Abstract:
A thread forming device for compression molding comprises a base positioned adjacent a compression molding device, a plurality of posts extending from the base, a carrier moveable along the posts and supporting at least one rotatable threaded shaft, each shaft being configured for forming a threaded bore during compression molding of a part, an assembly for rotatably driving the at least one threaded shaft for unthreading the at least one threaded shaft from the part, and one or more biasing members configured to push the carrier away from the base once the at least one threaded shaft is unthreaded from the part.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of provisional U.S. Application No. 61/738,971 filed on Dec. 18, 2012, the content of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to compression molding, and in particular to a thread forming device for compression molding and an apparatus incorporating same. 
       BACKGROUND OF THE INVENTION 
       [0003]    Compression molding of shaped composite material products involves compressing polymer resin in a mold under an applied pressure. To form an internal cavity within a compression molded part, it is necessary to provide an additional mold core that is separate from the mold, and that is withdrawn in a direction different from the direction of separation of the mold halves. As will be understood, when the internal cavity includes a threaded surface, the additional mold core cannot be simply axially withdrawn because doing so would interfere with the newly-formed threads. 
         [0004]    Several approaches for forming threaded bores during injection molding and compression molding have been described. For example, U.S. Pat. No. 5,135,700 to Williams et al. describes a method and apparatus for molding a product having internal threads that includes a threaded mold core and a device for rotating the mold core about the axis of its threads and for axially withdrawing the core from the mold at a rate that is related to the rate of rotation, according to the pitch of the threads. A cam is rotated in synchronism with the device that rotates the core, and a cam follower displaces the core as the cam is rotated. The cam follower is fixed with respect to the mold. The cam and threaded core are positioned on a carrier, along with the device for rotating the core, such that rotation of the cam displaces the carrier away from the mold. The core is inserted into the mold, prior to the molding process, by displacing the carrier toward the mold without rotating the core. In one embodiment, the cam is rotated by a unidirectional motor such that resetting of the cam after the core is withdrawn is accomplished by forward indexing of the cam a small amount to a zero position. 
         [0005]    U.S. Pat. No. 6,602,065 to Ingram describes an apparatus for compression molding plastic closures having a peripheral skirt with an internal thread. The apparatus 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. 
         [0006]    Improvements are generally desired. It is therefore an object of the present invention at least to provide a novel thread forming device for compression molding and apparatus incorporating same. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, in one aspect there is provided a thread forming device for compression molding, comprising: a base positioned adjacent a compression molding device; a plurality of posts extending from the base; a carrier moveable along the posts and supporting at least one rotatable threaded shaft, each shaft being configured for forming a threaded bore during compression molding of a part; an assembly for rotatably driving the at least one threaded shaft for unthreading the at least one threaded shaft from the part; and one or more biasing members configured to push the carrier away from the base once the at least one threaded shaft is unthreaded from the part. 
         [0008]    The device may further comprise: a push block moveable along the posts, the push block being configured to push the carrier toward the base for inserting the at least one threaded shaft into the compression molding device. The device may further comprise an actuator coupled to the push block for moving the push block along the posts. The device may further comprise a support block mounted to an end of said posts and supporting the actuator. The actuator may be a hydraulic actuator. 
         [0009]    The assembly may comprise a hydraulic motor. The hydraulic motor may be mounted on the carrier. The assembly may comprise a gear box configured for coupling the hydraulic motor to the at least one threaded shaft. 
         [0010]    The at least one rotatable threaded shaft may comprise two rotatable threaded shafts. 
         [0011]    Each threaded shaft may comprise a threaded portion, and a tapered portion adjacent the threaded portion. The device may further comprise a sleeve comprising at least one tapered stop for providing a surface against which the tapered portion of each threaded shaft abuts. 
         [0012]    In one embodiment, there is provided a compression molding apparatus, comprising: a compression molding device; and the above-described thread forming device coupled thereto. 
         [0013]    In another aspect, there is provided a method of forming a threaded bore in a compression molded part, comprising: positioning at least one rotatable threaded shaft in a mold cavity during compression molding of a part; rotatably driving the at least one threaded shaft for unthreading the at least one threaded shaft from the part; and pushing the at least one threaded shaft away from the mold cavity once the at least one threaded shaft is unthreaded from the part. 
