Forming apparatus and method for forming highly contoured composite structures

A method for constraining a composite charge and forming the composite charge into a highly contoured composite structure, the method comprising the steps of:

• positioning the composite charge between, and in contact with, a first die and a second die of a forming apparatus, the first die having pairs of first die portions spaced apart to define a die cavity into which the composite charge is formed into a contoured hat section having a cap, the pairs being slidably displaceable with respect to each other, and the second die having a tapered portion designed for at least partial insertion into the die cavity, and the second die having a composite charge engagement end with a width, and the first die having a constraining assembly with a constraining device positioned in the die cavity, the constraining assembly comprising a pneumatic assembly coupled to the first die and having the constraining device comprising an air cylinder coupled to an air cylinder rod, the air cylinder rod having a first end coupled to a cap presser having an engagement surface engaging a cap portion of the composite charge, the air cylinder and the cap presser being removably attached to the die cavity, the cap presser comprising an aluminum bar that is semi-rigid and having a uniform width that is less than a width of the air cylinder and less than the width of the composite charge engagement end of the second die, and the width of the air cylinder being less than the width of the composite charge engagement end of the second die;
• pressing the composite charge into the die cavity of the forming apparatus, to form the contoured hat section having the cap and having sides;
• constraining the cap portion of the composite charge between the engagement surface of the cap presser and the composite charge engagement end of the second die, as the contoured hat section is formed, and applying an upward resistive force, with the constraining device, against the cap portion, and against a downward compressive force applied by the second die, to transfer stress away from the cap and to provide wrinkle prevention in the cap;
• obtaining the highly contoured composite structure having the cap that is wrinkle-free;
• and
• retaining the constraining device with a retaining element of the constraining assembly, to prevent the constraining device from pushing the highly contoured composite structure upward, when the second die is retracted.

A method for forming an aircraft stringer having a cap that is wrinkle-free, the method comprising the steps of:

• positioning a composite laminate charge between, and in contact with, a female die and a male die of a high contour stringer former apparatus, the female die having pairs of die blocks spaced apart to define a die cavity, and the female die having a constraining assembly with a constraining device disposed in the die cavity, and the male die having a composite charge engagement end with a width, and the constraining assembly comprising a pneumatic assembly coupled to the female die and having the constraining device comprising an air cylinder coupled to an air cylinder rod, the air cylinder rod having a first end coupled to a cap presser having an engagement surface engaging a cap portion of the composite laminate charge, the air cylinder and the cap presser being removably attached to the die cavity, the cap presser comprising an aluminum bar that is semi-rigid and having a uniform width that is less than a width of the air cylinder and less than the width of the composite charge engagement end of the male die, and the width of the air cylinder being less than the width of the composite charge engagement end of the male die;
• pressing the composite laminate charge into the die cavity of the high contour stringer former apparatus, to form a contoured hat section of the aircraft stringer, the contoured hat section having the cap and sides;
• constraining the cap between the engagement surface of the cap presser and the composite charge engagement end of the male die, as the contoured hat section is formed, and applying an upward resistive force, with the constraining device, against the cap, and against a downward compressive force applied by the male die, to transfer stress away from the cap and to provide wrinkle prevention in the cap;
• forming remaining portions of the composite laminate charge into the aircraft stringer, while the cap presser continues to constrain the cap and apply the upward resistive force against the cap and against the downward compressive force;
• obtaining the aircraft stringer having the cap that is wrinkle-free;
• retracting the male die away from the aircraft stringer;
• retaining the constraining device with a retaining element of the constraining assembly, to prevent the constraining device from pushing the aircraft stringer upward, when the male die is retracted;
• removing the aircraft stringer from the female die;
• and
• releasing the retaining element.