Thermoplastic Welding Research Articles

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

Laser Transmission Welding of White Thermoplastics with Adapted Wavelengths

Different types of laser sources can be used for polymer welding. By the principle of laser transmission welding the wavelength of the laser is one of the most important criteria when selecting a laser source as the optical properties of the polymers are depending on the wavelength. Up to date white material cannot be welded to each other. The results show that by analysis of the optical properties, especially the absorption and the scattering coefficient and adaption of the laser wavelength the process limits can be extended.

Experimental Investigation of Laser Transmission Welding of Thermoplastics with Part-Adapted Temperature Fields

Abstract Laser transmission welding is known for high flexibility, extraordinary potential for process automation and outstanding weld seam properties. Problems may occur due to the poor gap-bridging capability of contour welding. Gaps of a few tens of microns can lead to processing issues such as welding failures, poor achievable process speed or low weld seam strengths. To overcome this, laser transmission welding with part-adapted temperature fields was developed, and is experimentally investigated here. Results concerning the process behavior, dependent on several oscillation types of the laser beam, as well as achieved tensile shear strengths and the monitored gap-bridging capability are presented.

Laser Transmission Welding of Thermoplastics With Dual Clamping Devices

Abstract Beside well chosen process parameters a geometrical joining partner design suitable for laser transmission welding and adequate clamping pressure appliance is necessary to form high quality welds. Amongst other clamping techniques Dual Clamping Devices (DCD) are a promising approach to fulfill the process needs and to get to a robust and also fail safe clamping. When using DCD the laser beam has to pass thin non transmitting bars while a weld seam is formed. So the laser beam is partial refracted, reflected and absorbed. Here the influences of the bars onto the laser beam intensity distribution behind the bars rather in the interaction zone of the laser beam and the two joining partners are investigated. Welding experiments with DCD are carried out and discussed. Based on the experimental results thermal process simulations are performed, to get a deeper knowledge about the effect of the bars onto the spatially and temporally changing temperature field within the joining partners.

Finite element simulation of laser transmission thermoplastic welding of circular contour using a moving heat source

In the present research, a three-dimensional transient numerical model is developed to simulate the laser transmission welding of circular contours. Polycarbonate, a thermoplastic material, is considered as work material for developing and validating the model. The welding simulation is performed, to compute the temperature distribution and shape and size of the molten pool, using the commercial finite element software ANSYS®. The results numerical analysis is further used for investigating the parametric effect on temperature field and weld bead profile. The results obtained revealthat laser power has a positive effect on all the responses investigated; whereas, welding speed has a negative effect. For constant line energy, higher temperature is obtained at a combination of high power with high welding speed.

Description of transmitted energy during laser transmission welding of polymers

It is important to describe the laser energy distribution in polymers properly when modeling heat transfer during laser transmission welding of thermoplastics. This paper presents an analytical model that describes the energy transmission in laser transmission welding of light scattering polymers. The model considers that the transmitted laser beam in a scattering polymer can be represented by scattered and unscattered components. The distribution of the scattered energy from any point in the incident beam is Gaussian. The transmitted power from the discretized input beam is summed to create a normalized power flux distribution model. The model was validated using the measured laser energy distributions after transmission through parts made of polypropylene, as well as unreinforced and glass fiber reinforced polyamide 6.

Optical design and development of a fiber coupled high-power diode laser system for laser transmission welding of plastics

Eva Rodriguez-VidalIban QuintanaJon EtxarriUrko AzkorbebeitiaDeitze OtaduyIK4-TEKNIKERPolo Tecnologico de EibarCalle Inaki Goenaga, 520600 Eibar, Gipuzkoa, SpainE-mail: erodriguez@tekniker.esFrancisco GonzalezFernando MorenoUniversity of CantabriaGroup of OpticsApplied Physics DepartmentAvenida de los Castros, s/n39005, Santander, SpainAbstract. Laser transmission welding (LTW) of thermoplastics is a directbonding technique already used in different industrial applications sectorssuch as automobiles, microfluidics, electronics, and biomedicine. LTWevolves localized heating at the interface of two pieces of plastic to bejoined. One of the plastic pieces needs to be optically transparent tothe laser radiation whereas the other part has to be absorbent, beingthat the radiation produced by high power diode lasers is a good alterna-tive for this process. As consequence, a tailored laser system has beendesigned and developed to obtain high quality weld seams with weldwidths between 0.7 and 1.4 mm. The developed laser system consistsof two diode laser bars (50 W per bar) coupled into an optical fiberusing a nonimaging solution: equalization of the beam parameter product(BPP) in the slow and fast axes by a pair of step-mirrors. The powerscaling was carried out by means of a multiplexing polarization technique.The analysis of energy balance and beam quality was performed consid-ering ray tracing simulation (ZEMAX®) and experimental validation. Thewelding experiments were conducted on acrylonitrile/butadiene/styrene(ABS), a thermoplastic frequently used in automotive, electronics and air-craft applications, doped with two different concentrations of carbon nano-tubes (0.01% and 0.05% CNTs). Quality of the weld seams on ABS wasanalyzed in terms of the process parameters (welding speed, laser powerand clamping pressure) by visual and optical microscope inspections.Mechanical properties of weld seams were analyzed by mechanicalshear tests. High quality weld seams were produced in ABS, revealingthe potential of the laser developed in this work for a wide range of plasticwelding applications.

