Abstract:
There is provided a duct and an associated preform and method. The duct is formed of a thermoplastic material and is lightweight, strong, flame and smoke resistant, and non-toxic. The duct is formed from a flat laminate and does not require a specialized, disposable mandrel for supporting conventional thermoset plies. Preferably, the duct is formed from a preform, which is cut according to the desired shape and size of the duct so that trimming after the duct has been formed is not required. Additionally, features such as holes and spud locations can be provided by cutting or marking the preform before forming the duct. Each duct can be formed of multiple articulated segments that are joined by connectors.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1) Field of the Invention  
           [0002]    The present invention relates to ducts and, more specifically, to ducts formed of thermoplastic laminates and preforms and methods for forming such ducts.  
           [0003]    2) Description of Related Art  
           [0004]    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.  
           [0005]    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 specially shaped ducts such as curved ducts and ducts that include a spud or attached fitting, a bead, a bell or flared portion, a conical section, or another contour. For example, curved ducts are conventionally formed around a disposable plaster mandrel. The plaster mandrel is formed in a specially shaped rotatable tool that acts as a mold to form the plaster mandrel according to the desired shape of the duct. First, a cavity of the tool is partially filled with uncured plaster, and the tool is rotated so that the plaster coats an inner surface of the tool cavity. When the plaster is partially cured to form the mandrel, the tool is stopped and opened so that the plaster mandrel can be removed and placed in an oven for subsequent curing. The mandrel is then treated with a sealant, cured again, and treated with a release agent. Plies of fabric, such as Kevlar®, preimpregnated with the thermoset material are cut and draped over the mandrel, often by hand, and a heat gun is used to mold the plies to the shape of mandrel. The mandrel is placed in a vacuum bag, which is fitted with one or more valves, and air is evacuated from the bag through the valves so that the bag urges the plies against the mandrel and consolidates the plies while heat is applied to cure the plies and form the duct. When the plies are cured, the vacuum bag is removed and the plaster mandrel is broken and removed from the duct. The duct is cleaned and trimmed to the desired dimensional characteristics. A jig that corresponds to the desired shape of the duct is often used for trimming the duct and for accurately locating additional features on the duct such as holes, spuds, brackets, and the like. Further processing is sometimes necessary for adding a bead or bell so that one or both ends of the duct can be secured and sealed to another duct. Typically, a bead is formed by adding additional material, thus adding weight to the duct. Insulation can also be added to the inside and/or outside of the duct.  
           [0006]    The manufacturing process for such reinforced thermoset ducts is complicated, time consuming, and expensive. The rotatable tool used to mold the plaster mandrel is 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 while being cured and consolidated 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.  
           [0007]    Additionally, ducts formed of common thermoset epoxies 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. For example, changes in environmental laws or proposed changes to performance requirements mandated by the Federal Aviation Administration would prevent the use of ducts formed from some thermoset composites in certain aircraft environmental control system applications.  
           [0008]    Thus, there exists a need for an improved duct and method of forming a duct that reduces complexity and increases cost efficiency. Preferably, the method should not require the formation of a special rotatable tool and plaster mandrel for each duct or special hand tool for hand lay-up. The duct should be compatible with efficient methods for forming beads and bells and for locating other features, preferably without the use of an expensive jig that is duct-specific. Additionally, the duct should be lightweight and strong and should meet strict flammability, smoke, and toxicity standards.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides a duct formed of a thermoplastic laminate and an associated preform and method. The duct is lightweight, strong, and performs well in flammability, smoke, and toxicity tests. The duct is formed by configuring a flat laminate and does not require a disposable plaster mandrel for supporting plies, a duct-specific rotatable tool for forming such a mandrel, or a special hand lay-up tool. Locations for features such as holes and spuds can be determined before the duct is formed from the laminate. Additionally, the duct can be formed to provide beads and bells without the addition of material.  
