Abstract:
A multi-material runner gate system allows effective de-gating of elastomeric materials that would otherwise stretch and/or flex reducing de-gating forces at the gate upon separation of the molds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to injection molding and more particularly to a mold that provides improved de-gating of parts molded from elastic material.  
           [0002]    In injection molding, a thermoplastic material is forced under pressure into a mold cavity that defines the shape of the part being molded. A nozzle, providing the source of plastic material, communicates with the mold cavity through a sprue passage joining with one or more runner passages, in turn connecting to the mold cavity at corresponding gates. Generally, the gates are somewhat narrower than the runner passages to allow the sprue and runners (referring respectively to plastic remaining in the sprue and runner passages at the completion of the molding) to be broken away from the molded part at the gate in a process termed “de-gating”.  
           [0003]    De-gating may occur as a separate process after the part is ejected from the mold or automatic de-gating may be accomplished with certain mold designs. For example, a tunnel-gate, also known as a “hook” or “submarine” gate, may be used to capture the runner in one part of the mold portion, so that when the molded part is ejected, the retained runner is pulled from the part. The tunnel-gate takes its name from the fact that the runner passage, which normally extends along the part line of the mold, “tunnels” below the part line into one mold part and then “hooks” upward into the mold cavity. The runner is removed from the mold with a slight twisting and/or flexing of the runner.  
           [0004]    Automatic de-gating using, for example, a tunnel-gate, is often unsuccessful with elastic materials. Elastic materials, by stretching, dissipate the de-gating force developed as the part is ejected. The inability to de-gate such parts without a secondary operation, increases the cost of molding parts using elastic materials.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention allows for automatic de-gating of parts molded of elastic materials. A runner support structure is molded within the runner passage from a material having low elasticity. The elastic material of the runner adheres to the runner support structure, the latter which serves to reinforce the runner. When the molded part is ejected, stretching of the runner is reduced, increasing the stress at the gate and ensuring proper de-gating. When the molded part is a “two shot” injection molding, using one shot of an elastic material and a second shot of a relatively inelastic material, the runner support structure may be molded during the injection of the inelastic material without the need to add process steps.  
           [0006]    Specifically then, the present invention provides a method of de-gating a molded part produced by injection molding of an elastic material in a mold cavity. The method includes the steps of: (1) molding a runner support structure within a portion of the runner passage leading to the mold cavity by the injection of a substantially inelastic material into the runner passage, and (2) molding the part by the injection of elastic material into the mold cavity via the runner passage. The part is de-gated by a force on the runner formed in the runner passage, at least in part, transmitted through the runner support structure.  
           [0007]    Thus, it is one object of the invention to provide automatic de-gating when molding parts from elastic materials. It is another object of the invention to allow runner passages to be freely designed without concern that cumulative stretch over the length of the runner may interfere with de-gating.  
           [0008]    The method may be used in two shot molding operations employing an elastic and substantially inelastic material. In this case, the molding of the runner support structure may take place during the molding of the substantially inelastic material.  
           [0009]    Thus, it is another object of the invention to provide a method of automatic de-gating of parts molded of elastic materials where the method is well adapted to two shot molding.  
           [0010]    The step of forming the runner support structure may include fitting a blocking structure into a portion of the runner passage to prevent complete filling of the runner passage with the inelastic material during the molding of the runner support structure and then removing that blocking structure prior to molding with the elastic material. Conversely, the blocking structure may be fit into the portion of the runner passage during the molding with the elastic material and removed to allow the introduction of the inelastic material.  
           [0011]    Thus, it is another object of the invention to make use of the same techniques used in two shot molding to produce the runner of the present invention.  
           [0012]    The mold may have a part line separating two mold sections and the runner passage may be a tunnel-gate passing below the part line completely into one mold portion prior to entering the cavity.  
           [0013]    Thus, it is another object of the invention to provide a de-gating system suitable for use with conventional tunnel-gate structures.  
           [0014]    The runner support structure may be tapered in the direction of the gate.  
           [0015]    Thus is another object of the invention to provide for a stress reducing interface between the relatively rigid runner support structure and the elastic portion of the runner such as minimizes separation or tearing at the interface between the runner support structure and the elastic material of the runner.  
