replication characterization in injection molding of microfeatures with high aspect ratio: influence...

Replication Characterization in Injection Molding ofMicrofeatures With High Aspect Ratio:Influence of Layout and Shape Factor

Can Yang,1,2 Han-Xiong Huang,1 Jose M. Castro,2 Allen Y. Yi21 Laboratory for Micro Molding and Polymer Rheology, South China University of Technology,Guangzhou 510640, People’s Republic of China

2 Department of Integrated Systems Engineering, The Ohio State University, Columbus, Ohio 43210

In this work, both experimental study and numericalsimulation were carried out to investigate the replica-tion capability and flow behavior of polymeric meltinside microchannels with high aspect ratio. For thispurpose, a mold insert with microchannels havingdifferent layouts (parallel and transverse to the mainflow direction) and various cross sectional shapes(triangle, rectangle, and semicircle) was designed andfabricated. The filled length and surface morphology ofthe microfeatures molded under different conditionswere characterized using scanning electron micro-scope and optical microscope. It was found that thecross sectional shape had clear influence on the filledlength of the microfeatures, with a dependence on themold layout. Moreover, serious hesitation of polymericflows in microchannels took place in the transverselayout, which led to eccentric line defects. However,such defects were not observed in the parallel layout.At last, the specific mechanisms of how the cross sec-tional shape affected the replication capability and howthe line defects in the microfeatures’ surface formedwere discussed. POLYM. ENG. SCI., 51:959–968, 2011.ª 2011 Society of Plastics Engineers

INTRODUCTION

Microinjection molding is one of the key technologies

for fabricating micro polymeric parts for its mass-

production capability. In recent years, extensive research

activities in this area have been reported. These include

investigating the influence of process parameters and geo-

metric factors [1–4], surface topography, internal structure

of the injection molded products [5–7], and so forth.

More details on the recent development of microinjection

molding are available in the topical reviews from Refs. 8

and 9. Although microinjection molding is gaining

popularity, there is no full agreement as to what can be

considered microinjection molded parts. The most

accepted classification can be described as follows [9]:

(a) Parts having a weight of less than 1 mg or being a

fraction of a polymer pellet that is approximately spheri-

cal in shape and 3 mm in diameter. (b) Conventional

sized parts with microstructures having a thickness typi-

cally around 100 lm. (c) Parts having any dimensions

with tolerances in micrometer range, typically between 2

and 5 lm. Among these categories, the second one, i.e.,

conventional sized parts with microstructures have

attracted more attention for their wide range of applica-

tions and easiness of handling. More specifically, accord-

ing to the orientation of the microfeatures with respect to

the flow direction in conventional sized part (defined as

the main flow direction in this work), this kind of parts

have two layouts as shown in Fig. 1: One is parallel lay-

out which involves microfeatures whose long dimension

is parallel to the main flow direction (see Fig. 1a), and

the other is transverse layout with microfeatures whose

long dimension is perpendicular to the main flow direc-

tion (see Fig. 1b). For the latter case, the microfeatures

can be located on either the lateral surface (Region I) or

the top surface (Region II). However, they can be consid-

ered to have similar flow dynamics due to the large

difference in dimensions between the base plate and the

microfeatures. This hypothesis has been verified by the

preliminary investigation using simulation (unpublished

work) which showed very similar temperature and pres-

sure histories at the entrance of the microfeatures located

in both regions shown in Fig. 1b.

In the authors’ previous work [10], the effects of

process conditions and geometric parameters on the repli-

cation quality of the microfeatures were investigated.

However, the aspect ratio (height-to-width ratio) of the

microfeatures was limited to less than 2. It is well known

that the aspect ratio is a key issue influencing the filling

capacity and the flow behavior of polymer flow into the

microchannel. Therefore, as the continuity of the authors’

previous work, the present article is intended to include

the replication capability and flow behavior of polymeric

melt inside high aspect ratio microfeatures. The advan-

tages of locating the microfeatures on the main flow plane

Correspondence to: Can Yang; e-mail: [email protected] or Allen Y. Yi

e-mail: [email protected]

Contract grant sponsor: National Science Foundation; contract grant num-

ber: EEC-0425626; contract grant sponsor: China Scholarship Council.

DOI 10.1002/pen.21914

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

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2011

(see Fig. 1a and Region I in Fig. 1b) are that diverse

cross sectional shape and high aspect ratio of the micro-

features can be easily machined using the method to be

discussed in this article. Therefore, only the microfeatures

located on the main flow plane will be discussed in this

work.

The two layouts shown in Fig. 1 corresponds to differ-

ent flow behavior when polymeric melt flows into the

cavity, since the temperature and pressure fields at the

vicinity of the microchannel entrance are different. For

the former case, the melt fills the base plate first, followed

by the microfeatures without altering the flow direction.

