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Experimental investigation of high fiber tow count fabric unsaturation during RTM
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 976–982 www.elsevier.com/locate/compscitech
Experimental investigation of high fiber tow count fabric unsaturation during RTM B. Gourichon, C. Binetruy *, P. Krawczak Ecole des Mines de Douai, Polymer and Composite Technology Department, 941 rue Charles Bourseul, BP 838, 59508 Douai Cedex, France Received 13 September 2004; received in revised form 21 July 2005; accepted 25 July 2005 Available online 3 October 2005
Abstract High fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements when manufacturing structural automotive composite parts at high production rates by RTM. Impregnation of these large fiber clusters may lead to local incomplete saturation of fabrics. Mechanical softening, early failure, or part rejection because of high voids content may be expected. A new experimental method has been proposed to measure the air volume entrapped within the wetted part of the fabric at any given time and to quantify air entrapment kinetics. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. This experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected. 2005 Elsevier Ltd. All rights reserved. Keywords: Manufacturing; Resin transfer molding; Saturation; Void
1. Introduction Resin transfer molding (RTM) is a versatile method of composite part manufacturing. A preform made of glass or carbon fabrics is placed into a mold cavity, then the mold is closed and a low viscosity thermoset or reactive thermoplastic resin is injected to infuse the preform. Automotive industry has shown growing interest for composite materials and RTM over the past 10 years, as it presents many advantages on other processes such as autoclave curing or hand lay-up. This processing technology allows one to produce parts with complex shapes and inserts, and since it is a closed mould process it limits VOC emanation. The use of composites to produce structural automotive parts could reduce total car weight up to 40%, and CO2 release up to 20%. Considering the chal-
*
Corresponding author. Tel.: + 33 3 27 71 21 75; fax: +33 3 27 71 29 81. E-mail address: [email protected] (C. Binetruy).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.07.032
lenges in reaching automotive production rates and mechanical part properties, RTM has to date the best potential in fulfilling those requirements. Moreover, high fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements. Recent efforts on manufacturing process modeling have made efficient simulation tools available, thus reducing the expensive prototyping phase. The study of high fiber tow count fabric impregnation has received little attention yet, and considering the large diameter of such tows as compared to usual ones, impregnation of large fiber clusters with a very fast curing resin system may lead to local incomplete saturation of fabrics due to gas entrapment within tows. Mechanical softening, early failure, or part rejection because of high void content may be expected. Air entrapment is a key parameter in producing large structural void free composite parts, the main objective being to optimize processing parameters (resin gel time, injection pressure or flow rate, bleeding time) to have the minimum final void content. This is
B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
critical for fast injection of highly reactive resin since the tow saturation time scale can be of the same order of magnitude than the whole part impregnation and resin gel time. Experimental results presented in this paper aims to highlight this unsaturation phenomenon, and to quantify air entrapment kinetics. The basic idea is to perform a 1D injection of a fluid through a glass fabric placed in a mold prior to injection. The flow front extent recorded during injection is then compared to the position that would result from instantaneous and complete saturation of the fabric. 2. Previous work Usually, DarcyÕs law has been used to describe the physics of flow through a fabric considered as a porous medium v ¼
K rP ; l
ð1Þ
where v is the volume-averaged velocity, $P the pressure gradient, l the viscosity and K the permeability tensor of the porous medium. Although it is valid when modelling flow in a single scale porous medium, it is no longer valid when applied to a dual scale porous medium [1]. In a dual scale porous medium, the difference in length scale between micropores (pores inside a fiber tow) and macropores (pores between fiber tows) is usually 2 or 3 orders of magnitude. It has been shown that tows are mainly impregnated transversally [2] and the global flow front position matches the macropores flow front. The direct result of this delayed impregnation is the presence of air bubbles within the fabric when resin starts exiting the mold. Different indirect experimental observations proved the existence of the unsaturation phenomenon. The microvoid formation processes has been studied [3–5] and this is the most critical consequence of unsaturation since voids can dramatically reduce mechanical properties of composites. As bubbles may be evacuated from the mold before resin cures, microvoid content cannot be directly linked to an unsaturation degree. Hayward and Harris [6] gave the evidence that applying a vacuum at the vent reduces microvoid content, the intensity of transmitted light decreasing with microvoid content and as such, with unsaturation. Jinlian et al. [7] studied analytically and numerically the formation of microvoids in multilayer woven fabrics. The model predicts the presence of voids in warp tows and size of voids is found to be function of the ratio between tow axial and transverse permeability. Flow within a woven or stitched fabric is driven both by viscous forces on the macroscale and capillary forces on the microscale and the formation of micro- or macro-voids is the result of the competition between these two phenomena. Lee et al. [8–11] carried out some experimental work to investigate the process of void formation. They found the void content could be expressed as a fonction of the modified capillary number Ca* (Eq. (2)) with macrovoids being formed at low Ca* and microvoids at high Ca*
Ca ¼
lul ; c cosðhÞ
977
ð2Þ
where ul is the superficial velocity, c the surface tension and h the contact angle between the resin and the fibers. Investigating the impregnation of unidirectional stitched fiber mat, a processing window corresponding to a minimal void content was obtained for flow along fiber tows while it was not for flow normal to fiber tows. According to these results, at a given flow rate, three different populations of voids may be observed: • Only macrovoids. • No voids (processing window). • Only microvoids. Fig. 1 is a schematic representation of the void population versus the modified capillary number. The occurence of a processing window with a minimum void content as presented in Fig. 1 will depend on the fluid/preform combination. The critical Ca* for microvoid formation decreases from 102 for axial flow to 104 for flow normal to fiber tows. In most industrial injection schemes, this processing window will whether not exist or be incompatible with fast production rates. Kang et al. [12] developed a mathematical model to analyse void formation during RTM, size and number of voids were found to vary with the modified capillary number for a given fiber preform. 3. Materials A glass 1 · 3 twill weave fabric has been used to create big fiber clusters (S31500, Chomarat, France). This woven fabric is unbalanced, 630 g/m2 in the warp direction and 870 g/m2 in the weft direction. As the main process of void formation is mechanical entrapment in transverse tows, all injections were run along warp tows in order to maximize
Fig. 1. Schematic representation of the void content variation versus the modified capillary number (2) [normal flow–axial flow].
