Patent Description:
Traditionally, stringers were comprised of a stiff and preferably lightweight covering material such as alloys of aluminum, steel, and titanium. More recently, however, advanced forms of stringers have been made from composite materials such as multiple layers, or plies, of suitable filler or fiber material bonded together using a resin matrix or the like.

For composite stringers having a constant cross-section, the composite stringer may be produced using pultrusion or a similar process. Pultrusion is a continuous manufacturing process in which multiple plies of pre-impregnated fibers or similar material are stacked and pulled through a stationary die in which the plies are heated or cooled and thus cure or harden, respectively, into the desired cross-sectional shape. After leaving the die, the stringer can be cut to length. Although pultrusion is a relatively inexpensive process, pultrusion can only be used to create continuous cross-section stringers due to limitations of the die. Thus, pultrusion and similar processes have limited applicability for creating complex stringers and other components.

Often, however, stringers require a complex contour to accommodate the complex geometry of the skin to which the stringer is to be attached or to otherwise conform to the local contouring of the skin. Composite stringers requiring a varying cross-sectional thicknesses are thus formed using ply drops-that is, a variation in the numbers of strips, or plies, of fibers along the stringer's length to produce a variation in cross-sectional thickness. Stringers with ply drops cannot be formed using pultrusion or similar processes because the varying crossprofile thickness prevents the use of a die. Thus, manufacturing composite stringers having varying profiles is time consuming and relatively expensive.

Namely, such composite stringers are batched produced by layering plies including ply drops on a forming tool and hardening the plies in place. These complex stringers are often formed using autoclave molding or the like utilizing flexible bags that conform to the ply stack during hardening, thereby accommodating the ply drops and other complex geometry. However, the combination of high pressure and temperature required for thermoplastics and some thermosets makes these autoclaves rare and expensive. Also, since the autoclave is a batch, not continuous, process, the size of the autoclave limits the size of parts made and the heat/cooling rates are limited by the convective heat transfer rates associated with a gas filled autoclave.

Thus, there remains a need for a continuous manufacturing process that can be utilized to create varying length strengthening parts such as aircraft stringers, utilizing lowcost, single-sided tools, which can accommodate ply drops and other variations in the stringer profile.

<CIT> describes a system and method for making a fibre reinforced polymer tape. A polymer impregnated roving is traversed through a system comprising an inlet and an outlet, applying a consolidation pressure within the system to the polymer impregnated roving, and applying a smoothing pressure within the system to the polymer impregnated roving. The method may further include adjusting a temperature of the polymer impregnated roving with a heat transfer device between the inlet and the outlet, the heat transfer device having a temperature different from a temperature of the polymer impregnated roving at the inlet. The heat transfer device may include one or more heat transfer chambers.

The present invention is directed to an improved process and tooling for creating a composite stringer or other composite reinforcing part. The process is a continuous manufacturing process used to create unlimited length parts without the need for implementing special or expensive tooling. The process and tooling includes the capability to accommodate various contours of stringers thus accommodating for ply drops and other irregular cross-sectional areas along the length of the stringer.

For example, some embodiments of the invention are directed to a pressure vessel for forming a composite part such as a stringer or other reinforcing part used for a vehicle. The pressure vessel includes an entrance opening at an upstream end of the pressure vessel, which is sized and shaped to receive a forming tool supporting a plurality of plies as the forming tool is conveyed through the pressure vessel. The pressure vessel further includes an exit opening provided at a downstream end of the pressure vessel, which is sized and shaped to allow the forming tool to exit the pressure vessel after it is conveyed therethrough. The pressure vessel includes an interior chamber between the entrance opening and the exit opening housing multiple spheres. The spheres are configured to press against the plurality of plies as the forming tool is conveyed through the pressure vessel, thereby pressure treating the plies and forming the composite part.