For the latter case, however, the melt flowing in the main

cavity corresponding to the base plate has to turn 908 to

flow into the microfeatures. In this regard, the replication

capability and flow behavior in the microfeatures must be

different, and is worthy of being studied. To the best of

authors’ knowledge there is no report available on

detailed comparison of the replication capability and flow

behavior encountered in injection molding of both designs

presented in Fig. 1. On the other hand, it is well known

that some micro scale factors which are normally

neglected in conventional injection molding may play an

important role in microinjection molding. In this study,

the cross sectional shape of the microfeatures is selected

as the geometric variable in terms of its effect on the rep-

lication capability.

Design and Fabrication of the Micro Mold Insert

A mold insert consisting of a main cavity and three

microchannels was designed as shown in Fig. 2a. The

square cavity has a length of 25 mm and a thickness of

1.5 mm, respectively. Three gate locations (Gates no.

1–3) are available for feeding the cavity. Gates no. 2 and

3 are equivalent. By using either Gate no. 1 or 2 one at a

time, this design offers the two different layouts shown in

Fig. 1. For some experiments, Gate no. 3 is used instead

of Gate no. 2 to see the effect of flow direction on the

flow behavior. To minimize the effect of the trapped air

in the microchannels during injection, the microchannels

were designed in such a way that they are long enough

(10 mm) to accommodate the trapped air without huge

pressure increase under general molding conditions. The

microchannels are located along one edge of the main

cavity with a gap of 1.5 mm (the middle one is exactly at

the center of the edge). As shown in Fig. 2b, to include

the effect of cross sectional shape on the replication capa-

bility, the microchannels were designed to have different

cross sectional shapes (triangle, rectangle and semicircle).

All microchannels have the same cross sectional area, i.e.,

5640 6 15 lm2.

The mold insert was made of the aluminum 6061,

which has a brinell hardness of 95 and compression

strength of 276 MPa. The designed mold insert with

microchannels was machined on an ultraprecision diamond

machine (350 FG, Moore Nanotechnology Systems,

Keene, New Hampshire) using micro carbide tools with

well-defined cutting edges. The main specifications of the

ultraprecision machine were detailed elsewhere [11]. After

diamond turning to create a flat surface on the mold insert,

the microchannels were machined by high speed micromil-

ling using an ultraprecision high speed air bearing spindle

designed and built by Professional Instrument (ISO 6000,

maximum speed 60,000 rpm or revolution per minute). A

50-lm diameter carbide flat end carbide mill (Performance

Tools, WI) was used and the spindle speed was set at

20,000 rpm. Finally, the machined insert with the main

cavity and microchannels was mounted into a two-platen

mold, which is schematically shown in Fig. 2c.

Experiments

A microinjection molding machine (LD30EH2, Sodick

Plustech) with a maximum clamping force of 30 ton and

a maximum injection velocity 250 mm s�1 was used for

the molding process. Unlike conventional injection mold-

ing machine, the Sodick LD30EH2 machine has an

injection system composed of a screw plasticizing unit

and a piston injection unit. The plasticizing screw has a

diameter of 14 mm, and a 12-mm diameter injection

FIG. 2. Schematic (a) top view of the entire cavity, (b) side view of

microchannels, and (c) the entire two-platen mold setup (unit: mm).

FIG. 1. Representative of microfeatures with their long dimensions (a)

parallel to, and (b) perpendicular to the main flow direction (The arrow

represents the main flow direction).

960 POLYMER ENGINEERING AND SCIENCE—-2011 DOI 10.1002/pen

piston allows an injection stroke up to 70 mm. A semi-crys-

talline polymer, high density polyethylene (HDPE, ExxonMo-

bilTM HD 6704.18) with a melt index of 4.0 g/10 min

(1908C/2.16 kg ASTM D1238) was used in the experiments.

During injection molding, the mold temperature and injection

pressure were fixed at 308C and 70 MPa, respectively. As

our interest is the filling stage of injection process, the pack-

ing stage is omitted, and accordingly the packing pressure is

set to 0 MPa. To investigate the influence of the controllable

process variables on the replication capability and flow

behavior, a full factorial experiment with two values of the

barrel temperature and injection velocity was carried out,

namely: 200 and 2408C, 100 and 200 mm s�1, respectively

resulting in 22 runs. To ensure a stable injection process,

around 100 test pieces were molded before the test samples

were collected. For each molding condition, ten samples were

collected and the last five were used for characterization.

To quantitatively characterize the replication capability of

the microchannels with different cross sectional shapes, an

optical microscope, Nikon optical measurescope (MM-11,

Nikon) was employed to measure the filled length. Addition-

ally, a scanning electron microscope (SEM, Hitachi S-

3000H) operating at 20 kV was used to evaluate the surface

morphology of the microfeatures. Before examination was

performed by the SEM, all microfeatures were sputter coated

with a thin layer of gold on the surface to prevent charging.

