1

SHEET MOLDING COMPOUND (SMC) CHARACTERIZATION USING SPIRAL FLOW

Michael Rabinovich1, Kate L. Olsavsky1, Burr (Bud) Leach2,

Mauricio Cabrera-Ríos3 and José M. Castro1

1 Department of Industrial, Welding & Systems Engineering

The Ohio State University Columbus, Ohio, USA 43202

2Ashland Specialty Chemical Company

Dublin, Ohio 43017

3 Graduate Program in Systems Engineering Universidad Autónoma de Nuevo León

San Nicolás de los Garza, Nuevo León, México, 66450

Abstract

Sheet Molding Compound (SMC) is a fiber-reinforced polymer composite. It is often used in automotive, marine, and industrial applications over other materials because of its high strength to density ratio, resistance to corrosion, and low cost. One of the fastest growing applications of SMC is compression molded truck body panels. Due to their large size and high strength requirement, the molding forces occurring during the molding process are a large enough factor to have a major influence in the molding cycle. This increases the need in the SMC industry to predict molding Forces from experimentally measured parameters. Previous work in our group developed a method using flat plate moldings to measure the SMC material parameters needed to predict molding forces. In this paper we first develop and experimentally test a model to predict the molding forces required for the SMC to flow in a spiral flow mold. We then propose a method to use the spiral flow mold to measure the SMC flow parameters and compare the measured values to the values obtained from flat plate moldings. Keyword: Spiral Flow, Sheet Molding Compound, SMC.

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Introduction

Sheet Molding Compound (SMC) is a fiber reinforced polymer matrix composite

material that is widely used in the automotive, aerospace, marine, and industrial/consumer

industries. Its superior strength to weight ratio and corrosion resistance and relatively low cost

makes it favorable over steel in many applications. SMC is composed of polymerized cross-

linked resin, reinforcement fibers, filler, and various additives [1].

The process of making the SMC starts off by mixing all the ingredients, with the

exception of the fibers, to form the resin paste. The SMC machine then chops and adds the

fibers. The fibers are usually from ½ to 2 inches in length and are fed in between two uniformly

spread resin paste sheets. The final step in producing raw SMC sheet is completed when the two

sheets are passed through the compaction zone, where the paste is impregnated into the fiber

bundles while eliminating most of the trapped air pockets [2]. The thickness of an SMC sheet is

typically from 0.15 to 0.24 inches with the resin thickened but not cured.

One of the fastest growing applications of SMC is compression molded truck body

panels. Due to their large size, the molding forces occurring during the molding process are

large enough to have a major influence in the molding cycle. As the SMC parts increase in size

while demanding higher strength, the fiber length and composition per volume increases,

requiring higher compression forces to mold. Thus, understanding how the SMC components

affect the forces required to fill the mold is critical. Previous work in our group [3, 4, 5]

developed a method using flat plate moldings to measure the SMC material parameters needed to

predict molding forces. In this paper, we present a method to predict these parameters using a

spiral flow mold.

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The spiral flow tool has evolved in its design and application in the past thirty years. The

concept behind the spiral flow tool is to use a relatively small amount of material to represent the

actual molding process. The initial spiral flow tool’s design was taken from the injection

molding industry, where the SMC paste was injected into a small channel in a hot mold. The

dimensions of the channel were 0.25”x 0.125”x 48”, and the tool was intended to be used only to

obtain relative comparisons of the final length for different materials. The biggest disadvantage

of the early spiral flow tool designs was that the fiber reinforcements prevented the SMC from

flowing in the channel [6, 7]. In order to use the early spiral flow tool, the SMC had to be

chopped up into small pieces. These experiments were inconsistent and unrepeatable. The main

application for the early spiral flow tool was to test the paste without the glass. The spiral flow

tool’s geometry was later improved to accommodate for the flow of glass fiber during molding.

