Methodicalapplication of liquid composite molding simulation€¦ · Methodicalapplication of liquid composite molding simulation Institutfur Verbzmdwerksto#e GmbH, Germany U. Huber,M

Methodical application of liquid compositemolding simulation

Institutfur Verbzmdwerksto#e GmbH, GermanyU. Huber, M. Maier

Abstract

about the stability of the process can be made.range of all input parameters is considered in simulation a reliable statementparameters due to measurement errors or qualitative oscillations. Only when thevalues, significant short comings can be shown for the necessary variation of theseemingly yields an optmal solution under the assumption of constant inputthe aspect of an error tolerant simulation will be discussed.While the simulationdemonstrated on an example taken from practice. Using the front wall of a carBased on these results the methodical optimsation of an injection process will bewith impregnation in thxkness directionof the preform.in thckness direction is considered. The formula is valid for an injection lineto the part thickness), the ratio of the in-plane permeability and the permeabilitypresented. As parameters in ths formula the relative flow path (flow path relatedfor the approximated error between a 2D model and a 3D model will bethe decision for the dimensionof the Finite-Element-Model,an analfic formulaIn a first step some aspects for model choice are highlighted.In order to simplifysimulation in a methodlcalway is discussed.application. In the presented paper the use of liquid composite moldingthere’s only little support for the users of thx software concerningits methodlcaltools for the RTM-process and its variations have been developed,but up to nowsimulation to support the process development. Over the years many simulationTo meet the requirements of short development cycles, there is a need for

1 Introduction

into the mold cavity containing a pre-placed dry fabric preform. Due to relativepolymer composite structures. During RTM a liquid thermoset resin is injectedResin Transfer Molding (RTM) is an efficient process for manufacturing

© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved.Web: www.witpress.com Email [email protected] from: High Performance Structures and Composites , CA Brebbia and WP de Wilde (Editors).ISBN 1-85312-904-6

modelling of the mold filling process is very important.optimsed parameters makes the process development expensive. Thus numericalformation and dry spots. The common trial and error tactic while determiningand in properly designing the mold in order to avoid problems such as voidHowever, in practice, much time is spent m optimsing processing parameterspotential for cost reduction m the fabrication of large parts of complex shape.low injection pressure applied in processing this techmque it is expected to offer546 High Petj&wance Stmctww and Composites

regard to the dimensionality of a simulation such an approach is essential.and benefit (computing time, detemnation of parameters). Particularly withpractice. Furthermore it is advisable, to analyse the proportionality of expensethat the necessary input parameters are nearly impossible to determine inreasonable and can be treated as a waste of time, if it results in the realizationthis. The compilation of a detailed model of an LCM-process is not verysimulation model and about the detemnation of the parameters necessary forprocess. One of these are methodical considerations about the choice of theconsiderations are necessary to be made in order to organise an efficient working

Before bemg able to start with the real work at a simulation, some prelimnary

model and t h s is nomally harder to supply than the 2D variant.very time-consuming. Furthermore a 3D-material-data-set is needed for a 3D-surface data available, consequently the conversion into a volume model can bethe modelling of the geometry of thin-walled structures most often there is onlyelements is less time-consuming and thus also achievable at a lower price. Forto the 2D-variant: h general the simulation of thm-walled structures using shell(2D) simulation. However the 3D-method also has some substantial drawbacksand more detailed outcome in comparison with the simplified 2-dimensional

In principle a 3-dimensional (3D) simulation of a structure gives more precise

simulation are possible.simulation should be preferred if both the 2-dimensional and the 3-dimensionalespecially regarding computation-time and costs. Thus a possible 2-dimensionalof advantage to use a 2-dimensional simulation for thin-walled structuresmiscellaneous 3D-RTM-simulation-tools have been developed lately - it can bewith the number of used nodes. Consequently it has to be realized that - although3d1mensional problem. This results from the CPU-time increasing quadraticallytime, which is much lower for a 2-dimensional computation as for a comparable

