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Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites
Composites: Part A 31 (2000) 1303–1310 www.elsevier.com/locate/compositesa
Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites M.O.W. Richardson*, Z.Y. Zhang Institute of Polymer Technology and Materials Engineering, Loughborough University, Leicestershire LE11 3TU, UK Received 29 July 1999; accepted 6 January 2000
Abstract Resin transfer moulding (RTM) of glass fibre reinforced polymeric composites offers the advantages of automation, low cost and versatile design of fibre reinforcement. A replacement of glass fibres with natural plant fibres as reinforcement in polymeric composites provides additional technological, economical, ecological and environmental benefits. The resin transfer mould filling process has significant effects on different aspects, such as fibre wetting out and impregnation, injection gate design, “dry patch” and void formation. Flow visualisation experiments were carried out using a transparent RTM mould to develop a better understanding of the mould filling process for hemp mat reinforced phenolic composites. The mould filling of unreinforced phenolics was characterised by a “quasi-one-dimensional steady state” flow. In the case of hemp non-woven reinforced system, the mould filling process can be considered as the flow of fluids through porous media. “Fibre washing” was a typical problem encountered during the injection process, leading to poor property uniformity. In addition, a preferential flow path was usually created near the edges and corners of the mould. The path exhibited low flow resistance and caused the resin flow front to advance much faster in these regions. The edge flow disturbed the steady flow, leading to difficulties in venting arrangement and “dry patch” formation. The edge flow and fibre washing were alleviated by reinforcement manipulation so steady state flow could be achieved. The relationships between the filling time and injection pressure and between filling time and different fibre weight fractions have been established for certain specific injection strategies. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: E. Resin transfer moulding; Phenolic composites
1. Introduction Many fibre reinforced polymer (FRP) composite materials exhibit distinct properties such as high specific modulus and strength, light weight, high productivity, excellent processability, environmental degradation resistance and cost effectiveness. They have sustained a rapid development for more than two decades and found ever-increasing applications as engineering components and structures. Resin transfer moulding (RTM) has proved to a viable and competitive technique for fabrication of large, complex and high performance FRP composite materials [1–3]. A growing environmental awareness all over the world has aroused interest in research and development of environmentally compatible materials [4–6]. The use of renewable natural resources will lead to a positive impact on the environment. A replacement of glass fibres with natural plant fibres as reinforcements in polymeric composites * Corresponding author. Tel.: ⫹ 44-1509-223161; fax: ⫹ 44-1509223949. E-mail address: [email protected] (M.O.W. Richardson).
provides technological, economical, ecological and environmental benefits. The advantages of natural plant fibres over traditional glass fibres are acceptable specific strength and modulus, economical viability, low density and additional weight-saving, reduced tool wear in machining operations, enhanced energy recovery, reduced dermal and respiratory irritation and biodegradability [7–8]. RTM of natural plant fibre reinforced phenolic composites will offer additional advantages because phenolic resin has good fire resistance and low smoke generation in a fire situation, high temperature stability and retention of modulus over a wide temperature range and excellent chemical resistance. Natural fibre reinforced composites are particularly suited to structural and decorative applications in the mass transit, marine, offshore and construction industries [9–12]. Successful implementation of RTM for different composites involves many parameters. Of these, the mould filling process plays a major part and is related to preform architecture and permeability, resin viscosity, injection pressure, resin temperature, gate location and configuration, vent control and preform placement techniques [13–14]. A
1359-835X/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-835 X( 00)00 008-7
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Fig. 1. Schematic of RTM system for mould filling visualisation (dimensions are in mm).
good understanding of the mould filling process is essential in order that detailed information can be generated which can be used as a guide to determining appropriate operating parameters. The mould filling process can be experimentally visualised using a purpose built transparent mould which enables observation of the dynamic mould filling process directly. In addition, experimental visualisation plays an indispensable role in validating the different mathematical model and numerical simulations related to different RTM processes. Although some research has been carried out in this field, most of it has been predominantly concerned with experimentation and numerical simulation of RTM using different glass fibre reinforced composites [15–17]. The objective of this current research was to investigate different aspects of the mould filling process, including fibre washing, edge flow, mould filling profiles and velocity profiles with different numbers of layers of non-woven hemp under different injection pressures.
