Patent Publication Number: US-2022234313-A1

Title: Pultrusion system with cooling stage and method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the priority of U.S. Patent Application No. 62/854,612, filed on May 30, 2019 and incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present application relates to a pultrusion system and to a method for pultruding beams using natural fibers or synthetic fibers, and the resulting pultruded beam reinforced by natural fibers or synthetic fibers. 
     BACKGROUND OF THE ART 
     Studies on pultrusion using natural fibers and/or biosourced matrices have demonstrated that biocomposite parts can be highly valuable for their mechanical properties, price and environmental benefits. In pultrusion, yarns of reinforcement fibers are fed into the system from a creel. Thermosets are added using a resin bath. Thermoplastics are fed into the system parallel to the reinforcement fibers or injected as melted pellets. When they are fed into the system parallel to the reinforcement fibers, they can be in the form of parallel yarns, comingled fibers, powder impregnated fibers or pre-impregnated tapes. Fibers and resin pass through a heated die of a certain cross-section shape. For thermoplastics, a tapered die entrance and resin overfilling are used to ensure pressure build-up for impregnation. A cooling system is added to prevent deconsolidation. The beam is pulled by a mechanism controlling the process speed. 
     Surface finish defects and deconsolidation have an impact on the commercial interest in pultruded products. Unlike thermoset composites, thermoplastic pultruded composites (TPCs) must be cooled while being constrained between mold surfaces. Otherwise, TPCs may deconsolidate in a porous structure having low mechanical properties. However, the thermoplastic polymer may tend to stick to metallic mold surfaces during cooling. This may create surface finish defects on pultruded products. Moreover, high thermoplastic polymer crystallinity requires specific temperatures and duration to be achieved. Unsolved post-impregnation challenges materialize in bad surface finish quality, sloughing and deconsolidation that results in high pulling forces. Indeed, when a thermoplastic polymer is cooled, it changes from a viscous flow state to a glassy state. During this change, the microscale adhesion behaviour of the thermoplastic polymer with metals can vary remarkably. 
     SUMMARY 
     It is an aim of the present application to provide a method for cooling pultruding beams of natural or synthetic fibers that addresses issues related to the prior art. 
     It is a further aim of the present application to provide a system for cooling pultruding beams of natural or synthetic fibers that addresses issues related to the prior art. 
     It is a further aim of the present disclosure to provide a method and system to pultrude and cool hollow beams. 
     It is a further aim of the present disclosure to provide a novel pultruded beam of natural fibers. 
     Therefore, in accordance with a first aspect of the present disclosure, there is provided a system for pultruding a beam comprising: a pulling mechanism continuously pulling on a preform of yarns including a thermoplastic matrix and fibers, the pulling mechanism being downstream of the system; and a sequence of at least one pultrusion die having a tapering channel portion heated such that the preform is at a desired low viscosity temperature for resin in the thermoplastic matrix to impregnate the fibers, a cooling tube at a downstream end of the pultrusion die, and a cooling module spaced from the pultrusion die by the cooling tube, the cooling module to cool the cooling tube before the preform reaches the pulling mechanism, wherein the cooling tube defines a cooling channel. 
     Further in accordance with the first aspect, for instance, the pulling mechanism comprises at least a pair of roller on opposite sides of the beam. 
     Still further in accordance with the first aspect, for instance, a pre-heating module pre-heats the preform upstream of the at least one pultrusion die. 
     Still further in accordance with the first aspect, for instance, the at least one pultrusion die includes a first pultrusion die, a vacuum module having a vacuum cavity to remove air from the preform exiting the first die, and at least a second pultrusion die to further impregnate the fibers. 
     Still further in accordance with the first aspect, for instance, the at least one pultrusion die has a taper in a range of 2° to 6° from a central axis of the tapering channel portion. 
     Still further in accordance with the first aspect, for instance, the at least one pultrusion die includes a straight channel portion downstream of the tapering channel portion. 
     Still further in accordance with the first aspect, for instance, the cooling tube is exposed to ambient. 
     Still further in accordance with the first aspect, for instance, the cooling module is displaceable along the cooling tube to vary a distance between the cooling module at the at least one pultrusion die. 
     Still further in accordance with the first aspect, for instance, at least one actuator displaces the cooling module relative to the cooling tube. 
     Still further in accordance with the first aspect, for instance, the cooling tube extends through the cooling module. 
     Still further in accordance with the first aspect, for instance, a longitudinal axis of the cooling tube is curved. 
