Crystal Distribution and Molecule Orientation of MicroInjection Molded Polypropylene Microstructured Parts

Zhen Lu, K.F. ZhangSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Polypropylene (PP) microstructured part which com-prises micro columns array and a macroscopical baseplate was manufactured by micro injection molding.The morphology distribution in micro columns is quitedifferent from that of the base plate. This article inves-tigates the crystal distribution and molecule orientationof the microstructured part by X-ray diffraction. Thehardness of shear zone of micro columns was eval-uated by Nano Indenter. Test results show that bothmicro columns and macroscopical base plate contain aand b phase. However, the relative proportion of bphase in micro columns is markedly higher than that ofthe base plate. b phase distributes only in the shearzone of the microstructured part. So, the mechanicalproperties of micro columns must differ from that ofthe base plate. In addition, the orientation of Ø100 lmmicro columns is slight, which indicates that the me-chanical anisotropy of micro columns induced by ori-entation could be ignored. POLYM. ENG. SCI., 49:1661–1665, 2009. ª 2009 Society of Plastics Engineers

INTRODUCTION

The use of microstructured parts in the field of MEMS

has been strongly increasing over the past decade [1–3].

Microstructured parts have outer dimensions of several

millimeters up to a few centimeters with three-dimen-

sional microstructures on their surface [4–7]. Micro injec-

tion molding (MIM) is a promising technology for the

replication of microstructured parts because of its low

production cost, mass production capability, applicability

for many materials, good tolerance [8, 9]. At present, the

investigation on MIM contains mold technology, special

machine, production process, filling performance analysis,

numerical simulation, etc. [10, 11]. However, seldom

research on the crystal characters and molecule orienta-

tion of microstructured parts produced by MIM is

reported. Large numbers of microstructured parts are pro-

duced from polymers especially polypropylene (PP) [12].

Isotactic polypropylene (i-PP) may have three different

crystal structures (a monoclinic phase, b hexagonal phase,

c orthorhombic phase), a mesomorphic and an amorphous

phase [13]. The presence of one or more polymorphic

forms in i-PP strongly depends on crystallization condi-

tions (temperature, cooling rate, pressure, etc.) and prod-

uct size. Furthermore, the crystal phase and molecule ori-

entation have a significant impact on the mechanical and

optical properties of the finished polymer article [14, 15].

MIM has many features which are different from conven-

tional injection molding such as higher injection pressure

and speed, higher mold temperature and mold vacuum

[16, 17]. On the other hand, the size of microstructures

decreases to micron or sub-micron. So, investigating the

crystal distribution and molecule orientation is helpful to

understand the special properties of microstructured parts.

In this article, the crystal style and molecule orientation

of PP microstructured parts were investigated by X-ray

diffraction. The hardness of shear zone of micro columns

was evaluated by Nano Indenter.

EXPERIMENTAL PROCEDURES

The experimental material i-PP with isotactic index of

98.3%, density of 0.91g/cm3 and melting point of 164–

1708C is a commercialized product from Harbin Huaao

plastic Ltd., China. These microcavities on the silicon

mold insert were produced by inductive couple plasmas

ion etching. The specimen designed in this article has

outer dimensions of 12 mm 3 7 mm 3 1.5 mm (L 3 W

3 H) with 10 3 10 micro columns (F100 lm 3 250

lm) array locating on one surface, as shown in Fig. 1.

Test specimens were fabricated by the Babyplast6/10

MIM machine (Cronoplast S.L., Espana). The process

parameters contain mold temperature of 908C, injection

pressure of 100 MPa and holding pressure of 3 s. To

understand the crystal style and molecule orientation of

the microstructured part, X-ray diffraction tests were

introduced. The base plate was test by RIGAKU X-ray

diffractometer (Rigaku, Japan). D8 DISCOVER-GADDS

X-ray micro beam (F100 lm) diffractometer (Bruker

AXS, USA) was used to detect micro column and differ-

ent areas of the base plate. Firstly, a row of micro col-

umns connecting with a part of base plate were cut off

from the whole mcirostructured part by a thin blade. Then

Correspondence to: Zhen Lu; e-mail: luzhen-hit@163.com

DOI 10.1002/pen.21167

Published online in Wiley InterScience (www.interscience.wiley.com).

VVC 2009 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2009

micro beam diffractometer tests were conducted on the

slide surface of micro column along the axial direction.

The hardness of shear zone in micro columns was eval-

uated by Nano Indenter XP (MTS, USA) at room temper-

ature with an indentation depth of 1000 nm. Before nano-

indentation test, the whole microstructured parts were em-

bedded in a supporting material to prevent the distortion

of micro columns during grinding and polishing. Then,

micro columns were ground along axial direction and

radial direction, respectively. Afterward, micro columns

were polished to form smooth cross sections and longitu-

dinal sections through the principal axis of micro col-

umns. Finally, nanoindentation tests were performed on

the shear zone of cross section and longitudinal section,

respectively.

RESULTS AND DISCUSSION

The morphology distribution of F100 lm micro col-

umn and a part of the base plate are shown in Fig. 2. As

displayed, both micro column and base plate present

‘‘skin-core’’ morphology which comprises a noncrystalline

skin layer, a shear zone, and a spherulites core. It is well

known that most conventional injection molded crystal

polymer macroscopical parts represent ‘‘skin-core’’ mor-

phology. Microstructures made by MIM still represent

‘‘skin-core’’ morphology, which has been proven in our

previous investigation [18]. However, the morphology dis-

tribution in micro columns is quite different from that of

the base plate. The relative proportion of different struc-

tures is defined as the quotient between the thickness of

different structures and of overall sample. It can be con-

cluded by comparing Fig. 2a and b that the thickness of

skin layer and shear zone do not decrease with the reduc-

tion of the microstructures size. However, the thickness of

the base plate and micro column are 1.5 mm and 100 lm,

respectively. Thus, the relative proportion of shear zone

in micro columns is markedly higher than that of the base

plate.

