optimizing process condition of compression …...interesting from a fluid mechanisms point of view...

OPTIMIZING PROCESS CONDITION OF COMPRESSION MOLDING: FROM MATERIAL PROPERTIES CHARACTERIZATION TO NUMERICAL SIMULATION

C-W. Wang, W-C. Tsai, S-B. Sun, C-H. Hsu and R-Y. Chang, CoreTech System (Moldex3D) Co., Ltd.,

Chupei City, Hsinchu County, 302, Taiwan

Abstract

Herein, we present the recent development in

viscosity measurement by squeeze flow method. We

applied this technique to investigate to fiber reinforced

plastic (FRP) systems including, polypropylene-based

glass mat thermoplastic (PP-GMT), and thermosetting

sheet molding compound (SMC). The effects of

compression rate, temperature and curing time are

systematically studied. In both cases, the squeeze flow

data deviate from simple power law model, and is

analyzed by the approach proposed by Laun et al (J.

Rheol. 1992, 36, 743) [1]. The results demonstrate the

promising potential of viscosity measurement by squeeze

flow method, and great relevance to industrially important

process such as compression molding. The measured

rheological material properties are then used in process

simulation to obtain optimal process conditions of

compression molding.

Introduction

These days modern industries are driven by fuel

consumption, performance and low cost; FRP is a

composite material, which is made of a polymer matrix

reinforced with fiber preform. FRP such as Glass Mat

Thermoplastic (GMT) and Sheet Molding Compound

(SMC) have attracted enormous attentions. These

complex composite materials have found applications in

the automotive, aerospace, electronics, defense, energy,

recreational and home-related industries [2-3].

However, manufacturing process has entered a new

era with sharpened demands on efficient manufacturing of

defect-free products. The computer-aided engineering

(CAE) simulation software is used by manufactures to

help improve product quality, shorten product-market

time and increase production efficiency. One of the key

factors to successful CAE simulation is accurate material

information [2].

Compression molding is considered to be the most-

cost-effective manufacturing methods of load carrying

fiber composites for long and very long production series.

In the SMC/GMT-process a set of male and female molds

are used. They are mounted in a high capacity hydraulic

pressure and heated up to a desirable temperature range

with for example electrical cartridge heaters. When the

molds have reached set temperature, a charge is prepared

consisting sheets of SMC. The size of is general about 20-

90 percent of the mold. Next, the press is closed as fast as

possible to force to charge to fill the mold. The hydraulic

pressure is build up (3-20 MPa) and held for a set period

of time until the desired product shape is molded.

The actual flow during the processing is rather

complex involving, for instance, high temperature

gradients with corresponding gradients in viscosity of

resin, near wall effects caused by relatively long fibers in

a thin geometry, and the multi-components interactions of

resin, fibers, fillers and air and an accelerated cross-

linking of the molecules in the resin. It is also apparent

that the long fibers and relatively high fiber volume

SPE ANTEC™ Indianapolis 2016 / 463

fraction (30%) lead to interaction between individual

fibers.

Due to the high volume fraction and semi-continuous

nature of the fiber within the GMT (Glass Mat

Thermoplastics) or SMC (Thermosetting Sheet Molding

Compound), CTS material testing lab currently uses the

compression apparatus to acquire viscosity data used in

CAE analysis. Compared with the typical shear testing of

plastic materials, compression of GMT/SMC between two

heated plates leads to a squeeze flow.

When a liquid is squeezed between two parallel plates,

a pressure driven flow is generated. Nonetheless, the flow

is quite complex as the boundary condition keeps

changing with time. Because the walls are moving

together, the radial flow rate keeps increasing with r.

(Figure 1). Thus there are gradients in both z and r

directions. This means that in addition to the usual

inhomogeneity that accompanies all pressure-drive flow,

0≠∂∂ zvr , there is also extension, 0>∂∂ rvr . In

addition, the flow is transient because thickness changes

are normally recorded from a rest state. Squeeze flow is

interesting from a fluid mechanisms point of view since it

simulates such polymer processes as compression

molding and stamping [4-7].

There are several advantages of using squeeze flow.

First, only moderate strain is applied to FRP in which the

intermingled fiber network is preserved. Past attempts of

both steady state and dynamic shear flows resulted in

severe slip and poor measurements after the GMT was

heated. Second, the compression process resembles how

the complex resin flow is pushed through the pores

between the fibers in the actual manufacturing process.

Despite the numerous advantages that squeeze flow

method has to offer, to the best of our knowledge, it has

not been applied to complex systems such as FRP to

obtain rheological properties.

