Int J Plast Technol DOI 10.1007/s12588-017-9194-3 RESEARCH ARTICLE

Effect of injection molding parameters on crystallinity and mechanical properties of isotactic polypropylene S. J. A. Rizvi1

Received: 17 March 2017 / Accepted: 18 October 2017  Central Institute of Plastics Engineering & Technology 2017

Abstract The effects of injection molding parameters namely mold temperature, melt temperature and injection speed on crystallization and mechanical properties of isotactic polypropylene were studied. The specimens were molded as per the Taguchi design of experiment and analyzed for signal to noise ratio and for analysis of variance test. The crystallinity and crystal sizes were determined by wide angle X-ray diffraction data. The specimens were tested for tensile, impact, flexural, stress relaxation and dynamic mechanical properties. It was observed that mold temperature was the most influential injection molding parameter governing the crystallinity and other mechanical properties. It was observed that increase in crystallinity is favorable from tensile and flexural strength point of view but impact properties deteriorate with increase in crystallinity. At high injection speed b-phase changes to a-phase due to shear induced orientation of polymer chains. It is interesting to notice that stress relaxation occurred in elastic as well as in plastic regions has the similar trends and can be modeled by the same decay model proposed in this paper. Keywords Taguchi design of experiment  ANOVA  a-PP  b-PP  Stress relaxation  Wide angle X-ray diffraction

& S. J. A. Rizvi [email protected]; [email protected] 1

Department of Petroleum Studies, Faculty of Engineering and Technology, A.M.U., Aligarh 202001, India

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Introduction Polypropylene is among the most useful commodity thermoplastics and has found many engineering applications in segments like automotives and medical because of its good mechanical properties, inert nature, easy processability and low cost [1–3]. The mechanical properties of iPP are highly dependent on crystallinity of molded parts. There are three types of crystal phases found in iPP, namely a, b, and c-phase [3]. The monoclinic a-phase is the most common and thermodynamically stable crystal structure and posses good mechanical strength due to dense packing of molecular chains forming the cross-hatched lamellar morphology but suffers from low impact toughness especially at low temperatures [4]. Whereas the hexagonal b-phase is thermodynamically metastable. The b-phase exhibits better impact properties and elongation at break even at low temperatures [5, 6]. The orthorhombic c-phase may be formed when iPP s is crystallized under high pressure of about 2000 bar [7]. Injection molding is one of the widely used polymer processing techniques capable of meeting the demand of very fast production rate of complex parts at reasonable production cost. Because of the transient nature of mold cooling, lower thermal conductivity of polymers and comparatively cold mold wall, the injection molded components show non uniform microstructure usually divided into quickly frozen skin and slowly cooled core of molding. And during the mold cooling crystallization occurs non-isothermally under the influence of shear and elongation [8, 9]. This kind of skin–core microstructure has got influence over mechanical properties of iPP. The thickness of skin and the type of crystal at skin layer significantly depends upon the mold temperature and melt flow rate inside the mold cavity. Many researchers have reported the formation of b-phase especially at skin layer. The b-phase was mainly present on skin where as a-phase was present throughout the thickness of sample [9]. The stretching of iPP during the processing may transform the b-phase into a-phase. Since the density of a-iPP is more than biPP therefore micro voids may form due to ba-transformation [10]. Therefore, it is interesting to explore the effect of injection molding parameters on the crystal structure and mechanical properties of iPP moldings. In the present work three injection molding parameters (mold temperature, melt temperature and injection speed) were selected as factors of Taguchi design of experiments. The responses for S/N ratio analysis were crystallinity, a- and bcrystal sizes, yield strength, tensile modulus, impact strength, flexural modulus, stress relaxation and dynamic mechanical properties. The exhaustive range of responses covered in this work has given better insight about the structure property relationship of injection molded iPP. Since in this work the parameters selected are true injection molding parameters and well understood by the molding professionals, therefore it is expected that this work will not only be of interest of academicians and researchers but also will serve to the molding professionals at shop floors as well.