     Still further in accordance with the first aspect, for instance, the cooling tube is integrally connected to the pultrusion die and extends therefrom. 
     Still further in accordance with the first aspect, for instance, the cooling module has at least one block defining another cooling channel, and wherein heating cartridges are in the at least one block. 
     Still further in accordance with the first aspect, for instance, a pultrusion mandrel extends in the at least one pultrusion die and the cooling channel. 
     Still further in accordance with the first aspect, for instance, the pultrusion mandrel has a thermal management core inside a forming sheath. 
     Still further in accordance with the first aspect, for instance, the thermal management core has a heating element forming a heating zone and a cooling element forming a cooling zone. 
     Still further in accordance with the first aspect, for instance, an actuator displaces the pultrusion mandrel relative to the pultrusion die and to the cooling channel. 
     Still further in accordance with the first aspect, for instance, the pultrusion mandrel has a variation of cross section. 
     In accordance with a second aspect of the present disclosure, there is provided a method for pultruding a beam comprising: continuously pulling on a preform of yarns including a thermoplastic matrix and fibers; while continuously pulling, sequentially impregnating the fibers by passing the preform through at least one pultrusion die having a tapering channel portion heated such that the preform reaches a desired low viscosity temperature for resin in the thermoplastic matrix, and cooling the preform by passing the preform in cooling tube downstream of the pultrusion die, and by passing the preform in a cooling module spaced from the pultrusion die. 
     Further in accordance with the second aspect, for instance, continuously pulling on the preform of yarns comprises pulling on the beam after the cooling to cause a continuous pull of the preform of yarns. 
     Still further in accordance with the second aspect, for instance, on the beam comprises passing the beam through at least a pair of roller on opposite sides of the beam. 
     Still further in accordance with the second aspect, for instance, the preform is pre-heated prior to passing the preform through the at least one pultrusion die. 
     Still further in accordance with the second aspect, for instance, impregnating the fibers includes passing the preform a tapering channel portion tapering in a range of 2° to 6° from a central axis of the die. 
     Still further in accordance with the second aspect, for instance, impregnating the fibers includes passing the preform through a straight channel portion downstream of the tapering channel portion. 
     Still further in accordance with the second aspect, for instance, the cooling module is moved along the cooling tube to vary a distance between the cooling module at the at least one pultrusion die. 
     Still further in accordance with the second aspect, for instance, impregnating the fibers and cooling the preform includes passing the preform around a mandrel, whereby the preform is a tube. 
     Still further in accordance with the second aspect, for instance, the mandrel is displaced relative to the pultrusion die and to the cooling channel. 
     In accordance with a third aspect of the present disclosure, a system for pultruding a beam comprises a pulling mechanism continuously pulling on a preform of yarns including a thermoplastic matrix and fibers, the pulling mechanism being downstream of the system; and a sequence of a pre-heating module to pre-heat the preform, at least one pultrusion die having a tapering channel portion heated such that the preform is at a desired low viscosity temperature for resin in the thermoplastic matrix to impregnate the fibers, and a cooling module to cool the beam before the beam reaches the pulling mechanism, and a pultrusion mandrel through the system. 
     Further in accordance with the third aspect, for instance, the pultrusion mandrel has a thermal management core inside a forming sheath. 
     Still further in accordance with the third aspect, for instance, the thermal management core has a heating element forming a heating zone and a cooling element forming a cooling zone. 
     Still further in accordance with the third aspect, for instance, an actuator displaces the pultrusion mandrel relative to the pultrusion die and to the cooling channel. 
     Still further in accordance with the third aspect, for instance, an actuator displaces the thermal management core relative to the forming sheath. 
     Still further in accordance with the third aspect, for instance, the pultrusion mandrel has a variation of cross section. 
     Still further in accordance with the third aspect, for instance, the pulling mechanism comprises at least a pair of roller on opposite sides of the beam. 
     Still further in accordance with the third aspect, for instance, the at least one pultrusion die includes a first die having a tapering channel portion heated such that the preform reaches a desired low viscosity temperature for resin in the thermoplastic matrix to impregnate the fibers, a vacuum module having a vacuum cavity to remove air from the preform exiting the first die, and at least a second die having a tapering channel portion heated such that the preform is at the desired low viscosity temperature for resin in the thermoplastic matrix to further impregnate the fibers, 
     Still further in accordance with the third aspect, for instance, at least one of the first die and the second die has a taper in a range of 2° to 6° from a central axis of the tapering channel portion. 