2. Experimental Liquid phenolic resin J2027L has low viscosity and can be cured using an acidic catalyst at low temperatures which is preferable for the RTM process. Phencat 382 is a blend of organic and inorganic acids and is a delayed action catalyst. It has an extended pot life so the exothermic risk in isothermal RTM can be significantly reduced. Differential scanning calorimetry (DSC) curves show that a chemical crosslink reaction is activated at approximately 60⬚C and reaches a peak at approximately 100⬚C. The catalyst concentrations were maintained at 7%. Both phenolic resin and catalyst were provided by Blagden Chemicals Ltd. Non-woven needle punched hemp was used as the reinforcement for phenolic resin and was provided by J B Plant Fibres Enterprises Ltd. The moisture absorption and desorption characteristics were investigated. Accordingly the non-woven hemp was heated for 2 h at 100⬚C in an oven
to expel moisture [18]. The non-woven hemp has a density of 250 g/m 2. Mould filling visualisation at different reinforcement concentrations were carried out by placing 0, 1, 2 and 3 layers of non-woven hemp, equivalent to fibre weight fractions of 0, 5.6, 11.1 and 16.7%, respectively. The mould filling visualisation involved using a Hypaject MK IV RTM machine in combination with a transparent flat panel mould 450 × 300 × 4 mm purpose built by Plastech Thermoset Tectonics Ltd. for the production of phenolic composites as schematically shown in Fig. 1. All components for the machines and mould were acid resistant due to the acidity of the catalyst. A transparent grid film was placed on the mould for direct and accurate data acquisition. Resin advancement profiles were recorded and the velocity calculated by differentiating the resin advancement values. The resin and catalyst were kept at room temperature and the mould was heated to 60⬚C during the experimental process. 3. Results and discussion It has been recognised that there are many variables associated with the RTM process. The experimental results and discussion here are mainly concerned with three aspects of the mould filling process for non-woven hemp fibre reinforced phenolic composites, namely fibre washing, edge flow and mould filling. 3.1. Fibre washing Fibre washing is referred to as the unexpected movement or displacement of reinforcement in the closed mould during the RTM process. When the mixture of resin and catalyst is injected into the closed mould, the prelaid reinforcement has a tendency to be displaced due to the injection pressure which is a function of fibre types, architecture, weight fractions, mould configuration and geometry. Fibre washing can ultimately lead to the failure of the RTM process due to the
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Fig. 2. Fibre washing distances for 2-layer non-woven hemp at different injection pressures.
fact that fibre displacements interrupt the uniformity of pre-determined reinforcement distribution. Resin rich and starved areas are created and unacceptable physical and mechanical property variations are introduced in the materials. Although fibre washing does not occur for every RTM fabrication process, it is a common problem in the case of non-woven hemp reinforced phenolic composites. In this study, fibre washing is characterised by an ultimate fibre washing distance which can be visualised and quantified during the mould filling process. The relationship between the injection time and “fibre washing distance” at different injection pressures with two layers of non-woven hemp is shown in Fig. 2. There is an obvious effect of injection pressure on fibre washing which is illustrated by the different times at which fibre washing occurs and precise values of “ultimate fibre washing
distance”. The relationship between the injection time and “fibre washing distance” for different numbers of layers of non-woven hemp is shown in Fig. 3. It can be seen that the “fibre washing distances” were significantly reduced with more layers of non-woven hemp. When four layers of nonwoven hemp are employed, “fibre washing distance” is virtually reduced to zero. Experimental observation indicates that fibre washing will commence at the edge nearest the injection gate when the injection pressure builds up. No splitting and thinning of non-woven hemp was observed around the injection gate area which suggested that it has sufficient inherent strength from mechanical entanglement and interlocking to withstand the injection pressure. Fibre washing is directly related to the clamping condition within the mould. Higher clamping forces were achieved when more fibre layers were used. There are two obvious solutions for this problem. The first is to pinch-off the fibres
Fig. 3. Fibre washing distance for different layers of non-woven hemp at 2 bar injection pressure.
