Abstract:
A method of sandwich molding employs independent runners for each of the first and second thermoplastic materials simplifying the construction of the mold and runner passages, facilitating use of hot runner systems and the like, and simplifying the balancing of flow in multiple cavity molds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to injection molding techniques and specifically to “sandwich” molding in which a second material is injected within a first material within a mold cavity.  
         [0002]     In conventional injection molding, an injection nozzle injects a thermoplastic material into a mold cavity formed by two or more mold portions. The thermoplastic material passes from the injection nozzle along a runner passage in the mold through a gate (normally a constricted portion of the runner passage), and into the mold cavity.  
         [0003]     When the thermoplastic material has cooled enough to solidify, the mold portions are separated, and the molded part and a “runner” formed in the runner passage are ejected. The ejection process may break the runner from the molded part or this may be done in a separate “degating” step.  
         [0004]     In sandwich injection molding, two or more nozzles inject two materials, one after another through common gate, into the mold cavity. By proper control of the injection process, the second material is injected inside the first material so that the first material envelops the second material in a “sandwich.” 
         [0005]     Sandwich molding is useful when the materials forming the center and outer surface of the molded part require different properties. For example, a recycled plastic material may be used in the center of the part while the outer surface may use a material having improved color, surface finish, and opacity.  
         [0006]     A typical sandwich injection-molding machine may include multiple hoppers, a single plasticizing screw, and a mold. In this system, each of the hoppers contains un-melted thermoplastic pellets and feed a pre-determined amount of each type of thermoplastic pellet to the plasticizing screw on a process-controlled interval. The thermoplastic pellets melt, but do not mix with the other melted thermoplastic materials in the plasticizing screw. The nozzle attached to the plasticizing screw will inject a first shot of the desired “skin” material into the mold and then at a second shot of the desired “core” material into the mold cavity via the same runner passage. Because both shots enter the mold cavity at the same location, the core material is injected within the skin material.  
         [0007]     Instead of using a single plasticizing screw for both thermoplastic materials, most sandwich injection molding machines include multiple plasticizing screws and injectors and a branched runner passage leading to a single gate. The branched runner passages include a valve or valves that open and close to control which branch supplies the flow of thermoplastic material to the common gate and the mold cavity. The valves that prevent backflow of the thermoplastic material from the active branch into the inactive branch are expensive to implement and prone to clogging.  
         [0008]     Lemke U.S. Pat. No. 6,284,726, assigned to the same assignee as the present invention and hereby incorporated by reference, discloses a system for sandwich molding involving a branched runner passage which eliminates the need for valves. In Lemke, as the first nozzle injects the first thermoplastic material through the first branch, the second nozzle blocks the second branch to prevent the flow of the first thermoplastic material into the second runner. The second nozzle then injects the second thermoplastic material into the second branch as the first nozzle blocks the first branch to prevent backflow and mixing of materials.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present inventor has discovered that sandwich molding can be performed using independent runner passages transporting only one type of thermoplastic material through separated gates. The present invention accordingly allows sandwich molding with simple runner passages that are more easily fabricated and adapted to hot runner systems and in which flow is more easily balanced in multiple cavity molds.  
         [0010]     More specifically, the present invention provides a method of sandwich molding comprising the steps of joining a first mold portion and a second mold portion at a part line to define a mold cavity including a first runner passage and a second runner passage, the first runner passage having a first inlet leading to a first gate at a first location within the mold cavity, and the second runner passage having a second inlet leading to a second gate at a second location within the mold cavity. A first thermoplastic material is injected through the first runner passage into the mold cavity without filling the mold cavity; and while the first thermoplastic material is still molten, a second thermoplastic material is injected through the second runner passage to join the first thermoplastic material and fill the mold cavity.  
         [0011]     It is thus an object of at least one embodiment of the invention to provide a method of fabricating molds for sandwich molding that does not require complex runner passage designs that can handle sequential flows of different materials.  
         [0012]     The first and second gates can be positioned and the injection shots sized so that the first thermoplastic material is injected until it covers the second gate and/or so that the first thermoplastic material wholly envelops the second thermoplastic material.  
         [0013]     Thus it is an object of at least one embodiment of the invention to provide a method of conventional sandwich molding where the different materials are injected at different points in the mold, simplifying mold design.  
         [0014]     The point at which the first thermoplastic material covers the second gate may come when the mold is as little as  33  percent filled or less than  66  percent filled.  
         [0015]     Thus it is an object of at least one embodiment of the invention to provide a sandwich molding technique in which a relatively thin skin of materials may cover a substantially thicker core material.  
