World Journal Of Engineering
RESIN FLOWING ANALYSIS IN SANDWICH LAMINATES
UNDER VARTM PROCESS
Cheng-Hsien Chung1*, Yu-Ti Jhan2, Ya-Jung Lee2 and Yu-Chieh Wang
1
2
United Ship Design and Development Center, Taipei, Taiwan, chcs@mail.usddc.org.tw
Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, Taiwan
Abstract and Introduction
area. Nevertheless, the race-tracking phenomenon is not
obvious when infusing thick laminates because many
fiber layers delay resin flowing through the lower
laminates, yet the in-plane flowing front has already
saturated the fabrics.
Utilizing a sandwich structure enables FRP assembly
to increase bending stiffness and achieve lightweight
requirements. Core surfaces are often cut and resin
infusion in the sandwich structure differs from pure
fibrous laminates because resin flows rapidly in the
grooves and then saturates into the laminates. This
research performed sandwich structure infusion
experiments under the VARTM process, and defined
four resin saturation stages inside the sandwich assembly
by observing and explaining the nonlinear experimental
flowing fronts. The research also executed infusion
simulations in the 3D sandwich model, including fiber
layers and grooves, to compare with the experimental
results. Realization of the permeant characteristics in the
sandwich structure establishes a base to ensure complete
saturation and to analyze the manufacture of large
sandwich structures.
Fig.1 Oblique drawing of sandwich assembly
Flowing Measurement and Description
Fig.2 (a)upper and (b)lower surface of thin laminate
Several scholars [1-2] have utilized the 1D flowing
Equation (1), derived from Darcy’s Law, to describe the
resin flowing process inside the porous space of fabrics
under the RTM or VARTM method from 1980 to date.
K


 L2
2P t
Because the infusion process starts from the injection
gate on the upper surface, the flowing front is detected
immediately, and the sectional schema of resin infusing
inside sandwich assembly at this time is signified as
Stage 1 of Fig.4. While resin saturates through the upper
fiber laminates and arrives at the middle core, it flows
quickly in the grooves and simultaneously saturates the
upper and lower fiber laminates, as Stage 2 of Fig.4
displays. Stage 2 of Fig.3 exhibits that the flow on the
upper surface slows down because a considerable
amount of resin forks into the grooves. Stage 2 of Fig.3
also shows no resin flowing detection on the lower
surface and the inferred resin saturated profile in each
layer displays in Fig.4. Stage 3 of Fig.3 illustrates that
the appearance of resin on the lower surface until resin
saturates the upper fiber laminates, flowing through the
core and saturating the lower fiber laminates. Therefore,
experimental observation reveals the initial time delay
on the lower surface and the time delay extends as the
thickness of fiber laminates increases. Resin rapidly
saturates the lower laminates because of fast flow in the
grooves of core. Flowing velocity on the lower surface is
even faster than the upper surface, as Stage 3 of Fig.3
displays. Stage 4 of Fig.3 and Fig.4 illustrates the
flowing front on the lower surface gradually catches up
with the upper front. These four stages show that
grooves on the core govern the infusion process of
sandwich assembly and the linear flowing process in
(1)
where ΔP is the pressure gradient,  is the porosity of
laminate, L is the flow front, and t is the infusion time.
Observing the experimental flowing process of the flow
front and the infusion time derives in-plane permeability
(K). Fig.1 shows the oblique drawing of sandwich
assembly and the laminates of sandwich assembly are
symmetric to the middle core material. Grooves cut on
the upper and lower sides of the core are crisscrossed
and intercommunicate between the upper and lower
surfaces. Observation of the 1D flowing experimental
process shows that the flowing front on the upper
surface gradually moves forward due to assistance of the
distribution medium, as Fig.2(a) displays. When
observing saturated conditions on the lower surfaces of
thin laminates (Fig.2(b)), the grille saturating shape
called the race-tracking phenomenon [3] appears. The
cause of this phenomenon is that resin flows in grooves
faster than it saturates inside the fabric porous space.
