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A STATISTICAL STUDY OF PARAMETERS INULTRASONIC WELDING OF PLASTICS

Atypical ultrasonic welding (USW) process startswhen a power supply changes 50 Hz of electricalenergy into high ultrasonic frequency of 20–40kHz. To transform this electrical energy into

mechanical energy, a converter, a lead zirconate titanate elec-trostrictive element, expands and contracts at the resonantfrequency. The converter is coupled to the mechanical imped-ance transformer called a horn. The horn transmits theenergy to the joint area where frictional heat is produced tomelt the plastic momentarily, causing it to fuse together. A pneumatically-activated press applies pressure to the part tobe welded. Figure 1 shows the USW process.

LITERATURE REVIEW

Thedistancebetween thesonotrode–workpieceinterface andthe joint interface affects the weld process and weld quality. Ultra-sonic joints are therefore classied intotwocategories, near-eldwelding and far-eld welding. In near-eld welding, alsoknown as contact sealing, the vibrating tool or horn/sonotrodeis less than 6 mm from the joint area. In far-eld welding orremote welding, the joint is placed at a distance ofmore than 6mm from the horn. It is obvious that the energy requirementsto affect fusion become less as the tool contact with the partapproaches the sealing area. It is therefore desirable to designthe part so that the seal area will be as accessible as possibleto the tool for the ultrasonic energy to be utilized and to pre-vent energy losses within the part itself. Industrial applica-tions do not always lend themselves to near-eld welding;it then becomes imperative to go in for ‘‘far-eld welding.’’Figure 2 shows the near-eld and far-eld conguration.Benetar et al. 1 investigated the near-eld welding of acrylo-nitrile butadiene styrene (ABS), polystyrene (amorphous),polyethylene, and polypropylene (semicrystalline) polymers.The estimated moduli were entered into a lumped parametermodel to predict heating rates and energy dissipation. Exper-imental results showed that variations in weld pressure hadlittle effect on energy dissipation or strength of joint.

Benetar and Cheng 2 also studied the far-eld welding of amorphous and semicrystalline polymers. They reported thatweld strength improved substantially with increasing ampli-tude of vibration at the joint interface. Increasing the weldtime or pressure also resulted in increased weld strengths.

If the contact area at the seal is reduced, the amount of ultra-sonic energy available there becomes concentrated, producingan immediate fusion. One way of concentrating the energy isby converging it to a point by molding a V-bead on the partcalled an ‘‘energy director.’’ Industry uses a triangular-shapedenergy director for simplicity of production. The other type of joint mostly used by the industry is the ‘‘shear joint,’’ which is

an interference type of joint. Figure 3 shows the two types of joints practiced by the industry.

Chuah et al. 3 studied three different types of energy direc-tor—triangular, rectangular, and an innovative semicirculargeometry using far-field samples of ABS and high-densitypolyethylene (HDPE) types of thermoplastic samples. Theyconcluded that the shape of the energy director affected thewelding efficiency significantly under the same welding con-ditions. The semicircular energy director was found to havethe maximum welding efficiency and the triangular shape thelowest. In a study by Van Wijk et al., 4 to optimize the USWprocess in series and mass production, a quantitative testmethod for measuring product quality by a design of experi-ments was required. They found no correlation between weldqualities and weld time measurements under uniform energycontrol.

In an article by Devine, 5 a number of joints have been dis-cussed. These include the butt joint, step joint, and tongueand groove joint. It has been suggested that the 90 8 energydirector is suitable for most of the amorphous resins, but 60 8

energy director is preferred for polycarbonate, acrylics, andsemicrystalline materials. Crystalline materials may receiveincomplete fusion when an energy director is used becausematerial displaced from the energy director may solidifybefore it ows across the joint to form a seal. For this reason,a shear joint is suggested for welding crystalline polymers.This has been conrmed by thepresent study. Frantz 6 studiedfactors such as material, joint design, and equipment optionsto determine the most appropriate method for joining crystal-line polymers. He suggested a continuous energy director joint such as saw tooth, which can give an air-tight or liq-uid-tight seal. But no comparative study has been taken upso far. Kenney 7 studied the USW of semicrystalline polymers.He suggested an alternate to the shear joint. This methodinvolves molded-in texturing of the welding surface oppositean energy director. The textured surface consists of peaks andvalleys about 3–6 mm deep. During welding, the texturedsurface produces more compact melt face and better heattransfer across the joint face. According to Kenney, this pro-cess can produce joints three times stronger than those withnontextured surfaces but are not as strong as the shear welds.His work is patented.

