Effect of Processing Parameters on the Mechanical Properties of Injection Molded Thermoplastic...

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Effect of Processing Parameters on theMechanical Properties of Injection MoldedThermoplastic Polyolefin (TPO) Cellular Foams

Steven Wong, John W. S. Lee, Hani E. Naguib,* Chul B. Park

In this study, the effects of processing parameters on the mechanical properties of injectionmolded thermoplastic polyolefin (TPO) foams are investigated. Closed cell TPO foams wereprepared by injection molding process. The microstructure of these foamed samples wascontrolled by carefully altering the processing parameters on the injection molding machine.The foammorphologies were characterized in terms of skin thickness, surface roughness, andrelative foam density. Tensile properties andimpact resistance of various injection moldedTPO samples were correlated with various foammorphologies. The findings show that the mech-anical properties are significantly affected byfoam morphologies. The experimental resultsobtained from this study can be used to predictthe microstructure and mechanical properties ofcellular injection molded TPO foams preparedwith different processing parameters.

Introduction

Structural foams are plastic foams manufactured by

using conventional preplasticating-type injection molding

machines. A physical blowing agent (PBA), a chemical

blowing agent (CBA), or both are employed in the process

to produce a cellular (foam) structure. The structural foam

molding technology was first invented and improved by

Angell et al.[1–3] Low-pressure preplasticating-type struc-

tural foam molding machines are most commonly used to

create structural foams, since the required size of the

S. Wong, J. W. S. Lee, H. E. Naguib, C. B. ParkDepartment of Mechanical and Industrial Engineering, Universityof Toronto, 5 King’s College Road, Toronto, ON, M5S 3G8, CanadaFax: (þ1) 416 978 7753; E-mail: [email protected]

Macromol. Mater. Eng. 2008, 293, 605–613

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

molding system for producing large products is smaller

than that of conventional injection molding due to lower

cavity pressure.[4] The advanced structural foam molding

technology[5,6] guarantees uniform gas dispersion and

complete (or substantial) dissolution in the polymer melt,

despite the non-steady molding process.

Since the generated cells compensate for the shrinkage

of injection molded parts during cooling, structural foams

typically have outstanding geometric accuracy. The

advantages of foam injection molding include the absence

of sink marks on the part surface, reduced weight, low

back pressure, faster production cycle time, and high

stiffness-to-weight ratio.[7–9] Due to this unique set of

advantages, low-pressure preplasticating-type structural

foam molding technology has been widely used for manu-

facturing large products that require geometric accuracy.

Poly(propylene) (PP) is widely used inmany engineering

applications. It has excellent physical and mechanical

DOI: 10.1002/mame.200700362 605

S. Wong, J. W. S. Lee, H. E. Naguib, C. B. Park

Figure 1. Schematic of advanced structural foammoldingmachine.

606

properties as well as good recycling ability. It is also

inexpensive. However, PP has poor mechanical properties

on impact and at low temperatures.[10] Therefore, polymer

alloys are used to enhance thesemechanical properties. For

instance, the addition of poly(ethylene terephthalate) (PET)

in PP could enhance the stiffness at higher tempera-

tures[11] and the addition of rubber, such as ethylene–

propylene rubber (EPR) or ENGAGE, could result in the

improvement of impact properties and scratch resistance

of the thermoplastic polyolefin (TPO) materials. Rubber

toughened PP is called TPO.[12,13]

The automotive industry has been increasingly using

TPO alloys in exterior and interior applications, such as

bumper fascias, claddings, and wire insulation, because of

their excellent weatherability, low density, and low

cost.[10,12,14] However, TPO foams have not been utilized

because the technologies for foaming TPO materials using

batch or injection foaming processes have not been

developed yet. Nonetheless, there is a significant amount

of interest in developing foamed TPO products since

replacing solid TPO with a foamed counterpart could

considerably reduce material costs and fuel consumption.

