Investigation of Fiber Orientation States inInjection-Compression MoldedShort-Fiber-Reinforced Thermoplastics

Can Yang, Han-Xiong Huang, Kun LiLaboratory for Micro Molding and Polymer Rheology, South China University of Technology, Guangzhou510640, People’s Republic of China

Injection-compression molding (ICM) has receivedincreased attention because of its advantages over con-ventional injection molding (CIM). This article aims toinvestigate the effects of five dominating ICM process-ing parameters on fiber orientation in short-fiber-rein-forced polypropylene (SFR-PP) parts. A five-layer struc-ture of fiber orientation is found across the thicknessunder most conditions in ICM parts. This is quite differ-ent from the fiber orientation patterns in CIM parts. Thefibers orient orderly along the flow direction in the shellregion, whereas most fibers arrange randomly in theskin and the core regions. Additionally, the fiber orienta-tion changes in the width direction, with most fibersarranging orderly along the flow direction at positionsnear the mold cavity wall. The results also show that thecompression force, compression distance, and com-pression speed play important roles in determining thefiber states. Thicker shell regions, in which most fibersorient remarkably along the flow direction, can beobtained under larger compression force or compres-sion speed. Moreover, the delay time has an obviouseffect on the fiber orientation at positions far fromthe gate. However, the effect of compression time isfound to be negligible. POLYM. COMPOS., 31:1899–1908,2010. ª 2010 Society of Plastics Engineers

INTRODUCTION

Compared with conventional injection molding (CIM),

injection-compression molding (ICM) possesses the

advantages of decreased molding pressure, reduced resid-

ual stress, minimized molecular orientation, reduced

uneven shrinkage, overcome sink mark and warpage,

reduced density variation, and increased dimensional

accuracy [1]. Because of these advantages, ICM is often

employed to produce parts with high-dimensional accu-

racy or heavily reinforced parts for which the fiber orien-

tation is an issue.

To date, numerous researchers have reported on the

fiber orientation patterns in thermoplastics parts molded

by CIM. Among them, some studies [2–4] were con-

cerned with the typical skin-core structure of fiber orienta-

tion. This involves two skin layers with the fibers highly

orienting along the flow direction and a core layer con-

taining fibers generally aligning normal to the flow direc-

tion. Shokri and Bhatnagar [5] investigated the effect of

packing pressure on the fiber orientation in fiber-rein-

forced parts. Their results showed that the packing pres-

sure at the instant of complete filling tries to reduce the

degree of fiber alignment in the flow direction. Silva

et al. [6] compared the fiber orientation patterns of short

glass fiber (SGF)-reinforced polypropylene (PP) in con-

ventional and nonconventional molding (using a mold

involving rotation, compression, and expansion). The

results showed that the induced high-fiber orientation

transverse to the radial flow direction is very pronounced

in the expansion mold. Malzahn and Schultz [7] investi-

gated the development of fiber orientation during the

mold-filling process by examining the through-thickness

microstructures in a series of controlled short shots. The

transverse orientation of fibers throughout the core layer

was found only after the mold was fully packed. They

attributed this phenomenon to stresses developed during

the packing stage of the injection molding process. Exper-

imental and numerical simulation work performed by Lee

et al. [8] focused on the three-dimensional fiber orienta-

tion states in injection-molded parts, with a conclusion

that the predicted fiber orientation tensors and orientation

states showed good agreement with the measured ones.

Additionally, they attributed some differences between the

predicted and measured results at the end of cavity to the

fact that the fountain flow phenomenon is not considered

in the numerical analysis. Han and Im [9] carried out the

simulation of the fiber orientation distribution in short-

fiber-reinforced injection-molded parts with consideration

Correspondence to: Han-Xiong Huang; e-mail: mmhuang@scut.edu.cn

Contract grant sponsors: Major Project of Key Fields in Guangdong-

Hong Kong; contract grant number: 2004A10402003.

Contract grant sponsors: Scientific Research Foundation for the Returned

Overseas Chinese Scholars, State Education Ministry.

DOI 10.1002/pc.20986

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2010 Society of Plastics Engineers

POLYMER COMPOSITES—-2010

of fountain flow effects. By comparing simulation results

with experimental data from literatures, they found that

the orientation components in shell structure can be accu-

rately predicted.

From the literatures, it is found that although research

on fiber orientation has been reported, it mainly focused

on those parts molded by CIM. However, the fiber orien-

tation patterns often seem quite complicated because of

their dependence on the mold geometry, material proper-

ties, and processing parameters. The fiber orientation

states in ICM parts may be quite different from those in

CIM parts. Therefore, the effects of processing parameters

on fiber orientation states in ICM parts were experimen-

tally investigated in this work.

