Plastic article or earthenware shaping or treating: apparatus – Means feeding fluent stock from plural sources to common...
Reexamination Certificate
2000-07-07
2001-07-24
Heitbrink, Tim (Department: 1722)
Plastic article or earthenware shaping or treating: apparatus
Means feeding fluent stock from plural sources to common...
C425S577000
Reexamination Certificate
active
06264455
ABSTRACT:
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,101,617 to Friederich discloses a method for producing plastic molded parts that have hollow sections using the so-called internal gas pressure method. An injection molding tool has a cavity, the walls of which define the surface of the part being produced. Plastic melt is injected into the closed tool through a plastic injection nozzle. Before the plastic melt hardens, a pressurized fluid, typically pressurized nitrogen gas, is injected into the interior of the plastic melt, so that the melt is pressed against the cavity walls thus forming hollow spaces. The gas pressure is maintained until the molded part cools down. The method advantageously achieves the result that the molded part is lighter, requires less plastic to produce, and has especially good surface characteristics, since the contraction of the material due to cooling is counteracted by the internal gas pressure. This effect prevents the formation of sink marks on the surface of the molded part.
The Friederich patent treats the simple case where the molded part being produced is essentially a tube that is to be formed as a hollow part by the internal gas pressure method. By analogy, the method can also be used to produce more complex molded parts having hollow regions as well as regions that are solid plastic.
SUMMARY OF THE INVENTION
One problem that arises, however, is that—regardless whether a particular volume of gas or gas under a defined pressure is injected into the melt—the dimensions of the gas bubble that forms in the interior, i.e., the hollow space in the interior of the molded body, is hard to reproduce consistently. The hollow space may be larger for one “shot” than for another, although the process parameters are the same. The parameters can only be held constant within the framework of a certain level of variation. The non-uniformities resulting from this variation lead to differently shaped hollow regions from workpiece to workpiece. This is especially disadvantageous in the case of parts for which the size of the hollow space produced by the internal gas pressure method must be precisely defined, certain regions needing to be solid for strength reasons while others being hollow for weight reasons.
The present invention concerns a method and an associated apparatus in which it is possible to produce molded parts by the internal gas pressure method, the molded parts having adjoining hollow and solid regions, but in which reliable and reproducible operation is achieved.
The invention achieves this result by blocking off a part of the cavity of a molding tool, so that no plastic melt can reach the blocked-off region. The plastic melt is then injected into the unblocked region of the cavity of the molding tool along a melt flow path, which extends from a plasticizing unit through a plastic injection nozzle into the molding tool.
The block is subsequently released, so that the plastic melt can reach the previously blocked-off region of the cavity. Simultaneously and/or subsequently to injecting the plastic melt, the pressurized fluid, especially pressurized gas, is injected into the melt by means of at least one fluid injection nozzle, so that the melt injected into the molding tool is distributed into the desired regions of the cavity. A hollow space is thus formed and the melt is pressed against the cavity walls of the molding tool. Once the molded part cools down to a temperature below the melting point of the plastic melt, the cavity is relieved of the pressure of the compressed fluid and the molded part demolded.
The method is therefore characterized in that first a divided-off region of the molded part or of the cavity is excepted from being filled with melt, while another region is filled. The division is then eliminated, allowing the melt to reach that previously divided-off region thereby creating the portion of the molded part that is supposed to be made solid. The redistribution of melt in the region of the cavity that has only now been released is accomplished by injecting pressurized gas into the melt which is initially situated in the filled region.
This has the advantageous effect that, without any other assisting devices—with the exception of a blocking element in the tool—and only by using that amount of melt which is needed to form the molded part, a workpiece can be produced which can be precisely defined with respect to the formation of the hollow space.
In specific embodiments, the method advantageously is further characterized in that the plastic melt of the process described above is injected until the unblocked region of the cavity is completely filled with plastic melt. Furthermore, the invention can provide that the blockage described above is not complete and, in order to avoid switch-over marks on the molded part, a small portion of plastic melt is allowed to penetrate into the blocked-off region.
In this process, the blocking and opening of the part of the cavity can be controlled or regulated. Advantageously, this is done in dependence on the time after the injection molding cycle begins or in dependence on the pressure in the cavity.
Still further, the region of blockage may be heated in order to avoid freezing.
In general according to another aspect, the invention is also directed to molding an apparatus. It comprises a molding tool, including at least two halves, with a cavity, a plasticizing unit, and a plastic injection nozzle that supplies plastic melt into the tool through at least one fluid injection nozzle.
The invention provides for means for essentially blocking off a part of the cavity whereby a slider element is used to block off a part of the cavity that is later withdrawn from it to release the melt into the previously blocked off region.
A further development of the invention provides that the means for blocking off a part of the cavity are influenced by control or regulation means. Finally, the invention can provide heat into the blocked-off region in order to stabilize the process.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.
REFERENCES:
patent: 2337550 (1943-12-01), Crosby
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patent: 5304341 (1994-04-01), Shah
patent: 5417916 (1995-05-01), Ladney
patent: 5423667 (1995-06-01), Jaroschek
patent: 5534216 (1996-07-01), Kamiyama
patent: 5612067 (1997-03-01), Kurihara et al.
patent: 5776412 (1998-07-01), Eckardt et al.
patent: 5849377 (1998-12-01), Horikoshi et al.
patent: 6143237 (2000-11-01), Eckardt et al.
Eckardt Helmut
Ehritt Jurgen
Gosdin Michael
Seuthe Alfons
Battenfeld GmbH
Hamilton Brook Smith & Reynolds P.C.
Heitbrink Tim
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