Bonding strength at solid-melt interface in a two ... · PDF file0 Bonding strength at solid-melt interface in a two-component injection moulding process H.H. Clements MT07.42 Professor:

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Bonding strength at solid-melt

interface in a two-component

injection moulding process

H.H. Clements

MT07.42

Professor: Prof. dr. ir. H.E.H. Meijer

Advisor: Ir. P.E. Neerincx

Technische Universiteit Eindhoven

Department Mechanical Engineering

Polymer Technology Group

Eindhoven, June 7th

, 2007

Abstract

A disposable bioreactor produced by a two-component (hard and soft polymer)

injection moulding process is developed to grow heart valves. The goal of this report

is to find two compatible polymers that can be used to produce the disposable

bioreactor and to find the optimal process conditions for these polymers to weld

together during the injection moulding process. In order to do this, the bonding

strength of two-component injection moulded tensile test bars is investigated both

theoretically and experimentally. Two different combinations of polymers are

investigated, a combination of Polycarbonate (PC) and Thermoplastic Polyurethane

(TPU) and a combination of Polypropylene (PP) and a Thermoplastic Elastomer

(TPE). A theoretical bonding strength model, which accounts for the cooling profile

of the solid-melt interface of the tensile test bars, was proposed. Under various

injection moulding conditions, such as injection speed, mould- and melt temperatures,

tensile test bars were produced and compared by the bonding strength. The tensile

tests show that a combination of PC and TPU gives the best results. The tensile test

bars break at a maximum tensile stress of 9.3 [MPa]. The bonding strength of the

tensile test bars is higher when the injection speed, mould- and melt temperatures are

kept low.

Table of Contents

1. Introduction 1

2. Weld line Weakness 2 2.1 Temperature profile of the solid-melt interface 2

2.2 Fountain flow 4

2.3 V-notch 5

2.4 Mechanisms of Adhesion 6

2.4.1 Mechanical interlocking 6

2.4.2 Adsorption 7

2.4.3 Diffusion 7

3. Experimental Setup 8 3.1 Injection moulding machine 8

3.2 Mould 8

3.3 Materials 9

3.4 Moulding Conditions 10

4.5 Tensile Tests 10

4. Numerical Approach of the bonding interface temperature 11

5. Results 14 5.1 Polycarbonate and Thermoplastic Polyurethane 14

5.2 Polypropylene and Thermoplastic Elastomer 15

5.3 Optimized process conditions 17

6. Conclusion 18

7. Recommendations 19

Appendix A: Mathematics 21

Appendix B: Cooling profile of the injection moulding process 23

Appendix C: Material properties and process conditions 27

Appendix D: Experiments 29

Appendix E: List of Symbols 41

Literature 42

1

1. Introduction

The heart is one of the most important organs of the human body. A common heart

problem is that the aortic heart valve is not functioning correctly. At present there are

two commonly used methods for replacing heart valves. The first is a mechanical

heart valve replacement. A disadvantage of this alternative is that the patient needs to

use medicines to prevent the blood from clotting. The second alternative is a donor

hart valve from a human or animal. The disadvantage of this method is that the

replacement heart valve is less durable.

Tissue-engineering of heart valves could serve as a solution to the problem mentioned

and several designs of bioreactors exists in which heart valves can grow. Problem

with these bioreactors is that they are either very large, occupying a complete

incubator for only one valve, or have limited function. Moreover, they are very

expensive. In the report “Design and Realization of a Disposable Bioreactor” [1] a

disposable bioreactor is described which is point-symmetric and consists of two

identical shells. The shells are produced with a two-component injection molding

process. This processing method has the following advantages:

• The disposable bioreactor needs little assembly work.

• Polymers are relatively cheap.

• Injection molding is a process that is capable for mass production.

• Combining hard and soft polymers in one part.

• The disposable bioreactor is produced in four sequential injections, this means that

the large bioreactor can be produced on a relatively small injection moulding

machine.

This makes the production of the disposable bioreactor cheaper, more integrated and

more flexible than the current bioreactors that are available. The disposable bioreactor

is made of two polymers, one soft polymer for the control valves, membranes and

seals, and a hard polymer for the casing. It is necessary for the bonding strength

between the polymers to be strong enough for the bioreactor to work properly.

The goal of this project is to find two compatible polymers that can be used to

produce the disposable bioreactor. An injection moulding machine and two sets of

polymers will be used to produce tensile test bars (made from a combination of

Polycarbonate and Thermoplastic Polyurethane or a combination of Polypropylene

and a Thermoplastic Elastomer and produced with different process conditions) in

order to find the optimal conditions for the polymers to be welded together.

First the phenomena that cause weld line weakness in injection moulding will be

discussed in chapter 2. Next, the experimental setup will be discussed in chapter 3.

After this, the injection moulding process is analyzed numerically with Matlab [2] in

chapter 4. Finally in chapter 5 the results of the measurement and the

recommendations will be presented.

2

2. Weld line Weakness

Two-component injection moulding process is a technique to form complex injection

moulded parts by two sequential injections. The bioreactor consists of two different

polymers: a relatively hard polymer for the casing and a soft polymer for the control

valves, membranes and seals.

According to Hagerman [3] the tensile strength at the weld line is determined by three

factors (Figure 2.1), because the bioreactor consists of two different polymers, there is

a fourth factor that will influence the strength of the weld line:

• The incomplete bonding at the interface of the two melt fronts.

• The frozen-in molecular orientation that is caused by the fountain flow [4].

• The existence of V-notches around the weld line.

• The adhesion between the polymers.