EXPERIMENTAL

Materials

The short-fiber-reinforced PP (SFR-PP) with 30 wt %

SGF used in this work was prepared by compounding PP

with SGF in a twin-screw extruder (with a diameter of 35

mm and a length-to-diameter ratio of 40) and then pellet-

ized. The PP used in this experiment was grade J501

(Sinopec Group Guangzhou, Guangdong, China) with a

melt index of 2.7 g/10 min (at 2308C and 2.16 kg). The

SGF had an average length of 3.0 6 1.0 mm, a diameter

of 11 6 1.0 lm, and was manufactured by Jushi Group

(Zhejiang, China).

ICM Part Molding

ICM experiments were conducted on an 80-ton injec-

tion molding machine (CJ80HE, Chende, China), which

can perform the function of ICM by changing its control

program. Both compression force and speed of the mova-

ble mold platen can be adjusted. To prevent back-flow of

the polymer melt, a special shut-off nozzle was connected

to the end of injection molding machine. Table 1 lists all

processing parameters as well as their values used in this

work. Only one parameter was changed at a time, while

the others were kept at a constant value (shown in bold in

Table 1). An injection-compression mold with an adjusta-

ble shim block that allowed a maximum compression dis-

tance of 3 mm was used to mold the rectangular parts

with the dimensions of 150 3 20 3 2 mm3.

Microscopic Observation

To observe the orientation of the glass fibers in the

ICM parts molded under various processing parameters,

or at different positions in the ICM parts molded under

the same processing parameters, small specimens were

cut at four different positions on the part, denoted as posi-

tions A, B, C, and D as shown in Fig. 1. Specimens were

studied from positions A and B for all the parts molded

in this work. Positions C and D were only analyzed from

parts molded under different compression distances and

compression speeds to better understand the fiber orienta-

tion along and transverse to the flow direction. The speci-

mens were deeply cooled in liquid nitrogen, and then bro-

ken parallel to the flow direction to prepare a surface for

scanning electron microscope (SEM) observation. A Phi-

lips XL-30FEG SEM was utilized to observe the fracture

surface at an acceleration voltage of 10 kV. All specimen

surfaces were gold coated before observation. Considering

the symmetry of each section across the thickness, only

half of the broken surface was observed for the specimen

under each condition.

RESULTS AND DISCUSSION

Fiber Orientation within ICM Parts

Figure 2 demonstrates the typical fiber orientation at

positions A and B within an ICM part. Clearly, three dif-

ferent regions across the thickness of the ICM part can be

found: a thin skin region located below the mold cavity

wall, a core region, and a shell region between the skin

and the core regions. By closer examination, it can be

seen that for positions A and B, there are more grooves

than holes in the shell region, whereas a contrary phe-

nomenon can be observed in the skin and the core

regions. This reveals that more fibers in the shell region

orient remarkably along the flow direction whereas more

fibers in the skin and core regions arrange randomly. It

can be observed that the boundaries between different

regions at position B are not as clear as those at position

A. Moreover, the thickness of the shell region at position

B is smaller than that at position A. The difference in

fiber state at positions A and B is due to the fact that the

dominant fiber orientation pattern originates from the

injection stage for position A, whereas from the compres-

sion stage for position B. Details are discussed later.

Considering the fact that the SEM images shown in

Fig. 2 only cover half of the part thickness, it is under-

standable that, despite some difference in fiber orientation

between positions A and B, a five-layer structure of fiber

TABLE 1. Values for various parameters used in this work.

Parameters Values

Melt temperature (8C) 210

Mold temperature (8C) 50

Injection pressure (MPa) 30

Injection time (s) 1.5

Cooling time (s) 30

Compression force (t) 1 3

Compression distance (mm) 1 2.5

Compression speed (mm/s) 1 3

Delay time (s) 0 2

Compression time (s) 5 13

1900 POLYMER COMPOSITES—-2010 DOI 10.1002/pc

orientation, that is, a core layer in the middle and two

skin and two shell layers on both sides, exists in ICM

parts. This is schematically shown in Fig. 3. This result is

quite different from that in CIM parts reported in Refs.

