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Manufacturing of centrifuged continuous fibre-reinforced precision pipes with thermoplastic matrix
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 66 (2006) 2601–2609 www.elsevier.com/locate/compscitech
Review
Manufacturing of centrifuged continuous fibre-reinforced precision pipes with thermoplastic matrix Max Ehleben a, Helmut Schu¨rmann b
b,*
a Group Research Materials, Volkswagen AG, 38436 Wolfsburg, Germany Department of Lightweight Design and Construction, Darmstadt University of Technology, Petersenstrasse 30, 64287 Darmstadt, Germany
Received 2 September 2005; received in revised form 9 March 2006; accepted 14 March 2006 Available online 15 May 2006
Abstract The paper describes details of a procedure developed for manufacturing continuous fibre reinforced precision pipes with thermoplastic matrices, using centrifugation technology. The centrifuged pipes are almost free of eccentricity and have high quality surfaces, both at the inside and at the outside. They are therefore particularly suitable as drive shafts, bearings or hydrocylinders. 2006 Elsevier Ltd. All rights reserved. Keywords: FRP-pipes; Centrifugal manufacturing; Thermoplastic matrix; Impregnation process
Contents 1. 2. 3. 4. 5. 6. 7. 8.
Introduction . . . . . . . . . . . . . . . . . . . . . . TEP centrifugal process . . . . . . . . . . . . . . Multiple use of the thermoplastic material . Calculation of the impregnation pressure . . Impregnation of the reinforcement fibres . . Characteristics of the centrifuged pipes . . . Fields of application . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Pipes made of FRP (fibre reinforced plastics) have a number of advantages that mean that they can replace steel or aluminium in structural constructions. Additional advantages result if a thermoplastic matrix is used. The *
Corresponding author. Tel.: +49 6151 162160; fax: +49 6151 163260. E-mail addresses: [email protected] (M. Ehleben), [email protected] (H. Schu¨rmann). 0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.015
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material has a practically unlimited shelf life as well as excellent industrial hygiene. It can be welded and later reformed and it can be recycled to a high degree. However, due to the usually high melt viscosities of thermoplastics fibre impregnation presents a special problem. For this reason so far no economical manufacturing method has been available for production of continuous fibre-reinforced precision pipes with high quality surfaces. In order to close this gap a new procedure was developed at the University of Technology Darmstadt.
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2. TEP centrifugal process
3. Multiple use of the thermoplastic material
The thermoplastic endless fibre reinforced pipes (TEP) centrifugal process is a variant of the conventional centrifugal technique [1]. The operational sequence consists of five manufacturing steps (Fig. 1):
The concept behind the TEP centrifugal process is based on multiple use of the thermoplastic:
1. The reinforcement fibres are wound on a non-reinforced thermoplastic pipe. The thermoplastic pipes used here can be manufactured conventionally in an extruder or an injection moulding process. The fibres are dry wound and aligned exactly according to the load the component is designed to carry during normal service. 2. The thermoplastic pipe with reinforcement material wound on it is then inserted into the centrifuge mould. In this way an appropriate fibre orientation is achieved in the centrifuge mould. 3. The thermoplastic core pipe is heated during slow rotation until the thermoplastic material is melted and functions as matrix material. 4. The actual centrifugation procedure takes place in the fourth step. The rotation rate of the centrifuge mould is increased. Due to centrifugal force the thermoplastic melt penetrates the fibre reinforcement and impregnation takes place. 5. The last manufacturing step is cooling and removal of the fabricated FRP pipe.
• The thermoplastic pipe is used as the winding mandrel making acquisition of an expensive steel mandrel with polished surface unnecessary. In contrast to the conventional centrifugal casting process this makes it possible to place the fibre reinforcement by filament winding according to the load and to insert it into the centrifuge. • The second task of the thermoplastic core pipe is to impregnate fibre material with melted thermoplastic that functions as matrix material. • With a correctly adjusted thermoplastic surplus a pure matrix layer is left on the inside of the pipe which can be used as a wear resistance layer or sealing layer or the like.
4. Calculation of the impregnation pressure During the centrifuge procedure an impregnation pressure results from centrifugal force that presses the matrix melt into the fibre layer. Inside of a thick-walled pipe the impregnation pressure is time dependent which changes its value within the duration of the impregnation process
Fig. 1. Operational sequence of the TEP centrifugal process.
M. Ehleben, H. Schu¨rmann / Composites Science and Technology 66 (2006) 2601–2609
Impregnation pressure ΔpRot [bar]
5
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Internal diameter of matrix pipe dM,i = 200 mm
4
150 mm Design data • Wall thickness of matrix pipe sM : 4 mm 1,13 g/cm³ • Density of matrix pipe ρM :
3
100 mm
2 50 mm 1
0 0
2000
4000
6000
Rotation rate n
8000
10000
[1/min]
Fig. 2. Dependence of the impregnation pressure for a fibre reinforced plastic pipe with a PA 6 matrix as a function of rotation rate and diameter.
due to the radial movement of the molten thermoplastic mass. However, thin-walled pipes are used for most practical applications and the much lower radial mass translations inside of thin-walled pipes during the impregnation process lead to a negligible change of the impregnation pressure. This calculation of the impregnation pressure of a thin-walled pipe is much easier because the impregnation pressure is no longer time dependent and hence can be assumed to have a constant value. The impregnation pressure of a thin-walled pipe due to radial acceleration DpRot can be calculated as DpRot ¼ 2p2 n2 qM sM d M;i
ð1Þ
where n is the rotation rate, qM the density of the matrix, sM the wall thickness and dM,i the internal diameter of the matrix pipe. In Eq. (1) the impregnation pressure depends quadratically on the rotation rate and linearly on the geometrical characteristics (i.e. pipe diameter and wall thickness). Eq. (1) is a suitable approximation formula as long as the ratio dM,i/sM P 10. In that case the maximum deviation from the calculation of thick-walled pipes is less than 4% [2]. In Fig. 2 the dependence of the impregnation pressure on the rotation rate is shown for different pipe diameters for a PA 6 matrix pipe with a wall thickness of 4 mm. From the diagram it is evident that the impregnation pressures attainable by the centrifugation procedure are small compared with other manufacturing methods, for instance moulding techniques with p 7 bar. The lightweight construction advantage of the polymers, i.e. their low density, is a disadvantage here. Thus for instance for a 4 mm thick thermoplastic pipe with a diameter of 100 mm and a rotation rate of 6000 1/min the impregnation pressure is just under 1 bar. Since the pipe geometry is usually given, a clear increase in the impregnation pressure is attainable only through a substantial increase in the rotation rate.