The tensile properties and impact resistance of cellular

polymeric foams are rarely reported. Sun and Mark[15]

studied the effect of cell morphology on mechanical

properties of microcellular polysulfone, polyethersulfone,

and polyphenylsulfone foams. They found that the tensile

modulus of these polysulfone foams increased with the

square of their relative densities. The tensile strengths

were proportional to foam densities. More research also

proved that the relative foam density is by far, the most

influential parameter over mechanical properties of

cellular liner low-density polyethylene (LLDPE),[16] poly-

(methyl methacrylate) (PMMA),[17] polyurethane,[18] poly-

carbonate,[19–21] glass fiber reinforced polycarbonate,[22]

polystyrene,[23] poly(phenylene oxide),[24] and poly(vinyl

chloride) (PVC).[25,26]

Matuana et al.[27] found that the impact resistance

increases as the relative density of the cellular PVC foams

decreased. The cellular structure in the foams improves the

impact resistance of the material. In addition, Doroudiani

and Kortschot[28] investigated the effect of cellmorphology

on the impact strength of polystyrene foams. They found

that the cell size does not affect the impact strength of the

foams. However, the impact strength is dominated by

the relative density of the foam. They also concluded

that the plastic and viscoelastic deformation in the cell

walls are the major sources of energy dissipation in these

materials. Other research also investigated the impact

strength of polystyrene[29] and polyethylene[30–32] foams,

which led to similar results.

Recently, the tensile behaviors of various unfoamed TPO

composites were investigated by Lee et al.,[33] Mehta

et al.,[12] Mishra et al.,[10] and Liu and Kontopouloua.[34]



However, the relationships of cell morphologies and

tensile properties, and the impact resistance of injection

molded TPO foams have not been previously studied.

The purpose of this study is to investigate the relation-

ships of the processing, structure, and properties for

injectionmolded TPO foams. Cellular closed-cell TPO foams

were successfully processed using injection molding

process. Nitrogen was used as the PBA to process the

injection molded TPO foams. By altering the processing

parameters, different foam morphologies of the injection

molded TPO foams could be successfully controlled. Tensile

and impact testing were conducted in order to investigate

the relationships between the foaming parameters, foam

morphologies, mechanical properties, and impact resis-

tance. The injectionmolded TPO foamswere preparedwith

different processing parameters in order to examine their

effects on the mechanical properties of the foam structure.

Elastic modulus, tensile strength, elongation at breaks, and

impact resistance of the foamed TPO samples were

characterized as functions of foam morphologies. The

results of this study will allow designers to predict the

microstructure and mechanical properties of microcellular

TPO foams prepared from different sets of processing

parameters for their specific applications.

Experimental Part

Material and Preparation of TPO Foams

The polymer material used in this study was TPO (Hifax TYC735C

from Basell). It had a density of 0.96 g � cm�3 and an MFI of

25 dg �min�1. The specimens of fine-celled TPO foams were

processed. An 80-ton injection molding machine (TR80EH) from

Sodick Plustech Inc. was modified into an advanced structural

foam molding machine.[5,6] Figure 1 shows a schematic of the

advanced foam injection machine. The ASTM standard (D638

type IV) foamed TPO specimens and a 3 mm thick foamed TPO

plate were produced with this injection molding machine for

tensile and impact testing, respectively. The information on the

processing parameters of injection molded TPO foamed samples

are listed in Table 1. Three major process parameters that are

controllable, including the amount of gas content in the molten

DOI: 10.1002/mame.200700362

Effect of Processing Parameters on the Mechanical Properties . . .

Table 1. Processing information of injection molded TPO foams.

Run

Number

N2 Gas

Content (%)

Relative

Density

GCP Mold

Opening

0 (Unfoamed) 0 1 N/A N/A

1 0.5 0.90 No No

2 0.5 0.85 No No

3 0.5 0.80 No No

4 0.5 0.75 No No

5 0.5 0.90 Yes No

6 0.5 0.85 Yes No

7 0.5 0.80 Yes Yes

8 0.5 0.75 Yes Yes

gas/polymer mixture, use of the gas counter pressure, and use of

mold opening, can be altered in this injection molding machine.

The PBA used was N2, supplied by BOC Canada.