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Experimental Materials In this study, polypropylene homopolymer, grade Repol H110MA was purchased from Reliance Industries Limited, India. This particular grade is suitable for injection molding purposes. The melt flow index is 11.0 g/10 min as reported by manufacturer as well as tested in laboratory according to ASTM D1238, under the conditions of 2.16 kg load at 230 C. The melting point of supplied polypropylene granule was 169 C, as determined by the first heating curve of DSC thermogram with a heating rate of 10 C/min. The PP raw material was molded without any pretreatment however due precautions were taken to avoid the moisture exposure during storage and use. Injection molding of samples The test specimens (tensile ASTM D 638 type-I, flexural D790 and impact D256) were molded on a table top micro injection molding machine make BabyPlast Italy, model 6/10P. This machine is equipped with mold temperature controller ranging from 5 to 120 C. The test specimens were molded as per Taguchi design of experiment (DOE). The mold temperature, melt temperature and injection speed were selected as factors of Taguchi design and three levels of each were set after the initial estimation of processing window for iPP. It is necessary to insure that any combination of these DOE levels should not fall outside the processing window for iPP. The moldings with defects like short shot or flash are indicative of processing parameter falling outside the process window. The factors and levels of design of experiments are summarized in Table 1. The experimental design is based on 3 factors and 3 levels i.e. Taguchi’s L-9 design of experiment, as given in Table 2. The processing windows for tensile, flexural and impact test specimens were individually established and the actual process parameters for them are listed in Table 3. The molding was carried out in automatic cycle mode and initial 8–10 shots were rejected/not considered so that molding process is established. Total numbers of nine experimental runs were carried out and for each run 10 numbers of samples were collected. Since three different test specimens namely tensile, flexural and impact were produced therefore a total of 270 samples were produced according to design of experiment and process conditions summarized in Tables 2 and 3.

Table 1 Factors and levels of L-9 Taguchi DOE

Factors

Levels L1

#

Maximum injection speed = 50 mm/s

L2

L3

1. Mold temperature (C)

20

40

60

2. Melt temperature (C)

190

220

250

60

70

80

3. Injection speed (%)#

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Factors Mold temperature (C)

Melt temperature (C)

Injection speed (%)

R1

20

190

60

R2

20

220

70

R3

20

250

80

R4

40

190

70

R5

40

220

80

R6

40

250

60

R7

60

190

80

R8

60

220

60

R9

60

250

70

Table 3 Injection molding parameters for tensile, flexural and impact test specimens Injection molding parameters

Values Tensile specimen (ASTM D638, type I)

Flexural specimen (ASTM D 790)

Impact specimen (ASTM D256)

1. Injection pressure (bar)

130

100

90

2. Injection time (s)

4.0

2.5

4.0

3. Hold-on pressure (bar)

25

45

120

4. Hold-on time (s)

1.0

1.2

3.0

5. Injection volume (shot size) (cm3)

11.0

6.23

7.0

6. Injection speed (%)

As per DOE (see Table 2)

7. Decompression (mm)

3.0

3.0

1.0

8. Cooling time (s)

25.0

25.0

25.0

9. Mold temperature (C)

As per DOE (see Table 2)

10. Melt temperature (C)

As per DOE (see Table 2)

Testing of specimens Tensile testing The tensile testing was performed on universal testing machine (UTM) make Lloyd, USA, model LS-5 equipped with 5 kN load cell and wedge action grippers. All the tests were performed at room temperature. The tests were performed as per ASTM D 638 standard with type-I tensile specimen. All the tests were conducted at the crosshead speed of 50 mm/min. For each run 5 samples were tested and the mean values of tensile properties are considered.

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Flexural testing Three point bending fixture was used for the determination of flexural properties in accordance to ASTM D790 standard. The above mentioned universal testing machine (UTM) was used for the flexural testing also. The support span was adjusted 85 mm apart and testing was performed at crosshead speed of 2 mm/min. The central deflection was limited to 30 mm. For each run 5 samples were tested and the mean values of flexural properties are considered. Impact testing Izod impact testing was performed on impact tester make Tinius Olsen, USA model IT 504 as per ASTM D256. The notch was cut by Ceast S.p.A., Italy notch cutter. The notch depth was maintained at 2.0 mm and the angle was 22.5. For each run 5 samples were tested and the mean values of impact properties are considered. Stress relaxation test The stress relaxation test was performed on UTM make Lloyd, USA. This test was conducted at room temperature in tensile mode for elastic and plastic regions separately. The sudden elongation was imposed on tensile test specimens. The sample was initially stretched up to 5 and 50 mm for the tests in elastic and plastic regions respectively. The rate of elongation was 50 mm/min. Then the crosshead was hold for 2 min for the test in elastic and plastic regions respectively. The stress value was calculated from the logged value of load cell data (force) and cross sectional area measured by caliper. Dynamic mechanical analysis (DMA) The dynamic mechanical properties were measured by equipment DMA 8000 make Perkin Elmer, USA. The sample size was 12 mm 9 3 mm 9 3 mm. These samples were machined from the identical location of impact test specimens. The tests were performed from 25 to 150 C at heating rate of 3 C/min. The samples were tested in the cantilever mode and the loading frequency was 1 Hz. The displacement at free end was set 0.05 mm. In the frequency sweep experiments, the frequency was varied from 1 to 100 Hz at constant temperature of 25 C. Wide angle X-ray diffraction (WAXD) In order to study the effect of molding conditions on the crystallinity of moldings, wide angle X-ray diffraction (WAXD) was carried out. The tests were performed on PANalytical XPert system having an X-ray tube producing monochromatic Cu-Ka ˚ . The scan was carried out for 2h value ranging radiation of wavelength 1.54060 A from 3 to 60. The scan rate was kept 0.2/min. Intensity (I) verses 2h data was recorded and plotted to give diffractograms. The crystallinity of polypropylene samples was estimated using Eq. 1.

