STR_V9_N3_02_FTG

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SIMTech technical reports (STR_V9_N3_02_FTG) Volume 9 Number 3 Jul-Sep 2008

119

In-situ plastic-to-metal adhesive bonding via injection moulding

S. L. Liu, T. T. Ng and K. P. Lim

Abstract This project investigates the feasibility ofmetal-to-plastic adhesive bonding. The bonding

process is performed and completed through normal

plastic injection moulding process. The effects of

surface pretreatment and moulding processing pa-

rameters on the bonding strengths were studied. It is

attempted to correlate the bonding strength with the

morphology of the fracture interfaces.

Keywords: Plastic-to-metal, Adhesive bonding, In-

jection moulding

1 BACKGROUND

In the integration process of a product, the

bonding of two parts made from two dissimilar ma-

terials is commonly encountered. There are a few

methods to bond two dissimilar materials, such as

physical, chemical and mechanical bonding. Physical

bonding, such as wire bonding, is commonly used in

the electronics industry, where one metal has a rela-

tively low melting point. Mechanical bonding with the

use of screws, rivets, and spot welds, has been com-

monly used to join two or more adherents together.However, chemical bonding is becoming more widely

used and is replacing mechanical bonding in manyapplications, including the bonding of metal to plastic

materials. Although joining of various adherents can

be challenging, with the use of adhesives, in many

cases, product performance and durability are in-

creased, component and assembly costs are reduced,

and fewer finishing operations are required. This is

especially useful when miniaturisation and aesthetics

of a product are of importance in the consumer elec-

tronics.Since the nature of the adhesive bonding is es-

sentially a chemical process, there are quite a fewfactors affecting the bonding strength and durability.

From the theoretical considerations and extensive

practical testing, the following factors need to be

considered to achieve satisfactory bonding: (a) suit-

able surface pre-treatment surface preparation is,

perhaps, the most important process governing thequality of an adhesive bond; (b) adhesive choice the

adhesive should be able to wet the adherent and so-

lidify under production conditions: time, temperature,

and pressure; (c) joint design adhesive joints are

generally more resistant to shearing, compressive, and

tensile stresses than they are to stress systems due to

peeling; and (d) service condition polymeric adhe-

sives generally have higher coefficients of thermal

expansion than metals and ceramics.

In the traditional adhesive bonding of plas-

tic-to-metal, the plastic parts shall be moulded in

advance. In this study, the in-situ bonding of plas-

tic-to-metal will be performed. Figure 1 compares thetwo bonding processes. In this project, the bonding

behaviour of polycarbonate to aluminium, which arethe materials commonly used in electronics products

will be examined with focus placed on the effects of

surface treatment and moulding conditions on the

bonding strengths will be investigated.

Mixing of hardener

with adhesiveApplying adhesive

Metal

substrate

Metal

substrate

Metal

substrate

Final product Assembly & Curing Drying of adhesive

(A)

Metal

substrate

Pre-molded plastic part

Mixing of hardener

with adhesiveApplying adhesive

Metal

substrate

Metal

substrate

Metal

substrate

Final product Assembly & Curing Drying of adhesive

(A)

Metal

substrate

Pre-molded plastic part

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Ap plying adhesive

Metal

substrate

Metal

substrate

Molding & final product Drying of adhesive

(B)

Metal

substrate

Ap plying adhesive

Metal

substrate

Metal

substrate

Molding & final product Drying of adhesive

(B)

Metal

substrate

Fig. 1. The comparison of: (a) traditional adhesive bonding; and (b) in-situ adhesive bonding.

2 OBJECTIVE

The objective of this project is to study the fea-

sibility of metal-to-plastic adhesive bonding and the

bonding process shall be completed in the plastic

injection moulding process.

3 METHODOLOGY

3.1 Materials

The metal used in this study is Aluminium 6106and the plastic is polycarbonate. The adhesive is a

latent reactive polyurethane provided by Bayer. The

chemical reaction for the cure crosslinking of the

polyurethane is illustrated in Fig. 2. As the cross-

linking reaction of polyurethane is fast, it must be

deactivated at storage. This is achieved by the surfacedeactivation reaction. NCO groups on the particle

surface react with the amine NH2 to form ureas, thus

preventing the isocyanate particles from reacting with

the water. Chemicals including hydro fluoride, so-

dium hydroxide were purchased from Sigma-Aldrich

and were used as received.