Simulation

To better understand the flow behavior in the injection

molding with a mold designed as shown in Fig. 1, a simu-

lation was carried out using the commercial software Mol-

dex3D R9.1 (CoreTech System, Chupei City, Taiwan). At

the filling stage, both polymer and air are assumed to be

incompressible. The polymeric melt is assumed to behave

as a generalized Newtonian fluid. The seven-parameter

Cross WLF (Williams-Landel-Ferry) viscosity model was

selected in the simulation, which is represented as follow:

Z ¼ Z0

1þ Z0gt�

8: 9;1�n(1)

Z0 ¼ D1 � exp �A1 � T � T�ð ÞA2 þ T � T�ð Þ

� �(2)

T* ¼ D2 þ D3 � P (3)

A2 ¼ ~A2 þ D3 � P (4)

where Z is the viscosity, Z0 is the zero shear viscosity,

and _c is the shear rate. The others are material constants,

whose values are listed in Table 1. Non-slip boundary

condition for all mold walls was adopted in the simula-

tion, and the heat transfer coefficient between polymer

and mold was set to 5,000 W (m K)�1.

As one of the greatest challenges encountered in

simulation of microinjection molding, it is extremely

difficult to mesh the entire model with large dimen-

sional changes using reasonable mesh sizes. Therefore,

a two-step simulation strategy was adopted to separate

the filling process in the main cavity and microfeatures,

which is schematically shown in Fig. 3. As the first

step, considering the fact that the microfeatures hardly

affect the filling of the main cavity, a filling simulation

of the main cavity without the microfeatures was car-

ried out, and the temperature and pressure histories

(T(t) and P(t) in Fig. 3) at the locations of the micro-

features were recorded by setting up sensor nodes dur-

ing building of the mesh model. In the second step, the

recorded T(t) and P(t) were imposed on the entrance

surface of the microfeatures as the inputs for the sepa-

rate simulation. To do so, the temperature and pressure

histories were further assumed to be uniform in the

entire entrance of the microfeatures (xy plane in Fig. 3)

owing to the extremely small area. Taking advantage of

the symmetry, only half of each model for the micro-

features was meshed for calculation. In addition, in

order to ensure the accuracy of the simulation in the

microfeature regions, all models were meshed using the

same three dimensional elements (prism) with the same

mesh size (5 lm). Finally, at least nine-layer meshes

have been generated across the smallest dimension (the

height, width and radius for triangle, rectangle, and

semicircle, respectively), which are shown in Fig. 4.

The same polymer material and molding conditions

used in the experiments were employed in the simula-

tion. The material data for simulation was from the

Moldex3D material database.

RESULTS AND DISCUSSION

Replication Capability of the Microfeatures

Effect of the Layout. Filling capability in the injection

stage is important for successful molding of microfea-

TABLE 1. Cross WLF viscosity model coefficients for HDPE.

n s* (Pa) D1 (Pa s) D2 (K) D3 (K Pa�1) A1 A~2 (K)

0.354 81700 1.19e þ 019 153.15 0 44.445 51.6

FIG. 3. Schematic process of the two-step simulation strategy: (a) sens-

ing temperature and pressure histories, and (b) imposing temperature and

pressure histories on microfeature entrance.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2011 961

tures, especially for those with high aspect ratio depicted

in this article. Specifically, the filled lengths of the micro-

features with various cross sectional shapes are obtained

under different layouts (parallel or transverse flow). Figure

5 and 6 present the filled lengths of the microfeatures

under both layouts in the experiment and simulation,

respectively. One can see that the simulation results agree

well with the experiments, not only in the trend of

change, but also in the magnitude of the filled lengths. It

is discovered that regardless of the cross sectional shape,

the filled length of the microfeature molded under trans-

verse flow is larger than that under parallel flow under a

given molding condition. One might expect the opposite

result of this observation, i.e., the microfeature molded

under parallel flow would be filled longer than that under

the transverse flow with the same molding condition,

since the filling is simpler without hesitation for the for-

mer case. To explain the unexpected results, the specific

changes of some important variables such as melt temper-

ature and filling pressure are needed. Figure 7 illustrates

the inlet pressure at the microfeature locations as a func-

tion of the filled volume of the main cavity. The barrel

temperature was set at 2408C while the injection velocity

was varied from 100 to 200 mm s�1. As shown in Fig.