10 and 11, characterized by most fibers orienting parallel

to the flow direction near the cavity wall but perpendicu-

lar to the flow direction in the core. The simulated fiber

orientation across the through thickness of position A in

CIM part is presented in Fig. 4. The Moldflow Plastic

Insight 6.1 was used to conduct the simulation. The same

mold cavity geometry and molding conditions as in

experiment were utilized in the simulation. The first prin-

cipal values of fiber orientation tensor are used to show

the probability of fiber alignment in the flow direction. A

value close to 1 in the result indicates a high probability

of fiber alignment in the flow direction, whereas a value

close to 0 indicates a low probability. From Fig. 4, one

can see the difference of fiber orientation states across the

thickness in CIM parts when compared with that in ICM

parts described earlier.

The different fiber orientation patterns in parts molded

by ICM and CIM can be explained by the way in which

the mold cavity is filled. In the ICM process, the molten

polymer is injected into the slightly opened mold cavity,

and after the filling stage is completed, the compression

process is activated to apply uniform pressure to the

entire cavity until the gate solidifies. This process influen-

ces the orientation of the fibers in the part to a certain

extent. Specifically, in the injection stage of ICM process,

the molten polymer is pushed into the cavity by a piston

having a constant velocity. The mold cavity in this study

has a constant cross-section area, thus based on the

assumptions that the polymer is incompressible and no

slip takes place on the mold cavity, the parabolic gap-

wised flow velocity distribution will move constantly

along the flow direction. In this stage, the molten polymer

next to the mold cavity experiences the highest shear rate.

At the same time, the cooling rate is also high next to the

mold cavity due to the thermal conduction between mol-

ten polymer and mold cavity. After the delay time, the

compression stage begins, in which the already injected

melt is pushed further by the movable mold platen. Dur-

ing this stage, the flow mass must be balanced. As a

result, the compressed melt in the core region near posi-

tion A must flow towards the end of the cavity. The main

characteristic of the compression stage is relatively low-

shear rate of the flow when comparing with the injection

stage. In a word, in ICM parts the fiber state in the skin

region is the result of high-shear rate and immediate cool-

ing rate once the melt contacts the cold mold cavity. The

fiber orientation parallel to the flow direction in the shell

region at position A is caused by the strong shear effect

induced by the injection flow, whereas that at position B is

caused by the compression flow with relatively low-shear

FIG. 2. Typical SEM micrographs of (a) position A and (b) position B

in ICM parts (compression force, 3 t; compression time, 13 s; compres-

sion distance, 1 mm; compression speed, 1 mm/s; and delay time, 2 s).

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 1. Part dimensions and the locations for SEM observation (unit:

mm). [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2010 1901

effect. The low-shear rate in the core region leads to a ran-

dom fiber orientation in both positions.

Effects of Processing Parameters on Fiber Orientation

Figure 5 shows the influence of compression force on

the fiber orientation at positions A and B in ICM parts. It

can be seen that a thicker shell region is formed at both

positions in ICM part molded under a higher compression

force. For example, the thickness of the shell region at

position A increases from 0.39 to 0.58 mm as the com-

pression force increases from 1 to 3 t. Recall that study

carried out by Shokri and Bhatnagar [5] showed that

when applying packing pressure to the melt in CIM, the

degree of fiber alignment along the flow direction reduces

significantly compared with the case without packing

pressure. The part geometry in their investigation is very

similar to that used in this study. However, this work

showed that the compression force in ICM exhibits a con-

trary effect on the fiber orientation. This phenomenon can

be explained by clarifying the difference between the

packing phase in the CIM and the compression phase in

the ICM. As shown in Fig. 6a, in the packing phase of

CIM process, the packing pressure is transferred from the

gate to the end of the cavity, which forms a radialized

pressure distribution from the gate. To simplify the analy-

sis, only two forces (denoted as F1 and F2 in Fig. 6a)

resulted from packing pressure are assumed to be applied

to a single SGF, which already aligned in the flow direc-

tion. Because of different pressure drop paths, the force

F1 is larger than the force F2, which results in a torque-

moment that can make the fiber rotate in a direction

inclined or even perpendicular to the flow direction (from

directions p1 to p2 as shown in Fig. 6a). Therefore,

increasing the packing pressure in the CIM process

decreases the degree of fiber orientation along the flow

direction. Unlike the packing pressure distribution in the

CIM, the compression force in the ICM is uniformly

applied on the melt through the movable mold platen.

Therefore, no torque-moment is caused by forces F1 and

F2 (as shown in Fig. 6b). Instead of making the aligned

fibers rotate, the uniform compression force encourages

the arrangement of fibers along the flow direction.

