Abstract:
There are provided apparatuses and related methods for preforming sheets to form preforms for forming ducts. The preforms can be formed of a thermoplastic material, such as flat sheets of reinforced thermoplastic, which can be lightweight, strong, and perform well in flammability, smoke, and toxicity tests. The apparatus includes a heater for heating the sheet to a processing temperature and a structure for configuring the sheet to a desired shape of the duct. For example, rollers, rods, tubes, or a funnel can be used to bend the sheet.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/215,815, filed Aug. 9, 2002, which is hereby incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1) Field of the Invention  
         [0003]     The present invention relates to apparatuses and methods for preforming thermoplastic materials and, more specifically, to apparatuses and methods for bending thermoplastic sheets to form preforms for ducts.  
         [0004]     2) Description of Related Art  
         [0005]     Ducts provide transport passageways for a wide variety of applications. For example, tubular ducts are widely used for air flow in aircraft environmental control systems. Similarly, ducts provide passageways for transporting gases for heating and ventilation in other vehicles and in buildings. Water distribution systems, hydraulic systems, and other fluid networks also often use ducts for fluid transport. In addition, solid materials, for example, in particulate form can be delivered through ducts. Ducts for the foregoing and other applications can be formed of metals, plastics, ceramics, composites, and other materials.  
         [0006]     One conventional aircraft environmental control system utilizes a network of ducts to provide air for heating, cooling, ventilation, filtering, humidity control, and/or pressure control of the cabin. In this conventional system, the ducts are formed of a composite material that includes a thermoset matrix that impregnates, and is reinforced by, a reinforcing material such as Kevlar®, registered trademark of E.I. du Pont de Nemours and Company. The thermoset matrix is typically formed of an epoxy or polyester resin, which hardens when it is subjected to heat and pressure. Ducts formed of this composite material are generally strong and lightweight, as required in many aircraft applications. However, the manufacturing process can be complicated, lengthy, and expensive, especially for ducts that include contours or features such as beads and bells. For example, in one conventional manufacturing process, ducts are formed by forming a disposable plaster mandrel, laying plies of fabric preimpregnated with the thermoset material on the mandrel, and consolidating and curing the plies to form the duct. The tools used to mold the plaster mandrel are specially sized and shaped for creating a duct of specific dimensions, so numerous such tools must be produced and maintained for manufacturing different ducts. The plaster mandrel is formed and destroyed during the manufacture of one duct, requiring time for curing and resulting in plaster that typically must be removed or destroyed as waste. Additionally, the preimpregnated plies change shape during curing and consolidation and, therefore, typically must be trimmed after curing to achieve the desired dimensions. The jigs required for trimming and for locating the proper positions for features such as holes and spuds are also typically used for only a duct of particular dimensions, so numerous jigs are required if different ducts are to be formed. Like the rotatable tools used for forming the mandrels, the jigs require time and expense for manufacture, storage, and maintenance. Additionally, ducts formed of conventional thermoset epoxies typically do not perform well in certain flammability, smoke, and toxicity tests, and the use of such materials can be unacceptable if performance requirements are strict. Further, features such as beads typically must be post-formed, or added after the formation of the duct, requiring additional manufacture time and labor.  
         [0007]     Alternatively, ducts can also be formed of thermoplastic materials. A thermoplastic duct can be formed by forming a thermoplastic sheet of material, cutting the sheet to a size and configuration that corresponds to the desired shape of the duct, bending the sheet to the desired configuration of the duct, and joining longitudinal edges of the sheet to form a longitudinal joint or seam. For example, apparatuses and methods for forming thermoplastic ducts and consolidation joining of thermoplastic ducts are provided in U.S. application Ser. Nos. 10/216,110and 10/215,833, titled “Thermoplastic Laminate Duct” and “Consolidation Joining of Thermoplastic Laminate Ducts”, both of which are filed concurrently herewith and the contents of which are incorporated herein by reference. Such thermoplastic ducts can be formed by retaining the thermoplastic sheet in the bent configuration until the ends are joined, and then releasing the duct so that the resulting joint continues to restrain the duct in the bent configuration. However, stresses induced in the thermoplastic material during bending can cause the duct to deform or distort from the desired configuration after joining, e.g., when released from the joining apparatus.  
         [0008]     Thus, there exists a need for improved apparatuses and methods of preforming ducts, i.e., providing a preform configured to correspond generally to the desired configuration of the duct in a substantially unstressed condition. The method should not require the laying of individual plies on a disposable plaster mandrel. Preferably, the method should be compatible with thermoplastic ducts, including reinforced thermoplastic ducts formed from flat sheets, which provide high strength-to-weight ratios and meet strict flammability, smoke, and toxicity standards.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides an apparatus and method for preforming sheets to form preforms for forming ducts. The preforms can be formed from thermoplastic materials, such as flat sheets of reinforced thermoplastic laminate. Thus, individual plies need not be laid on a disposable plaster mandrel. The thermoplastic material can be lightweight, strong, and perform well in flammability, smoke, and toxicity tests. Additionally, the method is compatible with ducts that are formed by consolidation joining thermoplastic laminates. In an unstressed condition, the preforms correspond generally to the desired configuration of the ducts. Thus, longitudinal edges of the preforms can be joined to form the duct, and the duct does not deform when released from the joining apparatus.  