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Fig. 4. Edge flow front isochrones of 2-layer non-woven hemp at 2 bar injection pressure (each isochrone represents 20 s).
across the mould seal but overdoing this will lead to unnecessary and premature resin overflow out of these locations. The second is to deliberately increase the clamping force by pre-laying additional narrow non-woven stripes along the edge to create intimate contact with the mould and higher clamping force. In practice, both methods are used in combination to substantially reduce fibre washing. 3.2. Edge flow For successful RTM process control and complete preform impregnation, the “quasi-one dimensional steady” flow state is required in a rectangular mould with a near edge injection gate. The “quasi-one dimensional steady state” flow is referred to as equal resin advancement distance or velocity at any moment during the injecting process in the resin advancement direction. In the process of resin transfer moulding, small clearances may exist between the fibre preform and mould edges because of
loose edge fibre bundles, poor fitting size, or deformation of the fibre preform. The clearance results in a preferential resin flow path during the mould filling stage. This edge flow can disrupt the uniformity of the flow pattern and cause incomplete wetting of the prefrom. This phenomenon intensifies with the decrease of preform permeability. A typical mould filling flow pattern with the edge flow effect is illustrated in Fig. 4. It can be seen that the edge flow effect is negligible just after the resin is fully expanded in the flow direction. The difference of resin advancement position at the same moment has been progressively increased with increasing the injection time. At later stages of injection, edge flow advances much faster than the flow in the middle position of the mould. The uniformity of flow pattern is completely disrupted. Once the edge flow arrives at the bottom edge, it changes direction and advances in a transverse direction. The vent position is preferably arranged along the bottom edge for good ventilation effect but early arrival of edge flow can cause overflow out of the mould leading to unnecessary spillage and waste. Additionally overflow would prematurely block the ventilation port, leading to the problem of air entrapment in the reinforcement. The combination of edge flow in the transverse direction and centre flow in the flow direction will inevitably lead to premature blocking of the ventilation port and formation of a dry spot. The flow distance/mould filling time curves for flows at edge and middle position are shown in Fig. 5. Edge flow effect can be characterised by mould filling distance differences between the centre and edge and the arrival time of edge flow at the bottom of the mould. Injection pressure and preform permeability influences this edge flow effect. Figs. 6 and 7 show the mould filling distance differences and the arrival time of edge flow at the bottom for different numbers of layers of non-woven hemp at different injection pressures. It can be seen that there is only a marginal increase for mould filling time at the edge with
Fig. 5. The edge and central flow distance differences for 2-layer non-woven hemp at 2 bar injection pressure.
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Fig. 6. Mould filling distance differences between the edge and centre at different injection pressures.
increasing injection pressure, implying that edge flow effect is less sensitive to pressure. Comparatively there is a dramatic increase when fibre preforms are increased from 1 layer to 2 and 3 layers. This implies that the edge flow effect is sensitive to changes of permeability. More layers of non-woven hemp in the mould result in high flow resistance and slow flow and impregnation. The resin has a tendency to flow along the channel of least resistance under injection pressure and therefore the effect is intensified. Consequently both the values of mould filling distance differences between the centre and edge and the arrival time of edge flow at the bottom are substantially increased. The edge flow has an adverse effect on the mould filling
process which leads to the formation of dry spots and spillage. The convenient solution to this problem is fibre preform manipulation. In practice, there is a difficulty in cutting the preform precisely to mould size. This is the case for the production of geometrically complicated components in particular. Experimental results show that the edge flow effect can be eliminated when the dimensions of the fibre preforms are slightly larger than that of the mould cavity. When the mould is closed, the preform will be squeezed and the clearance can be filled by the reinforcements. A local low permeability area may be introduced near the mould edge but it does not result in obvious interruption of global flow uniformity and mould filling can be successfully achieved.
Fig. 7. Time for the edge flow front to arrive at the end of the mould at different injection pressures.
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Fig. 8. The quasi 1D steady flow advancement profiles for different numbers of layers of non-woven hemp at 2 bar injection pressure.
3.3. Mould filling After the problems of fibre washing and edge flow were circumvented, successful mould filling was achieved. The resin advancement profiles with varying numbers of layers of non-woven hemp and under different injection pressures are shown in Figs. 8 and 9, respectively. As can be seen, after taking the fibre washing and edge flow influences into consideration, the profiles are characterised by smooth curves, implying that a “quasi-one dimensional steady state” flow is achieved. The resin fronts steadily advanced in the flow direction and arrived at the bottom edge of the mould nearly at the same time. The ventilation port remained open until injection was completed. In this case
the entrapped air can be removed from the reinforcement and complete impregnation achieved. Fig. 10 shows schematically the progressive advancements of resin fronts during the mould filling process. The injection pressure has an influence on the mould filling process. The injection time can be very long if the injection pressures are ⬍1 bar. Consequently higher injection pressures are preferably employed. It should be noted that there is a mechanism to prevent “over pressure” in this and other RTM machines. Over pressurisation can present a danger of poor sealing and reduced seal life, fast linkage wear and mould damage because in this present case a glass panel is used for visualisation and GRP composites for tooling to minimise the acidic corrosion. At the same time, over
Fig. 9. The quasi 1D steady flow advancement profiles of 2-layer non-woven hemp at different injection pressures.