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%XðXRDÞ ¼

AC  100 AT

ð1Þ

where Ac is area under the crystalline peaks and AT is the total area under the diffractograms. According to the Scherrer equation, if the size of a crystallite is small, the crystallite size may be estimated from wide angle X-ray diffraction peaks. The Scherrer equation assumes that all the line broadening of reflection peaks are results of the finite crystallite size [11]. Lhkl ¼

K k bhkl cos hhkl

ð2Þ

where Lhkl is the crystallite size in the perpendicular direction of diffraction plane qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (hkl), the crystallite shape factor, K, is considered as 0.89, and bhkl ¼ B2hkl  b20 with Bhkl as peak width (in radians) at half-maximum intensity and b0 is the instrumental resolution (Table 4).

Results and discussions Effect of molding parameters on crystallinity Polypropylene is known for polymorphism and types of crystal found in iPP are a, b, c and a mesomorphic crystal structure [12]. The most common form of iPP is a˚, form having monoclinic unit-cell structure (a = 6.66; b = 20.78; c = 6.495 A b = 99.6). The b-phase is formed under the condition of chain orientation or due to b-nucleating agent. The b-phase is thermodynamically less stable than a-phase and easily undergo b–a polymorphous transition when stretched [13]. The transformation of b-phase to a-phase may lead to formation of micro voids due to more densely packed a-phase [10]. The b-phase has hexagonal unit cell structure Table 4 Responses for crystal structure of samples as per Taguchi L-9 design Trial no.

Crystallinity XWAXD (%)

Average a-crystallite size L(110)(040)(130) (nm)

b-crystallite size L(300) (nm)

R1

58.49

12.37

21.24

R2

64.85

12.75

17.63

R3

65.46

12.76

15.94

R4

63.97

12.98

21.49

R5

64.70

14.00

20.56

R6

63.50

12.67

20.02

R7

69.00

15.17

24.69

R8

69.83

13.16

24.09

R9

70.38

14.40

26.21

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˚ ; a = b = 90; c = 120). The tensile yield strength (a = b = 11.03; c = 6.49 A of b-iPP is lower than the a-iPP. However the ductile nature of b-iPP helps in energy absorption due to high elongation at break thus enhances the toughness and impact resistance. The third type is c-form in iPP. The c-phase has orthorhombic ˚ , a = b = c = 90). c-phase is unit cell structure (a = 8.54; b = 9.93; c = 42.41 A observed in less stereo-regular isotactic polypropylene or in highly isotactic polypropylene crystallized under high pressure [12]. The different phases of iPP may be identified with the help of wide angle X-ray diffraction (WAXD). The (hkl) reflections for a, b, and c-phases are given in Table 5. However in this study the cphase was not observed. The iPP is an easily crystallizable thermoplastic with onset and peak crystallization temperature 110.28 and 102.71 C respectively as shown in Fig. 1. Crystallization exotherm observed in iPP (Repol-H110MA) pellets. The crystallinity of specimens, molded according to the design of experiments given in Table 2, were estimated by analyzing the wide angle X-ray diffraction data (I vs. 2h) for each run and the crystallinity values are reported in Table 4. The samples were analyzed for S/N ratio and the effects of molding parameters (i.e. mold temperature, melt temperature and injection speed) on the crystallinity are shown in Fig. 3. The S/N ratio trend indicates that crystallinity of molded specimens increase with the increase in mold temperature. This observation is in confirmation to the observations made of other researchers [3, 5, 9]. The increase in melt temperature and the injection speed support the crystallinity buildup up to a limited extent but attain saturation and does not increase with increase in melt temperature or injection speed. However the effect of melt temperature and injection speed is less pronounced than the effect of mold temperature on the crystallinity. The results of analysis of variance (ANOVA) are shown in Table 6 and indicate that only significant molding parameter is the mold temperature with p value of 7.7%. Table 5 (hkl) reflections of a, b, and c phases of isotactic polypropylene (iPP) a-phase (monoclinic)

