Abstract:
An apparatus for forming threaded molded parts such as closures for containers having improved means for unthreading the formed part from the mold core. The apparatus includes a cavity part and a core part, the cavity part being rotatable relative to the core part, such that simultaneous rotation of the cavity and opening of the mold results in unthreading of the part from the core. The mold cavity is rotationally coupled to a mold cavity rotator which is preferably driven by means of a rack. The rotation of the mold cavity is timed with the mold opening step such that the threaded part is unthreaded from the core as the mold plates are separated.

Description:
FIELD OF THE INVENTION 
     This invention relates to injection molding, and more particularly to an apparatus and method for removing internally threaded plastic parts, such as container closures, from the mold core of an injection mold. 
     BACKGROUND OF THE INVENTION 
     Injection molding generally involves introducing molten plastic under pressure into a space defined between a core part and a cavity part of an injection mold. The molten plastic injected into the space is allowed to coot and thereby solidify to form a “part”, after which the core and cavity parts are separated. The part generally shrinks a small amount upon cooling and remains on the core part of the mold from which it must be removed or “stripped”. 
     The removal of internally threaded parts from a threaded core has in the past presented a problem. Parts with shallow threads may sometimes be forced off the core using a stripper plate. However, deeper threads would be damaged by any effort to force them off the core with a stripper plate, and therefore are preferably removed by rotation or “unthreading” of the part from the core. 
     A number of devices and methods are known for simultaneously unthreading and pushing a threaded part from a mold core. One example of such a device is described in U.S. Pat. No. 5,383,780 (McCready et al.). The McCready apparatus includes a rotatable stripper ring which surrounds the mold core. After the part is formed and the mold is opened by separating the cavity part from the mold core, the stripper ring is rotated and simultaneously lifted relative to the core, simultaneously unthreading and pushing the threaded part away from the core. It is also known in the prior art to provide a rotatable core which is unthreaded from the part. Examples of patents which utilize a rotatable core are U.S. Pat. No. 2,306,205 (Crosman, Jr.) and Japanese Patent Application No. 62-264923. 
     Known devices for stripping threaded parts from a mold core have numerous disadvantages, such as increasing the size and complexity of the molding apparatus and reducing the speed of the molding process. Increased complexity can result in increased equipment and maintenance costs, while increased size can limit the number of mold levels which may be accommodated in a molding apparatus. 
     Accordingly, an improved method and apparatus for removing threaded parts from plastic injection molds is required. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems of the prior art by providing an apparatus and a method for removing threaded parts from plastic injection molds. In the apparatus of the present invention, the cavity part of the mold is rotatable relative to the core part, such that simultaneous rotation of the cavity and opening of the mold results in unthreading of the part from the core. 
     The apparatus for forming threaded molded parts according to the present invention comprises a first mold plate and a second mold plate. The apparatus has a mold closed position in which a mold is defined comprising the mold core and the mold cavity, and a mold open position in which the mold core and mold cavity are separated by a sufficient distance so as to permit removal of the part from the apparatus. The first mold plate (also referred to herein as the mold core plate) carries the mold core, the core having an external threaded surface. The second mold plate (also referred to herein as the mold cavity plate) carries the mold cavity, which is rotatable about a mold axis parallel to the direction of relative movement of the mold plates. 
     The mold cavity is rotationally coupled to a mold cavity rotator which is preferably driven by means of a rack. The rotation of the mold cavity is timed with the mold opening step such that the threaded part is unthreaded from the core as the mold plates are separated. The rotator for the mold cavity preferably comprises a rotatable shaft and a radially extending drive pinion. The shaft has gear teeth and is driven by the rack. Thus, movement of the rack results in rotation of the drive pinion, which meshes with a pinion extending radially about the mold cavity, causing rotation of the mold cavity. 
     The apparatus according to the invention preferably also includes a third mold plate (also referred to herein as the mold stripper plate) which carries a stripper ring. The stripper ring is axially movable relative to the core and is used to eject the part from the core after it is completely unthreaded. Preferably, the stripper ring is provided with at least one air passage through which pressurized air can be passed to assist in ejecting the unthreaded part from the core. 
     Since the part is unthreaded by the rotating mold cavity, it is desirable to prevent the part from rotating relative to the mold cavity during unthreading. Plastic closures for containers typically have an outer serrated surface to assist in unscrewing the closure from the container. These serrations on the part correspond to serrations provided on an inner axial surface of the mold cavity and advantageously provide sufficient resistance to relative rotation of the part and the mold cavity during unthreading. 
     In order to ensure that the part becomes separated from the mold cavity after it is unthreaded from the core, the apparatus preferably includes means to hold the part to the core until after the part is substantially completely unthreaded and the mold cavity is withdrawn from the part. In a preferred aspect of the invention, a negative pressure is created in a space which is formed between the part and the mold core as the part is unthreaded, and preferably near the end of the unthreading operation. This negative pressure is preferably applied through an air passage extending through the core. 