Figure 2.1: Origins of weld line weakness

2.1 Temperature profile of the solid-melt interface

Tensile test bars are produced with a two-component injection moulding process, to

investigate weld line weaknesses between two different polymers. In the first stage,

the polymer melt is injected into the mould and, after a certain cooling period, the

melt temperature of this polymer will drop below the glass transition temperature (Tg).

After the polymer is cooled down, a free surface is created by removing an insert and

the second polymer is injected. When the temperature of this melt is high enough, the

heat transfer through the solid-melt interface could cause a thin non-uniform layer of

the solidified part of the first polymer to melt (Figure 2.2).

3

Figure 2.2: Schematic representation of the formation of a non-uniform heat

penetration depth in a two-component injection moulding process.

The depth of this layer is greater at the center than it is near the mould surface,

because the temperature of the polymer is lower near the cooler mould surface.

Assuming that thermo physical properties are constant for the polymers, the unsteady

state three-dimensional heat transfer problem for the solid-melt interface can be

written as [5]:

2 2 2

2 2 2

,

i

i p i

k

t c x y z

δθ θ θ θ

δ ρ

∂ ∂ ∂= + +

∂ ∂ ∂ . (2.1)

Here, i s= for the solid polymer and i l= for the melted polymer, where θ is the

dimensionless temperature

m

l m

T T

T Tθ

−=

−. (2.2)

The tensile test bar is two and a half times thicker in the y-direction, than it is in the z-

direction, see figure 2.3. This means that the most heat loss will occur in the z-

direction. To simplify the model, the heat loss in the y-direction is neglected. With a

Fourier series approximation, the temperature profile of the z-direction can be

calculated using [6]

( )( )

( )( )

2 2

20 ,

2 14, sin 2 1 exp

2 1

i

p i i

n tzz t n

n L L c

π λπθ

ρ

∞ + = + −

+ ∑ . (2.3)

Figure 2.3: Tensile test bar.

4

To calculate the temperature in the x-direction we assume that the two polymer bodies

are infinitely long. The one-dimensional temperature profile between two infinite

bodies in thermal contact is defined by [6]:

,

4

e

i e i

i p i

T T xerf

T T t

c

αλ

ρ

− = −

, (2.4)

with i s= , 1α = − for the solid polymer, and i l= , 1α = for the melted polymer and

erf is the standard error function. The temperature of the solid-melt interface eT at

0x = is given by

s le s l

s l s l

k kT T T

k k k k= + , (2.5)

where ik is a coefficient for the conductivity of the contact surface of the polymer

,i i i p ik cλ ρ= . (2.6)

When equation (2.3) and (2.4) are combined the two-dimensional heat transfer

problem can be solved in Matlab [2].

2.2 Fountain flow

Liquid polymers generally show pseudo plastic behavior. In a pseudo plastic material

the viscosity decreases, when the shear rate increases (shear thinning) [7]. The

velocity of the polymer is higher at the centre than near the mould surface. The melt

entering the front region decelerates in de direction of flow and acquires a transverse

velocity, spilling outward toward the wall. The velocity field of the polymer is shaped

like a fountain, shown in figure 2.4. This phenomenon is called fountain flow.

5

Figure 2.4: The velocity field of a fountain flow.

The fountain flow has a negative effect on the bonding strength. When the liquid

polymer reaches the solid polymer, the molecular orientation of the liquid polymer

will be parallel to the bonding interface. Because the liquid polymer cools down

rapidly to a temperature below the glass transition temperature, the polymer

molecules have insufficient time to relax their orientation. The strength at the weld

line will decrease by the frozen orientation.

2.3 V-notch

In a two-component injection moulding process V-notches exist, when a polymer

cools down before the melt has reached the corners of the mould. V-notches cause a

concentration of the tensile stress, which could lead to brittle failure. A sharp notch

can be related to the apparent fracture strength through a material’s fracture toughness

[8]:

cK aσ π β= (2.7)

cK is the fracture toughness MPa m , β is a crack length and component

geometry factor that is different for each specimen [ ]− , σ is the applied stress

[ ]MPa and a is the crack length [ ]m .

Most polymers have a low cK value, which means that even a small crack at the weld

line could lead to brittle failure.

6

Figure 2.5: V-notch at the weld line

To reduce the effect of the V-notch on the tensile strength, the tensile test bars have to

be produced with high filling and holding pressures. Especially the holding pressure

will reduce the depth of the V-notch [9].

2.4 Mechanisms of Adhesion

The mechanisms of adhesion are still not fully understood and many theories can be

found in current literature. The four most accepted theories of adhesion are [10]:

1. Mechanical interlocking

2. Adsorption theory

3. Diffusion theory

4. Electronic theory

Only the first three theories will be discussed, since the electronic theory explains the

adhesion between metals and glass substrates.

2.4.1 Mechanical interlocking

This occurs when molecules connect as a consequence of their topology. In the case

of the tensile test bar: the second polymer is injected and reaches the surface of the

first polymer. This surface is not perfectly smooth, but shows irregularities like

cavities. The second polymer fills up those irregularities. When the polymer cools

down below the glass transition temperature the two polymers are hooked together.

7

2.4.2 Adsorption

This happens when adhesion is achieved by the forces between the atoms in the two

surfaces. These surface forces can be caused by primary forces (Ionic, covalent and

metallic bonds), but generally the adhesion is caused by secondary forces (van der

Waals forces).