b-phase (hexagonal)

c-phase (orthorhombic)

˚) dhkl (A

˚) 2h (at k = 1.54 A

–

–

(111)

6.39

13.86

(110)

–

–

6.54

14.14

–

–

(113)

5.86

15.11

–

(300)

–

5.49

16.1

(040)

–

(080)

5.24

17.02

(130)

–

–

4.78

18.55

–

–

(117)

4.38

20.27

(111)

–

(202)

4.17

21.31

–

(301)

–

4.09

21.7

 and (041) (131)

–

(026)

4.05

21.86

–

–

(206)

3.63

24.53

(150) and (060)

–

–

3.51

25.35

–

–

(00 12)

3.47

25.65

(200)

–

–

3.28

27.18

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Fig. 1 Crystallization exotherm observed in iPP (Repol-H110MA) pellets

Because of high p value the melt temperature and injection speed are not of much statistical significance. The crystallization of iPP during the injection molding is influenced by the elongation rate and shear inside the mold cavity. It has been observed that semi-crystalline polymers with shorter relaxation time undergo fast crystallization and exhibit the morphology composed of ‘‘skin–core’’ kind of structure [9]. The skin is composed of frozen oriented polymer layer developed when molten thermoplastic comes into contact of mold walls. The skin thickness Table 6 ANOVA results for the significance of molding parameters from WAXD crystallinity data Source

DF

Seq SS

Adj SS

Adj MS

F

p 0.077

Mold temp

2

1.3861

1.3861

0.69304

12.04

Melt temp

2

0.2652

0.2652

0.13261

2.30

0.303

Injection speed (%)

2

0.2370

0.2370

0.11850

2.06

0.327

Residual error

2

0.1151

0.1151

0.1151

Total

8

2.0034

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Fig. 2 WAXD diffractograms of iPP molded specimens (1–9 trials)

Fig. 3 SNR of processing parameters on crystallinity measured by WAXD

depends on the temperature of mold and melt, and on injection speed. During the flow of polymer melt inside the mold cavity, the chains get oriented in flow direction. The level of chain orientation is highest at skin and lowest at core. The orientation of chains at skin affects the crystallization process and leads to formation of b-form in iPP [9]. The presence of b-form in iPP crystals may be noted from the hkl reflections at 2h value at 16.1 and 21.7 respectively for plane (300) and (301) as shown in Fig. 2. Also it can be observed that the varying processing conditions, as per DOE, affect the fraction of b-form in molded iPP specimens.

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Fig. 4 SNR of processing parameters on b-crystallite size L(300)

Fig. 5 SNR of processing parameters on mean a-crystallite size L(300)(040)(130)

The crystallite sizes (L) were calculated according to Scherrer, Eq. 2, for a- and b-type crystallites and reported in Table 4. It may be noticed from S/N ratio analysis for a- and b-type crystallite size given in Figs. 4 and 5 that the high mold temperature is favorable for the growth of both type crystallites whereas high melt temperature leads to formation of smaller crystallites of both types. As explained by Write et al. [14] at high melt temperature rate of crystal nucleation becomes prominent and spherulites growth is checked by nucleation of new crystal growth sites. However the effect of melt temperature on the crystallite size is statically less significant than the mold temperature. The effect of injection speed is also quite prominent on crystallite size. An interesting observation was made here that increase in injection speed is favorable for the growth a-crystallite size but has adverse effect on b-crystallite size. At higher injection speed the molten polymer is subjected to high shear rate inside the mold cavity that causes the transformation of thermodynamically less stable b-phase to more stable dense a-phase [3, 10]. Effect of molding parameters on tensile properties The results for tensile tests are shown in Fig. 6. The S/N ratio analysis was carried out for yield strength and tensile modulus and the results are shown in Figs. 7 and 8 respectively. It may be noticed that high mold temperature is favorable for yield strength and tensile modulus. The results of ANOVA tests for yield strength and modulus reported in Tables 7 and 8 indicate that mold temperature is of statistical significance with the p value of 9.1 and 4.3% for yield strength and tensile modulus respectively. It may also be noticed from S/N ratio analysis that yield strength increases with the increase in mold temperature from 20 to 60 C however tensile