           [0016]    The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment and its particular objects and advantages do not define the scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a perspective view of a prior art runner with a tunnel-gate such as has been described above;  
         [0018]    [0018]FIG. 2 is perspective view of a multi-material runner with a tunnel-gate per the present invention showing a runner support structure positioned beneath elastic material of a runner;  
         [0019]    [0019]FIG. 3 is an exploded view of the multi-material runner of FIG. 2 showing the runner support structure separated from the remainder of the runner;  
         [0020]    [0020]FIG. 4 is a top plan view of a plastic part having a core of substantially inelastic material introduced by auxiliary gates and runners leading from the runner support structure of FIG. 2 molded with the core;  
         [0021]    [0021]FIG. 5 is a figure similar to that of FIG. 4 showing an over molding of the core with an elastic material and the formation of the elastic portion of the runner of FIG. 2;  
         [0022]    [0022]FIG. 6 is cross sectional view along line  6 - 6  of FIG. 4 showing first and second mold parts used producing the runner support structure in a restricted runner passage; and  
         [0023]    [0023]FIG. 7 is a cross sectional view along line  7 - 7  of FIG. 5 showing first and third mold parts forming a mold cavity having an unrestricted runner passage. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Referring now to FIG. 1, a conventional runner  10  and tunnel-gate  18  extend along the part line  12  of a first and second mold section  14  and  16  defined by a passage for the injection of a thermoplastic material into a mold cavity (not shown). The runner  10  is released by separation of the mold sections  14  and  16 , however, the tunnel-gate  18 , passing through mold section  16 , is held in place by mold section  16 . Thus, when the molded part (not shown) is ejected, the runner  10  and tunnel-gate  18  held by the mold section  14 , are pulled off the molded part.  
         [0025]    After the runner  10  and tunnel-gate  18  are separated from the molded part, they may be removed from the mold section  16  by upward rotation of the runner  10  and/or flexing of the tunnel-gate  18  withdrawing it from its passage. To facilitate this process, the tunnel-gate  18  tapers to a narrower cross-section toward the molded part.  
         [0026]    When a runner is composed of an elastic material, it may stretch sufficiently over its length to prevent the necessary concentration of stresses at the gate to allow automatic de-gating as described above. Accordingly, in the present invention, a multi-material runner is produced in which the elastic material of the runner is reinforced with a non-elastic material resisting runner stretching, better retention of the runner by the tunnel-gate, and improved ejection of the runner after molding.  
         [0027]    In the following description and claims, the term “runner” will be used generally to refer both to runner  10  and also associated materials removed from the molded part including portions of the gate and/or sprue as context requires.  
         [0028]    Referring now to FIGS. 2 and 3, the present invention provides a multi-material runner  20  having an elastic runner portion  22  molded together with a substantially inelastic runner support structure  24 .  
         [0029]    The elastic runner portion  22  includes a frusto-conical sprue  26  defined by a downwardly expanding passage receiving material directly from an injection nozzle (not shown). The sprue  26  connects to horizontally extending left and right runners  28  and  30  extending along a part line  12  of the mold (also not shown) and terminating in upwardly concave hook-shaped, left and right tunnel-gate portion  32  and  34 , respectively. The outward tips of the left and right tunnel-gate portion  32  and  34  join to the molded part (not shown).  
         [0030]    The under surface of the left and right runner  28  and  30  and the under surface of a portion of the left and right tunnel-gate portions  32  and  34  are joined to the upper surface of a conforming runner support structure  24 . Specifically, the runner support structure  24  includes left and right extending runners  36  and  38 , conforming and adhering, respectively, to the undersurface of upper, left and right runner  28  and  30 . Likewise, left and right tunnel-gate portions  40  and  42  of the runner support structure  24  conform and adhere to undersurfaces of upper, left and right tunnel-gate portions  32  and  38 .  
