The geometry influence of the active sonotrode surfaces over the quality of ultrasonic welding for the plastic materials.

Belgiu, George ; Oanca, Octavian ; Ruset, Vasile 等

1. INTRODUCTION

The quality of a ultrasonic welded joint depends highly by a series of technological, mechanical and acoustical parameters of the joining process such as: acoustic conditions for joint deformation, the amplitude and frequency of ultrasonic oscillations, the physical state of the contact surfaces, the physic-mechanical and physic-chemical properties of the joining materials, the intensity of the ultrasonic energy, the plastic deformation speed of the joining materials, the contact pressure and the duration of the ultrasonic activation process.

In the majority of cases, the research of different parameters influence on the quality of the ultrasonic welded joints has been done by determining the breaking force through traction stretching and shearing testing--also known as the classical de-buttoning test (Amza et al., 2009).

The paper seeks to highlight the important influence in the ultrasonic joining process of a secondary technological parameter, the sonotrode active surface geometry, together with basic energetic parameters of the ultrasonic welding process such as welding time, welding force, and ultrasonic micro vibrations amplitude.

The main criteria that characterizes the welding behavior of plastic materials is consisted by the damping factor B of the oscillations amplitude in the considered plastic material.

[beta] = [omega] x [eta]/2 x [([rho]/E).sup.1/2] (1)

where : [omega]--oscillation pulsation; [rho]--material density; E--elastic module of the plastic material; [eta]--ratio between the power and elasticity module.

The plastic materials with good ultrasonic welding proprieties are: PP (polypropylene), PVC (vinyl polychloride) etc, for which 0,35<[beta]<0,55 [cm.sup.-1] (Galvery & Marlow, 2001).

2. PROBLEM STATEMENT

The experimental program carried out by a partner from industry kept into consideration the realization of ultrasonic welded joints of two samples from PVC piping with 0.5 and 1 mm thickness on a metallic structure, an application used for cabling in automotive industry. The quality tests imposed by the specifications consisted in realization of high strength joints (Rm>10daN) and a corresponding visual aspect of the joints. In order to fulfill the requirements imposed to the application a ultrasonic welding equipment has been used that operates at a resonance frequency of 20kHz, foreseen with an active cylindrical work tool (sonotrode) in steps with 3 straight active lobes. The experimental tests that used a sonotrode with 3 straight active lobes with active surfaces (type A) have led to obtaining a 10 daN joint breaking strength placed at the higher limit imposed by the application (***, 2009b).

3. RESEARCH, METHOD AND SOLUTION

In order to realize some ultrasonic joints that will fulfill the requirements regarding quality and joint resistance norms a new work tool was conceived which in correlation with the energetic welding parameters (pressing force (F), welding time (t) and the oscillations amplitude at the top of the sonotrode (A)) will fulfill the requirements imposed by the application. Obtaining the forms and theoretical dimensions of the sonotrode and knowing the state parameters of internal load of the sonotrode was possible by using specialized simulation programs (***, 2009a).

In figure 1 the active zone of the sonotrode is presented (type B sonotrode), developed within the experimental researches, same as the welding setup for the joining materials. This sonotrode is also fitted with 3 active lobes. As a novelty item, in the active zone of the sonotrode we can observe that, at the level of the three lobes it is fitted with curvatures that take into consideration the form and size of the PVC piping that will be welded (Nonhof & Luiten, 1996).

[FIGURE 1 OMITTED]

4. RESULTS

The experimental results obtained after using a sonotrode with the active surfaces fitted with the curvature radius are presented in table 1.

In the table 1 the parameters are: F--welding force; [T.sub.s]--welding time; A--ultrasonic oscillations amplitude; [R.sub.m]--mechanical resistance at shearing traction; Q--joint quality taking into consideration its aspect (values from 1 to 10. Value 10 is given to a very good quality of joint).

In order to highlight the way that process parameters (F, [t.sub.s], A) influences the quality of the ultrasonic joints (strength of joint (Rm) and the quality factor--joint aspect (Q)) the data obtained after the ultrasonic welding experiment using the type B sonotrode (with active lobes with curvature radius) were interpreted by using the factorial experiment method.