Other embodiments of the invention are directed to a tooling assembly for forming such a composite part such as a stringer or other reinforcing part used for a vehicle. The tooling assembly includes an elongated conveying apparatus and a forming tool that supports the plies used to form the composite part, which is movably supported by the elongated conveying apparatus. The tooling assembly further comprises a pressure vessel disposed along the elongated conveying apparatus such as the pressure vessel described above.

Still other embodiments of the invention are directed to a method for forming a composite part. The method includes stacking pre-impregnated plies on a forming tool including creating at least one ply drop in the ply stack and conveying the ply stack and the forming tool through a pressure vessel. For example, the ply stack and the forming tooling may be conveyed through the pressure vessel described above. The method includes pressing the ply stack against the forming tool using multiple spheres located within an interior chamber of the pressure vessel. The method also includes cutting the composite part to length after it has exited the pressure vessel.

These and other features will be discussed in more detail below in connection with the accompanying drawings.

The present invention is described in detail below with reference to the attached drawing figures, wherein:.

The following detailed description of the invention references the accompanying drawings that illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the current invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the current invention is defined only by the appended claims.

In this description, references to "one embodiment," "an embodiment," or "embodiments" mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to "one embodiment," "an embodiment," or "embodiments" in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc., described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the current technology can include a variety of combinations and/or integrations of the embodiments described herein.

Generally, aspects of the invention are directed to a method of forming a composite part such as an aircraft stringer or the like from multiple plies of pre-impregnated fiber or other matrix and resin and the tooling used to form the same. More particularly, <FIG> shows a tooling assembly <NUM> used to produce composite reinforcing members having a constant or non-constant cross-sectional profile on a continuous basis. In a preferred embodiment, the tooling assembly <NUM> is used to produce a composite aircraft stringer having a varying (that is, non-constant) cross-sectional thickness, however the tooling assembly <NUM> can be used to produce other composite components having either a constant or non-constant profile without departing from the scope of the invention.

The tooling assembly <NUM> generally includes an elongated conveying apparatus such as an elongated table <NUM> including a plurality of rollers <NUM> on an upper side thereof. The plurality of rollers <NUM> are configured to spin as a forming tool <NUM> is conveyed along the rollers <NUM>. As seen in <FIG>, the tool <NUM> is conveyed in a right-to-left direction (indicated by arrow <NUM>) along the rollers <NUM> via an actuator <NUM>, which in the depicted embodiment is a winch operatively connected to the tool <NUM> via a cable <NUM>. In such embodiments, as the winch <NUM> retracts and wraps up the cable <NUM>, the winch <NUM> pulls the tool <NUM> along the rollers <NUM>. Any other suitable type of actuator <NUM> can be used to push, pull, or otherwise move the tool <NUM> along the rollers <NUM> without departing from the scope of the invention.

As best seen in <FIG>, the tool <NUM> has a cross-sectional shape corresponding to the cross-sectional shape of a composite part <NUM> being formed by the tooling assembly <NUM>. For example, in the depicted embodiment the tooling assembly <NUM> is being used to form a hat-shaped stringer <NUM>, and thus a cross-sectional shape of the tool <NUM> includes a generally horizontal upper surface <NUM> with a trapezoidal protrusion <NUM> extending therefrom. In this regard, when pre-impregnated plies of fiber, webbing, or the like are placed on the tool <NUM> and hardened or cured in place, the resulting composite part <NUM> will have a cross-sectional shape corresponding to the upper surface of the tool <NUM>-in this instance a hat shape. In other embodiments, tools having a different profile can be utilized to result in any desired cross-sectional shape of the resulting composite part, such as, for example, tools that result in I-, J-, Y-, and Z-shaped stringers or similar.