A substantial argument for the usage of a shell-model is the computation

2 Concept for the choice of the simulation model

2.1 Difficultieswith model choice

simulation, respectively.and volume elements are available which allow a 2-dimensional, 3-dimensionalmodel, i.e. of the used elements, has to be made in the beginning. Here shell-model on the base of a given geometry. Thus a decision about the type of theWhen processing a RTM-simulation, it is necessary first to generate a simulation


dependence of some of the parameters will influence the results of theanswered whether and to what extent the simplification of the model ma 2-dimensional model. In the following chapters the question has to beat first also in through-thicknessdirection. This can not be taken into account bydetermine. Due to the point injection the fluid has to penetrate the fibre materialHowever the rather thick panel m the middle makes the decision harder toclearly suffice for the very thin panel with the line injection on the left side.clearly only be simulated by a 3-dimensionalmodel, a 2-dimensionalmodel willmaking the choice are indicated in Fig. 1. While the cube on the right side canconcerning the simulation model is not always a simple one. The difficulties inwill be chosen when a model has to be generated. However the decision

According to the expected flow behaviour a shell-model or a volume-modelHigh Pet~jomanceS tmc twes and Composites 547

simulation.

using this criterion are shown with a simple experiment.for other cases without a thorough examnation. The dfficulties arising whenpart” comes from the subject of static computations, thus it should not be usedhowever, a volume model should be used. The decision criterion “thin-walledsufficiently accurate results. For more complex 3-dimensional structures,flow in the plane of the part and thus a 2-dimensional model will supplythe decision. For so called “thin-walled parts” it is assumed that there is only

So far only the geometry of the preform has been considered in order to make

2-dimensional 3dimensional~ ~ I , I ~ < : ~ % ; o I I

Fig. 1: Problems with choice of model for flow simulation

2.2 Example of the consequences of a wrong choice of model

seen in Fig. 2.front in dependence of the time is determined. The experimental setup can bea testing fluid is injected via a central point injection and the position of the flowAn injection experiment is made: In a panel of measurements 600 x 200 x 3 m m 3

Experimental setup


548 f l ighPerformance Strzctures m d ~ ' a m p o s i t e ~

preform and not in through-thickness direction.should guarantee that the flow will only be in the direction of the plane of theuniform distmbution of the test fluid over the thickness of the preform. Thisinto the preform right above the circular injection gate in order to p e m t awith only a slight difference. In the first experiment (no. 1) a circular hole is cut(Shell Tellus) with a viscosity of 70 mPas is used. Two experiments are made

The fibre volume fraction is 61 vol-% and as testing fluid a hydraulic oil

(Acryl glass)Upper half of mold

Camera

Lower half of mold

Resin reservoir trap

Fig. 2: Experimental setup for determination of flow behaviour

situation in the real manufacturing process when a point injection is used.flow is produced in the area of the injection gate. This experiment models thethe hole in the centre of the prefom. Thus a locally restricted, 3-dimensionalThe second experiment (no. 2) differs from the f i s t one only from the missing of

result in an identical prediction of the flow front.2-dimensional model will be absolutely identical for both cases, t i s will alsothe two experiments should give similar results. As the simulation when using a

As the structure has to be called ,,thm-walled" m the sense mentioned above,

Results

simulation were made isothermal.larger axis of the resulting ellipsoidal flow front. The experiment and theportrayed. The position of the flow front is exemplary given via the length of thesimulation. In each case the flow front position in dependence of time isrepresent the experimental data, the unmarked curve represents the results of theexperiments. The curves marked with rhombus and square, respectively,The following diagram (Fig. 3) shows the results of the simulation and of the

reason for this is the flow m though-thickness direction which creates a local 3-the second experiment (no. 2), however, differs in many points from that. Thedimensional flow, which is superbly represented by the simulation. The result ofThis is due to the hole above the injection gate which produces a pure 2-

It is evident that the simulation and the first experiment (no. 1) agree all in all.