7a, one can see that in parallel flow the inlet pressure at

the microfeature locations started building up at the

moment when the main cavity was almost filled (99% by

volume), and then jumped to the highest value immedi-

ately. Moreover, higher injection velocity required higher

filling pressure. Additionally, the time for the filling to

start and the filling pressure to develop were the same for

all microfeatures under a given molding condition. This

is because all microfeatures had the same distance to the

FIG. 4. Meshed models of microfeatures with (a) triangular, (b) rectan-

gular, and (c) semicircular cross section. The arrow represents the sym-

metric axis of the cross section. [Color figure can be viewed in the

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

FIG. 5. Filled lengths of microfeatures obtained under (a) parallel flow,

and (b) transverse flow (Molding Conditions 1: 2008C, 100 mm s�1; 2:

2008C, 200 mm s�1; 3: 2408C, 100 mm s�1; 4: 2408C 200 mm s�1).

FIG. 6. Simulated filled lengths of microfeatures obtained under (a)

parallel flow, and (b) transverse flow (Molding Conditions 1: 2008C,100 mm s�1; 2: 2008C, 200 mm s�1; 3: 2408C, 100 mm s�1; 4: 2408C,200 mm s�1).

FIG. 7. Predicted inlet pressure at microfeature locations under (a)

parallel flow, and (b) transverse flow (barrel temperature: 2408C).


gate and the flow was one dimensional before entering

the microfeatures. To the contrary, in the transverse flow

the inlet pressure started building up as the main cavity

was filled by nearly a half (60% by volume), and increas-

ing gradually until the sudden jump at the end of the fill-

ing of the main cavity, which is shown in Fig. 7b. In this

case, all microfeatures were located at various distances

from the gate, which led to different times to start being

filled. As a result, the pressure at the entrance varied

from one microfeature to another even under the same

molding condition. For instance, with an injection veloc-

ity of 200 mm s�1, the microfeature with triangular cross

section had the highest pressure during filling. Besides

the pressure, the inlet temperature change is also a crucial

factor affecting the filling of the microfeatures. Figure 8

shows the predicted inlet temperature at microfeature

locations as a function of filled percentage of main cavity

under parallel and transverse flow. As shown in Fig. 8a,

in parallel flow the temperature can be considered the

same as the barrel temperature since there was only a

slight decrease during the filling. However, from Fig. 8b,

one can see that in transverse flow the temperature at

microfeature locations increased as the melt front pro-

ceeded in the main cavity. This is especially the case

with a low injection velocity of 100 mm s�1, which has a

maximum temperature drop of 208C. It should be noted

that in transverse flow, the microfeature with longer dis-

tance from the gate had higher temperature because the

melt reached the entrance of different microfeatures at

different times, which subsequently led to faster cooling

for the microfeature near the gate.

The influence of the layout on the filled length of the

microfeatures should result from the interaction between

the pressure and the temperature. As far as the pressure is

concerned, compared to the parallel flow, the inlet pres-

sure developed in transverse flow had two characteristics,

namely, longer duration and higher value at the end of

filling. On the other hand, although the temperature at the

entrance was always kept above the flow temperature

(�1308C) of the material even for the transverse flow

having a relatively large temperature drop, the melt front

in microchannels would cool down in a short time.

Consequently, the specific filling sequence of the micro-

channels in transverse flow is more complicated and will

be discussed in the next section.

Effect of the Cross Sectional Shape. From Figs. 5 and

6, it can be clearly seen that under the same condition

there is a continuous increase in filled length as the cross

sectional shape changes in the order of triangle, rectangle

and semicircle. To better characterize the effect of cross

sectional shape, the relative filled length (RFL) was

defined as the ratio of filled length of the rectangular or

semicircular microfeature to that of triangular one. This

means the triangular microfeature always has an RFL

value of 1, and a value larger or smaller than 1 represents

an increase or decrease in filled length with the triangular

microfeature as the reference. Figure 9 shows the RFL of

microfeatures obtained in the parallel and transverse flow

with different molding conditions. From Fig. 9a, one can

see that the RFL reaches up to 1.2 for the semicircular

microfeature, indicating 20% increase in filled length. The

rectangular microfeature also gained 1.1–1.15 for RFL,

depending on the used molding condition. Similarly, as

shown in Fig. 9b, in the transverse flow, the RFL

increases when cross sectional shape is varied in the order

of triangle, rectangle, and semicircle. However, the differ-

ence from the former one is that the RFL is limited

within 1.1 regardless of the molding condition. Figure 10

indicates the simulated RFL of microfeatures for both lay-

outs. It is seen that the simulation result shows a good

agreement with experiment in general, with an underesti-

mate of RFL for rectangular microfeature in parallel flow.

The larger RFL in parallel flow than in transverse flow

can be explained as follows. In parallel flow, since the

pressure and temperature histories were the same for

all microfeatures during filling under the same condition

FIG. 8. Predicted inlet temperature at microfeature locations under

(a) parallel flow, and (b) transverse flow (barrel temperature: 2408C).

FIG. 9. RFL of microfeatures obtained under (a) parallel flow, and (b)

transverse flow.


