Patent Application
20080171100
The present invention generally relates to the field of injection molding. In particular, the present invention is directed to a constant-force Belleville spring and an injection molding melt-conveyance system incorporating such a spring as a seal spring.
In the art of injection molding of plastic parts/articles it is well known to utilize a hot runner to distribute molten plastic from an injection machine to the one or more mold cavities that receive the molten plastic and shape the one or more molded parts/articles. One type of hot-runner design, which is shown in U.S. Pat. No. 6,555,044 to Jenko, issued one Apr. 29, 2003, and titled “Hot Runner Valve Gate Piston Assembly” (incorporated by reference herein in its entirety), utilizes a manifold cavity plate and one or more plastic-distributing manifolds that are received in one or more corresponding manifold cavities formed in the plate. In this design, each plastic-distributing manifold is typically heated during use to maintain the molten plastic in the manifold at a proper temperature, e.g., using one or more electrical resistance heating elements that engage the manifold. A backing plate is usually used to enclose the manifold(s) within the respective cavity(ies). Each manifold has an outer surface that is typically spaced from the walls of its manifold cavity so as to provide an insulating air space between the manifold and each of the manifold cavity plate and backing plate.
This type of hot-runner design typically includes a plurality of nozzle assemblies, either thermal-gated or valve-gated, each having a nozzle housing secured to the manifold cavity plate and extending into a corresponding opening formed in the plate. Each nozzle housing includes a nozzle-housing melt-channel that carries the molten plastic from the manifold to a corresponding nozzle tip during molding. The manifold includes a corresponding manifold melt-channel that delivers the molten plastic to the nozzle-housing melt-channel. Of course, when the manifold and manifold cavity plate are at their operating temperatures, the manifold melt-channel and nozzle-housing melt-channel are highly aligned with one another so that their longitudinal central axes lie substantially along the same line. As mentioned, each manifold is heated, but the manifold plate is not. Consequently, due to the difference in temperature between these two components during operation, the matching ends of the nozzle-housing and manifold melt-channels move by differing amounts relative to a fixed reference during heat-up and cool-down. Therefore, when the manifold and manifold plate are cold, the matching ends of the nozzle-housing and manifold melt-channels are not in alignment.
A common way of handling this differential movement between the nozzle housing and manifold is to provide a metal-to-metal compression seal between the nozzle housing and manifold. This is often accomplished by providing a seal spring between the manifold cavity plate and the nozzle housing that urges the nozzle housing into sealing engagement with the manifold. This type of spring seal allows for both relative lateral translation between the nozzle housing and manifold and movement of the manifold and nozzle housing toward and away from one another due to heating and cooling.
While a spring seal between the nozzle housing and manifold can be very effective, much care must be taken to ensure that the various components that affect how much the seal spring is compressed, or deflected, are manufactured and milled to very close dimensional tolerances. This is so because the conventional seal spring used in these types of seals is a linear, variable force spring, i.e., a spring for which the spring force provided by the spring varies linearly with the spring's deflection from its relaxed state. Since the sealing force applied between the nozzle housing and manifold must be fairly precise, too much variation in the actual deflection of the spring relative to the design deflection will result in the sealing force being either too high or too low. Neither of these conditions is desirable. Consequently, tight dimensional tolerances affecting the deflection of the seal spring must be rigidly followed. Achieving such tight tolerances, however, typically leads to increased manufacturing costs.
In one embodiment, the present invention is directed to an urging device, comprising: a Belleville spring having: a thickness; an axis of rotational symmetry; an available deflection when the Bellville spring is in a relaxed state; and an overall height in the relaxed state; wherein the Belleville spring has a ratio of the available deflection to the thickness in a range of about 1.3 to about 1.7.
In another embodiment, the present invention is directed to an urging device, comprising: an annular cupped spring having: a thickness; an axis of rotational symmetry; an available deflection when the annular cupped spring is in a relaxed state; and an overall height in the relaxed state; wherein the thickness and the available deflection are selected to provide a spring force parallel to the axis of rotational symmetry that is substantially constant over a first range of deflection greater than 10% of the available deflection.
In a further embodiment, the present invention is directed to an injection molding melt-conveyance system for conveying a molten material from an injection machine to an injection mold, comprising: a first component having a first melt-channel; a second component having a second melt-channel for being in aligned fluid communication with the first melt-channel during use of the hot runner; and a constant-force seal spring urging the first component toward the second component toward so as to effect a seal between the first component and the second component so that the molten material is inhibited from leaking through the seal when the molten material moves between the first melt-channel and the second melt-channel.
For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein:
Referring now to the drawings,
Prior to describing the configuration of constant-force Belleville spring 104 that provides the desirable constant force over a range of deflection, however, exemplary hot runner 100 is described in more detail. It is noted that hot runner 100 is presented as but one example of many designs that may incorporate a constant-force Belleville spring made in accordance with the present disclosure, such as Belleville spring 104. Some of these alternative designs are mentioned below. However, those skilled in the art will readily appreciate that the alternatives mentioned are but a few of the many alternatives, and so, will understand that the concepts of the present invention have broad application.
Returning to exemplary hot runner 100, hot runner 100 includes one or more nozzle assemblies 116 (only one shown for convenience) that may include a nozzle 120, nozzle housing 108 and a nozzle heater 128, among other things. Nozzle assembly 116 may be engaged with a front plate 132, which may be part of a larger manifold cavity structure, e.g., plate 136, that defines a cavity 140 for receiving manifold 112 therein. Of course, alternative hot runners may have constructions that do not utilize a monolithic manifold cavity plate 136. For example, monolithic cavity plate 136 may be replaced by a separate front plate (not shown, but similar to front plate 132) with separate members (not shown) attached thereto to provide cavity walls similar to the cavity walls 144 of monolithic manifold cavity plate 136 shown. Nozzle assembly 116 may be partially held in place within front plate 132 via a generally annular insulator 148 made of a suitable thermally insulating material, such as titanium. Insulator 148 may be desirable to limit the amount of heat transferred from nozzle assembly 116 to front plate 132. Nozzle housing 108 may include one or more nozzle melt-channels 156 (one shown for convenience) that carries the molding material (not shown), e.g., molten plastic, to nozzle 120 during operation.
