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SCALING DOWN METHODOLOGY FOR COMPOSITE CAB FRONT PROTOTYPE USING RESIN TRANSFER MOULDING PROCESS

Raghu Raja Pandiyan K., Gautam Kundu, Swati Neogi*1

Composite Applications Laboratory, Department of Chemical Engineering, Indian Institute of Technology - Kharagpur, India, 721302.

Abstract

Most of the industrial composite parts that have large and complex geometry are manufactured by Hand Lay Up (HLU) method. Resin Transfer Moulding (RTM) process is a better substitute, but is not used readily due to the lack of proper manufacturing technology. Development of a proper RTM manufacturing process for a specific application requires a proper mould design. In addition, the difficulty in the tooling design and mould fabrication cost increases with increase in size and complexity of the component. The scale down strategy of full scale product avoids bigger size mould requirements, prototype bulk production for product testing and quality check at the starting phase of product development. Moreover, the mould scale down strategy paves the way to validate the process and the product with less capital input. Hence, there are tremendous beneficiations in scaling down large and complex shapes which represents the full scale product. In this work, we propose a mould scaled down strategy for a large and complex high speed train cab front prototype through RTM process. The mould scale down strategy was developed using isothermal moulding filling simulations based on the mould fill time and mould fill pattern comparisons between full scale and scale down model. From the simulations and actual experiments, it was found that the injection pressure at the full scale model has to be reduced to the times of reciprocal of square of geometrical mould scale down factor to meet the same mould fill time and mould fill pattern, provided the same injection strategy in both the scales. Similar scaled down methodology can be used for developing scaled down prototype other large and complex composite components.

Keywords: Composite Prototype Development, Resin Transfer Moulding, Mould Filling

Simulations, Mould Scaled Down Strategy, Injection Strategy

1. Introduction

Resin Transfer Moulding (RTM) offers a number of advantages over Hand Lay Up (HLU) for fabrication of polymer matrix composite components such as higher reinforcement volume percentage, better distribution of resin and reinforcement, good surface finish and uniform thickness, high production volume due to lower cycle time, and less styrene emission. However still, HLU is being practiced by most of the moulders for fabrication of complex and large components due to the very high cost of developing a proper mould design and determining most effective process parameters to produce the component with minimum mould filling time without the formation of dry spot. A proper mould design requires not only the geometry of the mould but also the effective injection strategy, pressure and temperature distributions. In addition, the difficulty in the mould design and also higher fabrication cost with increase in size and complexity of the component and chances of formation of dry spot is extremely high if the proper injection strategy is not used. Currently, RTM process is developed by trial and error

1 Corresponding Author. Tel: +91-03222 28 3924 Fax: +91-03222 28 2250 Email: swati@che.iitkgp.ernet.in

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methods, leading to high development cost and sub-optimum solution. Process simulation can be effectively used to determine the effective injection strategy including the position of injection gates and vents, and the most effective values of parameters for minimum mould filling time without formation of dry spot. The effective injection strategy, and the resulting pressure and temperature distribution can be utilized to design and fabricate the mould. A proper scaling down methodology based on process simulation can be used to manufacture a scaled down prototype for product testing and validating the process simulation of the full scale product. Thus the development of the RTM technology based on process simulation and scaled down prototyping will reduce the development cost and will enable the industry to use RTM technology instead of HLU for large and complex composite structures. While researchers have worked on different aspects on RTM, no effort has gone into develop RTM procedure with a scaled down component and use the data for scaling up to production level.