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Fig. 6 Stress–strain plots iPP molded specimens (1–9 trials)

Fig. 7 SNR of processing parameters on yield strength

modulus value saturates for mold temperature between 40 and 60 C. It may be observed from Figs. 7 and 8 that yield strength and tensile modulus are maximum for melt temperature level 220 C. Further increase in melt temperature (i.e. 250 C level) leads to deterioration in yield strength and tensile modulus. The rigidity may be imparted in samples due to formation of large size spherulites at high melt temperature [8, 11, 14]. According to the trend shown by the S/N ratio analysis, the yield strength and tensile modulus were found to increase with the increase in injection speed (below 70%) due to b–a transformation induced by melt orientation

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Fig. 8 SNR of processing parameters on tensile modulus Table 7 ANOVA results for the significance of molding parameters from yield strength data Source

DF

Seq SS

Adj SS

Adj MS

F

p

Mold temp

2

0.37698

0.37698

0.18849

9.96

0.091

Melt temp

2

0.07128

0.07128

0.03564

1.88

0.347

3.08

0.245

Injection speed (%)

2

0.11657

0.11657

0.05829

Residual error

2

0.03785

0.03785

0.01893

Total

8

0.60269

Table 8 ANOVA results for the significance of molding parameters from tensile modulus data Source

DF

Seq SS

Adj SS

Adj MS

F

p

Mold temp

2

0.210183

0.210183

0.105092

22.18

0.043

Melt temp

2

0.156870

0.156870

0.078435

16.55

0.057

Injection speed (%)

2

0.030760

0.030760

0.015380

3.25

0.236

Residual error

2

0.009477

0.009477

0.004739

Total

8

0.407290

Fig. 9 SNR of processing parameters on impact resistance

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Fig. 10 SNR of processing parameters on impact strength

Fig. 11 SNR of processing parameters on flexural modulus

at higher injection speed. However the high injection speed (beyond 70%) was found to be associated with detrimental effect on most of the mechanical properties as shown in Figs. 7, 8, 9, 10 and 11. It has been observed that at very high injection speed ([ 80%) the phenomenon of ‘‘jetting’’ may occurs during cavity filling. The melt jetting inside the cavity leads to turbulent cavity filling causing micro voids and other structural defects such as poor weld line that may lead to poor tensile, impact and flexural properties. Effect of molding parameters on impact properties The S/N ratio analyses were carried out for impact resistance and impact strength and the results are shown in Figs. 9 and 10 respectively. It is interesting to observe that the effect of mold temperature on impact properties has opposite trend when compared with tensile and flexural properties. Increase in mold temperature produces moldings with poor impact properties whereas high mold temperature is beneficial from tensile and flexural properties point of view. It can be observed from Figs. 4 and 5 that size of a and b crystallites increases with the mold temperature. Bigger crystallites may form bigger spherulites hence stiffness and strength increase but elongation before break reduces. Therefore toughness and impact properties are adversely affected by mold temperature in case of pure iPP. The effect of melt temperature is not much significant over the impact properties and found to be slightly decreasing the impact properties.

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Effect of molding parameters on flexural properties The S/N ratio analysis was carried out for flexural modulus as shown in Fig. 11. In this case also mold temperature was found to be the most significant process parameter affecting the flexural modulus or bending stiffness. It can be observed that flexural modulus increases with the mold temperature where as it reduces at very high injection speed (more than 70%). The noticeable trend is shown by the melt temperature on flexural modulus. The increase in stiffness with melt temperature is due to formation bigger spherulites causing brittleness in the test samples. Effect of molding parameters on stress relaxation The tensile specimens were subjected to constant strains of the magnitude of 4.34 and 43.47% for elastic and plastic deformations respectively for the duration of 120 s and decay in stress magnitude was recorded against time. The Fig. 12 shows the combined stress relaxation curves for elastic and plastic region for all the nine experiments. It may be observed that peak stress value for deformation under elastic region is sufficiently lower than the yield stress (r0) hence the relaxation behavior is, here, termed as relaxation–elastic whereas the relaxation behavior in plastic