     The apparatus and method for forming threaded molded parts have a number of advantages over the prior art. Firstly, the mechanism for rotating the mold cavity is relatively simple, which can reduce equipment and maintenance costs and improve reliability. Secondly, the mechanism for rotating the mold cavity is relatively compact, permitting it to be housed in a single mold plate. This may permit a greater number of mold levels to be stacked in the press of an injection molding apparatus. Thirdly, in the apparatus and method of the invention, the part is unthreaded from the core as the mold plates are separated, eliminating the need for a separate mold opening step, thus improving the speed of the molding process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectional view through a preferred apparatus according to the present invention in the “mold closed” position; 
     FIG. 2 is a sectional view through the apparatus of FIG. 1 after unthreading of the part from the core; 
     FIG. 3 is a sectional view through the apparatus of FIG. 1 in the “mold open” position, and prior to ejection of the part from the core; 
     FIG. 4 is a sectional view through the apparatus of FIG. 1 after ejection of the part by the stripper ring; and 
     FIG. 5 is a perspective view of a part which is molded in the apparatus of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred apparatus according to the present invention is generally indicated by reference numeral  10  in the drawings. The apparatus  10  comprises three mold plates, namely a mold core plate  12 , a mold cavity plate  14  and a mold stripper plate  16 . A mold core  18  is mounted in the mold core plate  12 , a mold cavity  20  is rotatably mounted in the mold cavity plate  14 , and an annular stripper ring  22  is mounted in the mold stripper plate  16  and surrounds the mold core  18 . The apparatus  10  further comprises a gate insert  24  including an injection nozzle  26  through which molten plastic is supplied into the mold. 
     In the mold closed position shown in FIG. 1, the mold cavity  20  and the stripper ring  22  engage one another along line A, and the core plate  12  and the stripper plate  16  engage one another along line B. The mold core  18  defines an inner surface of the part  30 , and the mold cavity  20 , stripper ring  22  and gate insert  24  together define an outer surface of the part  30  mold  28  in which a part  30  is formed. 
     In the preferred embodiment of the invention shown in the drawings, the part  30  comprises an internally threaded closure, such as a threaded lid for a container. The inner surfaces of the threaded part  30  are defined by the mold core  18 , which has an external threaded surface  32  extending generally parallel to mold axis M and a planar surface  34  transverse to the mold axis. The mold core  18  preferably comprises a mold core body  38  having a depression in its upper surface in which is received a generally cylindrical core insert  40  having a side wall  42 . Internal passages  44  are provided through the core body  38  and core insert  40  for coolant circulation and an air passage  46  extends along the mold axis upwardly through the core body  38  and into the core insert  40 , where it is redirected to an opening provided in the side wall  42  of core insert  40 . The air passage  46  thereby communicates with an annular venting gap  48  between the core body  38  and core insert  40 . Venting gap  48  opens into the mold  28  and permits escape of air from the mold  28  during injection of plastic. However, gap  48  is sufficiently narrow that molten plastic will not substantially enter the gap  48  during molding. 
     A major portion of the outer surface of the threaded part  30  is defined by the mold cavity  20  and the gate insert  24 . The mold cavity  20  comprises a generally annular body  50  having an inner surface which defines a portion of the outer surface of the part  30 . In the preferred embodiment shown in the drawings, the mold cavity  20  has an inner axial surface  52  which forms an outer circumferential surface  54  of the part  28 . Preferably, the inner axial surface  52  of mold cavity  20  is provided with serrations  56  (not shown) to form corresponding serrations  58  (FIG. 5) on the outer circumferential surface  54  of the part  30 . The mold cavity  20  also has an inner transverse surface  60  defining an outer peripheral surface  62  of the top of part  30 . The central portion  64  of the top of part  30  is defined by a transverse surface  66  of gate insert  24 . 
     The mold cavity  20  also comprises an annular pinion gear  68  having teeth  70 . Pinion gear  68  is secured to annular body  50  and extends radially outwardly therefrom. The mold cavity  20  comprising annular body  50  and pinion gear  68  is rotatable about the mold axis on bearing surfaces  72 ,  74  and  76 . 
     The apparatus  10  also comprises a mold cavity rotator  78  comprising an axially extending shaft  80  which is mounted in mold cavity plate  14  for rotation parallel to the mold axis. The upper end of shaft  80  is provided with a radially extending drive pinion  82  having teeth  84  which mesh with the teeth  70  of pinion gear  68 . The shaft  80  is provided with teeth  86  below the drive pinion  82  which engage teeth on a rack  88  which is movable transverse to the mold axis. Followers  90  are mounted in the mold cavity plate  14  in order to maintain engagement between the rack  88  and the shaft  80 . 
