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of 175 C and 162-173 C respectively. P OM is

s em i c ry st al lin e polyme r w i th 75-85 d eg re e o f

crystallinity.

As engineering plas t ics , PolyOxyMethylene

POM has two types, homopolymer and

copo lymerwhich d i ffer s ligh tly in i ts me l ting po in t

INTRODUCTION

ala unc i Analisis retak,Polioksimetilen,Derajat kristalinitas,Termal

ANALISIS RETAKPOLIOKSIMETILEN MENGGUNAKANSCANNING ELECTRON

MICROSCOPE, ANALISIS TERMAL DAN PROSES. Analisis retak dilakukan menggunakan

ScanningElectron Microscope SEM

dan

Differential Scanning Calorimetry DSC .

Perbandingan

karakteristik bahan antara sampel NG dan standar dilakukan.Analisis SEM menunjukkan bahwa

permukaanretakan padasampelNGcenderung lebihmudahpatahdibandingkanretakan pada sampel

standar.Retakan diduga berasal dari vicinity gate location.Derajat kristalinitas patahan NG adalah

59,6 lebihrendah dibandingkandengan sampel standard 65,8 . Tidak adaperbedaan yangjauh

antara suhu leleh 164C) dengan suhu kristalisasi 146 0C).Perubahan danjalur patahan disekitar

gate location berpotensi menyebabkan retakan selama proses. Berdasarkan hasil SEM dan DSC,

retakan diduga berasal dari setting proses yang tidak tepat yang menghasilkan derajat kristalinitas

yang rendah

danJrozen-in stress.

Beban luar yang diterima juga merupakan salah satu penyebab

terjadinya retakan.

ABSTRAK

eywords

Failure, Polyoxymethylene,Crystallinitydegree,Thermal

FAILURE ANALYSISONPOLYOXYMETHYLENE PRODUCT USING SCANNING

ELECTRON MICROSCOPE, THERMAL ANDPROCESSING ANALYSIS.Failure analysisof

PolyOxyMethylene POM) product were carried out using ScanningElectronMicroscope SEM),

DifferentialScanningCalorymetry DSC)and parameterprocess related factors.Comparationof the

failuremode andmaterialpropertiesbetween theNG andProvensampleswere taken. SEManalysis

showed that failure surface tent to have more brittle fracture compared with the proven sample

failure.Theorigin of crackwaspredictedfromthevicinityof gate location.Degreeof crystallinityof

failure part was 59.6 , which was lower than the proven products 65.8 ). Melting Tm) and

CrystallizationTemperature Tc)showedno significantdifferences, 164Cand 146Cformeltingand

crystallizationtemperature respectively. Whiteningand molded streamflowwere shownaround the

gate locationwhich potentiallycausedmolded in stress and crackpropagation during application in

chemicalenvironment.Externalloadalso found inthe vicinityof crack.Basedon the SEM,DSCand

process related factor analysis, the failure of product was supposed due to the combination of

impropersetting process which resulted in lower degree of crystallinity and frozen-in stress around

the gate and external load received have triggered the crack on product.

ABSTRACT

Accepted: 13December2013evised: 13November2013

eceived:

19

July 2013

R

Wijaya, A.Rifathin and

Afrinaldi

Centerfor Polymer Technology STP - BPPT

Kawasan Puspiptek;Serpong

15314,

TangerangSelatan

FAILURE ANALYSISON POLYOXYMETHYLENE

PRODUCT USING SCANNING ELECTRON MICROSCOPE,

THERMAL AND PROCESSING ANALYSIS

Vol.17,No.1, 2014, hal: 16-19

ISSN: 1410-7864

ajalak Polimer Indonesia

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approximately 10mg,placed intoalumina crucible.

The alumina crucible was then placed inThermal

GravimetryAnalysis (TGA) instrumentation. The

test is conducted under temperature program as

follow: Firstly the sample was heated up from

temperature of 50 to 600 under nitrogen gas.

The temperature was held at 600

for 5 minutes

nitrogen gas environtment. The temperature was

then continuing heated up until 900

under

oxygen gas. The rate of heating was 10

per

minute, and the nitrogen/oxygen gas flowratewas

50 mL per minute. After measurement was

completely finished the TGA-thermograms were

analyzed.