         [0014]    The method may further comprise pushing the at least one rotatable threaded shaft into the mold cavity prior to said compression molding of the part. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Embodiments will now be described more fully with reference to the accompanying drawings in which: 
           [0016]      FIG. 1  is a perspective view of a portion of a compression molding apparatus; 
           [0017]      FIG. 2  is a perspective view of a thread forming device forming part of the compression molding apparatus of  FIG. 1 ; 
           [0018]      FIG. 3  is a perspective view of a portion of the thread forming device of  FIG. 2 ; 
           [0019]      FIG. 4  is sectional side view of a portion of the thread forming device of  FIG. 2 ; 
           [0020]      FIG. 5  is a photographic view of portions of two threaded shafts forming part of the thread forming device of  FIG. 2 ; and 
           [0021]      FIGS. 6   a  to  6   c  are top views of the portion of the compression molding apparatus of  FIG. 1 , during operation. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0022]    Turning now to  FIG. 1 , a compression molding apparatus is shown and is generally indicated by reference numeral  20 . Apparatus  20  comprises a compression molding device  22  that has a first mold half  24  and a second mold half (not shown), which cooperate to define a mold cavity  28 . The shape of the mold cavity  28  is selected so as to produce a compression molded part having a corresponding desired shape, as is known in the art. 
         [0023]    Apparatus  20  further comprises a thread forming device  30  that is coupled to the compression molding device  22 , and which is configured to produce one or more threaded bores within the compression molded part. The thread forming device  30  may be better seen in  FIGS. 2 to 6 . Thread forming device  30  has a longitudinal body comprising a base  32  that is configured to be mounted to the compression molding device  22 , a set of parallel posts  34  extending from the base  32 , and a support block  36  mounted to the ends of the posts  34  distal from the base  32 . In the embodiment shown, the thread forming device  30  comprises four (4) posts  34 . Mounted to the support block  36  is an actuator  38 , which is in communication with a first set of hydraulic lines (not shown). The hydraulic lines are in communication with a supply of a pressurized hydraulic fluid (not shown) and a controller (not shown). The controller and the hydraulic lines are configured to supply pressurized hydraulic fluid in a controlled manner to the actuator  38  for controlled operation thereof. The actuator  38  has a push rod  46  that extends therefrom towards the base  32  and the mold cavity  28 . A distal end of the push rod  46  is connected to a moveable push block  48 , which is configured to slide linearly along the posts  34  in unison with the push rod  46 . In the example shown, the push block  48  comprises a planar block having four columns fastened thereto, so as to form a block having a generally “C-shape”. 
         [0024]    The thread forming device  30  also comprises a moveable carrier  56 , which is also configured to slide linearly along the posts  34 . Positioned between the carrier  56  and the base  32 , and accommodated within respective recesses (not shown) within the carrier  56  and the base  32 , are one or more return springs  57 , which provide a biasing force urging the carrier  56  away from the base  32 . Mounted on the carrier  56  is a hydraulic motor  58 , which is in communication with a second set of hydraulic lines (not shown). The hydraulic lines are in communication with the supply of pressurized hydraulic fluid and the controller, and the controller and the second set of hydraulic lines are configured to supply pressurized hydraulic fluid in a controlled manner to the hydraulic motor  58  for controlled operation thereof. The carrier  56  also comprises a gear box  66  that is mechanically coupled to a drive shaft  68  of the hydraulic motor  58 , as shown in  FIG. 3  in which carrier  56  is not included. The gear box  66  is also mechanically coupled to at least one rotatable threaded shaft  70 . In this embodiment, the thread forming device  30  comprises two (2) rotatable threaded shafts  70 . As will be understood, rotation of each threaded shaft  70  is caused by rotation of the drive gear of the hydraulic motor  58  via the gear box  66 . 
