CFD Modeling of the Closed Injection Wet-Out Process For Pultrusion

Transcription

1 CFD Modeling of the Closed Injection Wet-Out Process For Pultrusion Authors: Mark Brennan, Huntsman Polyurethanes, Everslaan 45, Everberg 3078, Belgium, and Michael Connolly & Trent Shidaker, Huntsman Polyurethanes, 2190 Executive Hills Boulevard, Auburn Hills, MI, USA Michael Connolly Product Manager-Urethane Composites Huntsman Polyurethanes 2190 Executive Hills Boulevard Auburn Hills, Michigan USA Phone: Fax: About the Authors: Michael Connolly, Ph. D. is Product Manager-Urethane Composites for Huntsman Polyurethanes in Auburn Hills, Michigan, USA. Michael has been employed in polyurethane product development for automotive, consumer and industrial applications for 12 years at Huntsman and has 19 years experience in a range of polymer materials development roles. He received his doctoral degree in Polymer Science and Engineering from the University of Massachusetts-Amherst in Mark Brennan is PU Expert in Computational Physics for Huntsman Polyurethanes in Everberg, Belgium. Mark has been employed in process and product research for 7 years at Huntsman and has 12 years experience in applying Computational Fluid Dynamics and Finite Element Analysis to solve industrial applications. He received his master s degree in Mathematics from University of Dublin, Trinity College in Trent Shidaker is Development Manager for Polyurethane Composites and Rigid Materials at Huntsman Polyurethanes in Auburn Hills, MI. Trent has been employed as manager or project leader in polyurethane foam and composite development at Huntsman for 12 years. Abstract: The practice of closed injection wet-out in pultrusion has become increasingly utilized over the last five years. This paper presents a mathematical model for simple geometries of the closed injection wet-out process used in pultrusion and represents an initial step to understanding wetting phenomena in complex shapes with complex reinforcement. The moving porous model used is both a generalization of the Navier- Stokes equations and Darcy s law used for flows in porous media. The model can predict pressure build-up, flow patterns and composite void content and evaluate the influence of process conditions, injection box design, and material properties on these parameters.

2 INTRODUCTION Over the past decade, the global pultrusion industry has increasingly adopted closed injection wet-out for profile processing. This trend has been driven by awareness of worker safety and mandates by regulatory agencies to reduce volatile organic compounds (VOC s) and by the availability of new resin technology. Polyurethane (PU) resins, in particular, have become an accepted alternate technology which contain no VOC s, especially for applications requiring high strength and durability combined with the potential for high pultrusion line speeds (Ref. 1-3 and references therein). However, the relatively fast reactivity of PU resins requires processing via closed injection wetout. The complex profile geometries and reinforcement schedules used in stateof-the-art profiles make efficient wetout challenging in a constrained state within an injection box. This challenge is even greater for PU resins, considering their inherently fast reactivity. Hence, a more thorough understanding of wet-out dynamics in injection processing for pultrusion is needed to expand acceptance of this process for PU resins and accelerate its implementation. This report describes a mathematical model for closed injection wet-out in the pultrusion process. The model predicts the impact of injection box geometry, process conditions, resin viscosity, reinforcement fraction and permeability on flow patterns, pressure profiles and final part density. EXPERIMENTAL CFD simulations were carried on an injection box for a die profile dimension of 5.21cm x cm (2.05 x ). The line speeds were varied between 0.6 and 1.5 m/min (24 to 60 inch/min) with injection box lengths from 30.5 to 53.3 cm (12 to 21 ). Resin viscosity was varied between 600 and 2400 cp. Experimental pultrusion runs were made to verify the CFD results. RIMLine SK polyol and Suprasec 9700 isocyanate and experimental variants were combined as the polyurethane matrix. Profiles were pultruded with 120 rovings for a total glass content of 77 wt% (60 vol %). A two-component metering unit (Electric Willie from GS Manufacturing) with gear pump flow control was used to ensure delivery of the polyurethane resin at constant pressure. Injection boxes were fabricated using aluminum with HDPE guide plates having small holes to prevent resin leakage. Digital pressure transducers or pressure gauges were placed at ~2.5 cm (1 ) and ~10 cm (4 ) from the die face as well as the injection port. The typical injection box setup is shown in Figure 1. MATHEMATICAL MODEL Due to difficulty in modeling macroscopic transport of resin and reinforcement based on microscopic variation of velocity between fibers, the resin and reinforcement are better treated as a moving porous fiber medium. The moving porous model used is both a generalization of the Navier-Stokes equations and Darcy s law used for flows in porous media. The balance equations for mass and momentum for steady incompressible flow are u 0 2 u u p u S u, p,,, and S are the superficial velocity, pressure, density, viscosity and momentum sources, respectively.