The tooling assembly <NUM> further includes an oven <NUM> or other heat source, and a pressure vessel <NUM> downstream of the oven <NUM>. As the tool <NUM> is pulled or otherwise conveyed along the rollers <NUM>, plies of pre-impregnated fibers <NUM> or other webbing are placed onto the upper side of the tool <NUM>. More particularly, the tooling assembly <NUM> may include multiple spools <NUM> of pre-impregnated fibers <NUM> such that as the tool <NUM> moves along the rollers <NUM> the plies <NUM> are spooled out and layered onto the tool <NUM> upstream of the oven <NUM>. In this regard, a thickness of the composite part <NUM> is varied by adjusting the number of plies of pre-impregnated fiber <NUM> or other material layered onto the tool <NUM> along the length of the composite part <NUM>. That is, the thickness is adjusted by adding ply drops, as discussed.

As the tool <NUM>, and thus the plies of pre-impregnated fiber <NUM> laying thereon, is conveyed through the oven <NUM>, the plies <NUM> are heated causing the resin contained therein to liquify. This results in initial bonding of the plies of material <NUM>, as the resin melds and holds the stack of plies <NUM> together. The tool <NUM> and plies <NUM> are then conveyed through wheels <NUM> that insure that the plies are close to the tool surface before they enter the pressure vessel <NUM> for further temperature and pressure treatment of the plies <NUM>, ultimately resulting in the finished composite part <NUM> exiting a downstream end of the pressure vessel <NUM>.

As best seen in <FIG>, the pressure vessel <NUM> extends along a portion of the elongated table <NUM> from an upstream end <NUM> to a downstream end <NUM>. The pressure vessel <NUM> generally includes a central portion <NUM> that receives the tool <NUM> passing therethrough, and one or more manifolds <NUM>, <NUM> extending outward from the central portion <NUM>. In some embodiments, the pressure vessel <NUM> includes a plurality of external rollers <NUM> along which the tool <NUM> rolls as it passes through the pressure vessel <NUM>, and an open interior chamber <NUM> that faces an upper surface of the tool <NUM> as it is conveyed through the pressure vessel <NUM>. More particularly, the rollers <NUM> push the plies against the tool <NUM> and position the tool <NUM> such that the plies <NUM> thereon clear the entrance of the chamber <NUM> as the tool <NUM> passes therethrough.

The interior chamber <NUM> includes a temperature-controlled sphere portion <NUM> including plurality of spheres <NUM>, and a regulated air portion <NUM> including a diaphragm <NUM>. In some embodiments the diaphragm <NUM> includes an inflatable bladder or similar component. Inflating or deflating the diaphragm <NUM> with a compressed gas increases or decreases, respectively, the pressure placed on the plurality of spheres <NUM>, which in turn press on the plies <NUM> thus pressure treating the curing or hardening material, as will be discussed in more detail below. Certain portions of the entrance opening <NUM> may extend farther forward than the other portions of the entrance opening thereby allowing the pressure of the balls to be applied to these area of the plies sooner than others. This will apply progressive pressure application as can be necessary to prevent wrinkling of the material that may occur when applying pressure to the entire part width simultaneously. In some embodiments, the diaphragm <NUM> may include a liner <NUM> separating the regulated air portion <NUM> and the temperature-controlled sphere portion <NUM>. The diaphragm liner <NUM> may include a low friction surface facing the temperature-controlled sphere portion <NUM> such that the plurality of spheres <NUM> can freely roll against the insert <NUM>.

Each manifold <NUM>, <NUM> is operatively connected to the interior chamber <NUM> via respective openings <NUM>, <NUM> in the sidewalls of the central portion <NUM>. The manifolds <NUM>, <NUM> circulate hot or cold air through the interior chamber <NUM> and in the space between the plurality of spheres <NUM> during the forming process. For example, when the composite part <NUM> is being formed using thermoplastic polymer-i.e., a polymer that turns to liquid when heated and turns solid when cooled-the manifolds <NUM>, <NUM> circulate cold air throughout the interior chamber <NUM> and in between the spheres <NUM> to cause the resin to freeze while being compacted by the pressurized spheres <NUM>. Alternatively, when the composite part <NUM> is being formed using a thermosetting polymer-i.e., a polymer that cures at elevated temperatures-the manifolds <NUM>, <NUM> circulate hot air throughout the interior chamber <NUM> and in between the spheres <NUM> to cause the resin to cure while being compacted by the pressurized spheres <NUM>.