(see Figs. 7a and 8a), it can be inferred that the different

RFL of microfeatures only resulted from different cross

sectional shape. While for transverse flow, because of

different distances from the gate, the semicircular and rec-

tangular microfeatures had lower inlet pressure than the

triangular one (see Fig. 7b), which partially lowered the

easiness of filling. From the RFL results, it is reasonable

to conclude that the semicircular microfeature is the easi-

est to fill, followed by the rectangular and triangular

shapes. The mechanism of how the cross sectional shape

affected the replication capability needs to be discussed.

Since the cross sections of all microfeatures have the

same area but different perimeters, the heat transfer

capacity and the pressure drop should be different. For

the convenience of analysis, the hydraulic diameter (dH)as expressed in Eq. 5 is used in this study:

dH ¼ 4A=L (5)

where A and L are the area and the perimeter of the cross

section, respectively. Using the dimensions shown in Fig.

2b, the dH values were calculated as 62.1, 67.6, and 73.3

mm for triangular, rectangular, and semicircular microfea-

ture, respectively. With this arrangement, the microfea-

tures with the same cross sectional area but different

shapes were equivalent to circular microfeatures with dif-

ferent diameters. Smaller diameter means larger surface

to volume ratio or perimeter to cross sectional area ratio,

which leads to faster heat transfer. In the equivalent circu-

lar (originally triangular) microfeature, the temperature

gradient existed in both radial and axial directions (r and

z direction, respectively, as shown in Fig. 11). However,

at a fixed distance from the entrance in the axial direction

(z coordinate), the heat transfer from the polymeric melt

to the mold is mainly determined by the heat flux in the

radial direction. To obtain the heat flux value, sensor

Nodes A and B were set in the radial direction in the sim-

ulation to record the temperature changes (see Fig. 11).

The distance from sensor Nodes A and B to the melt

entrance is 300 mm, and to the melt-mold interface is 3

and 6 mm, respectively. Figure 11 presents the predicted

temperatures at sensor Nodes A and B for triangular

microfeature in parallel flow with barrel temperature of

2008C and injection velocity of 100 mm s�1. It can be

seen that there is a clear temperature drop toward the

melt-mold interface. The temperature difference of sensor

Nodes A and B divided by the distance of 3 mm and mul-

tiplied by thermal conductivity of 0.5 W (m 8C)�1 made

the heat flux (W cm�2) through the polymer. Figure 12

presents the predicted heat flux in the radial direction for

all microfeatures in parallel flow with barrel temperature

of 2008C and injection velocity of 100 mm s�1. One can

see that there is a substantial increase in heat flux for

microfeatures with cross sectional shape in the order of

semicircle, rectangle, and triangle. Larger heat flux means

quicker heat transfer between the polymeric melt and the

mold. Consequently, the polymeric melt cools down

faster, which in turn results in a decrease of the filled

length. One can also see a sharp peak of the heat flux

exists at the beginning of the filling process for all micro-

features, indicating that most of the heat was transferred

at the beginning of the process. This is because at the lat-

ter part of the process, the heat transfer was inhibited by

the frozen layer of polymer developed right after it

touched the colder mold wall. In addition, the predicted

heat flux ranges from 60 to 100 W cm�2, which agrees

well with that found in experiments [12].

Besides heat transfer variation, the pressure drop of the

polymer flow in microchannels also changes with differ-

ent dH. Because of the extremely small dimension of the

microfeatures, the Reynolds Number (Re) is as small as

10�6 under normal injection conditions [13], which is far

less than the turbulent flow (Re ‡ 4000). This means that

the polymer flow inside the microchannels is laminar. For

laminar flow inside the microchannel with a hydraulic

FIG. 10. Simulated RFL of microfeatures obtained under (a) parallel

flow, and (b) transverse flow.

FIG. 11. Predicted temperature change at sensor Nodes A and B in

radial direction in parallel flow for equivalent circular (originally trian-

gular) microfeature (barrel temperature: 2008C, injection velocity:

100 mm s�1; sensor Nodes A and B are 300-lm away from the micro-

channel entrance).


diameter of dH, the pressure drop per unit length (DP/L)is as expressed in Eq. 6.

DP=L ¼ 128Qp�1md�4H (6)

where Q, and m are the volume flow rate, and the

dynamic viscosity of the material, respectively. Under a

given temperature, despite the dependency of viscosity on

shear rate, the polymer can be assumed at the second

Newtonian region since the shear rate is extremely high

(over 106/s) in microchannels [13]. Therefore, the viscos-

ity variation will not be a major factor causing pressure

drop since the viscosity variation is negligible at the sec-

ond Newtonian region. Assuming Q is uniform for all

microchannels, DP/L is inversely proportional to dH4.

Therefore, DP/L is mainly affected by dH, which means

that even small difference in dH may cause larger varia-

tion in DP/L. Figure 13 shows both dH and DP/L changes

versus the cross sectional shape. It can be seen that as the

dH increases gradually, the DP/L decreases drastically.

