Advanced Composites Processing
Exercise no. 6
Spring Semester 2012
Exercise no. 6: Prepreg Laboratory
Model Solution

Lay-up and vacuum bagging
1. Deep corners often give problems to get the prepreg laminate to conform
to shape. Vacuum bag bridging particularly becomes a problem. To aid
this, a pressure intensifier (often a wedge) is inserted, or a matched mould
can be used (as done in this laboratory exercise)
2. Other issues that may arise is getting the prepreg to conform to curved
geometries. Aids include:

Splicing (cutting the prepreg sheets at specific locations

Lightly heating the prepreg with a heat gun to give added flexibility

Debulking (drawing intermittent vacuum) after laying up every 3-4
plies.
3. If sufficient edge (in-plane) and through-thickness breathing is provided
and a clear air path to the vacuum port is present, the location of the port
is not crucial (where typically, edge breathing is considered more dominant
for gas extraction than through-thickness breathing due to prepreg gas
permeability). This is because the limiting factor in gas extraction is the
permeability of the prepreg, which is much lower than that of the breather,
despite any large distance from the laminate edge to the location of the
vacuum port.
Placement of the port however, should be considered issues such as part
mark-off, vacuum bag creases (also mark-off on part) and bag pleats.

Heat transfer and thick laminate exotherm
1. Due to the thickness of the laminate, a thermal gradient between the
top, middle, and bottom of the laminate will exist. The tool temperature
will more closely match the air temperature; however, the prepreg
laminate itself will lag behind due to its thermal properties (interface
thermal conductivity, internal thermal conductivity, and heat capacity).
When the laminate exotherms, the middle of the laminate increases in
temperature the greatest. It will take time for the exothermic heat in the
laminate to dissipate due to the relatively poor thermal conductivity of
the prepreg material. As the excess heat dissipates, the entire laminate
equilibrates to relatively the same temperature.
One remedy to help mitigate the thermal lag issue is to use slow
heating rates, and intermediate dwell(s) (isothermal hold) to reach the
final curing temperature.
2. Vacuum bagging consumables (breather, Teflon, peel-ply, etc.) are all
insulating materials. Therefore, with the exception of using a thick tool,
the thermal insulation provided by these materials will likely cause the
ETH Zurich,
Centre of Structure Technology
B. Louis
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Advanced Composites Processing
Exercise no. 6
Spring Semester 2012
top of the laminate to lag in temperature with respect to the bottom
(tool side) during heating.
3. The thermocouple data obtained in the experiment is shown in Figure 1
below.
Figure 1. Recorded thermocouple temperatures vs time. 50 layers of carbon/epoxy prepreg oven cured at
120°C.
From the thermocouple data, ΔT exotherm recorded was at its peak
over 80°C higher than the target oven temperature (approximately tool
temperature). The maximum exotherm occurred in the middle of the
laminate (with respect to thickness). This also corresponds to the
region that saw the most significant thermal lag during the heating
process.
Exothermic reactions can be beneficial in the cure-cycle as they are a
source of heat (the laminate self-heats), if this heat is accounted for the
in the cure cycle design. Exothermic reactions are a concern for thick
laminates, as the rise in temperature is greater. This poses problems
as the temperature of the laminate in particular locations can therefore
exceed the typical +/- 5°C (high-performance aerospace epoxy
systems) it’s allowed to vary from the target heat cycle curing
temperature. Going outside these bounds (above and below) can result
in degradation of the epoxy and lowering its mechanical properties. In
the laboratory example, the peak temperature was over 200°C, while
the target curing temperature was 120°C.
ETH Zurich,
Centre of Structure Technology
B. Louis
Page 2
Advanced Composites Processing
Exercise no. 6
Spring Semester 2012
4. The thermal issue that may arise is that an air-layer inside the vacuum
bag system may act as an additional insulation layer in addition to the
other vacuum bagging consumables (vacuum bag, breather, Teflon,
etc.).

Debulking and gas evacuation
1. The compaction data from two rotations of the experiment are
summarized below:
Test group rotation #1
ΔDepth
(thickness)
Time
Thickness
0
3.2mm
30sec
3.16
-0.04
1min
3.145
-0.015
2min
3.131
-0.014
5min
3.109
-0.022
Rebound
3.109
/
0
3.1
30sec
/
/
1min
3.07
-0.03
2min
3.051
-0.019
5min
3.018
-0.33
Rebound
3.022
+0.003
Scenario #1
Scenario #2
ETH Zurich,
Centre of Structure Technology
B. Louis
Page 3
Advanced Composites Processing
Exercise no. 6
Spring Semester 2012
Test group rotation #2
ΔDepth
(thickness)
Time
Thickness
0
3.39
30sec
/
/
1min
3.37
-.002
2min
3.3425
-0.0275
5min
3.3175
-0.025
Rebound
3.3475
+0.03
0
3.2
30sec
/
/
1min
3.195
-0.005
2min
3.195
0
5min
3.195
0
Rebound
3.21
+0.015
Scenario #1
Scenario #2
2. There was no major observable difference between the scenarios. For
a significantly observable compaction difference, the laminate samples
would have to be significantly larger. This would increase the in-plane
length of gas transport required; where scenario number two would
take significantly longer to fully evacuate the initial entrapped gases.
Other errors were the way in which the test was performed. The dial
gauge stand in some tests was placed on top of the vacuum bag.
When vacuum was drawn from the bag, this would shift the dial gauge.
Ideally, the dial gauge should be mounted fully independently from the
laminate tool vacuum bag setup. Additionally, the vacuum bag had
slack in the first reading. When vacuum was pulled, the bag is pulled
tight which leads to a large measurement change. This is why the first
reading on most tests were erratic and ignored. To remedy this, a
partial vacuum using a vacuum regular could be used to first pull “soft
vacuum” to “set the vacuum bag in place” (while minimizing the amount
of laminate gas extracted) prior to the first measurement. Lastly, the
dial gauge measurement pin was in direct contact with the vacuum bag
– which was in contact to the laminate surface. Depending on the
surface texture of the laminate, this would also add measurement error.
ETH Zurich,
Centre of Structure Technology
B. Louis
Page 4
Advanced Composites Processing
Exercise no. 6
Spring Semester 2012
To minimize this effect, a smooth caul sheet should have been placed
on top of the laminate.
3. A typical long debulking curve (thickness vs time) could look as follows:
Figure 2. Debulking curve MTM45-1 (carbon/epoxy prepreg) for 1m long edge breathed laminate. Curve
shows full edge breathing, and a scenario of an inhibited edge breathing (breathign edge is partially
blocked). Source: B. M. Louis, "Gas Transport in Out-of-Autoclave Prepreg Laminates," Master of Applied
Science, Materials Engineering, The University of British Columbia, Vancouver, British Columbia, 2010.
Initial compaction is due to nesting of the fiber tows (how the fall into
one another). After this, compaction is due to the removal of entrapped
air, and the collapsing of air void spaces previously occupied with
entrapped air. The observed plateau, or compaction-limit, is due to the
internal structure of the prepreg. Although, the air is extracted, the
internal structure of the evacuated void has enough support (due to the
fiber architecture, and nesting) to not fully collapse despite the removal
of air. Further reduction of the empty void space will only be
accomplished by resin flow in the prepreg during the curing process.
ETH Zurich,
Centre of Structure Technology
B. Louis
Page 5