However, this leads to a substantial increase in the cost of the machinery, in particular for bedding the mould. 5. Impregnation of the reinforcement fibres When realising the TEP procedure a central task was to achieve rapid impregnation of the reinforcement fibres also at low impregnation pressures. To aid in achieving this task fundamental investigations were carried out. After completion of many centrifugation tests it could finally be proven that such an impregnation is possible for a suitable choice and arrangement of the reinforcement material. This is possible through characteristic forming of flow channels. Microscope photographs prove that the inhomogeneous fibre distribution of the dry wound reinforcement material is retained also when applying a low impregnation pressure. The fibre bundle structure of the semi-finished material used made of twisted glass fibre yarn or roving is retained to a large extent. Impregnation of the reinforcement material takes place in two stages due to agglomeration of the filaments in fibre bundles. The thermoplastic melt moves primarily as a macroscopic flow through the channels between the fibre bundles and afterwards penetrates locally into the bundles as a microscopic flow (Fig. 3). The flow channels between the fibre bundles have relatively large cross-sections. Thus the macroscopic flow of thermoplastic melt can penetrate rapidly into the fibre layer, that is, impregnate over long distances. During the following process of impregnating the insides of the fibre bundles the channel cross-sections between the individual filaments are comparatively small. The microscopic flow is thus characterised by a significantly lower flow rate. However, since only short impregnation distances, of the order of magnitude of the fibre bundle radius, are considered short impregnation times are possible.
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Fig. 3. Impregnation of a fibre layer consisting of twisted yarns or rovings. Microscope photographs show an inhomogeneous fibre distribution, which causes formation of macroscopic and microscopic flows. In the mathematical model the fibre bundles are assumed to be ellipses with different eccentricities.
The impregnation time needed to completely impregnate a fibre layer can be computed according to the Darcy’s law [3]. As the centrifuge procedure causes an impregnation pressure working in radial direction only it can be expressed in the cylindrical coordinate system as S dp vr ¼ l dr
ð2Þ
where vr is the average flow rate in the radial direction, S is the transverse permeability, l is the viscosity of a Newtonian fluid and dp/dr is the derivative of the impregnation pressure with respect to the radial coordinate. The viscosity l in Eq. (2) is assumed as a constant value for a Newtonian fluid. In real flow processes like injection moulding the molten thermoplastic mass behaves like a generalized Newtonian fluid where the viscosity depends on the flow rate and is not constant anymore. However, in [2] it is shown that because of the very low velocity of the fluid during the centrifuge procedure of the TEP centrifugal process a difficult analytical modification of the Darcy’s law for a flow of generalized Newtonian fluids is not necessary. One can use the viscosity at zero rate of shear instead. The permeability S used in Eq. (2) is a measure of the permeance of the porous fibre layer. There are a number
of available formulations for the calculation of the permeability of a flow perpendicular to the fibres. Among the most well-known formulations are the equation of Kozeny–Carman [4], the formula of Gebart [5] and the formula of Gutowski et al. [6]. All these formulae produce similar predictions as shown in Fig. 4 where the dependence of the calculated permeabilities on the fibre volume fraction u is plotted. Also pictured in the diagram are the results of extensive measurement series. The measured data which are shown here were determined with a measuring apparatus – like a capillary rheometer – but instead of a capillary it was utilized a fibre layer as specimen [2]. Fig. 4 makes clear that the formula of Gutowski et al. is a good approximation for calculation the permeability when using the so called Gutowski constant kzz ffi 0.2. Several researchers [6– 10] worked out the analogous value kzz ffi 0.2 using the formula of Gutowski et al. which is given below: qffiffiffiffiffiffiffi 3 umax 1 u S qffiffiffiffiffiffiffi S ¼ 2 ¼ ð3Þ umax R 4 k zz þ 1 u with umax ¼ p4 for square fibre array, umax ¼ 2pp ffiffi3 for hexagonal fibre array. Here S* is the dimensionless or normalized transverse permeability. From Eq. (3) the permeability S
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1 Normalized transverse permeability S* [-]
Gutowski (kzz = 0,2), hexagonal fibre array 0.1
Gebart, hexagonal fibre array
0.01
Kozeny-Carman (kz = 11) Gutowski (kzz = 0,2), square fibre array
0.001 Gebart, square fibre array Measured data: • Twisted yarn (Rfb = 0,154 mm ; λ = 1,5) Roving (R fb = 0,268 mm ; λ = 9,5)
0.0001
0.00001 0
0.2
0.4 0.6 Fibre bundle volume fraction ϕmakro
0.8 [-]
1
Fig. 4. Comparison of different formulations for the calculation of the permeability perpendicular to the fibres as a function of the fibre bundle volume fraction with measured values.
still depends on both the actual fibre volume fraction u and on the square of the characteristic radius R. For the calculation of the permeability Smacro, which is active during the macroscopic flow, the radius R is assumed to be the fibre bundle radius Rfb and the fibre volume fraction is used to be u = umacro while for calculation the permeability Smicro for the microscopic flow R is assumed to be the filament radius Rfil and the fibre volume fraction is used to be u = umicro. The relationship between the different fibre volume fractions is given by ugl ¼ umicro umacro
ð4Þ
where ugl is the global fibre volume fraction of the whole fibre layer, umicro is the fibre volume fraction inside of a fibre bundle and umacro is the fibre bundle volume fraction, which is defined as the ratio of the volume of all fibre bundles to the volume of the whole fibre layer. ugl can be measured with the
determination of loss on ignition [11], umicro can be determined under a microscope by counting out the filament sections inside of a reference cross-sectional area and finally umacro can be calculated by division of the two values. The fibre bundles are deformed due to the impregnation pressure, which is preferentially applied perpendicularly to the fibres. The thus affected cross-sectional shapes of the bundles of twisted yarn or roving are modelled as ellipses with different eccentricities (Fig. 3). Using a formulation of Van West et al. [9] an equivalent fibre bundle radius Req can then be computed in each case as a function of the ellipse axes a and b: 2 pffiffiffi ab Req 2k a ð5Þ with k ¼ ¼ 2 Req ¼ 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) 2 b R 2 k þ 1 fb a þb Assuming that the cross-sectional areas of the circular and the elliptical shapes are equal, the ratio of the equivalent
1
Capillary pressure Δpc
[bar]
PA 6-matrix: • Temperature: 250˚C Fibre layer: • Material: E-glass fibres • Sizing: TD22
0.1
ϕ = 0,7
ϕ = 0,5
ϕ = 0,8
ϕ = 0,6 0.01
Range of microscopic flow 0.001 0.001
Range of macroscopic flow
0.01 Characteristic radius R
0.1
1
[mm]
Fig. 5. Computed value of the capillary pressure of a molten PA 6 matrix arising inside a glass fibre layer with a TD22 hybrid sizing.
