Ultrasonic Welding Of Thermoplastics Research Articles

A study on ultrasonic welding of thermoplastics with significant differences in physical properties under different process parameters

A study on ultrasonic welding of thermoplastics with significant differences in physical properties under different process parameters

Optimization of Ultrasonic Welding Process Parameters to Enhance Weld Strength of 3C Power Cases Using a Design of Experiments Approach.

Ultrasonic welding (UW) is a joining of plastics through the use of heat generated from high-frequency mechanical motion, which is known as an efficient process in many applications, such as textile, packaging, or automotive. UW of thermoplastics has been widely employed in industry since no polymer degradations are found after UW. However, the trial-and-error approach is frequently used to study optimum UW process parameters for new 3C plastic power cases in current industry, resulting in random efforts, wasted time, or energy consumption. In this study, Taguchi methods are used to study optimum UW process parameters for obtaining high weld strength of a plastic power case. The most important control factor influencing the weld strength is amplitude, followed by weld pressure, hold time, and trigger position. The optimum UW process parameters are amplitude of 43.4 µm, weld pressure of 115 kPa, hold time of 0.4 s, and trigger position of 69.95 mm. Finally, the confirmation experiments are performed to verify the optimum process parameters obtained in this study.

CALIBRATION AND VALIDATION OF A STEREO CAMERA SYSTEM AUGMENTED WITH A LONG-WAVE INFRARED MODULE TO MONITOR ULTRASONIC WELDING OF THERMOPLASTICS

Abstract. Ultrasonic welding of thermoplastics has become an important industrial manufacturing process in the fields of aerospace and transportation. High quality standards are demanded and a reliable quality assessment routine is fundamental. Most on-site and off-site approaches are time consuming or require additional active illumination. Especially temperature models have proven to be good indicators for welding quality. We present a methodology to measure the surface temperature in real-time and visualize it simultaneously as a 3D model. Through augmenting a stereo camera system with an additional passive thermal infrared camera, we are able to map the heat data of multiple successive welds of large, free-form structures into a common 3D data representation. A challenging calibration approach is used to derive the inner and exterior orientation for the trifocal camera system. Geometric and radiometric improvements for an aluminium chessboard allow the usage of wide-angle optics for the thermal infrared camera. Consequently, we verify the quality of each camera by means of their resolving power. Therefore, a Siemens star test pattern is used for the thermal camera as well. We demonstrate the effectiveness of our methodology on a robot-guided ultrasonic welding tool.

Double-Pulse Ultrasonic Welding of Carbon-Fiber-Reinforced Polyamide 66 Composite.

Ultrasonic welding of thermoplastics is widely applied in automobile and aerospace industries. Increasing the weld area and avoiding thermal decomposition are contradictory factors in improving strength of ultrasonically welded polymers. In this study, relations among the loss modulus of carbon-fiber-reinforced polyamide 66 composite (CF/PA 66), time for obtaining stable weld area, and time for CF/PA 66 decomposition are investigated systematically. Then, a double-pulse ultrasonic welding process (DPUW) is proposed, and the temperature evolutions, morphologies and structures of fractured surfaces, and tensile and fatigue properties of the DPUWed joints are measured and assessed. Experimental results show the optimal welding parameters for DPUW include a weld time of 2.1 s for the first pulse, a cooling time of 12 s, and a weld time of 1.5 s for the second pulse. The DPUW process enlarged the weld area while avoided decomposition of CF/PA 66 under appropriate welding parameters. Compared to the single-pulse welded joint, the peak load, weld area, and endurance limit of the DPUWed joint increased by about 15%, 23% and 59%, respectively. DPUW also decreases the variance in strengths of the joints.

Application of sheet-like energy directors to ultrasonic welding of carbon fibre-reinforced thermoplastics

A highly efficient joining method using ultrasonic welding is necessary for manufacturing and processing thermoplastic carbon fibre-reinforced plastic (CFRP) structures. The application of resin mesh as a sheet-like energy director is proposed, and its feasibility is assessed. The relationship between the welding conditions and the single-lap shear strength was investigated using specimens prepared by ultrasonic welding. The fracture surface observations indicated that resin mesh melted from the edges of the specimen, whereas the unmelted regions remained at the centre of the welding surface. It was necessary to supply a sufficient amount of resin to the welding surface and select a resin mesh suitable for the welding conditions to minimize variations in the bonding strength. Under the welding conditions used herein, the specimens with the coarsest resin mesh showed an average single-lap shear strength of 34 MPa and a coefficient of variation of 0.1. The study proves that the application of a sheet-like energy director for the ultrasonic welding of thermoplastic CFRP is a promising method to achieve practical bonding strength.