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B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
Numeric camera
Injection piston 1 Vent
Vent 2
3 PMMA Plate
Steel plate Woven fabric
Fig. 3. Schematic representation of the experimental setup.
Fig. 2. Viscosity of DOP versus temperature.
the entrapped air volume. The lay-up was four plies for a 4.9 mm thickness, giving a fiber volume fraction of 47%. The fluid used is a DOP and a red dye is used to magnify the contrast between the fluid and the fabric. Viscosity was measured using a Brookfield viscometer for temperature ranging from 15 to 35 C (Fig. 2). The temperature is measured just before experiment and viscosity is calculated from the fitting curve equation. 4. Experimental setup DOP is injected at a constant flow rate with a pistonbased machine (Matrasur, France) controlled by a numerical command. The accuracy was found to be ±47 mm3/ min within the range of flow rate used. The stiff tubing between the injection piston and the mold does not deform at the experimental pressures (less than 0.5 MPa), preventing any variation in flow rate. The mold is made of a 40 mm thick steel bottom plate with two holes for flush mount pressure transducers (0–1 MPa), and of a 25 mm thick PMMA top flat plate, with two holes for injection and vent. The mold cavity was created by inserting a seal and spacers between plates. The seal is bonded to the steel plate at the exact dimensions of the woven fabric to prevent racetrackings. Steel bars of dimension 300 · 80 · 30 mm3 are distributed over the mold surface to clamp the mold and prevent top plate deflection during filling experiments. The maximum deflection of the top PMMA plate was monitored throughout experiments and was found to be less than 0.1 mm. Finally, a numeric camera is placed 0.5 m above the mold to record flow front positions over time. The experimental setup is sketched in Fig. 3. Three valves can be opened or closed manually at the injection gate and vent. The injection tube is carefully filled prior to injection, preventing any air to remain trapped in the inlet tube. If air is trapped in the tube, this might lead to variation in flow rate because of air compression. The principle of the method relies on a comparison of the real injected volume known with very good accuracy from the delivered flow rate with the apparent injected volume calculated from the flow front position over time. The
real and apparent injected volumes Vr and Vapp were determined using the following equations: V r ¼ tQ; V app ¼ xSð1 V f Þ;
ð3Þ ð4Þ
where t is the impregnation time, Q the flow rate, x the flow front position, Vf the fiber volume fraction and S the cavity cross-section. To insure no variation of the mold cavity volume from experiment to experiment, the clamping pressure was set to a constant value for all experiments using a torque wrench. Pressure transducers were connected to a PC, with a Labview data acquisition program recording inlet and outlet pressures during experiments. 5. Results and discussion Experiments were conducted at three different flow rates, 80, 100 and 120 cm3/min. The data acquisition is run once the flow front starts to impregnate the woven fabric, so the recorded pressures are synchronized with the state of unsaturation. Fig. 4 shows snapshots of the mold filling with the recorded flow front position at different times during an injection run at 100 cm3/min. The flow front remains flat during injection and racetracking effects were eliminated by careful positioning of the woven fabric within the mold cavity.
Fig. 4. Position of the flow front at various time: (a) 28 s, (b) 42 s, (c) 60 s, and (d) 87 s during an injection at 100 cm3/min.
B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
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Fig. 5. Schematic of the flow distribution around tows.
Fig. 7. Inlet pressure profile during 1D injection run at 1203 cm/min (tinj = 66 s).
The determination of Vapp relies on a visual observation of the flow front position. Tows composing the woven fabric (weft tows) are about 6 mm wide. Even with a flat macrofront, Fig. 5 illustrates a disturbing effect. When the flow front reaches a transverse tow, the macropores are filled almost immediatley, and impregnation of the transverse tow is much slower. Thus, it is not possible to have a perfectly flat flow front. The width of weft tows (6 mm) was taken as the error on flow front position recording
the fluid pressure increases, those trapped bubbles may be removed from the tows, but at a different kinetic compared to the formation dynamics. Thus, the uncomplete tow impregnation explains why a steady state could not be reached. A steady unsaturation level might be reached if air bubbles were expelled out of tows. The evolution of inlet pressure versus time is plotted in Fig. 7. Four distinct zones are observed:
EðxÞ ¼ 3 mm.
ð5Þ
Finally, the error on Vapp is estimated at EðV app Þ ¼ 1:4 cm3 .