     Still further in accordance with the third aspect, for instance, at least one of the first die and the second die includes a straight channel portion downstream of the tapering channel portion. 
     Still further in accordance with the third aspect, for instance, the system has a channel smaller in cross-section in the second die than in the first die. 
     In accordance with another aspect of the present disclosure, there is provided a method for pultruding a hollow beam comprising: continuously pulling on a preform of yarns including a thermoplastic matrix and fibers; while continuously pulling, sequentially passing the preform through at least one pultrusion first die having a straight or tapering channel portion heated such that the preform reaches a desired low viscosity temperature for resin in the thermoplastic matrix to impregnate the fibers, also passing around a mandrel having a straight, tapered or expanding cross-section, passing around the said mandrel having a heated and a cooled section along the length, passing the preform in cooling tube downstream of the pultrusion die, passing the preform in a cooling module. 
     In accordance with another embodiment of the present disclosure, the position of the components of the pultrusion system can be adjusted during pultrusion to control the profile thickness and pultrusion forces. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a pultrusion system with a cooling module in accordance with the present disclosure; 
         FIG. 2  is a schematic view of a pultrusion module and cooling module as part of the pultrusion system of  FIG. 1 , along with an exemplary temperature graph; 
         FIG. 3  is an exemplary embodiment of a temperature graph using the pultrusion system with cooling stage of  FIG. 1 , the temperature graph showing the temperature read by a temperature sensor following the pultrudate; 
         FIG. 4  is a zoomed view of the cooling temperatures in the example of  FIG. 4 ; 
         FIG. 5  is a schematic view of a pultrusion module and curved cooling module as part of the pultrusion system of  FIG. 1   
         FIG. 6  is a schematic view the pultrusion module and cooling module as part of the pultrusion system of  FIG. 1 , with relative movement of the cooling module; 
         FIG. 7  is a schematic view of the pultrusion module and cooling module as part of the pultrusion system of  FIG. 1 , with a pultrusion mandrel for forming pultruded tubes; and 
         FIG. 8  is a schematic view of the pultrusion module and cooling module as part of the pultrusion system of  FIG. 1 , with a pultrusion mandrel having two different cross-sectional shapes. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings and more particularly to  FIGS. 1 and 2 , there is illustrated a pultrusion system  10  to operate a method for pultruding beams B including natural fibers in accordance with the present disclosure. The method therefore uses a preform of yarns A including a thermoplastic resin forming the matrix of the beam, and natural fibers used as reinforcement. However, even though the method is suited for the addition of natural fibers, it is considered to use synthetic fibers as well. As an example, the method and system  10  may use polylactic-acid (PLA) matrix as one of the yarns A, and a flax yarn as another one of the yarns A. For example, the PLA yarns may be trilobal 180 tex spun by Applied Polymer Innovations (from 4032D, NatureWorksLLC). This is one possible thermoplastic that may be used, as other thermoplastics, such as polypropylene, may be used as well, for this or for other natural fibers. Flax is a natural fiber that is well suited for being part of the present pultruded beam, due to its relatively low lignin content for high cellulose content, enabling higher process temperatures. Other natural fibers that may be used as well include hemp fibers and jute fibers, or mixes of natural fibers. Synthetic fibers can be used as well, such as glass fibers, carbon, etc, as well as mixtures of natural and synthetic fibers. The number of yarns A is determined by a nominal fiber volume content of 40%. Other materials that may be used for the yarns A include commingled yarns of carbon fibers and polyamide fibers (Dualon, Karijene Inc., Japan; Lexter, Mitsubishi Gas Chemical Inc., Japan). 
     Different types of yarn precursors can be used such as powder impregnated yarns where the reinforcing fibers are filled with polymer powder. Another type of precursor is a pre-impregnated tape where a fixed amount of reinforcement fibers are impregnated by a polymer. Other kind of yarns where the reinforcement fibers are mixed with the polymer can be used. Various arrangements are considered to feed the yarns to the system  10 , such as having polymer yarns and reinforcement yarns on different spools, or as part of a parallel hybrid yarn. As another embodiment, comingled yarns of polymers and reinforcing fibers may also be used. As another embodiment, the reinforcement fibers may be sheath coated with a polymer sheath. As another embodiment, polymer fibers may be part of microbraided yarns. 
