Abstract:
Apparatus for compression molding plastic closures having a peripheral skirt with an internal thread includes a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved relative to the other to bring the male mold core into the female mold cavity for compression molding a closure, and then is moved to open the cavity such that the closure is retained on the male mold core. The male mold core is rotated relative to the first mold assembly to unthread the core from within the closure, and thereby strip the closure from the core, without stretching the closure skirt over the core or wiping the internal thread over the external surface of the mold core.

Description:
The present invention is directed to an apparatus and method for compression molding plastic closures having a peripheral skirt with an internal thread, and more particularly to an apparatus and method for stripping the closure from a male mold core by rotating the core so as to unthread the closure. 
     BACKGROUND AND OBJECTS OF THE INVENTION 
     U.S. Pat. Nos. 5,554,327, 5,670,100 and 6,074,583, assigned to the assignee of the present application, disclose apparatuses and methods for compression molding plastic articles such as plastic closures. A plurality of male and female mold assemblies are mounted on a turret in opposed circumferential arrays. The assemblies are coupled to fixed cams for movement into engagement with each other to form a mold cavity and compression mold a plastic charge into a closure, and then to move the assemblies axially away from each other to open the mold cavity. As the male mold assembly is retracted from the female mold assembly, the molded closure is stripped from the male mold core by a stripper sleeve that pushes the closure axially off of the core. The internal thread or threads on the closure skirt are thus wiped across the outer face of the mold core. 
     Although the arrangements disclosed in the noted patents have enjoyed substantial commercial acceptance and success, further improvements remain desirable. In particular, although the apparatuses and methods disclosed in the noted patents work well with relatively flexible and resilient thermoplastic materials such as polypropylene, they do not work as well as desired with thermosetting materials or with relatively stiff thermoplastic materials such as PET. When employing such materials, the closure cannot be axially stripped from the male mold core by expanding the closure skirt and wiping the threads across the radial surface of the core. The closure skirts are not sufficiently resilient for stripping by this technique, and the threads can become damaged. It is therefore a general object of the present invention to provide an apparatus and method for compression molding plastic closures that include facility for rotating the male mold core with respect to the outer holding member(s) for stripping the closure by unthreading the male mold core from within the closure. Another and more specific object of the invention is to provide an apparatus and method of the described character suitable for implementation in a continuously rotating molding apparatus of the type disclosed in above-noted U.S. Pat. Nos. 5,554,327, 5,670,100 and 6,074,583. 
     SUMMARY OF THE INVENTION 
     Apparatus for compression molding plastic closures having a peripheral skirt with an internal thread (or threads) in accordance with a presently preferred embodiment of the invention includes a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved relative to the other to bring the male mold core into the female mold cavity for compression molding a closure, and then is moved to open the cavity such that the closure is retained on the male mold core. The male mold core is rotated relative to the first mold assembly to unthread the core from within the closure, and thereby strip the closure from the core without stretching the closure skirt over the core or wiping the internal thread over the external surface of the mold core. 
     In the preferred embodiment of the invention, the first mold assembly includes a first shaft mechanism carried by a housing for rotary and axial movement, with the male mold core being coupled to the end of the first shaft mechanism. The male mold core is rotated by rotating the first shaft mechanism relative to the housing. The first shaft mechanism is mounted between axially spaced brackets, and a ballscrew shaft is rotatably carried by the brackets parallel to the first shaft mechanism. Drive gears rotatably couple the first shaft mechanism to the ballscrew shaft in the preferred embodiment of the invention, and a ballnut actuator is coupled to the ballscrew shaft for controlling rotation of the ballscrew shaft. In the preferred embodiment of the invention, the ballnut actuator is coupled to a fixed cam for controlling position of the ballnut actuator and rotation of the ballscrew shaft with axial motion of the first mold assembly. The contour of the cam controls the rate of rotation of the ballscrew shaft, the first shaft mechanism and the male mold core as the first mold assembly rises. A stripper sleeve on the first mold assembly engages the skirt of the closure and holds the closure against rotation as the mold core is rotated and retracted from the closure. 
