2013 International Conference on Robotics, Biomimetics, Intelligent Computational Systems (ROBIONETICS) Yogyakarta, Indonesia, November 25-27,2013

Inspection of Ultrasonic Welding for Thermoplastic

Materials Joining

Rashiqah Rashli, Shahrul Kamaruddin School of Mechanical Engineering

Engineering Campus, Universiti Sains Malaysia Nibong Tebal, Pulau Pinang, Malaysia.

sheeqa _ 89@yahoo.com, meshah@usm.my

Abstract- In manufacturing industries, inspection process is

the most vital process in production line in order to certify the

quality of the product. At the end of every production line, there

would be one quality assurance section to do the inspection.

Sometimes quality inspection is not just needed at the end of the

production line, it is also needed at specific area or at the process

where usually problem occurs that denotes to bad quality

product. This phenomenon frequently occurs in joining

thermoplastic materials which used ultrasonic welding.

Ultrasonic welding had increasing popularity as it offers a

continued safe operation, faster and also low cost as it able to join

weld part less than one second. Despite of its advantages in

joining parts, ultrasonic welding is one of process that needs a

special care and its own inspection process since it leads to major

reject products because of its bad quality welding. Conventional

inspection for ultrasonic welding joining usually used a

destructive testing method, for an example pull/ push test. This

testing will brings to waste in time and material cost. In order to

encounter this matter, non destructive testing using ultrasonic

testing is used in this research. It is been done by detecting the

signal of ultrasonic welding vibration during the ultrasonic

welding process of thermoplastic materials joining. This obtained

signal will then be analyzed to acquire the abnormalities within

the signal which represents the defects between the joint, thus

concludes the quality of the product.

Keywords-ultrasonic welding; thermoplastic; processing; inspection; ultrasonic testing

I. INTRODUCTION (Heading 1)

signal

Michael J. Troughton [1] describes ultrasonic welding as one of the common used welding methods for joining thermoplastics where it uses high frequency current; typically between 20 - 40 kHz which are beyond the range of acceptance human hearing. This frequency is used to produce low amplitude (l - 25 11m) mechanical vibrations where the vibrations generate heat at the joint interface of the parts being welded, consequently melt the thermoplastic materials and finally weld formation after cooling. Ultrasonic welding is the fastest known welding technique where it abIes to weld less than one second. Figure 1 shows an ultrasonic welding machine with its welding sample. As can be seen in Figure 1, ultrasonic welding consists of power supply, converter or transducer, booster and a horn or sonotrode. The detail process

Knowledge Transfer Programming funding by MOHE and ERGS

978-1-4799-1208-7/13/$31.00 ©2013 IEEE 125

Elmi Abu Bakar, Abd Rahim Othman School of Aerospace Engineering

Engineering Campus, Universiti Sains Malaysia Nibong Tebal, Pulau Pinang, Malaysia

meelmi@eng.usm.my, merahim@eng.usm.my

of ultrasonic welding will be further explained in the next section of this paper.

The applications of ultrasonic welding are extensive and can be found in many manufacturing industries such as electrical and computer, automotive and aerospace, medical, and packaging. Though ultrasonic welding can be seen and applied in various manufacturing industries, ultrasonic welding is not applicable for thick parts specimen since it's difficult to melt the material between the joint interfaces. It is only suitable and applicable for wires, microcircuit connections, sheet metal, foils, ribbons and meshes which are thin and easier to melt including thermoplastic materials.

converter

booster

Fig. 1. Schematic of ultrasonic welding

Weldability of parts being welded is always affected by the properties of the material. Anixter in [3] exemplifies that thermoplastic is easier to be weld using ultrasonic welding if compared to thermoset as thermoplastic has the ability to be melted and take on a new shape as it cool plus it can be done over and over again. But it is different to thermoset material where it will take its final shape and cannot be recycled or reused. Figure 2 shows the molecular structures of thermoplastic and thermoset material. For thermoset, it can be seen that there are smaller molecules, (red line) which become a cross-linked between the long individual polymers chains

where it became the reason why thermoset cannot be reshape or recycle like thermoplastic.

Thermoplastic Molecular Chains Thermoset Cross-linked Molecule

Fig. 2. Thermoplastic and thermoset molecular chain

If take a look deeper into thermoplastic material types, amorphous thermoplastic is simpler to be welded rather than semi crystalline thermoplastics because material with higher melting temperature requires more energy for ultrasonic welding and resin with lower modulus of elasticity transmits energy to the join energy much lesser than the stiffer raisin.