     In an embodiment, yarns A are fed from a creel into the system  10 , as a possibility among others. A ring C may be used to ensure all yarns are aligned before entering the system  10 . Yarn precursors are wound onto multiple spools and placed onto the creel. Yarns are tensioned by adjusting the friction between the spools and the creel. This tension keeps the yarns straight before they enter the pultrusion modules of the system  10 . 
     The pultrusion system  10  may have or may not have a pre-heating module  20 , an impregnation module  30  including pultrusion die(s), a cooling module  40 , and a pulling mechanism  50  at the outlet of the cooling module  40  to exert the pulling action on the pultruded beam exiting from the cooling module  40 . The various components of the modules  20 ,  30 , and  40  of the system  10  may be arranged in a modular fashion, for example along rails to facilitate their handling and relative positioning and alignment. 
     If present, the pre-heating module  20  is used to pre-heat the yarns A entering the system  10 , gradually and uniformly, to reduce the risk of degrading of the preform due to an abrupt temperature increase. As an example, a pre-heating module  20  that may be used is as described in PCT Application Publication No. WO 2017/219143, incorporated herein by reference. At the exit of the pre-heating module  20 , the preform is heated to the point where the thermoplastic resin is in a near-liquid state, with high enough viscosity for the preform to remain integral, without significant dripping of the resin from the preform, without significant flowing to impregnate the reinforcement fibers. This may be achieved by an iterative control of the pre-heating parameters. Therefore, the preform enters the impregnation module  30  to be further heated and consolidated into a pultruded beam. 
     The impregnation module  30  is used for the impregnation of the natural fibers or synthetic fibers in the thermoplastics, to form a hot consolidated pultruded beam, by way of one or more pultrusion dies. The pultrusion dies can also be separated by heating ovens or heated vacuum chambers. 
     The cooling module  40  cools the hot pultruded beam at the exit of the impregnation module  30 . In other words, the cooling module  40  may be referred to as a “heat extraction module”, as the hot pultruded beam loses heat as it passes through the cooling module  40 . As explained below, the cooling module  40  removes heat from a cooling tube in which the hot pultruded beam passes (a.k.a., the pultrudate, i.e., the material passing in the pultrusion system  10  and the result of the pultrusion). While the expression cooling is used, the cooling device  41  may have to be heated above ambient conditions, as the cooling of the pultruded beam is achieved at temperatures below those involved in the impregnation module  30 , but nonetheless higher than ambient temperatures. 
     The pulling mechanism  50  is located at the outlet of the cooling module  40  to exert the pulling action on yarns A, becoming the pultruded beam B exiting from the cooling module  40 . 
     Referring to  FIG. 2 , the impregnation module  30  is shown having a pultrusion die  31 . The pultrusion die  31  may be standalone, or may be one of a plurality of pultrusion dies  31  of the impregnation module  30 . For instance, the impregnation module  30  has two pultrusion dies  31  with a vacuum die therebetween, although it is possible to provide the system  10  with a single one of the pultrusion dies  31 . Although the expression “vacuum die” is used, the vacuum component between the dies  31  could be called a vacuum chamber since there may be no contact with the pultrudate during passage through this section. Also, the vacuum chamber or die could be replaced or accompanied by a heated chamber without vacuum applied, essentially acting like an oven. The temperature is controlled, not necessarily at the polymer processing temperature. As another possibility, the impregnation module  30  may have a cascade of N pultrusion dies  31  separated by N−1 vacuum dies (e.g., N=3 or more). The pultrusion dies  31  may have a similar construction. For simplicity, a single pultrusion die  31  is described, if multiple pultrusion dies  31  are present the one being described is the downstream pultrusion die. The pultrusion dies  31  may be as described in PCT Application Publication No. WO 2017/219143. Hence, the pultrusion dies  31  may form a channel  32 . The channel  32  may have a tapering channel portion  32 A, with or without a subsequent straight channel portion  32 B (which may instead be present in a cooling tube described below). 