     In accordance with another aspect of the present invention, a method of compression molding plastic closures contemplates provision of a first mold assembly having a male mold core and a second mold assembly having a female mold cavity. At least one of the first and second mold assemblies is moved with respect to the other to bring the male mold core into the female mold cavity for compression molding a closure, and the at least one mold assembly is then retracted from the other such that the mold cavity is opened and the closure is retained on the male mold core The closure is removed from the male mold core by rotating the male mold core with respect to the closure so as to unscrew the mold core from the closure. This is accomplished by mounting a ballscrew shaft on the first mold assembly parallel to a first shaft mechanism to which the male mold core is secured, rotatably coupling the ballscrew shaft to the first shaft mechanism and rotating the ballscrew shaft, preferably by means of a ballnut actuator. The ballnut actuator may be held in fixed position so that the ballscrew shaft, the first shaft mechanism and the male mold core are rotated in opposite directions as the male mold core is lowered and raised, or more preferably is coupled to a fixed cam for controlling rotation of the ballscrew shaft, first shaft mechanism and male mold core as a function of axial movement of the male mold core. In the preferred embodiment, a plurality of first and second mold assemblies are mounted on a rotating turret in such a way that each assembly pair executes a complete operating cycle in sequence with the remaining assembly pairs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objects, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
     FIG. 1 is a sectioned elevation view of apparatus for compression molding plastic closures in accordance with a presently preferred embodiment of the invention in a fully open condition; 
     FIG. 2 is a sectioned elevational view similar to that of FIG. 1 but showing the mold assemblies in the closed position; 
     FIG. 3 is a sectioned view on an enlarged scale of the first or upper mold assembly in the embodiment of FIGS. 1 and 2; 
     FIG. 4 is a side elevational view of the first mold assembly illustrated in FIG. 3; 
     FIG. 5 is a fragmentary elevational view of cams for operating the first mold assembly illustrated in FIGS. 1-4; 
     FIGS. 6A-6E are sectioned elevational views that illustrate the apparatus of the invention, particularly the first or upper mold assembly, in sequential stages of operation; and 
     FIG. 7 is a fragmentary sectional view on an enlarged scale of the lower portion of the upper tool assembly in FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The disclosures of U.S. Pat. Nos. 5,554,327, 5,670,100 and 6,074,583 are incorporated herein by reference. 
     The drawings illustrate an apparatus  10  for compression molding plastic closures in accordance with a presently preferred embodiment of the invention. A plurality of first or upper mold assemblies  12  and a plurality of second or lower mold assemblies  14  are mounted in axially opposed circumferential arrays on a rotatable turret  16 . In lower or second mold assembly  14 , a female mold cavity  18  is mounted on tooling  19  at the upper end of a tooling actuator  20 . A plunger  22  is slidably mounted within actuator  20 , being urged upwardly by a coil spring  24 . Spring  24  is captured in compression within the lower end of plunger  22  by means of a plug  26  sliding within actuator  20 . A nitrogen cylinder  28  is mounted within actuator  20 , with a piston in engagement with the lower face of plug  26 . A plurality of lift pins  30  are mounted on a lift pin retainer  32  at the upper end of actuator  20 , and are biased upwardly by a plurality of coil springs  34  captured within actuator  20 . A pair of cam rollers  36 ,  38  are carried at the lower end of actuator  20 , and engage respective fixed cams  40 ,  42  that are carried in fixed position on a turret frame. Actuator  20  is slidably mounted in a support body  44  that is mounted on turret  16 , and vertical motion under control of cams  40 ,  42  is guided by a rod  46  slidably received in a bore  48  within support body  44 . Rod  46  and bore  48  prevent rotation of actuator  20  about its axis. Second or lower mold assembly  14  is substantially the same as that disclosed in above-referenced U.S. Pat. No. 5,554,327. 