A review of current approaches in modeling and simulation of the RTM process was

presented by Shojaei et al [1]. It has been reported that the mould of shell like geometry having thickness much smaller than the length and width, is often considered to be two-dimensional. Therefore, the resin flow in a three-dimensional thin cavity has been modeled as a two-dimensional problem. Researchers have employed different algorithms such as gradient-based optimization methods [2&3], genetic algorithms [4-7], branch and bound search [8] etc., as a key design tool to develop new injection schemes for the RTM procedure especially gate location and its optimization [9-11] with pressure requirements and reasonable fill time. Gokce et al [12] aimed to determine optimal vent locations along with race tracking in a mould. Jiang et al [13] have proposed a geometrical method to optimize gates and vents using genetic algorithm in minimizing entrapped air to avoid dry spots formation. Kang et al [14]. investigated two different multiple gate injection schemes namely sequential manner gate opening with resin front advancement and simultaneously gate opening with pressure control of each gate individually. Several numerical techniques have been used to resin flow mathematical model such as finite difference method [15&16], boundary element method [17] and finite element method [18-25]. Of these, finite element is often preferred, because it can be easily applied for all kind of geometries and boundary conditions can be modeled comparatively easy. The commercially available FEM packages such as ANSYS, LUSAS, NASTRAN, ABAQUS etc., cannot directly support RTM process simulation, whereas it can support mould filling simulations using homogenization technique with limitations of heat transfer and cure kinetics analysis. Moreover, these packages do not have separate process simulation module and this led research groups to develop their own codes such as LIMS [26&27], LIMS3D [28], RTMSIM [29] etc., to achieve their requirements. To find ever faster routes to RTM process simulation, commercially available specialized software packages such PAM-RTM, RTM-Work that simulates various moulding processes has been launched into the market. There have been a few work published on RTM process simulation for applications such as automobile parts [30-32] using PAM-RTM software.

While researchers have worked on different aspects on RTM, no effort has gone into develop RTM procedure with a scaled down component and use the data for scaling up to production level. The main objective of this work is to develop a scaling down methodology for manufacturing a scaled down prototype using RTM process. In this article, first section introduces the design for cab front with its dimensions and the details of component mesh. In the following section, the development of scaling down strategy using Darcy’s equation foe single dimension and single injection are reported. In the subsequent section, the comparisons between the simulated results of full scale and scaled down models of different scaling factors are discussed. Finally, the experimental validation of simulated results on scaled down component is presented.

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2. Cab Front Design and Discretization

The cab front, currently being manufactured for Indian Railways using HLU technology, was chosen as the object of this study. The design of the cab front, which is shown in Figure 1, is of complex shape with 12 mm uniform thickness and approximately 3.5 m width and 3.0 m height. Since the thickness used is much smaller compared to the height and the width, the mould filling in the cab front geometry was modeled as two-dimensional flow. The model was meshed with geometry meshing software Visual Mesh. Due to the complex shape of the mould, unstructured meshes composed of triangles are usually preferred and, hence the geometry was meshed with 2D triangular elements with an element size of 0.06 m as shown in Figure 2. Number of nodes and elements formed after meshing were 8084 and 15617, respectively. Further reduction in element size increases the number of nodes and elements which in turn increases the load on computation without significant improvement in the simulation results.

Figure 1: Solid Model and Finite Element Model of the Cab Front

3. Process Model

The RTM process can be modeled as an incompressible fluid flow through porous medium. Darcy’s law is the fundamental equation describing the flow through porous media which is given in Equation 1.

kv = .∇P

μ (1)

where v is the velocity vector, k is the permeability tensor, μ is the resin viscosity and ∇P is the pressure gradient. The boundary conditions associated to solve the pressure field given by Equation 1 are given in Equation 2.

P = Patm.front &

Pinj.

P =inlet or

Q = Qinlet inj. &

∂P= 0

∂n wall (2)

where Pinj.

is the pressure of injection, Qinj.

is the flow rate of the injection, Pfront

is the

pressure at the flow front and Patm. is the atmospheric pressure.

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In this work, the isothermal resin flow was simulated using commercial software PAM RTM 2008, developed by ESI Technologies.

4. Scaling Down Strategy

4.1. Scaling Down Criteria

The full scale product and process can be validated through the scaled down prototype, if the product quality at both scales are matched. RTM process for the full scale and scaled down model differs only in the moulding filling phase and the criteria to match the product quality at both the scales are given below.

Mould Fill Pattern = Mould Fill Patternfull scale scaled down or (3)

Mould Fill Time = Mould Fill Timefull scale scaled down (4)

4.2. Analytical Prediction

The scale down strategy can be predicted analytically by matching the mould fill times of full scale and scaled down model, obtained from Darcy’s equation. The scale down strategy using Darcy’s equation for single dimension and single injection scheme is shown below.