The analysis of the experimental data highlights the fact that the influence factors F, ts and A produce significant effects on the joint strength (Rm), it will vary between 1 / 24daN depending of the value attributed to the influence factors. The optimum value (maxim) for Rm is obtained for F=50N, [t.sub.s]=1,8s si A=18 [micro]m. By modifying even with small values the levels of the influence factors from the optimum values, generally lead to spectacular modification in performance.

The macro structural analysis of the ultrasonic welded joint with an optimum technological regime highlights welded areas well determined with a robust geometric configuration is shown in figure number 2.

[FIGURE 2 OMITTED]

The global and estimative analysis of type B experiment is presented in the graphics from figure 3. Global and estimative analysis for type B experiment the following are recorded:

* generally to operate with low values of the weld time (ts) and weld force (F) parameters and high values for the amplitude parameter is a disadvantage;

* performance evolution is spectacular as soon as exits it the optimum zone;

* the objective function values are highly dependant of the interactions between the influence factors (response surfaces are highly deformed);

* usually Q can obtain good values for A [member of][12; 18][micro]m and ts [member of] [1; 2]s, adjusted to A values in a direct proportion.

Interpretation of results for type A and B experiments regarding polymer welding joints realization with sample breakage in the base material (type B experiment) comparative to type A experiments with samples breakage in the weld is explained by sonotrode active surfaces form and configuration (see figure 3).

[FIGURE 3 OMITTED]

5. CONCLUSION

The experimental program has presented the important influence in joining ultrasonic process of the sonotrode active surface geometry together with basic energetic parameters of the ultrasonic welding process, the welding force (F), the welding time (ts) and the ultrasonic micro vibrations amplitude (A). Experimental tests that used a sonotrode with straight active surfaces (type A) have led to obtaining a joint breaking strength of approximately 10daN, resistance at the high limit imposed by the application. Tests during the shearing traction trial have highlighted the random breakage in the weld without the possibility to control the welding technology. Modifying the active zone of the sonotrode in correlation with the welding process had lead to an increase of the breakage resistance of the joint from 10 daN to 24 daN in the case of type B sonotrode (with lobes fitted with curvature radius). For the case of using type B sonotrode and values for F = 50N, ts =1,8s si A=18[micro]m, the breakage during shearing traction test has occurred in the base material.

6. REFERENCES

Amza, G.; Balaceanu, M.A. & Tasca, G.D. (2009). Contributions regarding the optimization of ultrasonic welding process for termorigide plastic materials, Available from: http://www.imsar.ro/Sisom_2009.pdf, Accessed on: 2009-07-03

Galvery, W.L. Jr. & Marlow, F.M. (2001). Welding Essentials: Questions & Answers, Industrial Press, Inc., ISBN 0831131519

Nonhof, C.J. & Luiten, G.A. (1996). Estimates for process conditions during the ultrasonic welding of thermoplastics, Polymer Engineering and Science, Available from: http://findarticles.com/p/articles/mi_hb3367/is_n9_v36/ai_n 28669850/, Accesed on: 2009-04-12

*** (2009a). http://www.krell-engineering.com--Resonator design, Accesed on 2009-06-17

*** (2009b) http://www.powerultrasonics.com--Polymer materials for ultrasonic plastic welding, Accesed on: 2009-07-03

Tab. 1. Experimental program parameters using type B
sonotrode

No. F [N] [t.sub.s] A [[micro]m] [R.sub.m] Q [1...10]
crt. [S] [daN]

 1. 38 1 19,5 1 1
 2. 63 1,4 17 19 9
 3. 75 1,4 14 20 8
 4. 88 1,5 12 16 7
 5. 50 1,2 18 1 2
 6. 50 1,5 18 10 5
 7. 50 1,6 18 15 7
 8. 50 1,8 18 24 10
 9. 50 18 22 8
10. 25 1,8 22,5 7 2
11. 38 1,8 19,5 9 4
12. 58 1,8 16,5 17 7
13. 63 1,8 17 14 6