Plastic article or earthenware shaping or treating: apparatus – Female mold and charger to supply fluent stock under... – With accumulator – trap chamber – or serially arranged valves...
Reexamination Certificate
1999-01-15
2001-03-13
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Female mold and charger to supply fluent stock under...
With accumulator, trap chamber, or serially arranged valves...
C425S562000, C425S567000, C425S568000, C425S572000
Reexamination Certificate
active
06200126
ABSTRACT:
1. FIELD OF THE INVENTION
The present invention relates to an injection moulding machine including a plasticizing cylinder, which is connected to at least one nozzle opening, and including a worm operating in the plasticizing cylinder.
2. DESCRIPTION OF THE PRIOR ART
Such injection moulding machines are used predominantly for processing thermoplastic materials. They currently operate on the worm thrust principle. Granular, plasticizable material is supplied to the plasticizing cylinder whilst the worm is rotating and is conveyed by the worm in the direction towards the nozzle opening. The plastic material melts under the pressure which is built up and optionally promoted by heating the plasticizing cylinder and at the same time the material is intensively mixed. During this process the worm moves increasingly away from the nozzle opening. As soon as the dosed amount of plastic material has liquefied, the worm is moved in the direction towards the nozzle opening, whereby the plastic material is injected through the nozzle opening into the mould. The machine is then ready for the next injection cycle.
It was previously problematic to manufacture small and very small components with such injection moulding machines. This is due to the fact that the diameter of the worm cannot be reduced indefinitely. If the diameter of the worm is too small the spaces in the worm threads in the inlet of the worm are filled to very different extents. This differential filling results in large variations in the dosing time which impair the quality of the liquefied material and the reproducibility of the entire injection cycle. The worm threads must consequently be adequately dimensioned in order to be able to accommodate substantially equal amounts of granular plastic material. Furthermore, strength considerations determine the size of the diameter of the core of the worm. The overall result of this is worms of relatively large diameter which currently have a minimum diameter of 14 millimeters. In order to inject smaller amounts of material, as are required for small and very small components, these known worms may only be moved through a minimal stroke. However, at minimal stroke the reproducibility of the process as regards the weight of the injected plastic composition is inadequate as a result of the given resolution of the controller and the mass moments of inertia etc. Small and very small components have for these reasons previously been produced with a large dead head which must be disposed of, principally as waste. The additional material consumption and the additional waste disposal costs are considerable.
It is the object of the invention to develop the injection moulding machine of the type referred to above so that the material consumption can be reduced when manufacturing small and very small components.
SUMMARY OF THE INVENTION
In order to solve this object the injection moulding machine referred to above is characterised in that formed upstream of the nozzle opening there is a cylinder chamber in which a needle-shaped piston element, preferably with a round cross-section, operates, the cross-sectional area of which is smaller than the cross-sectional area of the cylinder chamber, and that a reverse flow barrier is provided which separates the plasticizing cylinder from the cylinder chamber, when it is actuated.
The injection of the dosed amount of plastic material is thus no longer effected by the worm but instead by the needle-shaped piston element specially provided for this purpose. The plastic material is fed to the cylinder chamber already in liquid form so that no contribution to the pre-plasticization need be made at this point. Instead, this is taken care of by the worm whilst simultaneously intensively mixing the material.
Since the cross-sectional area of the needle-shaped piston element is smaller than the cross-sectional area of the cylinder chamber, the plasticized material moves during the dosing process past the piston element and creates a pressure in front of the piston element by which the piston element is pushed back. When the set travel has been reached, the reverse flow barrier is actuated and the needle-shaped piston element is moved a predetermined distance in the direction towards the nozzle opening, whereby it displaces the pre-dosed material. The reverse flow barrier prevents a proportion of the material being forced back into the plasticizing cylinder during the build up of the high injection pressure. During the injection process itself, the very advantageous First In-First Out effect is produced which prevents the material which is fed first being injected last and thus being overcooled or heated for too long when it reaches the nozzle.
The dimensions of the cylinder chamber and of the needle-shaped piston element can always be selected to be so small that dosing of small and very small amounts is possible with a reasonable piston stroke. Very small components with a weight of the order of significantly less than 0.1 g may be produced without difficulty.
A preferred embodiment of the invention is characterised in that the reverse flow barrier is defined by the tip of the worm and a correspondingly shaped sealing seat and that the worm is movable axially to a limited extent within the plasticizing cylinder.
If fresh material is processed in the plasticizing cylinder after termination of an injection process in this construction, the worm is forced back by the plasticized plastic material. This backward movement of the worm within the plasticizing cylinder is limited e.g. by the fact that a collar on the worm comes into contact with an abutment on the plasticizing cylinder. Its tip has come free from the sealing seat in this position so that plasticized material can flow into the cylinder chamber through the gap which is thus produced. The pressure created in the cylinder chamber then results in the needle-shaped piston element being pushed back. In the event that the opposing mass forces which occur are too large, an advantageous embodiment of the invention resides in the fact that the return movement of the worm and of the needle-shaped piston element is assisted by an external drive.
As soon as the cylinder has received the predetermined amount of plastic material, the worm is advanced. Its tip engages the sealing seat and thus seals the cylinder chamber. The reverse flow barrier can thus not influence the shot weight. The needle-shaped piston element is then moved forwardly and the liquid plastic material is forced through the nozzle opening into the mould.
A particularly advantageous embodiment of the invention is characterised in that the worm has an axial through-hole and that the piston element extends through the through-hole. Since the needle-shaped piston element is arranged in the previously unused interior of the worm, a particularly compact construction is produced. The space requirement of this embodiment is also minimal. A plurality of injection units can be arranged correspondingly closely next to one another in an injection moulding machine.
The space requirement of an injection unit can be further reduced if the plasticizing cylinder is connected to a plurality of cylinder chambers by means of a hot runner. Only one worm is thus required for the intensive mixing and plasticization of the material. The plasticized material flows via the hot runner to the different cylinder chambers. A respective needle-shaped piston element, which performs the actual injection process, is associated with the cylinder chambers.
One embodiment of the invention, which may be realised particularly simply technically, is characterised in that the cylinder chambers are arranged with their axes parallel to one another and that the hot runner extends perpendicular to the cylinder chambers.
A preferred embodiment of the invention is characterised by at least one central valve acting on the hot runner which separates at least two cylinder chambers from one another, when it is actuated. With a star-shaped arrangement of a plurality of cylinder chambers the central
Blakely & Sokoloff, Taylor & Zafman
Dr. Boy GmbH
Heitbrink Tim
LandOfFree
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