Fig. 12 Stress relaxation plots for elastic zone and plastic zone

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Fig. 13 Exponential decay of stress when stretched i elastically and ii plastically

deformation zone is termed as relaxation–plastic. The appearance of fall in stress value after yield point followed by necking phenomenon occurring at constant stress is indicative of deformation in plastic region. The decay in stress w.r.t. time was fitted with exponential decay model given by Eq. 3. rðtÞ ¼ r0 þ feðtt0 Þ=k

ð3Þ

where (r0) is the peak stress value; (t0) is the  time to reach peak stress; (k) is defined g as relaxation time and given as k ¼ E where g and E are the coefficient of r0 rR Þ ; where (rR) is the residual viscosity and modulus of elasticity. And, f ¼ ð0:1r 0 stress as shown in Fig. 13. The parameters of decay model given in Eq. 3 were calculated for all the nine sets of experiments and are listed in Table 9. It is quite interesting to observe that the behavior of relaxation–elastic and relaxation–plastic, as shown in Fig. 12, follow the same decay pattern and can be modeled by the Eq. 3 and the magnitude of residual stress (rR) will be approximately same. Therefore it can be concluded

Table 9 Parameters of exponential decay model rðtÞ ¼ r0 þ feðtt0 Þ=k

r0 (MPa)

t0 (s)

r0 rR Þ f ¼ ð0:1r 0

k (s)

R1

14.61800

7.2225

3.14128

7.9922

R2

15.39759

6.79664

3.48936

7.87557

R3

14.16412

6.06408

3.60015

6.46960

R4

15.05290

6.03693

3.89915

6.34460

R5

15.35782

6.80436

3.31859

7.66095

R6

15.10240

6.83294

3.27546

7.75448

R7

15.64293

6.04794

3.87805

6.45574

R8

16.43936

6.04853

4.10610

6.50367

R9

14.67971

6.79016

3.10810

8.00115

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Fig. 14 SNR response of processing parameters on peak stress (r0) produced in the test specimens at 4.34% strain during stress relaxation test within elastic limit

Fig. 15 SNR response of processing parameters on residual stress (rR) after 60 s at 4.34% strain during stress relaxation test within elastic limit

Fig. 16 SNR response of processing parameters on fractional stress decay strain during stress relaxation test within elastic limit

ðr0 rR Þ r0

after 60 s at 4.34%

that the relaxation behavior of isotactic polypropylene is similar during elastic and plastic deformation. In order to examine the effect of injection molding parameters on stress relaxation behavior of iPP, the peak stress (r0), residual stress (rR) and fractional RÞ were selected as response for the S/N ratio analysis as shown in stress decay ðr0rr 0 Figs. 14, 15 and 16. The variation of peak stress (r0) and residual stress (rR), when strained elastically (e = 4.34%, in this study), show resemblance with SNR plots for yield strength and tensile modulus shown in Figs. 7 and 8. It can be understood from Hook’s law that specimens having high modulus of elasticity (E) will exhibit higher peak stress (r0). It has already been discussed the effect of molding parameters under question on the crystallinity and tensile modulus and the same arguments are applicable to peak stress (r0) and residual stress (rR) as well.

