A simple analytical model for estimation of melt film variables in thermoplastic vibration welding

The strength of vibration welds of thermoplastics is governed by the weld zone microstructure, which in turn, is closely tied to the welding process variables, such as the thickness of the weld melt film and the temperature profiles therein. The mathematical model described in this report is aimed at describing the role of the rheology of the melt—specifically the magnitude and shear rate dependence of the melt viscosity—in governing the melt film variables during the steady state penetration phase (Phase III) of vibration welding. The steady state momentum balance and heat transfer within the melt film are solved by using the power law model for viscosity. Closed‐form analytical expressions are obtained for estimating the melt film thickness, the shear rates, and the temperature field within the film. This model has been used to estimate weld zone variables for four different polymers displaying a wide range of viscosities and shear thinning behaviors. POLYM. ENG. SCI., 54:499–511, 2014. © 2013 Society of Plastics Engineers     

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.     

Estimation of melt film variables during the steady‐state penetration phase of thermoplastic vibration welding using a generalized newtonian fluid model

The thickness of the melt film and the temperature profiles within the melt film in the weld zone are key process variables governing the development of weld‐zone microstructures and the resulting development of weld strengths, during vibration welding of thermoplastics. The mathematical model described in this report is aimed at investigating the role of the rheology of the melt—specifically the magnitude and shear‐rate as well as temperature dependence of the melt viscosity—in governing the process variables such as the molten film thickness and the viscosities, stresses, and the temperatures within the melt film during vibration welding. The analysis is focused on the steady‐state penetration phase (phase III) of vibration welding. The coupled steady‐state momentum balance and heat transfer within the melt film, formulated using the Cross‐WLF (Williams‐Landel‐Ferry) relationship for viscosity, are solved in an iterative finite element framework. The model has been implemented for two different polymers displaying significant differences in viscosities and shear thinning behaviors. An attempt has been made to correlate the trends in the estimated melt film variables with the experimentally measured weld quality. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Comparison of vibration and hot‐tool thermoplastic weld morphologies

Because of very different heating rates in hot‐tool and vibration welding, and the higher weld pressures used in vibration welding inducing more squeeze flow, the weld zones in these two processes see very different flows and cooling rates, resulting in different morphologies. The weld morphologies of bisphenol‐A polycarbonate (PC) and poly(butylene terephthalate) (PBT) for these two processes are discussed in relation to these differences. The thickness of the heat‐affected zone (HAZ) in hot‐tool welds increases with the melt time; this zone is thicker than in vibration welds. The HAZ thickness in hot‐tool welds increases from the center toward the edges. The HAZ thickness is more uniform in vibration welds. Hot‐tool welds of PC have large numbers of bubbles around the central plane; the bubble size increases from the center to the edges. PC vibration welds do not have bubbles except near the edges. Both hot‐tool and vibration welds of PBT do not have bubbles. The morphology of the HAZ in PBT is very different in hot‐tool and vibration welds. In hot‐tool welds, the resolidified material consists of a sandwich structure in which two thin layers with very small crystallites surround a thicker central layer in which the spherulites are almost as large as in the original molded material. In vibration welds, the HAZ has large crystallinity gradients across the weld zone as well as squeeze‐flow induced distortion of the small spherulites.     

Influence of radiation cross‐linking on fiber orientation in vibration welds of Polyamide 66

A conventional vibration welding process of fiber‐reinforced Polyamide 66 is characterized by a continuous melt flow in the quasi‐steady phase. This squeeze flow leads to a disadvantageous fiber reorientation in the weld zone. The fibers are oriented parallel to the melt flow and thus perpendicular to the common stress direction. This causes relatively low weld strength compared to the strength of the base material. Radiation crosslinking fiber‐reinforced Polyamide 66 with electron beams influences the material characteristics. As a consequence, the resulting energy balance during vibration welding is changed and the squeeze flow is impeded, thus averting the fiber reorientation in the weld seam. The scope of this article is to demonstrate the influence of radiation crosslinking on fiber orientation in vibration welds. Mechanical, calorimetric, rheological, scanning electron microscope, and light microscope investigations serve to highlight the influence of radiation crosslinking on the vibration welds of fiber‐reinforced Polyamide 66. POLYM. COMPOS., 38:489–495, 2017. © 2015 Society of Plastics Engineers     

