Patent Application
20070096364
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.
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.
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.
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.”
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
In at least one embodiment, the present invention may provide at least one runner passage branch extending to a plurality of mold cavities.
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.
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.
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.
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.
Referring now to
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.
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.
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.
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.
Referring now to
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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.
Referring now to
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.
Referring now to
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.
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.
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.
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.
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.
Referring now again to
Referring now to
Referring now to
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.
A pin 76, as is understood in the art, is then moved inward (per
Referring now to
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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.
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.