In some embodiments, a membrane <NUM> is overlaid on the plies <NUM> before the tool <NUM> enters the pressure vessel <NUM> in order to protect the plies <NUM> from direct contact with the spheres <NUM>. For example, the membrane <NUM> may be spooled onto the plies <NUM> after the tool <NUM> leaves the oven <NUM>, as shown in <FIG>. In other embodiments, the membrane <NUM> may be overlaid on the plies <NUM> anywhere along the tooling assembly <NUM> upstream of the pressure vessel <NUM> without departing from the scope of this invention. For example, in some embodiments the membrane <NUM> may be overlaid on the plies <NUM> upstream of the oven <NUM> without departing from the scope of this invention. In some embodiments, the membrane <NUM> is a metal foil overlaid the plies <NUM>. In other embodiments, the membrane <NUM> is a plastic film or bag that overlays or surrounds the plies <NUM>. In some embodiments, the membrane <NUM> is vacuumed sealed against the plies <NUM>. In any event, the membrane <NUM> is used to isolate the plies <NUM> forming the composite part <NUM> from the plurality of spheres <NUM> in the interior chamber <NUM>.

The pressure vessel <NUM> further includes an upstream opening <NUM> at the upstream end <NUM> and a downstream opening <NUM> at the downstream end <NUM>. The upstream and downstream openings <NUM>, <NUM> have generally the same shape as the outer perimeter of the tool <NUM>, however the openings <NUM>, <NUM> are slightly larger than the tool <NUM> thus forming a clearance between the openings <NUM>, <NUM> and the upper surface of the tool <NUM> so that the plies <NUM> can pass therebetween. The clearance formed at the openings <NUM>, <NUM> accommodate varying thicknesses of plies <NUM> entering and exiting the pressure vessel <NUM>. Thus, the tooling assembly <NUM>, and more particularly the pressure vessel <NUM> of the tooling assembly <NUM>, accommodates composite parts <NUM> being formed with integral ply drops, unlike known continuous process such as pultrusion processes in which the die cannot accommodate parts having varying thicknesses. However, the clearances formed at the openings <NUM>, <NUM> are less than the diameter of each the plurality of spheres <NUM> so that the spheres <NUM> do not escape the interior chamber <NUM> as the tool <NUM> and plies <NUM> are being conveyed there through. The spheres near the entrance and exit may be larger than the spheres near the midpoint of the chamber, which may enable larger clearances at the openings <NUM>, <NUM> while preventing smaller spheres from escaping the interior chamber <NUM>. Partitions may be present in the chamber to prevent or reduce the mixing of different sizes of spheres and thereby prevent or reduce the migration of smaller spheres to the entrance or exit regions. In some embodiments, the spheres may deviate from perfect sphericity. Therefore, the term sphere, as used herein, shall include particles, grains, aggregate materials, or the like having a sphericity of at least <NUM>. Preferably, the spheres may have an average sphericity of at least <NUM>. More preferably, the spheres may have an average sphericity of at least <NUM>. Moreover, in some embodiments the spheres <NUM> may be formed from a ferromagnetic material, and the pressure vessel <NUM> may further include an electromagnetic to suspend the spheres <NUM> when the tool <NUM> is not in place or when switching between tools <NUM> so that the spheres <NUM> do not escape through the upstream or downstream openings <NUM>, <NUM>.

In some embodiments the pressure vessel <NUM> includes a pair of wheels <NUM> at the upstream end <NUM> of the vessel <NUM>, which serves to help place and flatten the membrane <NUM> as the tool <NUM> enters the pressure vessel <NUM>. More particularly, the wheels <NUM> are placed just before the upstream opening <NUM> and have a cross-sectional profile mirroring the upper surface of the tool <NUM> (<FIG>) such that the wheels <NUM> compact the membrane <NUM> and plies <NUM> sufficiently to insure that the membrane <NUM> or the plies <NUM> does not contact the edges of the upstream or downstream openings <NUM>, <NUM>. In some embodiments the wheels <NUM> may extend into the oven or may be otherwise heated to prevent them from cooling the plies before pressure is applied in the chamber.