Fig. 2. Crosslinking for the formation of polyurethane.

3.2 Specimen Preparation

Two types of aluminium half tensile bars were

machined according to the following dimensions (Fig.

3). Both types of the half tensile bars can fit in the

mould which is used for injection moulding of plastic

tensile specimens. The differences of these two types

of tensile bars provide different joint designs. The

results of bonding strengths in this project were based

on the design (a). After machining, the tensile bars

were subjected to different surface treatments beforebonding with polycarbonates. In this study, two sur-

face treatments were conducted. They were acid

treatment and alkali treatment.

3.3 Moulding Process

The injection moulding was conducted on aNestal injection moulding machine. The moulding

conditions such as melt temperature, mould tem-perature, and cooling time were adjusted according to

experimental design.

(a)

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(b)

Fig. 3. The dimensions of Al half tensile bars.

3.4 Characterisation

The curing behaviour of the latent reactive poly-

urethane was characterised with a TA2910 differential

scanning calorimeter. The heating rate used was

10C/min. And the testing was carriedout from 40

C

to 150C under the nitrogen purge. The bonding

strength was measured using an Instron 4505 tensile

machine. The crosshead speed was maintained at 2

mm/min during tests. The fracture morphologies of

the interfaces and adhesives were observed with an

optical microscope.

4 RESULTS & DISCUSSION

4.1 Design of Experiments (DOE)

As there are a lot of moulding parameters influ-

encing the bonding strength, such as melt temperature,cooling time, and mould temperature, their effects on

the bond strength shall be determined first. As such, a

3 factor, 2 level design of experimental matrix was

designed. The three factors are mould temperature,melt temperature, and cooling time. The response is

the bond strength.The first step of the DOE is to determine the low

and high value of each experimental parameter. As

recommended from the supplier, polycarbonate can

be processed above 280C. Polycarbonate will de-

compose at high temperature. Therefore, the low end

was set to 280C, and the high end was set to 320C.

Mould temperature was determined according toboth the polycarbonate processing temperature and

the cure temperature of the adhesive. The latent reac-

tive polycarbonate starts to cure around 98.5C and is

fully cured around 140C. Considering that there is an

operation time after the insertion of the aluminium bar

coated with adhesive into the mould and before the

injection moulding, the mould temperature shall be

lower than 110C, which is the fast cure temperature.

The low end of the mould temperature shall take

the flowability of polycarbonate into consideration.

This is due to the poor flowability of polycarbonate at

low mould temperature. Therefore, the low end of

mould temperature is set to 70C. The cooling time isset based on the recommendation for material proc-

essing and past experience. As the injection time is

short, the adhesive is cured mainly during cooling.

Therefore, longer cooling time shall favour more

extensive curing, thus providing higher bonding

strength. However, longer cooling time will lowerproductivity. As such, the cooling time was set be-

tween 10 s and 60 s.

4.2 Bonding Results

After the determination of the critical processing

parameters, bonding experiments were conducted.

Table 1 gives the bonding results for the specimens

treated with acid. From this table it can be seen that

the bonding strength varies dramatically with the

change of moulding conditions. To find the most

critical parameters which affect the bond strength, the

DOE analysis was performed.

Figure 4 shows the effect of various moulding

conditions on the bonding strength. It can be seen

from Fig. 4 that the bonding strengths decrease withincreasing mould and melt temperatures, whereas the

cooling time has little effect on the bonding strengths

in the range investigated. From these experimentalresults, the moulding parameters can be optimised at:

melt temperature 280C, mould temperature 70C and

cooling time 10 s.

Table 1. Bonding results for the acid treated samples.

