SAMPE Journal - July/August 2016 - 8
Feature Article
Figure 1. Typical edge breathing
arrangement for a VBO part.
plane of the ply and laminate3 and the bagging
arrangement is such as to facilitate gas extraction at
the laminate edge. A typical arrangement is shown
in Figure 1.
Because in most parts there are some plies
which terminate internally and do not connect to
the laminate edge and breather arrangement, some
8
degree of through thickness gas transport does have
to take place, however the permeability of a prepreg
stack in this direction is significantly less than that
in the plane of the laminate at room temperature
although there is some evidence that this may
change for fabric prepregs at a mildly elevated or
'super-ambient' dwell temperature3 and the transport
of air or volatiles in the through thickness direction
may become more efficient at such temperatures.
Because the final laminate quality is dependent on
the physical extraction of entrapped air and volatiles,
a time dependent process, both the physical size
of the part and the complexity in terms of internal
ply terminations determine how long the cure cycle
needs to be to achieve an acceptable part porosity
result, generally accepted as <2% and preferably
<1% by volume. The format of the prepreg in respect
of the presence of internal evacuation channels also
has a significant effect on the required duration of the
cure cycle. A further factor, albeit one not extensively
documented in the published literature, is that
ambient humidity conditions during part manufacture
have an effect on the amount of moisture dissolved
within the prepreg resin and hence on the amount of
time required under vacuum to remove this potential
source of porosity. The combination of these factors
and uncertainties in how to quantify the associated
effects has inevitably led to conservative cure cycle
practices in much of the initial development work
on large VBO structures. Most notable among the
conservative practices as previously mentioned
is the use of a 16 hour or greater dwell at room
temperature prior to the commencement of the cure
cycle. It is believed that the principal effect of such
a long dwell is to facilitate the removal of dissolved
atmospheric moisture which may constitute a small
SAMPE Journal, Volume 52, No. 4, July/August 2016
http://www.expanded-materials.com
Table of Contents for the Digital Edition of SAMPE Journal - July/August 2016
Contents
SAMPE Journal - July/August 2016 - Cover1
SAMPE Journal - July/August 2016 - Cover2
SAMPE Journal - July/August 2016 - Contents
SAMPE Journal - July/August 2016 - 2
SAMPE Journal - July/August 2016 - 3
SAMPE Journal - July/August 2016 - 4
SAMPE Journal - July/August 2016 - 5
SAMPE Journal - July/August 2016 - 6
SAMPE Journal - July/August 2016 - 7
SAMPE Journal - July/August 2016 - 8
SAMPE Journal - July/August 2016 - 9
SAMPE Journal - July/August 2016 - 10
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