     As shown in the drawings, the gate insert  24  has an outer tapered surface  92  which has a truncated conical shape and which tapers inwardly and downwardly toward transverse surface  66  of gate insert  24 . The tapered surface  92  of gate insert  24  forms a seat against a conical inner surface  94  of the mold cavity  14  in the mold closed position shown in FIG.  1 . The gate insert  24  is movable along the mold axis relative to the mold cavity plate  14 , allowing the gate insert  24  to be slightly withdrawn out of engagement with the mold cavity  20  after molding of part  28  and prior to unthreading. For example, the gate insert  24  may be withdrawn by about 0.03 inches. Preferably, the initial separation of gate insert  24  from cavity  20  is produced by disc spring assembly  100  shown in FIGS. 2 to  4 . 
     The apparatus  10  further comprises a stripper ring  22  which surrounds the core  18  and is mounted in mold stripper plate  16 . After the part has become substantially unthreaded as shown in FIG. 3, the mold core plate  12  and the mold stripper plate  16  are axially moved away from one another, causing the stripper ring  22  to eject the part from the core  18  as shown in FIG.  4 . The stripper ring  22  is preferably provided with at least one air passage  96  through which pressurized air can be passed in order to assist in ejecting the part  30  from core  18  after unthreading has been substantially completed. Air passages are preferably directed upwardly and inwardly so as to direct the pressurized air inside the part  28 . More preferably, a plurality of such air passages  96  are provided. The term “substantially unthreaded” is used herein to indicate that after the unthreading operation a portion of the thread of the part  30  may remain engaged with the thread of the core  18 . Therefore, the apparatus  10  preferably includes stripper ring  22  to ensure that the part  30  will be removed from core  18 . 
     A preferred method for molding an internally threaded plastic part according to the invention is now described below. 
     The first step in the method of the invention is to mold part  28  with the mold plates  12 ,  14  and  16  in the mold closed position shown in FIG.  1 . In this position, the mold cavity  20  and stripper ring  22  engage one another along line A, and mold plates  12  and  16  engage one another along line B. In addition, gate insert  24  is completely inserted into mold cavity  20 , such that the outer tapered surface  92  of the gate insert  24  is sealed against the inner tapered surface  94  of mold cavity  20 . 
     After the molding operation has been completed, the gate insert  24  is axially withdrawn slightly from the mold cavity  16 , preferably by about 0.03 inches, such that a space is formed between the outer tapered surface  92  of the gate insert  24  and the inner tapered surface  94  of the mold cavity  20 . At this point, there is preferably no axial movement of mold cavity plate  14 , and therefore the serrations  56  on the mold cavity  20  remain engaged with the serrations  58  on part  30 . 
     The rack  88  is then moved transverse to the mold axis in order to cause rotation of the shaft  80  in the clockwise direction. Rotation of shaft  80  with drive pinion  82  causes rotation of the mold cavity  20  in a counterclockwise direction. 
     As the mold cavity  20  is rotated, the mold cavity  20  and mold core  18  become separated to open the mold  28 . Preferably, the rate of separation of the mold cavity  20  and the mold core  18  during unthreading is substantially the same as the rate of axial movement of the part  30  relative to core  18 , thus ensuring continued engagement between mold cavity  20  and part  30  during unthreading. The rate and timing of separating the mold cavity  20  and the mold core  18  relative to rotation of the mold cavity  20  is controlled by any convenient means (not shown), and is preferably controlled by program logic. The gate insert  24  moves with the cavity plate  14  so as to maintain the clearance between the gate insert  24  and the mold cavity  20  during unthreading of part  30 . 
     After the part  30  becomes substantially unthreaded from core  18  as shown in FIG. 2, the mold cavity  20  and the mold core  18  can be more rapidly separated from one another to the mold open position shown in FIG. 3, the separation being sufficient to allow ejection of the part  30  from core  18 . During separation of the mold cavity  20  and mold core  18  to the mold open position, the mold cavity  20  becomes separated from part  30 , which remains on the core  18 . In order to ensure that the part  30  remains on the core  18  during removal of cavity  16 , a negative pressure is created in the space  98  between the part  30  and the core  18  during separation of the mold cavity  20  and the mold core  18  and and until the mold cavity  20  becomes completely separated from part  30 . The negative pressure is created by vacuum means (not shown) remote from the mold  26 , and is applied to the space  98  between part  30  and core  18  through the air passage  46  and venting gap  48 . Preferably, the negative pressure is applied only during the last portion of the unthreading step and during withdrawal of the mold cavity  20  away from part  30  in order to prevent “dishing” of the top surface of part  30 , which may occur as a result of excessive application of negative pressure to space  98 . 
     After the apparatus  10  is opened to the mold open position, the unthreaded part  28  is ejected from the core  18 . In order to eject part  28 , the application of vacuum is discontinued and the mold core plate  12  and mold stripper plate  16  are axially separated from one another as shown in FIG. 4, thereby causing stripper ring  22  to push the part  30  from the core  18 . As discussed above, pressurized air is preferably passed through air passages  96  in stripper ring  22  to assist in ejection of part  30 . 
     Although the invention has been described with reference to certain preferred embodiments, it is not intended to be limited thereto. Rather, the invention includes all embodiments which may fall within the scope of the following claims.