DSC821-Mettler Toledo was used to

measure the thermal properties of the samples such

as melting point, glass transition temperature,

thermal history, degree and the growth of

crystallinity. The test was conducted according to

ASTM D 3418-2003. Sample was cut and

weighed approximately 20 mg, placed into Al

crucible. The Al-cruble was then placed in DSC

sample chamber. The test is conducted under

nitrogen gas environment with temperature

program of heating-cooling-heating as follow:

the temperature program was started from 30 to

200 C, then cooled down to -100 C and finally

heated up to 200 C. The rate of heating was

10 c per minute, and the nitrogen gas flow rate

was 50 mL per minute. After the test was

completely finished the DSC-thermograms were

analyzed.

Figure 2. FTIR test results of NG and proven

samples

._

The failure POM product was received as

failured-part applied inmotor cycle.

To identify the main material, possible

degradation and semi quantitatively analysis the

material composition, we used FT-IR-Bruker

Tensor 27, ATR method. Placed the sample on

ATRcrystal. Touched the sample surface perfectly

on the crystall by setting down the stainless steel

tip. Samples analysis were carried out by OPUS

software (Figure 2).

To measure the material composition.

The test was carried out in accordance toASTM

1131-1998. Sample was cut and weighed

EXPERIMENTAL METHOD

Figure 1. Frozen-in orientation (top) and poten

tial shrinkage near to the gate (bottom)

dge p le

\\~I

>:

~I : _____

POM are generally processed in injection

molding with special runner system. Hot runner

types are commonly applied in POM. Wrong

selection of runner type could result in loss of

pressure during process. Toovercome the pressure

reduction, operators usually increase the

temperature. Unfortunately this could severe the

material to degradation.

The melt entering the cavity will form

orientation during fillingphase.Andmelt adjacent

tothemoldwill freeze:firstandleadto highinterfacial

shear stress between the melt and the solid layer.

This frozen-in orientationwill continue to develop

duringholding pressure in order to compensate for

volume shrinkage, particularlynear the gate region

of the part (Figure 1). However, fast cycle time

during production is required. Those process

parameters influentthemicrostructureof partwhich

also affect the properties and performances.

In this paper, we analyzed the effect of

settingprocess to thermalpropertiesas contribution

ofpotential causes of failureby observing the crack

pattern on the failed surface.

Failure Analysis on PolyOxyMethylene Product Using Scanning Electron Microscope, Thermal and Processing

Analysis R. Wijaya)

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Figure

4. DSC test results ofNG (a) and proven

(b) samples oncooling phase after first heating

---; ::; ; ;; j

_ __ _ _ _ _ _ . ._ H ._ _ , _ __ . . . ,_ . __ _ ._ _ _ , _. ._ ..

~: I

to ~,(;~.

....~

,.

.~--....-......----- :;-;.;~=~--=~-- u=~=:=

;~~.:;;::~:.;\.~- ~-::::;i: ~;~~_,.:..

_-6 .'Ii

\ ,)U~.ll:

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REFERENCES

[1]. DuPont, 2008, Problems with Hot Runners.

[2]. DuPont, DuPont Delrin acetal resin

Molding Guide:TechnicalInformation.

[3]. ARCHODOULAKI VM, LUFTL S,

KOCH T, SEIDLER S, Property changes

in polyoxymethylene POM resulting from

processing, ageing andrecycling, Pol Degrad

Stab, 92 2007 2181-2189

[4]. MALLOY RA., Plastic Part Design for

Injection Molding: Introduction, Hanser

Gardner Publications Inc. 1994

The authors are grateful to The Center for

Polymer Technology for supporting this project.

ACKNOWLEDGMENTS

Extemalload onthesurfaceofhighly frozen

in stress part due to improper setting process tend

to form brittle on the surface and layer between

the surface. The inner core part resulted in crack

beginning from the outer surface part and

propagate toward the entire part of thickness

CONCLUSION

Figure 7.Delamination form on the fracture sur

face, supposed due to layer formation between

the skin and the core.

Figure

6. Different pattern surface fracture

Proven sample

N sample

Figure 5. Crack around the gate bottom and

top ofNG sample.

cooling rate. The setting process also tend to

increase the frozen-in stress orientation on the

surface near to the gate see Figure 2 . This

supposed cause two layers formation, surface part

with highly frozen-in stress orientation

smooth

brittle surface-patterncrack ; delamination formed

and ductile fracture mode in the inner core part.

This is in accordance to the information received

about the cycling time reduction duringproduction

Figure 7 .

Frozen-in stress inmicrostructure and the

external load applied from outer surface has

resulted in initial crack and propagate to entire

thickness product.

Failure Analysis on PolyOxyMethylene Product Using Scanning Electron Microscope, Thermal and Processing

Analysis R. Wijaya)