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4. Conclusions A good understanding has been established of the mould filling process of RTM involving non-woven hemp reinforced phenolic composites. This understanding could be useful for production control and optimisation as well as the validation of different mould filling mathematical models and numerical simulation schemes.
Fig. 10. Quasi 1D steady flow isochrones of 2-layer non-woven hemp at 2 bar injection pressure (each isochrone represents 20 s).
pressurisation can cause more serious fibre washing and edge flow. Reinforcement concentrations can have a pronounced influence on the mould filling process. This is clearly illustrated by observing mould filling velocity profiles as shown in Figs. 11 and 12. The velocity profiles at different locations in the panel demonstrate that the flow rates have been gradually reduced at constant injection pressure due to the increase in flow resistance. The reinforcement concentrations have a strong influence on mould filling velocity profiles. At an early stage of the injection process, the velocity differences are very high and then gradually reduce. This is due to the gradual increase in flow resistance which leads to smaller differences in velocity.
• Fibre washing is a common problem at low fibre concentrations due to poor clamping conditions leading to the failure of the injection process. With increase in fibre concentration, the fibre washing effect is gradually reduced due to the consequential improvement of clamping conditions. Employing pinch-off and localised increase of fibre concentrations near the edge is a convenient solution to this problem. • Edge flow is introduced due to the clearance between the preform and the mould edge. The presence of edge flow leads to the interruption of flow uniformity and the resin near the edge has a tendency to flow much faster than in the main area due to lower resistance. Edge flow is less sensitive to injection pressure variations but fibre concentrations have a dramatic influence. The deployment of preforms larger than the mould eliminates the problem. • “Quasi-one-dimensional steady” flow is preferred in order to achieve successful mould filling with complete impregnation and proper ventilation. Resin front advancement profiles and velocity profiles have been established which illustrate the influence of injection pressure and fibre concentrations on the mould filling process. Changes in injection pressure lead to corresponding variations in injection time. Also an increase in the number of layers of non-woven hemp leads to prolonged injection times.
Fig. 11. The quasi 1D steady mould filling velocity profiles for different numbers of layers of non-woven hemp at 2 bar injection pressure.
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Fig. 12. The quasi 1D steady flow mould filling velocity profiles of 2-layer non-woven hemp at different injection pressures.
Acknowledgements The authors would like to acknowledge the support of a DTI/EPSRC Technology Foresight Challenge Grant. Thanks are also due to The BioComposites Centre of University of Wales, Concargo Ltd and J B Plant Fibres Ltd.
References [1] Rudd CD, Long AC. Liquid moulding technologies. Cambridge, UK: Woodhead Publishing Limited, 1997. [2] Potter K. Resin transfer moulding. UK: Chapman and Hall, 1997. [3] Bickerton S, Advani SG. Experimental investigation and flow visualisation of the resin-transfer mould-filling process in a non-planar geometry. Composites Science and Technology 1997;57(1):23–33. [4] Richardson MOW, Madera-Santana TJ. Natural fibre composites-the potential for the Asian market. Progress in Rubber and Plastics Technology 1998;14(3):174–88. [5] Schloser T. Vehicle parts reinforced with natural fibres. Kunststoffe 1997;87(9):1148–52. [6] Satyanarayana KG, Sukumaran K. Fabrication and properties of natural fibre reinforced polyester composites. Composites 1986;17(4):329–33. [7] Mohan R, Shridhar MK, Rao RM. Compressive strength of jute-glass hybrid fibre composites. Journal of Materials Science Letters 1998;2:99–102.
