An experimental study of yellow shift in injection-molded light ...An experimental study of yellow...

© 2016 The Korean Society of Rheology and Springer 187

Korea-Australia Rheology Journal, 28(3), 187-196 (August 2016)DOI: 10.1007/s13367-016-0019-2

www.springer.com/13367

pISSN 1226-119X eISSN 2093-7660

An experimental study of yellow shift in injection-molded light guide plate

Inki Min1, Sungjun Lee

1, Sunghee Lee

2, Jongsun Kim

2 and Kyunghwan Yoon

1,*1Department of Mechanical Engineering, Dankook University, Yongin 16890, Republic of Korea

2Research Institute of Advanced Manufacturing Technology Molds & Dies R&D Group, Korea Institute of Industrial Technology, Bucheon 14441, Republic of Korea

(Received April 21, 2016; final revision received July 4, 2016; accepted July 8, 2016)

Recently, the size of light guide plate (LGP) in LCD-BLU (Backlight Unit) module is getting thinner andlarger than ever. The part reached critical thickness to mold it by conventional injection molding methodsdue to the low flowability, melt solidification, machine limitation, and so on. Therefore, severe conditionshave been applied to the part to increase the flowability such as high injection speeds and higher melt tem-perature. However, these approaches lead to the degradation of material and loss of optical properties. Thesedefects are connected to the invisible part failure, so called, yellowing and color shift. In the present studya series of injection molding experiments were conducted to understand the distribution of yellowness ininjection-molded LGP, and how the optical properties change under various injection molding conditions.Optical properties of yellow index (YI), CIE xy, and spectral transmittances of LGP sample were analyzedby the UV-visible spectrophotometer. Also, correlations between optical properties and process conditionswere investigated from the Design of experiment (DOE). Interestingly, the value of YI, i.e., yellow shift inCIE diagram showed the maximum near the gate and decreased as the distance from the gate increased. Fur-thermore, as far as yellow shift concerned the data of direct transmittance are much more useful than totaltransmittance for evaluating color behavior. Meanwhile, analysis of variance (ANOVA) was conducted tosee the effectiveness of chosen processing parameters. Mold temperature was found to be the most influ-ential factor on the color shift and injection speed, melt temperature, packing pressure were followed.

Keywords: color shift, LGP, high speed injection molding, CIE xy, direct transmittance, yellow index

1. Introduction

In display industry, LCD is still the most popular panel

type in the display history. In LCD, because the liquid

crystal cannot emit any light itself, additional light source

is required, so called, backlight unit (BLU). BLU com-

prises point light source of LED, many optical films to

enhance the optical efficiency such as prism, diffuser,

reflection films, and light guide plate (LGP). Fig. 1 shows

the structure of BLU. Among these, LGP converts the

point light source of LED to plane light. In view of BLU,

the module size, brightness level, and the thickness of

LGP were main interests in the past, but the trends are

changing to enhance color representation together with

thickness. Generally, BLU is designed to emit the white

and uniform lighting for whole area of display. Since the

source LED comes from the mixture of blue LED chip

and yellow phosphor, the color of LED emits the bluish

white which is not an ideal white. On the other hand, the

color in BLU is governed by additional factors not only

LED but other parts such as optical films and LGP.

Among the parts, LGP is a key factor changing the color

property of light. Because it goes through the most severe

history of processing during manufacturing. Hong et al.

(2014) investigated the structure of the LGP in view of

formation of birefringence, however, they did not give

interests to color performance. Since LCD display emerged

out in the market, the color representation becomes much

more important than ever to express the realistic and vivid

images. Recently, many display suppliers have been trying

to obtain the color repeatability in CIE xy diagram on the

critical thickness. Kim and Chung (2007) used RGB 3

bands of LED arrays to widen the color gamut in 5 inch

display. Also, Lee et al. (2012) arrayed the LEDs by turns

which have the relations of complementary color to obtain

the high purity of white and increase the usage of LED.

On the other hand, there was an approach in view of mate-

*Corresponding author; E-mail: [email protected] Fig. 1. (Color online) The structure of BLU.

