SAMPE Journal - January/February 2016 - 52
FEATURE
PHD STUDENT
ARTICLE
DIVISION AWARD ARTICLE
Figure 8. Temperature, pressure, and model α for cure cycle H (45 minutes intermediate dwell at 150°C).
The location of these points - to the left of the
corresponding αcrit points - presents an apparent
discrepancy. For all intermediate dwell temperatures,
the pressure drop occurred before the critical degree of
cure was reached. Accordingly, the samples for cycles
C - G all exhibited 2-5% surface porosity. However,
for cycle B, which started the final temperature ramp
8 minutes before the pressure drop would have
occurred at 130°C (as in cycle C), the pressure drop
did not occur until 27 minutes into the ramp toward
185°C, and the surface only developed porosity over
0.12% of the area.
The explanation for this delayed pressure drop
lies in the thermal expansion of the resin during
temperature ramps. In the case of Figure 6 (cycle A),
the cavity pressure increased during the temperature
ramp, but only slightly. Because the resin is initially
liquid and connected to a constant-pressure supply,
as it undergoes thermal expansion in the mold cavity,
the resin has the ability to back-flow through the inlet
port to maintain equilibrium hydrostatic pressure.
However, once the resin has gelled, it loses the ability
to flow. In the case of cycle B, the resin gelled during the
intermediate dwell at 130°C and the cavity pressure
52
began to decrease below the supply pressure due to
cure shrinkage. However, when the temperature was
ramped upwards, thermal expansion temporarily
caused the cavity pressure to increase, delaying the
pressure drop long enough for the cold-side α to
surpass αcrit.
The phenomenon of thermal expansion is critical to
maintaining mold pressure, because after the resin has
gelled, the supply pressure loses all ability to influence
cavity pressure (as clearly evidenced by the pressure
drops that occur in every test). To exploit the effect
of thermal expansion on cavity pressure, we must
first gel the resin at an intermediate temperature, and
only then ramp to the final cure temperature. We can
therefore define the minimum times for intermediate
dwells by αgel, which is indicated by the green points
on Figure 7.
The green shaded region - between the gel
times and the pressure drop times - represents
the allowable "process window" for intermediate
temperature dwells. For any of the temperatures in
the range shown, a dwell shorter than the gel time will
be ineffective (resulting in behavior similar to that of
cycle A), and a dwell that's too long will also result in
SAMPE Journal, Volume 52, No. 1, January/February 2016
Table of Contents for the Digital Edition of SAMPE Journal - January/February 2016
Contents
SAMPE Journal - January/February 2016 - Cover1
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SAMPE Journal - January/February 2016 - Contents
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