Plastic article or earthenware shaping or treating: apparatus – With means to gather excess or rejected stock material
Reexamination Certificate
1999-11-30
2002-05-21
Nguyen, Nam (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
With means to gather excess or rejected stock material
C425S547000
Reexamination Certificate
active
06390798
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an injection device for a plunger-type injection molding machine which injects a molten resin by means of an injection plunger. More specifically, the present invention relates to an injection device for a plunger-type injection molding machine equipped with a resin ejection opening, which reliably ejects the resin which, during the injection operation, has leaked out of the injection chamber to the area around the injection plunger shaft, from the rear edge of the injection cylinder.
BACKGROUND
Plunger-type injection molding machines supply molten molding resin to an injection device, and inject it into a molding die with an injection plunger. In practice, plunger-type injection molding machines are frequently used as a screw preplasticizing injection molding machine; therefore the explanation below refers to a screw preplasticizing injection molding machine.
A conventional screw preplasticizing injection molding machine contains a plasticizing screw within a plasticizing chamber in a plasticizing section; an injection plunger is contained within an injection chamber in an injection section, and a connecting path is provided between the plasticizing section and the injection section which connects the plasticizing chamber and the injection chamber. A synthetic resin material (hereinafter “resin”) supplied from a hopper is blended and melted, which is to say plasticized, within the plasticizing chamber by means of the rotation of a plasticizing screw and the heat of the plasticizing chamber, and is pressure fed to the injection chamber through the connecting path. At this point, the stoke of retraction of the injection plunger which retracts with the pressure feed is detected, and the amount of molten resin is measured (metered). Next, the injection plunger advances, and the molten resin is injected into a cavity in the die.
In such a plunger-type injection molding machine, in order to ensure that the injection plunger does not bite into the injection cylinder during injection, a minute gap exists between the plunger head and the injection cylinder inner wall. Also, the injection plunger shaft has a slightly smaller diameter than the injection plunger head, and a step is formed between the plunger head and the plunger shaft. Therefore, during the injection operation, in which molten resin is filled at high pressure into a die, an extremely small amount of molten resin leaks out of the gap between the plunger head and the injection cylinder inner wall and accumulates in the gap between the plunger shaft and the injection cylinder inner wall. Resin which thus leaks out (hereinafter “leaked resin”) is pressed into the aforementioned step with each retraction of the plunger head and is pushed backward, such that it is continuously ejected from the rear (base) portion of the injection cylinder, or from a resin ejection opening provided near the rear edge of the injection cylinder. However, when this leaked resin has not been perfectly ejected, the leaked resin is further compressed with each injection by the leaked resin, forming a dam, and the retraction of the injection plunger is prevented, thus impeding the metering operation.
Several injection devices equipped with resin ejection openings with which the leaked resin could be ejected outside the injection cylinder have thus been proposed. For example, Jpn. Patent H8-156049 discloses an injection device equipped with a scraper ring, which supports an injection plunger as well as scrapes off ejected resin which has adhered and emerged around the injection plunger shaft. In Jpn. Patent H8-229997, an injection cylinder is proposed in which, in order to eject the leaked resin in the molten resin state, a resin drain opening is provided directly on the injection cylinder, while at the same time a heater is added to the injection cylinder behind that drain opening. Jpn. Patent H9-85787 proposes, in addition to the structure of Jpn. Patent H8-229997, an injection device which provides a scraping sleeve immediately behind the resin drain, such that the molten resin is prevented from leaking out behind the scraping sleeve.
When molding is performed with a general use resin, such resin is thermally stable, and there is therefore no degradation even if the leaked resin is exposed over long periods to high temperatures around the plunger shaft. Therefore, if molding can be carried out while maintaining a molding temperature sufficiently above the temperature at which resin begins to melt, the leaked resin will be ejected in the molten state from the injection cylinder, and will fall from the ejection opening in the molten state.
However, depending on the type of resin used, or even with the same resin, depending on molding conditions, leaked resin is ejected in many different forms.
For example, there are many types of engineering plastics or super engineering plastic materials with narrow stable-melting molding temperature, which are easily heat decomposed; these resins are usually molded at temperture just above the lower molding temperature limit. When
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types of plastic materials having differing melting temperatures are mixed and used as an alloy in order to improve the strength of molded products, the molding temperature is adjusted to the lower limit molding temperature of the resin having the higher molding temperature, in order to prevent degradation of the resin having the lower molding temperature. In such cases, when the molding temperature falls somewhat below the lower limit temperature, the viscosity of the molten resin can increase, or internal hardening may occur. Under such circumstances leaked resin may readily harden around the ejection opening, where the temperature can easily drop.
When imparting flame resistance or other characteristics to a resin by admixture of additives to the resin, the molten resin may become more easily thermally degraded. When molding with an extremely small injection quantity per cycle, the length of the stroke which the injection plunger moves for a molding is extremely small compared to the total motion stroke inherent to the molding machine, and the leaked resin is exposed to high temperatures around the plunger shaft for a long period before being ejected, making it prone to thermal degradation. In such cases, as well, the molten resin viscosity increases, and the resin is prone to internal hardening.
When changing materials from a high molding temperature resin to a low molding temperature resin, lowering the injection cylinder heater temperature setting from high to low temperature before the pre-changeover leaked resin has been completely ejected from the ejection opening will result in a lowered temperature at the ejection opening, such that leaked resin will harden and not fall down at the ejection opening. To avoid this, it was previously necessary to sufficiently purge until the leaked resin had been completely ejected, maintaining the injection cylinder temperature setting at the pre-changeover high temperature, requiring extra time to be added to the purging operation.
An object of the present invention is to provide an injection device which will drop the leaked resin such that it does not plug up the ejection opening even when the resin is in one of the various states described above, whereby it is prone to plug up the ejection opening. Therefore, for resin capable of being ejected in the molten state, an injection device equipped with an ejection guide capable of controlling temperature independently from that of the injection cylinder is provided in order to enable reliable ejection in the molten state. For resins prone to hardening, an injection device is provided which is equipped with an ejection guide capable of ejecting the resin by dividing it into fine strands, as well as independently controlling temperature. The above ejection guides may also be made selectively detachable/attachable in response to the resin ejection state.
SUMMARY OF THE INVENTION
A plunger-type injection molding machine injection device is provided
Devinsky Paul
Luk Emmanuel
Nguyen Nam
Sodick Co. Ltd.
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