2.4.3 Diffusion

When the tensile test bars are produced by injection moulding the liquid polymers will

cool down fast. If the temperatures are still high enough for polymer self diffusion an

entangled network can form over the weld line. The maximum diffusion time will be

somewhere between 0 and 10 [sec] at the center of the bonding interface. The inter-

diffusion constants between compatible polymers were found between the order of 10-

11 to 10

-14 [cm

2sec

-1]. This means that the maximum diffusion depth for this injection

moulding process is very small. However, even a very small penetration depth can

still result in high joint strengths. For example an interpenetration of macromolecules

between 1 and 2 [nm] may result in a five- to nine-fold increasement in joint strength.

8

3. Experimental Setup

In this chapter the setup of the experiments will be shown. The injection moulding

machine and the mould that was used will be discussed. After this, information about

the materials and the process conditions that were used to produce the tensile test bars

will be listed. Finally, the approach of the tensile tests will be described.

3.1 Injection moulding machine

For the two-component injection moulding process an injection moulding machine,

model Feromatic Milacron K-TEC 60, was used. This machine has a horizontal and a

vertical injection cylinder, which makes it possible to use it for two-component

injection moulding processes. The injection moulding machine has a clamping force

is 60 tons and the maximum injection pressure is 240 [MPa]. The maximum injection

speed is 150 [cc/sec].

3.2 Mould

The mould that is used to produce the tensile test bars is displayed in figure 3.1. When

the mould closes the hydraulic cylinder is activated, separating chambers 1 and 2. The

first polymer is injected from the horizontal cylinder. The polymer will flow through

the runner and will start filling chamber 1. After a certain cooling time, when the

polymer’s temperature has dropped below the glass transition temperature, the

hydraulic cylinder is pulled back. Next the second polymer is injected from the

vertical cylinder. The second polymer will fill chamber 2. Because the hydraulic

cylinder is pulled back, the second polymer will heat up the first solidified polymer,

causing the polymers to weld together. When the second cooling period has expired,

the mould opens and the tensile test bar is ejected by the expulsion pens.

9

Figure 3.1: Schematic of the mould for the tensile test bar

3.3 Materials

Two different combinations of polymers are used to produce the tensile test bars. The

first combination consists of Polycarbonate and Thermoplastic Polyurethane. The

second combination is Polypropylene and a Thermoplastic Elastomer. The following

materials where used to make the tensile test bars:

- PC - Polycarbonate - (Lexan Resin 141R)

- TPU - Thermoplastic Polyurethane - (Desmopan 385S)

- PP - Polypropylene - (Borealis HD601CF)

- TPE - Thermoplastic Elastomer - (Evoprene G 969 Natural)

The properties of these materials can be found in Appendix C.

10

3.4 Moulding Conditions

The moulding conditions that were used to produce the tensile test bars are shown in

table 3.1. To determine the best conditions for the polymers to weld together, the

following process conditions were varied:

- Sequence of injecting the polymers.

- Temperatures of the polymers.

- Mould temperature.

- Injection speed.

TTPU [˚C] TPC [˚C] TPP [˚C] TTPE [˚C] Injection speed [cc.sec-1] TMould[˚C]

TPU on PC 210 300 10 30

220 330 50 40

50

PC on TPU 210 280 10 30

220 300 50 40

330 50

PP on TPE 180 180 10 30

200 50 45

220 60

TPE on PP 200 170 10 30

180 50 45

190 60

Table 3.1: Processing Variables of the injection moulding process

The remaining moulding conditions (holding pressure, injection pressure and cooling

time), that where used to produce the tensile test bars, are listed in Appendix C.

4.5 Tensile Tests

The tensile tests where carried out using a Zwick/Roell material tester, model 1475.

The following conditions were used for the tensile tests:

- Crosshead-speed: 360 [mm/min]

- Distance between the grips: 50 [mm]

- Pre-load: 2 [N]

- Pre-load speed: 10 [mm/min]

The measurements were done with a force cell of 10 [kN]. For each set of process

conditions 3 tensile test experiment where performed.

11

4. Numerical Approach of the bonding interface temperature

Diffusion between the materials is possible if the temperature of both polymers is

above their glass transition temperature. To investigate whether there is diffusion

between the polymers during the injection moulding process, equations (2.3) and (2.4)

are combined in a Matlab file (show in Appendix A). This M-file calculates and plots

the temperature of the polymers solid-melt interface during the cooling time of the

injection moulding process. In the approach two assumptions were made: the mould

temperature is constant during the injection moulding process and after the cooling

time the polymer that was first injected has the same temperature as the mould (see

Appendix B).

In figure 4.1 the temperature profile in the z-direction for injection TPU on PC is

plotted. The temperature of the solid-melt interface at t = 0 [sec] is below both glass

transition temperatures, even for the process conditions with the highest mould and

melt temperatures. This means that there is no diffusion between the polymers. The

polymers are connected by mechanical interlocking and adsorption.

Figure 4.1: Temperature profile in the z-direction (TPU 220 [˚C] injected on PC 50

[˚C])

12

In figure 4.2 the temperature profile in the z-direction for injection PC on TPU is

plotted. The temperature of the polymers is above the glass transition temperature for

approximately 5 [sec]. Because there will be diffusion between the polymers the

bonding strength for injection PC on TPU is expected to be stronger than injecting

TPU on PC.

Figure 4.2: Temperature profile in the z-direction (PC 330 [˚C] injected on TPU 50

[˚C])

The temperature profile of the solid-melt interface for injection PP on TPE and TPE

on PP are shown in figure 4.3 and 4.4. The figures show that in both cases diffusion is

possible, but because the diffusion time when injection PP on TPE is twice as long as

the diffusion time of TPE on PP, it is likely that the bonding strength of the tensile test

bars produced by injecting PP on TPE is stronger.