Thermoplastic vibration welding: Review of process phenomenology and processing–structure–property interrelationships

Vibration welding offers a robust method for physically joining thermoplastics to fabricate complex hollow assemblies from simpler injection‐molded articles without using an external heat source, adhesives, or mechanical fasteners. Vibration welding involves a complex interplay of several phenomena—solid (Coulomb) friction, melting, high strain‐rate, pressure‐driven, strong (high‐strain) melt flows, solidification, and microstructure development—which ultimately govern the strength and integrity of the weld. Defects in the weld region may lead to catastrophic failure of the welded assembly. In this article, the current understanding of the processing–structure–property relationships in the context of vibration welding of thermoplastics and polymer‐matrix composites is reviewed. Experimental as well as analytical methods of investigation of the vibration welding process phenomenology are presented. The interrelationships between the microstructure in the weld region and the resulting weld strength and fatigue behavior are then discussed in the light of this phenomenological information for neat polymers, filled polymers, polymer blends, and foams. This review is also aimed at identifying the areas requiring further investigation with regard to understanding vibration welding phenomenology and weld structure–property relationships. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Vibration welding of nano‐TiO2 filled polypropylene

Polypropylene (PP)‐based nanocomposites compounded by a twin‐screw extruder and injection molded into plates those were then joined by linear vibration welding. The mechanical performances of the welds and bulk materials were examined. While the incorporation of rigid particles slightly improves the impact strength of the bulk PP, the mechanical properties of the welds decrease with increasing nanoparticle contents. The best weld quality is obtained at low weld pressure without nanoparticles. The fracture surfaces and microstructure of the welds showed that the reduced weld quality is caused by the orientation of nanofillers parallel to the weld plane, the destruction of interphase between fillers and matrix, and the reduction of molten‐film thickness by incorporation of nanoparticles. POLYM. ENG. SCI., 55:243–250, 2015. © 2014 Society of Plastics Engineers     

Effects of welding procedures on mechanical and morphological properties of hot gas butt welded PE,PP, and PVC sheets

Mechanical and morphological properties of hot gas butt welds on polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC) sheets for four different procedures, which are single and double V‐welds with and without a welding shoe, were investigated. Besides, weldabilities of base materials were evaluated by rheological measurements. These revealed that weldabilities of PE and PP sheets were better than that of PVC. Welding energy (E_w), which is transferred onto weld surfaces, was calculated to evaluate weld quality. The results of tensile, impact, and bending tests indicated that the weld strengths of PVC sheets were lower than those of PE and PP sheets. When the welding shoe was used, weld strength increased significantly for each material because of the presence of sufficient welding pressure and the effective heating on surfaces. The best results were attained for the double V‐welds with the welding shoe. Morphology of welded regions was evaluated by polarized light, stereo, and scanning electron microscopy. Polarized light microscopy studies indicated that the heat‐affected zone (HAZ) consisted of welding rod core, molten zone, and deformed spherulitic zone, and the welding interface was indistinguishable from the base material when the welding pressure was enough. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Vibration welding of thermoplastics. Part I: Phenomenology of the welding process

In vibration welding of thermoplastics, frictional work done by vibrating two parts under pressure, along their common interface, is used to generate heat to effect a weld. The main process parameters in vibration welding are the weld frequency, the amplitude of the vibratory motion, the weld pressure, and the weld time. How these parameters affect weld quality, the conditions that result in the best welds, the weldability of dissimilar plastics, and the effect of fillers such as glass are of interest. To address these issues, a research vibration welding machine in which all the parameters can be independently and accurately controlled and monitored was designed and fabricated. The phenomenology of welding, as determined by experiments on the four thermoplastics polycarbonate, poly (butylene terephthalate), polyetherimide, and modified poly (phenylene oxide), is described.     