By conveying the tool <NUM> with the plies <NUM> on the upper surface thereof through the described tooling assembly <NUM>, a composite part <NUM> having an infinite length and varying cross-section (i.e. ply drops) can be formed using a continuous manufacturing process without the need for expensive tooling and without requiring time consuming batch manufacturing or the like. This will be more apparent with reference to <FIG>, which is a flowchart outlining a process of forming a composite part <NUM> using the above-described tooling assembly <NUM>.

At step <NUM>, multiple plies of a pre-impregnated fibers <NUM> are stacked onto a tool <NUM> having a cross-sectional profile mirroring the cross-sectional profile of the composite part <NUM> being formed. Again, when the composite part <NUM> is a hat-shaped stringer, the profile will be substantially similar to that shown in <FIG>, but other profiles can be used without departing from the scope of the invention. In some embodiments, the plies <NUM> are spooled onto the tool <NUM> from one or more spools <NUM>. Moreover, to create varying cross-sectional thicknesses along the length of the composite part <NUM>, the amount of plies <NUM> being spooled onto the tool as the tool <NUM> is conveyed through the tooling assembly <NUM> is periodically varied, thus creating ply drops and a non-constant cross-sectional thickness, as discussed.

At step <NUM> the tool <NUM> and plies <NUM> thereon are conveyed through an oven <NUM>. Although in the embodiment shown the tool <NUM> is conveyed via winch <NUM> attached by a cable <NUM> pulling the tool <NUM> through the oven <NUM> and the pressure vessel <NUM>, any other method of conveying the tool <NUM> in a generally horizontal direction can be employed without departing from the scope of this invention. As the tool <NUM> and plies <NUM> move through the oven <NUM>, the resin of the pre-impregnated plies <NUM> is heated and liquified, thus causing the stack of plies <NUM> to generally meld together.

At step <NUM>, the membrane <NUM> is applied to the plies <NUM> and the tool <NUM>. More particularly, the membrane <NUM> is overlaid on the uncured for thermosets, or cold for thermoplastics, pre-impregnated plies <NUM>. In some embodiments, the membrane <NUM> is vacuum sealed at step <NUM> over the tool <NUM> and plies <NUM>. At step <NUM>, as the tool <NUM> continues to move in the direction of travel <NUM>, the membrane <NUM> and plies <NUM> underneath the membrane <NUM> are compacted via the wheels <NUM> so that the plies <NUM> and membrane <NUM> do not contact the upstream opening <NUM> of the pressure vessel <NUM> as the tool <NUM> enters the pressure vessel <NUM>.

At steps <NUM>-<NUM>, the plies <NUM> are pressure treated and hardened or cured within the pressure vessel <NUM>. More particularly, at step <NUM> the tool <NUM> and plies <NUM> are conveyed through the vessel <NUM>, which in some embodiments includes either pulling or pushing the tool <NUM> with an actuator <NUM>, overcoming the drag of the spheres <NUM> rolling against the part and tool <NUM>. Simultaneously with step <NUM>, at steps <NUM> and <NUM> the diaphragm <NUM> is inflated such that the spheres <NUM> are pressed against the plies <NUM> and tool <NUM>. The diaphragm <NUM> may include any suitable compressed gas that is pumped into and out of the space above the diaphragm <NUM> as desired such that pressure is created by the compressed gas acting on a diaphragm <NUM> on the opposite side of the interior chamber <NUM> from the tool <NUM>. At steps <NUM>-<NUM>, relative motion between the pressure vessel <NUM> and the tool <NUM> is maintained in the direction of the prismatic profile axis such that the plies <NUM> become squeezed between the tool <NUM> and the plurality of spheres <NUM> (e.g. trapped ball bearings or the like) in a zone of high pressure over the length of the vessel <NUM>. The cumulative force from the pressure on the spheres <NUM> is reacted into the tool <NUM> via the rollers <NUM> that engage the back side of the tool <NUM>.