From triangular to semicircular microfeature, only 20%

increase in dH leads to 50% DP/L drop. Smaller pressure

drop means smaller resistance for the flow, which resulted

in larger filled length of the microfeature. In a word, the

pressure drop of the polymer flow inside the microchan-

nels is another important factor influencing the filling

capability.

Surface Morphology of the Molded Microfeatures

As mentioned earlier, the pressure and temperature his-

tories at the entrance of the microfeatures in parallel and

transverse flow are different, which resulted in different

flow behavior. This may further lead to different surface

morphology of the microfeatures. To verify this hypothe-

sis, the microfeatures molded under the test conditions in

this research were examined using SEM. Figure 14 shows

the surface morphology of the microfeatures molded in

parallel flow with various conditions. As can be seen from

Fig. 14a–d, all microfeatures have smooth surfaces

regardless of the molding condition. It should be men-

tioned that some flashing occurred along the perimeter of

the microfeatures. The thickness of the flashing is around

2 lm, far less than the dimension of the microfeatures,

thus their effect on the flow can be ignored. Figure 14a0

presents detailed view of the triangular microfeature

selected in Fig. 14a. A clear edge definition of the micro-

feature can be seen. There is no obvious surface defect

except small flashing around the edges. On the contrary,

FIG. 12. Predicted heat flux in the radial direction for all

microfeatures in parallel flow (barrel temperature: 2008C, injection

velocity: 100 mm s�1).

FIG. 13. Pressure drop and hydraulic diameter changes versus the cross

sectional shape.

FIG. 14. SEM microphotographs of microfeatures in parallel flow

obtained under a barrel temperature and injection velocity of: (a) 2008Cand 100 mm s�1, (b) 2008C and 200 mm s�1, (c) 2408C and 100 mm

s�1, and (d) 2408C and 200 mm s�1; (a0) detailed observation of the

selected area in Figure (a). (The arrow represents the melt flow direction

in the base plate; all micrographs use the same scale bar).


Fig. 15 presents the surface morphology of the microfea-

tures in transverse flow molded with barrel temperature of

2008C and injection velocity of 100 mm s�1. Figure 15a

shows that the microfeatures also have very smooth surfa-

ces and clear edge definition. However, in this case, the

microfeatures have line defects on their surface across

the width about one thirds of the filled length away from

the root, as shown from Fig. 15b–d. Under careful exami-

nation, it was found that the line defect was present

around the entire perimeter of the cross section, meaning

that the molded microfeature is consisted of two segments

(A and B in Fig. 15b) connecting to each other by the

inner material at the location of the line defect. The line

defects are remarkable for the triangular and rectangular

microfeatures under this molding condition. However, the

line defect narrowed to a separating line for semicircular

microfeature, as shown in Fig. 15d. Figure 16 gives the

comparison of surface morphology in transverse flow

molded under various molding conditions. It can be found

that by increasing injection velocity or/and barrel temper-

ature, the surface morphology of the microfeatures can be

improved, namely, the line defects on the surface became

smaller or even disappeared. For instance, at barrel tem-

perature of 2008C, the line defects of the triangular and

rectangular microfeatures narrowed to the separating

lines and the one of the semicircular microfeature disap-

peared with increasing injection velocity from 100 to

200 mm s�1 (see Fig. 16b). Furthermore, with a barrel

temperature of 2408C and injection velocity of 200 mm

s�1, only triangular microfeature had a separating line on

the surface (see Fig. 16d). It should be mentioned that

under some molding conditions, the molded microfeatures

also had torn-off skin near the root (see Fig. 16c and d),

which is probably caused by the experienced high shear

stress. At this point, it can be concluded that the micro-

features near the gate have a higher potential to have a

line defect. Therefore the formation of the line defect

around the cross section mainly resulted from the interac-

tion between the inlet pressure and temperature, not the

cross sectional shape of the microfeatures. To verify this

conclusion, an experiment with reversed main flow direc-

tion was performed by using Gate no. 3 shown in Fig. 2.

Figure 17 shows the surface morphology of microfeatures

molded under a barrel temperature and injection velocity

of 2008C and 100 mm s�1 with reversed gate. From Fig.

17b, one can see just as the triangular and rectangular

microfeatures, the semicircular microfeature has a wide

and deep line defect, which never happened when Gate

no. 2 was used.

