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BERT NEUHAUSCRISTOPH HINSE ET AL.
Many problems that can occur ininjection molding can be avoid-ed
by simulating mold filling in
advance. Current commercial softwaretools are mainly designed to
simulatethermoplastic materials. Simulating reac-tion-injection
molding processes (RIM),though, is much more complicated,which is
probably why there are as yet nosatisfactory solutions for this on
the mar-ket. In view of this, BASF PolyurethanesGmbH, Lemförde,
Germany, and Sim-paTec GmbH, Aachen, Germany, decid-ed to
collaborate on developing a simu-lation tool for polyurethane (PU)
RIMsystems.
The objective was to be able to predictthe filling behavior,
including the fiberorientation, the shrinkage and thewarpage of the
finished part, while allow-
ing for the chemical cross-linking reac-tion. The focus lay
mainly on simulatingfast-curing,quasi-compact
materials.Thedevelopment work was based on Mold-ex3D (manufacturer:
CoreTech SystemCo., Ltd), a commercial software packagefor
computational fluid dynamics.
Description of the SimulationMethod
The simulations are performed on a fieldpart and two other
“test-plate molds.”Simulation comprises stages of fil-ling,
hardening of the material and
RIM Simulation. So far, there has been a lack of simulation
tools suited to
routine polyurethane RIM processes. This could be about to
change: the tool
presented here has cleared the first test hurdles.
Simulating Reaction InjectionMolding Processes
The 75 % filling of aninjection molded frontspoiler
correspondswell with the result ofa simulation (see bot-tom)
(figures: SimpaTec,BASF Polyurethanes)
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© Carl Hanser Verlag, Munich Kunststoffe international
5/2010
Fig. 1. The Simulation of a 75 % filled front spoiler
corresponds well with the real part (see top)
Translated from Kunststoffe 5/2010, pp. 46–48Article as PDF-File
at www.kunststoffe-international.com; Document Number: PE110390
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warpage. In order to be able to accurate-ly depict every aspect
of what happensin reality, only 3-D simulations are per-formed.
The movement of the PU system,which becomes more viscous as
timepasses, can be described mathematicallyin compliance with the
laws of conserva-tion of mass, momentum and energy. Itis important
in this regard to couple theinfluences of viscosity and the
kinetics ofcrosslinking to each other so that a moreaccurate
picture of the filling behaviormay be obtained.
Discretization of the laws of conserva-tion is effected by means
of the finite vol-ume method (FVM), which is now usedsuccessfully
in all areas of flow simula-tion. The flow front is calculated
bymeans of the volume-of-fluid method(VOF).
Time-dependent Filling Study
For a detailed filling simulation, a knowl-edge of the following
material parametersis needed:� The cross-linking reaction as a
func-
tion of time and temperature,� the viscosity,� the thermal
conductivity, and� the heat capacity,each as a function of the
degree of cross-linking and temperature. Two different,established
methods are used to charac-terize the kinetics of the RIM-PU
system.First, a commercial IR spectrometer isused which has been
converted for track-ing the polyurethane reaction underisothermal
conditions. Second, the in-crease in temperature of the
polyurethanesystem during the reaction is measured asa function of
time,and the results are usedto determine the parameters of the
reac-
tion kinetics under assumed adiabaticconditions. The method of
adiabatic tem-perature increase [1] was used especially
for very fast systems. As part of the col-laboration, BASF
Polyurethanes and Sim-paTec managed to develop a cross-linking
model and to implement this in the soft-ware.
The first experiment consists in simu-lating Elastolit R
8719/105LT, a poly-urethane which cures in about 2 s. Forthis, an
automotive front spoiler filled to75 % is compared with the results
of thecorresponding simulation. The real, in-completely filled part
and the simulatedpart match in all important criteria (Titlephoto
and Fig. 1).
To check whether the modeling de-scribes the cross-linking
reaction in suf-ficient accuracy,“injecting into the cross-linking
material” is performed, both inthe experiment and the simulation.
Inother words, injection is performed soslowly that the material
has cured beforethe mold has been completely filled. Thereal part
can be removed after a fillingtime of 1.4 s. With the simulation,
moldfilling stagnates after 1.37 s due to thecross-linking reaction
and the resultantsharp increase in viscosity. The simula-tion is
therefore very adept at reproduc-ing curing of the material during
the shot(Figs. 2 and 3).
Fiber Orientation and Warpage
In the simulation, fiber orientation is de-scribed by the
Folgar-Tucker equation,while the change in orientation to
reflectthe flow conditions is described by themethod of Advani and
Tucker.