Experimental Section

We have carried out experimental investigation on

Glass Mat Thermoplastic (GMT) and Sheet Molding

Compound (SMC). In particular, their rheological

behaviors are the focus of this study. Instron 5966 tensile

testing machine equipped with compression module was

used in this study. The compression surface has contact

radius of r = 22.5mm. The instrument was equipped with

an environmental heating chamber.

Characterization of GMT: PP-GMT sample was

supplied by Moldex3D customer, Voestalpine. Prior to

rheological characterization, the bulk PP-GMT sample

was cut to a specific dimension (r = 22.5 mm) using

computerized sawing machine. Since PP-GMT was a

thermoplastic, it was hold at its melt temp for sometimes

to ensure complete melting of the sample. The upper plate

was brought to contact with the sample before

compression commenced. The processing temperature of

this sample is 180 to 220 ℃. During the measurement, a

constant force (f) is applied, and sample height (H) as a

function of time is recorded.

Characterization of SMC: Quantum SMC sample was

choose in this study due to its superior performance, and

wide usage in the many different industries such as

automotive, aerospace, sport equipment and etc. The

SMC sample is consisted of vinyl ester in styrene

monomer reinforced with 60% glass fiber. Compression

experiments were carried out at 150 ℃. Since SMC is a

thermoset, there was no thermal equilibrating period as

was in the case of GMT. Compression began as soon as

the upper plate was brought to contact with the sample.


During the measurement, a constant force (f) is applied,

and sample height (H) as a function of time is recorded.

We also carried out differential scanning calorimeter to

analyze curing kinetics behavior of SMC. PerkinElmer

DSC 8500 was used in this study: the non-isothermal

conversion profiles were determined by the dynamic

temperature scanning from 30 to 250℃ with several

heating rates: 10, 20, 40 and 60℃/min.

Results and Discussion

Laun et al has suggested a very simple way to

analyze squeeze flow data. The flow at any location

between plates is nearly that for a slit and the viscosity

can be obtained by through the following equations:

2ln

)ln(1−

−=

HdHd

n

! (1)

)12()(2 nHHR

WR+

−=

!!γ (2)

32)3(RHfnWR πτ += (3)

where n is power law index; H! is rate of change of the

gap; H is the initial gap; WRγ! is the shear rate; R is radius

of the parallel plate; WRτ is the shear stress; f is the

applied force. From a plot of log ( H! ) vs. log H，the

power law index can be obtained directly (Eq1). The

viscosity can be obtained from the result of equation 2 and

3.

The squeeze flow testing has also been adopted in

deriving material properties for other processing methods,

e.g. resin in the VARTM process (Composites A, 2001,

32, 1553) [8]. The sample was placed between two

parallel plates in the heated environment. (Figure 2). A

constant force was then applied to collect the gap

evolution with time, i.e. creep, data. (Figure 3) The strain

and strain rate were calculated based on the gap and its

time derivative. We observed that that during the initial

compression, there was a bounce back of compression

gap, and we attributed this phenomenon elasticity of fiber.

The expansion of the fiber material after compression is

seen in Figure 2.5b. The stress was then derived by

dividing them into a viscous part, i.e. pressure required to

drive the resin flow, and an elastic part, which compresses

the fiber network. We found both parts follow nicely the

Hooke's law (Figure 4), and experiments were conducted

at different temperatures and results were fitted to the

power law model (Figure 5 and 6). These models are then

incorporated into the material databank to describe the

behavior of a GMT material under compression. These

models are then incorporated into the material databank to

describe the behavior of a GMT material under

compression.

We further investigate the suitability of squeeze flow

testing method for the characterization of SMC. Similar to

the GMT measurement, a constant force was applied to

collect the gap evolution. The viscosity data was obtained

using equation 1-3. The typical viscosity behavior for a

thermoset material can be seen in Fig 7 in that the

viscosity first drops with increasing temp and reaching a

minimum before increasing rapidly (a U shape curve).

The experimental data of cure conversions were

fitted by numerical parameters using a combined model of

n-th order reaction and autocatalytic reaction since most

thermoset shows a combination of both curing kinetics.

The combined model is given by the following equation

and the fitting parameters are summarized in Table 1:


nmba KK

dtd )1)(( ααα

−+= (4)

)exp(TTAK A

a−

= (5)

)exp(TTBK B

b−

= (6)

where αrepresents the degree-of-cure of the reaction. 0=α Implies the reactants are not reacted at all,

while 1=α implies the curing reaction is completed.

α is defined as the ratio of the heat released in

reaction versus the total reaction heat of the curing

reaction. The result of the fitting parameters of

curing kinetic is shown in Table 1.