ð6Þ
When comparing real and apparent injected volumes, it is found that the real flow front is always behind the apparent front. The total void content (difference between real and apparent volumes) is plotted versus time in Fig. 6. One can see that Vapp Vr grows linearly over time. This result is in contradiction with the model proposed by Pillai and Advani [13], where tows are supposed to act like sinks and a steady state is reached when tows behind the flow front saturate at the same speed new sinks are open. This experiment shows that the total unsaturation is constantly growing. The discussed model does not account for the entrapment of air bubbles and is based only on the delayed saturation of fiber tows. Transverse impregnation of tows perpendicular to the macroscopic flow leads to entrap air pockets inside tows. As
(A) At the beginning of impregnation, one can observe a non-linear pressure build up as the fluid encounters flow resistance. This deviation from the behavior predicted by DarcyÕs law has already been observed [14]. (B) The second zone corresponds to a linear growth of pressure with time as observed in single scale porous media (random mat). (C) Another pressure rise occurs when the fluid is exiting the woven fabric. (D) Then the pressure stabilize to reach the saturated flow condition. The saturated permeability (Ksat) was calculated with 1D DarcyÕs law using pressure values at t = 2tinj. Saturated permeability does not reflect the real flow behavior in the fabric during RTM, the unsaturated permeability was computed from the following equation: K unsat ¼
lQx ; SDP
ð7Þ
where x is the flow front position. Comparison of saturated and unsaturated permeabilities is presented in Fig. 8. Variations in permeability ratio (Kunsat/Ksat) can be divided following the same zones as for pressure:
Fig. 6. Difference between real and apparent injected volumes as a fonction of time for an injection at 100 cm3/min (tinj = 79 s).
(A) Permeability ratio drops as fluid encounters flow resistance and initial wetting effects become negligible. (B) The unsaturated permeability increases gradually to a higher value than the saturated permeability, ratio passes over 1.
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Fig. 8. Permeability ratio during an injection run at 1203 cm/min (tinj = 66 s).
Fig. 10. Air content (cm3) entrapped within the woven fabric as the flow front reaches the vent (t/tinj = 1).
(C) The permeability ratio decreases. (D) The permeability ratio stabilizes at 1, fabrics being completely saturated. This four-step behavior has been observed for a 3D woven fabric by Luce et al. [14]. Series of experiment were conducted at 3 different flow rate (80, 100, and 120 cm3/min). Average values for pressure rise at the end of injection and air volume inside the fabric are presented in Figs. 9 and 10. Changing flow rates did not show any significant variation. The pressure rise observed in zone C could be related to the saturation state inside the mold, since the pressure would reach a constant value as the fluid exits the mold if saturation was complete. It might represent the pressure needed to compress air bubbles and allow them to move within fabrics. According to Lunstro¨m et al, as pressure reaches a critical value (Pc), bubbles start moving [15]. When the fluid reaches the end of the mold, air bubbles are trapped mostly within fiber tows because Ca* > 102 in our experiments. As inter-fiber space within fiber tows is small, bubbles cannot move freely.
Fig. 9. Pressure rise for t > tinj.
Fig. 11. Air bubbles expelled out of the fabrics.
Fig. 11 presents a picture of bubbles exiting the fabric during bleeding after injection was stopped and restored, and one can clearly see a gradient in bubble size, the smallest bubbles exiting at last. Bubbles expelling out of a fabric have been already observed and two different zones corresponding to differences in bubble sizes were distinguished [16]. Because large air bubbles could not fit into micropores, they must come from macropores where they remain trapped. Small air bubbles probably comes from micropores, the pressure required to displace them is clearly higher as they exited later. Due to the small size of micropores and the higher contact surface with fibers, higher pressure is needed to displace them. Patel et al. [11] observed microvoids that were difficult to eliminate even at much higher flow rates than those at which they were created. If small bubbles exit the fabric when bleeding at the same flow rate, it means the pressure in the mold must have passed Pc on the inlet side of the fabric. Thus, when expressing the void content as a function of Ca* (Fig. 1), there should be a critical Ca* related to Pc over which the void content could decrease. To validate this hypothesis, a set of experiments was run with injection flow rate varying from 10 to 200 cm3/min. These experiments aimed to compare our experimental results with existing data in the literature regarding the variation of void content versus the modified capillary number. The graph in Fig. 12 presents this comparison where the void content is measured at the end of injection.
B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
Fig. 12. Void content vs modified capillary number (2) at t/tinj = 1. Pc is a critical pressure for void mobilization.
