Ultrasonic welding of thermoplastics in the far-field

In 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.     

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

An 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

Order 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.     

Factors affecting the joint strength of ultrasonically welded polypropylene composites

Ultrasonic welding of thermoplastic composites has become an important process in industry because of its relatively low cost and resultant high quality joints. An experimental study, based on the Taguchi orthogonal array design, is reported on the effect of different processing factors on the joint strength of ultrasonically welded composites, including weld time, weld pressure, amplitude of vibration, hold time, hold pressure, and geometry of energy director. Three materials were used in the study: virgin polypropylene, and 10% and 30% glass‐fiber filled polypropylene composites. Experiments were carried out on a 2000‐Watt ultrasonic welding unit. After welding, the joint strength of the composites was determined by a tensile tester. For the factors selected in the main experiments, weld time, geometry of energy director and amplitude of vibration were found to be the principal factors affecting the joint property of ultrasonically welded thermoplastic composites. Glass‐fiber filled polymers required less energy for successful welding than the non‐filled polymer. The joint strength of welded parts increased with the fiber content in the composites. In addition, a triangular energy director was found to weld parts of the highest strength for virgin polypropylene and 10% glass‐fiber filled polypropylene composites, while a semi‐circular energy director was found to weld the highest strength parts for 30% glass‐fiber filled composites.     

Studies on High Density Polyethylene in the Far-field Region in Ultrasonic Welding of Plastics

Polyethylene (PE) is an extremely versatile plastic and has the largest sales turnover than other plastics. With new uses for PE, researchers continue to find innovative technologies to process and join the material. Ultrasonic welding is one such process that is rapidly emerging as a major joining process for thermoplastics because of its reliability, ease of operation, fastness, and economic feasibility. Amorphous polymers are ideal materials for ultrasonic welding, but semicrystalline polymers are difficult to weld in the far-field region. This paper deals with the far field welding of semicrystalline polymer/high-density polyethylene (HDPE). The temperature distribution has been modeled for varying lengths of the specimen using Ansys to predict the temperature spikes, which can be related to the performance of the joints achieved. Experimental work studied the temperature at the joint interface and the variation in tensile strength for different lengths of the specimen.     

Welding of polymer interfaces

Studies of strength development at polymer-polymer interfaces are examined and applications to welding of similar and dissimilar polymers are considered. The fracture properties of the weld, namely, fracture stress, σ, fracture energy, G_Ic, fatigue crack propagation rate da/dN, and microscopic aspects of the deformation process are determined using compact tension, wedge cleavage, and double cantilever beam healing experiments. The mechanical properties are related to the structure of the interface via microscopic deformation mechanisms involving disentanglement and bond rupture. The time dependent structure of the welding interface is determined in terms of the molecular dynamics of the polymer chains, the chemical compatibility, and the fractal nature of diffuse interfaces. Several experimental methods are used to probe the weld structure and compare with theoretical scaling laws, Results are given for symmetric amorphous welds, incompatible and compatible asymmetric amorphous welds, incompatible semicrystalline and polymer-metal welds. The relevance of interface healing studies to thermal, friction, solvent and ultrasonic welds is discussed.     

New results on the spin welding of plastics

Short welding times make spin welding particularly suitable for mass production. This paper presents an analysis of the friction phase, which makes it possible to estimate the influence of the welding parameters and the material being welded on the temperature in the welded zone, the melt rate, and the torque in the spin welding of semicrystalline thermoplastics. A comparison of experimental and calculated results shows an acceptable correlation. In addition, the influence of speed, axial pressure, and braking on the weld seam quality of different amorphous and semicrystalline thermoplastics is discussed.     

Ultrasonic and impulse welding of polylactic acid films

The weldability of polylactic acid (PLA) is examined in this article. Biaxially oriented PLA films of various thicknesses were joined with impulse and ultrasonic welding techniques. Relatively high weld strengths were achieved with impulse welding over a wide range of welding parameters. Ultrasonic welding produced high weld strengths with relatively short cycle times. In detail, ultrasonic welded samples had a weld factor (weld strength/base material strength) of 1 at cycle times of 0.25 sec. The weld factor was significantly lower at shorter weld times and weld times above 0.35 sec. In contrast, 100‐μm thick samples joined by impulse welding for 2–3 sec had a weld factor of 1 and a standard deviation of only ±5%. The peak temperature during the impulse welding was measured to determine the fusion temperatures of the films. Mechanical, thermal, and optical analysis was used to examine the properties of the PLA at various welding and annealing conditions. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Some Characteristics of Ultrasonic Welding of Polymers

Ultrasonic mechanical vibrations are usually introduced perpendicular to the welded surfaces, which coincides with the direction of the clamping force. The characteristics of the physical processes in ultrasonic welding (USW) of polymers cause heating of the parts to occur primarily at the site of their contact. For this reason, it is possible to form a high-quality weld at a lower temperature than with other kinds of welding. In some cases, USW of polymer parts is even possible at a temperature below their pour point (melting point), which allows obtaining welded articles made from different polymers and welding polymers whose degradation temperature is comparable to or lower than the melting point [fluoroplastic, poly(ethylene terephthalate)]. One of the most important characteristics of USW is the possibility of forming a high-quality weld at a relatively large distance from the site of introduction of mechanical energy, which allows obtaining sufficiently strong welded joint in articles of very complicated design. The possibility of manufacturing welded joints of polymers with contaminated surfaces significantly expands the range of application of USW, as it can be used for hermetic sealing of polymer containers filled with finished products. USW allows industrial production of nonwoven cloth both from purely thermoplastic polymer fibers and from blends with natural or other chemical fibers.     