Effect of contact area with fixture on dynamic behaviour of joint interface in ultrasonic welding of thermoplastics

Ultrasonic welding is one of the most common methods for joining thermoplastics. The two plastic parts to be joined are placed on the fixture before high frequency mechanical vibration is applied to the parts through an ultrasonic horn. This generates heat at the joint area and locally melts thermoplastics. The parts are welded together. In this paper, we investigate the effect of the contact area between the lower part and the fixture on the dynamic behaviour of the joint interface, which affects heat generation. The displacements and elastic strains of the interface are predicted using finite element dynamic contact analysis and compared for different contact areas. The results show that the dynamic behaviour of the interface depends on the dynamic characteristics of the two parts. When both natural frequencies are close to the horn vibration frequency, the displacements and strains are small. Conversely, when the lower part has no natural frequency near the horn vibration frequency, the displacements and strains are large. The contact area significantly affects the dynamic behaviour of the interface. This is because its dynamic behaviour depends on the natural frequencies of the parts to be joined while the natural frequency of the lower part is easily improved by the contact area.

Effects of the shape of the energy director on far‐field ultrasonic welding of thermoplastics

AbstractAn energy director is widely used in ultrasonic welding to increase the welding speed and quality. In the present work, three different types of energy directors were studied—namely, a triangular, a rectangular, and an innovative semicircular energy director. Experiments were performed using far‐field test samples made of amorphous‐type (ABS) and semicrystalline‐type (PE) thermoplastics. It was found that the weld time is an important parameter of ultrasonic welding for the three types of energy directors studied. Weld pressure has different effects for the types of plastics tested. Increasing the weld pressure will decrease the welding efficiency for ABS. But for PE, increasing the weld pressure to four bars will increase the welding efficiency. The shape of the energy director was found to significantly affect the welding efficiency. In comparison, a semicircular shape was found to yield the highest welding efficiency under the same welding conditions and the triangular shape the lowest. Temperature measurements at the triangular energy director during the welding process indicate that the energy director absorbed 48.5% of the welding energy for ABS and 21.1% for PE. The different energy absorption rates are probably due to the difference in elasticity and viscosity between amorphous (ABS) and semicrystalline (PE) plastics.

Estimates for process conditions during the ultrasonic welding of thermoplastics

AbstractOrder of magnitude estimates are presented for processes that play a role in ultrasonic welding. The fact that the sonotrode may not always be in contact with the product being welded, which results in the sonotrode repeatedly hammering the product, is accounted for in this study. The calculations do not use estimates for loss or storage modulus of plastics at 20 kHz around the glass or melting point for amorphous or semicrystalline polymers respectively. The flow of molten polymer in the weld zone is shown to be a laminar viscous squeeze flow driven by the welding pressure. An energy balance is used to show that the heat generated by the internal damping is, in part, used in heating cold material and is squeezed out into weld flash. The theoretical findings are correlated with existing practical pointers on ultrasonic welding in series and mass production in industry.

Ultrasonic welding of thermoplastics in the near‐field

AbstractUltrasonic welding is one of the most popular techniques for joining thermoplastics because it is fast, economical, and easily automated. In near‐field ultrasonic welding, the distance between the horn and the joint interface is 6 mm or less. This study investigated the near‐field ultrasonic welding of amorphous (acrylonitrile‐butadiene‐styrene and polystyrene) and semicrystalline (polyethylene and polypropylene) polymers. High frequency ultrasonic wave propagation and attenuation measurements were made in order to estimate the dynamic mechanical moduli of the polymers. The estimated moduli were entered into a lumped parameter model in order to predict heating rates and energy dissipation. Experimental results showed that variations in the welding pressure had little effect on energy dissipation or joint strength; Increasing the amplitude of vibration increased the energy dissipation and the weld strength. For the semicrystalline polymers, increasing the weld time improved strength up to weld times greater than 1.5 s, where strength leveled off. For the amorphous polymers, the weld strength increased with Increasing weld time up to times of 0.8 s; for longer weld times, the power required was too high, causing overloading of the welder. Monitoring of the energy dissipation and static displacement or collapse provided valuable information on weld quality.

Ultrasonic welding of thermoplastics in the far‐field

AbstractIn far‐field ultrasonic welding of plastic parts the distance between the ultrasonic horn and the joint is greater than 6 mm. This study investigated the farfield ultrasonic welding of amorphous (acrylo butadiene styrene and polystyrene) and semicrystalline (polyethylene and polypropylene) polymers. Far‐field welding worked well for amorphous polymers. Weld strength improved substantially with increasing amplitude of vibration at the joint interface. Increasing the weld pressure and/or the weld time also resulted in higher weld strengths. Far‐field ultrasonic welding was not successful for semicrystalline polymers. The parts melted and deformed at the horn/part interface with little or no melting at the joint interface. A model for wave propagation in viscoelastic materials, which was developed to predict the vibration amplitude experienced at the joint interface, indicates that increasing the length of the samples to a half a wavelength should improve the far‐field welding of semicrystalline polymers by maximizing the amplitude of vibration at the joint interface.