Four zones can be identified on this graph. (A) In the first zone, flow is dominated by capillary forces and macrovoids are formed. This result is consistent with Lee et al work where the limit of macrovoid formation was obtained for Ca* 3 · 103 for injection of DOP on unidirectional stitched fiber mat with flow along fiber tows and Ca* 102 for injection of DOP on unidirectional stitched fiber mat with flow perpendicular to fiber tows [8]. Although results cannot be directly compared from one preform to another, the results are within the same order of magnitude. (B) In the second zone, competition between viscous and capillary forces leads to an equivalent fluid velocity inside and outside fiber tows. Thus, the void content is minimum corresponding to the processing window described in Fig. 1. (C) In the third zone, flow is controlled by viscous forces and is much faster in macropores than in micropores. Voids are formed within micropores in transverse tows and in a less extent in axial tows, the void content is rising with Ca* due to the increasing difference between viscous and capillary forces. (D) In the last zone, the void content is expected to increase with Ca*, but a decrease in void content is actually measured. The phenomenon in zone D can be explained using the theory developed by Lundstro¨m et al. [15]. The pressure withstood by a bubble created at the flow front will increase during injection, thus, using the ideal gas law, the volume of the bubble will decrease. The authors suggested that there is a critical bubble volume at which the bubble may be mobilized. So the decrease in void content measured for Ca* > 1.5 · 102 expresses the fact that bubbles were compressed enough to be forced out the tow to the macropore where they are transported to the flow front. Patel and Lee [17] developed a model for void mobilization, microvoids may be expelled out of the fiber tow and aggregate into mesovoids that are easily transported within
981
macropores. Results presented in Fig. 12 show that void mobilization is a key parameter, and a short bleeding time at the end of injection with a high enough pressure could lead to very small void content. The critical capillary number expresses the ratio between viscous and capillary forces. But as viscous forces are expressed throughout the flow rate within Ca*, the influence of viscous pressure cannot be directly integrated. Thus, the critical pressure Pc should be specified for void mobilization and elimination. Another ideal processing window allowing bubbles mobilization and transport appears at high injection pressures. The critical pressure for our fluid/preform combination was found to be 0.18 MPa. One should note that the critical pressure was reached during experiments, however no bubbles exiting the fabric were observed before experiment was stopped and restarted for bleeding. The part of the fabric where pressure was over Pc was limited and located close to the injection gate. The better tow impregnation near the injection point due to the important action of capillary forces at the beginning of impregnation [2] has lead to the creation of smaller micro-voids. Thus, the critical bubble size for mobilization was reached before pressure reached Pc and bubbles were eliminated during the injection process. The existence of a processing window at high modified capillary number has been confirmed by Binetruy et al. [18] who performed high pressure RTM of an automotive complex shape part with high fiber tow count fabrics. Injection of resin excess for a few seconds at more than 2.5 MPa led to a very good part quality. The described method has been used to study the process of micro-void creation and elimination for the purpose of getting closer to industrial injection rate conditions. However, this method might be used for low modified capillary number flows where voids are mainly created between tows. In that case, one has to track the wicking front within fiber bundles to compute the apparent injected volume. 6. Conclusions and outlook A new experimental method to follow the real saturation process during the impregnation of a dual scale porosity fabric has been developed. The air volume entrapped within the wetted part of the fabric can be measured at any given time. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. Although the graph giving void content versus modified capillary number has been extensively demonstrated, this experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected. Modeling work is currently in progress to account for bubbles compression
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and mobilization. This seems to be compulsory for the improvement of RTM process simulation and the development of void free structural automotive parts, injected with very fast curing resin system. Acknowledgement The authors thank the FEDER organization for their financial support. References [1] Parnas RS, Salem AJ, Sadiq TAK, Wang HP, Advani SG. The interaction between micro- and macroscopic flow in resin transfer molding preforms. Compos Struct 1994;27:93–107. [2] Chan AW, Morgan RJ. Tow impregnation during resin transfer molding of bi-directionnal nonwoven fabrics. Polym Compos 1993;14(4):335–40. [3] Binetruy C, Hilaire B, Pabiot J. Tow impregnation model and void formation mechanisms during resin transfer molding. J Compos Mater 1998;32(3):223–45. [4] Peterson RC, Robertson RE. Flow characteristics of polymer resin through glass fiber preforms in resin transfer molding. In: Advanced composite materials: new developments and applications conference proceedings, Detroit, MI, USA; 1991. p. 203–8. [5] Chen Y, Davis HT, Macosko C. Wetting of fiber mats for composites manufacturing: I. visualization experiments. AIChE J 1995;41(10):2261–73. [6] Hayward JS, Harris B. The effect of vacuum assistance in resin transfer molding. Compos Manuf 1990;1(3):161–6.
[7] Jinlian H, Yi L, Xueming S. Study on void formation in multi-layer woven fabrics. Composites A 2004;35:595–603. [8] Patel N, Rohatgi V, Lee LJ. Microscale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat. Polym Eng Sci 1995;35(10):837–51. [9] Patel N, Lee LJ. Effect of fibermat architecture on void formation and removal in liquid composite molding. Polym Compos 1995;16(5):386–99. [10] Patel N, Rohatgi V, Lee LJ. Macro- and microvoid formation in liquid composite molding. In: 9th ASM/ESD advanced composites conference, Dearborn, MI, USA; 1993. p. 81–98. [11] Rohatgi V, Patel N, Lee LJ. Experimental investigation of flowinduced microvoids during impregnation of unidirectionnal stitched fibermat. Polym Compos 1996;17(2):161–70. [12] Kang MK, Lee WI, Hahn HT. Formation of microvoids during resin transfer molding process. Compos Sci Technol 2000;60:2427–34. [13] Pillai KM, Advani SG. A model for unsaturated flow in woven fiber preforms during mold filling in resin transfer molding. J Compos Mater 1998;32(19):1753–83. [14] Luce TL, Advani SG, Howard JG, Parnas RS. Permeability characterization. Part 1. A proposed standard reference fabric for permeability. Polym Compos 1995;16(6):429–45. [15] Lundstro¨m TS, Gebart BR, Lundemo CY. Void formation in resin transfer molding. J Reinforced Plastics Compos 1993;12:1339–49. [16] Wang TJ, Perry MJ, Lee LJ. Analysis of permeability and void formation in resin transfer molding. In: ANTEC 1992, Detroit, MI, USA; 1992. p. 756–60. [17] Patel N, Lee LJ. Modeling of void formation and removal in liquid composite molding. Polym Compos 1996;17(1):104–14. [18] Binetruy C, Lacrampe MF, Krawczak P, Piccirelli N. Flow simulation of an automative resin transfer molded complex shape part made with heavy tow carbon fabrics – comparison with experimental results. In: EUROPAM, Antibes, France; 2002. p. 81–98.