The major problem that occurs in most ultrasonic welding process is too many reject product due to bad quality welding. Inspection need to be done right after the ultrasonic welding process for each product to detect the abnormalities occurrence beforehand and passed to the next process without adding any defects to the parts. Current inspection for this process is done at the fust few parts and consequent parts at every 1 hour or others duration time depends on the management decision and that inspection is the destructive testing which will damage the parts. Inspection that had been done for every part that had gone through the process is done by the operator itself where it is judge by the visual analysis. If the defects cannot be seen visually, it will be considered as good welding. This is not an accurate and precise method. Defects or abnormalities may also occurred inside the parts where cannot be seen and recognized by normal operators.

Hence, the main purpose of this research is to inspect the ultrasonic welding for thermoplastic materials joining by calculating the occurrence of abnormalities percentage without adding any defects or damage into the parts or materials in easier way and defmitely low cost. In order to do so, a non contact measurement had been done by using ultrasonic testing. A transducer is attached to the horn of ultrasonic welder to detect the signal of vibration during the ultrasonic welding process. This signal is used to analyze and calculate the abnormalities occurred and thus concluded the quality of the parts.

This paper is divided into four chapters which are introduction, methodology, result and discussions and conclusions. Throughout this chapter, a brief introduction of ultrasonic welding with its application; its weldability with materials and major problem that ultrasonic welding faces had been briefly discussed. Next in chapter 2, methodology that relies behind this research is exposed and it is divided into six sections which are A, B, C, D, E and F. In chapter 3, a result and discussion of this research is explained in details and lastly a conclusion of this paper is concluded in chapter 4.

II. METHODOLOGY

Basically there are six steps involved in this research. Those steps are illustrated in Figure 3 and will be explained further in this section.

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Experimental setup

NG

Fig. 3. Overall process flow

A. Experimental Setup First, we need to setup the machines and the equipments

needed. During experiment, besides ultrasonic welder, we need a transducer to detect the mechanical vibrations during the ultrasonic welding process. This transducer is at the upper part of the horn as can be seen in Figure 4. The selection of this particular area because of the ultrasonic welding machine will produce a vibration signal after machine start operationg. This mechanical vibration will transmit to LMS SCADAS Mobile SCMO 1 to be processed and compute a signal wave. Through the channel output channel from LMS SCADAS, then the signal is transfer in display unit and the raw signal data will be processed with the signal acquisition algorithm.

Ultrasonic Welder

Hom

Transducer

LMS SCADAS Mobile SCMO 1

Fig. 4. Schematic of experimental setup

B. Ultrasonic Welding After the equipments had been setup besides the ultrasonic

welder, ultrasonic welding can be start. Technically, ultrasonic welding process starts by clamping the workpiece between the hom and anvil. Then, the hom vibrates and starts to produce heat which will automatically melt the contact surface consequently build up deformation layer. Finally weld area was performed and joined the workpiece together. This ultrasonic welding process can be seen much clearer in Figure 5 below.

Horn

Workpiece

Anvil

Step I:

Clamping the workpiece

Step 2: Step 3: Contact Build up surface melted &

deformation layer workpiece joint

Fig. 5. Schematic of ultrasonic welding process

C. Signal Detection Using the LMS SCADAS Mobile SCMOI with the aid of

LMS Test.Xpress software, a signal detection of ultrasonic wave from ultrasonic welding process can be easily plotted as in Figure 6. This graph shows the respond of welding speed against time for every specimen in welding process.

.-.---.-------.-,.-.-

.............. _ -, ....

....... Conttol Control:

Time T1OOU:111 Compru!.d

(a) Testing A

15!l_D31:AnaIYZlrdlfault(31)C3:[gl

---- ------.. _--,.- ... _----_ .... -,---

•• + • • • • +._--,-- .. ... ... - .. �- ..

Co�trol Contr9.1 .... ··· · ·· p(:)�nH .......... .

Time Traces: 111 CompJ."".d

(b )Testing B

127

1G!l-D32:Anall'nrdlbult(32) C3: [�l

--- -----.c0�rol -----------:----------------- �-­

Control

(c)Testing C

Fig. 6. Welding speed vs. time graphs

From the plotted graph, we identified the two features of welding behavior during the process. We highlighted in circle for specific area as Control point 1 and Control point 2. Control point 1 represents the condition where joined process takes place and during this time the bottom and upper specimen will be joined as on. Meanwhile Control point 2 is the abnormalities signal caused by electrical response before the welding process achieves the steady state condition.