13

Figure 4.3: Temperature profile in the z-direction (PP 220 [˚C] injected on TPE 60

[˚C])

Figure 4.4: Temperature profile in the z-direction (TPE 190 [˚C] injected on PP 60

[˚C])

14

5. Results

In this chapter the results from the tensile tests (listed in Appendix D) will be shown.

First the results of injecting PC with TPU will be discussed. After this the results of

injecting PP with TPE are shown. Finally the optimized process conditions are

summarized and the difference between the bioreactor and the tensile test bars will be

discussed.

5.1 Polycarbonate and Thermoplastic Polyurethane

In figure 5.1a and 5.1b two tensile test bars are shown. The tensile test bar in figure

5.1a has an average tensile stress of 9.0 [MPa] at break. The tensile test bar in figure

5.1b breaks at an average tensile stress of 3.7 [MPa]. The tensile test bars are

produced with the following process conditions:

Variable Figure 5.1a Figure 5.1b

Mould Temperature [˚C] 30 50

Temperature PC [˚C] 300 330

Temperature TPU [˚C] 210 220

Injection Speed [cc/sec] 10 50

Table 5.1: Process conditions of the tensile test bars shown in figure 5.1

When figure 5.1a and 5.1b are compared, a few things can be noticed. First the tensile

test bar in figure 5.1b shows more flash than the other tensile test bar. Because TPU is

injected with a higher temperature, the polymer is more liquid and more flash will

occur. Secondly the TPU of figure 5.1b is more opaque, which means that there is

more degradation. This will result in a poor bonding strength. Finally, the bonding

interface of the tensile test bar in figure 5.1b shows a lot of volumetric shrinkage. The

cavities that are at the bonding interface decrease the bonding area, which also will

decrease the bonding strength.

Figure 5.1a: Injecting TPU on PC with

low temperatures and injection speed

Figure 5.1b: Injecting TPU on PC with

high temperatures and injection speed

15

In figure 5.2a and 5.2b two tensile test bars are shown. The tensile test bar in figure

5.2a breaks at an average tensile stress of 17.5 [MPa]. The tensile test bar in figure

5.2b already breaks at 4.4 [MPa]. The process conditions are listed in table 5.2.



Temperature PC [˚C] 280 330

Temperature TPU [˚C] 210 210



When PC is injected at high temperatures large volumetric shrinkage occurs near the

bonding interface, this is shown in figure 5.2b. In figure 5.2a the cavities at the

bonding interface are smaller than at figure 5.2b. The measurements show that the

tensile test bar in figure 5.2a has a higher bonding strength.

The tensile test bars that are produced of TPU and PC can resist a tensile stress

between 3.7 and 9.3 [MPa]. This means that the best tensile test bars can achieve up to

23 % of the maximum tensile stress of TPU at break. The tensile test bars that are

produced by injecting PC on TPU are even stronger. These tensile test bars can reach

a maximum tensile stress of 17.5 [MPa].

5.2 Polypropylene and Thermoplastic Elastomer

In figure 5.3a and 5.3b two tensile test bars are shown. The tensile test bars are

produced by injecting TPE and PP. The tensile test bar in figure 5.3a has an average

tensile stress of 1.6 [MPa] at break. The tensile test bar in figure 5.3b breaks at an

average stress of 5.5 [MPa]. The process conditions are listed in table 5.3.



Temperature PP [˚C] 200 200

Temperature TPE [˚C] 170 190



Figure 5.2a: Injecting PC on TPU with


Figure 5.2b: Injecting PC on TPU with


16

Unlike PC and TPU, the tensile test bars that are produced by injecting TPE on PP

have a higher bonding strength, when TPE is injected on PP with high temperatures

and an average injection speed. Because TPE and PP have approximately the same

processing temperature, a high temperature will cause the other polymer to melt. This

will result in a higher bonding strength, due to diffusion and mechanical interlocking.

When PP is injected on TPE the tensile test bars generally do not break at the

interface. The diffusion and mechanical interlocking will cause a high bonding

strength. During the tensile test the TPE will extend and the cross-area will decrease

until the maximum tensile stress of TPE is exceeded.

The tensile test bars that are produced of TPE and PP can resist a tensile stress

between 1.6 and 5.5 [MPa]. This means that the best tensile test bars can achieve up to

67 % of the maximum tensile stress of TPE at break. A disadvantage of the use of PP

is the high volumetric shrinkage. With high temperatures of PP the bonding interface

shrunk enough for the TPE to flow along the bonding interface all the way up to the

runner’s of PP.

Figure 5.3a: Injecting TPE on PP with


Figure 5.3b: Injecting TPE on PP with


17

5.3 Optimized process conditions

The result from the tensile tests (Listed in Appendix D) show that the tensile test bars

made of TPU and PC, have a higher bonding strength when the injection speed,

temperatures of the mould and polymers are low. While the tensile test bars that are

made of TPE on PP have a higher bonding strength, when the injection speed,

temperatures of the mould and polymers are high. The results are summarized in table

5.4.

Table 5.4: Optimized process conditions for the tensile test bars

Material Melt Temperature Mould Temperature Injection Speed

TPU

TPU 210 [˚C] (low) Disadvantages of a high temperature:

• Flash.

• Short degradation time.

Mould 30 [˚C] (low) Disadvantages of a high temperature:

• Flash.

• A high temperature will increase the volumetric shrinking near the bonding interface.

TPU 10 [cc/sec] (low) Disadvantages of a high injection speed:

• The shear rate will cause a higher temperature at the flow front. This will increase the volumetric shrinking near the bonding interface.

