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ANALYSIS OF SEEMANN COMPOSITE RESIN INFUSION MOLDING PROCESS(SCRIMP)

Xiudong Sun, Ling Li , Shoujie Li, Jun Ni and L. James Lee

Department of Chemical Engineering

The Ohio State University, Columbus, OH 43210

Abstract

The objective of this paper is to analyze mold filling

and curing in the Seemann Composite Resin Infusion

Molding Process (SCRIMP). Mold filling experiments

were carried out to explore the influence of various

molding conditions on filling pattern and filling time.

Computation models were developed to predict the flow

pattern and filling time. In the cure study, a kinetic model

based on the free radical polymerization mechanism was

developed for simulating the reaction kinetics of a vinyl

ester resin. A heat transfer model combined with the

kinetic model was solved to simulate the cure behavior in

SCRIMP. The effects of room temperature and moldmaterials on the curing process were discussed.

Introduction

SCRIMP [1-2] is a new and promising resin infusion

technology that meets increasingly stringent environment

regulations and is capable of delivering large size

composite parts at low cost. In SCRIMP, the fiber preform

is placed on top of the mold and covered with a plastic

vacuum bag. The vacuum bag is sealed tightly around the

mold periphery. The air in the mold cavity is pulled out

and the resin is infused into the dry fiber preform by

vacuum. The vacuum pump is kept running until the resin

in the fiber preform gels. Generally, there are two types of

resin distribution systems to facilitate the resin infusion as

shown in Figure 1. One involves a highly permeable

medium placed between the fiber preform and the mold

surface, while the other involves grooves cut in the core or

the mold.

Although SCRIMP has already been used in industry

for small quantity productions, the process has not been

thoroughly analyzed. Since SCRIMP applications are often

for very large composite parts like sail boats, refrigerated

cargo boxes and bridge decks, the raw materials used are inlarge quantity and expensive, and the molding process is

often time consuming. To make SCRIMP more

economically attractive, the process needs to be optimized

and the product rejection rate has to be kept very low. This

requires a thorough understanding of both mold filling and

curing, and the development of efficient computation tools

for process design and material selection. Through both

experimental and numerical analyses, this paper will

address the resin flow and curing in the SCRIMP process.

Experimental

Mold Filling

In the first set of SCRIMP mold filling experiments

fiber preform (QM6408 from Brunswick Technologies,

with permeability of Kx = Ky = 0.6e-10 m2 and Kz = 0.12

m2, and thickness of 0.2 cm per layer ,) was placed on a

platform. A layer of high-permeable medium was pl

under the vacuum bag and a peel ply was laid between

permeable medium and the upper surface of the

preform. The length of the fiber preform was 64.77 cm

the width was 11.43 cm. Three fluids were used in

experiment: DOP oil (viscosity 43cp at 24.4 C), extra hMobil oil (viscosity 320cp at 26.2

C) and Mobil BB

(viscosity 530cp at 24.5

C). Since both the top vacuum

and the bottom mold were transparent, with the aid of

tilted mirrors the resin infusion patterns could be recorde

a CCD camera. In the second set of SCRIMP mold fi

experiments, a grooved acrylic plate (3 grooves of 0.317

x 0.3175 cm size, 2.54 cm and 5.08 cm spacing) was use

the 'core', and placed on the glass platform. The inlet cha

perpendicularly linked to all of the grooves was connect

the fluid tank. The fiber preform of the same size as th

the first set of experiments was wrapped around the ac

plate and covered with a large vacuum bag. The ochannel connected to the vacuum pump was 6.35 cm a

from the end of the grooves.