Heating and bonding mechanisms in ultrasonic welding of thermoplastics

AbstractAn experimental study of the heating and bonding mechanisms in ultrasonic welding is described. Polystyrene specimens were joined under a variety of welding conditions while the temperatures at the interface and within the interior of these specimens were measured. The power input, amplitude of vibrations, and amount of deformation during welding were measured concurrently. In general, the rate of heating at the interface is greatest at the beginning of the weld cycle, but slows markedly after the interface temperature reaches approximately 250°C. The interface temperature peaks well before the weld is completed. Temperatures within the body increase most rapidly at temperatures near the glass transition temperature. Welded specimens were broken on a special testing apparatus under combined torsional and compressional loads to determine the weld strength. The results show that weld strength is dependent on the amount of energy input and the degree to which material flows out of the interface region. Possible mechanisms for heating and bonding during ultrasonic welding are discussed in light of the observed behavior.

Ultrasonic welding of thermoplastics: Maher, A.D. and Walkin, P.British Plastics, 38, No. 7, 436 (1965)

Ultrasonic welding (UW) is not only a well-known industrial process but it has also been an active research area. Materials ranging from metals to non-metals e.g. polymers and from virgin materials to non-virgin materials e.g. composites are easily welded using this welding technique. Some research has already been carried out but more thorough analysis is needed on ultrasonic welding of thermoplastics. Two thermoplastics selected for this research are commercially known as Acrylo-nitrile-Butadiene Styrene (ABS) and Polypropylene (PP). ABS belongs to amorphous type of thermoplastic, whereas PP is semi-crystalline thermoplastic. Owing to this dissimilarity in their molecular structure, ultrasonic welding of these two plastics has already been considered to be different. Energy director (ED) is usually protruded on anyone of the samples to be welded. Ultrasonic energy is uniformly driven in the presence of energy director at a localized area between the samples. In this research, triangular (TRI) and semi-circular (SEMI) energy directors (ED) were protruded on the surface of specimen by designing and manufacturing injection molds. Tensile testing in shear was performed for measuring lap shear strength of joints after being welded ultrasonically at constant strain rate of 3.24 mm/minute. Maximum LSS (lap shear strength) of 17 MPa and 6 MPa were found for ABS with TRI ED and PP with SEMI ED respectively. In other words, these LSSs (lap shear strengths) were 34% and 14.63% of base material strength for ABS and PP respectively. In this work, a statistical analysis (General Linear Model) was also used to deduce meaningful information from experimentation for both materials. For different factor settings, percent change in LSS was also calculated. Maximum percent change in LSS was found for weld time e.g. 2682 and 76.67 from low to high level was computed for ABS and PP respectively. Similarly, 55 & 47.2 for amplitude and 41 & 6 for static force were calculated with ABS & PP respectively. Percent change in LSS of 409 and 47 was also evaluated from SEMI to TRI and TRI to SEMI EDs for ABS and PP respectively. All the factors appeared to be significant to affect the LSS but weld time was found to be the most significant to achieve the higher bond strength. After doing the GLM (General Linear Model) analysis, some interesting results were also highlighted. Experimental techniques were used to investigate the main reasons for these findings. Difference in softening temperatures, viscosities, temperature spreads, ED collapses, heat affected zones (HAZ) and fractured surfaces for both materials were used to determine the reasons for interesting effects. For example, low static force was required for ABS in gaining higher LSS due to its lower softening temperature and viscosity drop at weld zone. Thus this work presented new and deeper understanding of the ultrasonic welding of thermoplastics. Various hypotheses were also made after having gone through the literature. Experimental tools were also used to test these hypotheses. These technical tools included differential scanning calorimeter (DSC), melt flow index (MFI) tests, finite element analysis (FEA), high speed video camera (HSVC) and microscopy (optical and scanning electron). These tools helped approve or disapprove the hypotheses. Viscoelastic heating was considered to be crucial mechanism in joining thermoplastics ultrasonically. Viscoelastic heating was depending mainly upon loss modulus, applied frequency and strain amplitude. Weld strength further depended upon the temperature development at weld interface. Apart from above description, simulation based on FEA was also validated for its accuracy and precision. A good match was found between experimental and simulated results.