         [0010]     According to one embodiment, the present invention provides an apparatus for preforming a thermoplastic member to form a preform that generally corresponds to the desired configuration of the duct, which defines a passage. The apparatus includes first and second rotatable rollers, which are substantially parallel and define a nip, and a heater configured to heat the thermoplastic member to a processing temperature, for example, less than a glass transition temperature of the thermoplastic member and within about 70° F. of the glass transition temperature. In one advantageous embodiment, the processing temperature is between about 5° F. and 70° F. less than the glass transition temperature. At least one of the rollers is heated and at least one of the rollers is configured to rotate and thereby translate the thermoplastic member through the nip so that the thermoplastic member is heated, compressed, and bent generally to the desired configuration of the duct. A rotational actuator can be configured to rotate one of the rollers, and the first roller can be heated by the heater located therein. An actuator can also be configured to adjust at least one of the rollers in a transverse direction to adjust the nip. Additionally, a fastener can be provided to connect a longitudinal leading edge of the thermoplastic member to the first roller. A non-stick layer can be disposed on the rollers to facilitate release of the preform therefrom, and the rollers can be magnetically attracted.  
         [0011]     According to one aspect of the invention, the apparatus includes third and fourth rollers, which are also positioned substantially parallel and proximate to the first roller. The second, third, and fourth rollers are positioned at incremental angular positions about the first roller so that each of the second, third, and fourth rollers is capable of urging the thermoplastic member against the first roller in a configuration that generally corresponds to the desired configuration of the duct. According to another aspect, a deflection roller is positioned to intersect a tangent of the nip so that the thermoplastic member is deflected and bent about the first roller. The deflection roller can be offset from the tangent, and an actuator can be configured to adjust an offset position of the deflection roller.  
         [0012]     According to another embodiment, the present invention provides another apparatus for preforming a thermoplastic member to form a preform generally corresponding to the desired configuration of the duct. The apparatus includes a support structure extending longitudinally and at least partially defining a cavity. An elongate member with an outer surface corresponding to the desired configuration of the duct extends longitudinally in the cavity so that the thermoplastic member can be supported between the support structure and the elongate member. A heater is configured to heat the thermoplastic member to a processing temperature, for example, within about 70° F. of the glass transition temperature of the thermoplastic member. The support structure is configured to adjust from a first position in which the support structure supports the thermoplastic member in a flat configuration to a second position in which the support structure is adjusted radially inward to bend the thermoplastic member against the elongate member to the desired configuration of the duct. At least one actuator can be configured to adjust the support structure between the first and second positions. The support structure can include a plurality of rods that extend longitudinally in the first position and adjust to an angularly incremental configuration about the elongate member in the second position.  
         [0013]     According to another aspect, the support structure includes two partial hollow tubes that are rotatably adjustable between the first and second positions, each tube defining an interior surface corresponding to the outer surface of the elongate member. A heater can heat the interior surfaces of the tubes to the processing temperature. Each tube can define a first longitudinal edge joined by a hinge and a second longitudinal edge defining a radially inwardly extending stop, and the tubes can be configured to rotatably adjust from a first position in which the cavity is open to a second position in which the cavity is at least partially closed. In the first position, the tubes are configured to receive and support the thermoplastic member between the stops. In the second position, the tubes cooperably form the cavity and define an inner surface corresponding to the desired configuration of the duct.  
         [0014]     According to another embodiment, the preforming apparatus includes a hollow tube that defines a longitudinal cavity. A funnel extends longitudinally from an end of the tube and tapers in the longitudinal direction toward the tube from a cross-sectional size larger than the duct to a cross-sectional size about equal to the duct. The funnel is configured to receive the thermoplastic member and configure the thermoplastic member to the desired configuration of the duct as the thermoplastic member is urged longitudinally through the funnel. A heater is configured to heat the funnel and/or the tube to a processing temperature, for example, less than the glass transition temperature of the thermoplastic member and within about 70° F. of the glass transition temperature.  
         [0015]     The present invention also provides a method for preforming a thermoplastic member to form a preform generally corresponding to a desired configuration of a thermoplastic duct defining a passage. The method includes heating the thermoplastic member to a processing temperature, e.g., between about 5° F. and 70° F. less than a glass transition temperature of the thermoplastic member. A first and/or second roller is rotated, and the thermoplastic member is transported through a nip defined by the rollers so that the member is heated, compressed, and bent generally to the desired configuration of the duct. The rollers can also be magnetically urged together. The thermoplastic member can be heated before being transported through the nip and by the roller(s) as the member is transported through the nip. The thermoplastic member can be transported about the first roller through nips defined between the first and second rollers, the first roller and a third roller, and the first roller and a fourth roller. Additionally, the thermoplastic member can be continuously transported about the first roller an angular distance of more than one revolution, for example, by fastening a longitudinal leading edge of the thermoplastic member to the first roller. A deflection roller can be positioned to intersect a tangent of the nip so that a rotational axis of the deflection roller is offset from the tangent of the nip in the direction of the second roller, and the deflection roller deflects the thermoplastic member to bend about the first roller.  