[8] Bolton J. The potential of plant fibres as crops for industrial use. Outlook on Agriculture 1995;24(2):85–9. [9] Rossa EP. Reinforcing fibres give phenolics high-tech uses. Plastics Engineering 1998;44(1):39–41. [10] Mattigetz R. Phenolic composites in mass transit. International SAMPE Symposium and Exhibition 1991;36(2):1916–27. [11] Bishop GR, Sheard PA. Fire-resistant composites for structural sections. Composites Structures 1992;21(2):85–9. [12] Soni RP, Soni M. Studies on natural fibre-phenolic composites. Journal of Scientific and Industrial Research 1999;58(1):34–6. [13] Rohatgi V, Patel N, James L. Experimental investigation of flowinduced microvoids during impregnation of unidirectional stitched fibre glass mat. Polymer Composites 1996;17(2):161–70. [14] Chen YF, Stelson KA, Voller VR. Prediction of filling time and vent locations for resin transfer moulds. Journal of Composite Materials 1997;31(11):1141–61. [15] Liu B, Bickerton S, Advani SG. Modelling and simulation of resin transfer moulding (RTM)—gate control, venting and dry spot prediction, Composites Part A: Applied Science and Manufacturing 1996;27(2):135–41. [16] Wu CJ, Hourng LW, Liao JC. Numerical and experimental study on the edge effect of resin transfer moulding. Journal of Reinforced Plastics and Composites 1995;14(7):694–722. [17] Young WB, Lai CL. Analysis of the edge effect in resin transfer moulding. Composites Part A: Applied Science and Manufacturing 1997;28(9):817–22. [18] Richardson MOW, Zhang ZY. Development of plant fibre-phenolic resin RTM materials for the building industry. Interim Report, Loughborough University, UK, 1998.
Experimental investigation and flow visualisation of the resin transfer mould filling process for non-woven hemp reinforced phenolic composites M.O.W. Richardson*, Z.Y. Zhang Institute of Polymer Technology and Materials Engineering, Loughborough University, Leicestershire LE11 3TU, UK Received 29 July 1999; accepted 6 January 2000
Abstract Resin transfer moulding (RTM) of glass fibre reinforced polymeric composites offers the advantages of automation, low cost and versatile design of fibre reinforcement. A replacement of glass fibres with natural plant fibres as reinforcement in polymeric composites provides additional technological, economical, ecological and environmental benefits. The resin transfer mould filling process has significant effects on different aspects, such as fibre wetting out and impregnation, injection gate design, “dry patch” and void formation. Flow visualisation experiments were carried out using a transparent RTM mould to develop a better understanding of the mould filling process for hemp mat reinforced phenolic composites. The mould filling of unreinforced phenolics was characterised by a “quasi-one-dimensional steady state” flow. In the case of hemp non-woven reinforced system, the mould filling process can be considered as the flow of fluids through porous media. “Fibre washing” was a typical problem encountered during the injection process, leading to poor property uniformity. In addition, a preferential flow path was usually created near the edges and corners of the mould. The path exhibited low flow resistance and caused the resin flow front to advance much faster in these regions. The edge flow disturbed the steady flow, leading to difficulties in venting arrangement and “dry patch” formation. The edge flow and fibre washing were alleviated by reinforcement manipulation so steady state flow could be achieved. The relationships between the filling time and injection pressure and between filling time and different fibre weight fractions have been established for certain specific injection strategies. 䉷 2000 Elsevier Science Ltd. All rights reserved. Keywords: E. Resin transfer moulding; Phenolic composites
1. Introduction Many fibre reinforced polymer (FRP) composite materials exhibit distinct properties such as high specific modulus and strength, light weight, high productivity, excellent processability, environmental degradation resistance and cost effectiveness. They have sustained a rapid development for more than two decades and found ever-increasing applications as engineering components and structures. Resin transfer moulding (RTM) has proved to a viable and competitive technique for fabrication of large, complex and high performance FRP composite materials [1–3]. A growing environmental awareness all over the world has aroused interest in research and development of environmentally compatible materials [4–6]. The use of renewable natural resources will lead to a positive impact on the environment. A replacement of glass fibres with natural plant fibres as reinforcements in polymeric composites * Corresponding author. Tel.: ⫹ 44-1509-223161; fax: ⫹ 44-1509223949. E-mail address: [email protected] (M.O.W. Richardson).