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fibre bundle radius Req to the fibre bundle radius Rfb can be expressed as a function of the ellipse axis ratio k subsequently also called eccentricity. In summary the result of this method of calculation is that for increasing fibre bundle eccentricity k the necessary impregnation time is increased for the macroscopic flow. Because the equivalent fibre bundle radius is reduced and therefore the permeability for the macroscopic flow depending on the square of the fibre bundle radius reduces as well. In other words the flow channels for the macroscopic flow become smaller. For increasing fibre bundle eccentricity the impregnation time of the microscopic flow is shortened due to the reducing of the equivalent fibre bundle radius the impregnation distances inside of the fibre bundles become shorter. During the impregnation procedure inside the fibre bundles additionally capillary effects arise that are no longer of negligible order of magnitude due to the very small flow channels. In [2] several formulations for the calculation of the capillary pressure are compared and it is shown that the simple formula of Connor et al. [12] which is based upon an energy balance represents a quite good approximation. 2 r cos h u Dpc ¼ ð6Þ R 1u In Eq. (6) the capillary pressure Dpc depends on the surface tension of the molten matrix r, the wetting angle of contact between the molten matrix and the fibres h, the characteristic radius R and the actual fibre volume fraction u. Determining the surface tension r and the contact angle h extensive measurements are carried out in [2]. After this study the exemplary value of the surface tension of a molten PA 6 matrix with a temperature of 250 C is about 40 · 103 N/m and the contact angle between the PA 6 matrix and the glass fibres with a TD22 hybrid sizing is approximately 35. In Fig. 5 the dependence of the calculated capillary pressure on the characteristic radius
for different fibre volume fractions is plotted. The diagram clearly shows that during the centrifuge procedure the capillary pressure is relevant only for the microscopic flow (Dpc ffi 0.03–0.6 bar), while it is negligible for the macroscopic flow (Dpc ffi 0.02–0.001 bar). In the computation of the impregnation time for the microscopic flow this capillary pressure Dpc is therefore added to the impregnation pressure DpRot. Finally it is assumed that impregnation of the fibre bundles begins only after macroscopic flow around all the bundles has taken place. Hence the total time needed for a complete impregnation of the reinforcement material tim is given in Eq. (7) by the sum of the impregnation times for the macroscopic flow tim,macro and the microscopic flow tim,micro: k2 þ 1 2k tim;macro þ 2 tim;micro 2k k þ1 2 1 l ð1 umacro Þ d F;i with: tim;macro ¼ 16 S macro DpRot " # 2 2 d F;e d F;e d F;e 2 ln þ1 d F;i d F;i d F;i
tim ¼
tim;micro ¼
1 l ð1 umicro Þ R2fb 4 S micro ðDpRot þ Dpc Þ
ð7Þ
where dF,i is the internal pipe diameter of the fibre layer (which is analogous to the external diameter of the thermoplastic core pipe) and dF,e is the external pipe diameter of the fibre layer (which is analogous to the external diameter of the fabricated FRP pipe). The first line of Eq. (7) also contains the already mentioned influence of the needed impregnation times for the macroscopic and for the microscopic flow on the shape of the fibre bundles using terms with the eccentricity k. The complete derivation of the approximate formula above for the calculation of the impregnation time tim inclusive all the utilized formulas is worked out in [2].
6
tim, λ > 1 / tim, λ = 1
[-]
5 10 5 3 2 1.5
4 3
t im ,m acro t im ,m icro
2
=1
0.5
1 0 1
2
3
4 5 6 Eccentricity λ = a / b
7 [-]
8
9
10
Fig. 6. Influence of the eccentricity k on the calculated impregnation times tim for complete impregnation of the reinforcement material.
M. Ehleben, H. Schu¨rmann / Composites Science and Technology 66 (2006) 2601–2609
Design data • Wall thickness: 5 mm • Diameter: 100 mm • Matrix density: 1,13 g/cm³ • Viscosity: 119 Pa s • Fibre volume fraction: 50%
12
Impregnation time
[min]
10
Macroscopic flow Roving
Macroscopic flow Twisted Yarn
8
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Σ Macroscopic + microscopic flow Roving
6 4
Microscopic flowTwisted Yarn
2
Microscopic flowRoving
Σ Macroscopic + microscopic flowTwisted yarn
0 0
2000
4000 Rotation rate n
6000 [1/min]
8000
10000
Fig. 7. Computed value of the impregnation time for a centrifuged fibre reinforced plastic pipe as a function of the rotation rate. Reinforcement material: twisted glass fibre yarn and roving (tex number 408 g/km, filament diameter 11 lm), impregnation medium: PA 6 melt.
Fig. 6 makes clear the dependence of the eccentricity on the calculated impregnation times for complete impregnation of the reinforcement material. The diagram shows the ratio of the impregnation time tim,k>1 (for a layer consisting of elliptical fibre bundles) divided by the impregnation time tim,k = 1 (for a layer with circular fibre bundles) depending on the eccentricity k for different quotients of tim,macro/ tim,micro. Particularly when the quotient tim,macro/ tim,micro > 1 which follows from a high fibre layer thickness with small fibre bundle cross-sections the needed impregnation time for complete impregnation tim is increased for increasing fibre bundle eccentricity k. In order to reach a short impregnation time such reinforcement fabrics are suitable which keep an almost circular cross-sectional shape of the fibre bundle also during the impregnation process. In Fig. 7 examples of computed values of the impregnation time for the macroscopic and microscopic flows as well as the resulting total time are plotted against the rotation rate. Here the impregnation times for reinforcement materials made of twisted glass fibre yarn and glass fibre roving are compared with otherwise identical process parameters. For impregnation of the twisted yarns the total time consists of about equal contributions from the impregnation times for the microscopic and macroscopic flows especially in the higher speed range. However, the total time for complete impregnation of the roving is determined almost exclusively by the time needed for the macroscopic flow because of the higher eccentricity of the elliptical fibre bundles. Altogether reinforcement layers made of twisted yarn can be impregnated significantly faster than roving. The twist of the filaments in the twisted yarn leads to retention of an almost circular cross-sectional shape of the fibre bundle also under the impregnation pressure. Thus flow channels of large cross-section favourable to macroscopic flow can be
formed. A 5 mm thick reinforcement layer can be completely impregnated in 3 min at a rotation rate of 6000 1/min, which is still realisable without great technical expenditure.