Bay et al. [12] investigated the short-fiber orientation

in the CIM part with simple strip geometry. They found

that in general there is no in-plane stretching that occurs

during radial flow; the orientation of fibers in the core

layer is a result from the flow in the gate and is carried

down the cavity with little change. In this work, the part

has dimensions of 150 3 20 3 2 mm3 and hence can

also be considered as a strip. Unlike Bay et al.’s result,

there are more fibers orienting in a random state

(including the skin and the core regions) at position B

than position A under the same compression force, as

shown in Fig. 5. This may be attributed to the fact that

the ICM process involves a compression stage following

the injection stage. In the injection stage, the fibers in the

core region near position A orient randomly due to

the low-shear rate. In the compression stage, most of the

polymer melt containing randomly oriented fibers in the

core region near position A is pushed to fill the remaining

cavity. Because of the weak flow-induced shear rate in

the compression stage, the state of fiber orientation at

position B changes slightly. That is to say, at position B,

most fibers maintain the random state, similar to the core

region at position A.

Figure 7 shows the fiber orientation patterns at posi-

tions A through D in ICM parts molded under compres-

sion distances of 1 and 2.5 mm. As can be seen from

Fig. 7(1), more fibers orient orderly along the flow

direction under 2.5 mm compression distance than 1 mm

compression distance at position A. This can be

explained as follows. As compression distance increases,

the melt filling length in the injection stage becomes

shorter, providing a longer channel to be filled in the

FIG. 4. Simulated fiber orientation across the through thickness of posi-

tion A in CIM part. (melt temperature, 2108C; mold temperature, 508C;injection time, 1.5 s; injection pressure, 30 MPa; and cooling time,

30 s). [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]

FIG. 3. Schematic representation of fiber states in ICM parts.

[Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

1902 POLYMER COMPOSITES—-2010 DOI 10.1002/pc

compression stage. As a result, more melt in the core

region near position A, where most fibers randomly ori-

ent, is pushed forward to fill the void cavity. Meanwhile,

after compression stage the melt remaining near position

A contains more fibers that are already oriented along

the flow direction in the injection stage. Figure 7(2 and 3)

demonstrates that the changes of fiber orientation versus

compression distance at positions D and B are very sim-

ilar, with more fibers orienting randomly in the core

region under a larger compression distance. This is

because that as mentioned earlier, the melt at positions

D and B come from the core region near position A in

compression stage, meaning that the final fiber orienta-

tion at positions D and B is to a large extent determined

by the injection stage, in which most fibers in core

region near position A orient randomly. From Fig. 7(4),

it can be seen that at position C, most fibers across the

whole thickness orient parallel to the flow direction

except that a few fibers in the skin region have a tend-

ency to arrange normal to the flow direction. This is

also valid for the fibers in the core region that have a

weak dependence on the compression distance. This high

degree of fiber orientation along the flow direction may

FIG. 5. SEM micrographs of positions (1) A and (2) B in ICM parts molded under compression forces of (a) 1

and (b) 3 t. (compression time, 13 s; compression distance, 1 mm; compression speed, 1 mm/s; and delay time,

2 s). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 6. Comparison of pressure distributions between (a) packing phase

in CIM and (b) compression phase in ICM. [Color figure can be viewed

in the online issue, which is available at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2010 1903

result from high-shear rate due to the special location of

position C. The closeness of position C to the mold cav-

ity wall causes the velocity profiles of the flow with low

velocity in the thickness and width directions to overlap,

resulting in high-shear rate across the whole section near

position C.

FIG. 7. SEM micrographs of positions (1) A, (2) D, (3) B, and (4) C in ICM parts molded under compres-

sion distances of (a) 1 and (b) 2.5 mm. (compression force, 1 t; compression time, 13 s; compression speed,

1 mm/s; and delay time, 2 s).

1904 POLYMER COMPOSITES—-2010 DOI 10.1002/pc

Figure 8 shows the influence of compression speed on

the fiber orientation at positions A through D in ICM

parts. As can be seen, less fiber in the core region

arranges randomly as the compression speed increases,

especially at positions A and D. Stronger shear caused by

increased compression speed may account for this result.

FIG. 8. SEM micrographs of positions (1) A, (2) D, (3) B, and (4) C in ICM parts molded under compres-

sion speeds of (a) 1 and (b) 5 mm/s. (compression force, 1 t; compression time, 13 s; compression distance,

1 mm; and delay time, 2 s).

