SYSTEM AND METHOD FOR BENDING TOWPREG

Abstract

A method for bending towpreg to yield a desired amount of axial force during bending to form a preform is provided including inserting towpreg into a bending die, 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. Axial force 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. An apparatus for bending towpreg to form a preform is provided including bending dies disposed in fixed positions, and a follower. Axial involute cam mechanism having a drive shaft, a cam, a cam follower that follows an involute path around the cam, a gripper and a clamp.

Claims

1. A method for bending towpreg to yield a desired amount of axial force during bending to form a preform, comprising: (a) inserting towpreg into a bending die, the towpreg having a fixed, stationary end and a gripped end, the bending die having a bending die surface; (b) applying an initial axial force to the towpreg; (c) 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; whereby the axial force 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.

2. The method for bending towpreg of claim 1, wherein the step of moving the gripped end of the towpreg comprises gripping, by an end effector, the gripped end of the towpreg.

3. The method of bending towpreg of claim 2, wherein the end effector has 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.

4. The method for bending towpreg of claim 1, including the step of applying heat to the towpreg.

5. An apparatus for bending towpreg to form a preform, comprising: (a) at least one bending die disposed in a fixed position; (b) an involute cam mechanism, comprising: (i) a drive shaft rotatable about a drive shaft axis; (ii) a cam disposed on the drive shaft, the cam having a cam surface; (iii) at least one cam follower, said at least one cam follower following an involute cam path around the cam, said cam follower biased against the cam by a biasing member; (iv) the cam surface providing for movement of the cam follower in an involute path; (c) a gripper disposed on the cam follower, said gripper for gripping a first end of the towpreg (d) a clamp for fixing a second end of the towpreg adjacent to the bending die, and fixed relative to the first end of the towpreg.

6. The apparatus for bending towpreg of claim 5, wherein the at least one follower is dual followers.

7. The apparatus for bending towpreg of claim 5 wherein the at least one bending die is heated.

8. The apparatus for bending towpreg of claim 5 wherein the biasing member is a spring.

9. The apparatus for bending towpreg of claim 5, wherein the axial force is a tensile force.

10. The apparatus for bending towpreg of claim 4, wherein the axial force is a compressive force.

11. The apparatus for bending towpreg of claim 5, wherein the axial force increases during bending.

12. The apparatus for bending towpreg of claim 4, wherein the axial force decreases during bending.

13. The apparatus for bending towpreg of claim 4, wherein the axial force remains constant during bending.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:

[0020] FIG. 1 is a simplified plan view of a prior art apparatus for bending towpreg;

[0021] FIG. 2 is a simplified plan view of an apparatus for bending towpreg in accordance with an exemplary embodiment of the present invention;

[0022] FIG. 2A is one example of a preform that may be formed with the apparatus for bending of FIG. 2.

[0023] FIG. 3 is an isometric view of the apparatus for bending towpreg of FIG. 2;

[0024] FIG. 4 is a partial top, plan view of the apparatus for bending towpreg of FIG. 2;

[0025] FIG. 5 is another partial isometric view of the apparatus for bending towpreg of FIG. 2;

[0026] FIG. 6 is a partial bottom, plan view of the apparatus for bending towpreg of FIG. 2; and

[0027] FIG. 7 is a flowchart of a method for bending towpreg to yield a desired amount of axial force during bending in accordance with an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0028] The following terms, and their inflected forms, are defined for use in this disclosure and the appended claims as follows: [0029] “Involute path” means a curved path defined by the locus of a point on a piece of taut string as the string is either unwrapped from or wrapped around the circle. That is, the curve for which all the normals are tangent to the fixed circle. It is the curve traced by a hand unwinding a wire reel held in the other hand. [0030] “Preform” means a bundle of plural, unidirectionally aligned, same-length, resin-wetted fibers. The bundle is often (but not necessarily) sourced from a long length of towpreg. That is, the bundle is a segment of towpreg that has been cut to a desired size and, in many cases, is shaped (e.g., bent, twisted, etc.) to a specific form, as appropriate for the specific part being molded. The cross section of the preform, and the fiber bundle from which it is sourced typically has an aspect ratio (width-to-thickness) of between about 0.25 to about 6. Nearly all fibers in a given preform have the same length (i.e., the length of the preform) and, as previously noted, are unidirectionally aligned. Applicant's use of the term “preform” means a fiber-bundle-based preform, and explicitly excludes any size of shaped pieces of: (i) tape (typically having an aspect ratio—cross section, as above—of between about 10 to about 30), (ii) sheets of fiber, and (iii) laminates. [0031] “Tow” means an untwisted and unidirectional bundle of continuous fiber. The term “bundle” is used herein synonymously with the terms roving and tow. Tows usually contain multiples of 1000 fibers, such as a 1K tow (1000 fibers), a 12K tow (12,000 fibers), a 24K tow (24,000 fibers), etc. [0032] “Towpreg” means a fiber bundle (i.e., a tow) that is impregnated with resin. [0033] “About” or “Substantially” means+/−20% with respect to a stated figure or nominal value.