The measured viscosity is fitted by the

following Cross Castro Macosko’s model.

α

αα

α

τγη

ηη 21)(

)*

(1 10

0 cc

g

g

n

+

− −+=

(7)

TT

o

b

Be=η (8)

Where γ is shear rate, α is conversion, n is the power law

index, 0n the zero shear viscosity, *τ is the parameter

that describes the transition region between zero shear

rate and the power law region of the viscosity curve. The

fitting parameters for reactive viscosity are summarized in

Table 2 . The experimental data and the numerical fitting

line show good agreement, as shown in Figure 6. We note

that when compression time reaches ca. 15 S, squeeze

flow equations described by Laun et al can no longer be

applied. The conversion percentage has reached 30 % at

15 S, and as a result fluid seized to flow.

In order to validate the experimental parameters

obtained by squeeze flow measurements, simulations on

SMC compression molding process are also carried out

(Figure 7). The simulations are run using the idential

conditions from the squeeze flow measurements. The

simulation filling pattern has mached well with the

compression sample from squeeze flow experiments.

Conclusions

In this study, we have applied squeeze flow method

to study rehological behavior of complex material systems

such as GMT and SMC. For GMT, the stress can be

divided into two componenets, the elastic and the

viscosity component. Both componets can be descirbed

by Hooke’s Law and power law model. Using squeeze

flow method, the typical reactive viscosity behavior of

SMC (a U shape curve) has been observed and measured

viscosity data follow Cross Castro Macosko’s model.

Further more, a simulation on SMC compression molding

is carried out to validate the experimental parameters

obtained from squeeze flow measurment. Squeeze flow

method is a more suitable method in places where

traditional viscosity measurment by parallel plate on these

high fiber content systems are difficult.

References

1. H. M. Laun, Journal of Rheology, 36, 743 (1992).

2. M.K. Kang, W.I.Lee, H. T. Hahn, Composites Part

A: Applied Science and Manufacturing, 32, 1553

(2001).

3. T. Odenberger, Licentiae Thesis: Department of

Applied Physics and Mechanical Engineering, Luleå

University of Technology (2005).

4. P. J. Leider and R. B. Bird, Industrial & Engineering

Chemistry Fundamentals, 13, 336 (1974).

5. A. Matsoukas and E. Mitsoulis, Journal of Non-

Newtonian Fluid Mech. 109, 231, (2003).


6. SH. Chatraei and C. W. Macosko, Journal of

Rheology, 25, 433 (1981).

7. D. D. Pelot, R. P. Sahu, S. Sinha-Ray, and A. L.

Yarinm, Journal of Rheology, 57, 719 (2013).

8. M. K. Kang, W. I. Lee, H. T. Hahn, Composites Part

A: Applied Science and Manufacturing, 32, 1553,

(2001).

Key words: squeeze flow, compression molding, fiber

reinforced plastics (FRP), glass mat thermoplastic (GMT),

sheet molding compound (SMC)

Figure 1. The illustration of flow behavior during a

squeeze flow measurement

Figure 2. Squeeze flow measurement using Instron 5966

tensile testing machine equipped with compression

module. The sample was placed between two parallel

plates in the heated environment. A constant force was

then applied to collect the gap evolution with time, i.e.

creep, data

Figure 3. GMT under squeeze flow meaurment where

gap vs time is collected under different applied forced

Figure 4. Displacement of GMT as a function of applied

force during squeeze flow measurement

Figure 5. Displacement of GMT as a function of different

applied temperature during squeeze flow measurement


Figure 6. GMT viscosity data fitted by power law model

Figure 7. SMC viscosity data and curing kinetic data. The

viscosity raw data (not shown) is obtained using the

calculated shear rate (blue line) and fitted with Cross

Castro Macosko’s model (red line) (a). The curing kinetic

is fitted with combined model of n-th order reaction and

autocatalytic reaction (b)

Figure 8. Simulation on SMC compression molding

process using the identical conditions from the squeeze

flow measurements. Simulation (A) and squeeze flow

experiment (B)

Table 1: Numerical parameters for combined kinetics

model Parameter of

Kinetics Unit Value

m N/A 1.2404 n N/A 2.6498 A 1/sec 1.19e12 B 1/sec 6.24e16 Ta K 14144 Tb K 16610

Table 2: Numerical parameters for Cross Castro Macosko

model Unit Value

C1 --- 1 C2 --- -3.22 αg --- 6.62 A g/(cm.sec) 1.95e-3 Tb K 1.87e4 n --- 2.17e-1

Tau* dyne/cm^2 1.00


optimizing process condition of compression …...interesting from a fluid mechanisms point of view...

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