Fig. 8. Carbon and glass fibre reinforced pipes with PA 6 matrix manufactured using the TEP centrifugation process.
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Fig. 9. Comparison of the surface profiles of a glass fibre reinforced wound GF-EP pipe with a GF-PA 6 pipe manufactured using the TEP centrifugation process.
However, the computed impregnation times can serve only as approximate values since simplifying assumptions were made in their derivation. Extensive measurements executed in [2] have proved that an impregnation of all fibre bundles free from pores could only be reached after the double calculated impregnation time tim according to formula (7). The main reason for this deviation is the assumption made in Eq. (7) that requires a completely homogeneous distribution of the fibre bundles all over the reinforcement layer. However, in real reinforcement layers local regions with a higher fibre volume fraction and a lower permeability occur where a higher impregnation time is needed. Nevertheless the approximate formula (7) is quite suitable for dimensioning of material and processing parameters and for estimating the influence of special single parameters on the needed impregnation times for the macroscopic and for the microscopic flow. Furthermore the TEP centrifugation process can realize quasi-non-porous composite laminates only with a production under vacuum because otherwise inside the fibre bundles voids are enclosed and have no possibility anymore to escape. In principle, using the TEP centrifugation process with suitable choice of reinforcement material and processing variables an excellent non-porous impregnation of the fibre reinforcement is attainable in relatively short production times. 6. Characteristics of the centrifuged pipes Pipes manufactured using the TEP centrifugation process can be strengthened with directional fibre reinforce-
ment at an arbitrary angle with a high fibre volume fraction of from 50% to 60%. The centrifuged pipes have close tolerances concerning roundness and wall thickness. They can be manufactured with high quality smooth, shiny external surfaces (Fig. 8). Even for the internal surface, which is formed without mould contact, extremely small surface roughness are produced. Fig. 9 shows a comparison of the surface profile with that of a thermosetting polymer pipe produced conventionally using wet filament winding. While the average surface roughness of the wound pipe is about 19 lm, a centrifuged pipe can be manufactured with a surface roughness of for instance 1 lm, which corresponds to a honed surface. First results of measurements showed that pipes manufactured using the TEP procedure achieve the same static torsional strength as pipes manufactured conventionally using the wet filament winding with high quality epoxy resin matrix [13]. 7. Fields of application The TEP centrifugation process can be adapted to many fields of application. The following applications appear particularly suitable: • Drive shafts made of fibre reinforced composites for passenger automobiles and trucks are an interesting alternative to steel shafts due to their outstanding vibration characteristics. Pipes centrifuged using the TEP procedure can be manufactured with very small eccentricities and weldable remouldable thermoplastic matrices offer perspectives for various loading situa-
M. Ehleben, H. Schu¨rmann / Composites Science and Technology 66 (2006) 2601–2609
tions. Recycling of the thermoplastic materials is possible against the background of the end-of-the-life vehicle’s directive. • Internally pressurised hydrocylinders in lightweight constructions are particularly interesting for mobile hydraulic systems in crane vehicles and aircraft. The centrifuged pipes can be manufactured with very good surface quality so that sufficiently good surfaces are attainable also without finishing. • For bearings made of fibre reinforced plastic there is the possibility of centrifuging special tribological layers. Such a layer can be provided with teflon or molybdenum sulfide particles, for instance, in order to produce a low coefficient of friction.
8. Conclusions Only a relatively low impregnation pressure can be achieved using the TEP centrifugation process in a revolution range that is still realisable at justifiable structure mechanical expense compared with other manufacturing methods. Using suitable reinforcement materials it is nevertheless possible to completely impregnate the fibre reinforcing with a very viscous thermoplastic melt in a short cycle time. The TEP centrifugation process is thus a new procedure suitable for economically mass producing directed fibre reinforced precision pipes with thermoplastic matrices.
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References [1] Schuermann H, Kampke M. Manufacturing process for a tube made of fibre reinforced plastic, patent application, 1993. [2] Ehleben M. Herstellung von endlosfaserverstaerkten Rohren mit thermoplastischer Matrix im Schleuderverfahren. Dissertation TU Darmstadt. Aachen: Shaker, 2002. [3] Bird RB, Stewart WE, Lightfoot EN. Transport phenomena. New York, Chichester, Brisbane, Toronto, Singapore: Wiley; 1960. [4] Carman PC. Trans Inst Chem Eng 1937;15:150–66. [5] Gebart BR. Permeability of unidirectional reinforcements for RTM. J Compos Mater 1992;26(8):1100–33. [6] Gutowski TG, Cai Z, Bauer S, Boucher D, Kingery J, Wineman S. Consolidation experiments for laminate composites. J Compos Mater 1987;21:650–69. [7] Kim TW, Jun EJ, Um MK, Lee WI. Effect of pressure on the impregnation of thermoplastic resin into a unidirectional fiber bundle. Adv Polym Technol 1989;9(4):275–9. [8] Seo JW, Lee WI. A model of the resin impregnation in thermoplastic composites. J Compos Mater 1991;25:1127–42. [9] Van West BP, Pipes RB, Advani SG. The consolidation of commingled thermoplastic fabrics. Polym Compos 1991;12(6):417–27. [10] Kerbiriou V. Impraegnieren und Pultrusion von thermoplastischen Verbundprofilen. Dissertation, Universitaet Kaiserslautern. Duesseldorf: VDI Verlag, 1997. [11] ISO 1172: Textile-glass-reinforced plastics, prepregs, moulding compounds and laminates – determination of the textile-glass and mineral-filler content – calcination methods, 1996-12-01. [12] Connor M, Toll S, Manson J-A. On surface energy effects in composite impregnation and consolidation. Compos Manufact 1995;6(3–4):289–95. [13] Schuermann H, Ehleben M. On the production of continuous fibre reinforced thermoplastic tubes by a centrifugal casting process. In: Manual of the 2nd International AVK-TV Conference, Baden-Baden, 1999.