Mechanism of Line Defect Formation

To further investigate the formation of the line defect

in transverse flow a series of short shots were carried out

with a barrel temperature of 2008C and injection velocity

of 100 mm s�1. The metering system of the microinjec-

tion molding machine used in this work has a metering

tolerance of 62 mm3, allowing the precise control of the

shot size. Figure 18 presents the parts molded in the short

FIG. 15. SEM microphotographs of microfeatures in transverse flow

obtained under a barrel temperature and injection velocity of 2008C and

100 mm s�1: (a) overview of the microfeatures, (b), (c), and (d) detailed

observation of the triangular, rectangular, and semicircular microfeature,

respectively (the arrow represents the melt flow direction in the base

plate; all micrographs use the same scale bar).


obtained under a barrel temperature and injection velocity of: (a) 2008Cand 100 mm s�1, (b) 2008C and 200 mm s�1, (c) 2408C and 100 mm

s�1, and (d) 2408C and 200 mm s�1 (the arrow represents the melt flow

direction in the base plate; all micrographs use the same scale bar).


using reversed gate obtained under a barrel temperature and injection ve-

locity of 2008C and 100 mm s�1: (a) overview of the microfeatures, (b)

detailed observation of the semicircular microfeature (the arrow repre-

sents the melt flow direction in the base plate; all micrographs use the

same scale bar).


shot experiment by changing the shot size. The numbers

within the images indicate the filled percentage, which is

calculated based on the maximum shot size used in the

experiment. Figure 18a shows that all microchannels were

filled less than 0.2 mm with a filled percentage of

93.75%. This observation can be explained as follows. In

the filling process, the polymeric melt started to fill the

microchannels once the melt front in the main cavity

reached the entrance of the microchannels, when the part

had a corresponding filled percentage around 50%. How-

ever, the large surface-to-volume ratio of microchnnels

caused by drastic decrease in dimension led to quick heat

transfer between the polymeric melt and the mold, result-

ing in rapid cooling of the melt. Consequently, the melt

flow proceeding in the microchannels became very slowly

or even tended to stop, which is known as hesitation.

Once the flow hesitation occurred, a frozen layer is

formed at the melt front. At the same time, the melt in

the inner layer was still at high temperature and high

pressure. As the filling went on, the pressure along the

flow path increased. As can be seen in Fig. 18b, at the

time the part was filled by 96.88%, the melt in the semi-

circular microchannel close to the end of the main cavity

moved much farther than that in other microchannels.

This is because the melt reached the semicircular micro-

channel slightly behind compared with other microchan-

nels, meaning that the melt temperature in the semicircu-

lar microchannel was higher, which resulted in easier fill-

ing. Similarly, as shown in Fig. 18c, when the part was

filled 98.44%, the pressure along the flow path further

increased, which led to noticeable filling improvement for

the rectangular microchannel. However, no obvious

improvement for the semicircular and triangular micro-

channels can be observed. For semicircular microchannel,

this is mainly due to the fact that the pressure at the

entrance was inadequate to drive more melt into the

microchannel, although the melt front was still in high

temperature. To the contrary, for triangular microchannel,

despite enough pressure at the entrance, the melt could

not move much more due to the melt front temperature

being too low caused by long residence time. Figure 18d

shows that at the moment when the part was 100% filled,

the filled length for all microchannels gained great

improvement. This is because at the moment when the

main cavity was completely filled, the pressure along the

flow path greatly increased. Upon this drastic pressure

increase, the melt in the inner layer already filled in the

microchannels started to move forward again quickly,

which is referred as the ‘‘abrupt flow’’ in the microchan-

nels. The ‘‘abrupt flow’’ finished in a very short time with

a very high velocity. This is the reason why the microfea-

ture closer to the gate has wider and deeper line defect on

the surface. This is also the reason why the Segment B of

the microfeature has more flashing than Segment A (see

Fig. 15b). It should be pointed out that the ‘‘abrupt flow’’

for each microchannel took place at different moments.

The ‘‘abrupt flow’’ happened in the semicircular micro-

channel first, followed by the rectangular and triangular

microchannels. In a word, the surface line defect is

believed to mainly have been resulted from the ‘‘abrupt

flow.’’ The morphology of the line defect strongly

depends on the condition under which the ‘‘abrupt flow’’

happens. The lower melt front temperature and higher fill-

ing pressure led to wider and deeper surface line defect.

To further verify the proposed mechanism of the line

defect formation, an experiment was performed using

both high barrel temperature and high injection velocity

(2508C and 250 mm s�1). Figure 19 illustrates the surface

morphology of the molded microfeatures. It is found that

no visible line defects can be observed at the locations

where they used to appear under most investigated mold-

ing conditions (see selected regions in Fig. 19b–d). Note

that some surface defects near the root of the microfea-

tures in Fig. 19b and c are torn-off skins discussed earlier

and not the line defects caused by flow hesitation. With

increased injection velocity, on one hand the melt reached

FIG. 18. SEM microphotographs of short shot experiments with differ-

ent filled percentages (the arrow represents the melt flow direction in the

base plate; all micrographs use the same scale bar).