PC

PC 280 [˚C] (low) Disadvantages of a high temperature:

• A high temperature will increase the volumetric shrinking near the bonding interface.

---

PC 10 [cc/sec] (low) Disadvantages of a high injection speed:

• The shear rate will cause a higher temperature at the flow front. This will increase the volumetric shrinking near the bonding interface.

TPE

TPE 190 [˚C] (high) Advantages of a high temperature:

• A high temperature will cause more diffusion.

Mould 60 [˚C] (high) Advantages of a high temperature:

• A high temperature will cause more diffusion.

TPE 50 [cc/sec] (average) Advantages of a high injection speed:

• The shear rate will cause a higher temperature at the flow front. This will increase the diffusion.

PP

PP 180 – 220 [˚C] When the temperature of the PP melt is chosen between 180 – 220 [˚C]. The tensile test bars (injecting PP on TPE) usually do not break at the bonding interface.

Mould 45 [˚C] (average) The mould temperature for injecting PP on TPE is less important, because the tensile test bars are much weaker when TPE is injected on PP.

PP 10 [cc/sec] (low)

---

18

6. Conclusion

The numerical analyses in Matlab [2] show that when TPU is injected on PC the

temperature of the solid-melt interface is too low for diffusion to occur between the

polymers, unlike the process where PC is injected on TPU. In this case there are

approximately 5 [sec] for the polymers to diffuse. The numerical analyses for TPE

and PP show that in both cases the temperature of the solid-melt interface is high

enough for diffusion to occur between the polymers, but when PP is injected on TPE

the diffusion time is approximately 10 [sec]. This is twice as long as when TPE is

injected on PP.

From the results of the tensile tests show that high mould and melt temperatures and

an average injection speed have a negative effect on the bonding strength when TPU

is injected on PC. When the temperatures and the injection speed are low, TPU will

show less degradation, there will be fewer problems with flash and the volumetric

shrinkage near the bonding interface will be smaller. This will result in a higher

bonding strength between the polymers. The bonding strength for injecting PC on

TPU is higher than when TPU is injected on PC.

For injecting TPE on PP measurements show that high temperatures and an average

injection speed have a positive effect on the bonding strength. High temperatures

cause more diffusion and mechanical interlocking between the polymers. This has a

positive effect on the bonding strength. When PP is injected on TPE the bonding

strength between the polymers is even higher. Generally the tensile test bars do not

break at the bonding interface.

The results from the tensile tests show that both combinations of polymers can be

used to produce the bioreactor. The tensile test bars that are made from TPE and PP

can resist a tensile stress of 5.5 [MPa]. This should be enough to hold the parts of the

bioreactor together. However, the tensile test bars that are made of TPU and PC can

hold a tensile stress of 9.3 [MPa]. Because PP has a high volumetric shrinkage and the

combination of TPU and PC has a higher bonding strength, it is recommended to use

TPU and PC to produce the bioreactor.

19

7. Recommendations

The bioreactor has a more complex form than a tensile tests bar. Because of this

complex form it could be possible that the bioreactor could not be injection moulded

with the recommended process conditions. This is why the conclusions of this report

should be used as an indication to find the right process conditions for the bioreactor.

Some differences between the tensile test bars and the bioreactor are:

• At high melt and mould temperatures the tensile test bars have a big volumetric

shrinking near the bonding interface, which will weaken the bonding strength. The

walls of the bioreactor are thinner than the tensile test bars. Because thin parts

have less volumetric shrinking, higher melt and mould temperatures can be used.

• When the bioreactor is made by injection moulding, low viscosity is necessary to

stay under the maximum clamp force of the machine. This is achieved by using

high melt and mould temperatures.

• The tensile test bars have a small injection volume and a long cooling time

(thicker walls) compared to the bioreactor. At high melt temperatures the material

in the barrel will degrade. Because the bioreactor has a large injection volume, the

melt will not stay long in the barrel. This means that higher melt temperatures can

be used

• When the tensile test bars are injection moulded, the melt will arrive last at the

bonding interface, shown in figure 5.4. This means that the melt had time to cool

down before it reaches the other polymer. With the bioreactor the melt will flow

parallel over the other polymer, shown in figure 5.5, which results in a longer heat

transfer. This means that there is more time for diffusion.

Figure 5.4: Melt flow for the tensile test bars

Figure 5.5: Melt flow for the bioreactor

20

The numerical analyses show that when PC is injected on TPU, the temperature is

high enough for diffusion between the polymers. It is expected that these tensile test

bars are stronger than the tensile test bars produced by injecting TPU on PC. The

tensile tests confirm this, but it is incorrect to conclude that this only is caused by

diffusion. There are other factors that influence the bonding strength, such as

shrinkage and mechanical interlocking.

The tensile test bars had a large elongation during the tensile test. Because of this

elongation the stress-strain curve had an unusual form and some tensile test bar did

not break at the bonding interface. To improve this tensile test bars shaped like the

one in figure 7.1 could be used.

Figure 7.1: Tensile test bar to improve the tensile tests

In this report the Thermoplastic Polyurethane Desmopan 385 S was used, where the

designation S stands for a special additive. For Desmopan 385 S this additive is a

lubricant. Because the bioreactor is used for medical purposes, it is recommended to

use Desmopan 385, without the lubricant.