         [0016]     According to another embodiment, the thermoplastic member is heated to the processing temperature, supported with a longitudinally extending support structure in a generally flat configuration, and bent against an outer surface of the elongate member to the desired configuration of the duct as the support structure is adjusted radially inward, for example, by an actuator. The thermoplastic member can be supported by a plurality of rods, which extend longitudinally and adjust to an angularly incremental configuration about the elongate member. Alternatively, the thermoplastic member can be supported by two partial hollow tubes in an open configuration and urged against the elongate member by an interior surface of the tubes corresponding to the outer surface of the elongate member as the tubes adjust to a closed position. According to one aspect of the invention, a first edge of the thermoplastic member is urged against a second edge of the thermoplastic member and the edges are heated to above the glass transition temperature and consolidation joined. For example, the edges can be urged together by adjusting a consolidation joining head radially against the thermoplastic member such that an elastomeric portion of the head urges the edges against the elongate member.  
         [0017]     According to another embodiment, the thermoplastic member is heated to the processing temperature, supported between radially inwardly extending stops defined by two partial hollow longitudinal tubes, and bent to the desired configuration of the duct as the tubes are rotatably adjusted about a hinge from an open position to a closed position. According to yet another embodiment, a tapering funnel is provided for preforming the thermoplastic member. The thermoplastic member is heated to the processing temperature, inserted into a first end of the funnel that is larger than the duct and urged through a second, smaller end of the funnel and into a hollow tube. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0019]      FIG. 1  is a perspective view of a preforming apparatus according to one embodiment of the present invention;  
         [0020]      FIG. 2  is a perspective view of flat thermoplastic sheet for forming a preform according to one embodiment of the present invention;  
         [0021]      FIG. 3  is a perspective view of a preform formed from the sheet of  FIG. 2  according to one embodiment of the present invention;  
         [0022]      FIG. 4  is a perspective view of a duct formed from the preform of  FIG. 3  according to one embodiment of the present invention;  
         [0023]      FIG. 5  is a perspective view of a preforming apparatus according to one embodiment of the present invention;  
         [0024]      FIG. 6  is an elevation view of a preforming apparatus according to another embodiment of the present invention;  
         [0025]      FIG. 7  is an elevation view of a preforming apparatus according to another embodiment of the present invention in an open configuration;  
         [0026]      FIG. 8  is an elevation view of the preforming apparatus of  FIG. 7  in a closed configuration;  
         [0027]      FIG. 9  is a section view of the preforming apparatus of  FIG. 8  as seen along line  9 - 9  of  FIG. 8 ;  
         [0028]      FIG. 10  is a section view of the preforming apparatus of  FIG. 9  with the sheet partially preformed;  
         [0029]      FIG. 11  is a section view of the preforming apparatus of  FIG. 9  with the sheet fully preformed;  
         [0030]      FIG. 12  is a section view of the preforming apparatus of  FIG. 9  including a consolidation joining head adjusted to an open position according to one embodiment of the present invention;  
         [0031]      FIG. 13  is a section view of the preforming apparatus of  FIG. 12  with the consolidation joining head adjusted to a closed position;  
         [0032]      FIG. 14  is an elevation view of a preforming apparatus according to another embodiment of the present invention in an open position;  
         [0033]      FIG. 15  is an elevation view of the preforming apparatus of  FIG. 14  in a closed position;  
         [0034]      FIG. 16  is an elevation view of the preforming apparatus of  FIG. 14  including a consolidation joining head according to one embodiment of the present invention;  
         [0035]      FIG. 17  is an elevation view of a preforming apparatus according to another embodiment of the present invention in an open position with the thermoplastic sheet partially inserted;  
         [0036]      FIG. 18  is an elevation view of the preforming apparatus of  FIG. 17  in a closed position; and  
         [0037]      FIG. 19  is a perspective view of a preforming apparatus according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0038]     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
         [0039]     Referring now to  FIG. 1 , there is shown a preforming apparatus  10  for preforming a thermoplastic member, such as a thermoplastic sheet  50  as shown in  FIG. 2 . Preforming generally refers to bending the thermoplastic member to form a bent or curved preform  70  as shown in  FIG. 3 , which, in an unrestrained condition, generally corresponds to a desired configuration of a duct  90 . The preform  70  can be formed to have a diameter slightly larger or smaller than the desired diameter of the duct  90 , for example, so that the preform  70  can be subjected to a compressive or expansion force for holding the preform  70  during subsequent processing, such as consolidation joining, to arrive at the desired configuration of the duct  90 . The preform  70  and, hence, the duct  90 , shown in  FIG. 4 , extend from a first end  72  to a second end  74  and define a passage  76 . Preferably, longitudinal edges  78 ,  80  of the preform  70  are overlapped to form an interface portion  82 . The longitudinal edges  78 ,  80  of the preform  70  can be joined to form the duct  90  having a seam or joint  92 , preferably without significantly further bending or deforming the preform  70  so that the duct  90  is substantially free of internal stress. The longitudinal edges  78 ,  80  can be joined using adhesives, heat, or other joining methods. For example, joining can be achieved by applying heat and pressure to the edges  78 ,  80  to form the seam  92 . As the thermoplastic material of the preform  70  is heated above its glass transition temperature, the material becomes plastic and the pressure consolidates and joins the interface  82 . Joining can be performed by manual or automated methods, for example, as described in U.S. application Ser. No. 10/215,833, titled “Consolidation Joining of Thermoplastic Laminate Ducts”.  