Inki Min, Sungjun Lee, Sunghee Lee, Jongsun Kim and Kyunghwan Yoon

188 Korea-Australia Rheology J., 28(3), 2016

rial formulation. Matthew et al. (2014) investigated the

polycarbonate to obtain the high flowability and color sta-

bility. They have released the new material having color

stability and ductility for LGP application by optimizing

additives and molecular weight. Amon (2014) measured

the absorption and transmittance of the material across the

visible range and tried to find what affects the color shift

for the various materials. However, previous studies have

been mostly focused on material and optical design. None

of them showed the interest in the color shift from the pro-

cess conditions. In the present study, the transmittance and

color representation of LGP were investigated by con-

trolling the injection molding conditions of LGP.

2. Theory

2.1. Transmittance In optics, transmittance is a measure of the ability to

transmit radiation, equal to the ratio of the transmitted flux

to the incident flux. For a plate of transparent material or

medium, the transmittance is generally defined as the %

ratio between the spectral intensities of incident (I0) and

transmitted ray (I) as shown in Eq. (1).

(1)

The level of transmittance is generally used as spectral

interests across UV-visible wavelength range with spec-

trophotometer. Fig. 2a describes the principle for the mea-

surement of transmittance (Hunter and Harold, 1987). For

the spectral transmittance, it requires grating device to

separate the ray and get the spectral information at each

wavelength. Therefore, all transmitted information is

depicted based on the wavelength. On the other hand, the

transmittance can be divided into total and direct quanti-

ties. Because total transmittance deals with the all trans-

mitted rays from sample, all rays are captured in the

integrating sphere (Fig. 2b) and detector reads the inten-

sity of all rays. On the contrary, direct transmittance is not

interested in diffused/scattered information of rays. There-

fore, detector reads the intensity of rays directly propa-

gating which does not include diffused or scattered

information (Fig. 2c).

2.2. CIE’s color spaceHistorically, there were many researches to quantify the

color. In the history of art, the color was expressed as

RGB level only based on the Munsell color system defin-

ing the color as hue, value, and chroma. But it was insuf-

ficient to show the color as a number in digital color

processing. Thereafter, the international commission on

illumination (CIE) announced the standard color space in%T λ( ) =

I λ( )

I0 λ( )----------- × 100

Fig. 2. (Color online) Schematic diagrams for (a) the measurement of spectral transmittance and the details of A region for (a) total

or (b) total and (c) direct transmittance.


Korea-Australia Rheology J., 28(3), 2016 189

1931, which is a CIEXYZ colorimetric space. CIE desig-

nated the 3 factors as standard observer, light, and color

matching functions to define the color as a number. All the

spectral information of object can be expressed as tristim-

ulus XYZ value together with color matching functions

and simply normalized as xyz as shown in Eqs. (2) to (7).

(2)

(3)

(4)

(5)

(6)

(7)

where (λ), (λ), and (λ) are the color matching func-

tions and S(λ) is spectral information from the sample.

Fig. 3 shows the CIE’s (λ), (λ), and (λ), i.e., color

matching functions, which have the maximum weighting

values at 600 nm, 555 nm, and 450 nm of wavelength,

respectively.

Since 1931 many color spaces have been released such

as L*a*b*, HSV, and u'v', but CIEXYZ is the origin of all

color quantification and can be converted to other color

space. Another popular colorimetric diagram in display

area is CIE1976 u'v' space, which was developed from

CIE1931 to offer color uniformity as visual perception

(Berns, 2000; MacAdam, 1942; Koschan and Abidi,

2008). CIE1931 xy and CIE1976 u'v' spaces have follow-

ing relations as shown in Eqs. (8) and (9). Fig. 4 shows the

CIE1931 xy and CIE1976 u'v' color spaces.

(8)

(9)

From the CIE1976 u'v' space, color difference of ΔE*(u'v')

between two points can be calculated using the Euclidean

distance as following definition of Eq. (10). Color differ-

ence is a quantified expression of adjective concepts in

color such as redder or yellower, depending on the direc-

tion of difference in Fig. 4b, it gives the information as

number how the object of color is different from the ref-

erence color. The color difference of ΔE* is widely used as

an index for managing color uniformity in the display field

and industry.

(10)

2.3. Yellow index (YI)Yellow index or yellowness scale is a quantitative value

which expresses how the object looks yellow, and desig-

nated in ASTM based on CIE1931. YI is defined as a sin-

gle value for expressing the color. In the painting and

fabric industry, the definition of YI follows the rule of

ASTM E313. However, ASTM D1925 is used in plastic

industry. YI and CIEXYZ have following relations of Eq.