provides technological, economical, ecological and environmental benefits. The advantages of natural plant fibres over traditional glass fibres are acceptable specific strength and modulus, economical viability, low density and additional weight-saving, reduced tool wear in machining operations, enhanced energy recovery, reduced dermal and respiratory irritation and biodegradability [7–8]. RTM of natural plant fibre reinforced phenolic composites will offer additional advantages because phenolic resin has good fire resistance and low smoke generation in a fire situation, high temperature stability and retention of modulus over a wide temperature range and excellent chemical resistance. Natural fibre reinforced composites are particularly suited to structural and decorative applications in the mass transit, marine, offshore and construction industries [9–12]. Successful implementation of RTM for different composites involves many parameters. Of these, the mould filling process plays a major part and is related to preform architecture and permeability, resin viscosity, injection pressure, resin temperature, gate location and configuration, vent control and preform placement techniques [13–14]. A
1359-835X/00/$ - see front matter 䉷 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-835 X( 00)00 008-7
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Fig. 1. Schematic of RTM system for mould filling visualisation (dimensions are in mm).
good understanding of the mould filling process is essential in order that detailed information can be generated which can be used as a guide to determining appropriate operating parameters. The mould filling process can be experimentally visualised using a purpose built transparent mould which enables observation of the dynamic mould filling process directly. In addition, experimental visualisation plays an indispensable role in validating the different mathematical model and numerical simulations related to different RTM processes. Although some research has been carried out in this field, most of it has been predominantly concerned with experimentation and numerical simulation of RTM using different glass fibre reinforced composites [15–17]. The objective of this current research was to investigate different aspects of the mould filling process, including fibre washing, edge flow, mould filling profiles and velocity profiles with different numbers of layers of non-woven hemp under different injection pressures.
2. Experimental Liquid phenolic resin J2027L has low viscosity and can be cured using an acidic catalyst at low temperatures which is preferable for the RTM process. Phencat 382 is a blend of organic and inorganic acids and is a delayed action catalyst. It has an extended pot life so the exothermic risk in isothermal RTM can be significantly reduced. Differential scanning calorimetry (DSC) curves show that a chemical crosslink reaction is activated at approximately 60⬚C and reaches a peak at approximately 100⬚C. The catalyst concentrations were maintained at 7%. Both phenolic resin and catalyst were provided by Blagden Chemicals Ltd. Non-woven needle punched hemp was used as the reinforcement for phenolic resin and was provided by J B Plant Fibres Enterprises Ltd. The moisture absorption and desorption characteristics were investigated. Accordingly the non-woven hemp was heated for 2 h at 100⬚C in an oven
to expel moisture [18]. The non-woven hemp has a density of 250 g/m 2. Mould filling visualisation at different reinforcement concentrations were carried out by placing 0, 1, 2 and 3 layers of non-woven hemp, equivalent to fibre weight fractions of 0, 5.6, 11.1 and 16.7%, respectively. The mould filling visualisation involved using a Hypaject MK IV RTM machine in combination with a transparent flat panel mould 450 × 300 × 4 mm purpose built by Plastech Thermoset Tectonics Ltd. for the production of phenolic composites as schematically shown in Fig. 1. All components for the machines and mould were acid resistant due to the acidity of the catalyst. A transparent grid film was placed on the mould for direct and accurate data acquisition. Resin advancement profiles were recorded and the velocity calculated by differentiating the resin advancement values. The resin and catalyst were kept at room temperature and the mould was heated to 60⬚C during the experimental process. 3. Results and discussion It has been recognised that there are many variables associated with the RTM process. The experimental results and discussion here are mainly concerned with three aspects of the mould filling process for non-woven hemp fibre reinforced phenolic composites, namely fibre washing, edge flow and mould filling. 3.1. Fibre washing Fibre washing is referred to as the unexpected movement or displacement of reinforcement in the closed mould during the RTM process. When the mixture of resin and catalyst is injected into the closed mould, the prelaid reinforcement has a tendency to be displaced due to the injection pressure which is a function of fibre types, architecture, weight fractions, mould configuration and geometry. Fibre washing can ultimately lead to the failure of the RTM process due to the
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Fig. 2. Fibre washing distances for 2-layer non-woven hemp at different injection pressures.