OBJECTIVE As discussed in the literature review, studies were carried outby the researchers on the inuence of parameters in achievingdesired levels of performance of the joint. But the factors con-sidered are limited in number. A comprehensive study of theUSW process with the design of experiment considering allthese variables (weld pressure, weld time, hold time, materialtype, joint type, and eld type) was carried out in the presentstudy. Factors like amplitude (60 mm) and frequency (20 kHz)could not be studied as it is a restriction of the machinery.

A 26 2 2

fractional factorial design was chosen to study the

TECHNIQUESby R.M. Rani, K.S. Suresh, K. Prakasan, and R. Rudramoorthy

R.M. Rani (principal investigator [DST-WOS]), K.S. Suresh (research scholar), K. Prakasan (professor), and R. Rudramoorthy (professor and principal) are afliatedwith the Department of Production Engineering, PSG College of Technology, Peel-

ameedu, Coimbatore, India.

doi: 10.1111/j.1747-1567.2007.00182.xÓ 2007, Society for Experimental Mechanics September/October 2007 EXPERIMENTAL TECHNIQUES 53



individual effect of the welding variables and also some of theinteraction effects of these variables on the welding results.The response variables were energy consumed and ultimatetensile strength (UTS) of the joint formed.

EXPERIMENTAL DETAILSIn the present study, a commercial ultrasonic plastic weldingmachine (20 kHz and 1500 W) manufactured by NationalIndosonic, Bangalore, India, wasused. Hornsmade ofaluminum–titanium alloy were used for studying the near-eld and far-eldeffects. Theparameters investigated are shown in Table 1.

Variations in the experimental levels (Table 1) have beenbased on the literature survey and initial experiments con-ducted by the authors. Below this experimental range, therewas no welding, and above this range, there was overwelding

(excess of ash). Based on these observations, the experimen-tal range for parameters X 1, X 2, and X 3 has been selected. Asthe experimental range has to cover both amorphous andsemicrystalline polymers, there is a big difference in levelsfor parameters X 1, X 2, and X 3. Amorphous plastic–ABS andsemicrystalline plastic–HDPE specimens were made as perthe specication of the ZVEI or Philips test parts as in Fig. 4(Benetar et al. 1). The Philips test part can be used both innear-eld and in the far-eld welding with an energy directorfor shear joints. A semicircular energy director (Chuah et al. 3)found to have maximum welding efciency was chosen for thepresent study. The specimens were made by injection moldingat Sugi Plastics, Coimbatore, India. Figure 5 shows thewelded and unwelded specimens.

The energy consumed by the specimen during welding wasobtained by reading the energy meter provided in the control

Fig. 1: USW system for joining thermoplastics

Fig. 2: Near- and far-eld congurations Fig. 3: Energy director and shear joint practiced by the industry

STATISTICAL STUDY OF PARAMETERS INULTRASONIC WELDING OF PLASTICS

54 EXPERIMENTAL TECHNIQUES September/October 2007



The regression equations are:

Y 5 0:153 1 0:090X 1 1 0:694X 2 1 0:009X 31 0:079X 4 2 0:037X 5 2 0:016X 62 0:113X 1 X 2 2 0:015X 1 X 4 2 0:101X 1 X 2 X 2 X 4

ð1Þðfor energy consumed Þ

Y 5 2 0:048 1 0:218X 1 1 0:047X 2 1 0:069X 32 0:979X 4 1 0:314X 5 2 0:516X 61 0:534X 1 X 2 2 0:089X 1 X 4 2 0:004X 1 X 2 X 4

ð2ÞðforUTSÞ

Where Y is the response variable. For Eq. 1, it is the energyconsumed as percent of welding energy, and for Eq. 2, it is theUTS given in MPa. The independent variables on the right-hand side of the equation are the weld parameters given in

Table 2. The parameters X 4, X 5 , and X 6 take the values 1 1and 2 1 for the 1 and 2 levels of experimental setting. Tocheck the model for adequacy, ‘‘ F ’’ test was carried out, andit was found to be adequate at 1% level of signicance. 7

Table 3 gives the details of the F test.