Curing

The resin used in the mold curing is DERAKANE

350 vinyl ester resin from Dow Chemical. Cum

hydroperoxide (CHP) in a solution form (Akzo N

Chemical) was used as initiator and a solution of 6.0 w

cobalt naphthenate (Huls America) was used as prom

The reaction kinetics and the heat of reaction

determined using a differential scanning calorimeter (D

Model 2910, TA Instruments). The reactions were conduin volatile aluminum sample pans which may stan

atmosphere internal pressure after sealing. Iso-the

reactions were measured at different temperatures

various promoter contents, and two scanning runs

carried out successively from room temperature to 30

with a heating rate of 5 C/min to determine the residual

and the baseline. Since DSC can only measure the ov

reaction exotherm, a Fourier transform infrared (F

spectroscopy (Nicolet, Magna-IR 550) was applied


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differentiate overlapped multiple reactions of styrene vinyl

and vinyl ester vinylene groups.

The schematic of the molding set-up is shown in

Figure 2. Under the vacuum bag was a 7.62 cm thick foam

core with precut grooves. The fiber mats (QM6808) were

placed between the composite mold and the foam core.

Thermocouples were placed at the surface of each layer of

fiber mat to follow the temperature change during curing.

Two runs of experiments with different groove design andlayers of fiber mats were carried out. In run 1, the groove

size was 0.3175 cm x 0.3175 cm, the groove spacing was

2.54 cm and three layers of fiber mats were used. In run 2,

the groove size was 0.635 cm x 0.635 cm, the groove

spacing was 3.81 cm, and five layers of fiber mats were

used.

Results and Discussion

Mold Filling

Figure 3 shows the typical flow patterns obtained

from the SCRIMP experiments based on a high-permeable

medium. The flow patterns on both the bottom and the top

surfaces of the fiber preform are like plug flow, which are

in agreement with Darcys law. There is an apparent lead-

lag distance between the top and bottom flow fronts

because of the difference in permeability, which leads to

the transverse flow in the thickness direction. It was

observed from experiments that except a little decrease at

the beginning, this lead-lag distance remained nearly

constant during the filling process. Using Control

Volume/Finite Element Method (CV/FEM) [3], the

continuity equation and Darcys equation governing

SCRIMP mold filling could be solved and the filling timecould be predicted. Table 1 summarizes the comparison of

experimental and simulated mold filling time under various

molding conditions.

Figure 4(a) shows the typical flow patterns obtained

from SCRIMP experiments based on grooves. The filling

time for the one-layer fiber preform was very close to that

for the two-layer fiber preform, but much shorter than that

for thicker fiber preforms. Snapshots of the resin infusion

process on both the top and the bottom surfaces showed

that the grooves were filled first, then the resin infused

from the filled grooves into the fiber preform. For one-

layer fiber mat, the flow fronts on the top and the bottom

surfaces were almost identical. For four to ten-layer fiber

mats, when the bottom surface was almost filled, the top

surface was still dry. There were some transverse flows in

the thickness direction, and the filling became thickness

dependent.

For groove type of SCRIMP, the number of grooves

can be very large for large parts. It is impractical to use

CV/FEM or other conventional numerical methods for

process simulation because of unreasonable computation

time. A simple leakage flow model was proposed to s

this problem. The solution procedure of the leakage

model is as follows: 1). Describe the flow in the groove

one-dimensional flow through a porous medium, w

equivalent permeability is obtained using a correspon

close-form solution [4]. 2). Divide the fiber area between

neighboring grooves into rectangular blocks. Each bloc

treated as a sink, receiving the fluid diffused from the gr

segments. 3). Calculate the pressure distribution in

groove and the leakage flow from the groove toneighboring fiber sinks by using an iteration method. Up

the flow front positions in the grooves and in the sink

every time interval.

The leakage flow model was first compared with

conventional CV/FEM method using a simple case. A g

agreement was found between the two models, and

computation time of the former was 20 times less than th

the latter. The leakage flow model was then used to simu

SCRIMP, and the simulation results were compared wit

actual filling pattern taken from the experiment. Figure

shows the simulated flow patterns obtained from the leaflow model for SCRIMP with three grooves and 4 laye

fiber mats. The prediction of the flow pattern near the bo

side (i.e. the side with grooves) is very good, while

prediction of the flow pattern near the top side is reasona

Curing

Kinetic Characterization

To obtain the needed kinetic information,

isothermal runs were carried out at various temperatures

three different weight ratios of promoter to initiator, 1/10

and 1/3. Figure 5 shows the experimental results of

isothermal reaction rate profiles of the vinyl ester resin the promoter to initiator weight ratio of 1/5. The average

of reaction measured in the isothermal mode is 1475.5

Figure 6 shows the FTIR measurements of the reaction

profiles of styrene vinyl and vinyl ester vinylene groups.