Even though the spiral flow mold has a more complex geometry than the previously used

flat plate, its long flow channel allows for evaluation of the paste’s ability to carry the

reinforcement. This long channel also holds another advantage over the flat plate since the

material’s ability to flow can be quantified by simply measuring the flow length [7]. The spiral

flow used in this research has been designed to better represent the actual molding process than

the early spiral flow tool by accommodating for the long glass fibers and shear edges [6,7]. Its

dimensions were increased to a 6”x6” square charge cavity, with a 2” wide channel that is 48” in

length. The spiral flow tool is equipped with a data acquisition system that is able to collect five

temperature and pressure readings along the channel, hydraulic force, and mold separation all

with respect to time. Figure 1 shows the positions of temperature and pressure sensors. In the

absence of the data acquisition system, the spiral flow tool can be used to empirically compare

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materials on how well they flow based on the final length of flow inside the channel under the

same molding conditions [6,7].

Evaluating Glass Carrying Ability of SMC

One of the important factors in SMC manufacturing is the ability of the paste to carry

glass, which is vital to achieving uniform physical properties in a part. The spiral flow tool can

be used to evaluate the SMC’s ability to carry glass because the material is forced to flow for a

long distance. In order to demonstrate this, an experiment was performed in which one-inch

circular samples were drilled out of the molded part and measured to check for glass contents at

various locations, as shown in Figure 2. The glass burnout method was used to determine glass

compositions [7]. This method involves placing the samples in an oven and baking them

overnight at 450 Co . The organics, including the polymer matrix, reacted with the oxygen in the

oven to form carbon dioxide, leaving the filler and glass. The filler was reacted away with acid,

leaving the glass fibers, which were then washed and dried before measuring the amount of glass

in the sample. Three different materials that were molded at high and low pressures are shown in

Figure 3. The low variation of the glass amounts at each of the samples’ locations shows that the

materials tested are very good at carrying the glass reinforcement.

Mapping the SMC Flow inside the Spiral Flow Tool

The flow inside the spiral flow tool was mapped by replacing sections of the charge with

an alternative color and observing its flow path. Figure 4 shows an example of one trial of this

experiment. Further details on this experiment are provided in reference 7. A schematic

representation of the flow behavior observed is shown Figure 5. This representation allowed

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break down of the flow inside the spiral tool into a series of simple geometries, as discussed

later.

Relationship Between Part Thickness and Flow Length

A schematic of the spiral flow tool mold channel is shown in Figure 6. Both the length

and the width of the squared section, called Ls and Ws respectively, are 6 inches resulting in an

area of 36 in2. The width of the long channel is 2 inches. The mold separation is measured with

a Linear Variable Displacement Transducer (LVDT) calibrated to record the part thickness with

respect to time during molding. Assuming constant density and no leakage at the shear edges, a

relationship between the part thickness and the flow length in the spiral flow tool was derived

and is shown in Equation 1

6)(2

)(36)( +⋅⋅−

=th

thVtLm (1)

where Lm(t) is the flow length (in) inside the tool, V is the volume of compacted SMC used (in3),

and h(t) is the instantaneous part thickness (in).

Verification of Equation 1 was done by controlling the final distance between the mold

halves after closing, which in turn changed the final flow length of the material. In order to make

this possible, steel shims were placed at each of the four sides of the mold. The predicted and

measured flow length for three different trials with different part thicknesses are shown in

Figure 7. The final flow lengths and the final measured value of the molded parts differ by less

than one inch in most cases. This variation is considered to be insignificant in the SMC industry

since the spiral flow length for different trials for the same material under the same conditions is

repeatable only to within an experimental error of one inch

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Mathematical Model for Predicting the SMC Flow

Castro and co-workers [3, 4, 5, 8] proposed a model to represent the flow of SMC based

on the pioneering work of Tucker, Barone and Caulk, and Marker and Ford [9 - 13]. The model

consists of a core with lubricating layers at both mold surfaces. The lubricating layer is assumed

to contain only paste. Figure 8 shows a schematic representation of the model. Considering

unidirectional SMC flow, and using the continuity and linear momentum equations, Abrams and

Castro [3] derived the pressure relationship shown in Equation 2.