Run

Mould

Tempera-

ture ( C)

Melt Tem-

perature

( C)

Cooling

Time (s)

Bonding

Strength

(MPa)

1 100 320 60 0.312

2 70 320 10 0.725

3 100 280 10 0.972

4 70 280 60 1.410

Table 2 shows the bonding results for the speci-

mens treated with alkali. It seems that the variation of

bonding strengths is much less in this category,

compared with that in the acid treated group. The

DOE analysis of various bonding conditions on the

bonding strength was performed and the results are

shown in Fig. 5. For the alkali treated samples, the

mould temperature and melt temperature show oppo-

site effects on the bonding strength. Increasing mouldtemperature results in an increase of the bonding

strength, while increasing melt temperature will de-

crease the bonding strength. Similar to the samples

treated with acid, samples treated with alkali are alsoinsensitive to cooling time as evidenced in Fig. 5(c).

Table 2. Bonding results for the alkali treated samples.

Run

Mould

Tempera-

ture ( C)

Melt Tem-

perature

( C)

Cooling

Time (s)

Bonding

Strength

(MPa)

1 100 320 60 1.410

2 70 320 10 0.749

3 100 280 10 1.710

4 70 280 60 1.380

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(C)

(MPa)

(C)

(MPa)

(C)

(MPa)

(C)

(MPa)

(s)

(MPa)

(s)

(MPa)

Fig. 4. The responses (bonding strength) to moulding con-

ditions for the acid treated specimens: (a) mould tempera-

ture, (b) melt temperature, and (c) cooling time.

(C)

(MPa)

(C)

(MPa)

(C)

(MPa)

(C)

(MPa)

(MPa)

(s)

(MPa)

(s)

Fig. 5. The responses (bonding strength) to moulding con-

ditions for the alkali treated specimens: (a) mould tem-

perature, (b) melt temperature, and (c) cooling time.

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As the bonding strength is affected inversely to

the melt temperature and mould temperature at similarintensity (slopes of Figs. 5(a) and (b)), the moulding

parameters for the alkali treated samples can be op-

timised at: melt temperature 300C, mould tempera-

ture 85C and cooling time 10 s.

Figure 6 compares the effects of different treat-ments on the bonding strengths. It is quite obvious

that alkali treatment is much more efficient in ob-

taining good bonding. This could be resulted from a

better removal of oxides on the surface and the for-

mation of non-uniform crates on the surface, which

are helpful in locking the adhesive and improving the

bonding strength.

Bonding Strength

0.00E+00

5.00E+05

1.00E+06

1.50E+06

2.00E+06

2.50E+06

1 2 3 4

Run

BondingStrength(N

/m

2)

Acid

Alkali

4.3 Fracture Surface Analysis

From the above bonding results we can see that

the bond strength varies tremendously with different

moulding conditions and surface treatments. Figure 7

shows the morphology for the specimens showing low

and high bonding strengths, respectively. For the

specimen showing low bonding strength, the fracture

surface is smooth, and the island morphologies

dominate. This could be resulted from the incomplete

cure of the adhesive. For the specimen showing high

bonding strength, the fracture surface is rough and

fracture is mainly caused by the tearing of the adhe-

sive.

(a)

(b)

Fig. 7. The morphologies of the fractured surfaces of sam-

ples with: (a) low bonding strength, and (b) high bonding

strength.

5 CONCLUSION

This project has proved the feasibility of poly-

mer-to-metal adhesive in-situ bonding through injec-

tion moulding. Compared with traditional two-part

bonding method, in-situ bonding technique is much

faster and can reduce assembly process. The key

moulding parameters which affect the bond strengths

are identified through the design of experiments. It

was also found that alkali treatment is more effective

in improving the bonding strength than acid treatment.

6 INDUSTRIAL SIGNIFICANCEThe in-situ bonding process developed in this

project exhibits advantages over traditional two-part

adhesive bonding. The in-situ bonding process has

reduced a few process steps compared with the tradi-

tional two-part adhesive bonding process. The bond-

ing technology developed in this project will be very

useful for the bonding of dissimilar materials, espe-

cially for those electronics industry where thin wall

metal frames shall be bonded with plastic materials.

REFERENCES

[1] E.W. Thrall, R.W. Shannon, Adhesive bonding of

aluminum alloys, New York: M. Dekker , 1985.

[2] R.D. Adams, Adhesive bonding: science, technology

and applications, Cambridge: Woodhead Publications,

Boca Raton, Fla. CRC Press, 2005.

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STR_V9_N3_02_FTG