(11) in ASTM D1925.

(11)

YI is limited to the sample which has the dominant

wavelength range from 570 nm to 580 nm. Typically, the

YI is measured for a reference representing ideal white.

Samples are compared to reference. ΔYI is obtained from

the difference between samples and reference and positive

values give the sample became yellow and negative value

indicates that the sample became blue (Berns, 2000; Kos-

chan and Abidi, 2008).

X = 380

780

∫ x λ( )S λ( )dλ

Y = 380

780

∫ y λ( )S λ( )dλ

Z = 380

780

∫ z λ( )S λ( )dλ

x = X

X Y Z+ +----------------------

y = Y

X Y Z+ +----------------------

z = Z

X Y Z+ +---------------------- = 1 x– y–

x y z

x y z

u′ = 4X

X 15Y 3Z+ +------------------------------- =

4x

2x– 12y 3+ +-------------------------------

v′ = 9Y

X 15Y 3Z+ +------------------------------- =

9y

2x– 12y 3+ +-------------------------------

ΔE*

u′v′( ) = Δu′( )2

Δv′( )2

+

YI = 1.28X 1.06Z–

Y--------------------------------- × 100

Fig. 3. (Color online) The spectral plots of color matching func-

tions of (λ), (λ), and (λ).x y z

Fig. 4. (Color online) The color space of (a) CIE1931 and (b)

CIE1976 u'v'.



2.4. DegradationWhen the polymer is exposed to severe environment

such as chemical/physical attacks, its chemical structure is

easily changing and generates the new spices or chemical

structure continuously and loses intrinsic properties (Mur-

phy, 2003). In the degradation process, side branches fall

off from main branch and have a reaction with surround-

ing molecules in the air. These reactions induce a forma-

tion of chain reaction and the molecules rearrange

continuously until the structure becomes stable. These

processes are kinds of oxidation and called degradation.

Degradation of polymer is a consequence of chemical

reaction. If the polymer chains are activated by the exter-

nal stimulation such as photon and heat, they generate rad-

icals continuously and make a chain reaction together with

oxygen. The oxygen is the most important molecule and

the degradation of polymer is governed by the concentra-

tion and diffusion of oxygen. This oxygen based degra-

dation generates hydroperoxide(-OOH), carboxylic acid(-

COOH), ester(-COOR), alcohol(-OH) and trigger bridging

reaction, depending on the polymer series or reaction con-

dition. All polymers take processing such as compound-

ing, blending, extruding, molding etc., and it is always

exposed to degradation. For polycarbonate, a chemical

bonding between benzenes is strong enough to external

stimulation but single bonding between carbon and oxy-

gen is easy to break to UV or thermal stimulus. These dis-

connected carbon and oxygen molecules are being

unstable but taking rearrangement to get stable with sur-

rounding oxygen molecules soon (Rivaton et al., 2002).

Polymer degradation covers aging, deterioration, destruc-

tion, decomposition, and discoloration. In addition, these

phenomenon affect the part quality and accompanies by

the change of visual appearances, material properties, and

chemical compositions (Murphy, 2003; Rivaton et al.,

2002). Also, severe degradation induces the process distur-

bances such as drooling, silver streak, black spec, and so

on.

3. Experiments

3.1. Material and molding equipmentIn the present experiments, Iupilon® HL-4000 (PC, Mit-

subishi Co. Ltd.) was used as a molding material. HL-

4000 has 1.2 g/cm3 of density, 63 g/10 min of MFR

(ISO1133), 136°C of heat deflection temperature and 9.0

kJ/m2 of Charpy impact strength. HL-4000 is widely used

for LGP molding due to high impact strength and heat

resistance, especially, high flowability. The mold com-

prises 2-platen and 2-cavities. Cavities for parts are 80

mm (vertical)×52 mm (horizontal) and 0.43 mm of nom-

inal thickness as shown in Fig. 5. For molding trial, injec-

tion molding machine (LGE150III-DHS) from LSMTRON

was used shown in Fig. 6. It has 150 tons of clamping

force, 1,000 mm/s of maximum injection speed and covers

3,500 bar of maximum injection pressure. The general

specifications of injection molding machine are summa-

rized in Table 1.