1-D Darcy’s Equation,

dx k dP=

dt μ dx (5)

Dimensionless forms of the parameters in Equation 5, , ,x t P

X = t = P =L t Pm inj

(6)

Where X , t & P are the dimensionless form of length, mould fill time and pressure

respectively. L , tm &

Pinj are the total length, time to fill the complete mould and injection

pressure respectively.

Subsuiting Equation 6 in Equation 5,

P1 d X k d Pinj=

2t μd t d XLm (7)

Full scale process,

P1 d X k d Pinj(f)=

2t μd d XLm(f) (f)t

(8)

Scale down process,

P1 d X k d Pinj(f)=

2t μd d XLm(f) (f)t

(9)

Equation 9 divided by Equation 8 gives,

Ptm 1inj(f)(s)=

2t Pm L(f) inj(s)(f)

L(s)

(10)

Matching the mould fill time at both the scales, t = tm m(s) (f),

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Pinj(f)

P =inj(s) 2

S

(11)

where S is the scaling down factor given by the ratio of full scale length, L(f)

to the

scaled down length, L(s)

The same scaling down strategy can be applied to multi-dimensional and multi-injection systems and it can be deduced using the same procedure for single dimension and single injection systems.

5. Raw Material Parameters

Medium reactive unsaturated polyester resin specially formulated for RTM process and

woven roven mat of E-class glass fiber were the raw materials considered in this study. Characteristic properties such as density, gel time, and viscosity of a commercially available medium reactive unsaturated polyester resin were experimentally measured. The gel time test was performed by following ASTM D2471. Viscosity at constant shear rate of 60 s-1 and at a temperature of 25°C was measured using Bohlin Viscometer. To determine the permeability, the actual isothermal mould filling experiments were conducted using the unsaturated polyester resin and the woven roven mat. The flow front progression was tracked using visualization technique. Then, the isothermal mould filling simulations were carried out using PAM-RTM 2008 and a sensitivity analysis was performed using different values of permeability to match the experimental flow pattern with simulated flow pattern. The obtained characteristic properties of the polymer resin matrix and the woven roven mat which were used in the mould filling simulations are given in Table 1.

Table I: Properties of Raw Materials

Matrix Properties Reinforcement Properties

Property Value Unit Property Value Unit

Resin Density 1100 Kg/m3 Fiber density 2540 Kg/m3

Raw Resin Viscosity 0.25 Pa.s Porosity 0.498 No unit

Resin Gel Time (100: 1: 1, Resin: Accelerator: Catalyst, Volume proportion)

52 Minutes Permeability Kx 2x10-10 m2

Ky 5x10-10

6. Effective Injection Strategy

Prior to the development of scaling down strategy for the cab front prototype, the optimization of inlet and vent location at the full scale requiring minimum mould fill time without dry spots was completed using hit and trial isothermal mould filling simulations. The simulation results showed that the injection strategy of four injection points on the front face and four vents at the corners of the cab front as shown in Figure 2 with the constant pressure of 5 atm delivered an optimum mould fill time of 32 minutes without dry spots. The same injection configuration was used for the experimental validation of scaled down strategy.

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Figure 2: Effective Injection Strategy for the Cab Front

7. Scale Down Simulations

Isothermal mould filling simulations were performed on the geometrically scaled down cab front models with the different scaling factors from the full scale. The injection strategy and the raw material parameters were kept constant at the full scale and scale down models simulations. Results indicate that the mould fill pattern and pressure gradient pattern remains unaltered on both scales of the cab front model, provided the injection pressure at the scaled down model follows the developed scaling down strategy. The inferences of the full scale and scaled down simulations are tabulated in Table 2. The comparison of simulated results of case study 4 in the Table I are shown in Figures 3 and 4. In this case, the scale down cab front model to the factor of 3 was simulated with the injection pressure of 2.75 atm pressure (40 psi, which was used as the injection pressure in the actual mould filling experiments). The full scale cab front model was simulated with the injection pressure of 24.75 as per the developed scaling down strategy. From the simulated results as shown in Figure 3, it can be observed that the mould fill time distribution remains the same on both the scales and the obtained mould fill time on the both the scales were 11 minutes. From the Figure 4, it may be mentioned that the pressure gradient distribution remains the same on both the scales and the maximum pressure created inside the mould is at the injection point on both the scales

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Table I: Properties of Raw Materials

Sl. No. Scaling Down Factor

Pinj (full scale), atm

Pinj (scaled down), atm

Full Scale Model Mould Fill Time, Min.