         [0040]     The shape of the preform  70  is determined by projecting the desired shape of the duct  90  onto the flat sheet  50 . Although the ends  72 ,  74  and edges  78 ,  80  of the preform  70  are shown to be straight in  FIG. 3 , the preform  70  can alternatively define a variety of shapes that correspond to ducts that are straight, curved, tapered, or otherwise contoured. The sheet  50  and, hence, the preform  70  and duct  90  can also define a variety features such as holes  75 , for example, for connecting spuds, brackets, and the like to the duct  90 . Methods and apparatuses for forming preforms and for determining geometric patterns that correspond to ducts are provided in U.S. application Ser. No. 10/216,110, titled “Thermoplastic Laminate Duct”. It is also appreciated that marks can be provided on the preform  70 , for example, to accurately identify the location of post-formed features such as bead and bells or to facilitate the manufacture or assembly of the ducts, as also provided in U.S. application Ser. No. 10/216,110, titled “Thermoplastic Laminate Duct”.  
         [0041]     Preferably, the preform  70  is formed of a composite laminate that includes a thermoplastic matrix and a reinforcing material. Thermoplastic materials are characterized by a transition to a plastic state when heated above a glass transition temperature. For example, the preform  70  can be formed of polyetherimide (PEI) or polyphenol sulfide (PPS), both of which can be thermoplastic. Thermoplastic PEI is available under the trade name Ultem®, a registered trademark of General Electric Company. According to one embodiment of the present invention, each preform  70  is formed of a composite material that includes a matrix of thermoplastic PEI that is reinforced with a reinforcing material such as carbon, glass, or an aramid fabric such as a Kevlar® aramid, or fibers of such a material. Alternatively, the preform  70  can be formed of other thermoplastic materials, which can be reinforced by other reinforcing materials, or can include no reinforcing materials.  
         [0042]     The duct  90  formed from the preform  70  can be used in numerous applications including, but not limited to, environmental control systems of aerospace vehicles, in which air is delivered through the passage  76  of the duct  90  to provide heating, cooling, ventilation, and/or pressurization of an aircraft cabin. The ends  72 ,  74  of the duct  90  can be connected to other ducts or other devices such as ventilators, compressors, filters, and the like. Multiple ducts  90  can be connected so that a longitudinal axis of each duct  90  is configured at an angle relative to the longitudinal axis of the adjoining duct(s). Thus, the ducts  90  can be connected to form an intricate duct system (not shown) that includes numerous angled or curved ducts  90  for accommodating the devices connected by the duct system and for meeting layout restrictions as required, for example, on an aircraft where space is limited.  
         [0043]     The preforming apparatus  10  shown in  FIG. 1  includes a first roller  12  and a second roller  14 . The rollers  12 ,  14  extend longitudinally and are supported by a frame (not shown) such that the rollers  12 ,  14  are substantially parallel and define a nip. The rollers  12 ,  14  can be formed of a variety of materials such as aluminum, steel, and alloys thereof, and a non-stick layer can be disposed on the rollers  12 ,  14  to prevent the sheet  50  from sticking to the rollers  12 ,  14 . For example, the non-stick layer can be formed of Teflon® film, registered trademark of E.I. du Pont de Nemours and Company. An actuator  20 , such as an electric motor, is configured to rotate at least one of the rollers  12 ,  14 . In the embodiment of  FIG. 1 , the actuator  20  is configured to rotate the first roller  12  such that, as the sheet  50  is fed into the nip in a direction  13 , the rotating roller  12  transports the sheet  50  through the nip.  
         [0044]     The first roller  12  includes a heater  30 , which is configured to heat the roller  12  and, thus, the thermoplastic sheet  50  to at least a processing temperature. As the foregoing examples illustrate, the first roller  12  is therefore formed of a thermally conductive material. Preferably, the sheet  50  is heated to a processing temperature that is less than the glass transition temperature of the thermoplastic material of the sheet  50 . For example, the processing temperature can be between about 5° F. and 70° F. less than the glass transition temperature. In the case of PEI, which has a glass transition temperature of about 417° F., the sheet  50  can be heated to a processing temperature of between about 350° F. and 412° F.  