     In an embodiment, the channel  32  has a circular section, such that the pultruded beam is a rod. The channel  32  may therefore have a frustoconical channel portion  32 A or like tapering channel portion, followed up by a straight channel portion  32 B downstream of the frustoconical channel portion  32 A. In the illustrated embodiment, the tapering channel portion  32 A is longer than the straight channel portion  32 B, as the tapering is gradual to produce a change in sectional dimensions of the preform. For example, the tapering of the tapering channel portion  32 A is between 2° and 6°, with the tapering channel portion  32 A constituting roughly 80% (e.g., from 70-85%) of the overall length of the channel  32 , the straight channel portion  32 B taking up the remaining 20% of length (e.g., from 15-30%). The 2°-6° range represents the angle of the tapering wall relative to a central axis of the tapering channel portion  32 A. The range between 2° and 6° is merely provided as an example at which the pultrusion method can suitably involve natural fibers. However, larger angles can be present, for instance up to 45°, and different shapes (other than cone) can be used such as ellipsoidal, rounded polymers, rounded cone, among possibilities. If there are multiple dies pultrusion  31 , one difference between the similar dies  31  is the smaller size of the tapering channel  32 , in a downstream direction of the pultrusion process. According to an embodiment, the diametrical dimensions are 2.5% smaller in the downstream die  31 . For example, an upstream pultrusion die  31  may have a final diameter larger by 5 mm than a downstream pultrusion die  31 , to let additional resin flow into the downstream pultrusion die  31 , for overfilling purposes. 
     The die  31  is made of a material having suitable heat-transfer properties (e.g., a metal) as it must transmit heat to the preform passing in the tapering channel  32 . According to an embodiment, continuous bores extend through the pultrusion die  31 , to receive heating elements therein, or heating fluids by forming a fluid circuit. The heating elements or heating fluid in the bores will heat the base die  31 , in such a way that the preform in the tapering channel  32  is heated by the surfaces of the tapering channel  32 . Other bores may also be used for thermocouples, to monitor the temperature in the module. In an embodiment, 120V or 240V electric heating cartridges are used, capable of producing 350 W/m 2  for example. In another embodiment, external heating plates can be used over and under the die  31 . The heating by the dies  31  allows the thermoplastic matrix to reach a sufficiently low viscosity. The temperature reached may be above the melting temperature of the thermoplastic for this purpose. 
     In an embodiment, the system  10  has a single-die impregnation module  30  with a single one of the pultrusion dies  31 . For example, the single pultrusion die  31  may be 76 mm long, with a 5° taper in the tapering channel portion  32 A, followed by the straight channel portion  32 B being for example 20 mm long with a 4.78 mm constant diameter. 
     Referring to  FIG. 2 , the cooling module  40  is shown as having a cooling device  41 . The cooling device  41  may have a similar construction to that of the other components of the system  10 , with a base  41 A and a cover  41 B, though this is not necessary. The cooling device  41  may surround a cooling tube  42 , that defines a cooling channel  42 A. The cooling tube  42  is shown as being part of the cooling module  40 , in that cooling of the pultruded beam occurs in the cooling tube  42 , in spite of the fact that, in an embodiment, the cooling tube  42  may be integrally connected the impregnation module  30 . The cooling tube  42  may be a thin-walled extrusion for instance of thickness ranging between 0.05 and 100 mm-100 mm being for larger beams-, for an internal diameter ranging between 0.1 and 2000 mm, in the case of a circular cross section, but with equivalent cross-sectional areas if the cross section is not circular. In an embodiment, the cooling tube  42  defines a closed section, e.g., circular, rectangular, or other, in continuation with that of the pultrusion die  31 . The cooling tube  42  may project continuously from the pultrusion die  31 . Stated differently, an inner diameter is preserved from the straight channel portion  32 B to the cooling channel  42 A. In an embodiment, the cooling tube  42  is integral with the pultrusion die  31 , i.e., it is made with the same material and may be monoblock with it. Another assembly is considered in which the cooling tube  42  is fitted into the die  30  with a holding system such as a pipe compression fitting. The tube  42  and the die  30  may hence be two different metal parts that are mechanically joined together (e.g., welded, brazed, etc). The cooling tube  42  is surrounded by an air gap (e.g., ambient), fluid gap, or lower heat-transfer material, such that a heat sink between modules  30  and  40  is small. The cooling tube  42  may then contact the cooling device  41  and may even penetrate it as shown in  FIGS. 2-5 . In an embodiment, the cooling tube  42  abuts against an upstream surface of the cooling device  41  or penetrates partially into the cooling device  41 , with the cooling device  41  defining its own cooling channel  42 A ( FIG. 3 ). 
