BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to injection molding systems. More specifically, the present invention relates to a valve gating system found in injection molding systems.
2. Summary of the Prior Art
Injection molding nozzles are well known and are used to inject materials, such as plastic, into the cavity of a mold. For example, such nozzles receive molten material, such as plastic, from an injection molding machine and direct the same into a mold cavity through a passage called a gate. When an injection operation is complete, and prior to opening the mold cavity to eject the molded part, the transfer of molten material through the gate must be stopped. Generally, two methods exist for stopping the transfer of molten material through the gate, namely: thermal, or open, gating; and valve gating.
In thermal gating, the gate is an open aperture through which molten material passes during an injection operation. The gate is rapidly cooled at the end of the injection portion of the cycle, when the injection pressure is removed, to “freeze” the injected material into a plug. This plug remains in the gate to prevent drool of molten material from the gate when the mold is open for the ejection of the molded part. In the next injection portion of the cycle, the cooling applied to the gate is removed and hot molten material from the injection molding machine pushes the plug into the mold cavity, where it melts and mixes with the newly provided molten material.
In valve gating, the opening and closing of the gate is independent of injection pressure and/or cooling and is achieved mechanically with a valve stem. This stem can be moved between an open position, wherein flow of molten materials through the gate is permitted, and a closed position wherein the gate is closed by entry of the valve stem into the gate which establishes a seal, preventing molten materials from passing through the gate. Valve gating is well known and examples of such systems are shown in U.S. Pat. Nos. 2,878,515; 3,023,458; and 3,530,539, each being incorporated herein by reference.
Generally, for situations that require improved aesthetics, valve gating is preferable to thermal gating because it can reduce the undesired gate vestige which results on the finished molded part. However, there are problems with valve gating systems.
Specifically, the valve stem and gate each have a complementary sealing portion which are brought into contact to seal the gate. Typically there is a 0.001″-0.002″ diametrical clearance between the valve stem and the gate sealing portion. As the valve stem is moved into alignment with the sealing portion of the gate to effect sealing, a slight misalignment of the stem with the gate will cause the stem to strike the gate sealing portion. Over time, this will cause the gate area to wear and become misshapen. Now that the gate sealing area is worn, the stem no longer stops the flow of molten material and a small amount of molten material will migrate between the stem and the worn gate sealing area. This leakage adversely impacts the vestige quality because as the mold is opened, the now solidified material between the gate and the valve stem will cause a tear or blemish to form along the vestige of the part, and in extreme cases, the tearing can propagate to the surface of the molded article or preform.
Following the injection cycle, typically the mold halves will open and the molded article in a somewhat solidified state will be removed from the area of the stem/gate area. Due to the entrapped molten material between the worn gate area and the stem, the molded article will not break away cleanly when the mold is opened, but rather will tear away from the gate area, which results in a blemished vestige on the molded article.
Referring to FIGS. 1 and 2 this phenomenon can be clearly seen. As well known in the art, a nozzle assembly 10 is comprised of an elongated nozzle bushing 12 with a nozzle tip 16 affixed co-axially therein. Optionally, an insulator 14 is affixed to a proximal end of the nozzle tip 16 thereby thermally insulating the heated nozzle assembly 10 from the cooled cavity plate 34. A movable valve member 18 extends co-axially in the nozzle assembly 10 and is selectably positioned in or out of a passageway/gate area 22. A melt channel 20 surrounds the valve member 18 and runs the length of the nozzle assembly 10 to communicate a flowable material to a mold cavity 28. When the valve member 18 is placed in a fully closed position (as shown in FIG. 1), a sealing portion 25 in the cavity plate 34 sealingly surrounds the valve member 18 to shut off the flow of material to the mold cavity 28. As shown in FIG. 1, a face portion 21 of valve member 18 defines the entire top of the vestige 26 of the molded article. A chamfer 36 is typically provided along the face of the valve member 18 to help guide the valve member into the gate area and reduce wear of the valve member and cavity plate 34.
Due to the close fit of the valve member 18 to the sealing portion 25, any misalignment that exists between their respective interfaces will cause the valve member 18 to strike the surface of the sealing portion 25 which will ultimately lead to a deterioration of the seal portion 25 and/or the valve member 18.
At the end of the injection cycle, the valve member 18 is moved into its closed position as previously described and the mold cavity is held in a closed position with a core 30 for a predetermined cycle time to allow the molten material to cool and solidify, thereby forming the molded article. Once the molded article has been allowed to cool to a sufficient level, the core 30 with the molded article thereon is moved in the direction as denoted by arrow A, and the vestige 26 is pulled away from the face portion 21 of the valve member 18. If enough wear exists between the valve member 18 and the sealing portion 25, a small amount of molten material will migrate therein, and as the mold core 30 and the molded article 27 are moved to an open position, a peeled edge 38 will form on the vestige 26 of the molded article 27.
Also, as the valve member 18 is in the flow of molten material when the gate is open, it can become quite hot. When the gate is closed by the valve member 18, the hot tip of the valve member 18 can be difficult to cool as the mold cavity 28 is cooled and this can result in a need for increased cycle times to permit the necessary cooling, and/or can result in undesirable characteristics in the molded article 27. Specifically, as the material in the mold cavity 28 adjacent the valve member 18 is cooled less efficiently due to the hot tip, parts molded from thermally sensitive materials such as PET can suffer from an enlarged area of crystallinity 40 or other undesired characteristics. In addition, since the entire top surface of the vestige 26 is in contact with the face portion 21 of the hot valve member 18, the molten material adjacent the face portion 21 remains somewhat molten and stringing and an uneven edge forms when the mold is opened.
Therefore there is a need for an improved injection-molding machine with a valve gate system that reduces or obviates some or all of the drawbacks of the prior art.
SUMMARY OF THE INVENTION
The primary objective of the present invention is to provide an improved injection molding system with a valve gating system that reduces or obviates the drawbacks of the prior art.
Another object of the present invention is to provide an insert that interfaces with a valve member in an injection molding system that reduces or eliminates the formation of peeled edges along a vestige of a molded article.
Yet another object of the present invention is to provide a gate insert in the mold plate adjacent the valve member that may be easily replaced.
The foregoing objects are achieved by providing a mold cavity with a vestige cross-sectional area that is larger than the cross-sectional area of the valve member so that the periphery of the vestige is cooled quicker than the interior portion of the vestige. In another preferred embodiment, a replaceable insert is provided to help guide the valve member into a sealing position with the gate. Replacement of this insert can easily be performed whenever the wear of the insert reaches a predetermined and unacceptable level.
Further objections and advantages of the present invention will appear hereinbelow.