  which is moved relative to core  41  by ejector pins  44 . 
     Considering cavity  36  in greater detail and with further reference to FIG. 3, the upper portion  46  thereof determines the shape of the upper surface of top  11  and the inner surface of well  12 . There is an annular depending protrusion  51  below top  46 . The inside of protrusion  51  determines the shape of the exterior of skirt  13 . The lower surface or bottom edge  52  of protrusion  51  (which comprises the parting line between the two mold sections) is at the level of the bottom edge  14  of skirt  13 . The exterior of cavity  36  is illustrated in FIGS. 2 and 3 but the details thereof form no part of the present invention. For example, the external groove  48  is for cooling fluid. A central projection  49  from upper portion  46  determines the shape of the inside of well  12 . 
     Core insert  42  fixed to the upper end of core  41  has a top surface  56  which determines the shape of most of the underside of top  11 . The shape of the remainder of the underside of top  11  is determined by the tip  57  of core  41 . The exterior of the annular extension  64  of core  41  determines the shape of the inside of skirt  13 , including bead  16 . The bottom  59  of insert  42  fits against the bottom  61  of a bore in core  41 , the portion of core  41  outside such bore constituting the annular extension  64  of core  41 . Projecting up from surface  61  are dowels  63  which, as best shown in FIG. 3, fit into holes  65 , thereby locating insert  42  relative to core  41 . 
     The purpose of using an insert  42  within the extension  64  of core  41  is to facilitate cooling of the members  41 - 42 . Thus, as shown in FIG. 3, there is an annular cooling water groove  66  which communicates with a duct  67  in core  41  and is connected to a source of cooling water. Although not shown in the accompanying drawings, it will be understood that there is another duct  67  through which cooling water from groove  66  may be withdrawn. Seals  58  and  58   a  seal core insert  42  to core  41 . 
     A recess  71  is formed in insert  42  into which is positioned a threaded fastener (not shown) to hold the parts  41  and  42  assembled. 
     Poppet valves  76  are located at several positions recessed below the top  56  of insert  42 . The valves  76  communicate with ducts  77  which in turn communicate with connecting ducts  78  here shown disposed at an angle to the vertical. The ducts  78  communicate with longitudinally extending ducts  79  in core  41 . The purpose of the valves  76  is to break vacuum tending to retain cap  10  and particularly the top  11  thereof onto the mold when the cap  10  is stripped from core  41 . 
     As best shown in FIG. 2, the exterior of core  41  comprises a downward outward tapered flank  83  and below the same is a collar  84 . In closed position the bottom surface  85  of mold stripper ring  43  rests on top of collar  84  and its top surface  86  mates with the bottom surface  52  of protrusion  51  of cavity  36 , the two surfaces  52  and  86  comprising the parting line  38 . It will be noted that the interior of ring  43  has an upward-inward tapered surface  88  which mates with surface  83  in the closed position of the mold. Collar  84  is formed with holes in which ejector pins  44  slide to move ring  43  relative to core  41 . 
     In order to form a tear line or thinning of the skirt  13 , a rib  96  is fixed to the interior of protrusion  51  (see FIG.  3 ). To further thin the skirt  13  in the vicinity of bead  16 , an insert  97  is added to extension  64 . To form pull tab  18 , a groove  98  is formed in stripper ring  43  extending downward near to bottom surface  85  of stripper ring  43 . 
     Operation 
     At the commencement of the cycle of operation the mold halves are closed and the parts are in the position shown in FIG.  2 . Molten plastic is injected into the space between the two mold halves from hot tip  37 . The next step in operation is the withdrawal of cavity  36 , the completion of this step being shown in FIG.  4 . 
     The next step in the separation of cap  10  from the mold is accomplished by the ejector pins  44  pushing stripper ring  43  upward relative to core  41 . The upper surface  86  of ring  43  engages under the bottom edge or base  14  of skirt  13 . Poppet valves  76  supply air under top  11 , facilitating breaking any vacuum between of the cap and core  41  and core insert  42 . The completion of this step is shown in FIG.  5 . The cap  10  is for practical purposes free of the mold halves and in accordance with conventional practice drops by gravity onto a conveyor which removes it from the molding machine. It is noted that as stripper ring  43  begins to move relative to core  41 , surfaces  88  and  83  immediately separate, eliminating any potential for galling, which, as discussed above, is a significant problem with the sliding components used in prior art methods. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.