     Pultruded materials previously heated at processing temperature in the pultrusion die  31  are pulled through the cooling tube  42 . Heat is extracted from the pultruded product by the cooling tube  42  via its cooling with the cooling device  41 , installed on the cooling tube  42  and/or downstream of the cooling tube  42  at a predetermined distance from the pultrusion die  31 . This cooling system, defined by the cooling device  41  and the cooling tube  42 , may provide an accurate control on the thermal profile on the surface of the pultruded product, therefore enabling an improvement of the surface finish quality. In an embodiment, the beam cools sufficiently while in the cooling tube  42  upstream of the cooling device  41 , such that it contracts so as not to rub against the surface of the channel  42 A when in the cooling device  41 . The separation distance between the die  31  and the cooling device  41 , resulting from the use of the cooling tube  42  allows cooling to occur without deconsolidation. In addition, the arrangement of  FIGS. 2 and 3  may allow the hot molten polymer mixture to develop a controlled semi-crystalline microstructure due to the accurate cooling temperature profile control. Accordingly, whether the cooling tube  42  extends continuously or not through the cooling device  41 , the pultrusion is cooled sufficiently to contract before it may come into contact with any surface disruption or seam in the cooling channel  42 A. 
     Referring to  FIG. 6 , the cooling module  40  may be configured to vary a separation distance D between the pultrusion die  31  and the cooling device  42 . For example, this is achieved by allowing the cooling device  42  move relative to the cooling tube  42 . This variation of the separation distance D may be performed to accurately control the cooling temperature profile of the pultruded product. The distance can be adjusted mechanically in response to a sensor(s) reading(s), with the sensor(s) including one or more of a surface profilometer and/or a pultrusion pulling force sensor, as examples. The variation of the separation distance D can also be adjusted in relation to the pultrusion speed, as an example. A short separation distance will accelerate the cooling. The pultruded beam or rod will shrink and de-bond from the cooling channel faster. This will reduce the pulling forces generated by the pultrusion system  10 . On the other hand, a long separation distance will slow the cooling. The pultruded beam will be in contact for a longer time with the cooling channel surface, thus creating higher pultrusion forces. The longer exposition at hotter temperature will enhance the crystallization of a semi-crystalline thermoplastic polymer. Hence, a variation of the separation distance D may be adjusted as a function of the desired surface finish. This may done during the pultruding, for instance as a response to a variation of pulling speed. The mechanical adjustments may be done using appropriate actuator(s)  45 , such as linear actuators including electro-mechanical actuators, pneumatic cylinders, hydraulic cylinders. Position adjustment mechanisms, including sets of screws and bolts, may also be used, for instance for manual mechanical adjustment. For example, the adjustment may be done at the beginning of a new batch and/or adaptively during the pultrusion of the batch. 
     In an embodiment, the cooling channel  42 A has a circular section, like other components of the system  10 . Likewise, the components of the system  10  may produce beams in a variety of cross-sections, including square, rectangular, polygonal, oval, U, I, just as examples. The base  41 A, the cover  41 B, the cooling tube  42  are made of materials having suitable heat-transfer properties, i.e., low longitudinal thermal conductivity, (e.g., a metal) as they must cool the beam passing in the cooling channel  42 A. According to an embodiment, the base  41 A and cover  41 B have bores  43 A and  43 B ( FIGS. 3 and 4 ) in their side walls, to form bores extending through the cooling device  41 , to receive cooling elements therein ( FIG. 4 ), or cooling fluids. Other bores may also be used for thermocouples, to monitor the temperature in the module  40 . The cooling elements (e.g., fluid coils for cooling fluid) in the bores  43 A and  43 B will heat the base  41 A and cover  41 B, in such a way that the preform in the cooling channel  42 A is cooled by the surfaces of the channel  42 A, whether they be defined totally by the cooling tube  42  or also by the cooling device  41 . In an embodiment, the cooling elements are 120V or 240V electric heating cartridges, capable of producing 450 W/m 2  for example. In an embodiment, the cooling element is the bore  43 A is warmer than the cooling element in the bore  43 B. 