         [0016]     In at least one embodiment, the present invention may provide local heating elements such as hot runner passages, insulated hot runners, or a hot manifold to maintain the temperature of the thermoplastic material.  
         [0017]     Thus, it is another object of at least one embodiment of the invention to provide simple runner passages that may be readily adapted to standard hot runner systems or the like.  
         [0018]     In at least one embodiment, the present invention may provide at least one runner passage branch extending to a plurality of mold cavities.  
         [0019]     Thus, it is another object of at least one embodiment of the invention to provide a simple runner topology that aids in balancing flow to each of the mold cavities in multi-cavity molds.  
         [0020]     In at least one embodiment, the present invention may use a control system joining the mold portions and controlling the injection of the thermoplastic materials.  
         [0021]     Thus, it is another object of at least one embodiment of this invention to provide a molding technique that may be easily implemented on standard controls used for injection molding.  
         [0022]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a simplified perspective view of a sandwich injection molding system with independent runner passages according to a first embodiment showing independent runner passages that may inject thermoplastic material into the mold cavity at two separate locations;  
         [0024]      FIG. 2  is a cross-sectional view taken along B-B of  FIG. 1  showing injector nozzles positioned against runners comprising simple sprue bushings in preparation for injection into the mold cavity;  
         [0025]      FIG. 3  is a figure similar to that of  FIG. 2  showing the mold cavity after the first nozzle has injected the first thermoplastic material into the mold cavity from the gate of the first sprue bushing to a point covering the gate of the second sprue bushing;  
         [0026]      FIG. 4  is a figure similar to that of  FIG. 3  showing the mold after the second nozzle has injected the second thermoplastic material into the mold cavity to fill and become substantially wholly surrounded by the first thermoplastic material;  
         [0027]      FIG. 5  is a figure similar to that of  FIG. 4  showing a multi-cavity mold having branched independent runners according to a second embodiment of the invention;  
         [0028]      FIG. 6  is a figure similar to that of  FIG. 4  showing use of the invention to provide a molded part having the core material partially exposed for decorative or structural purposes;  
         [0029]      FIG. 7  is a figure similar to that of  FIG. 6  showing the use of the present invention to create a two material part in a single mold cavity;  
         [0030]      FIG. 8  is a view similar to that of  FIG. 6  of a mold for producing blow molding preforms showing use of the present invention for making preforms having different inner and outer layers;  
         [0031]      FIG. 9  is a top perspective view of a mold allowing the production of separate molded inclusions within a part; and  
         [0032]      FIG. 10  is a perspective view similar to that of  FIG. 9  showing the use of the present invention with hook or tunnel gates opening off of the parting line. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]     Referring now to  FIG. 1 , a mold  10  suitable for practice of the present invention may include of a front mold portion  12  and a rear mold portion  14  joined along a parting line  16  to define a mold cavity  18 . After the sandwich injection molding process, the mold  10  will contain a molded part  20  which may be removed by separating the mold portions  12  and  14  along a longitudinal axis  35 .  
         [0034]     The front mold portion  12  contains a first straight runner passage formed by a sprue bushing  22  extending along the longitudinal axis A. As used herein, the term “runner” shall be any feed channel providing a connection between the nozzle of the injector and the gate including sprues and runners. The term may also be used for the plastic piece formed in this channel as context will make evident.  
         [0035]     The first sprue bushing  22  has an inlet  24  located on a front face of the outside surface of the first mold portion  12  and a gate  26  located on the inside surface of the mold cavity  18 . The rear mold portion  14  contains a second straight sprue bushing  28  also extending along the longitudinal axis  35 . The second sprue bushing  28  has an inlet  30  located on the rear face of the outside surface of the mold portion  14  and gate  32  located on the inside surface of the mold cavity  18  opposite to the first gate  26  across the parting line  16 .  
         [0036]     It will be appreciated that these straight sprue bushings  22  and  28  are both simple to machine and simple to fit with heated sprue bushings or similar systems if desired. The coaxial opposition of the sprue bushings  22  and  28  further aids in locating the injectors described below. Generally, however, the gates  26  and  32  need not be opposite each other and may be arbitrarily close together so long as independent sprue bushings  22  and  28  are preserved.  
         [0037]     A first injector  34 , extending along axis  35 , has a first machine nozzle tip  36  which can be placed against the first inlet  24  of the first sprue bushing  22 . A second injector  38 , also extending along axis  35 , has a second machine nozzle tip  40  which can be placed against the second inlet  30  of the second sprue bushing  28 . Once placed against the first and second inlets  24  and  30 , the first and second injection nozzle tips  36  and  40  may inject thermoplastic material through the sprue bushings  22  and  28  to fill the mold cavity  18 . In this first embodiment, the sprue bushings  22  and  28 , the inlets  24  and  30 , the gates  26  and  32 , the injectors  34  and  38 , and the injection nozzle tips  36  and  40  all extend along the axis  35 , but it is not required that any of these elements be coaxial.  
