Method for incorporating rigid elements into the core of composite structural members in a pultrusion process

A pultrusion method produces a composite structural member having rigid elements embedded therein. The structural member may be a sandwich structure in which one or more rigid, pre-rigidized, or rigidizable composite or non-composite structural elements are introduced at regular or irregular positions within core elements. The structural member may also be formed from layers of resin-matrix fiber fabric into a structural cross-section, such as an I-beam or T-beam, with a bundle of pre-pultruded rods located at the bends or the web-flange intersection points within layers.

DETAILED DESCRIPTION OF THE INVENTION A first embodiment of the present invention is illustrated in FIGS. 1 - 3 , which depict a method of making composite sandwich panels 10 ( FIG. 3 ) using a pultrusion process. The resulting sandwich panel is composed of three distinct types of components integrated into a single consolidated unit: (1) two thin face or outer skins 12 , (2) a thicker core 14 of a homogeneous, lightweight material, such as a closed cell foam, honeycomb, or balsa, to hold the inner and outer skins at a fixed separation distance, and (3) one or more rigid, pre-rigidized, or rigidizable composite or non-composite structural elements 16 introduced at regular or irregular positions in the core. The structural elements are added for a variety of reasons, including providing reinforcing, extra strength, and/or stiffness beyond that normally achievable using a homogeneous core, improving impact protection, forming hard points for mounting equipment, and providing hollow sections for running wires or for blowing heating or cooling air. The structural elements are generally smaller than the core elements and may take any desired cross-sectional shape, such as channel-shaped, I-shaped, H-shaped, T-shaped, Z-shaped, C-shaped, or box-shaped. FIG. 4 illustrates examples of I-, box-, T-, and Z-shaped structural elements. The structural elements may be rigid elements, such as aluminum extrusions, or composite elements that have been pre-rigidized, such as pre-pultruded composite sections or elements. The structural elements may also be composite elements that are rigidized during the pultrusion process by impregnation and subsequent curing of resin. FIGS. 1 and 2 illustrate a schematic of a pultrusion processing system to make flat sandwich panels 10 containing composite skins and homogeneous foam core elements with the inclusion of an occasional rigid or pre-rigidized structural element inserted at appropriate locations between the opposed faces of adjacent core elements. The structural elements 16 , channel-shaped in the illustrated embodiment, are inserted between adjacent core elements 14 at desired discrete locations prior to the entrance of the pultrusion die 20 . The discrete structural elements and core elements are butted edge to edge as required by engineering requirements and fed into the pultrusion die as a continuous sheet 22 . A bonding agent, such as the same resin used as the matrix material for the skins, may be applied onto the interfaces between the core and structural elements prior to the assembly of the sequenced core elements and structural elements. Alternatively, the sequenced elements may be bonded together with resin that flows into the interfaces between the structural elements and core elements during the resin wet out and infiltration step of the pultrusion process. Fiber reinforcing materials in the form of individual tows of fiber and/or fabrics of the same or different fiber are positioned on creels 24 arranged to feed the dry fiber materials 26 continuously onto the surfaces of the sequenced core elements and structural elements and into the further pultrusion processing equipment. The fiber and cloth creels are usually followed by a set of guides (not shown) arranged to form the dry fiber into the general shape of the component being manufactured. The guides feed the formed fiber collection into the resin wet out processing station 28 , at which the previously dry fiber materials are fully wetted with the matrix resin. Any suitable type of resin wet out equipment may be provided, as would be known in the art. Typical examples include a wet bath (an open or closed vat of resin through which the fibers are pulled), a through-bath (a co-linear wet bath, usually holding a small quantity of resin), an external resin injection port (a close-fitting tool usually fed by a continuous supply of pumped resin), or a pumped injection port system integrated with the pultrusion die. During resin wet-out, the inserted structural elements 16 may also be impregnated with the same resin if desired. Typically, if the inserted elements are to be impregnated during this stage, a less viscous resin is used, the process is run at a slower speed and/or at a higher temperature, and/or vacuum or pressure resin assist may be used to ensure that the resin fully impregnates the inserted elements, as one of skill in the art may readily determine. The resin-impregnated reinforcing fiber and matrix combination next enters the pultrusion die 20 . This die is usually a multi-part steel tool having the mirror-polished cross section of the pultruded composite part machined through its length. The die is heated along its length. As the resin-impregnated assembly of fibers and/or fabrics is pulled through the heated tool, the resin reacts or cures, transforming from the liquid resin that enters the die to a solid matrix at the exit. In some cases, the curing of the resin continues after the part exits the die with additional inline heaters in the form of ovens, heat lamps, ultraviolet lights and other energy sources. The material flow is maintained at a steady pace, typically between one-tenth to ten meters per minute, by some form of pulling mechanism 30 such as a tractor, roller or hand over hand mechanism. The pultrusion production line may end with an automated cut off saw 32 arranged to slice the finished composite product to predetermined lengths, if desired for the particular product. In some cases, cut pieces are placed in an off-line oven for additional curing. Many variations on the general pultrusion process described above may