DOI 10.1002/pc POLYMER COMPOSITES—-2010 1905

FIG. 9. Average flow velocity distribution along the flow direction in compression stage of the ICM

process. (compression force, 1 t; compression time, 13 s; compression distance, 1 mm; compression

speed, 1 mm/s; and delay time, 2 s). [Color figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

FIG. 10. SEM micrographs of positions (1) A and (2) B in ICM parts molded under delay times of (a) 0

and (b) 2 s. (compression force, 1 t; compression time, 13 s; compression distance, 1 mm; and compression

speed, 1 mm/s).

In this study, the cross-section area of the mold cavity

keeps constant along the flow direction. Thus for CIM,

except around the gate area, the flow velocity keeps con-

stant as the melt front flows forward from the gate provid-

ing that a constant velocity control of the machine is used

for the injection stage. Whereas in the compression stage

of the ICM, the longer the melt front advances from the

gate, the higher the average flow velocity is. This can be

proved by the numerically simulated average flow veloc-

ity along the flow direction in the ICM using Moldflow

Plastic Insight 6.1, as shown in Fig. 9. Therefore, with

increased compression speed, it is easy to understand that

less randomly oriented fibers exist in the core region at

position D than position A. On the other hand, because of

the strong shear effect resulted from the strip geometry of

the molded part, the fibers across the whole surface at

position C have a perfect orientation along the flow direc-

tion regardless of the compression speed [see Fig. 8(4)].

In general, high orientation of polymer molecules results

in a significant anisotropy of the part properties, whereas

random orientation of polymer molecules gives more uni-

form properties in various directions. Hence, the ICM

parts may possess different properties across the thick-

ness. This deduction was also confirmed by the work of

Kang et al. [13]. They experimentally examined the bire-

fringence distribution in thickness direction of an ICM

optical part and found that the birefringence has a local

minimum value in the center and two local maximum val-

ues next to the core on both sides. They contributed the

increase of birefringence near the center to the thermal

stress caused by quenching. However, this study suggests

that the molecular orientation distribution developed in

the ICM process also accounts partially for that birefrin-

gence pattern.

As can be seen in Fig. 10(1), the fiber orientation

states are very similar at position A in ICM parts

molded under different delay times. This is because

most of the melt near position A is that close to the

mold cavity wall developed in injection stage. The

processing conditions in the injection stage are

the same in all the experiments, which results in the

same fiber orientation along the flow direction near

position A. Differently, all the melt at position B

comes from the core layer at position A developed in

injection stage, where the fibers orient randomly. In

compression stage, the melt temperature decreases as

the delay time increases. The increased viscosity caused

by decreased melt temperature in the core layer induces

stronger shear effect, leading to more fibers arranging

randomly to orient along the flow direction, as shown

in Fig. 10(2). Consequently, it can be concluded that

the variation of delay time has an obvious effect on

the fiber orientation at positions far from the gate, but

has little influence on the fiber orientation near the

gate.

Figure 11 illustrates the changes of the fiber orientation

at position B with different compression times. It can be

seen that the compression time has little effect on the

fiber orientation. The reason may be that the effective

time when the compression force is acted on the polymer

melt is much shorter than the set compression time. This

indicates that once the gate is solidified, further increasing

the compression time does not affect the fiber arrange-

ment. This observation agrees with the conclusion of

Huang et al.’s research [14] in which the shrinkage distri-

bution of ICM PP parts does not change with continually

increased the compression time.

CONCLUSIONS

In this work, experiments were carried out to investi-

gate the effects of processing parameters on fiber orienta-

FIG. 11. SEM micrographs of position B in ICM parts molded under compression times of (a) 5 and (b) 13 s.

(compression force, 1 t; compression distance, 1 mm; compression speed, 1 mm/s; and delay time, 2 s).

DOI 10.1002/pc POLYMER COMPOSITES—-2010 1907

tion states in ICM short-fiber-reinforced PP parts. Differ-

ent from the skin-core structure of fiber orientation in

CIM parts, a five-layer structure of fiber orientation is

found across the thickness under most conditions in ICM

parts. The fibers orient orderly along the flow direction in

the shell region, whereas most fibers arrange randomly in

the skin and the core regions. In addition, the fiber orien-

tation changes greatly in the width direction, with most

fibers orienting along the flow direction at positions close

to mold cavity wall. The results also show that the com-

pression force, compression distance, and compression

speed are important parameters influencing the fiber orien-

tation. Larger compression force or compression speed

leads to that more fibers orient along the flow direction in

the shell region. Moreover, the delay time has an obvious

effect on the fiber orientation at positions far from the

gate, and compression time is found to have a weak influ-

ence on the fiber orientation in ICM parts.

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