21

Appendix A: Mathematics

M-file Temperature profile of the solid-melt interface

clear

TMb= 330; %C Temperature (Melt)

TS= 50; %C Temperature Solid)

TgPC= 135;

TgTPU= 180;

lambdaS= 0.137; %W/(m.K) Thermal Conductivity

rhoS= 1372; %kg/m^3 Density

cpS= 1700; %J/(kg*K) Heat Capacity

lambdaM= 0.24; %W/(m.K) Thermal Conductivity

rhoM= 1033; %kg/m^3 Density

cpM= 1880; %J/(kg*K) Heat Capacity

kS= sqrt(lambdaS*rhoS*cpS);

kM= sqrt(lambdaM*rhoM*cpM);

aS=lambdaS/(rhoS*cpS);

aM=lambdaM/(rhoM*cpM);

L= 0.004;

thetas = 0;

i = 1;

j = 1;

begin = 0;

step = 2;

finish= 12;

figure;

for t=begin:step:finish

for z=0:0.0001:0.004

if t == 0

thetas = 1;

else

for n=0:1:1000

thetan=(4/((2*n+1)*pi))*sin((2*n+1)*(pi*z)/L)*exp(-

((2*n+1)^2*pi^2*aM*t)/(L^2));

thetas = thetas + thetan;

end

22

end

TM=thetas*(TMb-TS)+TS;

Te(j,i)= kS/(kS+kM)*TS+ kM/(kS+kM)*TM;

thetas=0;

i = i + 1;

end

leg=['t=',num2str(t)];

kleur=['b','g','c','m','y','k','r'];

tekst{j}=leg;

zp=0:0.0001:0.004;

plot(zp,Te(j,:),kleur(1,j));

hold on;

j = j + 1;

i = 1;

end

tekst{j}='Tg PC'

zlijn=0:4:4;

plot(zlijn,[TgPC TgPC],'r-.')

tekst{j+1}='Tg TPU'

zlijn=0:4:4;

plot(zlijn,[TgTPU TgTPU],'k-.')

title('Temperature profile of the solid-melt interface (TPU on PC)')

xlabel('z [m]');

ylabel('Temperature [^\circC]');

axis([0 0.004 0 220])

legend(tekst)

23

Appendix B: Cooling profile of the injection moulding process

Injection Processing Cooling time Cooling time

Sequence Variables after first injection after second injection

[sec] [sec]

TPU on PC - 17 60

PC on TPU - 50 40

PC on TPU Mould 50 [˚C] 60 40

TPE on PP - 45 40

TPE on PP Mould 60 [˚C] 55 40

PP on TPE Mould 30 [˚C] 40 30



PP 220 [˚C]


Table B.1: Cooling time

The M-file that is used for calculating and plotting the cooling profile of the mould-

melt interface is shown in Appendix A.

Figure B.1: Cooling profile of the mould-melt interface in the z-direction (TPU 220

[˚C], mould 30 [˚C])

24

Figure B.2: Cooling profile of the mould-melt interface in the z-direction (PC 330

[˚C], mould 30 [˚C])

Figure B.3: Cooling profile of the mould-melt interface in the z-direction (PP 200

[˚C], mould 30 [˚C])

Figure B.4: Cooling profile of the mould-melt interface in the z-direction (TPE 180

[˚C], mould 30 [˚C])

25

Figure B.5: Cooling profile of TPU in the z-direction

Figure B.6: Cooling profile of PC in the z-direction

Figure B.7: Cooling profile of PP in the z-direction

26

Figure B.8: Cooling profile of TPE in the z-direction

The M-file that is used for calculating and plotting the cooling profile of the polymers

is shown in Appendix A.

27

Appendix C: Material properties and process conditions

Material Properties The following properties where obtained from Moldflow [11]

Desmopan 385S (TPU) Unit

Density 1372 [kg/m3] Melt

Thermal Conductivity 0,137 [W/(m.K)] At 220 [˚C]

Specific Heat 1700 [J/(kg.K]) At 220 [˚C]

Tglass 180 [˚C]

Tensile Stress, brk 40 [Mpa] ISO 527-1,-3

Shore Hardness 85A 32D

Lexan Resin 141R (PC) Unit

Density 1033 [kg/m3] Melt


Specific Heat 1880 [J/(kg.K)] At 295 [˚C]

Tglass 135 [˚C]

Tensile Stress, brk 68 [Mpa] ASTM D 638

Rockwell Hardness 70M 118R

Borealis HD601CF (PP) Unit

Density 752,2 [kg/m3] Melt


Specific Heat 2600 [J/(kg.K)] At 180 [˚C]

Melting Temperature 119 [˚C]

Tensile Stress, brk 30-50 [Mpa] ISO 527-3

Evoprene G 969 Unit

Natural (TPE)

Density 900 [kg/m3] ISO 2781

Thermal Conductivity 0,18 [W/(m.K)] Approximation

Specific Heat 2000 [J/(kg.K)] Approximation

Melting Temperature 100 [˚C] Approximation

Tensile Stress, brk 8,2 [Mpa] ISO 37

Shore Hardness 64A

Table C.1: Material properties of the polymers

Steel Unit

Density 7800 [kg/m3]

Thermal Conductivity 46 [W/(m.K)]

Specific Heat 490 [J/(kg.K)]

Table C.2: Material properties of the steel mould

28

Process Conditions

Injection Processing Injection Holding Cooling time Cooling time

Sequence Variables Pressure Pressure after first injection after second injection

[bar] [bar] [sec] [sec]

TPU on PC - 1000 800 17 60

PC on TPU - 1000 800 50 40

PC on TPU Mould 50 [˚C] 1000 600 60 40

TPE on PP - 1000 800 45 40

TPE on PP Mould 60 [˚C] 1000 800 55 40

PP on TPE Mould 30 [˚C] 1000 400 40 30

PP on TPE Mould 45 [˚C] 1000 400 50 30

PP on TPE Mould 45 [˚C] 1000 300 60 30

PP 220 [˚C]

PP on TPE Mould 60 [˚C] 1000 300 60 30

Table C.3: Adjusted processing conditions to prevent fingers

The processing conditions where only adjusted if it was not possible to produce the

tensile test bars with the current settings.