be practiced, depending on the desired finished product and available starting materials. Referring to FIG. 5, a further embodiment is described in which cores 14 are prepared with their edges wrapped with a dry cloth 34 to form a composite structural member having occasional inserted fiber-reinforced C-stiffeners or I-stiffeners surrounded by homogeneous core. The cores are homogeneous lightweight pieces, such as foam or honeycomb, as described above. The cloth wrapping may cover one or more of the mating faces of the individual core elements. In other cases, the cloth wrapping material may also cover some of the top and/or bottom faces of the core element as well. Further examples of various wrapped configurations are illustrated in FIG. 6 . Core wrapping can occur using continuous in-line equipment or alternatively may be prepared off-line in a secondary operation in preparation for the pultrusion process. Wrapped cores are then sequenced into the pultrusion stream as described above. Resin from any selected in-line wet out scheme flows or can be made to flow with additional processing equipment, such as vacuum or pressure assist, into the cloth inter-core reinforcing sheets. It is also possible to pre-wet the cloth materials on each core piece off-line by rolling resin onto cloth sheets or otherwise applying resin to appropriate areas of the cloth wrapping. When sequenced into the pultrusion stream, the cloth layers cure along with the upper and lower face skins, either inside the pultrusion die or later in the process. The resulting product forms solid composite reinforcing structural elements between and around the core elements. Referring to FIG. 7, a further embodiment is provided in which a continuous sequence of originally homogeneous lightweight foam cores 14 , modified by the addition of occasional through-the-thickness stitching 36 of dry fiber at various angles and spacing, is fed into the pultrusion die along with the surfacing skins. The through-the-thickness stitching can be added to the core continuously by the inclusion of a sewing-type of machinery in-line and prior to the other pultrusion process equipment previously described, or alternatively pre-stitched unimpregnated cores can be made off-line and inserted into the pultrusion stream panel by panel. Dry-stitched core panels are available from WebCore Technologies, Inc. (See also U.S. Pat. Nos. 5,462,623, 5,589,243 and 5,834,082.) The stitching in the pre-stitched fiber cores may be pre-wet by soaking the cores in a bath of resin prior to feeding them sequentially into the pultrusion die. Alternatively or additionally, pressure and/or vacuum may be used to assist the resin flow into the through-the-thickness stitching fibers. Another approach is to wet the through-the-thickness stitching fibers of the core with resin using an in-line wet-out tool (a through bath, continuous in-line resin injection system, or the like). Another possibility is to conduct resin wet out in the pultrusion die itself, forcing resin through the reinforcing fiber layers on the surface of the core and down into the through-the-thickness fibers stitched through the core. In all of these implementations, heat from the pultrusion die, and/or possibly an in-line oven or off-line post curing oven then advances the curing of the resin in the skins and stitching fibers. The structural elements in the embodiments above are perpendicular to the pultrusion direction. The structural elements can also be inserted parallel to the pultrusion direction to provide lengthwise core inserts. For example, referring to FIG. 8 , blocks of core elements 14 are aligned for introduction into the pultrusion die. Long discrete lengths or continuously spooled or pre-pultruded or otherwise prepared structural elements 38 are fed in between the core elements, either in horizontal planes or in vertical planes, as desired. Some possible cross-sectional shapes of the lengthwise reinforcement elements are a hollow box, standard structural shapes such as I, T, C, H, or Z, or rods of circular or other cross-section. (See FIG. 4 for some examples.) In a further embodiment, illustrated in FIG. 9 , pre-pultruded rods 42 are assembled into a suitable shape, such as a triangle, and fed between layers of fiber fabric 44 at points 46 where the fabric is bent to form a particular structural shape. For example, multiple layers of fiber reinforcing fabric are shaped into a structure having a flange 48 and a web 50 . The layers of the web fabric are separated and bent to form the intersection with the flange, which tends to form a generally triangular-shaped gap at the intersection. In prior art structures, care must be taken to prevent formation of this gap. As illustrated in FIG. 9 , according to the present invention, the rods in a triangular bundle are introduced into the intersection between the flange and web before introduction to the pultrusion die, facilitating the manufacture of this structure and strengthening the finished structural member. Structural elements can also be provided at selected locations to provide localized hard points inside the panels by inserting blocks of different types, weights, and/or strengths of core or other materials. For example, for a door panel, a small region of higher density material can be implanted at the location where a doorknob will be attached. In the present invention, the lightweight foam or honeycomb core material used in processing the sandwich structures can be left inside the finished product. Alternatively, the lightweight core material can be removed by mechanical or chemical means, leaving only a now-rigid structure of solid fiber reinforced composite struts and/or thin vertical webs. Examples of the core-rigidizing elements include C- or I-section beam-like elements, or a distribution of many thin composite struts resulting from rigidization of the through-the-thickness perpendicular or angled stitching. The above examples are presented as representative examples of a few of the possible processing techniques that can be used for the pultrusion of structures with rigid-element-reinforced cores and are not intended to present all possible processing variations covered by the disclosed methods. The invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.