Residual Stresses and Thermoviscoelastic Deformationof Laminated Film Prepared for Film Insert Molding

Sung Ho Kim, Seong Yun Kim, Seung Hwan Lee, Jae Ryoun YounResearch Institute of Advanced Materials (RIAM), Department of Materials Science and Engineering,Seoul National University, Gwanak-Gu, Seoul 151 742, Korea

Residual stresses and thermoviscoelastic deformationof a laminated film utilized for film insert molding wasinvestigated through measurement of thermal expan-sion coefficient (CTE) and relaxation modulus. Thermo-viscoelastic deformation of the film was also analyzedwith numerical analysis by applying measured relaxa-tion modulus, CTE, and residual stress to finite ele-ment method (FEM). Stress relaxation of the pristinefilm showed significantly different behavior from thatof the unannealed film during annealing. Effects of theCTE and relaxation modulus on the thermoviscoelasticdeformation were predicted by considering thermalshrinkage and structural relaxation. Moreover, numeri-cal results on thermoviscoelastic deformation were ingood agreement with experiments when initial stressdistribution in the solid specimen was applied to thenumerical analysis. POLYM. ENG. SCI., 52:1121–1127, 2012.ª 2011 Society of Plastics Engineers

INTRODUCTION

Film insert molding (FIM), known as in-mold decorat-

ing, is one of the various coating technologies to perform

surface modification of injection molded parts. In general,

a film is inserted into a mold and then the cavity is filled

by a molten resin such that physical bonding is induced

at the interface between the film and the resin. Therefore,

it is possible to conduct the FIM process at short cycle

time and comparatively low cost due to one-step process

without post-processing, e.g., printing patterns on a film

and attaching it over the substrate. It has also been

applied to various products, such as mobile phone cases

with logo design, automotive parts, and packaging materi-

als, and will be utilized in more industrial fields. How-

ever, it is well known that many problems such as wash-

ing-off of printed ink, delamination, wrinkling, and

remelting of the laminated film occurred at the surface of

final film insert molded parts [1]. Frequently, cited studies

[2–4] reported that other processing defects, e.g., un-

desired creases, folded lines, and severe warpage, were

generated at the near weld-line region when high shear

rate was applied to FIM process. It has also been reported

that unusual problems were observed due to thermal con-

ductivity difference between the solid film and the molten

resin during FIM and asymmetric residual stresses over

the film and substrate regions [5–7].

It is essential to evaluate effects of various parameters

on internal structure and physical properties of final FIM

parts and understand FIM process. Processing parameters

such as mold geometries and molding conditions should

play an important role in determining physical properties

and decorating performance of final FIM products.

According to recent studies [8–10], effects of various

processing conditions on viscoelastic behavior and ther-

mal deformation of FIM parts were investigated by evalu-

ating injection speed, melt temperature, packing time, and

so on. On the other hand, material parameters, e.g., physi-

cal and thermal properties of biaxially oriented film and

polymer substrate used in FIM parts, should be investi-

gated accurately. Many studies have been reported to

understand effects of above parameters on internal struc-

ture, processability, and final properties of FIM parts.

However, there have been few studies on thermovisco-

elastic deformation of polymeric raw materials although

final FIM products are affected by their internal structure

and physical properties. Since most laminated films and

substrates are polymeric materials and have time-depend-

ent viscoelastic properties, it is essential to comprehend

effects of material properties on processability and long-

term deformation behavior of final FIM parts [11, 12].

It is well known that asymmetric deformation is

induced by the difference in thermal expansion coefficient

(CTE) and thermal shrinkage between the laminated film

and the polymeric resin used in FIM parts [13, 14]. Since

the films were produced by biaxial orientation and anneal-

ing, they showed anisotropic properties. When they are

employed in the FIM process, they experience thermal

treatment by the injected hot resin. Therefore, thermal

and physical properties of FIM parts are affected signifi-

Correspondence to: Jae Ryoun Youn; e-mail: jaeryoun@snu.ac.kr

Contract grant sponsor: National Research Foundation of Korea (NRF)

grant funded by the Korean government (MEST) through the Intelligent

Textile System Research Center (ITRC); contract grant number: R11-

2005-065.