In this regard, profile changes due to ramps, ply drops, or section geometry changes are accommodated by inherent flexibility of the spheres <NUM> and diaphragm <NUM> within the pressure vessel <NUM>. That is, unlike pultrusion with a rigid die where the profile thickness must be constant, the changing composite part <NUM> thickness can be accommodated because the spheres <NUM> can move slightly by rearranging or by deflecting the diaphragm <NUM>. Moreover, unlike dies used in pultrusion processes, the clearance between the pressure vessel <NUM> and the tool <NUM> at the upstream opening <NUM> further allows for changes in the profile of the composite part <NUM> to pass therethrough but prevents the spheres <NUM> from escaping. Put another way, the diameters of the plurality of spheres <NUM> are larger than the variation of the profile shape of the composite part <NUM> over its length.

The spheres <NUM> within the pressure vessel <NUM> have low rolling friction since each layer of spheres rotate in opposite directions throughout the depth of the vessel <NUM>. In some embodiments, a liner <NUM> is provided between the regulated air portion <NUM> and the temperature-controlled sphere portion <NUM>, which is covered with ball transfer rollers or roller bearings creating a low-friction surface for an uppermost layer of the plurality of spheres <NUM> to roll against. Moreover, in some embodiments the plurality of spheres <NUM> are rigid; that is, they are made of a material that can withstand much higher temperatures than elastomeric materials typically used to provide compliance to changing profiles. In any event, the spheres <NUM> force the plies <NUM> in compliance with the tool <NUM> and compact the plies <NUM> as they cure or solidify.

In some embodiments, hot or cold air is circulated through the interior chamber <NUM> of the pressure <NUM> via one or more manifolds <NUM>, <NUM> at step <NUM>. Namely, when a thermoset polymer is being used to construct the composite part <NUM>, heated air is circulated through the interior chamber <NUM> curing the resin. And when a thermoplastic polymer is being used, cooled air is circulated through the interior chamber <NUM> in order to solidify the resin,.

At step <NUM>, the tool <NUM> is removed from the pressure vessel <NUM> via the downstream opening <NUM>. At this point the resin will have cured or solidified, forming the composite part <NUM> on the upper surface of the tool <NUM>. Optionally, if the tool <NUM> is to be completely removed from the pressure vessel <NUM> such at the end of run or when switching between tools <NUM>, an electromagnet within the pressure vessel <NUM> is activated at step <NUM>, which in turn attracts and suspends the spheres <NUM> (when the spheres are constructed from a ferromagnetic material) so that the spheres <NUM> do not escape the vessel <NUM> via the upstream or downstream openings <NUM>, <NUM>. And finally, at step <NUM> the composite part <NUM> is cut to length or otherwise finished such that it is ready for installation on an aircraft skin or other vehicle component. In embodiments implementing a membrane <NUM>, the membrane <NUM> is removed and discarded during this finishing step <NUM>.

Claim 1:
A pressure vessel (<NUM>) for forming a composite part (<NUM>), the pressure vessel comprising:
an entrance opening (<NUM>) provided at an upstream end (<NUM>) of the pressure vessel configured to receive a forming tool (<NUM>) supporting a plurality of plies;
an exit opening (<NUM>) provided at a downstream end (<NUM>) of the pressure vessel configured to permit the forming tool to exit the pressure vessel after it is conveyed through the pressure vessel;
an interior chamber (<NUM>) between the entrance opening and the exit opening; and
a plurality of spheres (<NUM>) within the interior chamber, wherein the plurality of spheres are configured to press against the plurality of plies as the forming tool is conveyed through the pressure vessel.