         [0045]     As the sheet  50  is transported through the nip, the rollers  12 ,  14  exert a compressive force on the sheet  50  and heat the sheet  50 . One or both of the rollers  12 ,  14  can be adjusted toward or away from the opposite roller  12 ,  14  to adjust the compressive force on the sheet  50 . Preferably, the sheet  50  is heated disproportionately by the rollers  12 ,  14  so that the sheet  50  is bent or formed as the sheet  50  emerges from the nip, for example, due to thermal expansion or contraction of the reinforcing material in the sheet  50 . For example, the heater  30  in the first roller  12  can be used to heat the sheet  50  so that the reinforcing material that is closer to the first roller  12  as the sheet  50  passes through the nip is expanded or contracted and the sheet  50  is bent. If the reinforcing material is one that expands when heated, such as carbon or glass reinforcement materials, the sheet  50  is bent around the second roller  14 . If the reinforcing material is one that contracts when heated, such as an aramid reinforcement material, the sheet  50  is bent around the first roller  12 . Thus, the longitudinal edges  78 ,  80  of the sheet  50  are bent together to form the preform  70 , which generally corresponds to the desired shape of the duct  90 .  
         [0046]     As shown in  FIG. 5 , a preforming apparatus  10   a  according to the present invention can also include multiple rollers  14   a - 14   f  that are spaced at incremental angular positions about the first roller  12  so that each of the rollers  14   a - 14   f  defines a nip with the first roller  12 . The first longitudinal edge  78  of the sheet  50  is fastened to the first roller  12  by a fastener  16 , which is a strip of heat resistant adhesive tape. Other fasteners  16  can also be used, such as glue, screws, bolts, clips, hooks, and the like. The first longitudinal edge  78  precedes the rest of the sheet  50 , i.e., the first edge  78  is the “leading edge”. The fastener  16  retains the first edge  78  against the roller  12  and the sheet  50  is thus transported through the nips defined by the rollers  14   a - 14   f  and the first roller  12 . Although the first longitudinal edge  78  is connected to the first roller  12  in  FIG. 5 , the second edge  80  can also, or alternatively, be connected to the roller  12 . Additionally, the rollers  14   a - 14   f  can be adjustable radially relative to the first roller  12  to urge the sheet  50  against the roller  12 . For example, the rollers  14   a - 14   f  can be adjusted radially outward from the roller  12  during processing to receive the leading longitudinal edge  68 ,  80  of the sheet  50  and then adjusted radially inward toward the roller  12  to urge the sheet  50  against the roller  12 .  
         [0047]     As shown in  FIG. 6 , a preforming apparatus  10   b  according to another embodiment of the invention includes a deflection roller  40  for deflecting and bending the sheet  50  as the sheet emerges from the nip between the first and second rollers  12 ,  14 . The deflection roller  40  is positioned to intersect a tangent of the nip between the first and second rollers  12 ,  14 . Thus, the tangent of the nip, i.e., a line tangent to both of the first and second rollers  12 ,  14  at the nip therebetween, intersects the deflection roller  40 . Similarly, as the sheet  50  emerges from the nip and follows a course approximating a direction of the tangent of the nip, the sheet  50  contacts the deflection roller  40  and is thereby bent. Preferably, the deflection roller  40  is offset from the tangent of the nip, i.e., the tangent of the nip intersects a portion of the deflection roller  40  other than a rotational axis of the deflection roller  40 . Advantageously, the deflection roller  40  can be offset such that the axis of the deflection roller  40  is closer to the second roller  14  than the first roller  12  and the sheet  50  is thus deflected to bend about the first roller  12 .  
         [0048]     The deflection roller  40  is rotatably mounted to a pivot  42  and a deflection actuator  44  is configured to adjust the position of the deflection roller  40  and change the degree of bending of the sheet  50 . Adjustment of the deflection roller  40  can be desirable to change the bend of the sheet  50 , or to maintain a uniform bend despite changes in other system parameters such as temperature of the sheet  50 , thickness of the sheet  50 , material type of the sheet  50 , and the like. Additionally, a nip actuator  46  is configured to adjust the second roller  14  relative to the first roller  12  and thereby affect the compressive force exerted by the rollers  12 ,  14  on the sheet  50  as the sheet  50  is transported through the nip, for example, to adjust for different thicknesses of the sheet  50 . The nip actuator  46  can be a hydraulic, pneumatic, electric, or other type of actuation device.  
         [0049]     The sheet  50  can be supported by a support table  34  and heated by heaters  32  as the sheet  50  is fed into the nip. Cam rollers  48  are positioned at incrementally longitudinal locations to support the rollers  14 ,  40 . By supporting the rollers  14 ,  40  at longitudinal locations between the ends of the rollers  14 ,  40 , the cam rollers  48  decrease the longitudinal deflection of the rollers  14 ,  40 . A guard  49  is also provided to catch the sheet  50  and prevent the sheet  50  from continuously passing through the nip multiple times. Alternatively, the sheet  50  can be transported multiple times through the nip(s) of the forming apparatuses  10 ,  10   a ,  10   b . For example, the first roller  12  of the preforming apparatus  10   a  shown in  FIG. 5  can be rotated more than one revolution after the first longitudinal edge  78  has entered the first nip between the rollers  12 ,  14   a . Thus, the roller  12  and the sheet  50  can be rotated until the sheet  50  has been bent to the configuration of the preform  70 .  