         [0038]     Referring now to  FIG. 2 , during a first step of the sandwich molding process, the mold portions  12  and  14  close along the parting line  16  to provide the mold cavity  18 , and the first machine nozzle tip  36  and the second machine nozzle tip  40  are placed against the first inlet  24  of the first sprue bushing  22  and the second inlet  30  of the second sprue bushing  28 , respectively.  
         [0039]     Referring now to  FIG. 3 , in a first embodiment, separation of the gates  26  and  32  is such that thermoplastic injected through one of the gates  26  or  32  covers the other gate  32  or  26  before the mold cavity  18  is filled, and preferably before the mold cavity  18  is filled by an amount over the ultimately desired ratio of core to skin material. Typically the thermoplastic material from the first gate will cover the second case before the mold cavity  18  is 30 percent and rarely more than  70  percent filled depending on the ratio of core to skin material desired in the final project.  
         [0040]     During this step of the process, the first machine nozzle tip  36  injects the first thermoplastic material  42  into the mold cavity  18  via the first sprue bushing  22  and the first gate  26 . The first machine nozzle tip  36  continues to inject the first thermoplastic material  42  into the mold cavity  18  until the first thermoplastic material  42  at least covers the second gate  32  of the second sprue bushing  28 .  
         [0041]     Referring now to  FIG. 4 , during the next step of the process, while the first thermoplastic material  42  is still molten, the second machine nozzle tip  40  injects the second thermoplastic material  44  into the mold cavity  18  via the second sprue bushing  28  through the second inlet  30 . The first thermoplastic material  42  covers the second gate  32  of the second sprue bushing  28 , so that when the second injection nozzles  40  injects the second thermoplastic material  44 , the second thermoplastic material  44  flows inside the first thermoplastic material  42  with the first thermoplastic material  42  forming a skin about a core formed of the second thermoplastic material  44 .  
         [0042]     The skin may be selected to have desirable traits such as, for example, surface finish, color, hardness, wear resistance, high strength, or resistance to UV radiation. The core may be engineered or selected to have desirable traits such as low density, low cost, or high strength. The first thermoplastic material  42  will substantially wholly envelop the second thermoplastic material  44  as it is injected, so it is not necessary that the first and second thermoplastic materials  42  and  44  to bond. Often the fact the first thermoplastic material  42  encapsulates the second thermoplastic material  44  is sufficient to maintain structural integrity as the molded part  20  cools. The selection of materials with compatible cooling rates, temperatures of solidification, and volumetric changes upon solidification ensures that the molded part  20  forms properly for the particular size of the molded part  20  and the material properties of the first and second thermoplastic materials  42  and  44 .  
         [0043]     Referring now to  FIG. 5 , in a second embodiment of the invention, the mold portions  12  and  14  join along a parting line  16  to provide a plurality of mold cavities  18   a ,  18   b ,  18   c  and  18   d . Hot runner manifolds  50  and  52  are held to the opposed outer faces of the mold portions  12  and  14  by clamp plates  54  and  56 , respectively. The hot runner manifolds receive the nozzles tips  36  and  40  and provide balanced runners  60  and  62  branching to each of eight sprue bushings  22   a - 22   d  (for balanced runner  60 ) and  30   a - 30   d  (for balanced runner  62 ). Balanced runners provide identical flow resistance from the inlet at nozzle tips  36  and  40  to each of the cavities  18   a - 18   b  to provide comparable flow of thermoplastic along each branch.  
         [0044]     The balanced runners  60  and  62  add complexity to the mold, but are still far simpler than would be required of conventional runner passages used for sandwich molding in which a common channel must handle two different materials from two different injectors. The simplicity and symmetry possible in the sprue bushings  22  and  28  in the present invention makes it substantially easier to balance flow rates into the various mold cavities  18 , necessary if consistent sandwich molding among the mold cavities is to be obtained. Again the simple sprue bushings  22  and  28  make use of standard or simpler hot runner and similar systems practical.  
         [0045]     While the embodiments of the invention include only two mold portions  12  and  14 , sprue bushings  22  and  28 , and injectors  34  and  38 , it will be understood from this description that the sandwich molding of the molded part  20  may employ more than two mold portions  12  and  14 , sprue bushings  22  and  28 , and injectors  34  and  38 . Additionally, a single mold cavity  18  may have a constricting cross section or may be so large as to use more than two sprue bushings  22  and  28  or injectors  34  and  38  to adequately fill the entire mold cavity  18 . Such a demand is also compatible with the present invention.  