The effects of the variables can be inferred from the coef-cients of the above equations. From Eq. 1, it is clear thatweld time is a signicant factor for energy utilized. Theinteraction effect of weld pressure and weld time is consid-erable for energy utilized for welding. The application of weld pressure for a sufcient time inuences the joint for-mation and the energy utilized as pressure is responsible forkeeping the surfaces to be joined in intimate contact and

generating heat by friction. The intimate contact eliminates

the presence of air gaps and hence the reduction in energylosses. Hold time does not contribute signicantly to thewelding results.

Of the two materials studied, HDPE absorbs more energythan ABSunder the same experimental conditions. This couldbe due to the energy required for breaking the crystallinestructure in HDPE. Also, shear joints absorb more energythan ‘‘energy director joints.’’ This can be attributed to thecontact area, which is more in the case of shear joint. Near-eld welding consumed more energy than far-eld weldingpossibly due to better transfer of the ultrasonic energy overa short distance. In far–eld welding, some of the energy is

dissipated while traveling through the part and less amountof energy is available at the interface for joint formation. Theinteraction effect of weld pressure, weld time, and material isnot signicant. Practically, it was seen that increasing theweld time and weld pressure at the same time resulted inoverwelding (excess of ash and part marking at the horn/-part interface).

Similarly, from Eq. 2, it can be seen that weld pressure ismore signicant for UTS than weld time and hold time. ABSwas found to weld with low pressure, whereas HDPE neededhigh pressure to weld owing to its crystalline structure. Thetype of material being welded (ABS, HDPE) is a very signi-cant factor in determining the UTS of the joint. Strength of

the joint for a specimen made of ABS is more than that of HDPE under similar weld conditions. Near-eld joints hadmore strength compared to far-eld welding. Among theinteractions, the effect of weld time and weld pressure wasfound to be signicant for strength of the joint.

Figure 6 shows the details of the soundness of the joint withthe help of X-ray radiography. A few of the experimental con-ditions resulted in no welding; for example, in the experimen-tal setting ‘‘1,’’ there was no welding of the components. Thiscould be attributed to low pressure and low weld time. Exper-imental setting ‘‘9’’ reported no weldingas HDPE being a crys-talline polymer needs more weld pressure and weld time for joining. HDPE could not be welded in the far-eld region by an

energy director joint even with high pressure (experimental

Table 2 — Experimental settings

No. X 1 X 2 X 3 X 4 X 5 X 6

1 2 2 2 2 2 1

2 1 2 2 2 2 2

3 2 1 2 2 1 1

4 1 1 2 2 1 2

5 2 2 1 2 1 2

6 1 2 1 2 1 1

7 2 1 1 2 2 2

8 1 1 1 2 2 1

9 2 2 2 1 1 2

10 1 2 2 1 1 1

11 2 1 2 1 2 2

12 1 1 2 1 2 1

13 2 2 1 1 2 1

14 1 2 1 1 2 2

15 2 1 1 1 1 1

16 1 1 1 1 1 2

Table 3 — Details of F test for adequacy of the model

No. Source Eq. 1 Eq. 2

1 Regression meansquare (Reg)

0.041 6.075

2 Residual meansquare (Res)

0.006 3.032

3 Fisher ’s ratio F (exp) 5 Reg/Res 6.519 2.004

4 ‘‘F (tab) ’’ ratiotabulated at 1% levelof signicance and at9, 6 degrees of freedom*

7.98 7.98

5 Observation F (exp) , F (tab) F (exp) , F (tab)

6 Adequacy of model Yes Yes

Value of F (tab) taken from Johnson. 9





setting ‘‘10’’) as the crystalline polymer solidies before a weld

is achieved. Far-eld welding with shear joint and weld pres-sure of 2 bar did not weld HDPE (experimental setting ‘‘13’’),owing to low weld pressure. It is clear that HDPE is a difcultpolymer to weld, and can be ultrasonically welded, only withhigh weld pressure and weld time preferably with a shear joint and in the near-eld congurations. On the other hand, ABS, an amorphous polymer, is an ideal candidate for USW; itcan be welded in both near- and far-eld regions, preferablywith an energy director joint (although it can be welded usinga shear joint in the near-eld region) even with low weldpressure and weld time. Experimental setting ‘‘8,’’ whichreported high UTS, is found to have bright regions at the weldarea in the X-ray photograph (Fig. 6), denoting a good welded joint.