         [0050]     Additionally, one or more of the rollers  12 ,  14 ,  14   a - 14   f  can be magnetized so that the rollers  12 ,  14 ,  14   a - 14   f  are magnetically attracted and the nip therebetween is uniform along the length of the rollers  12 ,  14 ,  14   a - 14   f . The rollers  12 ,  14 ,  14   a - 14   f  can include a magnetized material, such as a ferrous metal, or an electromagnetic for generating the attraction between the rollers. For example, the first roller  12  can include an electromagnet and the second roller  14  can be formed of steel so that the second roller  14  is attracted toward the first roller  12  and the nip between the rollers  12 ,  14  is uniform along the length of the rollers  12 ,  14 .  
         [0051]     There is shown in  FIGS. 7-11  a preforming apparatus  110  that includes an outer support structure comprising ring supports  112 , each arranged about a common longitudinal axis. The ring supports  112  support actuators  114 , which are configured to support a plurality of parallel rods  116 , six in the illustrated embodiment, and adjust the rods  116  radially inward and outward. As shown in  FIGS. 9-11 , the rods  116  can be adjusted radially to define an adjustable cavity  111  therein and, thus, support and bend, or preform, the sheet  50  to the desired configuration of ducts of different diameters and/or shapes, thus forming the preform  70 .  
         [0052]     Each of the rods  116  can be heated during processing, for example, by heaters  136  disposed in the rods  116 , such that the rods heat the sheet  50 . Alternatively, the sheet  50  can be heated by a heater (not shown) in the beam  150  or a heater configured to irradiate the sheet  50 . For example, the preforming apparatus  110  can be positioned in an oven, or a directional radiation source, such as an infrared or a microwave source, can be configured to heat the sheet  50 . Preferably, the heater(s) are configured to heat the sheet  50  to a processing temperature that is less than the glass transition temperature of the thermoplastic material of the sheet  50 , for example, between about 5° F. and 70° F. less than the glass transition temperature.  
         [0053]     An inner beam  150 , which extends from a first end  170  to a second end  172 , is positioned in the cavity  111  defined by the rods  116  such that the sheet  50  can be positioned around the inner beam  150 . Although the ends  170 ,  172  of the inner beam  150  are supported by a base  113 , at least one of the ends  170 ,  172  of the inner beam  150  can be disconnected from the base  113  to facilitate the insertion of the sheet  50  into the cavity  111  of the preforming apparatus  110 . For example, a latch  168  can be adjusted between an open position and a closed position. With the latch  168  in the open position, shown in  FIG. 7 , the sheet  50  can be inserted longitudinally into the preforming apparatus  110  such that the sheet  50  is disposed around the inner beam  150 .  
         [0054]     The preforming apparatus  110  can be used to form the preform  70  of  FIG. 3  from the sheet  50  of  FIG. 2 . During operation, the axial actuators  114  are used to retract the rods  116  radially outward to a first position, as shown in  FIG. 9 , and the latch  168  is opened. The sheet  50  is longitudinally installed in the preforming apparatus  110  so that the sheet  50  is supported by at least one of the rods  116 . The latch  168  is then closed to secure the first end  170  of the inner beam  150  to the outer support structure or the base  113 , as shown in  FIG. 8 .  
         [0055]     With the preforming apparatus  110  assembled as shown in  FIG. 8 , a power supply (not shown) is connected to the heaters  136  in the rods  116  or other heaters for heating the sheet  50 , preferably to the processing temperature. The actuators  114  are actuated to extend the rods  116  radially inward so that the rods  116  urge the sheet  50  against the inner beam  150  and bend the sheet  50  about the inner beam  150  to the desired configuration of the duct  90 , thus forming the preform  70 . Preferably, the longitudinal edges  78 ,  80  are overlapped to form the interface  82 . After the preform  70  is formed, the heater  136  can be turned off so that the preform  70  is cooled to a temperature below the processing temperature before the latch  168  is opened and the preform  70  is removed from the preforming apparatus  110 .  