DOI 10.1002/pen.22182

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

VVC 2011 Society of Plastics Engineers

POLYMER ENGINEERING AND SCIENCE—-2012

cantly by thermoviscoelastic properties of laminated films

with or without thermal treatment. According to the previ-

ous studies [8–10, 15], thermoviscoelastic deformation,

i.e., warpage reversal phenomenon (WRP), occurred for

final FIM parts because asymmetric residual stresses were

generated owing to different shrinkage of laminated films

and substrates during FIM process. FIM parts prepared by

using unannealed films showed the WRP during anneal-

ing, and the degree of WRP was affected significantly by

the molding conditions and thermal shrinkage of the used

films.

Effects of material parameters on the WRP and ther-

moviscoelastic deformation of the laminated film prepared

for FIM parts were investigated in this study through

measurement of the CTE, relaxation modulus, and resid-

ual stresses. Numerical analysis was carried out by using

finite element method (FEM) with hexahedron meshes. In

general, FEM analyses are performed with elastic or

viscoelastic models by assuming zero initial stresses in

substrate domains. However, to comprehend the residual

stress distribution and thermoviscoelastic deformation of

the laminated films numerically, initial residual stresses

should be considered for the FEM analysis of the

laminated film. Accuracy of the numerical analysis on

thermoviscoelastic deformation of the laminated film was

improved by applying measured CTE and relaxation

modulus of the laminated film to the FEM analysis.

Numerical results were compared with the experimental

data to understand how thermoviscoelastic deformation

was affected by newly developed residual stresses and

thermal shrinkage of the laminated film.

EXPERIMENTAL

Materials

Poly(methylmethacrylate) (PMMA)/acrylonitrile-buta-

diene-styrene (ABS) laminated films (Type-P, Nissha

Printing, Japan) were used in this study to investigate re-

sidual stresses and thermoviscoelastic deformation of the

laminated film used for FIM. As shown in Fig. 1a, it has

total thickness of 0.5 mm and was composed of two dif-

ferent film layers, i.e., PMMA (Sumipex MH, Sumitomo

Chemical Co., Japan) layer of 0.05-mm thickness and

ABS (Techno ABS-545, Techno Polymer, Japan) layer of

0.45-mm thickness. PMMA layer was used for printing,

and ABS layer was used as the supporting sheet. Tensile

specimens were prepared from the laminated films by

following ASTM D638 to evaluate effects of the CTE,

residual stresses, and relaxation modulus on thermo-

viscoelastic deformation of laminated films. Three types

of laminated films, i.e., unannealed curved film (UCF),

annealed flat film (AFF), and annealed curved film

(ACF), were used to understand effects of annealing on

thermoviscoelastic deformation of the pristine laminated

film. After laminated films were annealed, both curved

and flat films were obtained depending on annealing con-

ditions. The CTE and relaxation modulus measured for

the UCF specimen were used as the initial input data for

numerical stress analysis of laminated films. Experimental

results obtained for AFF and ACF specimens were com-

pared with the output data of the FEM analysis.

Coefficient of Thermal Expansion

The CTE is one of the most important parameters to

understand deformation of laminated films and FIM parts.

It is well known that the biaxially drawn films suffer from

structural relaxation after first heating and cooling due to

molecular orientation, but they have the same CTE value

after second heating [2]. To investigate thermoviscoelastic

deformation of the laminated film, the CTE was measured

with thermomechanical analysis (TMA, Q400EM, TA

Instruments, Utah, USA) under nitrogen atmosphere and

temperature range of 2208C–1208C. The linear thermal

expansibility is defined as

aT ¼ l� l0ð Þl0 T � T0ð Þ (1)

where aT represents the lattice parameter at temperature

T0, and a298 is the corresponding value at 298 K. Constant

values are commonly applied to the film, but the lami-

nated films used in this study consist of two materials that

have different thermal properties. Therefore, the differ-

FIG. 1. (a) Composition of Type-P film (PMMA, printing and adhesive

layer: 0.05 mm, ABS layer for backing: 0.45 mm); strain gages bonded

on film specimens for IHD: (b) UCF using the curved jig to drill the

curved film, (c) AFF using a flat jig.

1122 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

ence of CTE between the PMMA and ABS layers

affected the deformation, warpage, final shape, and buck-

ling of the film as the applied temperature was varied.