     First or upper mold assembly  12  includes an upper bracket or support body  50  secured to turret  16  in axial overlying alignment with lower support body  44 . A first or actuator shaft mechanism or assembly  51  includes a first shaft  52  slidably mounted within body  50  by spaced linear bearings  54 . The upper end of actuator shaft  52  is connected to an upper actuator bracket  56 . A pair of cam rollers  58 ,  60  are mounted on upper actuator bracket  56  for engagement with respective fixed cams  62 , 64  carried by the turret frame. Shaft mechanism  51  also includes a gear shaft  66  that is rotatably mounted by axially spaced thrust bearings  68  within the hollow lower end of actuator shaft  52  for rotation about the axis of shaft  52 . A spur gear  70  is secured to the lower end of shaft  66 . A second or ballscrew shaft  72  is mounted by sleeve bearings  74  within axially spaced radially outwardly extending arms  76  integral with body  50 . Shaft  72  is rotatably mounted on bracket  56  by a thrust bearing  78 , and is rotatably mounted in a manifold bracket  80  by a thrust bearing  82 . Manifold bracket  80  encircles the lower ends of both shafts  72 ,  66 , and includes suitable fittings  84  for connection to a source of tooling coolant. Shaft  72  has a section  86  disposed between arms  76  having a spiral ballscrew slot encircled by a ballnut  88 . Ballnut  88  is coupled to a ballnut actuator  90 , which is secured to a pair of laterally spaced shafts  92  that are mounted by linear bushings  94  in arms  76 . A cam roller  96  is mounted on ballnut actuator  90 , and is disposed within the slot  98  of a cam  100 . Thus, ballnut actuator  90  and ballnut  88  move axially with respect to housing  50  under control of cam  100 . Motion of ballnut  88  relative to ballscrew section  86  rotates shaft  72  about its axis, which is parallel to the axis of actuator shaft  52 . The lower end of shaft  72  is coupled by a spur gear  102  to spur gear  70  on shaft  66 . Thus, axial motion of ballnut actuator  90  relative to shaft  72  causes rotation of shaft  66  about its axis. Cams  62 ,  64  and  100  are mounted in fixed position on the turret frame. 
     The lower end of shaft  66  is coupled by a drive slot  104  and a drive tenon  106  (FIG. 4) to a core assembly  107 . Core assembly  107  includes an inner core sleeve  108  (FIGS. 3,  4  and  7 ), a core bushing  112  secured to an end of inner core sleeve  108 , and a mold core  110  secured to an end of core bushing  112 . Core assembly  107  moves as a unit under control of drive slot  104 , both axially under control of shaft  52  and rotationally under control of shafts  66 ,  72 . Drive slot  104  and tenon  106  accommodate a small amount of axial movement between shaft  66  and core assembly  107 . An outer core sleeve  114  surrounds sleeve  108 , and core assembly  107  is urged upwardly relative to outer core sleeve  114  by a coil spring  116  captured in compression between a thrust race  120  and an upper flange  118  on core sleeve  114 . A stripper  122  has a flange  124  engaged by a plurality of coil springs  126  that are captured in compression between flange  124  and a spring retainer  128 . Springs  126 , retainer  128 , sleeve  118  and stripper  122  are surrounded by a tool body  130 , with retainer  128  being attached to body  130  by bolts  131 . A sleeve  132  is secured to core sleeve  114  to form a core sleeve assembly  133  that is slidable within body  130 . A bracket  135  on core sleeve assembly  133  surrounds drive slot  104 . Manifold bracket  80  is laterally slidably secured to bracket  135 . A coolant manifold  134  has fittings  84  for circulation of coolant through core assembly  107 . Thrust race  120  is held in contact with manifold  134  by spring  116 , and slidably surrounds sleeve  108  of core assembly  107 . 
     In operation, with the mold assemblies spaced from each other in the position of FIG. 1, a charge of molten plastic material is delivered to female mold cavity  18 . The preferred structure for delivering molten plastic charges to the open female mold cavities is illustrated in U.S. Pat. No. 5,603,964, the disclosure of which is incorporated herein by reference. Continued rotation of turret  16  brings the first and second mold assemblies from the fully open position of FIG. 1 to the fully closed position of FIGS. 2 and 6A by operation of rollers  36 ,  58  and cams  40 ,  62 . Mold core assembly  107  (comprising inner core sleeve  108 , core bushing  112  and mold core  110 ) is driven downwardly by cam  62 , shaft mechanism  51  (comprising shafts  52 ,  66 ) and core sleeve assembly  133  (comprising core sleeve  132 , sleeve  114  and bracket  135 ). In this closed position, the closure is molded and allowed partially to cure, as disclosed in referenced U.S. Pat. No. 5,554,327. Mold core  110  and core sleeve  114  are in the fully lowered position, with flange  118  of core sleeve  114  adjacent to stripper spring retainer  128 . Lower mold assembly  14  is fully raised, and cavity springs  34  are compressed. 