Experimental investigation of high fiber tow count fabric unsaturation during RTM B. Gourichon, C. Binetruy *, P. Krawczak Ecole des Mines de Douai, Polymer and Composite Technology Department, 941 rue Charles Bourseul, BP 838, 59508 Douai Cedex, France Received 13 September 2004; received in revised form 21 July 2005; accepted 25 July 2005 Available online 3 October 2005
Abstract High fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements when manufacturing structural automotive composite parts at high production rates by RTM. Impregnation of these large fiber clusters may lead to local incomplete saturation of fabrics. Mechanical softening, early failure, or part rejection because of high voids content may be expected. A new experimental method has been proposed to measure the air volume entrapped within the wetted part of the fabric at any given time and to quantify air entrapment kinetics. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. This experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected. 2005 Elsevier Ltd. All rights reserved. Keywords: Manufacturing; Resin transfer molding; Saturation; Void
1. Introduction Resin transfer molding (RTM) is a versatile method of composite part manufacturing. A preform made of glass or carbon fabrics is placed into a mold cavity, then the mold is closed and a low viscosity thermoset or reactive thermoplastic resin is injected to infuse the preform. Automotive industry has shown growing interest for composite materials and RTM over the past 10 years, as it presents many advantages on other processes such as autoclave curing or hand lay-up. This processing technology allows one to produce parts with complex shapes and inserts, and since it is a closed mould process it limits VOC emanation. The use of composites to produce structural automotive parts could reduce total car weight up to 40%, and CO2 release up to 20%. Considering the chal-
*
Corresponding author. Tel.: + 33 3 27 71 21 75; fax: +33 3 27 71 29 81. E-mail address: [email protected] (C. Binetruy).
0266-3538/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.07.032
lenges in reaching automotive production rates and mechanical part properties, RTM has to date the best potential in fulfilling those requirements. Moreover, high fiber tow count fabrics have been developed by fibers and fabrics suppliers to meet automotive cost and performance requirements. Recent efforts on manufacturing process modeling have made efficient simulation tools available, thus reducing the expensive prototyping phase. The study of high fiber tow count fabric impregnation has received little attention yet, and considering the large diameter of such tows as compared to usual ones, impregnation of large fiber clusters with a very fast curing resin system may lead to local incomplete saturation of fabrics due to gas entrapment within tows. Mechanical softening, early failure, or part rejection because of high void content may be expected. Air entrapment is a key parameter in producing large structural void free composite parts, the main objective being to optimize processing parameters (resin gel time, injection pressure or flow rate, bleeding time) to have the minimum final void content. This is
B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
critical for fast injection of highly reactive resin since the tow saturation time scale can be of the same order of magnitude than the whole part impregnation and resin gel time. Experimental results presented in this paper aims to highlight this unsaturation phenomenon, and to quantify air entrapment kinetics. The basic idea is to perform a 1D injection of a fluid through a glass fabric placed in a mold prior to injection. The flow front extent recorded during injection is then compared to the position that would result from instantaneous and complete saturation of the fabric. 2. Previous work Usually, DarcyÕs law has been used to describe the physics of flow through a fabric considered as a porous medium v ¼
K rP ; l
ð1Þ
where v is the volume-averaged velocity, $P the pressure gradient, l the viscosity and K the permeability tensor of the porous medium. Although it is valid when modelling flow in a single scale porous medium, it is no longer valid when applied to a dual scale porous medium [1]. In a dual scale porous medium, the difference in length scale between micropores (pores inside a fiber tow) and macropores (pores between fiber tows) is usually 2 or 3 orders of magnitude. It has been shown that tows are mainly impregnated transversally [2] and the global flow front position matches the macropores flow front. The direct result of this delayed impregnation is the presence of air bubbles within the fabric when resin starts exiting the mold. Different indirect experimental observations proved the existence of the unsaturation phenomenon. The microvoid formation processes has been studied [3–5] and this is the most critical consequence of unsaturation since voids can dramatically reduce mechanical properties of composites. As bubbles may be evacuated from the mold before resin cures, microvoid content cannot be directly linked to an unsaturation degree. Hayward and Harris [6] gave the evidence that applying a vacuum at the vent reduces microvoid content, the intensity of transmitted light decreasing with microvoid content and as such, with unsaturation. Jinlian et al. [7] studied analytically and numerically the formation of microvoids in multilayer woven fabrics. The model predicts the presence of voids in warp tows and size of voids is found to be function of the ratio between tow axial and transverse permeability. Flow within a woven or stitched fabric is driven both by viscous forces on the macroscale and capillary forces on the microscale and the formation of micro- or macro-voids is the result of the competition between these two phenomena. Lee et al. [8–11] carried out some experimental work to investigate the process of void formation. They found the void content could be expressed as a fonction of the modified capillary number Ca* (Eq. (2)) with macrovoids being formed at low Ca* and microvoids at high Ca*
Ca ¼
lul ; c cosðhÞ
977
ð2Þ
where ul is the superficial velocity, c the surface tension and h the contact angle between the resin and the fibers. Investigating the impregnation of unidirectional stitched fiber mat, a processing window corresponding to a minimal void content was obtained for flow along fiber tows while it was not for flow normal to fiber tows. According to these results, at a given flow rate, three different populations of voids may be observed: • Only macrovoids. • No voids (processing window). • Only microvoids. Fig. 1 is a schematic representation of the void population versus the modified capillary number. The occurence of a processing window with a minimum void content as presented in Fig. 1 will depend on the fluid/preform combination. The critical Ca* for microvoid formation decreases from 102 for axial flow to 104 for flow normal to fiber tows. In most industrial injection schemes, this processing window will whether not exist or be incompatible with fast production rates. Kang et al. [12] developed a mathematical model to analyse void formation during RTM, size and number of voids were found to vary with the modified capillary number for a given fiber preform. 3. Materials A glass 1 · 3 twill weave fabric has been used to create big fiber clusters (S31500, Chomarat, France). This woven fabric is unbalanced, 630 g/m2 in the warp direction and 870 g/m2 in the weft direction. As the main process of void formation is mechanical entrapment in transverse tows, all injections were run along warp tows in order to maximize
Fig. 1. Schematic representation of the void content variation versus the modified capillary number (2) [normal flow–axial flow].