High Pe$ornmnce Structures and Composites 549

type are crucial. These factors should be analysed and quantified.experiments) supplementary parameters for the choice of the suitable simulationcan be stated that m addition to the geometry (whch was identical in botht h s experiment the production of the simulation is obviously not usable. Thus itdimensional flow, whch retards the advance of the flow fiont considerably. For

50

60

70

80

E0 30

g 40n

EY

0

c

;20J

0

10

0 200 400 600 800 1000 1200 1400

Injectiontime [S]

Fig. 3: Comparison of simulation and experiments

2.3 Modelling for the numerical determination of the dimensionality

denoted as relative error of the 2-dimensional simulation.denoted by absolute error, the variation referring to the total flow time will bevariation of the results of the 2-dimensional and the 3-dimensional model will becompared with the results of the 2-dimensional model. In doing so the absolutethe results of the 3-dimensional simulation are postulated to be correct and are2-dimensional simulation via an example of a line injection. In t h s comparisonparameter study is made, where a 3-&mensional simulation is compared with aIn order to determine the infhence of the locally restricted 3-dimensional flow a

notations is shown below (Fig. 4).complete covering of the injection line by the preform. The panel with the usedinjection over the whole side of the part. Attention should be paid to the

As geometry for the comparison a simple panel part was chosen with line


550 High Perjimzance Structures and ~ompos i t e s

permeabilities:--yRatio of

K xLength of flow path (si)

Flow front/

injection line (b = 2 mm) \ xThickness of preform d

constant pressuremould

Fig. 4: Geometry and denotation of parameters used in simulation

isoviscous newtonian fluid (100 t i a s ) .respectively. As boundary conditions a constant injection pressure (1 bar) andThe given geometry was simulated with shell-elements and volume-elements

range of 1 to 1000 m m .

range of measured values. The advance of the flowfront si was taken to be in apermeabilities KJK, was chosen from the interval [1,100], which matches thepreform d was chosen to have values between 1m and 15 m m , the ratio of thethe advance of the flow S, were varied and observed. The thickness of thepermeability ratio of the plane and of the through-thckness direction (KJK,) and

For the following survey of parameters the thickness of the panel, the

2.4 Discussion of the results

below. The discrepancy of the two model predictions is marked.experiment with a sidewise line injection and the second one an injection fromgives a sideways view of the two panels where the first one represents theThe trend of the error induced by the model is graphically described in Fig. 5 . It

The simulation error is calculated byis defined as the ratio of the flow front position to the thickness d of the panel.to the relative advance of the flow front. This relative advance of the flow frontshows the dependence of the relative simulation error when using a shell-model

In the following graphs the results of the parameter survey is given. Fig. 6

where erel: relative discrepancy of the2D simulation from the 3D simulation.

it is noticeable that even for a large ratio siid the relative error is too large to beadvances of the flow front and for thick parts, i.e. for a small si/d ratio. However,via the functional correlation: ere]- dq.As expected the error is large for shorton the relative advance of the flow front. This dependence can be approximatedbe observed that the deviation ofthe results of the simulation is highly dependent

The two curves result from two different ratios of permeabilities KJK,. It can


High Performance Structures and Composites 55 1

relative advances of the flow front.the permeabilities KJK,. Here the two different curves result from 2 differentneglected. Fig. 7 shows the dependence of the relative error erel from the ratio of

... S:"

+Fig. 5: Comparison of the prediction of 2D- and 3D-model

80

90

100

B 60S

70

k 40eL 50a-

Y

0

10

20

relative flow path s,ld [ l ]0 20 40 60 80 100 120 140 160 180 200

Fig. 6: relative error in dependence of relative length of flow path

this function predicts a slightly enlarged error for a large ratio of permeabilities

Mathematically the correlation can be described by ere,-E. However

The dependence of this ratio is declining for growing values of K,&


60

correlation.(<50), but then ths can be tolerated regarding the rough calculation of the