Foam Characterization

The TPO foam morphology was characterized by utilizing a

scanning electron microscope (SEM, JOEL JSM-6060). The samples

were cooled in liquid nitrogen and fractured to produce a clean

and intact surface with minimum plastic deformation. They were

then gold-coated by using a sputter coater for enhanced

conductivity. The average cell size and density were analyzed

by utilizing ImageJ software from the National Institutes of

Health, USA. The cell density was calculated as the number of cells

per unit volume with respect to the unfoamed polymer. The cell

density was calculated from the ImageJ software by using the

following equation:

Macrom

� 2008

N ¼ n

A

� �3=2�rPrf

(1)

where n is the number of cells in the defined area A, rp, and rf is

the density of the unfoamed and foamed polymers, respectively.

The foam density was measured by the buoyancy method. The

relative foam density is defined as the ratio of the foamed density

to the unfoamed polymer density. The volume expansion ratio is

defined as the ratio of the unfoamed polymer density to the

foamed density.

Figure 2. Cell density obtained from low pressure structural foammolding and gas counter pressure molding.

Mechanical Testing

The tensile mechanical testing of injection molded TPO foams

were conducted following the ASTM D638 on an Instron 4465

mechanical testing machine at room temperature. The capacity of

the load cell was 5 kN. All TPO foamed samples used for tensile

testing had a thickness ranging from 3 to 4 mm, depending on the

expansion ratio of the sample under processing parameters. The

ol. Mater. Eng. 2008, 293, 605–613

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

displacement rate of the crosshead was 50 mm �min�1.[35] The

tensile strength was calculated as the load force divided by

the initial cross-sectional area of the specimen. The strain was

calculated as the elongation divided by the initial length of a

specimen. Three tensile properties were determined from the

resulting stress–strain curves for each foaming condition:

modulus of elasticity, stress at break, and strain at break. The

modulus of elasticity was obtained by calculating the slope of the

stress–strain curves in the elastic region. The elongation at break

of a sample was calculated in terms of percent elongation. A

minimum of five specimens were tested for each set of foaming

conditions. The surface roughness of TPO foamed samples was

measured by using a roughness tester (Times Inc. TR200). The

elastic modulus, tensile strength, and elongation at break of the

foamed TPO specimens were characterized as functions of foam

morphologies and surface roughness. The average value of each

foaming condition is reported in this study.

The impact testing was carried out in accordance to ASTM

standard D5420[36] by using a Gardner impact tester (Qualitest

IG-1142). This method determines the impact energy and impact

resistance of rigid polymers. The impact energy is defined as the

minimum energy that is required to fracture the TPO specimen.

The impact resistance is defined as the impact energy to fracture

the sample per unit thickness. Aminimum20 sampleswere tested

for each processing condition. The average value of each foaming

condition is reported in this study.

Results and Discussion

Effect of Processing Parameter on the Cell Densityand Skin Thickness

Figure 2 shows the cell density obtained from low pressure

structural foam molding and gas counter pressure

molding. The overall trend of the cell density in the two

processes is very similar. The cell density is the lowest at a

relative density of 0.9 and increases as the relative density

decreases to 0.75. Above a relative density of 0.8, the

increase in the cell density is not significant. An interesting

observation is that the cell densities obtained from the two

www.mme-journal.de 607


Figure 3. Skin thickness obtained from low pressure structuralfoam molding and gas counter pressure molding.

Figure 4. Engineering stress–strain curves of injection moldedTPO foams.

608

processes are almost the same. Usually, the cell density

obtained from low pressure structural foam molding is

significantly higher than that from gas counter pressure

molding because of higher pressure drop rates at the

gate.[37,38] There are two possibilities that explain this

phenomenon. The first possibility is that the mold cavity

pressure in low pressure structural foam molding might

have been higher than the solubility pressure, thereby

preventing cell nucleation at the gate with a high pressure

drop rate as the cells should have nucleated along themold

cavitywith a low pressure drop rate. The second possibility

is that there might have been some cell coalescence during

filling, thereby decrease in the final cell density.