3 The momentum sources, S, defining the moving porous medium are S u U K K, and U are the permeability tensor, porosity and velocity of the fiber bundle. The geometry of the injection box is shown in Figure 2. This approach is similar to the model demonstrated by Jeswani et al. [Ref. 4] The approach is a phenomenological one with the main assumption being that the surface tension effects are not important or are captured in the permeability tensor. The reinforcements are initially dry and the resin must penetrate the fiber bundles. Capillary forces of attraction and repulsion act at the flow front. These forces, which depend on the surface tension of the resin and on its ability to adhere to the surface of the fibers, have an effect of increasing or decreasing the effective pressure at the resin front. However, they are assumed to be sufficiently small to be neglected by the model. This assumption can be checked with micro-scale modeling of air-resin interface flow in fiber bundles. A further assumption in the model is that degree of conversion and hence build-up of viscosity is small and, hence, can be ignored in the injection box region. It is also assumed that there is no shear rate dependence of viscosity. These complexities can be added to the model without great difficulty. Porosity and Fiber Volume Fraction The porosity of the fiber bundle varies with axial distance in the model hence, the stream wise and transverse permeability throughout the injection is not constant. An example of porosity variation throughout the injection box is shown in Figure 3. Permeability Model The Gebart permeability model [Ref. 5] with a hexagonal fiber arrangement was chosen to model the permeability in both the axial and transverse fiber directions. The model is particularly suited to pultrusion processes as the fibers are unidirectional. In the axial direction, the permeability is defined by K z 2D 2 3 f 2 c (1 ) where D f is the effective fiber diameter and c a model constant. The permeability in the transverse direction is K x K y C D V 2 1 f f,max 1 4 (1 ) where V f,max is the maximum volume fraction of fibers and C 1 a model constant. Profiles of stream wise and transverse permeability are shown in Figure 4. Boundary Conditions Where the fiber reinforcement enters the injection box, the pressure is set to atmospheric pressure and a positive pressure is set at the injection port. At the exit of the computational domain, a small distance into the die, the axial velocity is set to the porosity times the pull speed Vf ( z)

4 Flow Solver The mathematical model was solved by the flow solver ANSYS-CFX 11.0 with additional source terms added to account for the moving porous medium. RESULTS AND DISCUSSION Flow Patterns in the Injection Box Streamlines, created by releasing massless particles from the feed ports, show how the resin impregnates the fiber bundle in Figure 5. Resin injected through the feed ports spreads over the top of the bundle, then impregnates into it and finally is pulled through the injection box, achieving good wet-out. The streamlines are colored to indicate the pressure gradient. Pressure Profiles in the Injection Box Figure 6 shows the centerline pressure profiles in the injection box for different line speeds. As expected the final pressure rise increases with line speed. For example, at a line speed of 0.6 m/min, the pressure rise at the die face is 23 bar, rising to 57 bar at a line speed of 1.5 m/min. The experimental results (blue squares in Fig. 6) compare favorably with the CFD results at ~10 cm from the die face. However, the injection box pressure measured at ~2.5 cm from the die face is much higher than predicted. The model assumes a uniform reinforcement matrix and does not take into account factors such as roving twist or resin/fiber interactions which would likely be magnified in the constrained state near the die face. Further experimental work is in progress to verify and expand on these results. effective taper angle of the box) on the final pressure rise in the injection box. As expected for a fully wet-out part, this relationship is linear for viscosity and box length. While the physics of the pressure rise are not completely understood, the model predicts nonlinear relationships for box height and glass fraction. The model can also look at the influence of other geometrical factors such as injection location(s) and other operating parameters including injection pressure on the pressure profiles in the box. Fiber Wet-Out in the Injection Box Figure 8 shows the resin flow-front development in the injection box. To achieve good wet-out, both flow fronts should reach the center of the fiber bundle but also spread to all regions of the injection box. In Figure 8, the resin is colored to indicate residence time in the box. It can be observed in this case that the pultruded part is completely wet-out by the time it enters the die. The model can be used to illustrate process conditions under which profile wet-out becomes an issue. It can also be used to evaluate the influence of injection box geometry and operating parameters such as injection pressure on wetting efficiency. Figure 7 shows the influence of the viscosity of the resin, glass fraction, box length and box height (i.e., the