Figure 3 shows the diffraction profiles of different

areas of i-PP microstructured part by X-ray diffractometer

and micro beam diffractometer, respectively. The diffrac-

tion profile of the whole base plate is displayed in Fig.

3a. As shown, the 2y corresponding to the main peaks

contain 148, 178, 18.58, 218, and 228 of a phase and 168and 218 of b phase which indicates that the whole base

plate contains both a and b phase. Micro column also

comprises a phase and b phase, which can be concluded

from Fig. 3b. a phase is the most common and stable

crystal form of i-PP. b phase was found to increase by

the influence of high shear rates. The fraction of b phase

(Kb) was calculated using the following relation proposed

by Turner-Jones et al. [19].

K ¼ Ib300=ðIa110 þ Ia040 þ Ia130 þ Ib300Þ (1)

where Ib300, Ia110, I

a040, I

a130 correspond to the diffraction

intensities of the b phase (at 2y ¼ 168) and a phase (at

2y ¼ 148, 178, 18.58), respectively. The Kb of the whole

base plate and micro column are 0.276 and 0.413, respec-

tively, calculated from the diffraction intensities shown in

Fig. 3a and b. It means that the fraction of b phase in

micro columns is markedly higher than that of the base

plate. To understand the distribution of b phase in the

base plate, X-ray micro beam diffraction was introduced.

Figure 3c shows the diffraction result of the core zone

FIG. 1. Microstructured part with micro columns array.

FIG. 2. Morphology of (a) micro column and (b) a part of the base plate.

1662 POLYMER ENGINEERING AND SCIENCE—-2009 DOI 10.1002/pen

from the base plate which reveals that a phase is the only

crystal phase of the core zone. Test result of the area

including skin layer, shear zone, and core zone from the

base plate indicates that this area contains both a and bphase, as shown in Fig. 3d. On the other hand, the skin

layer is a noncrystalline layer. So, b phase must distribute

in the shear zone of the base plate. There is no special

nucleator in i-PP used in the experiments. The appearance

of b phase is induced by the effect of shearing action.

During the filling course, molecule in shear zone gets

high shearing stress, and crystal could appear in this zone.

The relative proportion of shear zone in micro column is

markedly more than that of the base plate which induces

the increase of Kb in micro column. The b i-PP has sev-

eral different characteristics in comparison with the tradi-

tional a phase. Studies have proved that b i-PP phase

shows an enhanced toughness and impact strength at

room temperature. On the other hand, the hardness will

decrease with the increase of b phase fraction [20]. So,

the mechanical properties of these micro columns must

differ from that of the base plate.

Figure 4 shows the Debye diffraction patterns of the

base plate’s core zone and micro column by X-ray micro

beam diffractometer. As shown in Fig. 4a, the shape of

the arcs is changeless and the brightness of the arcs is homo-

geneous which indicates that the crystals in the base

plate’s core zone are not oriented. The reason is that mol-

ecule of core zone gets low shearing action during the

filling stage and experiences a relative long time for

release during the cooling stage. Furthermore, the shear-

ing rate of molecular increases markedly when melt PP

flows into microcavities. Molecular of micro column must

experience a higher shear stress during the filling stage

compared with the base plate. However these micro col-

umns only hold slight orientation, as shown in Fig. 4b,

the brightness of these arcs is slightly nonhomogeneous.

The slight orientation is induced by the special process

conditions of MIM. The cooling method used in MIM

FIG. 3. X-ray diffraction profiles of different areas: (a) the whole base plate, (b) micro column, (c) core

zone of base plate, (d) a part of the base plate which contains skin layer, shear zone, and core zone.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2009 1663

process is water cooling. The average cooling rate is

small and the mold temperature is high up to 908C. Theseconditions give the molecule of micro columns a long

time to get relaxation after the injection stage. Despite the

skin layer holding marked orientation, it is too thin to

affect the whole micro column. Thus, only slight orienta-

tion is determined by micro beam X-ray diffraction for

micro columns.

Anisotropy is a special mechanical property of many

microstructures with the decrease of their size. Molecule

orientation resulted in the injection cycle is one of the

typical factors which induce the anisotropy. So, nanoin-

dentation tests were used to analyze the effect of slight

orientation of micro columns on their mechanical prop-

erty. Nanoindentation was conducted on the shear zone of

micro column along axial direction and radial direction,

respectively. Figure 5 shows the seriate values of hardness

from the nanoindentation tests on the cross section and

longitudinal section of micro column, respectively. As

shown, these values become constant gradually with the

increase of indentation depth. It is clear that the hardness

of shear zone along the axial direction of micro column is

approximate with that along the radial direction. There-

fore, the mechanical anisotropy of micro columns induced

by orientation could be ignored, such as the hardness of

shear zone.

CONCLUSIONS

I-PP micro columns (Ø100 lm 3 depth 250 lm) array

locating on a macroscopical base plate was produced by

MIM. Micro columns have the same types of crystal as

the macroscopical base plate, which are a and b crystal. bphase crystal is detected only in the shear zone of the

microstructured part. The relative proportion of shear

zone in micro columns is markedly more than that of the

base plate, which induces the increase of Kb in micro col-

umns (0.413) compared with the base plate (0.276). Mole-

cule of micro columns retains slight orientation after the

solidification stage due to the special process parameters

of MIM. The hardness of shear zone along the axial

direction of micro column is approximate with that along

the radial direction.

ACKNOWLEDGMENTS

The authors thank the 49th research institute of China

Electron Science and Technology Combine Company for

making the silicon insert.

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