Since nozzle 120 shown is of the valve-gate type having a valve stem 152 actuated by a valve-gate actuator 156, manifold 112 may include a valve-stem bushing 160 that extends therethrough and includes a manifold melt-channel 164 that carries the molding material to nozzle housing 108. As described in the background section above, when manifold 112 and front plate 132 are cold, manifold melt-channel 164 and nozzle-housing melt-channel 156 are typically not in alignment with one another due to the differing thermal growths of manifold 112 and front plate 132 as these components are heated to their operating temperatures. However, a good seal is needed between manifold 112 (in this case, valve-stem bushing 160, but in other embodiments, such as a thermal-gate embodiment, could be another part of manifold 112) and nozzle housing 108 under operating conditions so as to inhibit the molding material from flowing between manifold 112 and nozzle housing 108 and into the insulating air space 168 between manifold 112 and manifold cavity plate 136. In this example, the sealing force needed to effectuate the seal between manifold 112 and nozzle housing 108 is essentially provided by constant-force Belleville spring 104, which is described in more detail below.
As those skilled in the art will readily appreciate, hot runner 100 may include other components as needed, such as a backplate 172 and a backup pad 176 that may be provided to transfer compressive forces from backplate 172 to manifold 112 when backplate 172 is properly fastened to manifold cavity plate 136. Generally, this compressive force is due to the forces provided by the one or more constant-force Belleville springs 104 provided for the one or more corresponding nozzle assemblies 116 that form part of the entire hot runner 100.
Turning now to the configuration of constant-force Belleville spring 104 that makes it a substantially constant-force spring, attention is directed to
Implications of having constant-force Belleville spring 104 (
However, with constant-force Belleville spring 104 in this example having an hit ratio of 1.6 and an available deflection h of 3.7 mm, spring 104, by varying its physical dimensions, can be configured to provide the necessary sealing force of 31 kN over a range of deflection from about 0.6h to 1.0h, by using six springs, for example. Designing constant-force Belleville spring 104 to have an available deflection h of 2.8 mm, this means that the cumulative effect of manufacturing deviations, according to general design practice of Belleville spring can be as much as about ¾ of 0.4h, or 1 mm in this example, which is much larger than the range given above for the conventional, non-constant-force Belleville spring. Consequently, manufacturing of the components that affect the compression of constant-force Belleville spring 104 need not be as precise as for a conventional, non-constant-force Belleville spring. Less precision typically translates into cost savings, which are important to manufacturers and customers alike. In addition, it is noted that, if desired, constant-force Belleville spring 104 can be made by a manufacturing process less rigorous than the usual turning and grinding processes used to make conventional, i.e., non-constant-force, Belleville springs, such as stamping. This is so because the wide range of deflection over which constant-force Belleville spring 104 is designed to provide a substantially constant spring force allows for greater manufacturing tolerances. This, too, can lead to reduced manufacturing cost of the constant-force Belleville springs 104 themselves, as well as the assemblies that include them.
Whereas
In the example of
With manifold and backing plates 520, 524 secured to one another, constant-force spring pack 504 works against this arrangement to compress the overlapping portion of manifolds 508, 512 between spring holder 528 and locating insulator 532 at a design compressive force Fc. This compressive force Fc is designed to effect a proper seal at an interface 540 between manifold 508 and manifold 512 so that the molding material, e.g., molten plastic, flowing from a melt-channel 508A of manifold 508 does not leak from the interface 540 as it passes into a melt-channel 512A of manifold 512. Interface 540 may be any suitable interface such as a direct face-to-face interface between two corresponding respective machined faces of the two manifold 508, 512 or, alternatively, an interface that utilizes a compression disk 544 or other type of intermediate structure or gasket, among others. Like the seal formed at the interface between nozzle housing 108 and valve-stem busing 160 of
In spring pack 504 shown in
In other embodiments, Belleville springs 500A-C and/or other Belleville springs may be arranged in a “series” configuration, i.e., stacked in a non-nested manner along a common central stacking axis similar to common central stacking axis 548 of
Whereas
In this example, each of constant-force Belleville springs 604A-D has been selected to provide substantially the same constant spring force over a relatively wide range of deflection. The physical parameters of each constant-force Belleville spring 604A-D that permit that spring 604A-D to provide such a constant force are discussed above in connection with
Other components of sprue bar assembly 600 include a sprue bar 608, a telescopic bushing 612, a pair of sealing wedges 614A-B and a housing 616 comprising a tubular sleeve 620 and end-piece 624 secured to the tubular sleeve 620. In this design, constant-force Belleville springs 604A-D allow telescopic bushing 612 to telescope into and out of sprue bar 608 by an amount limited by the complete flattening of springs 604A-D at one extreme and the interference between a flange 628 on the bushing 612 and a stopping surface 632 at the other extreme. Whether constant-force Belleville springs 604A-D are working against stopping surface 632 of end-piece 624 or against a mating portion of a mold (not shown) (thereby compressing springs 604A-D more than depicted in
As those skilled in the art will appreciate, when constant-force Belleville springs 604A-D are in the partially compressed states illustrated in
Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.
