Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
2001-04-27
2004-03-02
Heitbrink, Jill L. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C264S328190, C425S566000, C425S571000
Reexamination Certificate
active
06699422
ABSTRACT:
The invention concerns the field of molding of plastic parts with compression injection molding using a repository space which communicates with the mold for temporary storage of the molding material melt.
The fabrication of plastic semimanufactured products such as granulates or similar materials is performed by preparing a melt and then pressing it in a mold at high-pressure, cooling the mass, and ejecting the molded part after the mold is opened. The pressures required for this are an important criterion for the constructive implementation of the machines and molds, above all in regard to the locking pressures obtaining. Therefore, injection molding technology has very high demands in this connection if surface-structured workpieces, such as those which require sharp-edged filling, are to be manufactured. To reduce the process pressure, solutions are known in which the material melts are fed into a repository space before the actual injection and are transferred from this repository space into the mold. Thus, the German patent application DE 196 31 209 A1 describes a process and a device for injection molding of plastic parts of this type, according to which a predetermined amount of a plastic melt is fed into a mold cavity of a mold via a sprue channel, with at least a part of the amount first fed into a reserve cavity located in the mold and then transferred into the mold cavity. The volume of the reserve cavity can hereby be approximately the same as or smaller than the volume of the predetermined quantity of melt. This known process is performed by means of the device described. This device has a mold with a mold cavity, an injection unit which can be attached to the mold, and at least one channel which connects the mold cavity with the available injection unit. Furthermore, the mold has a reserve cavity which is connected with the channel and which is provided with means for transfer of the melt from the reserve cavity into the mold cavity.
In a version characterized as preferred, the reserve cavity is implemented as a cylindrical boring and the transfer means is implemented as a piston which is axially displaceable in the cylinder, with the transfer of the melt from the reserve cavity into the mold cavity performed by application of a force to the piston, with the force exercised by a pressure spring or an adjustable pressure cylinder with a second piston.
This known solution has the disadvantage that a not insignificant amount of the molding material melt reaches the form cavity at high pressure directly from the injection cylinder before the reserve cavity is filled, so that the molding material melt goes into the solidification phase at varying times. This, however, leads to structure faults in the workpiece. These type of structure faults do not have the same effect in the production of very flat workpieces, i.e. essentially two-dimensional workpieces such as smartcards or compact discs, as they do in the production of workpieces with complicated geometry.
Another technology is compression injection molding. In a publication of the firm Hoechst AG of Frankfurt am Main titled “Einführung in die Technologie der Kunststoffe. [Introduction into the Technology of Plastics]”, Dipl.-Ing. Hans Domininghaus describes this process. To manufacture large-area injection molded parts which exceed the capacity of an injection molding machine, particularly in regard to closing and injection pressures, the mold is forced open a small amount by the effect of the injection pressure. After the entire volume has been injected, the full closing pressure is applied in a stamping stroke. The use of this procedure requires a plunger piston mold which sufficiently seals the inner cavity even if the mold halves are not completely closed. The advantageous effect consists of a better utilization of the machine and articles with lower stress due to the pressure dwell acting equally on the entire injection surface during production. The capacity range of an injection molding machine is increased and the quality of the articles is improved by means of an applied process in which the contraction compensation is performed by means of an elastic tie bar extension in the range of hundredths of a millimeter.
The known versions of compression injection molding processes have the disadvantage that the mold halves are completely closed only after the molding material has been injected, whereby an excess of molding material must be fed into the cavity, leading to mold mark formation on the crimping edges of the mold halves, so that costly mechanical post processing of the workpieces is necessary.
The fabrication of plastic semimanufactured products such as granulates or similar materials is performed by preparing a melt and then pressing it in a mold at high-pressure, cooling the mass, and ejecting the molded part after the mold is opened. The pressures required for this are an important criterion for the constructive implementation of the machines and molds, above all in regard to the locking pressures obtaining. Therefore, injection molding technology has very high demands in this connection if surface-structured workpieces, such as those which require sharp-edged filling, are to be manufactured.
An injection molding machine for the processing of thermoplastics is known from the German disclosure document 11 74 491, according to which the injection materials are not fed directly from an injection nozzle attached to an extruder into a mold, but rather by means of a cylinder in which a piston is positioned so that it is translationally movable in order to form a repository for the injection material, which is emptied by means of the piston into the mold. For this purpose, the piston has a channel whose outlet is implemented as a nozzle and which corresponds with a sprue boring in the mold. The piston is suspended via cables running over rollers from a yoke rigidly attached to the extruder outlet, with counterweights located on the free cable ends. The piston stroke occurs when the mold is pressed against the piston and displaces it in the cylinder until the cylinder is emptied through the piston channel into the mold, which corresponds via the sprue boring. The counterweights hereby have the purpose of reducing the process pressures.
This known injection molding machine has the disadvantage that the emptying of the repository occurs via a very narrow path consisting of the piston channel terminated by a nozzle and the corresponding sprue boring, and therefore high process pressures must be applied, which must in turn be reduced by use of the potential energy: of the counterweights. A further disadvantage is the technological expense of mechanically removing the sprue slugs of the injection molded parts produced.
Providing an accommodation space into which the melt is fed and then pressed by means of a plunger into the mold cavity, in order to be able to produce the variety of forms of injection molding technology as well as larger volumes and formats than in injection molding by reduction of the operating pressures, is known from the publication of the international application WO 98/09768, which is based on the priority of the South African applications 96/7509 and 97/4923. The known device for production of molded parts comprises a transfer unit which includes an accommodation space for accommodation of a batch of molding material. The molding material reaches a tube in a granulated or molten state through a heated passage by means of a supply spindle and is then displaced by means of plunger into the accommodation space. The transfer unit is then brought into a second position in which the accommodation space aligns with an opening in a mold plate. The opening is connected with an injection cavity in the shape of the molded part to be produced. In this position, the transfer unit displaces the batch of molten molding material via a second plunger out of the accommodation space through the opening in the injection cavity. In the filling position, the accommodation space is open on one side,
Collard & Roe P.C.
Heitbrink Jill L.
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