fact that fibre displacements interrupt the uniformity of pre-determined reinforcement distribution. Resin rich and starved areas are created and unacceptable physical and mechanical property variations are introduced in the materials. Although fibre washing does not occur for every RTM fabrication process, it is a common problem in the case of non-woven hemp reinforced phenolic composites. In this study, fibre washing is characterised by an ultimate fibre washing distance which can be visualised and quantified during the mould filling process. The relationship between the injection time and “fibre washing distance” at different injection pressures with two layers of non-woven hemp is shown in Fig. 2. There is an obvious effect of injection pressure on fibre washing which is illustrated by the different times at which fibre washing occurs and precise values of “ultimate fibre washing
distance”. The relationship between the injection time and “fibre washing distance” for different numbers of layers of non-woven hemp is shown in Fig. 3. It can be seen that the “fibre washing distances” were significantly reduced with more layers of non-woven hemp. When four layers of nonwoven hemp are employed, “fibre washing distance” is virtually reduced to zero. Experimental observation indicates that fibre washing will commence at the edge nearest the injection gate when the injection pressure builds up. No splitting and thinning of non-woven hemp was observed around the injection gate area which suggested that it has sufficient inherent strength from mechanical entanglement and interlocking to withstand the injection pressure. Fibre washing is directly related to the clamping condition within the mould. Higher clamping forces were achieved when more fibre layers were used. There are two obvious solutions for this problem. The first is to pinch-off the fibres
Fig. 3. Fibre washing distance for different layers of non-woven hemp at 2 bar injection pressure.
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Fig. 4. Edge flow front isochrones of 2-layer non-woven hemp at 2 bar injection pressure (each isochrone represents 20 s).
across the mould seal but overdoing this will lead to unnecessary and premature resin overflow out of these locations. The second is to deliberately increase the clamping force by pre-laying additional narrow non-woven stripes along the edge to create intimate contact with the mould and higher clamping force. In practice, both methods are used in combination to substantially reduce fibre washing. 3.2. Edge flow For successful RTM process control and complete preform impregnation, the “quasi-one dimensional steady” flow state is required in a rectangular mould with a near edge injection gate. The “quasi-one dimensional steady state” flow is referred to as equal resin advancement distance or velocity at any moment during the injecting process in the resin advancement direction. In the process of resin transfer moulding, small clearances may exist between the fibre preform and mould edges because of
loose edge fibre bundles, poor fitting size, or deformation of the fibre preform. The clearance results in a preferential resin flow path during the mould filling stage. This edge flow can disrupt the uniformity of the flow pattern and cause incomplete wetting of the prefrom. This phenomenon intensifies with the decrease of preform permeability. A typical mould filling flow pattern with the edge flow effect is illustrated in Fig. 4. It can be seen that the edge flow effect is negligible just after the resin is fully expanded in the flow direction. The difference of resin advancement position at the same moment has been progressively increased with increasing the injection time. At later stages of injection, edge flow advances much faster than the flow in the middle position of the mould. The uniformity of flow pattern is completely disrupted. Once the edge flow arrives at the bottom edge, it changes direction and advances in a transverse direction. The vent position is preferably arranged along the bottom edge for good ventilation effect but early arrival of edge flow can cause overflow out of the mould leading to unnecessary spillage and waste. Additionally overflow would prematurely block the ventilation port, leading to the problem of air entrapment in the reinforcement. The combination of edge flow in the transverse direction and centre flow in the flow direction will inevitably lead to premature blocking of the ventilation port and formation of a dry spot. The flow distance/mould filling time curves for flows at edge and middle position are shown in Fig. 5. Edge flow effect can be characterised by mould filling distance differences between the centre and edge and the arrival time of edge flow at the bottom of the mould. Injection pressure and preform permeability influences this edge flow effect. Figs. 6 and 7 show the mould filling distance differences and the arrival time of edge flow at the bottom for different numbers of layers of non-woven hemp at different injection pressures. It can be seen that there is only a marginal increase for mould filling time at the edge with
Fig. 5. The edge and central flow distance differences for 2-layer non-woven hemp at 2 bar injection pressure.
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Fig. 6. Mould filling distance differences between the edge and centre at different injection pressures.
increasing injection pressure, implying that edge flow effect is less sensitive to pressure. Comparatively there is a dramatic increase when fibre preforms are increased from 1 layer to 2 and 3 layers. This implies that the edge flow effect is sensitive to changes of permeability. More layers of non-woven hemp in the mould result in high flow resistance and slow flow and impregnation. The resin has a tendency to flow along the channel of least resistance under injection pressure and therefore the effect is intensified. Consequently both the values of mould filling distance differences between the centre and edge and the arrival time of edge flow at the bottom are substantially increased. The edge flow has an adverse effect on the mould filling
process which leads to the formation of dry spots and spillage. The convenient solution to this problem is fibre preform manipulation. In practice, there is a difficulty in cutting the preform precisely to mould size. This is the case for the production of geometrically complicated components in particular. Experimental results show that the edge flow effect can be eliminated when the dimensions of the fibre preforms are slightly larger than that of the mould cavity. When the mould is closed, the preform will be squeezed and the clearance can be filled by the reinforcements. A local low permeability area may be introduced near the mould edge but it does not result in obvious interruption of global flow uniformity and mould filling can be successfully achieved.