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B. Gourichon et al. / Composites Science and Technology 66 (2006) 976–982
Numeric camera
Injection piston 1 Vent
Vent 2
3 PMMA Plate
Steel plate Woven fabric
Fig. 3. Schematic representation of the experimental setup.
Fig. 2. Viscosity of DOP versus temperature.
the entrapped air volume. The lay-up was four plies for a 4.9 mm thickness, giving a fiber volume fraction of 47%. The fluid used is a DOP and a red dye is used to magnify the contrast between the fluid and the fabric. Viscosity was measured using a Brookfield viscometer for temperature ranging from 15 to 35 C (Fig. 2). The temperature is measured just before experiment and viscosity is calculated from the fitting curve equation. 4. Experimental setup DOP is injected at a constant flow rate with a pistonbased machine (Matrasur, France) controlled by a numerical command. The accuracy was found to be ±47 mm3/ min within the range of flow rate used. The stiff tubing between the injection piston and the mold does not deform at the experimental pressures (less than 0.5 MPa), preventing any variation in flow rate. The mold is made of a 40 mm thick steel bottom plate with two holes for flush mount pressure transducers (0–1 MPa), and of a 25 mm thick PMMA top flat plate, with two holes for injection and vent. The mold cavity was created by inserting a seal and spacers between plates. The seal is bonded to the steel plate at the exact dimensions of the woven fabric to prevent racetrackings. Steel bars of dimension 300 · 80 · 30 mm3 are distributed over the mold surface to clamp the mold and prevent top plate deflection during filling experiments. The maximum deflection of the top PMMA plate was monitored throughout experiments and was found to be less than 0.1 mm. Finally, a numeric camera is placed 0.5 m above the mold to record flow front positions over time. The experimental setup is sketched in Fig. 3. Three valves can be opened or closed manually at the injection gate and vent. The injection tube is carefully filled prior to injection, preventing any air to remain trapped in the inlet tube. If air is trapped in the tube, this might lead to variation in flow rate because of air compression. The principle of the method relies on a comparison of the real injected volume known with very good accuracy from the delivered flow rate with the apparent injected volume calculated from the flow front position over time. The
real and apparent injected volumes Vr and Vapp were determined using the following equations: V r ¼ tQ; V app ¼ xSð1 V f Þ;
ð3Þ ð4Þ
where t is the impregnation time, Q the flow rate, x the flow front position, Vf the fiber volume fraction and S the cavity cross-section. To insure no variation of the mold cavity volume from experiment to experiment, the clamping pressure was set to a constant value for all experiments using a torque wrench. Pressure transducers were connected to a PC, with a Labview data acquisition program recording inlet and outlet pressures during experiments. 5. Results and discussion Experiments were conducted at three different flow rates, 80, 100 and 120 cm3/min. The data acquisition is run once the flow front starts to impregnate the woven fabric, so the recorded pressures are synchronized with the state of unsaturation. Fig. 4 shows snapshots of the mold filling with the recorded flow front position at different times during an injection run at 100 cm3/min. The flow front remains flat during injection and racetracking effects were eliminated by careful positioning of the woven fabric within the mold cavity.
Fig. 4. Position of the flow front at various time: (a) 28 s, (b) 42 s, (c) 60 s, and (d) 87 s during an injection at 100 cm3/min.
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Fig. 5. Schematic of the flow distribution around tows.
Fig. 7. Inlet pressure profile during 1D injection run at 1203 cm/min (tinj = 66 s).
The determination of Vapp relies on a visual observation of the flow front position. Tows composing the woven fabric (weft tows) are about 6 mm wide. Even with a flat macrofront, Fig. 5 illustrates a disturbing effect. When the flow front reaches a transverse tow, the macropores are filled almost immediatley, and impregnation of the transverse tow is much slower. Thus, it is not possible to have a perfectly flat flow front. The width of weft tows (6 mm) was taken as the error on flow front position recording
the fluid pressure increases, those trapped bubbles may be removed from the tows, but at a different kinetic compared to the formation dynamics. Thus, the uncomplete tow impregnation explains why a steady state could not be reached. A steady unsaturation level might be reached if air bubbles were expelled out of tows. The evolution of inlet pressure versus time is plotted in Fig. 7. Four distinct zones are observed:
EðxÞ ¼ 3 mm.
ð5Þ
Finally, the error on Vapp is estimated at EðV app Þ ¼ 1:4 cm3 .