           [0010]    In one embodiment, the present invention provides an articulated thermoplastic laminate duct that includes a plurality of duct segments, which are formed of thermoplastic laminate, for example, a composite material that includes a thermoplastic matrix such as polyetherimide or polyphenol sulfide and a reinforcing material such as fabric or fibers formed of an aramid, carbon, or glass. First and second duct segments are connected to define a continuous passage therein, for example, by a flexible connector between ends of the duct segments, which are otherwise offset with a longitudinal axis of the first duct segment being angled relative to a longitudinal axis of the second duct segment. According to one aspect of the invention, the flexible connector defines first and second elongate portions which are partially enclosed. Each portion defines an opening for receiving the end of a respective duct segment, and the portions are connected so that the openings are generally outwardly opposed to each other.  
           [0011]    The present invention also provides a thermoplastic laminate preform for manufacturing a duct segment of an articulated duct. The preform includes a flat sheet of thermoplastic laminate that defines a flat geometric pattern corresponding to the desired shape of the duct segment so that the sheet can be configured to generally define the desired shape of the duct segment, which defines a passage. The sheet can be formed of a composite material that includes a thermoplastic matrix such as polyetherimide or polyphenol sulfide, and a reinforcing material such as an aramid, carbon, or glass. Construction data can be disposed on the perform, including a duct segment identification mark, an orientation mark, a radial alignment mark, a spud location mark, a detail location mark, and/or a cuff location mark. According to one aspect of the invention, the flat geometric pattern is at least partially defined by a sinusoidal curve and/or arc that is based on a number of duct segments required to form the articulated duct, a curvature of the articulated duct, a diameter of an inlet of the articulated duct, a diameter of the outlet of the articulated duct, a bend radius of the duct segment, a desired location for the seam of the duct segment, a seam overlap, and a desired deformation of the inlet and outlet of the articulated duct.  
           [0012]    The present invention also provides methods of forming a thermoplastic laminate preform and duct. The preform, which generally corresponds to a desired shape of a first duct segment, can be formed by impregnating a reinforcement material with thermoplastic to form a flat sheet, determining a flat geometric pattern that generally corresponds to the desired shape of the first duct segment, and cutting the sheet according to the flat geometric pattern. Preferably, the geometric pattern is determined such that the duct segment is consolidated to the desired shape of the first duct segment without trimming the segment after consolidation. For example, the dimensions of the flat geometric pattern can be calculated at least partially according to a sinusoidal curve and/or arc based on a number of duct segments for an articulated duct, a curvature of the articulated duct, a diameter of an inlet of the articulated duct, a diameter of the outlet of the articulated duct, a bend radius of the first duct segment, a desired seam location, a seam overlap, and desired deformations of the inlet and outlet of the duct. The duct is formed by configuring the perform to approximate the desired shape of the first duct segment, which defines a passage, and consolidating the preform to form the first duct segment. According to one aspect of the invention, the methods also include marking the sheet with construction data such as a duct identification mark, an orientation mark, a radial alignment mark, a spud location mark, a detail location mark, and/or a cuff location mark. According to another aspect, the first duct segment is connected to at least a second duct segment so that the passage defined by the first duct segment is fluidly connected to a passage defined by the second duct passage. For example, the ducts segments can be connected by joining a flexible connector to opposing ends of the duct segments. The first duct segment can be connected to the second duct segment such that longitudinal axes of the duct segments are angled relative to one another and the segments in combination at least partially form an articulated duct. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    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:  
         [0014]    [0014]FIG. 1 is a perspective view of a duct formed of four duct segments according to one embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is an elevation view of the duct of FIG. 1 illustrating several dimensions of the duct;  
         [0016]    [0016]FIG. 3 is plan view of a preform for forming a duct segment according to one embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a plan view of a flat sheet of a thermoplastic laminate, showing the layout of four preforms according to another embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is a perspective view of an elongate flexible connector according to one embodiment of the present invention;  
         [0019]    [0019]FIG. 6 is a perspective view of an elongate flexible connector according to another embodiment of the present invention;  
         [0020]    [0020]FIG. 7 is a perspective view of the connector of FIG. 5 disposed on a duct segment according to one embodiment of the present invention;  
         [0021]    [0021]FIG. 8 is a perspective view of an elongate flexible connector disposed on a duct segment according to another embodiment of the present invention;  
         [0022]    [0022]FIG. 9 is a perspective view of two duct segments joined by a flexible connector according to one embodiment of the present invention;  
         [0023]    [0023]FIG. 9A is a plan view of an elongate connector according to another embodiment of the present invention;  
         [0024]    [0024]FIG. 9B is a side view of the right side of the elongate connector of FIG. 9A;  
         [0025]    [0025]FIG. 9C is a side view of the bottom side of the elongate connector of FIG. 9A;  
         [0026]    [0026]FIG. 9D is a partial view of two duct segments joined by the elongate connector of FIG. 9A;  
         [0027]    [0027]FIG. 10 is a plan view of a flat thermoplastic laminate sheet, showing the layout of a preform with spud and detail marks according to one embodiment of the present invention;  
         [0028]    [0028]FIG. 11 is a perspective view of a duct segment with a spud according to one embodiment of the present invention; and  
         [0029]    [0029]FIG. 12 is a perspective view of a duct segment with a bead according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may 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 satisfy applicable legal requirements. Like numbers refer to like elements throughout.  