Relaxation Modulus

Relaxation modulus is obtained by measuring the stress

with respect to time while maintaining constant strain. It

is one of the fundamental properties of materials that

determine viscoelastic stress development in polymeric

materials and can also be obtained from the creep compli-

ance test under static loading. Relaxation modulus was

measured for each layer to predict relaxation behavior of

the laminated film composed of two different polymeric

materials. Relaxation modulus of each PMMA and ABS

layer was measured at the temperature of 258C, 408C,608C, 808C, and 1008C under nitrogen atmosphere with

the dynamic mechanical analysis (DMA, Q800, TA

Instruments, Utah, USA) equipment. Fixed strain was

applied to the laminated film and then held stationary

during measurement. Stress required to maintain the fixed

deformation was measured with respect to time.

Residual Stresses

A special three element strain gage, rosette (CEA-XX-

062UL-120, Vishay Micro-Measurements, NC, USA) was

installed at a point of the film surface to evaluate the

residual stress variation. An incremental hole drilling

(IHD) method was adopted, and a blind hole was gener-

ated by drilling 10 incremental depth of 0.05 mm at 400

rpm. Instead of drilling five incremental depth of 0.1 mm

at 2000 rpm as in the previous work, 10 incremental dril-

ling was conducted at lower velocity to improve thermal

stability. Each interval between incremental drilling lasted

for about 70 sec for stabilization (45–180 sec depending

on materials) [9]. Some considerations should be made to

obtain good results from the IHD method for injection

molded parts. The IHD method should be conducted care-

fully to obtain consistent raw data for calculation of resid-

ual stresses in an injection-molded polymeric product.

Proper amount of the adhesive should be applied to the

rosette for the whole area to be attached and hardened for

about 12 hr to achieve perfect bonding between the

rosette and the specimen. Soldering is required for wiring

of the strain gage circuit, and the soldering should be fin-

ished as soon as possible. Centering and fixing of the drill

determine the quality of the hole drilled incrementally.

After each incremental drilling, a time interval is needed

for stabilization of the residual stresses before reading the

signal from the strain gage amplifier. The hole of 1.575-

mm diameter is normally drilled up to 1-mm depth at the

center of a rosette (062UL type), and the maximum hole

depth is 0.4 times the hole diameter as described by the

ASTM E837. The equation employed to calculate elastic

strain at the periphery of the drilled hole is given as

err ¼ Aþ B cos 2b� �

smax þ A� B cos 2b� �

smin (2)

where b is an angle measured counterclockwise from

maximum principle stress direction to the axis of the

strain gage, A ¼ �a 1þ mð Þ= 2Eð Þ and B ¼ �b= 2Eð Þ are

calibration constants, and rmax and rmin are principle

stresses [10, 11].

As shown in Fig. 1b, a curved jig made from paper

clay was used to measure residual stress distribution in a

curved film. The films were not flattened during IHD,

because they were supported by the curved jig. Residual

stress distribution of the annealed film was also measured

by drilling an incremental hole on the curved surface of

the film. Because flattened films are inserted in the cavity

for FIM process, residual stresses in the flattened film

should be measured as shown in Fig. 1c. Residual stresses

were measured at three different points, i.e., center, inter-

mediate, and edge regions of each layer to understand

effects of the film geometry. Since residual stresses were

changed during injection of molten resin, experimental

results were obtained by averaging five measured values

for each point to lessen experimental errors.

THREE-DIMENSIONAL NUMERICAL ANALYSIS

Thermoviscoelastic Stress Analysis

Thermoviscoelastic stress analysis was performed to

investigate the long-term deformation of laminated films.

It is well known that residual stresses are developed in

the FIM parts due to nonuniform temperature and pres-

sure distribution during cooling. Residual stress distribu-

tion of the film has been predicted by thermal stress anal-

ysis considering thermal shrinkage and viscoelastic prop-

erties [12–16]. It was assumed in this study that the

laminated film is transversely isotropic, and the visco-

elastic behavior satisfies the assumption of the thermo-

rheological simplicity. Initial residual stresses of the film

have not been usually considered for the structural analy-

sis. However, they were considered for residual stress

analysis in this study. The constitutive equation for the

linear viscoelastic material is expressed as follows.

t tð Þ ¼ G0 yð Þ g�Z t

0

gR x Sð Þð Þ � g t� sð Þds� �

(3)

gR nð Þ ¼ dgR=dn;

where the instantaneous shear modulus G0 is temperature

dependent, and n(t) is the reduced time.