3.2. Experimental conditionsFor the preparation of molding trial material was fully

dried at 100°C and samples were taken at cyclic steady

state. In thin-walled part molding, rather higher melt tem-

perature and injection speeds than normal were used to get

high flowability and to prevent frozen layer growing in the

cavity. In the current study, the LGP model has the part

thickness of 0.43 mm and it includes numerous micro

Fig. 5. (a) Descriptions of mold and (b) LGP parts.

Fig. 6. (Color online) A photograph of injection molding

machine used. (LGE150III-DHS, LSMTRON)



optical patterns at stationary core. For the purpose of

obtaining high melt flowability and pattern replicability,

initial melt temperature was set as 340°C. To identify the

change of optical property by the molding conditions, melt

temperature, mold temperature, injection speed, and pack-

ing pressure of four factors were set as processing param-

eters. In addition, these factors were arrayed by L9 to

analyze statistically in DOE test set. The detailed process-

ing conditions are given in Tables 2 and 3. As explained

above, the process window of melt temperature was cho-

sen narrower than recommended range of PC due to

exceptionally thin part thickness in L9 experiment table.

3.3. Measurement of optical propertiesFor evaluating the optical properties, UV-visible spec-

trometer of Solidspec-3700 from Shimadzu was used. It

can measure the transmittance versus wavelength from

3,300 nm to 165 nm using the halogen (visible range) and

deuterium lamp (UV range). In addition, it uses double

beam and separate detector to analyze the reference and

sample signal, respectively. The colorimetric function is

included in the program, therefore, the information of

CIExy and YI level can be extracted from the intensity

data. General specifications of spectrometer are listed in

Table 4.

Also, optical properties were measured along the flow

front of center line. From the gate, 3 regions were selected

as measuring points which were near the gate, middle, and

flow end regions as shown in Fig. 7. For each run 20 sam-

ples were taken and optical properties were evaluated

including transmittance, YI level, and color value based

on CIE diagrams.

Table 1. General specifications of injection molding machine

(LGE150III-DHS).

Item Value Unit

Screw diameter 25 mm

Injection capacity 62 cc

Max. screw speed 470 rpm

Clamping force 150 ton

Max. injection pressure 3,500 bar

Max. injection speed 1,000 mm/s

Max. injection rate 491 cc/s

Table 2. Process factors and levels for molding trial.

Level 1 Level 2 Level 3

Melt temp. (°C) 340 360 380

Inj. speed (mm/s) 400 600 800

Mold temp. (°C) 70 80 90

Packing (MPa) 30 40 50

Table 3. L9 orthogonal array for DOE.

Exp. # Melt temp. Inj. speed Mold temp. Packing

1 1 1 1 1

2 1 2 2 2

3 1 3 3 3

4 2 1 2 3

5 2 2 3 1

6 2 3 1 2

7 3 1 3 2

8 3 2 1 3

9 3 3 2 1

Table 4. Specifications of Solidspec-3700 UV-visible spectrometer.

Item Properties

Photometric system Double beam

Light source 50W halogen lamp for visible, Deuterium lamp for UV range

Wavelength range 165 nm~3,300 nm

Resolution 0.1 nm

Wavelength accuracy ±0.2 nm in ultraviolet and visible region

Wavelength repeatability ±0.08 nm in ultraviolet and visible region

Photometric accuracy ±0.003Abs (1Abs), ±0.002Abs (0.5Abs)

Photometric repeatability 0.001Abs (0~0.5Abs), 0.002Abs (0.5~1Abs)

Detector InGaAs and cooled PbS cell for both R-928

Fig. 7. (Color online) Descriptions of measurement locations and

paths of ray for optical properties of LGP.



4. Results

In the present study, parametric studies were carried out

to understand the change of optical properties in LGP. One

is to find the distribution of the colorimetric value in injec-

tion-molded LGP and the other is to find the relations

between optical properties and process factors including

injection speed, packing pressure, and melt and mold tem-

peratures.

