Valve nozzle having a pressure-relief means

Details Valve nozzle having a pressure-relief means Valve nozzle having a pressure-relief means

1999-04-05

2001-08-14

Heitbrink, Tim (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Female mold and charger to supply fluent stock under...

With means between charger and mold to cut off flow of...

C425S566000

Reexamination Certificate

active

06273706

ABSTRACT:

SPECIFICATION
The present invention relates to a valve-type nozzle.
In injection molds, such nozzles are installed with hot runner systems through which a plastic material melt is fed at a temperature of e.g. 200° C. under high pressure to a separable tool block that has a mold cavity and can be cooled so that the injection-molded item will quickly solidify. Depending on the material, nonuniformities such as “cold slugs” or stress-loaded areas may develop at the gate because of the abrupt temperature transition and owing to possible flow obstructions.
Furthermore, very accurate dosing of material is important and has to be warranted even with rapid succession of shots, especially for products of small dimensions. In valve-type nozzles, usually a conical tip of a generally piston-driven needle valve is employed to open and close a gate opening periodically. Since extremely high pressures of e.g. far more than 1,000 bars are effective, precise sealing in the closed position is just as necessary as exact guidance of the needle valve.
DE 3403 603 A1 describes a positively controlled needle valve for injection nozzles wherein piston guides for needle valves are provided above a distributor or manifold plate penetrated thereby; underneath said distributor, the needle valves are supported in sliding guides. Feeding channels below lead into a hot runner for a melt coming from the distributor, which flows around each needle valve and out of the hot runner so long as a needle-valve end journal will not seal a closing piece (a gate ring).
Later on, special needle sections were proposed, e.g. according to DE 197 17 381 A1, a projection or triangular bevels adjacent the needle tip. Relatively difficult to produce are radial webs provided at the lower end of the nozzle or needle sleeve, respectively, which enclose the needle valve concentrically. For a nozzle with internal heating in the upper part, EP 0 781 640 A2 provides a guide head that is integral with the needle valve, slides into an externally heated bushing and has inclined material outlets.
According to DE 32 45 571 A1, there is in the area of the needle tip a precentering body which has radial passages. However, owing to its close fit in a heat-conducting nozzle, heat losses occur that are particularly undesirable near the gate. EP 0 374 346 B1 provides lips that partially enclose the needle valve and leave only a lateral longitudinal channel open for feeding material. According to DE 296 09 356 U1, such a feed channel may be designed as an inclined needle-valve recess that passes over to a narrow centric channel on which a narrow circumferential slot forms a material outlet for injection molding of very flat items from their narrow side.
The known arrangements have in common a drawback that, in spite of considerable mechanical design efforts, reliable guidance of the needle valve is not always guaranteed under high injection pressures. Eccentric loads will not only cause a high degree of wear, but will also provoke flow irregularities in the molten material whereby the quality of finished products may be degraded.
It is an important object of the invention to overcome these and other disadvantages of the prior art by simple means and to produce an improved valve-type nozzle in which an off-balance flow load is avoided and perfect needle guidance is warranted. It is another object of the invention to apply its design principle to positively controlled valve-type nozzles as well as to individual nozzles provided with needle valves that are piston-driven either directly or via shift levers.
The features of the invention are specified in the appended claims. In accordance with the invention, a valve-type nozzle as mentioned initially for hot runner systems through which a plastic material melt is fed under high pressure to a separable and coolable tool having at least one mold cavity for injection-molded items, which nozzle is heatable and comprises at least one needle valve supported by sliding guides within the nozzle, such that the melt fed through a material pipe passes around at least a lower section of the needle valve, and further comprising upstream of a mold cavity in an antechamber at least one outlet adapted to receive a closely fitting needle, the invention provides that the needle valve includes, in a lower section upstream of the needle end, pressure-relief means in direct flow-connection with the antechamber. Therefore, as the needle end enters the outlet during the closing phase and the melt mass present in the antechamber is inevitably displaced, the resulting material compression and pressure boost will automatically even out.
According to one aspect of the invention, the pressure-relief means comprises in a guide piece at least one branch-off bore that runs e.g. radially to the longitudinal direction of the needle, at any rate at an angle which, in accordance with another feature of the invention, is smaller than or equal to 90°. In particular, according to a further feature, the or any branch-off bore may extend through the guide piece in a transverse or inclined direction. It will be recognized that such pressure relief means can be produced with a minimum of manufacturing effort.
The same applies if optionally or additionally there is, according to still another feature of the invention, in the lower section of the material pipe and in a transverse or inclined direction thereto, at least one branch-off bore an end of which opens exactly into a top zone of a free space that leads to the antechamber and is provided between the material pipe and a mouthpiece. This bore will permit or assist melt flushing and will also contribute to the pressure relief in the antechamber. In order to obtain a pressure distribution as uniform as possible, another aspect of the invention provides a group of bores at the periphery of the material pipe and/or of the guide piece, advantageously in a radial or star-type array.
Still another feature of the invention provides in the nozzle a two-stage centering device for the or any needle valve, whereby secure fit at the valve seat is guaranteed, e.g. in the fashion of a cylindrical or conical seal. In particular, according to yet another aspect of the invention, precentering means for the valve needle serve to pilot it towards the sealing face so that wear which cannot be totally avoided under heavy material load will be reduced to an absolute minimum. For this purpose, the lower needle section may, according to still another feature of the invention, have at least one taper or shoulder to be introduced in a self-centering manner into a matching internal profile of a guide piece associated to the material pipe. Although it has been known from DE 38 33 220 C2 to use, in addition to a sealed upper piston guide, a double cone at a needle tip as a lower precentering device, no pressure relief at all from the antechamber is provided in the conventional design so that a large pressure boost will occur each time the needle valve moves into its closing position. According to the present invention, however, pressure relief will always subsequently be effected via a gap between the needle valve and the mouthpiece.
According to still another aspect of the invention, the shaft of the needle is advantageously arranged above an upper taper or shoulder in an upper guide by which it is slidably enclosed. Optionally or in addition, a smaller diameter shaft portion of the needle may, by yet another aspect of the invention, be shiftable with close fit in a lower guide located above a lower taper.
In line with still other concepts of the invention, the guide piece may advantageously have two transverse bores arranged at an axial distance, which bores can be arranged at an angle to each other, in particular at right angles and staggered in height. By extremely simple means, this arrangement will thus warrant pressure relief under especially favorable flow conditions.


REFERENCES:
patent: 4582224 (1986-04-01), Proksa et al.
patent: 4836766 (1989-06-01), Gellert
patent: 5830524 (1998-11-01), Braun
patent: 32

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