         [0031]    Referring now to FIG. 1, there is shown an articulate duct  10  according to one embodiment of the present invention. The duct  10  is formed of straight duct segments  20 , four in this embodiment, which are connected by connectors  80  such that the duct segments  20  define a continuous passage  12  extending from an inlet  14  to an outlet  16 . The duct  10  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  12  of the duct  10  to provide heating, cooling, ventilation, filtering, humidity control, and/or pressurization to an aircraft cabin. The inlet  14  and outlet  16  of the duct  10  can be connected to other ducts or other devices such as heaters, air conditioners, ventilators, filters, compressors, and the like. The duct segments  20  are connected so that a longitudinal axis of each duct segment  20  is configured at an angle relative to the longitudinal axis of the adjoining duct segment(s)  20 . Thus, the duct  10  defines an articulated shape and the inlet  14  and outlet  16  are angled relative to one another. Alternatively, the duct segments  20  can be connected to form a straight duct  10 .  
         [0032]    The ducts segments  20  are formed of a thermoplastic laminate. Thermoplastic materials are characterized by a transition to a plastic state when heated above a glass transition temperature and a return to a non-plastic state when cooled to a temperature below the glass transition temperature. For example, the duct segments  20  can be formed of polyetherimide (PEI) or polyphenol sulfide (PPS), both of which can be thermoplastic. Thermoplastic PEI, available under the trade name Ultem®, a registered trademark of General Electric Company, typically becomes plastic at temperatures above about 417° F. According to one embodiment of the present invention, each segment  20  is formed of a composite material that includes a matrix of thermoplastic PEI that is reinforced with a reinforcing material such as a fabric or fibers of carbon, glass, or an aramid fabric such as Kevlar®. Fabrics of these and other reinforcing materials can be prepared by various methods as are known in the art. For example, numerous types of chopped fiber and weave patterns can be used, including a 0/90 degree crowfoot fiber weave. Alternatively, the segments  20  can be formed of other thermoplastic materials, which can be reinforced by other reinforcing materials, or can include no reinforcing materials.  
         [0033]    For clarity of reference, there are illustrated in FIG. 2 several dimensions of the duct  10 . As shown, the inlet  14  of the duct  10  has first diameter D 1  and the outlet  16  has a second diameter D 2 . The diameters D 1 , D 2  can be different, but as shown in the embodiment of FIG. 2, the diameters D 1 , D 2  are equal and hereinafter referred to similarly as the diameter D. Each duct segment  20  defines a first segment width W 1  and a second segment width W 2 . In the illustrated embodiment, the second segment width W 2  of each duct segment  20  is longer than the first segment width W 1 , and the duct segments  20  are configured so that the duct  10  is articulated about a duct center  18 . The articulation of the duct  10  depends on an angle σ between adjoining duct segments  20 . The angle σ is measured between adjoining duct segments  20  at the outermost portion of the duct segments, i.e., where the second segment width W 2  occurs. However, the angle σ can alternatively be measured elsewhere, for example, between the longitudinal axes of adjoining duct segments  20 . An angle B is equal to the sum of the individual angles σ of the duct segments  20 . Thus, the angle B is the total articulation of the duct  10 . A bend radius R is measured as the distance between the duct center  18  and the outermost portion of the duct segments  20 , though the bend radius R can also be measured elsewhere, for example, between the center  18  and the longitudinal axes of the duct segments  20 .  