Review
Manufacturing of centrifuged continuous fibre-reinforced precision pipes with thermoplastic matrix Max Ehleben a, Helmut Schu¨rmann b
b,*
a Group Research Materials, Volkswagen AG, 38436 Wolfsburg, Germany Department of Lightweight Design and Construction, Darmstadt University of Technology, Petersenstrasse 30, 64287 Darmstadt, Germany
Received 2 September 2005; received in revised form 9 March 2006; accepted 14 March 2006 Available online 15 May 2006
Abstract The paper describes details of a procedure developed for manufacturing continuous fibre reinforced precision pipes with thermoplastic matrices, using centrifugation technology. The centrifuged pipes are almost free of eccentricity and have high quality surfaces, both at the inside and at the outside. They are therefore particularly suitable as drive shafts, bearings or hydrocylinders. 2006 Elsevier Ltd. All rights reserved. Keywords: FRP-pipes; Centrifugal manufacturing; Thermoplastic matrix; Impregnation process
Contents 1. 2. 3. 4. 5. 6. 7. 8.
Introduction . . . . . . . . . . . . . . . . . . . . . . TEP centrifugal process . . . . . . . . . . . . . . Multiple use of the thermoplastic material . Calculation of the impregnation pressure . . Impregnation of the reinforcement fibres . . Characteristics of the centrifuged pipes . . . Fields of application . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction Pipes made of FRP (fibre reinforced plastics) have a number of advantages that mean that they can replace steel or aluminium in structural constructions. Additional advantages result if a thermoplastic matrix is used. The *
Corresponding author. Tel.: +49 6151 162160; fax: +49 6151 163260. E-mail addresses: [email protected] (M. Ehleben), [email protected] (H. Schu¨rmann). 0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.03.015
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material has a practically unlimited shelf life as well as excellent industrial hygiene. It can be welded and later reformed and it can be recycled to a high degree. However, due to the usually high melt viscosities of thermoplastics fibre impregnation presents a special problem. For this reason so far no economical manufacturing method has been available for production of continuous fibre-reinforced precision pipes with high quality surfaces. In order to close this gap a new procedure was developed at the University of Technology Darmstadt.
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2. TEP centrifugal process
3. Multiple use of the thermoplastic material
The thermoplastic endless fibre reinforced pipes (TEP) centrifugal process is a variant of the conventional centrifugal technique [1]. The operational sequence consists of five manufacturing steps (Fig. 1):
The concept behind the TEP centrifugal process is based on multiple use of the thermoplastic:
1. The reinforcement fibres are wound on a non-reinforced thermoplastic pipe. The thermoplastic pipes used here can be manufactured conventionally in an extruder or an injection moulding process. The fibres are dry wound and aligned exactly according to the load the component is designed to carry during normal service. 2. The thermoplastic pipe with reinforcement material wound on it is then inserted into the centrifuge mould. In this way an appropriate fibre orientation is achieved in the centrifuge mould. 3. The thermoplastic core pipe is heated during slow rotation until the thermoplastic material is melted and functions as matrix material. 4. The actual centrifugation procedure takes place in the fourth step. The rotation rate of the centrifuge mould is increased. Due to centrifugal force the thermoplastic melt penetrates the fibre reinforcement and impregnation takes place. 5. The last manufacturing step is cooling and removal of the fabricated FRP pipe.
• The thermoplastic pipe is used as the winding mandrel making acquisition of an expensive steel mandrel with polished surface unnecessary. In contrast to the conventional centrifugal casting process this makes it possible to place the fibre reinforcement by filament winding according to the load and to insert it into the centrifuge. • The second task of the thermoplastic core pipe is to impregnate fibre material with melted thermoplastic that functions as matrix material. • With a correctly adjusted thermoplastic surplus a pure matrix layer is left on the inside of the pipe which can be used as a wear resistance layer or sealing layer or the like.
4. Calculation of the impregnation pressure During the centrifuge procedure an impregnation pressure results from centrifugal force that presses the matrix melt into the fibre layer. Inside of a thick-walled pipe the impregnation pressure is time dependent which changes its value within the duration of the impregnation process
Fig. 1. Operational sequence of the TEP centrifugal process.
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Impregnation pressure ΔpRot [bar]
5
2603
Internal diameter of matrix pipe dM,i = 200 mm
4
150 mm Design data • Wall thickness of matrix pipe sM : 4 mm 1,13 g/cm³ • Density of matrix pipe ρM :
3
100 mm
2 50 mm 1
0 0
2000
4000
6000
Rotation rate n
8000
10000
[1/min]
Fig. 2. Dependence of the impregnation pressure for a fibre reinforced plastic pipe with a PA 6 matrix as a function of rotation rate and diameter.
due to the radial movement of the molten thermoplastic mass. However, thin-walled pipes are used for most practical applications and the much lower radial mass translations inside of thin-walled pipes during the impregnation process lead to a negligible change of the impregnation pressure. This calculation of the impregnation pressure of a thin-walled pipe is much easier because the impregnation pressure is no longer time dependent and hence can be assumed to have a constant value. The impregnation pressure of a thin-walled pipe due to radial acceleration DpRot can be calculated as DpRot ¼ 2p2 n2 qM sM d M;i
ð1Þ
where n is the rotation rate, qM the density of the matrix, sM the wall thickness and dM,i the internal diameter of the matrix pipe. In Eq. (1) the impregnation pressure depends quadratically on the rotation rate and linearly on the geometrical characteristics (i.e. pipe diameter and wall thickness). Eq. (1) is a suitable approximation formula as long as the ratio dM,i/sM P 10. In that case the maximum deviation from the calculation of thick-walled pipes is less than 4% [2]. In Fig. 2 the dependence of the impregnation pressure on the rotation rate is shown for different pipe diameters for a PA 6 matrix pipe with a wall thickness of 4 mm. From the diagram it is evident that the impregnation pressures attainable by the centrifugation procedure are small compared with other manufacturing methods, for instance moulding techniques with p 7 bar. The lightweight construction advantage of the polymers, i.e. their low density, is a disadvantage here. Thus for instance for a 4 mm thick thermoplastic pipe with a diameter of 100 mm and a rotation rate of 6000 1/min the impregnation pressure is just under 1 bar. Since the pipe geometry is usually given, a clear increase in the impregnation pressure is attainable only through a substantial increase in the rotation rate.