x tð Þ ¼Z

ds

A y Sð Þð Þ; (4)

where A(y(S)) is a shift function at time S. The reduced

time concept for temperature dependence is usually

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 1123

referred to as thermorheologically simple (TRS) tempera-

ture dependence. The shift function is often approximated

by the following Williams-Landel-Ferry (WLF) form.

log Að Þ ¼ � C1 y� y0ð ÞC2 þ y� y0ð Þ ; (5)

where y0 is the reference temperature based on the glass

transition temperature at which the relaxation data are

given, and C1 and C2(K) are calibration constants

obtained at the temperature. If h � h0 � C2, elastic defor-

mation will occur based on the instantaneous moduli [17–

23]. Normalized shear relaxation modulus of the two

polymeric melts measured by the DMA is shown in

Fig. 2 and was used directly for the viscoelastic stress

analysis.

Residual stress distribution and deflection of the

annealed laminated film were predicted for three loca-

tions, i.e., center, intermediate, and edge regions of the

film after annealing for 30 min. Rectangular specimens

were modeled numerically by considering 10 layers of

hexahedral meshes within the 0.05-mm thick film for con-

vergence. It was assumed for numerical analysis that each

layer of the laminated film has the same residual stress

distribution. Numerical analysis was performed to obtain

viscoelastic stresses for three homogeneous regions of

symmetrical half model.

Finite Element Analysis

Residual stresses measured for 10 layers (incremental

drilling) were applied to FEM analysis of thermoviscoe-

lastic deformation of the laminated film to obtain more

accurate numerical results. Incremental depth used in the

IHD method was selected as 0.05 mm, which was the

same as thickness of the PMMA layer. It is well known

that the WRP of FIM parts came from the difference in

physical properties between the inserted film and the

injected substrate. Although the film used in this study

was very thin, residual stress distribution in the film was

needed for numerical analysis on thermoviscoelastic de-

formation of FIM parts, because the laminated film was

composed of two layers. Material properties needed for

the thermoviscoelastic finite element analysis are listed in

Table 1. A commercial stress analysis code, ABAQUS,

was used for viscoelastic stress analysis of the laminated

film by using eight noded hexahedral meshes listed in

Table 2.

RESULTS AND DISCUSSION

CTE and relaxation modulus of each layer should be

investigated to analyze thermoviscoelastic deformation of

the laminated films since biaxially oriented laminated

FIG. 2. Normalized relaxation modulus versus time: (a) master curve

of PMMA layer at 258C and (b) master curve of ABS layer at 258C.

TABLE 1. Material properties of PMMA and ABS resins for numerical

analysis.

Properties

PMMA

(Sumipex MH,

Sumitomo Chem.)

ABS (Techno

ABS-545,

Techno Polymer)

Elastic modulus (GPa) 2.74 2.24

Poisson’s ratio 0.355 0.392

Thermal conductivity

(W/m 8C)0.116 @ 758C(Temperature

dependent)

0.184 @ 908C(Temperature

dependent)

CTE (1/K) 21.16609E24 29.1268E26

Specific heat (J/kg 8C) 2444 @ 2208C 2202 @ 2008CSolid density (kg/m3) 1044.6 kg/m3 1054.1 kg/m3

TABLE 2. Finite element employed for thermoviscoelastic stress

analysis.

Mesh Hexahedron

Type C3D8RT

Number of elements 38,400

(3,840 EA 3 10 layers)

Initial condition 258CAnnealing condition 808C for 30 min

Viscoelastic property Relaxation tested by DMA

CTE Tested by TMA

1124 POLYMER ENGINEERING AND SCIENCE—-2012 DOI 10.1002/pen

films were used in FIM process. They were determined

by TMA and DMA measurements. As shown in Fig. 3,

negative CTE value was obtained because biaxial molecu-

lar orientation had been developed in the film during film

fabrication processing. The measured CTE was applied to

numerical analysis to take the thermal shrinkage of each

layer into account. Master curve of the normalized relaxa-

tion modulus is shown in Fig. 2, and stress relaxation of

the ductile ABS layer needs longer time than that of the

PMMA layer. Thermoviscoelastic property of the film

was identified by considering the master curve of relaxa-

tion modulus and the WLF model at the same time.

One of the typical methods of measuring residual stress

is the layer-removal method for polymeric parts [4]. How-

ever, it is difficult to apply the method to curved poly-

meric parts or those with complicated geometries.