         [0034]    A preform  60  for forming one of the duct segments  20  is shown in FIG. 3. The preform  60  is formed from a flat sheet  58  formed of a thermoplastic laminate, as shown in FIG. 4. The preform  60  defines first and second ends  62 ,  64  and first and second side edges  66 ,  68 . The preform  60  is configured to form one of the duct segments  20  by bending the preform  60  so that the first and second side edges  66 ,  68  are brought together. The preform  60  can be bent in a circular cross section or another shape such as a square, rectangle, triangle, ellipse, and the like. The first and second side edges  66 ,  68  are joined to form a seam  26  so that the duct segment  20  defines a closed polygonal cross section defining the passage  12 . Preferably, one of the first or second side edges  66 ,  68  overlays a portion of the other edge  66 ,  68  by a distance designated as a seam overlap L OL  as shown in FIG. 2. An angular location S of the seams  26  can be staggered on the duct  10 .  
         [0035]    The preform  60  can be bent and configured manually or by an automated machine. Methods of configuring a preform are provided in U.S. Application Ser. No. ______, titled “Preforming Thermoplastic Ducts,” filed concurrently herewith, the entirety of which is incorporated herein by reference. Preferably, the portion of the duct segment  20  at the seam  26  is consolidated and joined using glue, heat, or other joining methods. Joining is typically achieved by applying heat and pressure to the edges  66 ,  68  to form the seam  26 . As the thermoplastic material of the duct segment  20  is heated above its glass transition temperature, the material becomes plastic and the pressure consolidates and joins the overlapped material that makes up the seam overlap L OL . Joining can be performed by manual or automated methods, for example, as described in U.S. Application Ser. No. ______, titled “Consolidation Joining of Thermoplastic Laminate Ducts,” filed concurrently herewith, the entirety of which is incorporated herein by reference.  
         [0036]    The shape of the preform  60  is determined by projecting the desired shape of the duct segment  20  onto the flat laminate sheet  58 . For example, the duct segments  20  shown in FIGS. 1 and 2 can be formed from preforms  60  as shown in FIG. 4. The preform  60  shown in FIG. 3 can also be used to form the duct segments  20  of FIGS. 1 and 2, though the symmetrical preform  60  of FIG. 3 would result in a duct segment  20  with the seam  26  located at the narrowest section of the duct segment  20 , i.e., the radially innermost portion of the duct  10  relative to the duct center  18 . Thus, each duct segment  20  can be formed from preforms  60  of different shapes, and the shape of the preform  60  can be modified to affect the angular seam location S for each duct segment  20 . In the embodiment illustrated in FIG. 2, the seams  26  are staggered so that the seam  26  of each duct segment  20  is located at an angular position S that is 180° away from the seams  26  of the adjoining duct segments  20 .  
         [0037]    Preferably, at least one of the first and second ends  62 ,  64  of each preform  60  are curved as shown in FIG. 3 so that the first end  62  of one of the duct segments  20  can be engaged with and connected to the second end  64  of another one of the duct segments  20  without a gap between the duct segments  20 . According to one embodiment of the invention, at least one of the first and second ends  62 ,  64  of each preform  60  is at least partially defined by a sinusoidal curve that is based on the following characteristics of the duct  10  and duct segments  20 : a number N of duct segments  20  connected to form the duct  10 , the angle B of articulation of the duct  10 , the duct diameter D, the bend radius R, the desired angular position S for the seam  26  of each of the duct segments  20 , the seam overlap L OL , and a desired deformation of the inlet and/or outlet  16  of the duct  10 . Possible deformations that can be formed in the duct segment  20  include bells and beads, as described below. For example, in one embodiment, the diameter D of the duct  10  is uniform throughout, and a length L of each preform  60 , measured as the distance between the first and second side edges  66 ,  68  is equal to  
         D×π+L OL    
         [0038]    where π is an irrational constant approximately equal to 3.1415. The first segment width W 1  is equal to  
         2        (     R   -     D   2       )     ×     sin        (     B     2      N       )         ,                         
 
         [0039]    and the second segment width W 2  is equal to  
       2        (     R   +     D   2       )     ×       sin        (     B     2      N       )       .                           