552 High Perjormmce Stmct~wesand Composites

--_ _ _ ~ ~

l rl ~

50

10

0

1 -relativelength of flow path 30 1 i i II

0 20 40 60 80 100

I -40 20 40 60 80 100

Fig. 7: Dependence of relative error of the ratio of the permeability K.&

Ratio of permeabilitiesKJK, [ l ]

2.5 Characteristic value for the decision for a model

calculated to beinjection is used. This error value will in the following be denoted by Cflow.It isapplication of a 2-dimensional simulation for a specific component where a linecombined into a new characteristic value, which gives the expected error for theThe factors ,,relative advance of flow front" and ,,ratio of permeability" can be

direction.thckness of the part near the injection gate and Kdz is the permeability m x or zHere the characteristic length of the flow is denoted by sn ,d is the characteristic

the outer measurements of t ie part are not relevant.CFlowfor each gate separately. It is important to notice that apart from thicknessgate. Thus m a part with more than one injection gate it is necessary to determinethe maximal flow which is necessary to fill the part based upon the injection

The characteristic length of the flow is an estimated value giving the length of

3 Error tolerating simulation

optimal filling strategy a satisfying solution can not necessarily be achievedthe model and also of the spread of the input parameter. h spite of the apparentlyWhen using the simulation it is important to be aware of the simplifications in


High Perfonnance Structures and ~'omposites 553

parameters is considered.vent placing variant can be unusable, when the whole spread of the simulation[2]). As can be shown in the following example, a front wall of a car, an optimalalways due to an additional consideration of the spread of the parameters ([l],

time, the injection line runs over thewhole width of the part here.a line injection at the upper edge of the part. In order to m m m z e the filling

The following figure (Fig. S) shows the simulation of the filling process using

Node S c o l a r ZConlo"r

Lf&Wldh.B 3 . B90Ewl3--.- R 3 . ~ m m 3

-0 2 . 4 7 6 E m 39 2.829E+03

-0 3. lRRE+03

? 2.12ZEtD3-- 6 1,76llE+O3--- 5 1 . 4 1 5 E t 0 34 1.061E103

1 0.000c.00

n l n = o . m m m x t mV n x i 4.244175E+032:KG?a[S]

1 1 " sFrl"ge-8:

Areas with high risks of voids

Fig. 8: Flow front position, line injection on upper edge, injection pressure: 6 bar

a high risk of voids.be highly sensitive to oscillations of the properties of the preform. This results inpredict the optimal vent position and in addition to that the optimal position willapproximately at the same time over a quite big area. Thus it is difficult toof the part and reach the opposing edge in the lower right and left areaWith t h s injection method the flow fronts stay nearly parallel to the upper edge

behaviour is associated by a prolongationof the filling time by 50% .left comer, thus the vents should be placed in these comers. This improved flowvoids (Fig. 9). The last areas to be filled in the part are now the lower right andthe part, whch makes the flow behave more positively concerning the danger of

Hence the injection line is shortened to half t i e length of the upper edge of

temperature changes uniformly overt i e whole part.behaviour) if the temperature changes. For t i s it has to be assumed that theare only changed quantitatively (filling time) but not qualitatively (fillingtemperature. Nevertheless, the viscosity is only a scalar factor, thus the resultscan be kept low easily. The viscosity, however, reacts sensitively to changes inparameters. The parameter injection pressure is not very critical here, the spread

Now the results are studied with the regards of the spread of the input


554 H i g h Per~jortmnxx Stmcttwr.~and Composites

K,= 3,05. IO-" Line iniectionN o d e S c a l a r 2c o n t o u r-ll S.726Et03

&wen&..