( ) ( )111 )(

1)()(2

)()(4)( ++

+ −⋅+⋅

⋅⋅+

⋅⋅= nn

mn

n

xtLnth

tUMth

tUtP η (2)

where P(t) is the pressure; η is the resistance to extension; M is the lubricating layer parameter

defined as nmδ

, with m as the power law consistency index and δ as the lubricating layer

thickness; U(t) is the instantaneous press closing speed; h(t) is the instantaneous part thickness;

Lm(t) the instantaneous flow length; and x is the position measured from the center of the mold.

The composition of the SMC used in this study is the same as that used in the earlier flat plate

work of Boyland et al [4].

For ease of modeling, the flow of the SMC in the spiral flow tool was broken down into

three process stages. These three stages are I) filling of the square mold cavity, II) flow through

the channel until the maximum applied force is reached, and III) flow through the channel after

the maximum, constant force had been reached. Each stage is identified in the plot of force

exerted on the material versus time in Figure 9. It should be noted that the third stage is several

times longer than indicated on the plot, which was shortened for clarity of stage identification.

Stage I takes place when the SMC charge fills the square mold cavity of the spiral flow

tool. The actual charge that is used during molding trials is 5 inches by 5 inches and is placed in

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the center the mold cavity which is 6 inches by 6 inches. Abrams and Castro derived a

relationship between the molding parameters and the molding force [3]. The relationship

describes a 2-directional flow and is shown in Equation 3 [3].

( )( )1132

2

2 22)()(2

)()(4)( +++

+

+⋅⋅⋅

+⋅⋅

= nnn

n

nWthVtUM

thtUtF η

(3)

where F(t) is the Force exerted on the SMC, W is the part width, and the rest of the variables and

parameters follow the same notation used for Equation 2.

It is possible to determine the force required to mold the SMC material by applying

equation 3 and the material parameters measured by Boylan et al [4]. Equation 3 is valid only

while the square section of the mold is being filled. In the case shown in Figure 10, the

prediction is only valid until 1.2 seconds.

Stage II starts after the square part of the mold has been filled and the SMC flows into the

channel. This stage lasts until the maximum applied force is reached, which is approximately

one second in this particular case. The relationship between the measured parameters and the

molding force was obtained using Equation 2 as a basis. By studying the flow patterns of the

charge with colored pieces, it could be observed that the flow in the spiral flow tool can be

represented by a combination of unidirectional flows, as shown in Figure 11. The total force

required to mold could be represented by the summation of the force required to flow the

material in the long channel, plus the force required to flow the two sides. The long channel is

assumed to be surrounded by rigid boundaries, while the sides are assumed to be flowing into the

long rectangular channel. The flow of material from the two sides is assumed to be resisted by

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the pressure in the central channel. The resulting equations are shown below. The details can be

found in reference 7

sidelength ForceForcetF ⋅+= 2)( (4)

+

−⋅⋅+

⋅⋅+

+

+

−⋅+

⋅⋅+

⋅⋅⋅+

+

+

−+

⋅⋅+⋅

⋅⋅

⋅=

++

+

++

+

++

+

26)(6

)1()()(4

222

)1()()(2

)()(166

2)()(

)1()()(2)(

)()(4

2)(

21

1

22

1

22

1

ntL

nthtUM

nnthtUM

thtU

ntLtL

nthtUMtL

thtU

tF

nn

mn

n

nn

n

n

nmn

mn

n

m

η

η

(5)

where the notation is consistent with the previous equations.

The measured and predicted plot for one trial is shown in Figure 12. In the beginning of

stage II, the predictions do not match the experimentally measured values as they do in the end

of stage II. This could be attributed to the transitioning from one model to another.