[0034] Other than in the examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and in the claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are understood to be approximations that may vary depending upon the desired properties to be obtained in ways that will be understood by those skilled in the art. Generally, this means a variation of at least +/−20%.

[0035] Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges encompassed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of about 1 and the recited maximum value of about 10, that is, having a minimum value equal to or greater than about 1 and a maximum value of equal to or less than about 10.

[0036] Referring now to the drawing figures wherein like reference numbers refer to like elements throughout the several views, there is shown in FIG. 1 a prior art bending apparatus 10 that depicts a segment of towpreg 12 being processed. The towpreg 12 is shown mounted in a bending die (including left bending die 14a and right bending die 14b) of the bending apparatus 10. The prior art bending apparatus 10 includes an end effector 16 having a grip point 18. Relative positions of the end effector grip point 18, rotational path 20 of the grip point 18, end effector center of rotation 22, and die 14a, 14b centers of rotation 24a, 24b are also shown. The feed axis Y--Y of the towpreg 12 can be considered a bend angle of zero degrees, with +/−ninety degree bends represented by the axis −X--+X. It should be noted that the end effector center of rotation 22 is coincident with neither the left bending die 14a nor the right bending die 14b centers of rotation 24a, 24b. Further, the radius R of the rotational path 20 of the grip point 18 remains constant.

[0037] To preform all relevant shapes, the bending apparatus 10 for towpreg 12 must bend in both the positive and negative ranges of angles, as shown in FIG. 1. This necessity mandates the lack of coincidence between the end effector center of rotation 22 and the die centers of rotation 24a, 24b, assuming that the end effector center of rotation 22 is fixed (i.e., does not translate to coincide with the die centers of rotation 24a, 24b). For reasons of mechanical precision and bend accuracy in both bend directions (−X, +X), it is advantageous to keep the end effector center of rotation 22 as a fixed point relative to the bending dies 14a, 14b.

[0038] Given the lack of coincidence in centers of rotation 22, 24a, 24b, the towpreg 12 is not tensioned during bending. In such a bending apparatus 10, this negative effect worsens as the bend angle increases, since the radial distance from the grip point 18 to the die center of rotation changes accordingly (compare length D.sub.1 to length D.sub.2). This dynamic radial change between the grip point and die center causes buckling in the viscous bend region of the towpreg 12 through a lack of tension.

[0039] Maintaining a precisely located end effector center of rotation 22, while simultaneously maintaining the necessary distances and alignments between the grip point 18 and die 14a center of rotation 24a and grip point 18 and die 14b center of rotation 24b to apply tension, therefore, cannot be achieved through rotation having a constant radius.

[0040] Referring now to FIGS. 2 and 3-6, there is shown a bending apparatus 110 in accordance with an exemplary embodiment of the present invention. An example of a preform made with the bending apparatus 110 is depicted in FIG. 2A. The bending apparatus 110 includes an involute cam mechanism 130 (see FIGS. 3-6) that includes a drive shaft 132 (rotatable about drive shaft axis A) and a cam 134 disposed on the drive shaft 132. The cam 134 has a cam surface 136 upon which one or more cam followers 138 follows an involute path 120. A gripper 140 (having grip axis 118) is disposed on the cam follower 138 such that the gripper 140 moves in the involute path 120 of the cam follower 138. The cam follower 138 is biased against the cam surface 136 of the cam 134. A pair of bending dies 114a, 114b, preferably heated as known in the prior art, is disposed adjacent to the cam mechanism 130. Towpreg 112 is secured in the bending apparatus 110 by the clamp 148 at a first end 146 of the towpreg 112 and the gripper 140 at a second end 150 of the towpreg 112 adjacent to the bending dies 114a, 114b. The bending dies 114a, 114b have bending die surfaces 115a, 115b.

[0041] In the bending apparatus 110, the involute path 120 of gripper 140 is determined to be superior to the constant radial path 20 of the prior art bending apparatus 10 of FIG. 1 for producing quality bends in tow pregs 112 having a given level of complexity (i.e., having positive and negative bend angles).

[0042] The involution of the cam 134 is specified to maintain the appropriate distance between the gripper 140 and bending die 114a or 114b centers of rotation 124a or 124b during rotation of the cam 134, with a displacement necessary to properly tension the bending towpreg 112. If the involution is too small, the towpreg 112 will not be adequately tensioned while the matrix of the towpreg 112 is viscous. Inversely, too large of an involution will apply inordinate tension that risks misforming the viscous towpreg 112 against the die surfaces 115a, 115b, or causing slippage between the towpreg 112 and the gripper 140.

[0043] The amount of grip point displacement (see FIG. 2) resultant from the involute path 120 is a function of the bend radius 142a or 142b present in the associated die 114a or 114b, in addition to diameter of the towpreg 112. A larger radius 142a, 142b, for example, requires more displacement due to the involute path being derived from a larger radius. Varying the radii 142a or 142b of the bending die 114a or 114b thus requires a concomitant varying of the involute cam 134.