To be able to validate the simulatedfiber orientation,
experiments with a per-forated plate are carried out. The mate-rial
used (grade: Elastolit R 4503) has afiber volume fraction of 15 %.
Figure 4shows the fiber orientation in the fullyreacted perforated
plate as measured byultrasound (across the plate thickness)[2]. The
length of the arrows is indica-
Fig. 2. Partially filled front spoiler obtained by “injecting
into cross-linking material” after1.4 s filling time
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Kunststoffe international 5/2010
Fig. 3. Simulation ofthe partly filled frontspoiler. After 1.37
sthe filling stagnates –a result that is almostidentical with the
ex-periment
x
20
cm
16
14
12
10
8
6
4
2
00
y
2 4 6 8 10 12 14 16 18 20 22 24 26 28 cm 32
Fig. 4. Experimentally determined fiber orientation of a
perforated plate with 15 % fiber by volume
© Kunststoffe
Flow direction
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tive of the degree of orientation. Figure 5shows the simulated
fiber orientation ofa perforated plate at the end of filling.Only
areas of extensive orientation, anddegrees of orientation greater
than 0.9(where 1 is optimum orientation), areshown. The comparison
reveals goodagreement between simulation and ex-periment.
The warpage behavior of injectionmolded thermoplastics stems not
onlyfrom the fiber orientation but also fromthe volume change of
the matrix as afunction of pressure and temperature, i.e.pvT
behavior. In reactive PU systems, thebehavior depends also on the
degree ofcross-linking c. The pvT model musttherefore be expanded
by the degree ofcross-linking to yield a pvTc model. Theassumption
here is that the volume de-creases with increase in
cross-linking.
Corresponding experimental meth-ods are being developed with a
view tobeing able to describe the relevant pa-rameters of the
cross-linking-dependentshrinkage regardless of thermal effects.The
pVTc model is validated by mold-ing plate samples, both with and
with-out fibers. The measurements are thencompared with the
simulation results(Figs. 6 and 7). The simulation shows vol-umetric
shrinkage of 0.45 % across thefiber and 0.25 % along the fiber.
Thesevalues agree well with the measured val-ues of 0.48 % and 0.27
%.
Conclusion
As shown, the new model in the Mold-ex3D software package is a
useful tool forsimulating fast-reacting PU-RIM sys-tems. Comparison
between simulationand injection molding shows very goodagreement.
The program can help usersto design components which are perfect-ly
matched to the polyurethane system,and can be used for all compact
poly-urethane systems. It also enables theprocessor to quickly and
inexpensively re-spond to problems in tool design andparts
filling.�
REFERENCES1 Macosko, C. W.: RIM: Fundamentals of Reaction
Injection Molding. Hanser Publishers, New York(1989), pp.
139–145
2 Predak, S. et al.: Faserorientierungsmessung
ankurzfaserverstärkten PUR-RIM-Bauteilen: Kom-bination
zerstörungsfreier Prüfmethoden zurOptimierung von Simulation und
Herstellung-sprozess. Technisches Messen 73 (2006) 11,pp.
617–628
THE AUTHORSBERT NEUHAUS, born in 1967, has been project
manager in the Development Compact Systems de-partment (AD/C) at
BASF Polyurethanes GmbH, Lem-förde, Germany, since 2005 and is
responsible for PU-RIM development.
DR. MAX RÜLLMANN, born in 1972, was incharge of BASF’s Polymer
Physics Unit in the GlobalPolyurethane Specialties Research
Department atBASF Elastogran GmbH from 2005 to 2008 and hasworked
in polymer research at BASF SE, Ludwigs-hafen, Germany, since
2008.
CRISTOPH HINSE, born in 1977, is CEO of SimpaTec GmbH, Aachen,
Germany.
DR. REINHARD HAAG, born in 1960 is managingdirector of SimpaTec
GmbH, Bangkok, Thailand.
BASF Polyurethanes GmbHD-49448 LemfördeGermanyTEL +49 5443
12-0www.pu.basf.de
SimpaTec Simulation & TechnologyConsulting GmbHD-52072
AachenGermanyTEL +49 241 9367-1500www.simpatec.com
Contacti
Fig. 5. Simulationof fiber orienta-
tion. The fiberalignment >91 %
is shown (bluemeans a high
degree of orien-tation)
Fig. 6. Simulation of part deformation in flow direction (z)
Fig. 7. Simulation of part deformation across the flow direction
(z)
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© Carl Hanser Verlag, Munich Kunststoffe international
5/2010
z z
z