         [0046]     Additionally, any given one of the mold portions  12  and  14  may include more than one of the sprue bushings  22  and  28  or none at all. That is, the first mold portion  12  or the second mold portion  14  may house both the first and the second sprue bushings  22  and  28 . Conversely, one or more of the sprue bushings  22  and  28  may travel through one or more of the mold portions  12  and  14  or along the parting line  16 .  
         [0047]     When the sprue bushings  22  and  28  are on opposite mold portions  12  and  14 , one injector may be advantageously mounted on the moving platen per U.S. application 2002/0102320, filed Aug. 1, 2002, entitled: “Multi-Shot Injection Molding Arrangement”, assigned to the present assignee, and hereby incorporated by reference.  
         [0048]     As mentioned, the mold  10  as depicted in either the first or second embodiment may include a heated runner passage system. A hot runner passage, an insulated hot runner passage, or a hot manifold may keep the thermoplastic materials  42  and  44  fluid while in the sprue bushings  22  and  28 . Such a mold modification can reduce gating scrap and improve material flow, especially in cases where the sprue bushings  22  and  28  are long.  
         [0049]     Referring now again to  FIG. 1 , a standard controller  51  of a type known in the art may control the entire injection process including the opening and closing of the mold portions  12  and  14 , the injection of the thermoplastic materials  42  and  44  into the sprue bushings  22  and  28  by the injectors  34  and  38 , and the ejection of the part from the mold cavity  18 . The controller  51  may coordinate the timing and volumes of the injection of the thermoplastic materials  42  and  44  by the injectors  34  and  38  by a simple timing mechanism or may include a feedback or triggering system that includes sensors  53  on the injectors  34  and  38  or at an in-mold location to determine when the first machine nozzle tip  36  has injected a sufficient amount of the first thermoplastic material  42  into the mold cavity  18 .  
         [0050]     Referring now to  FIG. 6 , in an alternative embodiment of the invention, the gates  26  and  32  may be positioned with respect to the mold cavity  18 , and the shot sizes may be controlled so that the inner thermoplastic material  44  is exposed through the outer thermoplastic material  42  while still being mechanically linked together. As shown in  FIG. 7 , when the inner thermoplastic material  44  is to be exposed, greater separation of the gates  26  and  32  can reduce the enveloping of the inner material  44  by the outer thermoplastic material  42  when it is desired to segregate the materials along an axis and/or allow for some degree of simultaneous injection.  
         [0051]     Referring now to  FIG. 8 , an important application of the present invention may be in molding preforms  70  used in a blow molding procedure. Such preforms  70  provide a generally test-tube shaped molded element with threads at the open end  72 . The present invention provides a simple mold structure to be used in producing preforms  70  with different inner thermoplastic material  44  and outer thermoplastic material  42 , the two materials providing, for example, a structural material and a gas blocking material.  
         [0052]     In the mold, sprue bushings  22  and  28  or other runner systems are positioned across a temporary mold cavity  80  joined to the bottom of the preform cavity  71 . A core pin  74  provides for the inner surface of the perform cavity  71 , the outer surfaces provided by mold portions  12  and  14 . Inner thermoplastic material  44  and outer thermoplastic material  42  are injected through opposed gates of the sprue bushings  22  and  28  into the temporary mold cavity  80  and the preform cavity  71  with the inner thermoplastic material  44  enveloped by the outer thermoplastic material  42 .  
         [0053]     A pin  76 , as is understood in the art, is then moved inward (per  FIG. 6 ) after completion of the injections of materials  42  and  44  to force the sandwich molded material completely into the preform cavity  17  eliminating a sprue attached to the perform  70 .  
         [0054]     Referring now to  FIG. 9 , the present invention provides for the ability to create a molded part  81  (shown in cross section) having multiple separate inclusions  82  formed of inner thermoplastic material  44  injected through different gates  26   b  and  26   a . In the molded part  81 , thermoplastic material  42  is introduced through a runner (not shown) in the bottom of the mold portion  14 , and the inclusions  82  then formed within the molded part  81  at the gates  26   b  and  26   a , for example, to provide for local reinforcement of the molded part  81 .  
         [0055]     Referring to  FIG. 10 , the present invention is not limited to runners which pass along the parting line  16  or perpendicular thereto, but may allow for the production hook gates  84  used to provide separate injection of materials  44  and  42 .  
         [0056]     Generally, it will be understood to those of ordinary skill in the art from the above description that the invention is not limited to thermoplastic materials but may be used with other commonly injected materials.  
         [0057]     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.