Plastic article or earthenware shaping or treating: apparatus – With interposed non-adhering web or sheet type parting means
Reexamination Certificate
1998-03-11
2001-07-24
Silbaugh, Jan H. (Department: 1732)
Plastic article or earthenware shaping or treating: apparatus
With interposed non-adhering web or sheet type parting means
C425S391000, C425S544000, C264S145000, C264S257000, C264S281000, C264S285000
Reexamination Certificate
active
06264454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates generally to a method and an apparatus for forming SMC charges for molding processes and specifically to a method and an apparatus for forming wrapped charges from strips of SMC for use in molding fiber reinforced plastic components.
2. Discussion
Fiber reinforced plastic (FRP) parts are being increasingly considered for use in a wide variety of applications. An FRP part generally consists of a plastic shape in which carbon, glass fiber, or other reinforcing fibers are dispersed in order to provide strength to the resin.
An FRP product made from a thermosetting resin, particularly in the form of a sheet molded compound (SMC), can be formulated to have critical characteristics that are similar to the steel part it is intended to replace For example, an SMC part generally has a coefficient of thermal expansion which is equivalent to that of steel and is able to sustain an E-coat (metal protection) temperature of 400° F. Furthermore, an SMC part can be installed by mechanical fasteners or by bonding to metal in the production body shop of an original equipment manufacturer, side-by-side with an equivalent steel body part.
There are also competing thermoplastic materials which can be used instead of thermosetting resins. A number of thermoplastic materials, both reinforced and non-reinforced, have been evaluated in the automotive industry for primarily vertical components such as fenders and outer door panels. This class of materials has generally been limited to vertical panels because their stiffness is not sufficient (even in the reinforced version) for horizontal panels such as hoods and decklids.
These attempts at using thermoplastics have not been totally successful. Unlike thermosets, thermoplastics are not compatible with the automotive assembly line processes. Thermoplastics generally have to be processed separately from the E-coat bake ovens and require greater-than-desired gaps between mating surface panels to allow for their high coefficient of thermal expansion.
There are three primary processes to produce higher volume chopped fiber thermoset composite components, and there are features of each process which allow one to be the selected choice for a particular application. These three processes are compression molding, injection molding and transfer molding.
In compression molding, a charge such as a sheet molded compound containing a curable resin is placed between upper and lower heated die members defining a mold cavity. The dies are then brought to a closed position during which the dies compress the charge causing it to flow and fill the mold cavity. After the resin cures, the dies are opened and the finished part is removed. Compression molding has been historically the process of choice in making fiber reinforced thermoset composite components which require surface finish, mechanical properties and dimensional stability.
In thermoset injection molding, a plastic is injected into a cavity defined between two die halves. After cross-linking of the polymers is completed, the molds are separated and the finished part is ejected. Injection molding offers design flexibility benefits through enhanced part integration.
In transfer molding, a charge is deposited into a preheating transfer pot. A movable platform is used to drive the heated, flowable charge out of the transfer pot and through a series of channels into a plurality of mold cavities. Transfer molding is used for parts that are too small and intricate for compression molding.
Each approach of the known prior art suffers some disadvantages. Compression molding is relatively expensive. Injection molding and transfer molding cause severe degradation in mechanical properties of the resulting component because of deterioration in the integrity of the chopped fibers resulting from transportation of the material.
An additional disadvantage suffered by the prior art is the difficulty in forming and manipulating conventional stamped stack charges, as well as loading this type of charge into the molding dies. Thus, there remains the desire for an apparatus and process, which efficiently enables the molding of fiber reinforced plastic components of relatively high complexity, where the formation and manipulation of the charge used in the molding process is simple and efficient, the transfer of the prepared charge to the molding die is also efficient and the strength of the resulting component is not compromised because of fiber deterioration during the molding process.
SUMMARY OF THE INVENTION
The broad teachings of the present invention provide an improved apparatus and process for molding relatively complex fiber reinforced components at a high rate of production. The invention finds particular utility in providing an efficient and cost effective means for producing components of relatively high complexity. The present invention is advantageous over its traditional predecessors in that the formation and manipulation of the charge used in the molding process, as well as the transfer of the prepared charge to the molding die, is made simple and efficient.
Generally, the present invention incorporates a molding unit comprising upper and lower die halves with a charge overflow assembly incorporated into the lower die half. The lower die half further incorporates a hydraulically driven gate ram positioned within an apertured gateway. According to a preferred process of the present invention, a wrapped charge is prepared and automatically loaded into the apertured gateway, while the molding unit is opened and the gate ram is retracted to the depth of the pre-calculated charge height. The upper die half is lowered onto stops on the lower die half, thereby closing the molding unit and forming a mold cavity between the die halves. The gate ram then advances via a hydraulic cylinder so that the wrapped charge is pressed into the mold cavity. Excess charge pressed into the mold cavity is received by the charge overflow assembly.
According to one embodiment of the present invention, the charge overflow assembly includes an overflow pin positioned within an overflow cavity. The overflow assembly also includes a hydraulic cylinder placed behind the overflow pin for actuating movement of the overflow pin and a displacement sensing transducer attached to the hydraulic cylinder for measuring the displacement of the overflow pin caused by the excess charge from the mold cavity. According to the process of the present invention, excess charge from the mold cavity is displaced into the overflow cavity, thereby overcoming the oppositely directed force exerted on the overflow pin from the hydraulic cylinder. Prior to displacement of the overflow pin by the excess charge, the overflow pin is maintained in an upwardly biased position that is level with the lower surface of the mold cavity. Once the displacement of the overflow pin is measured by the transducer, the measurement data may either be manually gaged and adjusted for subsequent runs or automatically gaged and adjusted by a microprocessor, which averages the overflow pin displacement for a set number of moldings and progressively sends adjustment signals to the feeding assembly to adjust, accordingly, the amount of the SMC sheet advanced, and therefore cut, for each molding. The microprocessor may also communicate with a regulator valve on the hydraulic cylinder of the charge overflow assembly to automatically adjust the hydraulic pressure required to keep the overflow pin in its upwardly biased position, so that the SMC molding pressure can be applied to the mold cavity without displacing the overflow pin, while still allowing displacement of the overflow pin in response to any excess SMC material forced into the mold cavity by the gate ram.
A preferred embodiment of the present invention incorporates a charge forming unit for preparing and handling the charge. The charge forming unit includes a feeding assembly for advancing the sheet material, a cutting assembly for cutting the sheet into strips, a rolling a
Eashoo Mark
Harness & Dickey & Pierce P.L.C.
Silbaugh Jan H.
The Budd Company
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