Stage III begins when the maximum molding force has been reached by the press, after

which, the force exerted on the SMC remains constant. Most of the SMC flow takes place

during this stage, which also happens to last the longest period of time. In this stage, the

molding force remains constant, and we predict the closing speed. Equation 5 also applies to

this stage, however, the closing speed cannot be explicitly solved and an iterative method needs

to be used. Predictions for this stage were compared against experiments, some of which are

shown in Figure 13. These predictions were made using the SMC material parameters measured

by Boyland et al [4] using the flat plate molding method. As mentioned earlier, the SMC used

here is of the same composition as the one used by Boyland et al [4]. Generally, the same trend

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between the predictions and experimental measurements could be observed. The predictions

slightly over predict the experimental values. A possible explanation for the over prediction

could be attributed to the chemical reaction that slows down SMC flow [7].

Obtaining Material Parameters Using the Pressure Transducers of the Spiral Flow Tool

The spiral flow tool is equipped with pressure transducers positioned along the length of

the 2 inch channel, which are a part of the tool’s data acquisition system. The pressure responses

are recorded five times per second and stored for later analysis. These responses could be used

to calculate the lubricating layer parameter (M) of the SMC. The pressure transducer data along

with Equation 1 were used to determine the lubricating layer parameter. To obtain the

lubricating layer parameter a difference in pressures was taken, as shown by equation 6.

( ) ( )( )( )( )11

1

11

)(21)(

+++

++

−⋅+⋅−

== ni

ni

n

nii

n xxtUnthppmM

δ (6)

In this research only transducers 2, 3, and 4 were utilized. The data for the lubricating

layer parameter, shown in Figure 14 has some noise, but is fairly consistent and can be

represented by a straight line. The noise is typical of SMC experiments.

SMC material of the same composition as used by Boyland et al [4] was reproduced for

this research in order to make a comparison against the material parameters obtained using the

flat plate molding. The SMC material was molded under the same molding conditions with both

methods. Table 1 shows M for both the flat plate and the spiral flow tool. The values from both

methods are very close and follow the same trend, which means that the spiral flow tool can be

used to obtain the lubricating layer parameter. More details can be found in reference 7.

The M value for high filler-high glass is lower than anticipated. One possible explanation

for the lower measured value than expected could be because the 56% and 65% filler used to

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represent the low and the high filler amounts was not a significant enough amount to make a

difference in the obtained M, as verified with statistical analysis. Figure 15 shows the main

effects plot for the filler and glass levels. From this plot, it could be said that the glass

percentage has the greatest influence on the lubricating layer parameter. The filler levels chosen

as high and low did not make a significant impact on the lubricating layer parameter. The trends

of M as glass and filler composition followed the same trends as the values presented in Boyland

et al [4].

From the two material parameters, resistance to extension is the most important

parameter when it comes to predicting the required force to mold for typical SMC parts. This

could be shown by breaking up Equation 3 into two separate terms (Equations 7 and 8) to

observe how each contributes to the total force to mold.

( )2

)(4)(th

tUVtF ⋅⋅⋅=

ηη (7)

( ) ( ) 1132

2

22)(2)(

+++

+

⋅+⋅⋅

⋅⋅=

nnn

n

M nWthVtUtF (8)

Figure 16 shows a plot for predicting the required force to mold and the contributions

from each one of the material parameters. From Figure 16 it could be seen that the required

force to mold is greatest in the final seconds of the molding process, which is dominated by the

M term. The resistance to extension for Figure 16 was taken from reference 4.

Using the Spiral Flow Tool for Empirical Evaluation of Molding Performance

As previously mentioned, one of the advantages of the spiral flow tool with respect to flat

plate moldings is that the final flow length could be used to empirically judge the material’s

ability to flow. For example, trends could be observed in Figure 17 as to how the temperature

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and molding pressure affect the final flow length for a material with 56% filler and 36.5% glass.

Figure 18 shows the effect of glass filler composition and molding force at a molding

temperature of 300 F. The results show the expected trends.

Conclusions

The spiral flow tool, which was previously used only as a qualitative tool to evaluate

SMC, can also be used to determine the lubricating layer parameter, M, with similar results to

flat plate molding. The flow of SMC inside the spiral flow tool could be predicted with

parameters obtained from flat plate molding. The resistance to extension parameter cannot be

obtained from the spiral flow tool easily and needs to be investigated further.