 
         [0040]    When the diameters D 1 , D 2  are the same, i.e., equal to the diameter D, the curved first and second ends  62 ,  64  are symmetric as shown in FIGS. 3 and 4, and one end  62 ,  64  is described by the function  
         f        (   x   )       =       ±   D                       sin        (     B     2      N       )            [     R   ±     0.5                   sin        (         360      x       D                 π       +   S     )           ]                               
 
         [0041]    where the desired angular seam location S is expressed as an angle, the variable x is defined along the length L of the duct segment, and the function f(x) is measured perpendicular to the variable x. As shown in FIG. 4, multiple preforms  60  can be formed from a single thermoplastic laminate sheet  58 , and the preforms  60  can be configured in a nested arrangement on the sheet  58  to minimize wasted material.  
         [0042]    There is shown in FIG. 5 the flexible connector  80  that is used to connect the duct segments  20  to form the duct  10  according to one embodiment of the present invention. The flexible connector  80  is an elongate member, which defines first and second partially enclosed portions  82 ,  84  that define first and second openings  86 ,  88  respectively for receiving one of the ends  62 ,  64  of one of the duct segments  20 . The first and second openings  86 ,  88  are generally outwardly opposed to one another such that the duct segments  20  attached to the connector  80  extend therefrom in generally opposing directions. In the embodiment shown in FIG. 5, the connector  80  defines an elongate web  90  from which extend first and second flanges  92 ,  94  that form the partially enclosed portions  82 ,  84 . Either or both of the flanges  92 ,  94  can define gaps  96  that increase the flexibility of the connector  80 . The gaps  96  can also extend through the web  90 , as illustrated by an alternative connector  80   a  illustrated in FIG. 6, which provides separate webs  90   a  for supporting individual flange tabs  92   a . Preferably, the connector  80  is attached to a duct segment  20  such that the first partially enclosed portion  82  receives one of the ends  62 ,  64  of the duct segment  20  through the first opening  86 , and such that the gaps  96  are directed radially inward, or toward the passage  12  of the duct  10 , as shown in FIG. 7. Another alternative connector  80   b  is shown in FIG. 8. The connector  80   b  defines two connected elongate tubes  97 ,  98  each of which defines one of the openings  86 ,  88  and one of the partially enclosed portions  82 ,  84 . FIG. 9 illustrates two duct segments  20  that are connected via a connector  80 . Preferably, the connector  80  holds the two segments  20  together and also hermetically seals the passage  12  of the duct  10 . The connector  80  can be glued in place. Alternatively, the connector  80  and/or the duct segments  20  can be heated until at least partially plasticized and consolidated to join the connector  80  to the duct segments  20 . Consolidation can be performed by pressing the connector  80  against the duct segments  20  manually or using an automated mechanism.  
         [0043]    The connector  80  can define a uniform cross-sectional shape along its length, as shown in FIGS.  5 - 9 , or the cross-sectional shape can vary along the length of the connector  80 , for example, to better accommodate the articulated configuration of the duct segments  20  joined by the connector  80 . The connector  80  shown in FIGS.  9 A- 9 D is elliptical to correspond to the shape of ends  62 ,  64  of the two cylindrical duct segments  20 , as shown in FIG. 9D. The first and second flanges  92 ,  94  of the connector  80  do not define gaps  96 , but the flanges  92 ,  94  are angled relative to the web  90 . Further, the flanges  92 ,  94  are non-uniform along the connector  80 . Specifically, the flanges  92 ,  94  are perpendicular to the web  90  at one or more locations along the connector  80 , as shown in FIG. 9B, and vary from the perpendicular configuration by an angle A along the length of the connector  80 . For example, the angle A can vary to a maximum of B/2N as shown in FIG. 9C. Thus, the partially enclosed portions  82 ,  84  define the angle A therebetween, and the angle A varies along the connector  80  such that the connector  80  is configured to receive the ends  62 ,  64  of the duct segments  20 , which meet at an oblique angle, which can be equal to the angle A, as shown in FIG. 20 to form the articulated duct  10 .  
