Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to...
Reexamination Certificate
1998-09-28
2001-09-25
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
C264S519000, C264S572000, C264S328160
Reexamination Certificate
active
06294126
ABSTRACT:
BACKGROUND OF THE INVENTION
Pressurized gas injection between a molten or cooling plastic and a molding tool wall has been used as a technique for improving the surface quality of molded articles. For example, German patent document DE 195 16 290 C2 describes a plastic molding process in which, after the introduction of the plastic melt into the mold cavity, pressurized gas is injected through a fluid injection nozzle, so that a fluid blanket is created between the cavity wall and the plastic. A gas nozzle with a gap-shaped opening is used for this injection. Constrictions or wings are provided to limit the spread of the gas blanket.
Another process is known from WO 90/06220. A quantity of plastic melt, sufficient for the formation of a molded part, is introduced into the cavity of an injection molding tool. It is recognized that the plastic is subject to shrinking during the cooling process, and consequently, sink marks will typically occur. In order to avoid this, the pressurized gas is also introduced here, between the cavity surface and the molten plastic, as a blanket. Thus, the blanket is built up in a targeted manner and can be later removed.
WO 90/06220 describes two solutions for controlling the gas blanket. First, valves are provided that make possible gas sealing. By actuating, i.e., axial adjustment, of the valve shaft, the valve is selectively converted between the “open” and “closed” positions. Thus, the desired gas blanket can be created and removed in combination with the appropriate gas control. Alternatively, a porous sintered metal can be used. At the transition site in the molding tool where the gas blanket will be produced, a porous metal element is provided, through which pressurized gas, but not molten plastic, can flow. Therefore, targeted gassing is achieved without backflow of molten plastic. Detailed information on control of the extent of this gas pocket, however, is not found in the publication.
Others, however, have proposed techniques to control the size of the gas pocket. WO 93/01039, like DE 195 16 290, concerns the creation of the gas blanket between the cavity surface and the molten plastic and its restriction such that the blanket does not spread undesirably. The solution is to roughen the cavity surface where the gas blanket will be constructed to facilitate gas diffusion, and form smooth cavity surfaces elsewhere.
SUMMARY OF THE INVENTION
The solutions proposed for limiting the gas blanket, however, are not applicable to or successful in all cases.
The present invention concerns a process and device that makes it possible to achieve a precisely defined limit for the gas blanket, so that it is controlled to spread only in a desired region.
The invention concerns a process for the injection molding of plastic objects from thermoplastic, duraplastic or elastomeric material, which has the steps of injecting a sufficient quantity of molten plastic into the cavity of a mold along a plastic melt flow path, which extends from a unit for plasticizing and injecting the plastic through a plastic injection nozzle into the mold; and introducing pressurized fluid, particularly a pressurized gas, from one wall of the cavity by means of at least one fluid injection element, so that a fluid blanket forms between the wall of the cavity and the plastic melt. The molted plastic part thus produced is then allowed to harden. Finally, the cavity is relieved of the pressure of the pressurized fluid and the part demolded.
Further, the invention concerns a device for conducting the process.
The invention is characterized in that at least one surface projection is provided in the cavity wall of the mold. The upper plateau of the projection should be essentially parallel to the cavity wall and is preferably flat. The pressurized fluid is injected into the region of the upper plateau of the projection by the fluid injection element, so that a fluid blanket forms between the plastic and upper plateau.
The process is based on the understanding that good surface characteristics for the molted part cannot be obtained without the operation of the gas blanket between the cavity wall and the plastic. The extent of the blanket, however, is controlled by means of a defined face of the plateau, which is gas sealed at its side surfaces. The gas blanket can form only in the region of the upper plateau; the gas bubble cannot extend or bleed out from the region of the surface projection.
In order to be certain that the gas pocket does not spread out over the upper plateau of the projection, it is further provided that the side surfaces or walls of the surface projection are tempered so that the localized solidification behavior of the plastic is controllable. Independent of whether thermoplast or duroplast is processed, lateral or side regions of the projection are preferably either heated or cooled for a predetermined time.
For example, if the lateral regions are cooled for a specific time in the processing of a thermoplast, a more rapid solidification of the plastic occurs there. The solidified regions form a gas seal surrounding the upper plateau of the surface projection, where the gas blanket is found, so that spreading of the blanket is prohibited.
Advantageously, it is further provided that the injection of the plastic melt is conducted in such a way that prior to the introduction of the pressurized fluid between the wall of cavity and the plastic melt, the cavity of the mold is completely filled with the plastic melt using a plastic melt pressure such that the plastic material remains prestressed during the first phase of the solidification. Thus the entire surface of the flat projection is reliably brought into contact with the molten plastic, so that defined marginal conditions at the side surfaces are created in order to carry-out the invention. Generally, this could not necessarily be assured, if the cavity were only partially filled, since then the sealing at the side surfaces of the flat projection would not be complete. This effect could lead to the failure of the inventive process.
According to a specific embodiment, it is further provided that the introduction of the fluid through the fluid injection element is controlled or regulated as a function of the pressure prevailing in the cavity and/or as a function of the time that has elapsed since the beginning of the injection of the plastic melt and/or as a function of the injection path of the extruder screw. Further, the pressure of the pressurized fluid, which is introduced into cavity via fluid injection element, can be controlled or regulated according to a predetermined time profile.
In general according to another aspect, the device according to the invention has a mold with a cavity comprised of typically of two or more sections. A unit for plasticizing and injecting the plastic produces the plastic melt for filling a cavity of the mold via a plastic injection nozzle. At least one fluid injection element introduces pressurized fluid, particularly pressurized gas, from one wall of the cavity into the cavity, so that a fluid blanket forms between the wall and the plastic melt, which has been introduced into mold. At least one projection is introduced on a wall of the cavity and preferably runs generally parallel to the cavity wall in the region of its upper plateau. At least one fluid injection element is integrated in such a way that fluid blanket forms between the plastic and the upper plateau of the flat projection after gas injection through the fluid injection element.
As has been explained above, tempering elements are preferably arranged on or near the lateral side wall of the projection, and the temperature of the plastic is locally controlled with these elements. The tempering element may heat or cool as dictated by the type of plastic used.
It is possible that the fluid injection nozzles have a gap-shaped nozzle that opens into the cavity in a flush arrangement with the upper plateau of the flat projection. Alternatively, it may also may be provided that the fluid injection nozzles are comprised of m
Eckardt Helmut
Gosdin Michael
Battenfeld GmbH
Hamilton Brook Smith & Reynolds P.C.
Heitbrink Jill L.
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