0 LUlOE+00-; 5:206E+02__r 3 1.041E+03-4 I ,5GZE103----+ 5 2.082L+03-6 2.603€+03----+ 7 3.123Et03.- B 3.644EWJ3

9 4.16SEt03-__ 0 4.SBSE103-R 5.20E.Et03

m h = o . ~ m m ~ ~ mb x :G.24S960Et03

K E&%3 [S]

1 1 " sF r l w e 2 :

Fig. 9: Flow front position, line injection on upper edge (half), injectionpressure: 6 bar

\,,,,= 3,50.l0-l'L i n e injection\l

I

pressure: 6 bar, vaned permeabilitiesFig. 10: Flow front position, line injection on upper edge (half), injection

(l+o)and Ky,,,m= K (1-0) yields the flow behaviour plotted in Fig. 10.Now the simulation is repeated for extrema of the permeability. Using K,, = Kdetermined up to a spread of 0=0. 1 5 . Ths spread also meets own expectations.Surveys of different authors ( [ 3 ] , [4]) showed that the permeability can beresults, and thus t i e deviations have to be taken into account for the simulation.Deviations in the anisotropic permeability however lead to qualitatively changed


Bn 8 . 9 9 7 E W

td3w?m-&..R 8.179E102-- 0 7.361E+02-9 6 . 5 4 3 E W

3 1.636EtOZ2 8.17!X+Ol1 O.DLUlEtO0

Fringe-S:M a x 10: 2248n i n ID: 4 9 4Nsx = 9.815150EW2N i n q 0 . 0 0 0 ~ O ~ E W 0

Time [ S ]

Fig. 11: Flow front position, line injection on upper edge (half), injectionpressure 6 bar

optimisation is required here.will probably result in voids for extrema of the permeability. Furtherthe vent in the lower left comer of the part as calculated by the previous model,the edge of the part on the left side almost simultaneously everywhere. PlacingApparently this yields a very instable flow behaviour, as the flow front reaches


stable in spite of a hgh spread of the values for permeability (Fig. 11).If the injection line is shortened even more, results will be produced whch are556 High P e r f o r - m m c e S t n r c t w e s and Composites

4 Conclusion

used CPU-time, as for this an entire 3D-model is needed.results for point injections should be verified and adjusted. A problem here is theimpregnation in through-thckness direction of the preform. In further work thethickness permeabilities. The formula is valid for a line injection withto the thickness of the part) and the ratio of in-plane permeabilities to through-model. Parameters in this formula are the relative flow path (flow path referringapproximate error arising from the use of a 2D-model in comparison to a 3D-model, an analykal formula has been developed which calculates theIn order to simplify the decision about dimensionality of the finite-element-

parameters has been taken into account.statement about t ie process stability can not be made until the spread of all inputvaned due to measuring errors and oscillations in the quality. A trustworthyshortcomings show in the calculated solution if the parameters are necessarilysolution if constant input parameters are assumed whereas substantialpaid to the error-tolerating simulation. Simulation seems to give an optimal

In an example from practice (front wall of passenger car) special attention is

References

1998.Symposium and Exhibition, Part 2, Anaheim, CA, USA 43, pp. 1275-1288,non-isothermal mold filling during RTM. 43rd International SAMPE

[ l ] Padmanabhan, S.K. & Pitchumani, R.: Effects of parameter uncertainties on

Transfer,42(16), pp. 3057-3070, 1999.flow during resin transfer molding. International Journal of Heat and Mass

[2] Padmanabhan, S.K. & Pitchumani, R.: Stochastic modelling of nomsothermal

permeability. Polymer Composites, 19(6), pp. 429-445, 1998Characterization Part 1: A proposed standard reference fabric for

[3] Pamas, R., Howard, G., Luce, T. & Advani, S.G., Permeability

2000.measurements: a nordic round-robin study. Composites, A 31, pp. 29-43,

[4] Lunstrom, T.S., Stenberg, R. &L Bergstrom, R. et al.: In plane permeability


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Methodicalapplication of liquid composite molding simulation€¦ · Methodicalapplication of liquid composite molding simulation Institutfur Verbzmdwerksto#e GmbH, Germany U. Huber,M