The actual reaction mechanism of styrene-vinyl

system may be complicated. In this study, we assume tha

reaction follows the typical free radical co-polymeriz

mechanism. Based on the diffusion limitation model [

the reaction rates of styrene vinyl and vinyl ester viny

groups are developed as follows:

Styrene vinyls:

4

32

2/12

321

11

1

dt1

d

(1)

Vinyl ester vinylenes:


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'4

'3

'2

2/12

'3

'2

'1

12

1

dt2

d

(2)

where 1, 2, 3, and 4 are lumped-parameters accounting

for kinetic and diffusion effects on propagation and

termination. For a redox type of initiation system such as

cobalt naphthenate and cumene hydroperoxide, the

promoter concentration changes little during reaction.

According to Hiatt et al. [7], the decomposition rate of

initiator can be expressed as follows:

d I

d tk I Ii d p i

1 2/

(3)

The kinetic parameters in Eqs. (1)-(3) are estimated from

isothermal FTIR measurements by using a dynamic non-linear search program. Figures 5 and 6 show the

comparison of the model results with the DSC and FTIR

measurements.

Mold Curing

SCRIMP molding experiments were carried out at

room temperature. Figure 7 shows the measured

temperature profiles inside the composite during curing. It

can be seen that the curing reaction occurred from the very

beginning of molding, and the temperature increased

because of reaction exotherm. Depending on the groove

design and thickness of fiber mats, the highest temperature,which was in the resin rich area, i.e. grooves, varied. In the

thickness direction, the lowest temperature was observed at

the mold surface, which means the reaction would be

slower there.

In practical SCRIMP curing, which starts after the

mold has been filled, only cure reaction and heat transfer

need to be considered in the analysis. The basic equation of

heat transfer is written as follows:

C

T

tk

T

x

T

yGp

( )

2

2

2

2(4)

Where

is density, Cp heat capacity, k thermal

conductivity and G

the heat source term related to

reaction exotherm. Since heat transfer occurs in the foam

core, the grooves, the fiber preform and the mold wall, and

these areas have different thermal properties, separate

governing equations are needed to describe the heat

transfer in each area. These equations coupled with the

kinetic model of the vinyl ester resin were solved by a 2-D

finite difference program using the alternating implicit

method. Figure 7 shows the comparison of the simu

temperature distribution and the experimental results. A

be seen, the prediction fits the experimental re

reasonably well.

In SCRIMP, resin curing occurs at room tempera

Hence, the only heat source is the reaction exotherm o

reactive, room-temperature curable resins. Ideally,

reaction exotherm should be stored in the curing comp

so that a higher temperature can be reached during cuwhich would in turn accelerate the reaction, produce

heat and drive the curing reaction to completion. How

this ideal situation can not always be achieved because o

heat dissipation into the mold and heat loss to

surroundings. In winter when the room temperature is

such heat loss may result in a very long curing time. U

cure simulation, we investigated various factors affectin

SCRIMP curing, such as room temperature and

materials. Figures 8 and 9 show the effect of mold mate

on the temperature and conversion profiles inside

composite at different room temperatures respectively.

this vinyl ester resin, the gelation occurs at the conversio10%. If we assume that demolding can be proceeded w

70% conversion is achieved, Figure 10 reveals

relationship between the gel time and the demold tim

various room temperatures for the composite, aluminum

steel molds. It can be seen that at the same room tempera

the demold time is shorter for the composite mold tha

the steel mold, and the difference becomes significan

lower room temperatures. The gel time is affected by r

temperature, but is not sensitive to mold materials.