Figure 3 shows the skin thickness obtained from low

pressure structural foammolding and gas counter pressure

molding. For low pressure structural foam molding, the

skin thickness decreases as the relative density is

decreased. The decrease in skin thickness in low pressure

structural foam molding might have been caused by a

shorter injection time and lower mold cavity pressure due

to the smaller shot size. The lower mold cavity causes the

cells to nucleate during filling and the nucleated cells are

able to be located more closely to the mold surfaces,

thereby decrease in the skin thickness. On the other hand,

in gas counter pressure molding, there are no significant

changes in skin thickness with respect to the set relative

density of the foams. This is because in gas counter

pressure molding, cell nucleation always occurs after the

counter pressure is released. Since the counter pressure is

released after injection is completed, regardless of the shot

sizes, the set relative density does not have any effect on

the skin thickness.

Figure 5. The effect of skin thickness on tensile modulus ofinjection molded TPO foams.

Effect of Foam Morphology on Mechanical Properties

The effect of cell morphology on mechanical properties of

injection molded TPO foams is presented in this section.



Figure 4 shows the engineering stress–strain curves of TPO

foams prepared under various processing parameters

which are listed in Table 1. The effects of the relative

density, skin thickness, and surface roughness on tensile

and impact properties will be discussed in the following

sub-sections in detail.

Effect of Skin Thickness

The skin thickness is defined as the thickness of the

unfoamed skin layer in the foamed specimen. The

unfoamed skin layer exists in foams which were produced

by the injection molding technique. As the molten gas/

polymer mixture is injected into the mold, the rapid

decrease in pressure in the mold causes the gas molecules,

which are close to the surface, to diffuse very quickly from

the material, resulting in no cells nucleating at the surface

of the sample. As a result, the solid skin on the sample is

created and therefore, the skin thickness could affect the

mechanical properties of the polymeric foams.

DOI: 10.1002/mame.200700362


Figure 6. The effect of skin thickness on tensile strength ofinjection molded TPO foams.

Figure 7. The effect of skin thickness on the % elongation ofinjection molded TPO foams.

Figure 8. The effect of skin thickness on impact energy of injec-tion molded TPO foams.

Figure 9. The effect of surface roughness on tensile modulus ofinjection molded TPO foams.

Figure 5–7 show the effects of skin thickness on the

tensile properties of injection molded TPO foams. Obser-

vation of the figures show that the tensile modulus,

tensile strength, and percent elongation increase with

the skin thickness. The tensile modulus and strength have

an increase of approximately 10%. The percent

strain increases rapidly from 100 to 600% as the skin

thickness increases from 100 to 500 mm. These are

expected results since as the unfoamed region increases,

the amount of material per unit area increases, which

causes the tensile properties of the foams to increase. The

increasing elongation at break possibly results from

the small bubbles blunting the crack tips which increased

the energy needed to propagate the crack.[17]

The effect of skin thickness on impact resistance is

shown in Figure 8. In this figure, the impact resistance of

the injection molded TPO foams increases with skin

thickness. The impact resistance of the foams increases

approximately three-fold as the skin thickness increases

from 90 to 450 mm. When the impact occurred directly on

the face of the unfoamed skin layer, this solid skin layer

acted as an energy absorber in the first zone. The foamed



structure in the core of the sample was the second zone in

absorbing the impact energy. A thicker skin layer resulted

in more energy that can be absorbed and it was harder to

fracture. Therefore, the impact property increases with the

skin thickness of the foams.

Effect of Surface Roughness

The effects of surface roughness on tensile properties of

injection molded TPO foams are shown in Figure 9–11. The

tensile modulus and strength did not change noticeably as

the average surface roughness increases from 0 to 2.5 mm.

This finding shows that the effect of surface roughness

on tensile modulus and strength is negligible. However,

the percent strain decreases as the surface roughness

increases. The percent strain decreases significantly from

700 to 100% as the average surface roughness of the TPO

foams increases from 0 to 2.5 mm. Similarly, the impact

resistance decreases as the surface roughness increases as

shown in Figure 12. The impact resistance decreases from

approximately 6 to 3 kJ �m�1 as the surface roughness

increases from 0 to 2.5 mm. This is attributed to the



Figure 10. The effect of surface roughness on tensile strength ofinjection molded TPO foams.

