Modelling the pultrusion process of an industrial L-shaped composite profile
Introduction
Pultrusion is in principle a simple process to manufacture constant cross sectional composite profiles. The process has a low labor content and a high raw material conversion efficiency since it is a continuous processing technique. The pultruded products have consistent quality and there is no need for any secondary finishing steps before the usage in service. A schematic view of the pultrusion process is shown in Fig. 1. The reinforcements in the form of continuous unidirectional (UD) roving or continuous filament mat (CFM) are held on creel racks and fed continuously through a guiding system. These reinforcements are being impregnated with the desired matrix system in a resin bath. The wetted-out reinforcement pack is then collimated into a preformed shape before entering the heating die. A polymerization takes place inside the die with the help of the heat coming from the heaters. The cured profile is advanced via a pulling system to the cut-off saw where the finished product is cut to desired lengths.
The CFM is generally used in combination with UD roving for pultruded industrial profiles. The CFM consists of long, continuous lengths of fiber strands overlying each other in a totally random swirl pattern. It provides stiffness and strength in the transverse as well as the pulling (longitudinal) directions. On the other hand, the UD roving provides longitudinal strength in the length of the profile. Therefore, the UD layer is transversely isotropic (TI), however the CFM layer can be considered as quasi-isotropic (QI) having equal material properties in the in-plane directions [1] and the out-of-plane properties are different than the in-plane properties [2].
The use of pultruded profiles in several industries such as construction, transportation and marine has grown significantly. Their main advantages over traditional materials are high strength-to-weight ratio, high corrosion resistance as well as good electrical and thermal insulation properties. In order to have a better understanding of the mechanical response or the failure mechanisms of pultruded structures under service loading conditions, the process induced residual stresses have to be characterized since they may lead to cracking during curing [3]. In addition, the dimensional changes during processing have to be controlled in order to improve the product quality in terms of geometrical tolerances. The thermal and cure history together with highly non-linear resin phase transitions (viscous-rubbery-glassy) [4] make the process complex to control and have a significant influence on the quality of the final composite part. During phase transitions, the resin undergoes large changes in its material properties, most significantly in its thermal expansion and elastic modulus [5], [6]. The main mechanisms generating the process induced stresses and shape distortions in pultrusion are summarized in [3], [4], [5], [6], [7].
A numerical process simulation tool is essential to address the main challenges in pultrusion such as process induced residual stresses and shape distortions together with the prediction of the thermal and cure history. Expensive trial-and-error approaches for designing new products and process conditions can be avoided using the developed process models.
The temperature and cure evolutions have been investigated in detail for the pultrusion process in literature [8], [9], [10], [11], [12], [13], [14], [15], [16]. Both numerical simulations [8], [9], [10], [11], [12], [13] and experiments [14], [15] were carried out to characterize the thermal and curing history for the pultruded products having simple cross sectional geometries such as rods and flat plates. Pulling force models were developed to calculate the viscous and frictional forces at the die-part interface [17], [18]. The effects of uncertainties in process parameters and material properties on the product quality were examined in [23] by means of a probabilistic simulation tool developed for pultrusion. The process parameters such as pulling speed and heater temperatures were optimized to maximize the cure quality or production rate by carrying out optimization studies based on thermo-chemical process models [19], [20], [21], [22]. In addition to the thermo-chemical studies of pultrusion [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], state-of-the-art process models have recently been developed by the authors to calculate the process induced residual stresses and shape distortions in pultrusion of UD profiles [24], [25], [26], [27]. In this thermo-chemical–mechanical model, the temperature and the degree of cure fields were obtained in a three dimensional (3D) thermal model which was sequentially coupled with a 2D quasi-static mechanical model [24]. It should be noted that all these contributions have been dealing with pultruded products containing only UD reinforcements.
A novel thermo-chemical–mechanical analysis of the pultrusion process containing both UD and CFM layers is presented in this work. A numerical simulation tool is developed to calculate the process induced stresses and dimensional variations in an industrially pultruded L-shaped profile. An “orthophthalic” polyester resin system specifically prepared for the pultrusion process is utilized to wet-out the reinforcements. The resin system provided by the pultruder already contains the required fillers, initiators and chemical additives. The cure kinetics model was developed using isothermal and dynamic differential scanning calorimetry (DSC) experiments. The temperature- and cure-dependent resin modulus was defined by conducting dynamic mechanical analyser (DMA) tests in tension mode. A modified cure hardening instantaneous linear elastic (CHILE) model [28], [29] was employed for the modulus development during processing. The temperature and degree of cure distributions are calculated in a 3D thermo-chemical analysis formulated in a Eulerian frame. Subsequently, the 3D thermo-chemical model is coupled with a 2D quasi-static mechanical model to predicted the stresses and displacements [24]. Two separate micromechanics approaches are utilized for the UD and the CFM layers. The spring-in formation is predicted and compared with measurements on real pultruded products. Subsequently, the residual spring-in angle is further calculated using the developed simulation tool for different pulling speed values. The classical laminate theory is employed to verify the predicted through-thickness residual stress field.
Section snippets
3D thermal model
The steady state energy equations are solved simultaneously for the UD layer (Eq. (1)), the CFM layer (Eq. (2)) and the die (Eq. (3)) (see Fig. 10 for the positioning of the layers). Here, is the pulling or longitudinal direction; and are the transverse directions for the UD layer. On the other hand, (pulling direction) and represent the in-plane directions and is the out-of-plane direction for the CFM layer.
Material characterization of the polyester
In this study, a summary of the results in [34] which has been carried out by the authors are presented. An industrial “orthophthalic” polyester system specifically prepared for the pultrusion process is considered.
Experimental
An L-shaped profile was pultruded in a commercial pultrusion company. Some photographs of the product are shown in Fig. 7. The cross sectional dimensions of the part were 50 50 5 mm and it contained glass/polyester based UD and CFM layers. A CFM having a density of 450 g/m2 was used in the process. The heating die was made of chrome steel.
The die length was specified as 1 m and the heater temperatures were set to 110 °C (near die the inlet) and 140 °C (near the die exit) by the pultruder. The
Results and discussions
Fig. 12, Fig. 13 show the predicted temperature and degree of cure distributions, respectively, for the specified pulling speed (600 mm/min). The temperature evolution for a point inside the part is depicted in Fig. 12(left) and the corresponding contour plots at the die exit and end of the process are shown in Fig. 12(right). The temperature gradually increases inside the heating die and decreases at the post die region owing to the convective cooling to the ambient temperature. It is seen that
Conclusions
A numerical process simulation was carried out for the pultrusion of an industrial L-shaped profile containing both UD and CFM layers. The reinforcements were impregnated using a polyester resin system specifically prepared for the process. The curing behavior was characterized by performing DSC experiments for the resin sample. A cure kinetics model was developed using the isothermal and dynamic DSC data. A temperature- and cure-dependent resin modulus was defined by conducting DMA tests. A
Acknowledgments
The author wishes to thank Dr. Roy Visser, Ivo Vrooijink and Nadia Vleugels from University of Twente (The Netherlands) for guidance and valuable discussions of the experiments. This work is a part of DeepWind project which has been granted by the European Commission (EC), Grant 256769 FP7 Energy 2010, under the platform Future Emerging Technology.
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