29

Appendix D: Experiments

Results Tensile Tests PC on TPU Table D.2 shows the test results of the tensile test bars that are produced by injecting

PC on TPU. The denomination of the series (example 1221a) is explained in table D.1.

The first number is the temperature of TPU. The second number indicates the

injection speed. The third number is the temperature of PC and the last number shows

the temperature of the mould. The characters ‘1221a’, ‘1221b’, ‘1221c’ and ‘1221d’

indicate that the tensile test bars are from the same series, but that it is a different

tensile test bar.

1 TPU T 210 1

220 2

2 Injection v 10 1

50 2

3 PC T 300 1

330 2

280 3

4 Mould T 30 1

50 2

40 3

Table D.1: Denomination of the tensile test bar series

1111 1211 1121 1221 1131 1231

a 19,4 10,4 9,3 9,0 14,5 13,8

b 11,5 10,1 8,1 10,9 16,7 17,4

c 15,4 10,0 14,9 10,2 12,6 18,0

Mean 15,4 10,2 10,8 10,0 14,6 16,4

2111 2211 2121 2221 2131 2231

a 11,8 7,5 11,7 5,2 8,5 13,2

b 10,1 7,2 7,7 5,8 12,7 11,5

c 10,4 9,5 7,1 4,5 12,0 13,1

d 7,8

Mean 10,8 8,1 8,6 5,1 11,1 12,6

1112 1212 1122 1222 1132 1232

a 13,5 11,3 12,4 11,3 10,9 6,1

b 11,8 10,8 13,5 8,4 10,7 11,4

c 11,8 10,2 10,2 6,8 14,7 6,4

Mean 12,4 10,8 12,1 8,8 12,1 8,0

Table D.2a: Test results of the tensile test bars (PC injected on TPU)

30

2112 2212 2122 2222 2132 2232

a 10,0 11,7 7,7 9,0 6,5 6,1

b 10,4 12,2 6,5 7,3 6,2 6,2

c 10,9 9,5 12,4 7,3 5,7 7,0

Mean 10,4 11,2 8,8 7,9 6,1 6,4

1113 1213 1123 1223 1133 1233

a 12,3 7,6 9,1 4,5 18,1 15,4

b 11,9 7,9 9,2 4,8 17,4 14,9

c 13,3 7,6 8,3 4,0 17,0 12,1

d 9,1

Mean 12,5 7,7 8,9 4,4 17,5 14,1

2113 2213 2123 2223 2133 2233

a 9,3 6,1 4,0 5,0 12,0 9,8

b 9,7 5,1 5,5 3,7 11,1 10,4

c 9,0 5,3 5,9 4,8 12,7 10,1

Mean 9,3 5,5 5,2 4,5 11,9 10,1

Table D.2b: Test results of the tensile test bars (PC injected on TPU)

Does not break at the bonding interface.

Tensile test bar has a finger through the bonding interface.

Figure D.1: Bonding strength (PC injected on TPU) shown by injection speed

31

Figure D.2: Bonding strength (PC injected on TPU) shown by the temperature of

TPU

Figure D.3: Bonding strength (PC injected on TPU) shown by mould temperature

32

Figure D.4: Bonding strength (PC injected on TPU) shown by the temperature of PC

Results Tensile Tests TPU on PC Table D.4 shows the test results of the tensile test bars that are produced by injecting

TPU on PC. The denomination of the series (example 1221a) is explained in table D.3.

The first number is the temperature of TPU. The second number indicates the

injection speed. The third number is the temperature of PC and the last number shows



tensile test bar.

1 TPU T 210 1

220 2

2 Injection v 10 1

50 2

3 PC T 300 1

330 2

280 3

4 Mould T 30 1

50 2

40 3


33

1111 1211 1121 1221

A 8,9 7,9 8,0 7,4

B 9,2 8,6 7,8 7,7

C 9,0 7,9 7,6 7,8

Mean 9,0 8,2 7,8 7,7

2111 2211 2121 2221

a 8,5 7,9 7,7 7,1

b 9,6 8,1 8,0 7,3

c 8,2 8,0 7,8 6,9

Mean 8,8 8,0 7,8 7,1

1113 1213 1123 1223

a 9,3 8,4 6,7 5,8

b 9,4 8,2 6,7 6,1

c 9,1 8,6 6,9 6,1

Mean 9,3 8,4 6,7 6,0

2113 2213 2123 2223

a 4,9 6,0 6,9 5,3

b 5,3 6,2 6,9 5,1

c 5,1 5,9 7,5 5,4

Mean 5,1 6,0 7,1 5,3

1112 1212 1122 1222

a 8,7 5,9 7,7 5,5

b 8,1 5,4 7,4 5,2

c 8,1 6,2 6,8 5,0

Mean 8,3 5,9 7,3 5,3

2112 2212 2122 2222

a 7,5 4,7 4,0 3,9

b 7,6 4,6 4,7 4,0

c 8,6 5,2 4,7 3,4

d 5,5 4,1

Mean 7,9 5,0 4,3 3,7

Table D.4: Test results of the tensile test bars (TPU injected on PC)

34

Figure D.5: Bonding strength (TPU injected on PC) shown by injection speed

Figure D.6: Bonding strength (TPU injected on PC) shown by the temperature of

TPU

35

Figure D.7: Bonding strength (TPU injected on PC) shown by mould temperature

Figure D.8: Bonding strength (TPU injected on PC) shown by the temperature of PC

36

Results Tensile Tests PP on TPE Table D.6 shows the test results of the tensile test bars that are produced by injecting

PP on TPE. The denomination of the series (example 1221a) is explained in table D.5.

