BACKGROUND OF THE INVENTION

The present invention relates to fiber-composite parts. More particularly, the present invention relates to processing of fiber-composite parts.

A fiber composite part includes fibers that are dispersed within a matrix. The matrix, which is a polymer resin, surrounds and supports the fibers by maintaining their relative positions in addition to preventing the fibers from abrasion and environmental attack. The fibers impart their mechanical and physical properties to enhance those of the matrix. The combination is synergistic; the composite possesses material properties unavailable from the individual constituents, such as an exceptionally high strength-to-weight ratio.

A fiber-composite part exhibits anisotropic behavior. Such a part exhibits best achievable mechanical properties for a given part geometry when the fibers therein are aligned with the direction of principal stress everywhere throughout the part per the anticipated loading conditions. For example, such a part will have a higher strength and stiffness than a part in which fibers are unidirectionally aligned.

Optimally arranging fibers during manufacturing is typically not possible, and certainly not practical, with prior-art methods. Specifically, since the fibers within the tapes or fabric typically used to make such parts are fixed in one (tape) or two (fabric) directions, any such directional “optimization” is usually limited to offsetting successive layers of the tape/fabric by standard offsets, such as +/− increments of 30°, 45°, etc.

Applicant has taught the use of fiber-bundle-based preforms for creating fiber-composite parts. Such preforms may be formed from segments of towpreg, wherein the towpreg is cut to a desired size and bent into a desired shape. These fiber-bundle-based preforms provide an unparalleled ability to align fibers with the anticipated principal stress vectors within a part.

However, obtaining a desired bend angle, etc., in a length of towpreg can be problematic.

SUMMARY OF THE INVENTION

In the towpreg preforming process for fabricating fiber composite parts, maintaining tension while bending is desirable to achieving a repeatable and consistent bend angle. The benefit of tension is attributable to the fact that constituent fibers of the relevant towpreg must remain taught while the resin is viscous (e.g., brought above the glass transition temperature). Absent any tension while the matrix is viscous, the fibers can deform unpredictably during the bend process, resulting in undesirable fiber alignment in the bent region. U.S. Patent Publication Nos. 2018/0345604 (Escowitz) and 2018/0345605 (Escowitz), as well U.S. Patent Publication No. 2020/00171763 (Escowitz), owned by the present applicant, all teach preforming towpreg.

as the specifications of which are fully incorporated by reference hereto,

The present invention is directed to preforming towpreg into desired shapes utilizing an apparatus for bending towpreg that utilizes an involute path about a bending die. The involute path is determined to be an ideal path for maintaining tension via a preforming end effector (i.e., towpreg grippers). An involute path ensures that the towpreg is always under constant force tangent to the bending die surface where the towpreg is in contact with the bending die surface. Further, such geometry also maintains constant contact between the towpreg and the heated surface of the bending die through the entire range of motion, a property commonly used in the meshing of gear teeth.

An involute rotational path during bending may be achieved, for example, via the complex motion of a programmable three-axis SCARA (Selective Compliance Assembly Robot Arm). While effective, a SCARA robot may be cost prohibitive in a typical environment for bending towpreg. Prior to the use of a SCARA, prior art apparatus for bending towpreg did not follow an involute curve, and thus suffered bend quality issues (e.g., buckling out of plane). Rather, its rotation was configured to follow a path of constant radius. Its center of rotation was situated relative to the rotational center of the bending die such that undesirable effects were minimized to the extent possible, yet still present. Although cost effective, bend quality issues are thus unavoidable with such radial rotation due to a lack of constant tension at all bend angles. The present invention is a cost-effective means of applying tension and maintaining alignment to towpreg during the bending process.

An additional critical feature of the towpreg preforming process is ensuring the preforming end effector (e.g., towpreg grippers) is aligned with the axis of the tow. If during the bending the end effector becomes misaligned with the segment of tow that it is gripping, the end effector can introduce an undesirable kink in a segment of the towpreg. On prior art preformers, the end effector is aligned prior to bending, and as the end effector rotates to make a bend, by the nature of the geometry, the end effector becomes misaligned with the axis of the segment of the towpreg it is gripping. Using a SCARA, the orientation of the end effector may be controlled to ensure the end effector is aligned with the tow during the entire bend, but, again, does so at prohibitive cost. The present invention addresses the challenge of maintaining alignment between the bending end effector and the towpreg axis while the end effector follows an involute path to make a bend, in a system having a substantially reduced cost as compared to a system using a SCARA. It is estimated that such a mechanism is at least an order of magnitude less expensive than a SCARA mechanism, yet capable of achieving equivalent bend quality.

Design of a novel, effective, and cost-efficient mechanical method to create an apparatus for preform bending having an involute path is disclosed herein. The present invention consists of an end effector mounted to a dual-follower rotary cam system, the cam geometry providing an involute path along which the end effector moves during rotation. While the end effector translates, the end effector also rotates to maintain alignment with the axis of the towpreg being processed. Simultaneous translation and rotation is resultant from the involute cam follower mechanism.

The involute cam geometry is determined by the radius of the associated bending die, the towpreg diameter, and an additional adjustment that results in a path for the followers that provides the necessary translational displacement for tensioning towpreg. As the end effector assembly rotates, the radial distance from the grip point to the die rotational center displaces by means of the involute cam path. Thus, constant, increased or decreased axial force, as desired, and alignment can be maintained on the towpreg at all angles during the bending process.

A method for bending towpreg to maintain a desired amount of tension during bending to form a preform is provided, including inserting a towpreg into a bending die, the towpreg having a fixed, stationary end and a gripped end, the bending die having a bending die surface, applying an initial axial force to the towpreg, and moving the gripped end of the towpreg in an involute path using a cam, a cam follower and a gripper, the gripper disposed on the cam follower. The axial force to the towpreg between the gripped end and the stationary end of the towpreg from a point where the towpreg contacts a surface of the bending die is applied tangent to the surface of the bending die while bending the towpreg.

The step of moving the gripped end of the towpreg may include gripping, by an end effector, the gripped end of the towpreg. The end effector may have a grip axis and the towpreg has a longitudinal axis, wherein alignment of the grip axis and the longitudinal axis of the towpreg is maintained during bending. A step of applying heat to the towpreg may be provided.

An apparatus for bending towpreg is also provided. The apparatus includes at least one bending die disposed in a fixed position and an involute cam mechanism. The involute cam mechanism includes a drive shaft rotatable about a drive shaft axis, a cam disposed on the drive shaft, the cam having a cam surface, and at least one cam follower following an involute cam path around the cam. The cam follower is biased against the cam by a biasing member. The cam surface provides for movement of the cam follower in an involute path. A gripper is disposed on the cam follower for gripping a first end of the towpreg. A clamp is provided for fixing a second end of the towpreg adjacent to the bending die, which is fixed relative to the first end of the towpreg.

The follower may be dual followers. The bending die may be heated. The biasing member may be a spring. The axial force may be a tensile force. The axial force may increase, decrease or remain constant during bending. The axial force may be a compressive force.

