Semiempirical,squeeze flow,and intermolecular diffusion model. II. Model verification using laser microwelding

This article reviews the application of a coupled squeeze flow and intermolecular diffusion model, which was used to predict the quality and size of microwelds in plastics. Weld widths predictions were compared with previously presented experimental results using moving heat source models and temperature fields. The motivation for this work was to develop and verify a model based on fundamental principles that could accurately predict weld size and strength for conventional plastic welding techniques as well as novel techniques such as laser microwelding. It is envisioned that the resulting model could be used to predict proper welding parameters, including laser power and travel speed, to produce welds of varying size. Although insight into weld quality can be derived from this model, it was not the goal of this work to accurately predict weld strength for laser microwelding because of the difficulty in measuring weld strength on the micron scale. However, as reported in Part 1, weld strength for impulse welds were accurately predicted. In this model it was found that variable temperature histories, rather than a single value of maximum weld temperature, allows more accurate modeling of the welding process. In this work (Part 2), microwelds as small as 11 μm in width were produced with transmission infrared welding. In addition, welds over 150‐μm wide were also generated and the model was able to predict the range of weld widths that were found experimentally. It was found that the predictions were in very good agreement with the experimental results. There was some deviation between the experimental data and the model at the extreme parameters and it is believed that this was due to the temperature‐dependent material properties as well as optical aberrations. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Hot pin welding of thin poly(vinyl chloride) sheet

This paper develops a method of welding two thin sheets of poly(vinyl chloride) (PVC) with a heated pin, thus allowing construction of a relationship between the weld temperature and weld strength. Constructing a relationship between weld strength and temperature is necessary for modeling many welding processes, including laser transmission welding. An experimental approach to establishing this relationship is required because of the complex melting behavior of PVC. The designed experimental device uses a single heated pin to weld samples by using varying pressure and temperature for one second dwell time. An electro‐mechanical loadframe pulled the welded samples until joint failure occurred, thereby allowing determination of the weld strength. An experiment varying welding pin temperature and joining pressure found the temperature to be a highly significant determiner of weld strength, while the pressure was found to be not significant. A transient numerical heat transfer model was used to calculate the weld interface temperature for each pin temperature. The relationship established in this paper can be used to predict the weld strength from the temperature output from models of alternative welding methods. J. VINYL ADDIT. TECHNOL., 13:110–115, 2007. © 2007 Society of Plastics Engineers.     

Activation energy for diffusion and welding of PLA films

The bonding of polylactic acid (PLA) films was investigated for a broad range of temperatures and contact times above the glass transition temperature in a lap shear joint geometry using an impulse welding system. It was observed that interfacial strength was linearly dependent to the fourth root of welding time until it approached the bulk material strength. Using models based on reptation theories, the interfacial strength of lap shear welds was estimated based on thermal histories. In more detail, the activation energy for interfacial healing and self‐diffusion coefficient was calculated based on shear strength measurements of samples welded with well‐defined thermal histories. The parameters were then used to predict interfacial strength with varying temperature histories. This is the first work to measure the activation of energy for the interfacial welding PLA. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

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.     

Vibration welding of thermoplastics. Part II: Analysis of the welding process

The vibration welding process for thermoplastics is known to consist of four phases: (1) initial heating of the interface to the melting temperature by Coulomb friction; (2) unsteady melting and flow in the lateral direction; (3) steady-state flow; and (4) unsteady flow and solidification of the film after the vibratory motion is stopped. Simple analytical models are developed for the first three phases. These models are used for estimating the molten film thickness, the size of the heat affected zone, and the weld time as functions of the weld parameters: the amplitude and frequency of the weld motion, and the weld pressure. The steady-state film thickness and the heat-affected zone are shown to be very small.     

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     

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     

Evaluation of healing models to predict the weld line strength of the amorphous thermoplastic polystyrene by injection molding simulation

In many injection molded parts weld lines are often unavoidable. These cause optical defects and a reduction of the mechanical properties of the part. Therefore, the predictability of the weld line strength at an early stage of development would provide a significant advantage by avoiding costly iterations of the mold and increases the understanding of the correlation between process history of the melt and weld line strength. For this purpose, a calculation routine has been developed to predict the weld line strength based on injection molding simulation. Different models to calculate the healing of a weld line are compared and analyzed. By adding a factor to consider the shear rate in addition to the temperature and the pressure and after calibration to one design of experiment setting of the experimental data, the prediction of the weld line strength shows good agreement for all examined process setpoints of the experimental data for polystyrene.     

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     

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.     

A non-isothermal healing model for strength and toughness of fusion bonded joints of amorphous thermoplastics

A study to investigate the influence of processing on the fusion bonding of graphite (AS4) poly(etheretherketone) (PEEK) thermoplastic composites (BASF commingled PEEK/graphite NCS woven fabric) using a polyetherimide (PEI) film at the interface is presented. Fundamental to all fusion bonding processes is the intermolecular diffusion between surfaces in intimate contact. A model based on the healing theory of amorphous polymers has been proposed to predict strength and toughness as a function of non-isothermal process history. This model considers two different microscopic failure mechanisms of a healed interface. For the first time, using non-isothermal data and proper data reduction procedures, it is possible to differentiate between these two mechanisms, which are otherwise indistinguishable from isothermal data. Temperature dependent reptation times representative of the kinetics of chain diffusion in the polymer have been evaluated for both mechanisms over a large range of process temperatures using fracture tests conducted on lap shear specimens manufactured using a hot press. Three alternate and independent techniques to estimate the reptation time in PEI indicate that the model based on the average interpenetration distance is most representative of the physical system. Lap shear strength predictions based on this formulation have been generated for various non-isothermal conditions measured in the hot press and are within 20% of the experimental data. The model was used to show that in isothermal processes, maximum strength and toughness can be achieved in less than 1 s for temperatures exceeding 290°C. Application of the model to a highly non-isothermal technique such as resistance welding using amorphous film technology is also presented. Model predictions show that asymptotic strength may be achieved in relatively short process times with appropriate welding conditions.     