However, this leads to a substantial increase in the cost of the machinery, in particular for bedding the mould. 5. Impregnation of the reinforcement fibres When realising the TEP procedure a central task was to achieve rapid impregnation of the reinforcement fibres also at low impregnation pressures. To aid in achieving this task fundamental investigations were carried out. After completion of many centrifugation tests it could finally be proven that such an impregnation is possible for a suitable choice and arrangement of the reinforcement material. This is possible through characteristic forming of flow channels. Microscope photographs prove that the inhomogeneous fibre distribution of the dry wound reinforcement material is retained also when applying a low impregnation pressure. The fibre bundle structure of the semi-finished material used made of twisted glass fibre yarn or roving is retained to a large extent. Impregnation of the reinforcement material takes place in two stages due to agglomeration of the filaments in fibre bundles. The thermoplastic melt moves primarily as a macroscopic flow through the channels between the fibre bundles and afterwards penetrates locally into the bundles as a microscopic flow (Fig. 3). The flow channels between the fibre bundles have relatively large cross-sections. Thus the macroscopic flow of thermoplastic melt can penetrate rapidly into the fibre layer, that is, impregnate over long distances. During the following process of impregnating the insides of the fibre bundles the channel cross-sections between the individual filaments are comparatively small. The microscopic flow is thus characterised by a significantly lower flow rate. However, since only short impregnation distances, of the order of magnitude of the fibre bundle radius, are considered short impregnation times are possible.
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Fig. 3. Impregnation of a fibre layer consisting of twisted yarns or rovings. Microscope photographs show an inhomogeneous fibre distribution, which causes formation of macroscopic and microscopic flows. In the mathematical model the fibre bundles are assumed to be ellipses with different eccentricities.
The impregnation time needed to completely impregnate a fibre layer can be computed according to the Darcy’s law [3]. As the centrifuge procedure causes an impregnation pressure working in radial direction only it can be expressed in the cylindrical coordinate system as S dp vr ¼ l dr
ð2Þ
where vr is the average flow rate in the radial direction, S is the transverse permeability, l is the viscosity of a Newtonian fluid and dp/dr is the derivative of the impregnation pressure with respect to the radial coordinate. The viscosity l in Eq. (2) is assumed as a constant value for a Newtonian fluid. In real flow processes like injection moulding the molten thermoplastic mass behaves like a generalized Newtonian fluid where the viscosity depends on the flow rate and is not constant anymore. However, in [2] it is shown that because of the very low velocity of the fluid during the centrifuge procedure of the TEP centrifugal process a difficult analytical modification of the Darcy’s law for a flow of generalized Newtonian fluids is not necessary. One can use the viscosity at zero rate of shear instead. The permeability S used in Eq. (2) is a measure of the permeance of the porous fibre layer. There are a number
of available formulations for the calculation of the permeability of a flow perpendicular to the fibres. Among the most well-known formulations are the equation of Kozeny–Carman [4], the formula of Gebart [5] and the formula of Gutowski et al. [6]. All these formulae produce similar predictions as shown in Fig. 4 where the dependence of the calculated permeabilities on the fibre volume fraction u is plotted. Also pictured in the diagram are the results of extensive measurement series. The measured data which are shown here were determined with a measuring apparatus – like a capillary rheometer – but instead of a capillary it was utilized a fibre layer as specimen [2]. Fig. 4 makes clear that the formula of Gutowski et al. is a good approximation for calculation the permeability when using the so called Gutowski constant kzz ffi 0.2. Several researchers [6– 10] worked out the analogous value kzz ffi 0.2 using the formula of Gutowski et al. which is given below: qffiffiffiffiffiffiffi 3 umax 1 u S qffiffiffiffiffiffiffi S ¼ 2 ¼ ð3Þ umax R 4 k zz þ 1 u with umax ¼ p4 for square fibre array, umax ¼ 2pp ffiffi3 for hexagonal fibre array. Here S* is the dimensionless or normalized transverse permeability. From Eq. (3) the permeability S
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2605
1 Normalized transverse permeability S* [-]
Gutowski (kzz = 0,2), hexagonal fibre array 0.1
Gebart, hexagonal fibre array
0.01
Kozeny-Carman (kz = 11) Gutowski (kzz = 0,2), square fibre array
0.001 Gebart, square fibre array Measured data: • Twisted yarn (Rfb = 0,154 mm ; λ = 1,5) Roving (R fb = 0,268 mm ; λ = 9,5)
0.0001
0.00001 0
0.2
0.4 0.6 Fibre bundle volume fraction ϕmakro
0.8 [-]
1
Fig. 4. Comparison of different formulations for the calculation of the permeability perpendicular to the fibres as a function of the fibre bundle volume fraction with measured values.
still depends on both the actual fibre volume fraction u and on the square of the characteristic radius R. For the calculation of the permeability Smacro, which is active during the macroscopic flow, the radius R is assumed to be the fibre bundle radius Rfb and the fibre volume fraction is used to be u = umacro while for calculation the permeability Smicro for the microscopic flow R is assumed to be the filament radius Rfil and the fibre volume fraction is used to be u = umicro. The relationship between the different fibre volume fractions is given by ugl ¼ umicro umacro
ð4Þ
where ugl is the global fibre volume fraction of the whole fibre layer, umicro is the fibre volume fraction inside of a fibre bundle and umacro is the fibre bundle volume fraction, which is defined as the ratio of the volume of all fibre bundles to the volume of the whole fibre layer. ugl can be measured with the
determination of loss on ignition [11], umicro can be determined under a microscope by counting out the filament sections inside of a reference cross-sectional area and finally umacro can be calculated by division of the two values. The fibre bundles are deformed due to the impregnation pressure, which is preferentially applied perpendicularly to the fibres. The thus affected cross-sectional shapes of the bundles of twisted yarn or roving are modelled as ellipses with different eccentricities (Fig. 3). Using a formulation of Van West et al. [9] an equivalent fibre bundle radius Req can then be computed in each case as a function of the ellipse axes a and b: 2 pffiffiffi ab Req 2k a ð5Þ with k ¼ ¼ 2 Req ¼ 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ) 2 b R 2 k þ 1 fb a þb Assuming that the cross-sectional areas of the circular and the elliptical shapes are equal, the ratio of the equivalent
1
Capillary pressure Δpc
[bar]
PA 6-matrix: • Temperature: 250˚C Fibre layer: • Material: E-glass fibres • Sizing: TD22
0.1
ϕ = 0,7
ϕ = 0,5
ϕ = 0,8
ϕ = 0,6 0.01
Range of microscopic flow 0.001 0.001
Range of macroscopic flow
0.01 Characteristic radius R
0.1
1
[mm]
Fig. 5. Computed value of the capillary pressure of a molten PA 6 matrix arising inside a glass fibre layer with a TD22 hybrid sizing.