         [0031]    As mentioned, the material of the runner support structure  24  is selected to be relatively inelastic so as to reduce the flexing and stretching of the elastic runner portion  22 . The materials of the runner support structure  24  and elastic runner portion  22  are preferably selected to adhere to each other when molded together, however, materials having low adhesion may be accommodated by introducing a surface roughness or teeth, molded at the interface of these elements, to promote such adhesion.  
         [0032]    Referring still to FIGS. 2 and 3, the runner support structure  24  includes a post  44  extending downward from the junction of left and right runners  36  to engage with an injector pin used during the ejection of the molded part. In this way, the runner support structure  24  provides a rigid structure that may be acted upon by conventional ejector pin  46 .  
         [0033]    The runner support structure  24  also includes a frusto-conical sprue  48 , similar and parallel to sprue  26 , but laterally displaced from the sprue  26  to receive a different injector nozzle (not shown). Sprue  48  joins to runners  36  and  38  by means of laterally extending offset portion  50 .  
         [0034]    Extending from the runners  36  and  38  in the opposite direction of offset portion  50  are auxiliary runners  52  and  54  which provide path to auxiliary gates (not shown) to a mold cavity as will be described.  
         [0035]    Referring now to FIGS. 4 and 5, the present invention is particularly well suited to “two shot” molding processes in which the molded part  60  to be produced includes, for example, a relatively inelastic core  62  over molded with an elastomeric outer coating  66 . As used herein, “two-shot molding” refers to any molding operation where more than one material is injected into the mold cavity before ejection of the part.  
         [0036]    When the invention is used in a two shot molding process, the runner support structure  24  may be molded at the time of the molding of the core elements, eliminating the need for additional process steps or materials. In this case, sprue  48  receives inelastic material into the offset portion  50  to form the lower left and right runners  36  and  38  and left and right tunnel-gate portions  40  and  42 , and this same material is conducted through auxiliary runners  52  and  54  and auxiliary gates  67  to the part core  62 .  
         [0037]    Referring now to FIG. 5, the elastic material may now be injected through sprue  26  over the top of the runner support structure  24  to cover the lower, left and right runners  36  and  38  and lower left and right tunnel-gate portions  40  and  42  of the runner support structure  24  (shown in FIG. 4) forming the elastic runner portion  22  (consisting of upper, left and right runners  28  and  30  and upper, left and right tunnel-gate portions  32  and  34 ). Upper, left and right tunnel-gate portions  32  and  34  connect to the outer elastomeric coating  66  covering the core  62  and completing the part  60 .  
         [0038]    Referring now to FIGS. 4 and 6, the multi-material runner  20  of the present invention may be formed by a changing combination of different mold sections. During the molding of the core  62 , (shown in FIG. 4) a first mold section  70  and a second mold section  72  join along a parting line  74  to provide restricted mold gate passage  78  and a mold cavity  80 , defining the core  62 . Injection of thermoplastic material through the sprue  26  by a first injector nozzle  82  simultaneously forms the runner support structure  24  and core  62 .  
         [0039]    A passageway  84  providing the remainder of the tunnel-gate is blocked from flow of material out of the runner support structure  24  by downwardly extending portions  86  of the second mold section  72 . Similarly, flow of material from the core  62  into the passageway  84  is blocked by pins  88  of lateral mold sections  90  which press on the left and right about the first mold section  70 .  
         [0040]    Referring now to FIGS. 5 and 7, after the runner support structure  24  and core  62  are molded, the second mold section  72  is removed and replaced with a third mold section  92  having new downwardly extending portions  86  that provide an unrestricted runner cavity that may receive elastic material from injector nozzle  82 ′. The unrestricted runner cavity provides a passage conforming to left and right runners  28  and  30  over the runner support structure  24  and joining the passageway  84 .  
         [0041]    The lateral mold sections  90  are replaced with mold sections  90 ′, removing blocking pins  88  and allowing flow of elastomeric material through the passageway  84  into an expanded mold cavity section  96  provided by the third mold section  92  and mold section  90 ′.  
         [0042]    When the molded part  60  (including core  62  and elastomeric outer coating  66 ) is removed, the multi-material runner  20  (including runner support structure  24  and elastic runner portion  22 ) is retained by the first mold section  70 , causing a de-gating at the points  98 .  
         [0043]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.