Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
Exercise no. 3: Liquid Composite Molding – RTM & VARI
Model Solution

Injection strategies
1. The geometry shown in figure 1 has to be impregnated without dry spots
by setting one position for the inlet and another position for the outlet. It
was defined, that in this geometry, the inlet and the outlet have to be
placed at the outer circle in opponent locations. The challenge is to set the
points in such a manner that different flow fronts are meeting in the nodes
of the 1D-grid and finally in the position of the vent. Therefore it has to be
avoided, that flow fronts will meet between two nodes.
For learning purposes, two trials were provided to the students.
2. The main idea is that if two cross sections of different cross sectional area
are meeting in one node, the fluid velocity is higher in the thin cross
section than in the thick cross section. If an injection is directly at a node,
the outflows of this node are decoupled, meaning, the flow velocity in the
adjacent 1D-channels is the same.
Regarding this information, due to symmetry reasons, the best position for
an injection is at the node at the ring adjacent with the 80mm wide domain
(see Figure 2). The flow will have approximately the same velocity during
the first filling phase. As the distances between the nodes is approximately
the same, the flow fronts might meet in the nodes of the ring and the 40
mm domain and 20 mm respectively. A possible problem might appear in
the node of the 20 mm domain and the outer ring: As the ratio of widths
from 80 to 20 mm is quite high, there is a high risk, that the flow front in the
20 mm wide domain reaches the node adjacent to the circle faster than the
outer circle. This problem could be addressed by moving the injection gate
a small amount lower than it is displayed in Figure 2.
3. The continuity equation for RTM-processes states, that in a representative
volume the sum of inflows is equal to the sum of outflows:
𝑄1 − 𝑄2 = 0.
Assuming the same permeability 𝐾1 in the two domains, applying Darcy’s
Law at the node leads to
−
𝐾1 𝐴1
𝐾2 𝐴2
∇𝑝1 +
∇𝑝2 = 0
𝜂
𝜂
𝐴
∇𝑝
⇔ 𝐴2 = ∇𝑝1.
1
2
which shows that the pressure gradient is higher, if the cross section is
lower, therefore also the volume averaged velocity, which is
𝑣𝑖 = 𝑄𝑖 /𝐴𝑖 = −
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Centre of Structure Technology
𝐾1
𝜂
∇𝑝𝑖 , 𝑖 = {1,2}.
C. Di Fratta, B. Louis, F. Klunker
Page 1
Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
4. The part itself usually showed a flow front pattern as it was expected in the
discussions. The challenge, that the flow fronts meet in the node, was not
successfully reached in reality. There were minor deviations which can be
also due to a)modeling problems (RTM vs VARI) and b) possible edge
effects appearing, due to the complex setup.
Figure 1. Part geometry. Lengths are in millimeters.
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
Figure 2: Example of an RTM-flow simulation, where colors represent the fluid pressure and the flow
fronts are meeting in the nodes, the injection point is on the right side

Material and process parameters sensitivity
1. Sketches will vary. An approximate sketch is as follows:
2. In the lab demonstration experiment, fluid fully impregnates the distribution
media (mesh) first. Then the fluid (silicone oil in the lab, resin in real
composite processing) flows from the distribution mesh into the fiber
perform. This occurs predominantly occurs in the through-thickness
direction from the wetted distribution mesh.
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
3. Sample numbers are provided. Velocities will vary from experiment to
experiment.
Apparent Flow Front Velocity
Injection stage
No mesh
Mesh (top)
Mesh (bottom)
Start
5s (0.45cm/s)
3s (2.50cm/s)
4s (1.67/s)
Middle
45s (0.11cm/s)
8s (0.50cm/s)
10s (0.31cm/s)
End
150s (0.06cm/s)
31s (0.26cm/s)
36s (0.36cm/s)
4. Because fiber wetting occurs predominantly in the through-thickness
direction, air entrapment may occur in the corners. This occurs under the
following conditions:
The top portion of the fiber (or distribution mesh) is wetted out on both the
horizontal and vertical flange prior to the in-plane of the fiber being fully
impregnated. The progressing flow front (partial saturation) passes from
the horizontal flange to the vertical flange, without the fiber being fully
saturated. The result is any entrapped air in this region effectively
becomes entrapped now by two converging flow fronts: vertical flange, and
horizontal flange.
Figure 3. Actual in-plane impregnation progress as a result of using distribution media (mesh) in a 90°
corner. Effectively, two flow fronts form, and converge at the corner (gravity effect is also present in the
vertical flange impregnation).
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
Figure 4. Resulting air entrapment at 90° corner due to converging flow fronts.
Possible strategies to avoid this are as follows:
a. Stop the distribution mesh prior to the 90° corner, and let the flow
front progress fully impregnate the fiber in the through-thickness
direction. This is followed by letting the flow front progress in-plane
up the vertical flange. This can be achieved by leaving the vertical
flange bare of distribution media.
b. Again stop the distribution mesh prior to the corner. However, the
distribution mesh may begin again on the vertical flange at some
distance away from the corner region.