Scaled Down Model Mould Fill Time, Min.

Inferences

1 2 5 1.25 32 32 Same mould fill time & pressure gradient distributions

2 3 5 0.556 32 32 Same mould fill time & pressure gradient distributions

3 4 5 0.3125 32 32 Same mould fill time & pressure gradient distributions

4 3 24.75 2.75 11 11 Same mould fill time & pressure gradient distributions

(a)

(b)

Figure 3: Comparison of Mould Fill Time Distribution (a) Full Scale Cab Front Model (b) Cab Front Model Scaled Down to Factor 3

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(a)

(b)

Figure 4: Comparison of Pressure Distribution (a) Full Scale Cab Front Model (b) Cab Front Model Scaled Down to Factor 3

8. Experimental Validation

Figure 5: Schematic of Simplified Cab Front Mould Geometry with a Scaling Down Factor of 3 to the Actual Scale

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Figure 6: Scaled Down Cab Front Mould Assembly

A simplified scaled down cab front mould with a scaling ratio of 1:3 from the actual

production scale was designed as shown in Figure 4 and constructed to validate the isothermal mould filling simulations. The scaled down mould was designed without considering the complex projections to validate the 2-D flow pattern obtained from the simulations. The scale down mould retained the same angles at the bents from the full scale. The mould was made out of an acrylic material to be transparent to observe the resin flow in the mould. The two halves of the mould were assembled with the gaskets and bolts as shown in Figure 5 to ensure the good contact between fiber mat and the walls to prevent flow channeling. A digital video recorder was set above the mould to record the flow advancement. An orthotropic glass fiber woven roven mat of 1.6 mm thickness was used for the experiments. The isothermal mould filling experiments were performed under constant injection pressure of 2.75 atm (40 psi) at the optimized four injection ports. Flow fronts progression at the four injection ports were observed only at the top mould surface. An elliptical shape flow fronts were obtained at all the four injection gates for an isotropic preform before the flow fronts merge. The flow fronts advancement obtained from isothermal mould filling simulations and actual experiments on the scaled down cab front model are shown in Figure 7. The flow fronts positions at different time interval was obtained with the digital frames of experiments and simulations using X-Y Extractor software. The comparison of experimental and simulated flow fronts positions in 3, 6, 9 and 12 seconds are shown in Figure 8. From the same figure, it can be noticed that the flow fronts positions from the experimental results agrees well with simulated flow front positions at each intervals of time. It can also be shown that the difference between the flow fronts position in the x-direction becomes constant with equal intervals of time. In the y-direction, the difference in the flow front positions remains same until it reaches the boundaries of the mould.

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3 Seconds

6 Seconds

9 Seconds

12 Seconds

Figure 7: Experimental and Simulated Flow Fronts Progression

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Figure 8: Comparisons of Experimental and Simulated Flow Fronts

9. Conclusions

In this investigation, a scaling down strategy is proposed to develop a composite prototype using RTM process simulation. The proposed scaling down methodology based on full scale and scaled down model simulations can be used to validate the developed RTM technology for the full scale product through actual experiments on scaled down mould. The same methodology can also be used to test the product quality by manufacturing the scaled down prototype in the product developmental stage. The mould scale down strategy was developed using comparisons on mould fill time and mould fill pattern between full scale and scale down models. From the simulations and actual experiments, it was found that the injection pressure at the full scale model has to be reduced to the times of reciprocal of square of geometrical mould scale down factor to meet the same mould fill time and mould fill pattern, provided the same injection strategy is followed in both the scales. The injection pressure based on the developed scaled down strategy was tested on the scaled down mould and a very close agreement was observed between the flow fronts of experimental and simulated results, which validated the isothermal mould filling simulations as well as the developed RTM process in the full scale model.

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