[0044] Another challenge with the basic, constant bending radius apparatus of the prior art bending apparatus 10 of FIG. 1 are kinks that can be introduced into the towpreg 12 due to a misalignment between the end effector 16 and its grip axis 118 and towpreg 12 axis Y--Y of the tow segment being gripped. As the end effector 16 rotates, it is always facing the end effector center of rotation 22. However, the axis of the bent section of towpreg 12 is along a path that is tangent to the bending die radius 42a, 42b. Therefore, during bending, the end effector 16 is misaligned from the bent towpreg segment's major axis by an angle equal to the angle between the bent towpreg axis C and the radial line B between the end effector and the end effector center of rotation. This misalignment angle can cause a kink in the towpreg 12.

[0045] The bending apparatus 110 of the present invention addresses this misalignment between end effector 116 and the towpreg axis. The involute curve has an inherent property that a line normal to the involute curve is tangent to the circle that the involute curve is derived from. Since the involute cam surface 136 of the cam 134 is derived from the bending die radius 142a, 142b, any line normal to the involute curve is necessarily tangent to the bending die surface 115a, 115b. Therefore, these normal lines are coincident with the bent towpreg axis as the tow is bent. See FIG. 2. The present invention aligns the end effector 116 with the towpreg axis E (see FIG. 6) by using two cam followers 138 on a carriage 138a that can rotate to enable the alignment. With the two cam followers 138 following the involute path 120, the carriage 138a naturally rotates such that a line L--L (see FIGS. 4 and 6) between the centers of the followers 138 is approximately parallel to the tangent of the involute curve halfway between the followers 138. Then, by having the grip axis 118 oriented perpendicular to the drive shaft axis A, the grip axis 118 of the end effector 116 is always approximately coaxial with the towpreg axis during the entire bend. It is noted that the gripper 140 moves in a single degree of freedom (see lines F--F in FIG. 6), and acts in a plane initially normal to the towpreg axis E.

[0046] This alignment eliminates the kinks seen using the prior art bending apparatus of FIG. 1.

[0047] As the dual cam followers 138 travel along the involute cam surface 136, the end effector 116 translates along the axis E shown (see FIG. 6). The spring compliance of the spring or springs (biasing member or members 144) applies a force that pushes the cam followers 138 against the involute cam surface 136, enabling the end effector 116 to follow the involute path. The biasing members 144 are specified such that their applied force under displacement is small enough to not interfere with rotation, but large enough to ensure the forces generated during a bend do not overcome the spring forces, causing a divergence from the proper involute path and orientation resulting from followers 138 separating from surface 136.

[0048] The method 200 of the present invention will be described, which makes use of the bending apparatus 110 of the present invention, as described above. As shown in FIG. 7, the method for bending towpreg to maintain a desired amount of tension during bending includes inserting towpreg 112 into a bending die 114a, 114b having a bending die surface 115a, 115b (Step 210) and subsequently applying heat to the towpreg 112. Constraining the towpreg 112 prevents undesired deformation while the towpreg is hot. The towpreg 112 has a fixed, stationary end 146 and a gripped end 150. Zero tension (i.e., axial force) is applied to the towpreg 112 at the gripped end 150 (Step 212). The gripped end 150 of the towpreg 112 is moved in the involute path 120 (Step 214), thereby applying force to the towpreg achieved by displacement of the gripped end 150 along the involute cam surface 136. Axial force between the gripped end and the stationary end of the towpreg is maintained tangent to the bending die surface while bending the towpreg 112. By varying the amount of involution, the axial force can be increased or decreased as desired while bending. In some aspects, small amounts of axial compression can result in acceptable bends.

[0049] As can be seen in FIG. 2, the gripped end 150 of the towpreg 112 may be gripped by an end effector 116. The end effector 116 has a grip axis and the towpreg has a longitudinal axis at its gripped end (that effectively moves during bending). Alignment of the grip axis and the longitudinal axis of the towpreg 112 is maintained during bending.

[0050] For comparative context, all three mechanisms (present invention, SCARA, and constant radius) operate in a common reference frame. Bending occurs in the XY plane, with the major axis of the towpreg feed defining zero degrees (axis Y--Y of FIGS. 1 and 2). Defining left or right bends as positive or negative is arbitrary, provided the definition is consistent across all mechanisms.

[0051] The die and end effector axes of rotation are all normal to the XY bending plane, and the distances between these centers and the grip point lie in the XY plane. The die is static, forming towpreg about either of its rotational centers, while the end effector is dynamic, rotating about driveshaft axis A while gripping towpreg.

[0052] Further, all three share an end effector design in which a gripper actuates to hold the towpreg as rotation occurs about the end effector center. The gripper has a single degree of freedom, and acts in a plane initially normal to the towpreg feed axis.

[0053] During bending, the gripper actuation (line F--F of FIG. 6) remains normal to the outgoing side of the towpreg, while transitioning to an acute angle relative to the incoming towpreg. The angle formed between the gripping actuation line and towpreg axis is complementary to the displacement angle of the initial plane upstream of the bending dies 114a, 114b.

[0054] It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.