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However the ANOVA analysis show that melt temperature is the only parameter above 90% confidence therefore we may say that only melt temperature is the molding parameter having statistical significance on peak stress (r0) and residual stress (rR). However the trends of mold temperature and injection speed do have RÞ , qualitative significance here. Further SNR plot for fractional stress decay ðr0rr 0 shown in Fig. 16, suggest that fractional stress decay decreases with rise in mold temperature (when analyzed with ‘‘smaller is better’’ option). This is in line with our understanding that the crystallinity of iPP increases with increase in mold temperature. Because the polymer with high crystallinity exhibits more elastic behavior therefore fractional stress decay reduces with rise in mold temperature. Further injection speed is associated with degree of orientation in molded specimen. It may be observed that fractional stress decay reduces with rise in injection speed due to orientation in the direction of stretch. It has been stated that rise in melt temperature is helps in formation of large number of smaller crystallites. Also it may be observed in Fig. 3 that crystallinity initially increases with melt temperature. The increase in crystallinity is due to formation of smaller crystallites, the elastic behavior prevails over viscous nature of molding and thus residual stress value does not fall much during stress relaxation. This result in decrease in RÞ fractional stress decay ðr0rr with melt temperature. 0 Effect of molding parameters on dynamic mechanical properties The results of DMA temperature sweep are shown in Fig. 17. It was observed that the specimens (run 1 to 9) show variation in the values of storage (E0 ) and loss modulus (E00 ) at 25 C however these curves converge to a close value at temperature 150 C, near to crystalline melting point of iPP i.e. * 167 C. It also can be seen, Fig. 17i, that storage modulus (E0 ) plots are parallel and do not intersect  0 each other. The rate of fall in storage modulus dE dT changes around 83–86 C (at 1 Hz). Initially, the segmental mobility of chains in amorphous zone increases due to rise in temperature and may be accounted for the higher rate of fall in storage modulus w.r.t. temperature. However the rates of modulus fall reduce due to constrained mobility of the polymer chains in crystalline regions. The stiffness of crystalline zone is the major contributor towards the value of storage modulus at higher near the crystalline melting point. Signal to noise ratio analysis, as shown in Fig. 14, was carried out for the response storage modulus (E0 ) at 25 C and 1 Hz. It was observed that only melt temperature was of statistical significance with p value 0.042 and residual error of 3.44% in ANOVA test. It may be noted that the trend on S/N response for melt temperature on storage modulus (E0 ) is very much similar to S/N response for melt temperature on tensile modulus (E) as shown in Fig. 8. The value of E0 is found to be maximum in samples processed at 220 C melt temperature whereas E0 decreases at lower or higher temperatures (i.e. 190 and 250 C). It has been reported that the formation of b-iPP occurs at slower rate of cooling [8] therefore lower melt temperature favors formation of b-phase having lower value of tensile modulus [1, 3, 10, 15]. In case of high melt temperature (* 250 C), the crystallization

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Int J Plast Technol Fig. 17 DMA temperature sweep plots (at 1 Hz) of iPP molded specimens (1–9 trials) temperature verses i storage modulus E0 ; ii loss modulus E00 and iii loss factor tand

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yields large number of crystals with reduced size [16, 17] thus the rigidity is reduced and flexibility is imparted to iPP moldings. The formation of large number of smaller crystals may be accounted for decrease in storage modulus (E0 ) value. In another set of dynamic mechanical experiments were performed for frequency sweep range 1–100 Hz at 25 C. The Fig. 18 shows the variation of (i) storage modulus (E0 ), (ii) loss modulus (E00 ) and (iii) dissipation factor (tand) with respect to frequency at 25 C. It can be seen from Fig. 18i that the value of storage modulus (E0 ) increases with the increase in frequency and attains a limiting value. At higher frequency the viscous regions of polymer freeze down and slippage of chains over each other is restricted. The hindrance in chain mobility thus enhances the stiffness hence the storage modulus of polymer. However, the rate of increase in storage  0 modulus w.r.t. frequency dE dx yields and finally storage modulus attains near constant value do not further increases appreciably w.r.t. frequency. Although the values of storage modulus (E0 ) at 1 and 100 Hz vary significantly for the all nine runs but it was observed that the yielding frequencies were almost same (i.e. 12 Hz) in all the nine runs as listed in Table 10. Therefore it may be concluded that yielding frequency (x*) is near independent of molding parameters (mold and melt temperature and injection speed). The fractional increase in storage modulus between 1 and 100 Hz frequency was calculated as under;  0  E100 Hz  E01 Hz 0 DE ¼ ð4Þ E01 Hz where DE0 is fractional increase in storage modulus between 1 and 100 Hz. E01 Hz and E0100 Hz are the values of storage modulus at 1 and 100 Hz respectively. The DE0 was calculated for each experimental run and the values are listed in Table 10. These values were taken as response in S/N analysis and the results of Taguchi analysis are shown in Fig. 19. According to ANOVA only mold temperature and melt temperature have statistical significance with p value 0.038 and 0.086 respectively. The residual error is 2.53% which is well within acceptable limit. It can be seen in Fig. 19 that DE0 decreases with the increase in mold temperature. The increase in mold temperature is associated with increase in crystallinity, see Fig. 3, thus provides inverse relation between crystallinity and DE0 . The segmental mobility (above Tg) is hindered due to higher loading–unloading frequency in amorphous zone and polymer exhibits higher stiffness when subjected to dynamic loading at higher frequency. In case of highly crystalline samples, the fractional increase in storage modulus (E0 ) may not be as prominent as in case of less crystalline or amorphous samples. Figure 19 also shows the effect of melt temperature on DE0 . It can be observed that highest SNR for response DE0 is for 190 C and the lowest for 220 C. The low crystallinity and presence of b-phase at 190 C (see Figs. 3, 4) may be responsible for lower S/N ratio. However at 250 C temperature, the rate of heat transfer in mold increases due to increase in thermal gradient between molten polymer and mold’s wall. This condition does not favor the formation of b-phase crystals in iPP rather smaller a-phase crystals, in large numbers, are produced. The smaller sized a-phase crystals are connected with other