         [0044]    Preferably, the preform  60  is formed of a thermoplastic laminate that can be formed and consolidated uniformly and predictably so that features that are desired to be located at specific positions on the duct segment  20  can first be located on the preform  60  and, as the preform  60  is configured to form the duct segment  20 , the features are moved to the desired positions. For example, FIG. 10 illustrates a layout of the preform  60  on the thermoplastic laminate sheet  58 . The perimeter of the preform  60  is demarcated by a line  69 , along which the preform  60  can be cut from the sheet  58 . The preform  60  includes spud holes  70  for attaching spuds  71 , or fittings, to the duct segment  20 , as shown in FIG. 11. The spuds  71  can be connected to the duct segment  20  by heating one or both of the spud  71  and the duct segment  20  and pressing the spud  71  into the spud hole  70 . Adhesives, mechanical fasteners such as screws, bolts, clips, or rivets, and other joining devices or methods can be also be used to fasten the spuds  71  to the duct segment  20 . Preferably, the spud holes  70  are bores defined by the preform  60  before the preform  60  is formed into the duct segment  20 , but the location of the spud holes  70  can alternatively be marked on the preform  60  and formed by drilling the duct segment  20  after the preform  60  has been formed into the duct segment  20 . Spud location marks  72  and/or spud orientation marks  73  are used to identify each spud hole  70  and/or indicate the desired position and orientation of the spud  71 . The spud location marks  72  can show part of the outline of the spud  71  that is to be connected to the duct segment  20 , e.g., the corners, so that the spud  71  can be quickly and accurately positioned on the duct segment  20 , preferably without the use of a special jig. If there is more than one orientation of the spud  71  in which the spud  71  corresponds to the location marks  72 , and a particular orientation of the spud  71  on the duct segment  20  is desired, the orientation marks  73  can also be provided. For example, each spud orientation mark  73  can include an arrow, as shown in FIG. 10, which indicates the desired directional orientation of the spud  71  that is to be connected to the duct segment  20 .  
         [0045]    The preform  60  can include other holes and/or marks for additional features, such as detail holes  74  and detail marks  75  for receiving detail devices. The detail holes and marks  74 ,  75  are similar to the spud holes  70 , location marks  72 , and orientation marks  73 . For example, two detail holes  74  can be provided for receiving bolts or other fasteners for connecting a bracket (not shown) or other detail device to the duct segment  20 . The detail marks  75  provide an easily identifiable indicator of the desired placement and orientation of the detail device, simplifying the assembly and installation of the duct segments  20  and the duct  10 . Duct orientation marks  76  can be used to indicate the desired orientation of the duct segment  20  or the duct  10 . For example, the duct orientation marks  76  can include an arrow that indicates which end  62 ,  64  of the duct segment  20  should be connected to another duct segment  20  to form the duct  10 . The duct orientation marks  76  can also indicate the location of the inlet  14  and/or outlet  16  of the duct  10  formed from the duct segment  20  and, therefore, how the duct  10  should be positioned during installation in a duct system. Angular alignment marks  77  indicate the desired angular alignment of the duct segment  20 , for example, relative to a corresponding mark on another duct segment  20 . Thus, an operator assembling two duct segments  20  can align the angular alignment marks  77  on the duct segments  20  to achieve the correct angular alignment therebetween. Cuff marks  78  located near the side edges  66 ,  68  of the preform  60  identify the location or length of a cuff  79  of the preform  60  or duct segment  20 , i.e., the portion of the preform  60  or duct segment  20  that is used to join the duct segment  20  to another duct segment  20 . For example, each cuff mark  78  can include a line parallel to the ends  62 ,  64  of the duct segment  20  that indicates how much of the duct segment  20  should be inserted into another duct segment  20  for joining thereto. A seam overlap line  69   a  can also be provided on the preform  60  to indicate the extent to which the side edges  66 ,  68  should be overlapped to form the seam  26 , i.e., the length of the seam overlap L OL .  
