Plastic article or earthenware shaping or treating: apparatus – Control means responsive to or actuated by means sensing or... – Temperature control
Patent
1985-12-30
1988-02-23
Hart, Charles
Plastic article or earthenware shaping or treating: apparatus
Control means responsive to or actuated by means sensing or...
Temperature control
425549, 425588, B28B 1700
Patent
active
047267518
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a plastic molding system, and more particularly to a hot-runner plastic molding system.
BACKGROUND OF THE INVENTION
In contrast to a cold-runner plastic injection molding system in which molten resin loaded in a resin passage connecting the nozzle of a plastic injection machine to each cavity of a mold (i.e., a runner) is cooled to harden together with the molten resin loaded in each cavity through the resin passage (to form an article) and ejected from the mold upon the opening of the mold, there has been known a hot-runner plastic injection molding system in which only the resin in each cavity is hardened by cooling and ejected from the mold upon the opening of the mold while the resin in the resin passage is kept in a molten state and is loaded into the cavity in the following molding cycle.
In hot-runner plastic injection molding, "severing" of the resin at the gates upon mold opening has been a major problem. For example, though there is known a hot-runner plastic injection molding system in which the resin passage for introducing molten resin from the nozzle of a plastic injection machine into each cavity of a mold is heated with an electric resistance heater from outside the resin passage to keep resin in the resin passage in a molten state, it is difficult to keep constant the temperature of resin in the resin passage near the gate of the cavity since the part of the resin passage near the gate is positioned near a cavity plate which is generally being cooled with cooling water and accordingly the temperature thereof changes by a large amount when the mold is opened or closed. When the resin temperature at the gate is too high, resin in the gate is apt to string upon mold opening, and when the resin temperature at the gate is too low, resin in the gate can harden to close the gate and prevent injection therethrough in the next molding cycle. Further, when the resin temperature at the gate is excessively high, drooling is apt to occur.
In order to overcome such problems, there has been developed a hot-runner plastic injection molding system in which a mechanical valve means is provided in each gate and the valve means is closed upon mold opening to prevent stringing and drooling while resin near the gate is constantly heated to a temperature sufficient to keep the resin in a molten state. However, this molding system is disadvantageous in that the valve means is apt to break down due to high pressure imparted thereto during the injection stroke of each molding cycle, and since opening and closure must be repeated a great number of times. Further, due to the complicated structure of the valve means, the overall size of the system is enlarged.
In another hot-runner plastic injection molding system, a pointed heater element is disposed in the resin passage in opposition to the gate and the resin in the gate is positively cooled to harden it before mold opening thereby preventing stringing and drooling, and the heater element is quickly heated immediately before the injection stroke of the next molding cycle to melt the hardened resin in the gate to permit injection therethrough. This type of hot-runner plastic injection molding system is generally referred to as an intermittent heating type hot-runner plastic injection molding system. However, various problems have been encountered with the intermittent type hot-runner plastic injection molding system. For example, it takes a long time to melt the hardened resin in the gate, lengthening the injection cycle time; also the pointed tip is apt to break or abrade especially when glass fiber filled resin is being molded. Further, since the temperature of the heater element is apt to be higher at the base portion than at the tip, if the tip of the heater element is heated to a temperature sufficient to instantly melt the hardened resin in the gate, the resin surrounding the base portion of the heater element is apt to be burnt or decomposed.
Further, since the conventional hot-runner plastic injection moldi
REFERENCES:
patent: 3339227 (1967-09-01), Ehrenfreund
patent: 3520026 (1970-07-01), Stidham et al.
patent: 4279588 (1981-07-01), Gellert
patent: 4370115 (1983-01-01), Miura
patent: 4386898 (1983-06-01), Sera
patent: 4438064 (1984-03-01), Tsutsumi
Shibata Itsuo
Uchida Tetsuo
Ferguson Jr. Gerald J.
Foycik, Jr. Michael J.
Hart Charles
Hoffman Michael P.
Ju-Oh Trading Company, Ltd.
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