D. Signal Processing The first control point's data signals which represent the

joining process for all testing are shown in Figure 7 below and the red line shows the noise of the signal of the abnormalities occurrence.

·2 ,,-----�--,---�-�--�____,

., ,

·10 . . .. . . . .. : . ...... . .;.. . ...... . � . -:- .

·12 Noise on .. -=+! the si2:nal '

(a) Testing A

., c---.,------,-----,--�--�----,

., , I I

-10 J I

Noise on : : -12 . . . . . th�'sl'gn'ar-----';"

(b) Testing B

-2 .------_-�-___,____-�---__,

, I -10 :----- - --------------:-1

Noise on. . : -12 - thesignaF�

( c) Testing C

Fig. 7. Control point 1 signal graphs

For second control point's data signals that represent the points prior to steady state are plotted as in Figure 8 and red line shows the noise exist in the signal which lead to abnonnalities occurrence.

(a) Testing A

Peak

-2

-,

-6

-8

·1DO�--------;�--------;;';;---------,;,50

(b) Testing B

A' )�""I� Peak B ",;.;.Oi?"

C

-2

-,

-6

-8

·100�--------;�--------;;';;---------,;,50

( c) Testing C

Fig. 8. Control point 2 signal graphs

128

These two control points are selected and been focused because these are two critical points where control point 1 is the ultrasonic welding takes place and defects deformation due to ultrasonic welding may occur. While for control point 2, it seems that there is noise detected from the earlier signal detection as can be seen in Figure 6 before it reaches the steady state.

E. Analysis From the earlier graphs in Figure 6, it can be said that the

welding had been successfully done since the graphs for all three testing are uniform. Figure 9 below shows the abnormalities percentage concept that had been used in this research.

perfect=O; void=O; for a = l:l:length(sample)

for b = 1

limit)

end end

if(sample(a,b»lower limit && sample(a,b)<upper

perfect = perfect+1; else

void = void+1; end

percent of_void =( (void/length (sample) )*100);

percent of void

Fig. 9. Abnormalities percentage calculation

This calculation is based on out of range data in the signal. For this case, we had considered control point 1 and control point 2 to be focused since it is the area of noise existence as shown in Figure 6 earlier. So, this process is done twice for control point 1 and control point 2 and the lower limit and upper limit are difference and based on the signal graph of the testing. While the length sample for each testing are the same where we set 3 seconds or 2400 data for each testing.

III. RESULTS AND DISCUSSIONS

In this section, the percentage of abnormalities existence in ultrasonic welding of thermoplastic material joining are specified. The calculation is based on out of range data in the signal and the equation that had been used is as follows where a is the abnormalities percentage, number of voids is the out of range data in the signal and length of sample is the total number of data in the signal and it is the same for each testing with total of 2400 data.

a = no. of voids / length of samp Ie x 100% (1)

The abnonnalities percentage results for each testing are compute in Table 1 below.

TABLE T. ABNORMALITIES PERCENTAGE

Testing Abnormalities Percentage

Control point 1 Control point 2

A 2.2917 0.7917

B 2.5417 0.8333

C 1.8750 0.8750

From the percentage value of abnormalities exist, it can be said that, all three testings shows good significant result which is the total of abnormalities percentage are below than 3% and Testing C is the best quality welding and smallest value of abnormalities percentage of control point 1 which is 1.875% if compared to Testing 1 with 2.2917% of abnormalities and Testing B with 2.5417% of abnormalities. While abnormalities percentage for control point 2 shows not much difference of value where all testings which are between 0.75-0.9% range.

Besides, this research had been successfully done with safe continued operation without adding any defects and to the parts itself and wasted any materials. This inspection is also done in a short of time which is less than 1 minutes for every testing.

IV. CONCLUSION

Therefore, an inspection of ultrasonic welding for thermoplastic material joining had been successfully done with the aid of ultrasonic testing of non destructive testing and by detennining the percentage existence of abnormalities through the signal detection. The objective of this research had also been successfully accomplished by inspecting the ultrasonic welding of thermoplastic materials joining in safe continued operation, short processing time and also low cost since it does not need any expertise to do this operation and there is no waste material.

Further research could be done by inspecting other types of materials with different properties and the abnormalities percentage calculation should consider all area of the signal despite on focusing on a few control points that the abnormalities occurs the most to get more accurate result.

129

ACKNOWLEDGMENT

This project is a part of Knowledge Transfer Programming funding by MOHE, ERGS and collaboration project between University Sains Malaysia and industry.

REFERENCES

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