         [0056]     As shown in  FIGS. 12 and 13 , the preforming apparatus  110  can also include a consolidation joining head  160  that is configured to be adjusted radially relative to the inner beam  150 . The head  160  can be retracted from the cavity  111  during preforming, as shown in  FIG. 12 , and then positioned proximate to the preform  70  and in alignment with the interface  82  of the preform  70  as the preform  70  is held in the desired configuration of the duct  90  as shown in  FIG. 13 . The head  160  includes a heater  162  that is supported by an elastomeric block  164 , such that the heater  162  is disposed on or in the block  164 . After the sheet  50  has been configured to form the preform  70 , i.e., in the desired configuration of the duct  90  as shown in  FIG. 13 , the head  160  can be adjusted radially inward so that the block  164  and/or the heater  162  contact the preform  70 . The head  160  compresses the edges  78 ,  80  of the preform  70  together at the interface  82 . Preferably, the heater  162  is flexible, and flexibly supported by the elastomeric block  164 , so that the heater  162  conforms to the preform  70  and exerts a substantially uniform pressure thereon. For example, the heater  162  can comprise a flexible silicone heater disposed on the elastomeric block  164 . As the head  160  compresses the interface  82  against the inner beam  150 , the heater  162  heats the interface  82  and the edges  78 ,  80  are thus consolidation joined to form the longitudinal seam  92 , thereby forming the duct  90 . The inner beam  150  can also include an inner heater  166 , in addition or in alternative to the heater  162 . Thus, the interface  82  can be heated by the heater  162 , the inner heater  166 , or both heaters  162 ,  166 . Preferably, the heater(s)  162 ,  166  are configured to heat the edges  78 ,  80  to a temperature above the glass transition temperature of the thermoplastic material. Consolidation joining is further discussed in U.S. application Ser. No. 10/215,833, titled “Consolidation Joining of Thermoplastic Laminate Ducts”.  
         [0057]      FIGS. 14 and 15  illustrate an alternative preforming apparatus  210 , in which the outer support structure includes two partial tubes  212   a ,  212   b  connected by a hinge  213 . The partial tubes  212   a ,  212   b  can be rotated about the hinge  213  by actuators  238  from an open position, shown in  FIG. 14 , to a closed position, shown in  FIG. 15 . In the closed position, the partial tubes  212   a ,  212   b  define an internal cavity  211  that corresponds to the desired shape of the preform  70  and the duct  90 . A rigid inner member  236  is positioned proximate to the tubes  212   a ,  212   b  so that the tubes  212   a ,  212   b  at least partially enclose the inner member  236  when adjusted to the closed position. The rigid inner member  236  can be formed of a rigid material, such as steel, aluminum, or titanium, or the inner member  236  can be formed of a device that can be configured to be rigid, such as an inflatable bladder. The inner member  236  corresponds to the shape of the partial tubes  212   a ,  212   b  and, in the illustrated embodiment, is cylindrical although the partial tubes  212   a ,  212   b  and the inner member  236  may have other shapes if desired. Thus, the sheet  50  can be positioned between the partial tubes  212   a ,  212   b  and the inner member  236 , and the preforming apparatus  210  can be used to bend the sheet  50  from the flat configuration to the bent or preformed configuration by adjusting the partial tubes  212   a ,  212   b  from the open position to the closed position and urging the sheet  50  around the inner member  236 . Advantageously, the partial tubes  212   a ,  212   b  and/or the inner member  236  can be heated to thereby facilitate the bending or forming of the sheet  50 . For example, heaters  240  can be provided in or on each of the partial tubes  212   a ,  212   b , which, in turn, are constructed of a material such as aluminum, steel, titanium, alloys thereof, or a composite material, that is at least partially thermally conductive. Alternatively, the partial tubes  212   a ,  212   b  and/or the inner member  236  can be heated by an independent heater, such as an oven, configured to receive the partial tubes  212   a ,  212   b  when rotated to their open positions.  
         [0058]     The preforming apparatus  210  can also include a consolidation joining head  260  positioned proximate to the inner member  236  and in alignment with a gap between the partial tubes  212   a ,  212   b  once the partial tubes  212   a ,  212   b  have been closed. The head  260  is adapted to be adjusted radially relative to the inner member  236 . The head  260  can include a heater  262  that is supported by an elastomeric block  264 , such that the heater  262  is disposed on or in the block  264 . After the sheet  50  has been configured to form the preform  70 , i.e., in the desired configuration of the duct  90  as shown in  FIG. 16 , the head  260  can be adjusted radially inward so that the block  264  and/or the heater  262  contact the preform  70 . Advantageously, the preform  70  may be positioned such that the edges  78 ,  80  of the preform  70  are also in general alignment with the gap between the partial tubes  212   a ,  212   b  once the partial tubes  212   a ,  212   b  have been closed. In this advantageous embodiment, the head  260  compresses the edges  78 ,  80  of the preform  70  together at the interface  82 . Preferably, the heater  262  is flexible, and flexibly supported by the elastomeric block  264 , so that the heater  262  conforms to the preform  70  and exerts a substantially uniform pressure thereon while concurrently heating at least one edge  78 ,  80  of the preform  70  to consolidation join the edges  78 ,  80  and form the longitudinal seam  92  along the length of the preform  70 , thereby forming the duct  90 . The inner member  236  can include an inner heater  266 , in addition or in alternative to the heater  262 , so that the preform  70  can be heated on its inner and outer surfaces, preferably to a temperature above the glass transition temperature. The sheet  50  can be held in position about the inner member  236  by one or more straps and/or tape (not shown) instead of the partial tubes  212   a ,  212   b . Preferably, the tape is heat shrink tape, i.e., tape that constricts in length as the tape is heated to a processing temperature. Thus, the sheet  50  is wrapped around the inner member  236 , and the straps, which can be formed of heat resistant cloth, are secured around the sheet  50  to hold the sheet  50  in the desired configuration of the duct  90 . The heat shrink tape is then disposed around the sheet  50  such that the tape, when heated, constricts and urges the sheet  50  tightly against the inner member  236 . The thermal energy for heating the sheet  50  and the tape can be generated by an oven configured to receive the inner member  236  and the sheet  50  or by a heater located within the inner member  236 .  
