Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – Shaping against forming surface
Reexamination Certificate
1998-11-16
2001-01-16
Pyon, Harold (Department: 1722)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
Shaping against forming surface
C425S394000, C425S412000, C425S417000, C249S142000
Reexamination Certificate
active
06174484
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to compression molding and, more particularly, is concerned with an apparatus and method for producing a compression molded product.
In a compression molding process, a body of material is typically placed directly in a cavity of the mold of a die set and compression molding takes place under application of pressure with the plug of the die set. Many fiber-reinforced composite structures are formed using this process. In order to obtain good bonding between the reinforcing fibers and polymer matrix of a composite material as well as to protect the fibers during subsequent molding, the reinforcing fibers are surface treated by impregnation to enhance adhesion to the polymer matrix. The resulting partially cured material is commonly called a prepreg.
Pressure is applied to the mold and/or plug of the die set to accomplish the compression into the desired form of the prepreg lay-up in the mold. Pressure can be applied by a vacuum-bagging technique wherein the pressure required to form the shape and create good bonding is obtained by covering the lay-up prepreg with a plastic bag and then creating a vacuum in the mold cavity. Heat is also applied to fully cure the material. If additional heat and pressure are desired, the entire die set and prepreg are placed in an autoclave.
A problem of the above-described conventional compression molding process is the high cost and complexity associated with the conventional application of heat and pressure to the process. Consequently, a need still exists for an innovation which will provide a solution to the aforementioned problem.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a compression molding apparatus and method designed to satisfy the aforementioned need. The compression molding apparatus and method of the present invention employs thermal expansion differential between the plug and mold of the molding die set to produce the pressure necessary to make structures with repeatable results ensuring a quality finished product. The molding apparatus and method of the present invention eliminates the need for vacuum-bagging and autoclaving to produce the required pressure and temperature to cure the fiber-reinforced composite prepreg structure.
In an embodiment of the present invention, a compression molding apparatus is provided which comprises a mold formed of a first material having a first coefficient of thermal expansion, and a plug formed of a second material having a second coefficient of thermal expansion different from the first coefficient of thermal expansion of the first material of the mold. One of the mold and plug has a cavity into which the other of the mold and plug is placed such that the one is disposed relative to the other so as to provide an air gap between them having a first volume for holding a body of a third material at least partially filling the gap when the mold and plug are at a first temperature and for molding the body into a molded product when the mold and plug are subjected to a second temperature that causes one of the first and second materials of the mold and plug to change in size more than the other and thereby change the air gap to a second volume different from the first volume and compress the body between the plug and mold into the molded product. The change in the volume of the gap is caused essentially only by the change in temperature.
As an example, the second coefficient of thermal expansion of the second material of the plug is greater than the first coefficient of thermal expansion of the first material of the mold such that the first material of the mold expands during the change in temperature from the first temperature to the second temperature but to a lesser degree than the second material of the plug during the change in temperature from the first temperature to the second temperature. Furthermore, the second temperature is greater than the first temperature. The first temperature can be about room temperature whereas the second temperature can be about 250° Fahrenheit. The third material of the body can be a composite structure, such as a structure having graphite fibers embedded in an epoxy matrix.
The present invention also provides a compression molding method which comprises the steps of providing a mold of a first material having a first coefficient of thermal expansion and defining an open cavity, providing a plug of a second material having a second coefficient of thermal expansion different from the first coefficient of thermal expansion, positioning the mold relative to the plug with a body of a third material in the cavity of the mold and at a first temperature such that an air gap having a first volume is established between the plug and mold and the body at least partially fills the gap, and changing from the first temperature to a second temperature so as to cause one of the first material of the mold and the second material of the plug to expand relative to the other and thereby decrease the volume of the air gap and compress the body to a predetermined size and configuration between the plug and mold. The change in the volume of the gap is caused essentially only by the change in temperature.
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Plastics Mold Engineering Handbook, 3rd ed. New York, Van Nostrand Reinhold Company, 1978. pp. 112-137.
Thompson Paul Shadforth
van Oort Johannes Martinus
General Electric Company
Nguyen Thukhanh T.
Pyon Harold
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