ð6Þ
When comparing real and apparent injected volumes, it is found that the real flow front is always behind the apparent front. The total void content (difference between real and apparent volumes) is plotted versus time in Fig. 6. One can see that Vapp Vr grows linearly over time. This result is in contradiction with the model proposed by Pillai and Advani [13], where tows are supposed to act like sinks and a steady state is reached when tows behind the flow front saturate at the same speed new sinks are open. This experiment shows that the total unsaturation is constantly growing. The discussed model does not account for the entrapment of air bubbles and is based only on the delayed saturation of fiber tows. Transverse impregnation of tows perpendicular to the macroscopic flow leads to entrap air pockets inside tows. As
(A) At the beginning of impregnation, one can observe a non-linear pressure build up as the fluid encounters flow resistance. This deviation from the behavior predicted by DarcyÕs law has already been observed [14]. (B) The second zone corresponds to a linear growth of pressure with time as observed in single scale porous media (random mat). (C) Another pressure rise occurs when the fluid is exiting the woven fabric. (D) Then the pressure stabilize to reach the saturated flow condition. The saturated permeability (Ksat) was calculated with 1D DarcyÕs law using pressure values at t = 2tinj. Saturated permeability does not reflect the real flow behavior in the fabric during RTM, the unsaturated permeability was computed from the following equation: K unsat ¼
lQx ; SDP
ð7Þ
where x is the flow front position. Comparison of saturated and unsaturated permeabilities is presented in Fig. 8. Variations in permeability ratio (Kunsat/Ksat) can be divided following the same zones as for pressure:
Fig. 6. Difference between real and apparent injected volumes as a fonction of time for an injection at 100 cm3/min (tinj = 79 s).
(A) Permeability ratio drops as fluid encounters flow resistance and initial wetting effects become negligible. (B) The unsaturated permeability increases gradually to a higher value than the saturated permeability, ratio passes over 1.
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Fig. 8. Permeability ratio during an injection run at 1203 cm/min (tinj = 66 s).
Fig. 10. Air content (cm3) entrapped within the woven fabric as the flow front reaches the vent (t/tinj = 1).
(C) The permeability ratio decreases. (D) The permeability ratio stabilizes at 1, fabrics being completely saturated. This four-step behavior has been observed for a 3D woven fabric by Luce et al. [14]. Series of experiment were conducted at 3 different flow rate (80, 100, and 120 cm3/min). Average values for pressure rise at the end of injection and air volume inside the fabric are presented in Figs. 9 and 10. Changing flow rates did not show any significant variation. The pressure rise observed in zone C could be related to the saturation state inside the mold, since the pressure would reach a constant value as the fluid exits the mold if saturation was complete. It might represent the pressure needed to compress air bubbles and allow them to move within fabrics. According to Lunstro¨m et al, as pressure reaches a critical value (Pc), bubbles start moving [15]. When the fluid reaches the end of the mold, air bubbles are trapped mostly within fiber tows because Ca* > 102 in our experiments. As inter-fiber space within fiber tows is small, bubbles cannot move freely.
Fig. 9. Pressure rise for t > tinj.
Fig. 11. Air bubbles expelled out of the fabrics.
Fig. 11 presents a picture of bubbles exiting the fabric during bleeding after injection was stopped and restored, and one can clearly see a gradient in bubble size, the smallest bubbles exiting at last. Bubbles expelling out of a fabric have been already observed and two different zones corresponding to differences in bubble sizes were distinguished [16]. Because large air bubbles could not fit into micropores, they must come from macropores where they remain trapped. Small air bubbles probably comes from micropores, the pressure required to displace them is clearly higher as they exited later. Due to the small size of micropores and the higher contact surface with fibers, higher pressure is needed to displace them. Patel et al. [11] observed microvoids that were difficult to eliminate even at much higher flow rates than those at which they were created. If small bubbles exit the fabric when bleeding at the same flow rate, it means the pressure in the mold must have passed Pc on the inlet side of the fabric. Thus, when expressing the void content as a function of Ca* (Fig. 1), there should be a critical Ca* related to Pc over which the void content could decrease. To validate this hypothesis, a set of experiments was run with injection flow rate varying from 10 to 200 cm3/min. These experiments aimed to compare our experimental results with existing data in the literature regarding the variation of void content versus the modified capillary number. The graph in Fig. 12 presents this comparison where the void content is measured at the end of injection.
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Fig. 12. Void content vs modified capillary number (2) at t/tinj = 1. Pc is a critical pressure for void mobilization.
Four zones can be identified on this graph. (A) In the first zone, flow is dominated by capillary forces and macrovoids are formed. This result is consistent with Lee et al work where the limit of macrovoid formation was obtained for Ca* 3 · 103 for injection of DOP on unidirectional stitched fiber mat with flow along fiber tows and Ca* 102 for injection of DOP on unidirectional stitched fiber mat with flow perpendicular to fiber tows [8]. Although results cannot be directly compared from one preform to another, the results are within the same order of magnitude. (B) In the second zone, competition between viscous and capillary forces leads to an equivalent fluid velocity inside and outside fiber tows. Thus, the void content is minimum corresponding to the processing window described in Fig. 1. (C) In the third zone, flow is controlled by viscous forces and is much faster in macropores than in micropores. Voids are formed within micropores in transverse tows and in a less extent in axial tows, the void content is rising with Ca* due to the increasing difference between viscous and capillary forces. (D) In the last zone, the void content is expected to increase with Ca*, but a decrease in void content is actually measured. The phenomenon in zone D can be explained using the theory developed by Lundstro¨m et al. [15]. The pressure withstood by a bubble created at the flow front will increase during injection, thus, using the ideal gas law, the volume of the bubble will decrease. The authors suggested that there is a critical bubble volume at which the bubble may be mobilized. So the decrease in void content measured for Ca* > 1.5 · 102 expresses the fact that bubbles were compressed enough to be forced out the tow to the macropore where they are transported to the flow front. Patel and Lee [17] developed a model for void mobilization, microvoids may be expelled out of the fiber tow and aggregate into mesovoids that are easily transported within