4.1. Spectral transmittance from total and direct mea-

surementTo identify the color properties on the process condi-

tions, systematic molding trial has been conducted using

the L9 orthogonal array in Table 3. In this test, total and

direct transmittances were compared by which evaluation

is more effective to assess the color variation in the LGP

molding. Fig. 8 shows the results of total and direct trans-

mittances measured at middle point. The spectral trans-

mittances of visible and UVB range are shown in detail in

Figs. 8c-f, respectively. No big differences can be caught

among the changes of molding conditions in the spectrum

of visible range in Figs. 8a and b. Looking at blow up plot

of Fig. 8c, total transmittance hardly changes its values

across the visible range, therefore, it is hard to detect the

color shift by the process conditions. However, direct

transmittance varies about 2% across the visible range

among different process conditions as shown in Fig. 8d. In

addition, the maximum level showed about 70% for the

whole range. LGP includes a lot of micro patterns on sur-

Fig. 8. (Color online) Comparisons of the spectra of total (a, c, e) and direct transmittances (b, d, f); (For the whole (a, b), blow up

of visible (c, d), and UVB range (e, f), respectively.)



face, which make the rays scattered. That is why the direct

transmittance showed lower value than total one. In view

of the principle of BLU, rays from LED’s are travelling

through the LGP inside with total internal reflection (TIR)

angle and leave the LGP when it meets the micro optical

patterns due to the deviation from TIR angle. Therefore,

when it comes to analysis in yellowing effect in LGP,

direct transmittance is considered to be more reasonable.

In the spectrum at UVB range shown in Figs. 8e and f,

direct transmittance also shows more systematic deviation

among different process conditions than total transmit-

tance.

Because the color level and YI value are very sensitive

to small variation of transmittance in visible range, the

data obtained from Figs. 8c and d showed valuable results

as described in the following sections.

4.2. Color shift under different process conditionsBased on spectral results calculated from colorimetric

values were summarized in Table 5 and 6 and plotted in

Figs. 9 and 10. For all 9 experimental sets, the colors

based on total and direct transmittances were compared.

Reference point specifies the color value of light source in

the instrument (standard illuminant C), which is located at

(0.31040, 0.31910) in CIE xy. Apparently systematic color

shift was found after passing the LGP sample, and the

direction of shift was yellow. For the xy color level based

on the results of total transmittance shown in Fig. 9a, it

changes from the minimum point of (0.31087, 0.31962)

near the gate region to the maximum point of (0.31110,

0.31973) at the flow end region. And the color value in

CIE xy coordinates showed little variation in spite of the

change of molding conditions. Especially, the color values

at middle region showed similar values to flow end region

as shown in Fig. 9a. Therefore, it is hard to find the cor-

relations between process conditions as expected in Fig.

8c. In the CIE1976 u'v' colorimetric space, the same

results were found as in the CIE1931 xy space. The color

of reference point is located in (0.19999, 0.46258) and

moves to the maximum point of (0.20024, 0.46304) at

flow end region by passing the minimum point of (0.20012,

0.46294) at the gate region of as shown in Fig. 9b.

On the other hand, looking at the changes of color coor-

dinate based on the direct transmittance in Fig. 10a, the xy

color level changes from the minimum point of (0.31250,

0.32107) at flow end region to the maximum point of

(0.31547, 0.32363) near the gate region. Interestingly, it

shows systematic yellow shift from flow end to gate region,

which shows different trend from total transmittance.

Comparing total transmittance data with direct transmit-

Table 5. Results of colorimetric values obtained from total transmittance for each experiment set.

Exp. #Flow end Middle Gate

CIE xy CIE u'v' CIE xy CIE u'v' CIE xy CIE u'v'

1 (0.31106, 0.31971) (0.20022, 0.46302) (0.31106, 0.31974) (0.20021, 0.46304) (0.31089, 0.31965) (0.20012, 0.46296)

2 (0.31106, 0.31973) (0.20021, 0.46303) (0.31108, 0.31975) (0.20022, 0.46305) (0.31087, 0.31963) (0.20012, 0.46295)

3 (0.31109, 0.31974) (0.20023, 0.46304) (0.31109, 0.31977) (0.20021, 0.46306) (0.31087, 0.31962) (0.20012, 0.46294)

4 (0.31107, 0.31973) (0.20022, 0.46303) (0.31106, 0.31975) (0.20020, 0.46304) (0.31086, 0.31963) (0.20011, 0.46295)

5 (0.31110, 0.31973) (0.20024, 0.46304) (0.31108, 0.31977) (0.20021, 0.46306) (0.31087, 0.31963) (0.20012, 0.46295)