ACKNOWLEDGEMENT

The authors would like to thank Hardcore DuPontDow Chemical for financial support and material donatio

REFERENCES

1. W.H. Seemann, U.S. Patent 4,902,215, Feb. 20 (199

2. W.H. Seemann, U.S. Patent 5,316,462, May 31 (199

3. W. B. Young, K. Rupel, K. Han, L. J. Lee, and M

Liou, Polymer Composotes, 12, 30 (1991)

4. J. Ni, Y. Zhao, L. James Lee and S. Nakamura, Pol

Composites, 15, 134(1997).

5. Y. J. Huang and L. J. Lee, AICHE J., 31, 1585 (1985

6. Y.J. Huang and L. J. Lee, Polym. Eng. Sci., 30,(1990)

7. R. Hiatt, K.C. Irwin, and C. W. Gould, J. Org. Chem

1431 (1968)

Key words: vinyl ester resins, SCRIMP, mold fillingand curing


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vacuum bag

fiber

core

fiber

mold

high-permeable

medium

vacuum bag

fiber

fiber

core

mold

groove

(a)

(b)

Figure 1.Resin distribution systems used in SCRIMP.

(a) high-permeable medium, (b) grooves cut in the core

Fiber matlayers

Oil type Viscosity (cp) Filling time (sec)Experiment

Filling time (sec)Simulation

2 Extra heavy 325 807 863.4

2 DOP 40 133 105.9

2 Mobil BB 425 1247 1124.5

3 Extra heavy 270 960 920.2

Table 1. The molding condition and filling time of SCRIMP based on high-permeable medium.

Figure 4. Comparison of flow patterns obtained from experiment and the leakage flow model

for SCRIMP based on grooves, with four layers of fiber mat and fluid of viscosity of 270 cp.

(a) experiment, (b) leakage flow model.

(a)

t=2.0 s t=39.0 s

(b)

t=1.9 s t=40.9 s

top

bottom

(a) (b)

Figure 3. Flow patterns obtained from SCRIMP based on high-permeable medium, with

three layers of fiber mat and fluid of viscosity 270 cp. (a) t=67 s, (b) t=463 s

top

bottom

MoldT1

T2

Groove spacing

(Middle layer)

(Top layer)

Vacuum bag

Groove

(Bottom layer)

Foam core

T3

Figure 2. Schematic of cure experiment


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Figure 7. Comparison of experimental and simulatedtemperature profiles inside the composite

Figure 10. Demold time and gel time as a function

of room temperature for composite, steel and

aluminum mold

0

10

20

30

40

50

60

70

80

90

100

0 30 60 90 120

time (min)

temperature(C)

T1-simulation

T 2

T 3

T1-experiment

T 2

T 3

0

10

20

30

40

50

60

0 30 60 90 120 150 180

time (min)

temperature(C)

composite

steel

aluminum

0

20

40

60

80

100

120

140

160

180

200

0 10 20 30 40temperature (C)

time(min)

composite

steel

aluminum

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

0 30 60 90 120 150

time (min)

r

eactionrate(1/s)

Simulation

25 C

30 C

35 C

45 C

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

0 30 60 90 120

time (min)

reactionrate

(1/s)

Simulation

St, Co+/CHP=1/5

VE, Co+/CHP=1/5

Figure 5. Reaction rate vs. time at different

temperatures(1.5% wt. CHP, Co+/CHP=1/10)

Figure 6. Reaction rate profiles of vinyl ester resins

at 45C measured by FTIR(1.5% wt. CHP, Co+/CHP=1/5)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 30 60 90 120 150 180time (min)

conversio

n

composite

steel

aluminum

Figure 9. The effect of mold materials on the

conversion profile of the composite (fiber thickness

=0.2 cm, i.e. 1 layer, mold thickness=0.635 cm)

Figure 8. The effect of mold materials on the

temperature profile of the composite (fiber thickness

=0.2 cm, i.e. 1 layer, mold thickness=0.635 cm)

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