Radiographicstudies areone of theusefulnondestructive testscarried out by the industry. The principal concept is that radi-ation will penetrate light materials better than denser materi-als. Heavier, denser materials offer greater resistance toradiation penetration because they absorb more of the energy.The difference in absorption of the X-rays by various portionsof the exposed area is recorded on an X-ray lm and can besubsequently viewed once the lm has been developed. Theresulting image is basically a type of ‘‘shadow’’ of the materi-al/component in the path of the X-rays. Regions that appearbright are denser than the dark regions in the X-ray photo-graph. In Fig. 6, the X-ray radiograph shows bright regions atthe welded zone for experimental settings 7, 8, and 2. These

welded joints also reported high UTS values in comparison to

the other joints probably due to good mixing and homogeneityof the polymer at the weld region. Hence, we may concludethat X-ray radiographic studies can be carried out to ascertainthe quality of ultrasonic welded joint successfully.

CONCLUSIONSFrom this study, based on a fractional factorial design, thefollowing salient inferences can be highlighted.

1. Weld time has a signicant effect on the welding resultsfollowed by weld pressure. The interaction effect of these two parameters is also signicant. Hold time doesnot contribute signicantly to the welding results. Thisis because the pressure is essential in developing anintimate contact between the surfaces to be joined fora specic duration (weld time) to achieve sound joint.

2. Specimens made of ABS are found to be easier to weldultrasonically with better levels of energy utilizationthan HDPE. This is attributed to the amorphous struc-ture of ABS. HDPE is crystalline.

3. Energy director joint is suitable for ABS (can bewelded even with low pressure and low weld time),although it can be welded using a shear joint. HDPEcan be welded by an energy director joint in the near-eld conguration, only with high pressure and highweld time.

4. ABS can be welded both in the near-eld and in the far-eld region, whereas HDPE can be welded in the far-eld region using a shear joint only.

5. Strength of the joints for specimens of ABS was found tobe more than that of components of HDPE for the sameexperimental conditions.

6. Radiographic studies with X-rays correlated well withUTS of the welded components explaining the quality of welds for different weld conditions.

These observations will be useful when designing a new prod-uct for manufacture by USW and can provide the researcherwith initial data.

ACKNOWLEDGMENTSThe authors express sincere gratitude to the Department of Science and Technology, New Delhi, for funding this project(No-SR/WOS-A/ET-20/2003). Thanks are due to the manage-ment and the principal, PSG College of Technology, Coimba-tore, for providing the basic infrastructure to set up the

laboratory with Department of Production Engineering. Authors are thankful to the help and cooperation of Sugi Plas-tics, Coimbatore, in making the test specimen, xtures, andtooling for tensile testing. The support extended by theDepartment of Civil Engineering, for the tensile testing of specimens, is gratefully acknowledged.

References1. Benetar, A., Eswaran, R.V., and Nayar, S.K., ‘‘Ultrasonic

Welding of Thermoplastics in the Near Field,’’ Polymer Engineeringand Science 29 (23):1689–1698 (mid-December 1989).

2. Benetar, A., and Cheng, Z., ‘‘Ultrasonic Welding of Thermo-plastics in the Far Field,’’ Polymer Engineering and Science 29 (23):

1699–1704 (mid-December 1989).

Fig. 6: X-ray radiographs of specimens for weld conditions2–8, 11, 12, 14, and 16 (from top to bottom)


September/October 2007 EXPERIMENTAL TECHNIQUES 57



3. Chuah, Y.K., Chien, L.-H., Chang, B.C., and Liu, S.-J., ‘‘Effectsof the Shape of the Energy Director on Far-eld Ultrasonic Weldingof Thermoplastics,’’ Polymer Engineering and Science 40 (1):157–167(2000).

4. Van Wijk, H., Luiten, G.A., and Van Engen, P.G., ‘‘ProcessOptimization of Ultrasonic Welding,’’ Polymer Engineering and Sci- ence 36 (9):1165–1176 (mid-May 1996).

5. Devine, J., ‘‘Ultrasonic Plastic Welding Basics,’’ Welding Journal : 29–33 (2001).

6. Frantz, J., ‘‘Joining Plastics the Sound Way,’’ Machine Design69 (7):61–65 (1997).

7. Kenney, W.E., ‘‘Designing Plastic Parts for Ultrasonic Assem-bly,’’ Machine Design 64 (10):65–68 (1992).

8. Systat 10.2, Statistics I, Systat Software, San Jose, CA (2001).

9. Johnson, R.A., Miller and Freund’s Probability and Statistics for Engineers , 5th ed., Prentice Hall of India Private Limited, NewDelhi (1995). n



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