Method for forming cap with tear line

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.

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.

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 58a 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.