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macropores. Results presented in Fig. 12 show that void mobilization is a key parameter, and a short bleeding time at the end of injection with a high enough pressure could lead to very small void content. The critical capillary number expresses the ratio between viscous and capillary forces. But as viscous forces are expressed throughout the flow rate within Ca*, the influence of viscous pressure cannot be directly integrated. Thus, the critical pressure Pc should be specified for void mobilization and elimination. Another ideal processing window allowing bubbles mobilization and transport appears at high injection pressures. The critical pressure for our fluid/preform combination was found to be 0.18 MPa. One should note that the critical pressure was reached during experiments, however no bubbles exiting the fabric were observed before experiment was stopped and restarted for bleeding. The part of the fabric where pressure was over Pc was limited and located close to the injection gate. The better tow impregnation near the injection point due to the important action of capillary forces at the beginning of impregnation [2] has lead to the creation of smaller micro-voids. Thus, the critical bubble size for mobilization was reached before pressure reached Pc and bubbles were eliminated during the injection process. The existence of a processing window at high modified capillary number has been confirmed by Binetruy et al. [18] who performed high pressure RTM of an automotive complex shape part with high fiber tow count fabrics. Injection of resin excess for a few seconds at more than 2.5 MPa led to a very good part quality. The described method has been used to study the process of micro-void creation and elimination for the purpose of getting closer to industrial injection rate conditions. However, this method might be used for low modified capillary number flows where voids are mainly created between tows. In that case, one has to track the wicking front within fiber bundles to compute the apparent injected volume. 6. Conclusions and outlook A new experimental method to follow the real saturation process during the impregnation of a dual scale porosity fabric has been developed. The air volume entrapped within the wetted part of the fabric can be measured at any given time. An important observation is that the whole unsaturation grows linearly with time for 1D flow. The modified capillary number has been correlated to the amount of air entrapped during the injection process. However, results of this study show that it cannot account for void mobilization and elimination. A critical pressure for the onset of void mobilization has been identified for one fluid/preform combination. Although the graph giving void content versus modified capillary number has been extensively demonstrated, this experimental work carried out with proper calibration provided the evidence that for high modified capillary number, a decrease in void content is to be expected. Modeling work is currently in progress to account for bubbles compression
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and mobilization. This seems to be compulsory for the improvement of RTM process simulation and the development of void free structural automotive parts, injected with very fast curing resin system. Acknowledgement The authors thank the FEDER organization for their financial support. References [1] Parnas RS, Salem AJ, Sadiq TAK, Wang HP, Advani SG. The interaction between micro- and macroscopic flow in resin transfer molding preforms. Compos Struct 1994;27:93–107. [2] Chan AW, Morgan RJ. Tow impregnation during resin transfer molding of bi-directionnal nonwoven fabrics. Polym Compos 1993;14(4):335–40. [3] Binetruy C, Hilaire B, Pabiot J. Tow impregnation model and void formation mechanisms during resin transfer molding. J Compos Mater 1998;32(3):223–45. [4] Peterson RC, Robertson RE. Flow characteristics of polymer resin through glass fiber preforms in resin transfer molding. In: Advanced composite materials: new developments and applications conference proceedings, Detroit, MI, USA; 1991. p. 203–8. [5] Chen Y, Davis HT, Macosko C. Wetting of fiber mats for composites manufacturing: I. visualization experiments. AIChE J 1995;41(10):2261–73. [6] Hayward JS, Harris B. The effect of vacuum assistance in resin transfer molding. Compos Manuf 1990;1(3):161–6.
[7] Jinlian H, Yi L, Xueming S. Study on void formation in multi-layer woven fabrics. Composites A 2004;35:595–603. [8] Patel N, Rohatgi V, Lee LJ. Microscale flow behavior and void formation mechanism during impregnation through a unidirectional stitched fiberglass mat. Polym Eng Sci 1995;35(10):837–51. [9] Patel N, Lee LJ. Effect of fibermat architecture on void formation and removal in liquid composite molding. Polym Compos 1995;16(5):386–99. [10] Patel N, Rohatgi V, Lee LJ. Macro- and microvoid formation in liquid composite molding. In: 9th ASM/ESD advanced composites conference, Dearborn, MI, USA; 1993. p. 81–98. [11] Rohatgi V, Patel N, Lee LJ. Experimental investigation of flowinduced microvoids during impregnation of unidirectionnal stitched fibermat. Polym Compos 1996;17(2):161–70. [12] Kang MK, Lee WI, Hahn HT. Formation of microvoids during resin transfer molding process. Compos Sci Technol 2000;60:2427–34. [13] Pillai KM, Advani SG. A model for unsaturated flow in woven fiber preforms during mold filling in resin transfer molding. J Compos Mater 1998;32(19):1753–83. [14] Luce TL, Advani SG, Howard JG, Parnas RS. Permeability characterization. Part 1. A proposed standard reference fabric for permeability. Polym Compos 1995;16(6):429–45. [15] Lundstro¨m TS, Gebart BR, Lundemo CY. Void formation in resin transfer molding. J Reinforced Plastics Compos 1993;12:1339–49. [16] Wang TJ, Perry MJ, Lee LJ. Analysis of permeability and void formation in resin transfer molding. In: ANTEC 1992, Detroit, MI, USA; 1992. p. 756–60. [17] Patel N, Lee LJ. Modeling of void formation and removal in liquid composite molding. Polym Compos 1996;17(1):104–14. [18] Binetruy C, Lacrampe MF, Krawczak P, Piccirelli N. Flow simulation of an automative resin transfer molded complex shape part made with heavy tow carbon fabrics – comparison with experimental results. In: EUROPAM, Antibes, France; 2002. p. 81–98.