Microwave welding of high density polyethylene using intrinsically conductive polyaniline

The use of intrinsically conductive polymers in welding of plastics and composites offers the possibility of developing new welding methods. Intrinsically conductive polyaniline (PANI) composite gaskets were used to microwave weld high density polyethylene (HDPE) bars. Two composite gaskets were made from a mixture of HDPE and PANI powders in different proportions. Adiabatic heating experiments were used to estimate the internal heat generation and electric field strength in the gasket. During welding, the effects of heating time, heating pressure and welding pressure were evaluated. It was found that increasing the heating time and the welding pressure increased the joint strength. The maximum tensile joint strength was achieved using a 60 wt% PANI gasket with a heating time of 60 sec and a welding pressure of 0.9 MPa; this resulted in a tensile weld strength of 24.79 ± 0.34 MPa, which equals the tensile strength of the bulk HDPE.     

Proposal of ultrasonic welding technique and weld performances applied to polymers

A novel ultrasonic welding technique is proposed and the apparatus based on it is produced accordingly. The effects of weld technology on the strength have been discussed when evaluating with identical kind of polymers. According to the results, the protruded length and the surface roughness give poor effects on the welding strength. The higher welding stress and shorter welding time are more effective. There is an optimal welding stress in this method because the welding strength can be lower when the stress turns too strong. Moreover, the non‐welded areas and the air bubble of the interface have given powerful effects to the strength. It is considered that the ultrasonic welding technique and the apparatus are quite effective to the weld process between polymer materials. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

A microcellular processing study of poly(ethylene terephthalate) in the amorphous and semicrystalline states. Part I: Microcell nucleation

Microcellular semicrystalline polymers such as poly(ethylene terephthalate) show great promise for engineering applications because of their unique properties, particularly at higher densities. Recent studies reveal some high density microcellular polymers have longer fatigue lives and/or equal strengths to the neat polymer. Relatively few microcellular processing studies of semicrystalline polymers have been presented. In general, semicrystalline polymers are relatively difficult to microcellular process compared to amorphous polymers. In this paper and a companion paper, the microcellular processing of poly(ethylene terephthalate) in the amorphous and semicrystalline states is studied in order to quantify the processing differences. The microcellular processing steps addressed in this paper include gas/polymer solution formation and microvoid nucleation. Particular emphasis is given to microvoid nucleation comparing the processing characteristics of semicrystalline and amorphous materials. Moreover, this study identifies a number of critical process parameters. In general, the semicrystalline materials exhibit ten to one thousand times higher cell nucleation densities compared with the amorphous materials, resulting from heterogeneous nucleation contributions. The amorphous materials show a strong dependence on cell density, while the semicrystalline materials show a weaker dependence. Moreover, classical nucleation theory is not adequate to quantitatively predict the effects of saturation pressure on cell nucleation for either the amorphous or semicrystalline polyesters. Both the semicrystalline and amorphous materials exhibit constant nucleation cell densities with increasing foaming time. Foaming temperatures near the glass transition are found to influence the cell density of the amorphous polyesters, indicating some degree of thermally activated nucleation. Furthermore, classical nucleation theory is not adequate to predict the cell density dependence on foaming temperature. Similar to the amorphous polyesters above the glass transition temperature, nucleation in the semicrystalline materials is found to be independent of the foaming temperature.     

Effects of calcium carbonate,talc, mica,and glass‐fiber fillers on the ultrasonic weld strength of polypropylene

The objective of this work was to study the differences in the ultrasonic weld strength of polypropylene compounds with different fillers. The fillers were calcium carbonate, talc, mica, and glass fibers. The welder parameters were varied to determine the optimum set. These welder parameters were the weld time, weld force, trigger force, and amplitude. The results indicated that the weld time had the greatest effect on the weld strength of each of the filled compounds. Unfilled polypropylene had the highest weld strength under the optimum welding conditions, which were used as the baseline welding conditions. For each given filler, the weld strength was reduced as the filler loading increased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 1986–1998, 2004     

RF welding of PVC and other thermoplastic compounds

Many of the acclaimed advances in medical sciences have been possible because of the improvements in material sciences, particularly plastic. Radio frequency (RF) sealing of plastics has adopted an increasingly important role as a means of manufacturing medical devices. A high intensity radio signal is used to impart increased molecular vibration in two similar or dissimilar polymers. RF sealing is one of the methods for fabrication that allows the designer to seal or connect different types of plastics. This study addresses the effectiveness of this welding method applied to various commercially significant medical grade polymers with emphasis on PVC of varying hardness. The study compares measured weld strength and weld quality (inspected microscopically) at ambient and elevated temperatures. Also, selected samples of polymeric compounds were exposed after RF welding to gamma radiation, a method of medical device sterilization, in order to study the effects on the weld strength and color shift in the weld area. Of prime interest is the determination of optimum welding parameters for various constructions of plastic compounds.     