6 (0.31109, 0.31974) (0.20023, 0.46304) (0.31111, 0.31979) (0.20022, 0.46307) (0.31088, 0.31964) (0.20012, 0.46295)

7 (0.31109, 0.31975) (0.20022, 0.46305) (0.31107, 0.31976) (0.20021, 0.46305) (0.31090, 0.31966) (0.20012, 0.46297)

8 (0.31111, 0.31976) (0.20023, 0.46306) (0.31110, 0.31978) (0.20022, 0.46307) (0.31089, 0.31966) (0.20012, 0.46297)

9 (0.31112, 0.31977) (0.20024, 0.46306) (0.31112, 0.31981) (0.20022, 0.46308) (0.31089, 0.31966) (0.20012, 0.46297)

Table 6. Results of colorimetric values obtained from direct transmittance for each experiment set.

Exp. #Flow end Middle Gate

CIE xy CIE u'v' CIE xy CIE u'v' CIE xy CIE u'v'

1 (0.31250, 0.32107) (0.20071, 0.46398) (0.31357, 0.32197) (0.20112, 0.46464) (0.31490, 0.32310) (0.20162, 0.46546)

2 (0.31280, 0.32130) (0.20083, 0.46416) (0.31407, 0.32240) (0.20130, 0.46495) (0.31517, 0.32337) (0.20170, 0.46564)

3 (0.31317, 0.32160) (0.20098, 0.46438) (0.31440, 0.32273) (0.20141, 0.46518) (0.31523, 0.32347) (0.20171, 0.46571)

4 (0.31283, 0.32137) (0.20083, 0.46420) (0.31390, 0.32230) (0.20122, 0.46487) (0.31497, 0.32317) (0.20164, 0.46550)

5 (0.31320, 0.32170) (0.20096, 0.46444) (0.31440, 0.32270) (0.20142, 0.46516) (0.31547, 0.32363) (0.20181, 0.46583)

6 (0.31263, 0.32113) (0.20078, 0.46404) (0.31393, 0.32240) (0.20121, 0.46493) (0.31523, 0.32340) (0.20174, 0.46567)

7 (0.31330, 0.32173) (0.20102, 0.46447) (0.31413, 0.32253) (0.20130, 0.46503) (0.31537, 0.32357) (0.20177, 0.46578)

8 (0.31277, 0.32123) (0.20084, 0.46412) (0.31390, 0.32227) (0.20124, 0.46485) (0.31503, 0.32327) (0.20165, 0.46557)

9 (0.31303, 0.32150) (0.20092, 0.46430) (0.31417, 0.32257) (0.20131, 0.46506) (0.31537, 0.32357) (0.20177, 0.46578)



tance plotted in Figs. 10a and b, relatively small amount

of yellow shift can be found and the deviation among dif-

ferent conditions is in error range for the case of total

transmittance data. Revisiting Fig. 8d, the spectrum of

direct transmittance indicates distinguishable gaps among

different experimental conditions and these lead to more

significant and valuable color shift than total transmittance

test.

For all three measurement locations the color shift coor-

dinates are marked with experimental numbers in Fig. 10a

and b. First, as the melt and mold temperatures increase at

the location of flow end, the color coordinate shows the

tendency to move to yellow with the order of experiment

numbers of 1, 6, 8 to 3, 5, 7 sequentially, and experimental

number 7 locates the yellowest point which has 380°C,

90°C of melt and mold temperatures, respectively. In mid-

dle and gate locations, the order of experimental number

making yellowish is slightly different, but overall trends

are similar to the region of flow end. In the color space of

CIE1976 u'v' diagram, same trend can be found as shown

in Fig. 10b. Reference of light source is located in (0.19999,

0.46258) and shifting to yellow color occurs from (0.20071,

0.46398) at flow end to (0.20181, 0.46583) at gate location.

As shown earlier, total transmittance among different

conditions shows narrower color shift than direct one and

the color values are dispersed within error range. So, it is

Fig. 9. (Color online) The representation of color shift from total

transmittance at 3 measurement locations. (The data points were

from Table 5 and shown in CIE1931 xy (a) and CIE1976 u'v'

diagrams (b)).

Fig. 10. (Color online) The representation of color shift from

direct transmittance at 3 measurement locations. (The data points

were from Table 6 and shown in CIE1931 xy (a) and CIE1976

u'v' diagrams (b)).



hard to recognize any dependency on process conditions.