         [0059]     As shown in  FIGS. 17 and 18 , the preforming apparatus  210  can also be used without the inner member  236 . For example, each of the partial tubes  212   a ,  212   b  can define a longitudinal stop  242  that extends radially inward toward the cavity  211 . With the partial tubes  212   a ,  212   b  in the open position, as shown in  FIG. 17 , the sheet  50  can be inserted between the stops  242  such that the stops  242  retain the sheet  50  as the partial tubes  212   a ,  212   b  are adjusted by the actuators  238  to the closed position, as shown in  FIG. 18 . The sheet  50  can be inserted into the cavity  211  through a gap  214  between the longitudinal stops  242 , as shown in  FIG. 17 . The sheet  50  can also be inserted in a longitudinal direction into the cavity  211  from a longitudinal end of the tubes  212   a ,  212   b , and the tubes  212   a ,  212   b  can be in the closed position while the sheet  50  is inserted. As described above, the sheet  50  can be heated with the heaters  240  or other heaters (not shown) to the processing temperature and, after forming, the preform  70  can be cooled in the preforming apparatus  210  before the partial tubes  212   a ,  212   b  are opened to release the preform  70 . A consolidation joining head and/or an inner heater as described in connection with  FIG. 16  can also be used to join the edges  78 ,  80  as the preform  70  is held in the configuration shown in  FIG. 18 .  
         [0060]      FIG. 19  illustrates an alternative preforming apparatus  310  according to the present invention, which includes a hollow tube  312  and a funnel  320 . The hollow tube  312  can define a cylindrical cavity  314  or another shape that corresponds to the desired configuration of the preform  70  and the duct  90 . Additionally, the tube  312  can include an inner member (not shown) that can be received by the passage  76  of the preform  70  and defines an outer surface that corresponds to the desired configuration of the duct  90 . The funnel  320  extends longitudinally from a first end  316  of the tube  312 . The funnel  320  extends from a first end  322  to a second end  324 , which is smaller than the first end  322  and generally corresponds in size to the first end  316  of the tube  312 , so that the funnel  320  tapers toward the tube  312 . The sheet  50  can be inserted into the funnel  320  in a flat or partially bent configuration and urged longitudinally toward and into the cavity  314  of the tube  312 . As the sheet  50  slides longitudinally in the funnel  320 , the tapering shape of the funnel  320  causes the sheet  50  to bend to the diameter of the cylinder  312  and, hence, the desired configuration of the preform  70  and the duct  90 . The sheet  50  can be inserted into the funnel  320  and the tube  312  manually by an operator, or an automated insertion device (not shown) can be provided. Heaters  330  can be provided on the funnel  320  and/or the tube  312  such that the sheet  50  is heated to the processing temperature while the sheet  50  is urged into the funnel  320  and/or the tube  312 . For example, the heaters  330  can be electrical resistive heaters disposed on the tube  312  and the funnel  320  such that the heaters  330  can be connected to a power supply (not shown) and energized to heat the sheet  50 . The sheet  50  can be held at the processing temperature for a processing hold interval, such as 10 minutes, and the heaters  330  can then be turned off so that the resulting preform  70  is cooled in the tube  312  before being removed through the first end  316  or a second end  318 . Further, the tube  312  can comprise a consolidation joining apparatus or other joining apparatus for joining the longitudinal edges  78 ,  80  of the preform  70  and forming the duct  90 , for example, as discussed in U.S. application Ser. No. 10/215,833, titled “Consolidation Joining of Thermoplastic Laminate Ducts”.  
         [0061]     After the preformed  70  has been processed to form the duct  90 , the duct  90  can be post-formed to provide additional contours or features, such as bells, beads, and the like. A discussion regarding the formation of duct features such as bells and beads through post-forming, i.e., after the preforming and/or the consolidation joining of the sheet  50 , is provided in U.S. application Ser. No. 10/215,780, titled “Post-Forming of Thermoplastic Ducts” filed concurrently herewith and the contents of which are incorporated herein by reference. It is also appreciated that marks can be provided on the preform  70 , for example, to accurately identify the location of such post-formed features or to facilitate the manufacture or assembly of the ducts, as provided in U.S. application Ser. No. 10/216,110, titled “Thermoplastic Laminate Duct”.  
         [0062]     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. For example, it is appreciated that each of the working surfaces of the apparatuses can include a low friction layer or release layer, e.g., Teflon®, registered trademark of E.I. du Pont de Nemours and Company. The release layer can be a durable layer of material or a release agent that is wiped or sprayed onto the working surfaces before each consolidation joining process. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.