     Referring to  FIG. 7 , an embodiment of the pultrusion system  10  is shown, in which the system  10  has the capacity to pultrude hollow profiles, a.k.a., tubes. In this case, the pultrusion system  10  is equipped with a pultrusion mandrel  60 . The pultrusion mandrel  60  is attached to and supported at the upstream end of the system  10 , such as before the pre-heating die  20 . For simplicity, only the portion of the pultrusion mandrel  60  in the impregnation module  30  and cooling module  40  is shown, but the pultrusion mandrel  60  extends upstream to be present in the pultrusion system  10  as the yarns A penetrate the pre-heating die  20 , for the yarns A to be distributed in a hollow arrangement. The mandrel  60  may be composed of a thermal management core  61  incorporated into a forming sheath  62  (e.g., a tube) that comes into contact and shapes the pultruded beam. In an embodiment, the thermal management core  61  may have a heating component(s)  61 A forming a heating zone, and a cooling component(s)  61 B defining a cooling zone. The thermal management core  61  may be free to move relative to the forming sheath  62 , to adjust a position D′ of the heating zone and the cooling zone relative to the forming sheath  62  and thus relative to the dies of the various modules  20 ,  30  and  40 . In an embodiment, the position of the thermal management core  61  may be adjusted during pultrusion to modify the cooling temperature profile of the inner surface of the hollow pultruded profile, for example. The change of position may be as a function of the pultrusion parameters, such as the pultrusion speed and/or sensor readings on temperature, etc. The mechanical position adjustments of the thermal management core  61  and/or of the forming sheath  62  may be done using appropriate actuator(s)  65 A for the core  61  and/or  65 B for the sheath  62 , such as linear actuators including electro-mechanical actuators, pneumatic cylinders, hydraulic cylinders. The actuators  65 A and  65 B are shown in separate figures, i.e.  FIGS. 7 and 8 , but may be present concurrently to allow relative movement between core  61  and sheath  62 . A position adjustment mechanism(s), including sets of screws and bolts, may also be used, for instance for manual mechanical adjustment. For example, the adjustment may be done at the beginning of a new batch and may be adjusted during the pultruding. In an embodiment, such components access the thermal management core  61  via the upstream end of the system  10 , as may do the pipes or conduits that can direct coolant or wires that may power to the component(s)  61 A and to the cooling component(s)  61 B. 
     Referring to  FIG. 8 , the forming sheath  62  of the pultrusion mandrel  60  may have geometry variations along its longitudinal axis (i.e., parallel to the pultrusion process direction). For example, in  FIG. 8 , the forming sheath  62  has two different diameters, for instance with a taper (e.g., frusto-conical segment) or like smooth transition between the two diameters. The forming sheath  62  may be moved during pultrusion to adjust the hollow pultruded profile&#39;s thickness. The cross-sectional shapes of the forming sheath  62  may be the same, but may also vary. 
     Although different dimensions are possible, the cooling channel  42 A is a 140 mm long segment with a constant diameter of 4.78 mm, to be put in perspective with the dimensions of the other preform passages of the system  10 . 
     Referring to  FIG. 5 , the cooling tube  42  and/or cooling channel  42 A can be bent to pultrude a curved pultrusion product. In the illustrated embodiment, the curvature occurs directly downstream of the pultrusion die  31 , and extends all the way to the downstream end of the cooling channel  42 A. 
     The pulling mechanism  50  is located at the outlet of the cooling module  40  to exert the pulling action on yarns A. The pulling mechanism  50  may have any appropriate embodiment. For example, the pulling mechanism  50  may include multiple pairs of opposing rollers ( FIG. 1 ) between which the pultruded beam is squeezed. The superior rollers may be actuated by an electric motor, the inferior ones are free wheels, or vice-versa. A rotary encoder may be used to monitor the pultrusion speed. Load cells may also be placed between the pulling mechanism  50  and the module  40  of the system  10  to measure the pulling force on the beam. The pulling mechanism can also be a reciprocating puller, in which two clamps are moving to pull, one after the other, the beam. 
     In operation, the yarns A are continuously pulled by the pulling mechanism  50 , through the various modules of the system  10 . The thermoplastic matrix and natural fibers or synthetic fibers are gradually and uniformly pre-heated in the pre-heating module  20 , if present, to approach impregnation temperatures, at which the viscosity of the resin of the thermoplastic matrix will be at or near its minimum. The preheating initiating the impregnation and lowering void content in the preform. In the impregnation module  30 , the preform reaches this impregnation temperature, while being exposed to a reduction in diameter. There results a consolidating pressure in the resin, by which the resin will impregnate the natural fibers. The impregnation of the natural fibers is the soaking and saturating of the natural fibers with the resin, with resin penetration between fiber filaments. It is observed that the system  10  allows impregnation without air pressure beyond ambient. The intervening step of vacuuming the preform, in the vacuum die if present, between the two reductions in diameter of the pultrusion dies  31  if multiple pultrusion dies  31  are present, allows removal of air from the resin heated to low or minimum viscosity, and likely the deconsolidation of the preform. The deconsolidation preform has air-evacuation pathways to facilitate air removal from the pre-form. The removal of air, and the subsequent expose to diameter reduction while impregnation temperatures are maintained, allows the beam to reach a desired cross-sectional diameter, with reduced air content (less than 10%). The cooling of the resulting beam allows same to stabilize into its resulting dimension and shape. 