On the other hand, direct transmittance displays the dis-

tinct results of yellow color shift under the varying mold-

ing conditions.

From the CIE1976 u'v' color coordinate given in Tables

5 and 6, color difference of ΔE* from light source was cal-

culated and plotted with YI values as shown in Fig. 11.

The measured data of YI values are given in Table 7 for

total and Table 8 for direct transmittance test, respectively.

Looking at the relations between YI and ΔE* based on

total transmittance in Fig. 11a, it is difficult to find any

correlation between them. On the other hand, in the rela-

tion of YI and color difference based on direct transmit-

tance in Fig. 11b, ΔE* varies proportional to YI as expected

in Fig. 10. Once again, data of direct transmittance are

much more useful than total transmittance as far as yel-

lowness concerned.

The yellowness near the gate region is higher than flow

end region for the case of direct transmittance, which can

be explained as follows. Taking into account the polymer

Fig. 11. (Color online) A plot of YI and color difference based on

(a) total and (b) direct transmittance at 3 measurement locations.

Table 7. Results of YI obtained from total transmittance for each

experiment set.

Exp. # Flow end Middle Gate

1 2.1167 2.1200 1.9650

2 2.1333 2.1433 1.9600

3 2.1433 2.1567 1.9500

4 2.1300 2.1333 1.9467

5 2.1500 2.1567 1.9700

6 2.1400 2.1800 1.9700

7 2.1567 2.1467 1.9767

8 2.1667 2.1700 1.9833

9 2.1833 2.1933 1.9800

Fig. 12. (Color online) Main effect plot for mean of YI at (a)

flow end, (b) center, and (c) gate locations.



filling mechanism in the cavity, polymer goes through the

fountain flow and starts to solidify rapidly when it touches

the mold surface. Frozen layer at gate region experiences

all the processing history of high injection pressure and

shear rate for the first time and the history continues to the

termination of filling and packing. Also, the freshest poly-

mer fills the flow end region by the fountain flow. There-

fore, polymer near the gate region has relatively longer

residence time than flow end region and it is likely to oxi-

dize more and leads to yellowness (Coyle et al., 1987; Evan,

2010; Friedrichs and Guceri, 1993; Mehr et al., 2014).

4.3. Correlations between YI and process conditionsBecause previous color shifting trends are highly cor-

related with YI values, the direct transmittance was cho-

sen to find the effect of process conditions on yellowness.

ANOVA results of DOE data are supporting these trends

as shown in Fig. 12. All experimental sets show different

behavior but yellow index is increasing from flow end to

gate at individual test run in Table 8. In view of YI level,

mold temperature was shown the most influential factor

among the designed parameters and injection speed, melt

temperature, and packing pressure were followed as pre-

sented in Fig. 12. The reason why mold temperature

showed more influential than melt temperature was that

the processing window for melt temperature in this exper-

iment was chosen narrower than usual as explained earlier.

5. Conclusions

In the present study, color behaviors were investigated

under various process conditions by analyzing the trans-

mittance and colorimetric quantities of optical properties.

The spectrum of direct transmittance was proved to give

more useful information than the total transmittance for

analyzing the color shift embedded in LGP sample. The

color properties from direct transmittance showed system-

atic increase of yellow shift from the flow end to the gate

region and the color difference of ΔE* was shown linear

relation to YI values. It is believed that the high tempera-

ture, pressure, and shear history while molding cycle gen-

erated oxidative residues and resulted the yellowing and

reduction of transmittance at short wavelength for differ-

ent locations. Finally, mold temperature was found to be

the most influential factor to YI change for the present

process windows in our DOE test.

Acknowledgements

This research was financially supported via “Measure-

ment of rheological polymer property platform DB (Proj-

ect #JA160028)” by the Ministry of Strategy and Finance.

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Table 8. Results of YI obtained from direct transmittance for each

experiment set.

Exp. # Flow end Middle Gate

1 3.620 4.667 5.980

2 3.920 5.160 6.280

3 4.260 5.520 6.367

4 3.960 4.990 6.057

5 4.337 5.497 6.547

6 3.733 5.067 6.317

7 4.410 5.250 6.490

8 3.840 4.987 6.130

9 4.120 5.297 6.473

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