Figure 11. The effect of surface roughness on the % elongation ofinjection molded TPO foams.

Figure 13. The effect of relative density on relative tensilemodulus of injection molded TPO foams.

610

increase in stress concentration on the surface of the

samples as the surface roughness increases. Therefore, less

energy is needed to initiate cracks on the surface of the

sample under impact loading, thus causing the percent

Figure 12. The effect of surface roughness on impact energy ofinjection molded TPO foams.



strain and impact resistance to decrease. Therefore, the

percent strain and impact resistance are also affected by

the surface roughness of the injection molded TPO foams.

Effect of Relative Density

The effect of relative density on tensile properties of

injection molded TPO materials is shown in Figure 13–15.

Similarly, both tensile modulus and strength decrease

with relative density. By observing the figures, the

tensile modulus decreases from 420 to 320 MPa and the

tensile strength decreases from 18 to 15MPa as the relative

density decreases from 1 to 0.7%. This is attributed to the

material reduction per unit volume of the foams. However,

the tensile strength only decreases 16.7% as the material

weight is reduced by 30% as shown in Figure 14. Therefore,

the introduction of the cellular structure by the injection

molding technique successfully improves the mechanical

strength of the material. By observing Figure 15, the effect

of relative density on percent elongation is not noticeable.

The effects of relative density on the impact resistance

of injection molded TPO foams are shown in Figure 16. By

Figure 14. The effect of relative density on relative tensile strengthof injection molded TPO foams.

DOI: 10.1002/mame.200700362


Figure 15. The effect of relative density on the % elongation ofinjection molded TPO foams.

Figure 16. The effect of relative density on impact energy ofinjection molded TPO foams.

observing the figures, the impact energy decreases with

the relative density of the foams. The impact energy

decreases by approximately half at 10% weight reduction

of the TPO materials. For a 25% weight reduction of the

foamed samples which were produced without using gas

counter pressure, the impact resistance further decreases

to one sixth of the unfoamed material.

By using gas counter pressure, the impact resistance

remains at approximately 5 kJ �m�1 at a 25% weight

reduction of the foamed TPO materials. Since the foamed

samples which had better surface roughness can be

produced by using gas counter pressure, the stress

concentration on the surface of the samples was reduced,

causing the impact resistance to increase.

For unfoamed TPOmaterials, rubber that was treated as

a second phase in the PP matrix contributes to the overall

energy absorption in an impact loading. Thus, incorporat-

ing rubbery particles can toughen the unfoamed PP

polymer. However, the interfacial adhesion between the

rubber and PP phase is relatively weak. For TPO foams, the



cells in the structure reduce the crack length between two

regions of the second phase. As the crack of the cells is

initiated by impact loading, less energy is required to

propagate the crack. Therefore, the impact resistance of the

TPO foams are significantly reduced in comparison to the

unfoamed TPO materials.

Overall, the results show that the impact resistance is

very sensitive to the relative density, skin thickness, and

surface roughness. The cellular structure of the foams

significantly reduces the ability of the TPO materials to

withstand the shock loading.

Constitutive Model for Injection Molded TPO Foams

The structure–mechanical property of microcellular TPO

foams was experimentally obtained in the previous

section. In this section, constitutive models are proposed

to correlate the structure–mechanical relationship of

microcellular TPO foams. Most of these studies attempt

to relate the mechanical properties to the relative density

of the foam. Gibson and Ashby[39] suggested that the

tensile modulus and strength could be related to the

relative foam density in the following equations:

EfEp

� C1 frfrp

!2

þ C01 1� fð Þ rf

rp

!(2)

sf

sp� C5 f

rfrp

!3=2

þ C005 1� fð Þ rf

rp

!(3)

where Ef is the tensile modulus of the foams, Ep the tensile

modulus of the unfoamed polymer, sf the tensile strength

of the foams, sp the tensile strength of the unfoamed

polymer,rf the density of the foam, rp the density of the

unfoamed polymer, f the fraction of solid in cell struts, and

C1, C01, C5, and C 0

5 are constants representing the micro-

structure of the foams. These constitutivemodelswould be

able to assist engineers in predicting the mechanical

properties through knowledge of the relative density of the

polymeric foams.