FIG. 19. SEM microphotographs of the microfeatures free of line

defects molded under a barrel temperature and injection velocity of

2508C and 250 mm s�1 (the arrow represents the melt flow direction in

the base plate; all micrographs use the same scale bar).


the microchannel locations faster, giving less chance for

cooling, and on the other hand more material can be filled

into the microchannels at a given time. Meanwhile, the

melt viscosity decreased with elevated barrel temperature,

making it easier for the melt to flow inside the micro-

channels. More importantly, as the filling time is reduced,

the residence time of the melt inside the microchannels

decreased, which means the ‘‘abrupt flow’’ took place

with higher melt temperature. With high enough melt

temperature, the ‘‘abrupt flow’’ could not cause line

defects any more.

Molding Issues

From Figs. 5 and 6, one can find that at high injection

velocity (200 mm s�1), high barrel temperature (Condi-

tion no. 4) produced negative effect on the filled length

compared with the case using low barrel temperature

(Condition no. 2) in parallel flow. This phenomenon is

probably due to higher pressure of the trapped air during

injection, which has been reported in the authors’ previ-

ous work [10]. However, in transverse flow, the filled

length of the microfeatures maximized with Condition no.

4. This suggests that the resistance of trapped air in

microchannels in transverse flow was not as high as that

in parallel flow. This is because in transverse flow, the

microchannels were located in the center of the main cav-

ity edge, which is not the last region to be filled for poly-

meric melt. However, in parallel flow, all the trapped air

originally existed in the cavity before injection would be

pushed into the microchannels, which are the last regions

to be filled.

Based on the observation, under the same molding

condition, larger filled length can be obtained in the trans-

verse flow, while better surface quality in the parallel

flow. Increasing the barrel temperature or injection veloc-

ity can eliminate the surface defects caused by hesitation.

However, this will produce other defects caused by mate-

rial degradation. Therefore, for injection molding of

microfeatures requiring perfect surface quality, the design

involving the parallel flow is preferred.

CONCLUSIONS

In this study, the replication capability and flow behav-

ior of polymeric melt inside microchannels with different

layouts and cross sectional shapes were investigated using

both experimental and numerical methods. The filled

length of the molded microfeatures increased with cross

sectional shapes in the order of triangle, rectangular and

semicircle. This was determined to have been resulted

from the heat transfer and pressure drop variation with

different cross sectional shapes. In addition, the layouts of

the microfeatures had great influence on not only the

filled length but also the surface morphology. In trans-

verse flow, microfeatures had larger filled length mainly

because of the longer duration and higher value of the

inlet pressure. As far as the surface morphology is con-

cerned, microfeatures molded in parallel flow had very

smooth surface and clear edge definition. However, flow-

induced line defects appeared on the microfeatures under

most molding conditions in transverse flow due to the

special pressure and temperature histories during filling

process. Furthermore, among the designs investigated in

this work, it was discovered that air can be easily trapped

in the microchannels in parallel flow, which had negative

effect on filling capability.

ACKNOWLEDGMENTS

The authors acknowledge the assistant by David L.

McCray in mold fabrication at The Ohio State University.

The ISO high speed air bearing spindle system was

provided by the Professional Instruments Company in

Hopkins, Minnesota.

REFERENCES

1. Y.C. Su, J. Shah, and L.W. Lin, J. Micromech. Microeng.,14, 3 (2004).

2. L.Y. Yu, C.G. Kon, and L.J. Lee, Polym. Eng. Sci., 42, 5(2002).

3. F. Sammoura, J. Kang, and Y.M. Heo, Microsyst. Technol.,13, 5 (2007).

4. D.G. Yao and B. Kim, J. Micromech. Microeng., 12, 5

(2002).

5. S.H. Yoon and C. Srirojpinyo, SPIE, 5763, 1 (2005).

6. J. Chu, A. Hrymak, and M.R. Kamal, ANTEC, 4, 1973

(2007).

7. K.F. Zhang and Z. Lu, Microsyst. Technol., 14, 2 (2008).

8. J. Giboz, T. Copponnex, and P. Mele, J. Micromech. Micro-eng., 17, 6 (2007).

9. U.M. Attia, S. Marson, and J.R. Alcock, Microfluid. Nano-fluid., 7, 1 (2009).

10. C. Yang, L. Li, H.X. Huang, J.M. Castro, and A.Y. Yi,

Polym. Eng. Sci., 50, 10 (2010).

11. C.N. Huang, L. Li, and A.Y. Yi, Microsyst. Technol., 15, 4(2009).

12. C. Yan, M. Nakao, T. Go, K. Matsumoto, and Y. Hatamura,

Microsyst. Technol., 9, 3 (2003).

13. S. K. Kim, S.-J. Choi, K.-H. Lee, D.-J. Kim, and Y.-E. Yoo,

Polym. Eng. Sci., 50, 7 (2010).


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