Post‐crosslinking behavior of radiation crosslinked polyamide 66 during vibration welding

Vibration welding of radiation crosslinked polyamide 66 leads to a different relationship between process, structure, and properties. The radiation‐induced altered material properties result in an impeded squeeze flow and higher temperatures in the weld. It is possible to achieve weld strengths up the value of the base material. General theories cannot completely explain the adhesion for these crosslinked polymers. A possible explanation could be an additional temperature‐dependent adhesion mechanism. The high temperatures during vibration welding could affect the degree of crosslinking and lead to a post‐irradiation crosslinking of the polyamide. These additional links across the joining zone may be an additional adhesion mechanism and could explain the high weld strength of crosslinked polymers. The scope of this paper is to investigate the influence on the degree of crosslinking from material and welding parameters and to correlate these results with temperatures and weld strengths generated in a vibration welding process. POLYM. ENG. SCI., 56:735–742, 2016. © 2016 Society of Plastics Engineers     

Influence of radiation cross‐linking of polyamide 66 on the characteristics of the vibration welding process

The conventional vibration welding process of polyamide 66 only has a continuous and steady melt flow during the quasi‐steady phase. The process and resulting welds have been thoroughly investigated. Radiation cross‐linking of polyamide 66 with electron beams alters the material's characteristics. Consequently, the resulting energy balance during vibration welding changes and the squeeze flow is impeded. Additionally, this causes the cross‐linking to attain a residual stiffness above the crystallite melting temperature, thereby influencing the characteristics of the vibration welding process. Further, higher weld temperatures and a change in meltdown behavior can be observed. This leads to a varied relationship amongst the process, structure, and properties for vibration welding cross‐linked polyamide. Hence, weld strengths up to the value of the base material strength are possible. The scope of this article is to investigate the influence of radiation cross‐linking on the material characteristics and, by extension, the resulting processing and welding characteristics. Calorimetric, chemical, rheological, mechanical, and optical investigations serve to highlight the influence of radiation cross‐linking on the vibration welding process of polyamide 66. POLYM. ENG. SCI., 55:2493–2499, 2015. © 2015 Society of Plastics Engineers     

Morphology of PA‐6 vibration welded joints and its effect on weld strength

The microstructure of the heat affected zone (HAZ) in vibration welded joints of polyamide‐6 were studied using polarized light microscopy (PLM), Fourier transform infrared (FTIR) microspectrometry, scanning electron microscopy (SEM), and modulated differential scanning calorimetry (MDSC). PLM images showed the existence of two distinct HAZ layers: an inner layer (HAZ‐I) with small spherulites and an outer deformed layer (HAZ‐II) adjacent to the bulk zone. The thickness of the HAZ‐I and HAZ‐II layers and the size of spherulites in HAZ‐I depended on welding parameters, such as weld pressure and amplitude. The microstructure study of molecular orientation, using birefringence, and crystal phase type, using FTIR, suggests that HAZ‐I originates from a molten polymer film, while HAZ‐II arises in the polymer in the rubbery state. Etched HAZ surfaces also show clear distinction between the two HAZ layers. Therefore, HAZ‐I is renamed as the recrystallized layer and HAZ‐II as the deformed layer. Fracture tests show that the weakest part of the joint occurs in the recrystallized layer region adjacent to the deformed layer. MDSC analysis indicates that the determining factor of the weld strength is the crystallinity of the recrystallized layer, not the thickness of HAZ. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Improved weld strength of vibration welded polyoxymethylene/multiwalled carbon nanotubes hybrid nanocomposites