Experimental Studies on Thermo-Mechanical Behavior of Ultrasonically Welded PC/ABS Polymer Blends

This research paper attempts to investigate the performance of blended PC/ABS joints using the ultrasonic material joining process. The key focus is on examining the thermal aspects during the joining of PC/ABS blends using ultrasonic welding and the subsequent mechanical testing to determine the strength of the weldments. Thermal behavior of the blends during welding may govern or alter the mechanical properties and integrity of the joints. Hence, investigations on thermal characteristics involved in PC/ABS blends when subjected to high vibrational heat generated during the ultrasonic welding process is imperative. DSC is used to measure the glass transient temperature (T_g) after subjecting it to welding. Mass loss is calculated with TGA. TGA and DSC results indicate change in T_g which are attributed to the molecular alignment occurring when the specimens are subjected to ultrasonic vibrations. Initially, two step mass losses occur that is contributed by ABS in which long single chains are associated and alters PC. SEM images reveal the absence in intermolecular compounds or impurities that tend to weaken weld joints. The diffusion of these molecules is uniform in the welded region. The amorphous nature enhances the integrity of weld joints. Molded part illustrates the higher strain rate in comparison with the welded specimens. The RSM model proposed is sufficient and has limited possibility for violating the independence or the assumption of constant variance.     

Properties and weldability of plasticized polylactic acid films

The objectives of the presented work were to investigate films based on polylactic acid (PLA) and polyethylene glycol (PEG) in order to improve ductility and weldability of PLA films. The effect of plasticizer amount on the thermal, rheological, and mechanical properties of PLA plasticized films was investigated. The PEG content does affect the glass transition and the cold crystallization temperature of PLA in blends, while the melting temperature was not affected by the addition of PEG. The complex viscosity of the neat PLA granules and of plasticized films showed strong temperature and angular velocity dependence. The Young's modulus and tensile strength of plasticized films were improved with increasing plasticizer concentration, while the elongation at break stays rather constant. Plasticized PLA films were furthermore heat welded. These investigations showed that plasticized PLA films can be welded by heat welding. The obtained weld strength is strongly depending on the PEG amount as well as on selected welding parameters. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40394.     

Polymer weld strength predictions using a thermal and polymer chain diffusion analysis

A new simplified model for strength prediction of welded polymer-polymer interfaces is proposed. Two schools of thought exist for strength prediction of polymer-polymer interfaces: microscopic analysis of polymer chain behavior, and macroscopic analysis of bulk polymer properties during welding. The microscopic analysis is based on De Gennes' reptation theory for macromolecules (1), and has been described extensively by both analytical and empirical techniques. Reptation models are based on constant temperature interfaces, which are not found in any actual welding process. Conventional macroscopic analyses of welding empirically relate strength to important thermal process parameters, such as power density and heating time, but do not address the behavior of the polymer chains. Little interaction exists between these two schools of thought. This study seeks to combine these two areas, using reptation theory to explain the polymer chain interactions on a macromolecular level, and relate the interface thermal signature to strength prediction. The result of the new model is a method for strength prediction that takes into account fundamental materials properties as well as the engineering conditions imposed in a realistic welding process to predict weld strength. The model is adapted to run for similar conditions as two separate empirical endeavors. The results show that the model is effective in predicting overall trends, which emphasizes the importance of examining heat transfer effects in any polymer welding process.     

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     

Ultrasonic welding of PEEK graphite APC-2 composites

The ultrasonic welding process is modeled using a five part model that includes mechanics and vibration of the parts, viscoelastic heating, heat transfer, flow and wetting, and intermolecular diffusion. The model predicts that melting and flow occur in steps, which has been confirmed by experiments. The model also indicates the possibility of monitoring joint quality by measuring the dynamic mechanical impedance of the parts during welding, which has also been verified experimentally by indirectly monitoring the magnitude of the impedance. via measurements of both the power and the acceleration of the base. When the melt fronts of the energy directors meet, at the end of welding, the dynamic impedance of the composites' interface is shown to rise rapidly. This raises the possibility of developing closed loop control procedures for the ultrasonic welding of thermoplastic composites. Ultrasonic welding of polyetheretherketone (PEEK) graphite APC-2 composites produced joints with excellent strengths.     

Boundary layer model for char combustion and gasification

A non-steady boundary layer model is developed for numerical simulation of combustion and gasification of a single shrinking char particle. The model considers mass and energy conservation coupled with heterogeneous char reactions producing CO and homogeneous oxidation of CO to CO₂ in the boundary layer surrounding the char particle. Mass conservation includes accumulation, molecular diffusion, Stefan flow and generation by chemical reaction. Energy conservation includes radiation transfer at the particle surface and heat accumulation within the particle. Simulation results predict experimentally measured conversion and temperature profiles of a burning Spherocarb particle in a laminar flow reactor. Effects of bulk oxygen concentration and particle size on the combustion process are addressed. Predicted particle temperature is significantly affected by boundary layer combustion of CO to CO₂. With increasing particle size, char gasification to char combustion ratio increases, resulting in decreasing particle temperature and increasing peak boundary layer temperature.     

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     

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