Mechanical relaxations and moduli of oriented semicrystalline polymers

A systematic investigation was carried out on the mechanical relaxations and moduli of four drawn semicrystalline polymers: polyoxymethylene, polypropylene, polyvinylidene fluoride and polychlorotrifluoroethylene. Low-frequency tensile and torsional measuremnts were made between-140 and 140°C, and ultrasonic measurements of all five moduli were made by the water-tank method between 0 and 60°C. The patterns of relaxations remain essentially unchanged upon orientation, but there is a marked reduction of the height of relaxation peaks associated with the amorphous phase and, correspondingly, a smaller drop of moduli in the relaxation region. This reflects a lowering of molecular mobility in the amorphous phase due to the constraining effect of taut tie-molecules. The modulus C₃₃ increases sharply with draw ratio λ while the other moduli show little variation, which result from the alignment of molecular chain axes and the production of taut tie-molecules. The λ-dependence of the moduli is consistent with the aggregate model only when the polymer is glassy, that is, when its amorphous phase is comparable in stiffness to the crystalline phase and the polymer can reasonably be regarded as a one-phase material for which the aggregate model is valid.     

Energy transfer and bond strength in ultrasonic welding of thermoplastics

This paper describes an investigation into some fundamental aspects of ultrasonic welding of thermoplastics. A simple model was developed to characterize the temperature rise at the weld interface up to the glass transition temperature. Beyond this point, the temperature increases more rapidly and almost directly proportional to weld time. The rate of temperature rise increases with increase of amplitude of vibration. The correlation between weld strength and interface temperature was established using the method of dimensional analysis. It was found that the process can be optimized in terms of weld strength by monitoring the power input. There is an optimal load one can apply to achieve high weld strength. The overall efficiency of the process is rather low in terms of energy usage.     

Experimental characterization of processing-performance relationships of resistance welded graphite/polyetheretherketone composite joints

A study to investigate fusion bonding (welding) of AS4 graphite/polyetheretherketone (PEEK) thermoplastic composites is presented. Processing studies are conducted for resistance welding preconsolidated AS4/PEEK laminates in both unidirectional and quasi-isotropic configurations using PEEK and polyetherimide (PEI) film at the joint interface. All bonding was done under a constant displacement process. The influence of processing time, initially applied consolidation pressure, and the rate of heat generation on weld performance is examined through lap shear and Mode I interlaminar fracture toughness testing. A rapid increase in strength with processing time that asymptotically approaches the compression molded baseline is measured. Weld times for quasi-isotropic lap shear coupons are significantly shorter than those with a unidirectional lay-up. Variation of the initially applied consolidation pressure is shown to have little influence on the lap shear strength of PEEK film welded lap joints. A discussion of the mechanisms allowing void formation during the welding process is given. Bond strength test results are correlated with ultrasonic C-scans of the weld regions.     

Steady melting of rectangular thermoplastic bars induced by hot contacting surfaces

The steady melting of rectangular thermoplastic bars in contact with hot surfaces is analyzed by solving a simplified set of the momentum and energy balance equations, assuming a temperature and shear-rate dependent melt viscosity. A numerical model is developed for predicting the flow field and the temperature distribution in the solid and molten regions of the bar and the location of the solid/melt interface. Computer simulations show that the steady melting rate of the thermoplastic solid is mainly affected by the temperature sensitivity of the melt viscosity, by the pressure applied on the end of the bar, and by a balance between heat conduction and the convection of colder material into the molten region. For the amorphous and semicrystalline polymers considered, heat convection in the outflow direction of the molten material, viscous dissipation, and shear-thinning of the melt viscosity have a much smaller effect on the melting process. These results provide an insight into conduction-induced melting with forced melt removal caused by pressure-induced flow; they also provide a basis for developing a transient model for the hot-tool welding process.     

Heating and bonding mechanisms in ultrasonic welding of thermoplastics

An 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.     

Morphology and strength of polycarbonate to poly(butylene terephthalate) vibration-welded butt joints

Under the right conditions, the strength of vibration-welded butt joints of amorphous polycarbonate (PC) to semicrystalline poly(butylene terephthalate) (PBT) are shown to be as high as the strength of PBT, the weaker of the two materials. Optical, scanning and transmission electron microscopy are used to examine the morphology of the weld zone. Acoustic microscopy is used to visualize poorly bonded regions. The effects of the weld parameters on weld strength and weld morphology are considered in detail.