         [0046]    It is appreciated that holes, marks, and other details can be provided on the preform  60  for locating detail devices other than spuds, brackets, and cuffs. For example, the preform  60  can be marked with lines or other marks to indicate the position of features that are to be formed subsequently in the duct segment  20 , such as the desired location for a bell or bead for facilitating the connection of the duct segment  20  to another segment  20  or other device. FIG. 12 illustrates the duct segment  20  in which a bead  50  has been formed. Beads formed on conventional ducts formed of thermoset materials are typically formed by adding plies to the duct to build up the bead shape. The bead  50  of the present invention can be formed by such a build-up process, but preferably the bead  50  is formed by heating the thermoplast duct segment  20  to a temperature at which it becomes plastic and urging a tool against the duct segment  20  to deform the duct segment radially outward and form the bead  50 . A discussion regarding the formation of duct features such as bells and beads is provided in U.S. Application Ser. No. ______, titled “Post-Forming of Thermoplastic Ducts,” filed concurrently herewith, the entirety of which is incorporated by reference. The bead  50 , which extends radially outward, can be used to connect the duct segment  20  to another segment  20  or other device by inserting the beaded segment  20  into the other segment  20  or device. The formation of the bead  50  can also result in a recess  51  on the inner surface of the duct segment  20 . The recess  51  can be used to receive the bead  50  of another duct segment  20  for connecting the duct segments  20 . The calculated length L and/or widths W 1 , W 2  of the preform  60  can be adjusted for the formation of a bead  50 , bell, or other feature, which may affect the overall dimensions of the duct segment  20 . Additionally, marks can be provided for identifying the duct segment  20  or the duct  10 . For example, identifier marks (not shown) can be provided for indicating a model number, serial number, material type, and the like. One or all of the marks can correspond to data provided in manufacturing or assembly drawings.  
         [0047]    The preform  60  can be cut from the sheet  58  using an automated oscillating knife, rotating blade, waterjet, scissors, or other cutting devices, and the holes  70 ,  74  can be drilled or cut using a drill, punch, or other such devices. The lines and marks  69 ,  69   a ,  72 ,  73 ,  75 ,  76 ,  77 ,  78  can be formed before, during, or after the preform  60  is cut from the sheet  58 . For example, in one embodiment, a numerically controlled ultrasonic knife with a pen capability is used to cut the preform  60  and form the lines and marks  69 ,  69   a ,  72 ,  73 ,  75 ,  76 ,  77 ,  78  while the preform  60  is still flat.  
         [0048]    According to one embodiment of the present invention, the duct  10  is formed by first forming the preform  60  from a flat sheet of thermoplastic laminate. The preform  60  is marked with construction data such as the preform demarcation lines  69 , the spud holes  70 , the spud location marks  72 , the spud orientation marks  73 , the detail holes  74 , the detail marks  75 , the orientation marks  76 , the angular alignment marks  77 , the cuff marks  78 , the seam overlap line  69   a , and the identification marks. Preferably, the preform  60  is cut according to a flat geometric pattern that generally corresponds to the desired shape of the duct segment  20  so that the preform  60  can be configured and consolidated to form the desired shape of the duct segment  20  without trimming the duct segment  20  after forming. For example, the preform  60  is cut along the demarcation lines  69 , and the spud holes  70  and detail holes  74  are drilled in the preform  60 . The preform  60  is configured, consolidated, and connected to one or more other duct segments  20  to form the continuous passage  12  therethrough using the elongate flexible connector  80 . Preferably, the duct segments  20  are connected such that a longitudinal axis of each duct segment  20  is angled relative to a longitudinal axis of the duct segment(s) that are connected thereto, and the duct segments  20  form an articulated duct  10 . The marks  72 ,  73 ,  75 ,  76 ,  77 ,  78  can be used as guides during construction, for example, when connecting the duct segments  20  to one another or when connecting spuds  71  or other devices to the duct segments  20 .  
         [0049]    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. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.