The Effect of Relative Density on Tensile Modulus

The effect of relative foam density on relative tensile

modulus of injection molded TPO foams is shown in

Figure 13. Similarly, the tensilemodulus is found to be very

closely related to the relative density of the foams. The dots

are the data obtained experimentally from the previous

section. The constitutive model based on Gibson and



612

Ashby is proposed in accordance to the experimental data

of the following:

Macrom

� 2008

EfEp

� 0:8 0:5rfrp

!1=2

þ 0:6 0:5ð Þ rfrp

!(4)

In Equation (4), the constants that fit the experimental

data were C1¼ 0.8, C 01¼ 0.6, and f is 0.5. The formula is

plotted as the solid line in Figure 13. This constitutive

model fits the experimental data very well.

The Effect of Relative Density on Tensile Strength

Figure 14 presents the effect of relative foam density on

relative tensile strength of injection molded TPO foams.

The dots plotted on Figure 14 are the experimental data.

The proposed constitutive model based on Gibson and

Ashby is plotted as the dashed line on the figures shown in

the following equation:

sf

sp� 0:9

rfrp

!3=2

þ 0:1ð Þ rfrp

!(5)

where the constants are C5¼ 1, C05¼ 1, and f is 0.9. The

suggested use of the values of these constants was based

on the experimental data obtained from convectional

foam.

By observing Figure 14, the constitutive model (Equa-

tion 5) suggested by Gibson and Ashby has under-

estimated the performance of the TPO foams. Similarly,

this is due to the estimation of the constants used in

Equation (5) being based on low density conventional

foams which have a relative density between 0.4 and 0.7.

Since all the injection molded TPO foams have a relative

density above 0.75, the constitutive model from Gibson

and Ashby had underestimated the tensile strength of the

injection molded TPO foams.

As a result, an empirical formula is proposed in

accordance to the experimental data as follows:

sf

sp� 0:8 0:8

rfrp

!1=2

þ 0:2ð Þ rfrp

!(6)

where the constants are C5¼ 0.8, C05¼ 1, and f¼ 0.8. The

formula is plotted as the solid line in Figure 14 and fits the

experimental data very well.

Conclusion

The mechanical properties and impact resistance of

injection molded TPO foams are examined. The effects

ol. Mater. Eng. 2008, 293, 605–613

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

of relative density, skin thickness, cell density, and surface

roughness on tensile and impact properties of injection

molded TPO foams are correlated. Several conclusions are

made in this study: (i) the tensile strength only decreases

16.7% at a 30% weight reduction of injection molded TPO

foams. Therefore, the tensile strength is successfully

improved by introducing the cellular structure in the

TPO material by injection molding method. (ii) The tensile

properties and impact resistance increase with the skin

thickness of the foams. (iii) The percent strain is only

affected by the surface roughness of the TPO foams due to

the stress concentration on the surface of the foamed

samples. (iv) The impact resistance is very sensitive to the

relative foam density, skin thickness, and surface rough-

ness of the foams. (v) For foamed TPO materials that are

rubber toughened, the impact resistance is significantly

decreased by the cellular structure. (vi) The impact

resistance and surface roughness of the foams can be

improved by using the gas counter pressure.

Acknowledgements: The authors would like to acknowledge thefinancial support of the AUTO21 Network of Centres of Excellenceand the Government of Ontario.

Received: November 8, 2007; Revised: March 12, 2008; Accepted:March 13, 2008; DOI: 10.1002/mame.200700362

Keywords: cellular foam; impact resistance; injection moldingprocess; mechanical properties; processing parameters; tensiletesting; thermoplastic polyolefin; TPO

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Effect of Processing Parameters on the Mechanical Properties of Injection Molded Thermoplastic...

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