5 CONCLUSIONS A mathematical model of a pultrusion injection box has been created. A permeability model for unidirectional reinforcements based on porosity and direction has been used. The model can predict injection box pressure profiles and flow patterns giving an estimation of reinforcement wet-out. The model has proven to be versatile and can evaluate a number of factors influencing process performance including injection box geometry, the number and location of the injection ports, injection pressure, line speed, resin physical properties, fiber permeability and fiber fraction on pressure rise in the injection box. The model predicts the appropriate trends when compared with experimental data and experience. Further experimental work is planned to verify additional geometric and process variations. With the use of this model, the typical empirical approach taken with injection box development can be reduced or eliminated. Hence, the development time line can be shortened and adoption of closed injection wet-out and PU resin systems may be accelerated in the pultrusion industry. All information contained herein is provided "as is" without any warranties, express or implied, and under no circumstances shall the authors or Huntsman be liable for any damages of any nature whatsoever resulting from the use or reliance upon such information. Nothing contain in this publication should be construed as a license under any intellectual property right of any entity, or as a suggestion, recommendation, or authorization to take any action that would infringe any patent. The term "Huntsman" is used herein for convenience only, and refers to Huntsman Corporation, its direct and indirect affiliates, and their employees, officers, and directors. REFERENCES 1. Connolly, M., King, J.P., Shidaker, T.A., Duncan, A.C., Pultruding Polyurethane Composite Profiles: Practical Guidelines for Injection Box Design, Component Metering Equipment and Processing, Proceedings of Composites 2005, American Composites Manufacturers Association, Sept Connolly, M., King, J.P., Shidaker, T.A., Duncan, A.C., Processing and Characterization of Pultruded Polyurethane Composites, Proceedings of the 8 th World Pultrusion Conference, European Pultruders Technical Association, March Connolly, M., King, J.P., Shidaker, T.A., Duncan, A.C., Characterization of Pultruded Polyurethane Composites: Environmental Exposure and Component Assembly Testing, Proceedings of Composites 2006, American Composites Manufacturers Association, Oct Jeswani, A.L., Roux, J.A., Vaughan, J.G., Impact of Axial Location of the Injection Slot for High Pull Speed Resin Injection Pultrusion, Proceedings of the 1 st Global Pultrusion Conference, Baltimore, MD, June Gebart, B.R., Permeability of Unidirectional Reinforcements for RTM, Journal of Composite Materials, 26: (1992).

6 Mixhead Static Mixers Pressure Guage at Tee Inlet Digital Pressure Transducer Fiberglass Rovings Mounting Bracket Figure 1: Typical Injection Box Setup Figure 2: CFD model of injection box

7 Figure 3: Porosity variation of the fiber bundle in the injection box Figure 4: Stream wise and transverse permeability profiles in the injection box

8 Figure 5: Flow streamlines in the injection box colored with pressure for a line speed of 1.2 m/min Figure 6: Centerline pressure profiles for 45.7 cm (18 ) injection box at different line speeds with a viscosity of 1200 cp

9 Figure 7: Influence of viscosity, glass fraction, box length and box height on final pressure rise in the injection box Figure 8: Development of resin flow front indicating fiber wet-out