In this study, carbon nanotubes reinforced polyoxymethylene with different filler loadings was joined by using linear vibration welding technique. The tensile properties of vibration welded polyoxymethylene nanocomposites with different carbon nanotube contents were studied as functions of filler loading and weld pressure. The results showed that the addition of carbon nanotubes into polyoxymethylene slightly improved the matrix tensile strength and pronounced decreased the ductility of pure polyoxymethylene. Interestingly, the weld strength of the nanocomposites was also higher than the polymer matrix strength even at high weld pressure of 2 MPa. Possible reasons for this high weld strength are discussed based on the morphological investigations. POLYM. ENG. SCI., 56:636–642, 2016. © 2016 Society of Plastics Engineers     

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     

Vibration joining of fiber‐reinforced thermosets

Adhesive bonding is known to be particularly suitable for thermoset composites with continuous fiber reinforcement as it does not interrupt the fibers because of drilled holes. The frequently used two‐part adhesives often require long curing times for the chemical reaction. At the Institute of Polymer Technology (LKT), a vibration‐assisted hot melt bonding technique (vibration joining) was developed, which offers short cycle times and represents a modification of hot melt bonding, using the machine technology from vibration welding. It is suitable to join thermoplastics with thermoset materials or thermosets using a thermoplastic interlayer, by taking advantage of short cycle time and good lap shear strength, compared to bonding with reactive adhesives. Polym Compos 2009 POLYM. COMPOS., 31:1205–1212, 2010. © 2009 Society of Plastics Engineers     

Friction welding of polyamides

Results from different experiments on friction welding are used to characterize the behavior of polyamide over a wide range of welding conditions. Several types and grades of polyamide were joined using the vibration and spin welding processes. The quality of the welds was evaluated by short time tensile tests and microscopy. In addition to the geometry of the parts being joined, the process parameters and the material were found to affect the quality of the weld, so that associated with each application is a different set of optimum welding parameters.     

Hot‐tool and vibration welding of poly(vinyl chloride)

The weldability of poly(vinyl chloride) (PVC) is assessed through hot‐tool and 120‐Hz vibration welds. Equivalent strengths have been demonstrated for welds made by both of these welding techniques. For two grades of PVC, relative weld strengths of 85 and 97%, with corresponding failure strains of about 2.5 and 3.3%, respectively, have been demonstrated.     

The effect of flame retardants on the hot‐plate welding of talc‐filled polypropylene

The effect of a flame retardant on the weldability of polypropylene with two different talc loadings was studied by microscopy and mechanical tests on hot‐plate welded injection molded tensile test bars. Welding changes the orientation of the talc particles, which align parallel to the weld interface, and can cause voiding of the material with a consequent decrease of weld strength. Welds of the material containing a flame retardant, which melts and volatizes at the temperatures used in welding, exhibited higher voiding and lower relative weld strengths on welding than the grade without a flame retardant. Voiding may be reduced using lower hot plate temperatures and higher welding displacements, but that results in an increase in the transverse orientation of the talc particles in the weld zone. The relative weld strength, which is affected by the composition of the material, was about 50% for the 20% talc‐filled polypropylene and about 45% for the 30% talc‐filled grade containing a flame retardant.     

On the microstructure and tensile strength of PC/ABS polymer blend joints

Large articles of polymeric materials which can not be molded require welding to join the components. Weld zones result in a morphology that differs from the adjacent areas. This difference in structure represents a defect in the article that can result in premature failure during service. Experiments with a Pulse 830 (a polycarbonate/acrylonitrile-butadiene-styrene blend) engineering resin showed that weld zones made using hot plate techniques, retained only 30% of the unwelded tensile strength, while 80% was retained if vibration welding was applied. Examination of the weld zone by transmission electron microscopy (TEM) revealed a dramatic difference in the microstructure.The weld zone morphology in Pulse" 830 engineering resin by hot plate welding is highly laminar and oriented while a much more homogeneous structure, similar to that in the bulk, is produced by vibration welding. This morphology difference accounts for the variation of the tensile strength of the joints.