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fibre bundle radius Req to the fibre bundle radius Rfb can be expressed as a function of the ellipse axis ratio k subsequently also called eccentricity. In summary the result of this method of calculation is that for increasing fibre bundle eccentricity k the necessary impregnation time is increased for the macroscopic flow. Because the equivalent fibre bundle radius is reduced and therefore the permeability for the macroscopic flow depending on the square of the fibre bundle radius reduces as well. In other words the flow channels for the macroscopic flow become smaller. For increasing fibre bundle eccentricity the impregnation time of the microscopic flow is shortened due to the reducing of the equivalent fibre bundle radius the impregnation distances inside of the fibre bundles become shorter. During the impregnation procedure inside the fibre bundles additionally capillary effects arise that are no longer of negligible order of magnitude due to the very small flow channels. In [2] several formulations for the calculation of the capillary pressure are compared and it is shown that the simple formula of Connor et al. [12] which is based upon an energy balance represents a quite good approximation. 2 r cos h u Dpc ¼ ð6Þ R 1u In Eq. (6) the capillary pressure Dpc depends on the surface tension of the molten matrix r, the wetting angle of contact between the molten matrix and the fibres h, the characteristic radius R and the actual fibre volume fraction u. Determining the surface tension r and the contact angle h extensive measurements are carried out in [2]. After this study the exemplary value of the surface tension of a molten PA 6 matrix with a temperature of 250 C is about 40 · 103 N/m and the contact angle between the PA 6 matrix and the glass fibres with a TD22 hybrid sizing is approximately 35. In Fig. 5 the dependence of the calculated capillary pressure on the characteristic radius
for different fibre volume fractions is plotted. The diagram clearly shows that during the centrifuge procedure the capillary pressure is relevant only for the microscopic flow (Dpc ffi 0.03–0.6 bar), while it is negligible for the macroscopic flow (Dpc ffi 0.02–0.001 bar). In the computation of the impregnation time for the microscopic flow this capillary pressure Dpc is therefore added to the impregnation pressure DpRot. Finally it is assumed that impregnation of the fibre bundles begins only after macroscopic flow around all the bundles has taken place. Hence the total time needed for a complete impregnation of the reinforcement material tim is given in Eq. (7) by the sum of the impregnation times for the macroscopic flow tim,macro and the microscopic flow tim,micro: k2 þ 1 2k tim;macro þ 2 tim;micro 2k k þ1 2 1 l ð1 umacro Þ d F;i with: tim;macro ¼ 16 S macro DpRot " # 2 2 d F;e d F;e d F;e 2 ln þ1 d F;i d F;i d F;i
tim ¼
tim;micro ¼
1 l ð1 umicro Þ R2fb 4 S micro ðDpRot þ Dpc Þ
ð7Þ
where dF,i is the internal pipe diameter of the fibre layer (which is analogous to the external diameter of the thermoplastic core pipe) and dF,e is the external pipe diameter of the fibre layer (which is analogous to the external diameter of the fabricated FRP pipe). The first line of Eq. (7) also contains the already mentioned influence of the needed impregnation times for the macroscopic and for the microscopic flow on the shape of the fibre bundles using terms with the eccentricity k. The complete derivation of the approximate formula above for the calculation of the impregnation time tim inclusive all the utilized formulas is worked out in [2].
6
tim, λ > 1 / tim, λ = 1
[-]
5 10 5 3 2 1.5
4 3
t im ,m acro t im ,m icro
2
=1
0.5
1 0 1
2
3
4 5 6 Eccentricity λ = a / b
7 [-]
8
9
10
Fig. 6. Influence of the eccentricity k on the calculated impregnation times tim for complete impregnation of the reinforcement material.
M. Ehleben, H. Schu¨rmann / Composites Science and Technology 66 (2006) 2601–2609
Design data • Wall thickness: 5 mm • Diameter: 100 mm • Matrix density: 1,13 g/cm³ • Viscosity: 119 Pa s • Fibre volume fraction: 50%
12
Impregnation time
[min]
10
Macroscopic flow Roving
Macroscopic flow Twisted Yarn
8
2607
Σ Macroscopic + microscopic flow Roving
6 4
Microscopic flowTwisted Yarn
2
Microscopic flowRoving
Σ Macroscopic + microscopic flowTwisted yarn
0 0
2000
4000 Rotation rate n
6000 [1/min]
8000
10000
Fig. 7. Computed value of the impregnation time for a centrifuged fibre reinforced plastic pipe as a function of the rotation rate. Reinforcement material: twisted glass fibre yarn and roving (tex number 408 g/km, filament diameter 11 lm), impregnation medium: PA 6 melt.
Fig. 6 makes clear the dependence of the eccentricity on the calculated impregnation times for complete impregnation of the reinforcement material. The diagram shows the ratio of the impregnation time tim,k>1 (for a layer consisting of elliptical fibre bundles) divided by the impregnation time tim,k = 1 (for a layer with circular fibre bundles) depending on the eccentricity k for different quotients of tim,macro/ tim,micro. Particularly when the quotient tim,macro/ tim,micro > 1 which follows from a high fibre layer thickness with small fibre bundle cross-sections the needed impregnation time for complete impregnation tim is increased for increasing fibre bundle eccentricity k. In order to reach a short impregnation time such reinforcement fabrics are suitable which keep an almost circular cross-sectional shape of the fibre bundle also during the impregnation process. In Fig. 7 examples of computed values of the impregnation time for the macroscopic and microscopic flows as well as the resulting total time are plotted against the rotation rate. Here the impregnation times for reinforcement materials made of twisted glass fibre yarn and glass fibre roving are compared with otherwise identical process parameters. For impregnation of the twisted yarns the total time consists of about equal contributions from the impregnation times for the microscopic and macroscopic flows especially in the higher speed range. However, the total time for complete impregnation of the roving is determined almost exclusively by the time needed for the macroscopic flow because of the higher eccentricity of the elliptical fibre bundles. Altogether reinforcement layers made of twisted yarn can be impregnated significantly faster than roving. The twist of the filaments in the twisted yarn leads to retention of an almost circular cross-sectional shape of the fibre bundle also under the impregnation pressure. Thus flow channels of large cross-section favourable to macroscopic flow can be
formed. A 5 mm thick reinforcement layer can be completely impregnated in 3 min at a rotation rate of 6000 1/min, which is still realisable without great technical expenditure.