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References

[1] Castro, Jose M., and Melby, Earl G. Comprehensive Polymer Science. New York: Pergamon Press, 1989.

[2] Castro, Jose M., and Griffith, Richard M. “Press Molding Process.” Composite

Engineering Handbook. New York, 1997. [3] Abrams Lisa and Castro Jose M., “Predicting Molding Forces in SMC Compression

Molding I: Model Development”, Polymer Composites, 2002. [4] Abrams Lisa, Boylan Sarah and Jose M. Castro, “Predicting Molding Forces in SMC

Compression Molding II: Effect of SMC Composition”, Polymer Composites, December 2003, 731 .

[5] Boyland Sarah and Jose M. Castro, “Effect of Reinforcement Type and Length on

Physical Properties, Surface Quality and Cycle Time for Sheet Molding Compound (SMC) Compression Molded Parts”, Journal of Applied Polymer Science Volume 90, Ossie 9, P 2557 (2003)

[6] Castro, J., Rabinovich, M., and Leach, B. Mathematical Modeling of the Spiral Flow

Tool for Sheet Molding Compound (SMC). Numiform, The Ohio State University, 2004. [7] Rabinovich, Michael, Sheet Molding Compound (SMC) Processing: Spiral Flow Test and

Physical Properties. M.S. Thesis, The Ohio State University, 2004. [8] Castro, Jose M., and Griffith, Richard M. “Sheet Molding Compound Compression-

Molding Flow.” Polymer Engineering and Science. 1989: 632-638. [9] Osswald, T.A. and C.L. Tucker, Compression Mold Filling. Simulation for Non-Planar

Parts. International Polymer Processing, 1990. p.79. [10] Lee, C.C. and C.L Tucker, Flow and Heat Transfer in Compression Mold Filling. Journal

of Non-Newtonian Fluid Mechanics, 1987. p. 245. [11] Barone, M.R. and D.A. Caulk, A Model for the Flow of a Chopped Fiber Reinforced

Polymer Compound in Compression Molding. Journal of Applied Mechanics, 1986. p. 361.

[12] Barone, M.R. and D.A. Caulk, The Effects of Deformation and Thermoset Cure on Heat

Conduction in a Shopped Fiber Reinforced Polyester During Compression Molding. Int. J. Heat Mass Transfer, 1979. p. 1021-1032.

[13] Marker, L. and B. Ford. Rheology and Molding Characteristics of Glass Fiber

Reinforced Sheet Molding Compounds. Reinforced Plastics/Composites Institute. 1977.

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Spiral Flow M Flat Plate M

⋅+2n

n

inslb

⋅+2n

n

inslb

Low Filler-Low Glass 1.3 1.27

Low Filler-High Glass 4.1 4.36

High Filler-Low Glass 1.5 2.17

High Filler-High Glass 3.4 3.96

Table 1: M Calculated with the Flat Plate and the Spiral Flow Tool.

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Figure 1 - Positions of the thermocouples and pressure transducers

Center of Square Section

10”

20”

30”

40”

Reference (0”)

15

Figure 2 – Positions of samples taken to test the material ability for carrying glass reinforcements

2

1

3

4

5

6

7

8 9

16

38% Glass - Low Filler High Molding Force

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

45.0%

0 2 4 6 8 10 12Location

Gla

ss P

erce

nt

38% Glass - Low Filler Low Molding Force

28% Glass - Low Filler High Molding Force 28% Glass - Low Filler Low Molding Force

23% Glass - High Filler High Molding Force 23% Glass - High Filler Low Molding Force

38%

28%

23%

Figure 3 – Percent Glass Carry for three different materials with varied glass and filler compositions; refer to figure 2 for location map

17

Figure 4 - Black rectangles indicate the original positions of the colored rectangles

18

Figure 5 - Flow pattern model: arrows represent flow direction of SMC inside the mold cavity