Process monitoring
1. The LCM processes in closed molds (e.g. RTM) can be monitored by
suitable a sensing system, using for example cameras, electric
sensors, pressure transducers, ultrasound sensors, thermocouples,
etc. In this experiment, pressure sensors are used to track the flowfront during the process: when the fluid reach the pressure sensors,
their output signals increase; analyzing the pressure values by the
sensors and comparing them to value from simulations it is possible to
estimate the position of the flow-front. Possible “flow irregularities” can
be monitored by an irregular signal from the sensor, in respect to the
ideal response. Variations in the flow pattern can be detrimental to the
final composite parts, since they can lead to voids or dry spots,
resulting in a diminishment of the mechanical qualit: this is why a
process should be monitored.
2. “Runners” are flow channels occurring close to edges or boundary of
the cavity, usually because a bad alignment of the fabrics inside the
mold; this fact leads to a local lack of “material” and so creates
channels where the resin can flow faster. If runners are present, they
will disturb the flow pattern (the flow is uneven and for example the
resin can reach the vent before a full wet-out of the fabrics) and
therefore dry spots can arise. Pressure sensors close to a runner will
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
sense these disturbances by an abnormal increase of fluid pressure, in
comparison with other sensors or with ideal flow conditions.
3. At the beginning, the sensor measures only a constant value equal to
the fabric compaction pressure in dry state. Then the flow arrives and
the pressure decreases initially because the fabric wet compaction
pressure is lower than the dry compaction one. In the same time the
pressure starts to increase again because of the increasing resin
content in the cavity. The actual fluid pressure is equal to zero before
the resin arrival, at roughly 100 s, and then, after that point, it can be
obtained subtracting dotted line (fabric compaction pressure in wet
condition) from the sensor output pressure.
Figure 5. Sensor typical response
4. Since it is a 1D (constant) vacuum driven injection, the pressure
distribution in the cavity is always linear at any time (see graphics
below, with Pi = 1 bar, ΔP = Pi – Pf = 1 bar and xs = 0.55 m). Therefore
the following formula can be used to find the flow front xf and
approximated values are listed in the table below:
PS = Pi − (Pi − Pf )
ETH Zurich,
Centre of Structure Technology
xs
⇒
xf
xf = xs
C. Di Fratta, B. Louis, F. Klunker
Pi − Pf
Pi − Ps
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
Pi
Ps
Pf = 0
xi = 0
xs
xf
Figure 6. Pressure Distribution for each time
Time
Sensor output pressure Resin pressure Flow-front position
150 s
~0.42 bar
0.37 bar
0.87 m
200 s
~0.50 bar
0.45 bar
1m
250 s
~0.54 bar
0.49 bar
1.12 m
5. In Figure 7, the blue curve corresponds to the constant injection
pressure, while the red one is for constant flow rate. These trends can
be deducted by the 1D flow solution. Leaving out mathematical
formulas, when just looking at Figure 8 (pressure distribution along the
laminate at different time), it is noticeable that constant flow rate
injection imply a linear growth of pressure in each points of the
laminate (and thus also at sensor point), because the injection
pressure is linearly increasing with time. In case of constant pressure
of injection, instead, it is clear that the more the resin advances, the
less will be the increment of pressure at the sensor position.
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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Advanced Composites Processing
Exercise no. 3
Spring Semester 2012
Figure 7. Resin pressure at sensor location for constant flow/pressure injection
Figure 8. Constant injection pressure vs. flow rate
ETH Zurich,
Centre of Structure Technology
C. Di Fratta, B. Louis, F. Klunker
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