JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition - 78

dielectric analysis

Fig. 5: Selection of available sensors for the DEA
288 Epsilon: implantable IDEX sensor (left) and
reusable tool mount sensor (TMS) (right)

be equipped with two classes of sensors, an implantable or reusable type. A
typical implantable sensor is the standard
IDEX (interdigitated electrode) sensor,
consisting of comb electrodes on an inert
film substrate, which can be positioned
at the desired location in the composite
part. The reusable tool mount sensors
(TMS) can be mounted into a mould to
monitor the curing of composites inside
of the cavity. TMS contain an integrated
thermocouple and offer a long service
life due to their mechanical and thermal
robustness (Fig. 5).
The DEA 288 Epsilon is available with
different housings, one for laboratory use
and one for purposes closer to the manufacturing process. The instrument supports up to 16 parallel working channels,
which is an advantage to map the curing
progress through the part thickness or to
detect the flow front during composite
infusion processes. Parallel to the DEA
sensor (IDEX, TMS) connection, each
channel supports a thermocouple to monitor the resin temperature locally. For
further tasks, a wide range of accessories
are available, including a lab furnace for
curing measurements under controlled
temperature conditions. If required, the
lab furnace can be extended with a UV
light source to monitor cross-linking
reactions in photo-active cross-linking
systems. Finally, a lab-scale pneumatic
hot press for the simulation of real compression moulding processes is available,
which can be equipped with tool mount
or monotrode sensors to mould coupon
specimens out of SMC, BMC, neat resins
or prepregs (Fig. 6). The versatile acces-

Fig. 6: DEA 288 Epsilon (lab version) with a DEA furnace, equipped with a UV lamp; pneumatic heated
press with a plate-shaped mould for process-relevant parameters

sories of the DEA 288 Epsilon offer a
comprehensive characterization solution
for the thermoset composite industry.

Application of DEA to RTM
RTM is a resin infusion process in which
a mixture of a liquid resin, hardener and
catalyst is injected into a closed-cavity
mould already containing a dry fibre preform. After curing at high temperature,
the mould is opened and the finished
composite part is removed. RTM is an
out-of-autoclave process that is increasingly used in the high-volume manufacture

of products such as automotive parts and
sporting goods. Material suppliers and
manufacturer focus on reducing cycle
times in order to increase the number of
units produced while lowering manufacturing costs to a competitive level
with metal-based solutions. The cycle
time for an RTM process is governed by
two parameters: resin infusion and resin
curing. DEA sensors provide a detailed
insight into the closed cavity during the
whole process. For process development,
IDEX sensors can be located in different
layers and at different flow lengths of
the textile laminate to monitor the flow
front during filling and the progress of
the subsequent curing stage. To monitor
the curing of composites reinforced with
electrically conductive fibres and fillers
(carbon fibres, carbon black, etc.), the
DEA electrode has to be protected with
a non-conductive layer to avoid short circuits, for which purpose filtered IDEX or
coated TM sensors have been developed.
For mass production, however, integrating reusable tool mount sensors into
the mould is a cost-effective solution to
monitor curing during processing.
Fig. 7 shows three ion viscosity curves as
a function of process time. The middle
layer shows a higher signal than the two
outer layers, which is due to a delayed

Fig. 7: Cure monitoring of an epoxy-carbon fibre RTM process measured with 3 IDEX sensors located on the
top, bottom and middle layers of the laminate

78 jec composites magazine / No88 April 2014

JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #88 - April 2014 - Special JEC Americas Spring Edition