Fig. 7. Time for the edge flow front to arrive at the end of the mould at different injection pressures.
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Fig. 8. The quasi 1D steady flow advancement profiles for different numbers of layers of non-woven hemp at 2 bar injection pressure.
3.3. Mould filling After the problems of fibre washing and edge flow were circumvented, successful mould filling was achieved. The resin advancement profiles with varying numbers of layers of non-woven hemp and under different injection pressures are shown in Figs. 8 and 9, respectively. As can be seen, after taking the fibre washing and edge flow influences into consideration, the profiles are characterised by smooth curves, implying that a “quasi-one dimensional steady state” flow is achieved. The resin fronts steadily advanced in the flow direction and arrived at the bottom edge of the mould nearly at the same time. The ventilation port remained open until injection was completed. In this case
the entrapped air can be removed from the reinforcement and complete impregnation achieved. Fig. 10 shows schematically the progressive advancements of resin fronts during the mould filling process. The injection pressure has an influence on the mould filling process. The injection time can be very long if the injection pressures are ⬍1 bar. Consequently higher injection pressures are preferably employed. It should be noted that there is a mechanism to prevent “over pressure” in this and other RTM machines. Over pressurisation can present a danger of poor sealing and reduced seal life, fast linkage wear and mould damage because in this present case a glass panel is used for visualisation and GRP composites for tooling to minimise the acidic corrosion. At the same time, over
Fig. 9. The quasi 1D steady flow advancement profiles of 2-layer non-woven hemp at different injection pressures.
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4. Conclusions A good understanding has been established of the mould filling process of RTM involving non-woven hemp reinforced phenolic composites. This understanding could be useful for production control and optimisation as well as the validation of different mould filling mathematical models and numerical simulation schemes.
Fig. 10. Quasi 1D steady flow isochrones of 2-layer non-woven hemp at 2 bar injection pressure (each isochrone represents 20 s).
pressurisation can cause more serious fibre washing and edge flow. Reinforcement concentrations can have a pronounced influence on the mould filling process. This is clearly illustrated by observing mould filling velocity profiles as shown in Figs. 11 and 12. The velocity profiles at different locations in the panel demonstrate that the flow rates have been gradually reduced at constant injection pressure due to the increase in flow resistance. The reinforcement concentrations have a strong influence on mould filling velocity profiles. At an early stage of the injection process, the velocity differences are very high and then gradually reduce. This is due to the gradual increase in flow resistance which leads to smaller differences in velocity.
• Fibre washing is a common problem at low fibre concentrations due to poor clamping conditions leading to the failure of the injection process. With increase in fibre concentration, the fibre washing effect is gradually reduced due to the consequential improvement of clamping conditions. Employing pinch-off and localised increase of fibre concentrations near the edge is a convenient solution to this problem. • Edge flow is introduced due to the clearance between the preform and the mould edge. The presence of edge flow leads to the interruption of flow uniformity and the resin near the edge has a tendency to flow much faster than in the main area due to lower resistance. Edge flow is less sensitive to injection pressure variations but fibre concentrations have a dramatic influence. The deployment of preforms larger than the mould eliminates the problem. • “Quasi-one-dimensional steady” flow is preferred in order to achieve successful mould filling with complete impregnation and proper ventilation. Resin front advancement profiles and velocity profiles have been established which illustrate the influence of injection pressure and fibre concentrations on the mould filling process. Changes in injection pressure lead to corresponding variations in injection time. Also an increase in the number of layers of non-woven hemp leads to prolonged injection times.
Fig. 11. The quasi 1D steady mould filling velocity profiles for different numbers of layers of non-woven hemp at 2 bar injection pressure.
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Fig. 12. The quasi 1D steady flow mould filling velocity profiles of 2-layer non-woven hemp at different injection pressures.
Acknowledgements The authors would like to acknowledge the support of a DTI/EPSRC Technology Foresight Challenge Grant. Thanks are also due to The BioComposites Centre of University of Wales, Concargo Ltd and J B Plant Fibres Ltd.
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