         [0025]    The threaded shaft  70  may be better seen in  FIGS. 4 and 5 . Each threaded shaft  70  has a free end  71  that is sized and shaped to form a corresponding threaded bore in the compression molded part, and within the mold cavity  28 , during compression molding. The threaded shaft  70  comprises a threaded portion  72  adjacent the free end  71  having one or more screw threads  74  disposed on a surface thereof. Each threaded shaft  70  further comprises a tapered portion  76  adjacent to the threaded portion  72 , and a cylindrical portion  78  adjacent to the tapered portion  76 . 
         [0026]    Each threaded shaft  70  is accommodated within a respective sleeve  80  that is positioned within the molding device  22 . In this embodiment, the sleeve  80  forms part of the first mold half  24 . The sleeve  80  comprises a tapered stop  82  that is shaped to provide a surface against which the tapered portion  76  of the threaded shaft  70  abuts. The sleeve  80  further comprises a straight portion  84  adjacent to the tapered stop  82 . The sleeve also comprises one or more venting bores  88  through which excess material or debris resulting from operation of the thread forming device  30  may be discharged. Additionally, the thread forming device  30  comprises a cylindrical bushing (not shown) adjacent to the sleeve  80  for supporting rotational and axial movement of the threaded shaft  70 . 
         [0027]    During operation, the first mold half  24  and second mold half are brought together, so as to close the compression molding device  22  and to define the mold cavity  28 . The actuator  38  is then operated by pressurized hydraulic fluid supplied through the first set of hydraulic lines, causing the push rod  46  to extend from the actuator  38 , in turn pushing the push block  48  along the posts  34  toward the base  32 . During extension of the push rod  46 , the push block  48  abuts against the carrier  56  and pushes the carrier  56  along the posts  34  toward the base  32 , and against the opposing force provided by the return springs  57 . Extension of the push rod  46 , and unified movement of the push rod  46 , the push block  48  and the carrier  56 , continues until the tapered portions  76  of the threaded shafts  70  abut against the tapered stops  82  of the sleeves  80 . Once in this position, shown in  FIG. 6   a , the threaded portion  72  of each threaded shaft  70  is located at its desired position within the mold cavity  28 . The actuator  38  remains actuated during formation of the compression molded part, such that the threaded shafts  70  remain fixed in position against the high counteracting pressure used during the compression molding process. 
         [0028]    The compression molded part is then formed within the mold cavity  28  in the conventional manner known in the art. As will be understood, the compression molded part is formed around the threaded portion  72  of each threaded shaft  70 , such that each threaded portion  72  forms a corresponding threaded bore within the compression molded part. 
         [0029]    After the compression molded part has been formed, the actuator  38  is deactivated by release of pressurized hydraulic fluid therefrom. As a result, the push rod  46  is retracted into the actuator  38 , pulling the push block  48  along the posts  34  and away from the carrier  56  until the push block  48  abuts against the support block  36 , as shown in  FIG. 6   b . The hydraulic motor  58  is then operated by pressurized hydraulic fluid supplied through the second set of hydraulic lines, which in turn causes the threaded shafts  70  to be rotated via the gear box  66 . The direction of rotation of each threaded shaft  70  is selected such that, owing to the handedness of the screw threads  74 , the rotating threaded shafts  70  together with the carrier  56  are caused to be backed away from the compression molded part within the molding cavity  28 . Once the screw threads  74  of the threaded shafts  70  are clear of the threads of the threaded bores within the compression molded part, the carrier  56  and the rotating threaded shafts  70  are pushed by the opposing force provided by the return springs  57  to abut against the push block  48 , as shown in  FIG. 6   c . With the carrier  56  in this position, operation of the hydraulic motor  58  is then stopped. The molding device  22  may then be opened, and the compression molded part may be removed without the threaded shafts  70  interfering with its removal. 
         [0030]    Although in the embodiment described above, the thread forming device comprises two (2) rotatable threaded shafts, in other embodiments, the thread forming device may alternatively comprise one (1) rotatable threaded shaft or more than two (2) rotatable threaded shafts. 
         [0031]    Although embodiments have been described above and with reference to the accompanying drawings, those of skill in the art will appreciate that variations and modifications may be made without departing from the scope of the invention.