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Fig. 18 DMA frequency sweep plots (at 25 C) of iPP molded specimens (1–9 trials) frequency verses i storage modulus E0 ; ii loss modulus E00 and iii loss factor tand

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Int J Plast Technol Table 10 The values of storage modulus at 1 Hz, 100 Hz, yielding frequency (x*) and fraction of modulus buildup in case of frequency sweep DMA test   Yielding frequency (x*) (Hz) E01 Hz (MPa) E0100 Hz (MPa) E0100 Hz E01 Hz 0 E1 Hz R1

639,811,454

726,186,000

12.35

0.1350

R2

817,741,333

919,959,000

12.15

0.1250

R3

715,060,900

810,164,000

12.18

0.1330

R4

852,498,664

957,356,000

11.76

0.1230

R5

717,877,460

802,587,000

12.32

0.1180

R6

851,589,616

957,868,000

11.70

0.1248

R7

760,493,597

855,099,000

12.37

0.1244

R8

779,738,403

867,381,000

11.84

0.1124

R9

766,842,247

854,569,000

12.76

0.1144

Fig. 19 SNR response of processing parameters on



0 E100 Hz E1 Hz E10 Hz



at 25 C

crystals with the help of long chains crystallizing into various crystallites. The smaller crystals are not as rigid as large sized crystals thus they exhibit lower value of modulus at low frequency however at high frequency the chains connecting these crystals become rigid and thus polymer shows significant increase in storage modulus.

Conclusions The aim of this paper was to study the effect of key injection molding parameters on the crystalline structure and its aftereffects on the mechanical properties of the molded specimens. The experimentation plan was based on Taguchi’s Design of experiments for greater reliability and authenticity of the results. The experimental results (response) were analyzed for the signal to noise ratio and also the analysis of variance (ANOVA) was performed. On the basis of these results, the followings may be concluded; •

The mold temperature is the most influential parameter for the crystallinity of molded iPP and increase in mold temperature favors the formation of crystalline

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•

•

•

•

phase. However the increase in melt temperature favors nucleation and results in large number of small crystals. This trend is applicable for both, a- and b-types iPP crystals. Whereas the increase in injection speed leads to smaller b-crystal size but increase in a-crystal size was observed due to shear induced ba-phase transition. As a result of increase in the crystallinity at higher mold temperature, the tensile yield strength, tensile modulus and flexural modulus also increase with mold temperature. However it may be noticed that impact properties deteriorates with rise in mold temperature because crystallinity induces brittleness in sample and failure in more of rigid failure than ductile in nature hence lower toughness and poor impact properties are noticed in highly crystalline iPP specimens. The relaxation behavior of iPP in elastic and plastic zone, follow the same decay RÞ pattern and can be modeled by the Eq. 3. The fractional stress decay ðr0rr , 0 decreases with increase in mold temperature, melt temperature and injection speed because higher values of these parameters favors crystallinity hence more elastic nature of iPP do not allow the fractional stress to decay much. The S/N ratio trend of storage modulus (E0 ) for melt temperature resembles the trend of tensile modulus. The maximum value of storage modulus (E0 ) was observed at 220 C level. At 190 C melt temperature level slower rate of cooling is supposed to favor b-phase with low stiffness and at 250 C high nucleation rate will produce smaller crystals hence low stiffness and modulus. In case of DMA—frequency sweep at 25, the yield point occurs at * 12 Hz frequency.

Acknowledgements The author is thankful and acknowledge the extension of testing and characterization facilities by Prof. Alim Hussain Naqvi, Prof. Ameer Azam and Dr. Wasi A. Khan, Center of Excellence in Nanotechnology, Department of Applied Physics, Aligarh Muslim University, Aligarh, for this experimental study.

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