The first number is the temperature of TPE. The second number indicates the

injection speed. The third number is the temperature of PP and the last number shows



tensile test bar.

1 TPE T 170 1

190 2

180 3

2 Injection v 10 1

50 2

3 PP T 180 1

200 2

220 3

4 Mould T 30 1

45 2

60 3


3111 3211 3121 3221 3131 3231

a 5,3 4,6 5,2 4,6 6,7 4,5

b 4,6 4,6 6,3 4,4 7,1 4,4

c 4,9 4,5 6,4 4,7 6,4 4,6

Mean 5,0 4,5 5,9 4,6 6,7 4,5

3112 3212 3122 3222 3132 3232

a 6,2 4,3 6,8 4,8 6,3 4,5

b 4,5 4,8 5,7 4,8 6,4 4,7

c 4,6 4,6 6,5 4,9 6,2 4,8

Mean 5,1 4,6 6,3 4,8 6,3 4,6

3113 3213 3123 3223 3133 3233

a 5,3 3,8 4,7 3,9 4,4 3,9

b 5,1 4,5 4,4 3,7 4,4 4,2

c 5,1 3,8 4,5 4,2 4,7 4,3

Mean 5,2 4,0 4,5 3,9 4,5 4,1

Table D.6: Test results of the tensile test bars (PP injected on TPE)


37

Figure D.9: Bonding strength (PP injected on TPE) shown by injection speed

Figure D.10: Bonding strength (PP injected on TPE) shown by mould temperature

38

Figure D.11: Bonding strength (PP injected on TPE) shown by the temperature of PP

Results Tensile Tests TPE on PP Table D.8 shows the test results of the tensile test bars that are produced by injecting

PP on TPE. The denomination of the series (example 1221a) is explained in table D.7.

The first number is the temperature of TPE. The second number indicates the

injection speed. The third number is the temperature of PP and the last number shows



tensile test bar.

1 TPE T 170 1

190 2

180 3

2 Injection v 10 1

50 2

3 PP T 180 1

200 2

220 3

4 Mould T 30 1

45 2

60 3


39

1121 1221 2121 2221 3121 3221 1231

a 1,6 3,4 2,4 2,4 2,0 2,7 3,6

b 1,6 3,7 2,3 2,8 2,0 2,8 3,3

c 1,6 3,7 2,3 3,1 2,0 2,9 3,7

Mean 1,6 3,6 2,3 2,8 2,0 2,8 3,5

1122 1222 2122 2222 3122 3222

a 2,2 3,6 6,4 3,8 2,6 2,8

b 2,2 3,6 4,2 3,8 2,6 3,7

c 2,2 3,3 6,0 3,3 2,5 2,9

Mean 2,2 3,5 5,5 3,6 2,6 3,1

1123 1223 2123 2223 3123 3223

a 2,9 2,9 4,3 3,4 2,9 3,0

b 4,0 2,8 4,1 3,6 4,2 3,5

c 3,8 3,1 4,6 4,1 2,9 2,6

Mean 3,6 3,0 4,3 3,7 3,4 3,0

Table D.8: Test results of the tensile test bars (TPE injected on PP)


Figure D.12: Bonding strength (TPE injected on PP) shown by injection speed

40

Figure D.13: Bonding strength (TPE injected on PP) shown by mould temperature

Figure D.14: Bonding strength (TPE injected on PP) shown by the temperature of PC

41

Appendix E: List of Symbols

Tm Melt temperature

Ts Temperature of the solid polymer

Te Temperature of the solid-melt interface

Tl Temperature of the liquid polymer

Tg Glass transition temperature

T Current temperature

Cp Specific heat

k Coefficient of conductivity of the polymer contact surface ρ Density

θ Dimensionless temperature

n Step number of the Fourier-series

λ Thermal conductivity

x X-coordinate

y Y-coordinate

z Z-coordinate

L Thickness of the tensile test bar in the z-direction

α Constant that shows the phase of the polymer

Kc Fracture toughness

β Factor related to the crack length and component geometry

σ Applied stress

a Crack length

w The finite plate width

Table E.1: List of symbols

42

Literature

[1] P.E. Neerincx, Design and Realization of a Disposable Bioreactor, Internal

MSc Thesis Eindhoven University of Technology (2007)

[2] http://www.mathworks.com/products/matlab/description1.html

[3] E.M. Hagerman, Plastics Eng., 29, 67 (1973).

[4] J. Tadmor, Appl. Polyrn. Sci., 18, 1753 (1974).

[5] D.Y. Huang and R.S. Chen, Polymer Eng. & Sci., 39, 11 (1999)

[6] M.E.H. van Dongen and P.P.J.M. Schram, Fysische Transportverschijnselen

deel II, (2004)

[7] A.K. van der Vegt and L.E. Govaert, Polymeren van keten tot kunststof,

ISBN 90-71301-48-6

[8] http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/

Mechanical/FractureToughness.htm

[9] K. Tomari, S. Tonogai and T. Harada, Polymer Eng. & Sci., 30, 15 (1990)

[10] A.J. Kinloch, Journal of Mat. Sci., 15, 2141 (1980)

[11] Moldflow Plastics Insight 6.1 Properties Database

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