Fig. 8. Carbon and glass fibre reinforced pipes with PA 6 matrix manufactured using the TEP centrifugation process.
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Fig. 9. Comparison of the surface profiles of a glass fibre reinforced wound GF-EP pipe with a GF-PA 6 pipe manufactured using the TEP centrifugation process.
However, the computed impregnation times can serve only as approximate values since simplifying assumptions were made in their derivation. Extensive measurements executed in [2] have proved that an impregnation of all fibre bundles free from pores could only be reached after the double calculated impregnation time tim according to formula (7). The main reason for this deviation is the assumption made in Eq. (7) that requires a completely homogeneous distribution of the fibre bundles all over the reinforcement layer. However, in real reinforcement layers local regions with a higher fibre volume fraction and a lower permeability occur where a higher impregnation time is needed. Nevertheless the approximate formula (7) is quite suitable for dimensioning of material and processing parameters and for estimating the influence of special single parameters on the needed impregnation times for the macroscopic and for the microscopic flow. Furthermore the TEP centrifugation process can realize quasi-non-porous composite laminates only with a production under vacuum because otherwise inside the fibre bundles voids are enclosed and have no possibility anymore to escape. In principle, using the TEP centrifugation process with suitable choice of reinforcement material and processing variables an excellent non-porous impregnation of the fibre reinforcement is attainable in relatively short production times. 6. Characteristics of the centrifuged pipes Pipes manufactured using the TEP centrifugation process can be strengthened with directional fibre reinforce-
ment at an arbitrary angle with a high fibre volume fraction of from 50% to 60%. The centrifuged pipes have close tolerances concerning roundness and wall thickness. They can be manufactured with high quality smooth, shiny external surfaces (Fig. 8). Even for the internal surface, which is formed without mould contact, extremely small surface roughness are produced. Fig. 9 shows a comparison of the surface profile with that of a thermosetting polymer pipe produced conventionally using wet filament winding. While the average surface roughness of the wound pipe is about 19 lm, a centrifuged pipe can be manufactured with a surface roughness of for instance 1 lm, which corresponds to a honed surface. First results of measurements showed that pipes manufactured using the TEP procedure achieve the same static torsional strength as pipes manufactured conventionally using the wet filament winding with high quality epoxy resin matrix [13]. 7. Fields of application The TEP centrifugation process can be adapted to many fields of application. The following applications appear particularly suitable: • Drive shafts made of fibre reinforced composites for passenger automobiles and trucks are an interesting alternative to steel shafts due to their outstanding vibration characteristics. Pipes centrifuged using the TEP procedure can be manufactured with very small eccentricities and weldable remouldable thermoplastic matrices offer perspectives for various loading situa-
M. Ehleben, H. Schu¨rmann / Composites Science and Technology 66 (2006) 2601–2609
tions. Recycling of the thermoplastic materials is possible against the background of the end-of-the-life vehicle’s directive. • Internally pressurised hydrocylinders in lightweight constructions are particularly interesting for mobile hydraulic systems in crane vehicles and aircraft. The centrifuged pipes can be manufactured with very good surface quality so that sufficiently good surfaces are attainable also without finishing. • For bearings made of fibre reinforced plastic there is the possibility of centrifuging special tribological layers. Such a layer can be provided with teflon or molybdenum sulfide particles, for instance, in order to produce a low coefficient of friction.
8. Conclusions Only a relatively low impregnation pressure can be achieved using the TEP centrifugation process in a revolution range that is still realisable at justifiable structure mechanical expense compared with other manufacturing methods. Using suitable reinforcement materials it is nevertheless possible to completely impregnate the fibre reinforcing with a very viscous thermoplastic melt in a short cycle time. The TEP centrifugation process is thus a new procedure suitable for economically mass producing directed fibre reinforced precision pipes with thermoplastic matrices.
2609
References [1] Schuermann H, Kampke M. Manufacturing process for a tube made of fibre reinforced plastic, patent application, 1993. [2] Ehleben M. Herstellung von endlosfaserverstaerkten Rohren mit thermoplastischer Matrix im Schleuderverfahren. Dissertation TU Darmstadt. Aachen: Shaker, 2002. [3] Bird RB, Stewart WE, Lightfoot EN. Transport phenomena. New York, Chichester, Brisbane, Toronto, Singapore: Wiley; 1960. [4] Carman PC. Trans Inst Chem Eng 1937;15:150–66. [5] Gebart BR. Permeability of unidirectional reinforcements for RTM. J Compos Mater 1992;26(8):1100–33. [6] Gutowski TG, Cai Z, Bauer S, Boucher D, Kingery J, Wineman S. Consolidation experiments for laminate composites. J Compos Mater 1987;21:650–69. [7] Kim TW, Jun EJ, Um MK, Lee WI. Effect of pressure on the impregnation of thermoplastic resin into a unidirectional fiber bundle. Adv Polym Technol 1989;9(4):275–9. [8] Seo JW, Lee WI. A model of the resin impregnation in thermoplastic composites. J Compos Mater 1991;25:1127–42. [9] Van West BP, Pipes RB, Advani SG. The consolidation of commingled thermoplastic fabrics. Polym Compos 1991;12(6):417–27. [10] Kerbiriou V. Impraegnieren und Pultrusion von thermoplastischen Verbundprofilen. Dissertation, Universitaet Kaiserslautern. Duesseldorf: VDI Verlag, 1997. [11] ISO 1172: Textile-glass-reinforced plastics, prepregs, moulding compounds and laminates – determination of the textile-glass and mineral-filler content – calcination methods, 1996-12-01. [12] Connor M, Toll S, Manson J-A. On surface energy effects in composite impregnation and consolidation. Compos Manufact 1995;6(3–4):289–95. [13] Schuermann H, Ehleben M. On the production of continuous fibre reinforced thermoplastic tubes by a centrifugal casting process. In: Manual of the 2nd International AVK-TV Conference, Baden-Baden, 1999.