     Lower mold assembly  114  is then retracted to the lowered position of FIGS. 1 and 6B by operation of roller  38  and cam  42 . At this point, the molded closure  136  is held on core  110  by the internal thread or threads molded onto the closure being still engaged with the corresponding grooves on the core. Sleeve  114  will have moved down to its lower stop position, and the closure will remain on the core engaged by the core threads and under the force control of core spring  116 . Compression of spring  116  causes a small separation between core  110  and core sleeve  114 . Ballnut  88  moves upwardly synchronously with upward retraction of mold core assembly  117 . As retraction continues from the position of FIG. 6B to that of FIG. 6C, and upper mold assembly  12  continues to move upwardly under control of cam  64  and roller  60  (FIG.  3 ), shoulder  138  on connecting sleeve  132  engages thrust race  120 . At this point, there is an overwhelming force exerted on core assembly  107  through thrust race  120  and bushing  134 , which retracts core  110  in unison with upper mold assembly  12 . However, cam track  98  of cam  100  (FIGS. 3 and 5) moves ballnut actuator  90  and ballnut  88  downwardly relative to body  50 , thereby imparting rotary motion to shafts  72  and  66 , slot  104 , tenon  106  and core assembly  107 , rotating core  110  in a direction to unthread from closure  136 . Selection of a profile for cam track  98 , in conjunction with cam  64 , can add to or subtract from rotation of ballscrew shaft  72 . Stated differently, ballscrew actuator  90  could be held in fixed position on turret  16 , so that core  110  would be rotated in one direction as it is lowered and in the other direction as it is raised. However, cam  100  and track  98  provide additional flexibility for tailoring the rotation of the mold core. By selecting the profile of cam  100 , a condition can be established during unscrewing such that the unscrewing rate maintains the condition whereby the mold core assembly remains axially separated from the core sleeve, but shoulder  138  does not contact thrust race  120  until the closure is unscrewed and ready to be ejected. This motion control ensures that the closure is unscrewed with the substantially controlled axial force of the core spring, maintaining the closure in engagement with the stripper and ensuring that the stripper remains in engagement with the closure skirt, effectively resisting rotation of the closure during unscrewing and allowing final ejection of the closure after unscrewing is completed. Thus, in FIG. 6C, closure  136  is partially ejected, and in FIGS. 6D and 6E, the closure is fully ejected from core  110 . 
     Due to tolerance variations in the manufacture and assembly of apparatus  10 , there will be variation in the timing of retraction of core  110  and rotational position of core  110 . This variation is accommodated by core spring  116  and the axial separation of core  110  from core sleeve  118  (FIGS.  6 B and  6 C). As soon as the unscrewing is completed to the extent that the closures is ready to be ejected, cam  100  raises ballnut actuator  90  to the reset position of the ballnut in its upper position of FIG.  6 A. This upper position is high enough that the remaining downward travel of the ballnut (FIGS. 6B to  6 D) is sufficient to complete the unscrewing function. Cam  100  preferably is profiled such that the ballnut would move down synchronously with the ball shaft and thus substantially no rotation of the core would occur during molding. However, slight rotation could be imparted to the core during forming to reduce the effect of “weld fronts” in the molded part, or to impart a spiral flow to the plastic material during molding. 
     There has thus been disclosed an apparatus and method for molding plastic closures that fully satisfy all of the objects and aims previously set forth. A number of modifications and variations have been disclosed. Other modifications and variations will readily suggest themselves to persons of ordinary skill in the art. For example, intermeshing gears  70 ,  102  could be replaced by a belt or chain drive. If the closure is molded with a left-hand thread, it may be necessary to increase the length of ballscrew shaft  72  so as to increase the differential speed of the ballnut as the tooling is retracted upwardly. As an alternative, the ballscrew shaft could be changed to one having opposite pitch. The invention is intended to embrace all such modifications and variations as fall within the spirit and broad scope of the appended claims.