     There results a pultruded beam comprising a thermoplastic matrix reinforced with natural fibers, the pultruded beam having a void percentage lower than 10% of an overall volume of the pultruded beam, and further wherein the natural fibers constitute between 40% and 60% of an overall volume of the pultruded beam. 
     Other than the use of the pultruded beams directly as components, other uses for the pultruded beams include pultruded insert overmolded with thermoplastic resin, injection precursor with oriented pellets, and compression molding precursor, with oriented strands, for instance to form panels incorporating randomly oriented stands. 
     Consequently, the arrangement of a cooling tube  42  between the dies  31  and  41  may improve the overall surface finish as well as the impregnation quality in thermoplastic pultrusions. The cooling tube  42  may prevent composite deconsolidation since cooling to a rigid state is made when the material is constrained between surfaces. This allows an enhanced control on the beam shape and dimension. Moreover, the cooling tube  42  may cause a lower void content since no voids are reintroduced in a deconsolidation of the beams. The cooling tube  42  may limit the temperature interference between the dies  31  and  41  since there is less material to conduct heat. This improves the energy efficiency of the system. 
     The arrangement of cooling die  31  and cooling tube  42  may also allow the hot molten polymer mixture to cool relatively rapidly, therefore limiting adhesion forces. This effectively reduces the pulling forces at the pulling mechanism  50 . 
     In the embodiments of  FIGS. 2 and 5 , in which the cooling tube  42  extends through and beyond the cooling device  41 , the position of the cooling device  41  onto the cooling tube  42  may be modified towards or away from the pultrusion die  31  by sliding the cooling device  41 . Such an adjustment of the relative position of the die  31  and device  41  may be performed as a function of the pultrusion speeds, material properties and process temperatures. In such an embodiment, the cooling thermal profile can be modified to tailor the crystallinity content in semi-crystalline polymers. 
     Another benefit from a more rapid cooling is the thermal shrinkage of the pultruded component along its cross-sectional dimensions. As schematically shown in  FIGS. 2-5 , the thermal shrinkage will result in the pultruded component being smaller than the dimensions of the cooling channel  42 A. This effectively creates a small separation between the pultruded component B and the surface of the cooling channel  42 A, therefore reducing the pultruded component to die surface friction. This effectively reduces the pultrusion pulling forces. 
     A method for pultruding a hollow beam may include any of: continuously pulling on a preform of yarns including a thermoplastic matrix and fibers; while continuously pulling, sequentially passing the preform through at least one pultrusion first die having a straight or tapering channel portion heated such that the preform reaches a desired low viscosity temperature for resin in the thermoplastic matrix to impregnate the fibers, also passing around a mandrel having a straight, tapered or expanding cross-section, passing around the mandrel having a heated and a cooled section along the length, passing the preform in cooling tube downstream of the pultrusion die, passing the preform in the cooling module  40 . 
     Exemplary Embodiment 
       FIGS. 3 and 4  show actual temperature measures during a C/PEI pultrusion experiment, given solely as an example. The temperatures were measured by inserting a wire-thermocouple in the middle of C/PEI pre-consolidated precursor and pulled with the material along the pultrusion line. During this pultrusion experiment, T p  was set at 380° C. and pulling speed was set at a low speed of 50 mm/min. The pre-heater temperature was set at 200° C. PEI is an amorphous polymer. Its T g  is around 210° C. T c  was set at 109° C., which is, a temperature below T g . As can been seen in the figure, a sloughing critical zone was 25.0 mm. The sloughing critical zone can be defined as a zone where the polymer is in contact with the cooling tube and where the polymer is cooler than Tp while still hotter than Tg. Within this zone, the polymer can adhere to the cooling tube and break-off from the pultruded composite. The broken polymer chunk then acts as a fixed mechanical feature plowing in the moving pultruded beam. The defect created is called sloughing. The pultruded beam also left the cooling at a temperature below T g . This prevented any potential deconsolidation after leaving the cooling device. 
     Based on these surface measurements, it can be assumed that the temperature drops rapidly under T g  while the material is still in the cooling tube  42 . This high cooling rate results in a shorter sloughing-critical distance of approximately 32 mm.