Although the IHD method has been developed for com-

plex metallic parts, there have been many attempts to uti-

lize the method for polymeric parts. However, there have

been few studies on its applicability to polymeric films.

In this study, residual stresses in the curved laminated

film were investigated with the IHD method. Residual

stress distribution of the UCF and films annealed at 808Cfor 12 hr (AFF and ACF specimens) is shown in Fig. 4

with respect to the depth in the thickness direction. The

residual stress distribution of the UCF specimen was var-

ied as compression, tension, compression, and tension

ranged from PMMA layer of 0.05 mm to ABS layer of

0.45 mm, respectively. However, residual stresses of the

AFF specimen were varied conversely, i.e., tension, com-

pression, tension, and compression. It was found from the

experimental results that residual stresses of the UCF

specimen had been relaxed during annealing and then

new residual stress distribution was developed owing to

the deformation generated by the difference in the CTE

between PMMA and ABS layers. As shown in Fig. 2,

thermal shrinkage of the film was observed in the early

stage of annealing due to the relaxation of molecular ori-

entation that had been generated by the biaxial drawing

during film manufacturing. Figure 1b and c shows that

the pristine film used in this study was curved such that

the thin PMMA layer was protruded and compressive re-

sidual stresses existed in the PMMA layer. However, the

laminated film was almost flattened by stress relaxation of

both PMMA and ABS layers during annealing and then

the residual stress distribution was reversed.

It is well known that thermal shrinkage and residual

stresses play an important role in predicting the thermo-

viscoelastic deformation of final FIM products composed

of two polymeric parts [24, 25]. Therefore, thermal

shrinkage and residual stresses of the biaxially drawn film

should be investigated to perform thermoviscoelastic

stress analysis. All the CTE and residual stress data

obtained by linear dimensional measurement and IHD

method were used as the input to the FEM analysis.

Shape of the film predicted by the numerical method

before and after annealing is shown in Fig. 5, and the

numerical results gave a good agreement with the experi-

mental results. Residual stress contours in the specimen

were obtained as shown in Fig. 5a. Residual stress distri-

bution of the film specimen is shown in Fig. 5b and com-

pared with experimental one. Numerical results indicated

that application of proper thermal expansion coefficients

to numerical analysis can describe the irreversible relaxa-

FIG. 3. CTE of PMMA and ABS layers calculated by linear dimen-

sional change at 808C with respect to annealing time; CTE of PMMA

and ABS were 21.16609E24 (1/K) and 29.1268E26 (1/K).

FIG. 4. Experimental and numerical results: (a) film shape, numerical

before and after annealing, (b) length and height of the annealed curved

film (ACF) and annealed flat film (AFF) before and after annealing.

DOI 10.1002/pen POLYMER ENGINEERING AND SCIENCE—-2012 1125

tion behavior of the laminated film, and the viscoelastic

behavior can be modeled by using a shift function based

on the time temperature superposition theory. As shown

in Fig. 6, compressive stress in the PMMA layer turns

into tensile stress as the curved film was flattened after

annealing. Residual stresses in the laminated film were

influenced significantly by thermal shrinkage, and residual

stresses newly developed as a function of temperature.

CONCLUSIONS

Thermoviscoelastic deformation of a laminated film

composed of PMMA and ABS layers was investigated

with measurement of the CTE, residual stress, and relaxa-

tion modulus. Numerical analysis of the thermoviscoelas-

tic deformation was carried out by applying the measured

CTE and relaxation modulus to FEM. Residual stresses

and deformation of the annealed laminated film were

measured and predicted by the numerical analysis. Experi-

mentally measured residual stresses were used as the ini-

tial stress condition for numerical analysis, and more

accurate numerical results were obtained for stress distri-

bution and thermoviscoelastic deformation of the lami-

nated film. The negative CTE was also measured and

applied to the numerical analysis to describe thermovis-

coelastic deformation of the biaxially drawn PMMA/ABS

film. The numerical method was useful to perform the

structural stress and deformation analyses of laminated

films composed of two different types of polymers. It was

concluded from experimental and numerical investigations

that the predicted results were in good agreement with ex-

perimental results at the center region of the laminated

film, but they were different from the experimental results

obtained at the edge region due to asymmetric geometry

of the laminated film at the location.

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