Therefore, fiber plies near the grooves saturate first and
then resin gradually fills from each circuit of groove
blocks to the center of them. The basis of flowing
velocity judgment is modified in estimating the saturated
235
World Journal Of Engineering
Stage 3 and 4 of Fig.3 is discussed and compared to the
simulation program in the study.
sandwich structure model with channel elements can be
utilized to simulate the infusion process and replace the
time-wasting permeability experiments.
Fig.5 3D sandwich structure model in RTM-Worx
Fig.3 Resin saturating stages in sandwich structures
Fig.6 Lower surface simulation of thin laminates
Fig.4 Resin saturating profiles in sandwich structures
Numerical Simulation
Conclusions
The software RTM-Worx executed in this research
utilizes the CV-FEM to simulate resin saturating
behavior in the porous substance. However, resin
flowing inside the grooves of core, distinct from
saturating into fibrous laminates, is similar to fluid
flowing inside channels. Accordingly, the simulation of
infusing sandwich assembly in this study not only
considers the permeability characteristics of fibrous
laminates, but uses circular pipe elements to substitute
for core grooves including surface grooves and vertical
channels in the model. The cross-section value of core
grooves in the model is the same as the experimental
core material. The 3D sandwich structure model that
contains upper and lower laminates, surface grooves,
and vertical channels is displayed in Fig.5. Material
coefficients of fiber laminates includes thickness,
porosity, and permeability determined by 1D in-plane
flowing experiments [4]. Fig.6 is comparative pictures of
the experiment and simulation on the lower surface of
thin laminates, and comparison of the saturating front
square (L2) versus spending time (t). The difference of
initial time-delay phenomenon on the lower surface
between experiments and simulations is caused by the
limitation of 21/2D calculation. The error of time-delay
increases as the number of laminates increase. The
observed acceleration of infusion speed on the end
experimental process is the resin backflow phenomenon
caused by the resin stopper. The simulation model
excludes the stopper and the backflow phenomenon does
not occur in the simulation process. However, the central
linear flowing region which is not affected by the
boundary conditions exhibit fine agreements between
simulations and experiments, and it clarify that the 3D
The flowing behavior of resin in the sandwich structure
which contains fibrous laminates and core with grooves
is more complicated and different from resin saturated in
the assembly which includes pure fiber laminates. The
appearance of race-tacking phenomenon in experiments
necessitates changing the original infusion velocity
estimation from flowing fronts to the saturation area.
The infusion front measurement is an extremely
important basis for calculating permeability. Four stages
of infusion description inside the sandwich structure
explains the nonlinear infusion and also benefits
executing other manufacturing applications of sandwich
structures in the future. The 3-D model which includes
the upper and lower laminates and core channels is
brought up to simulate the flowing process of sandwich
assembly and different cutting types of core material in
the future to reduce time and cost in performing
experiments.
References
1. Wang, T.J., Wu, C.H. and Lee, L.J. In-Plane Permeability
Measurement and Analysis in Liquid Composite Molding,
Polym. Compos., 15(1994) 278-288.
2. Bickerton, S., Advani, S.G., Mohan, R.V. and Shires, D.R.
Experimental Analysis and Numerical Modeling of Flow
Channel Effects in Resin Transfer Molding, Polym.
Compos., 21(2000) 134-153.
3. Ni, J., Li, S., Sun, X. and Lee, L.J.. Mold Filling Analysis in
Vacuum-Assisted Resin Transfer Molding, PartⅡ:
SCRIMP Based on Grooves, Polym. Compos., 19(1998)
818-829.
4. Lee, Y.J., Wu, J.H., Hsu, Y. and Chung, C.H. A Prediction
236
World Journal Of Engineering
Method on In-Plane Permeability of Mat/Roving Fibers
Laminates in Vacuum Assisted Resin Transfer
Molding, Polym. Compos., 27(2006) 665-670.
237