19

Figure 6 - Spiral flow tool schematic.

x y

Ls

Ws

L(t)

20

Predicted vs. Measured Flow Length for 3 Different Thicknesses

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120time (sec)

flow

leng

th (i

n)

Predicted 0.1229 in Predicted 0.1364 in Predicted 0.1746 inMeasured 0.1229 in Measured 0.1364 in Measured 0.1746 in

Figure 7- Predicted versus measured flow length for three samples with varied final thickness

Flow

Len

gth

(in)

Time (sec)

21

Figure 8 - Visualization of the plug flow with a lubricating layer

t=0 t>0

LUBRICATING LAYER

LUBRICATING LAYER

PLUG FLOW

22

Figure 9 - Three stages of SMC flow, identified on the force-versus-time plot

Stage I Stage II Stage III Force (Tons)

20

15

10

5

0 0 1 2 3 4 5

Time (sec)

23

Figure 10 - The dots show experimentally measured values, while the line represents predicted force using Equation 2

0 1 2 3 4 5 Time (sec)

Force (Tons)

20

15

10

5

0

24

Length Side

Figure 11 - The setup is broken down into three sections: the length and two symmetrical sides. The sides can be represented by calculating the force on one side and multiplying by 2.

Side 1

Side 2

Length

2 2”

Lm(t)

6” L(t)

2”

6”

25

Figure 12 - The dots show experimentally measured values, while the line represents predicted force using Equation 5.

Stage II

transition

Force (Tons)

0 1 2 3 4 5

20

15

10

5

0

Time (sec)

26

0

10

20

30

40

50

60

2 7 12 17 22 27 time (sec)

Leng

th

A1 1000-300 Predicted

A1000-300 Measured

A1-250-300 Predicted

A1- 250-300 Measured

Figure 13 - Predicted and measured flow lengths for high glass-low filler SMC molded at two different pressures.

27

A1 - 1000 psi - 300F

0

1

2

3

4

5

6

7

6.4 7.4 8.4 9.4 10.4 11.4 12.4 13.4 14.4 15.4

time (sec)

M

M2-3 M3-4

Figure 14 - Calculated lubricating layer parameter from pressure differences for high glass-low filler material molded with 49 tons. The thick horizontal line represents the best-fit constant value.

2n

n

inslb+⋅

6.4 7.4 8.4 9.4 10.4 11.4 12.4 13.4 14.4 15.4

Time (Sec)

M2-3 M3-4

28

GlassFiller

35%

18%

65%

56%

3.5

3.0

2.5

2.0

1.5

M

Main Effects Plot - Data Means for M

Figure 16 - Main effects plot for filler and glass factors for the lubricating layer parameter.

29

0 1 2 3 4 50

50

100

150

200

250Force Contributions

time (sec)

Forc

e (to

ns)

F_total t( )

F_eta t( )

F_M t( )

t

Figure 17 - Predicted force to mold 17”x22” plaque and individual contributions from material parameters.

0 1 2 3 4 5 Time (sec)

200

150

100

50

0

250Force (Tons)

30

Figure 17 - Mold temperature and pressure effects on the final flow length of SMC with 56% filler and 36.5 % glass

0

5

10

15

20

25

30

35

40

45

50

250 600 1000 1200 1500

Flow

Len

gth

(in)

Mold Temp 280F Mold Temp 300F Mold Temp 320F

250 600 1000 1200 1500Hydraulic Pressure (psi)

31

Glass-Filler Flow Study at 300F

0

5

10

15

20

25

30

35

40

45

50

36.5% Glass - 56% Filler 19.7% Glass - 56% Filler 33.2% Glass - 65% Filler 17.3% Glass - 65% Filler

Material Composition

Flow

Len

gth

(in)

250 psi 600 psi 1000 psi 1200 psi 1500 psi

Figure 18 - Glass and filler composition and molding pressure effects on the final flow length of SMC at 300 F

Flow

Len

gth

(in)

Material Composition