Computational modeling and experimental studies of the dynamic performance of ultrasonic horn profiles used in plastic welding

Ultrasonic horns are tuned components designed to vibrate in a longitudinal mode at ultrasonic frequencies. Reliable performance of such horns is normally decided by the uniformity of vibration amplitude at the working surface and the stress developed during loading condition. The horn design engineer must pay particular attention to designing a tool that will produce the desired amplitude without fracturing. The present work discusses horn configurations which satisfy these criteria and investigates the design requirements of horns in ultrasonic system. Different horn profiles for ultrasonic welding of thermoplastics have been characterized in terms of displacement amplitude and von-Mises stresses using modal and harmonic analysis. To validate the simulated results, five different horns are fabricated from Aluminum, tested and tuned to the operating frequency. Standard ABS plastic parts are welded using these horns. Temperature developed during the welding of ABS test parts using different horns is recorded using sensors and National Instruments (NIs) data acquisition system. The recorded values are compared with the predicted values. Experimental results show that welding using a Bezier horn has a high interface temperature and the welded joints had higher strength as compared to the other horn profiles.

New Concepts for Laser Transmission Welding of Dissimilar Thermoplastics

New Concepts for Laser Transmission Welding of Dissimilar Thermoplastics

WELDING OF THERMOPLASTIC MATERIALS USING CO2 LASER

The welding of thermoplastics using CO2 laser achieved, two different types of thermoplastic materials used which are; Perspex (PMMA) which is the abbreviation of polymethyl methacrylate and (HDPE) which is the abbreviation of high density polyethylene. Similar or different materials can be welded together by laser beam but in this work similar polymers have been welded (i.e PMMA tube to PMMA tube and HDPE plate to HDPE plate). Mechanical properties for these materials measured after welding, two CW CO2 lasers were used in the welding process have the wavelength of 10.6µm. The maximum power of the 1st one is 16W, and for the 2nd one is 25W. The experimental result showed that the penetration depth increases with increasing laser output when the welding speed is constant. Also a relation between spot width and depth calculated using MATLAB software program version 6.5 taken into account the effect of the following parameters, power used in welding, melting temperature of the materials, welding speed and the spot diameter of the laser beam.

Use of thermal imaging to characterize laser light reflection from thermoplastics as a function of thickness, laser incidence angle and surface roughness

Laser light reflection during the laser transmission welding (LTW) of thermoplastics has the potential to overheat and/or cause unintentional welding of adjacent features of the part being welded. For this reason, and in order to assess how much light is being absorbed by the transparent part (after measurement of the light transmitted through the transparent part), it is important to be able to quantify the magnitude and distribution of reflected light. The magnitude and distribution of the reflected light depends on the total laser input power as well as its distribution, the laser incidence angle (angle between the normal to the transparent part surface and the laser beam), the laser light polarization as well as the surface and optical properties of the transparent part. A novel technique based on thermal imaging of the reflected light was previously developed by the authors. It is used in this study to characterize the magnitude and distribution of reflected light from thermoplastics as a function of thickness (1–3.1mm), laser incidence angle (20–40°) and surface roughness (0.04–1.04μm). Results from reflection tests on nearly polished nylon 6 (surface roughness between 0.04 and 0.05μm) have shown that, for the various thicknesses tested (1–3.1mm), the total reflection was larger than the specular top surface reflection predicted via the Fresnel relation. From these observations, it is conjectured that, in addition to top surface reflection, the bulk and/or bottom surface also contribute to the total reflection. The results also showed that reflection decreased slightly with increasing thickness. As expected, for the p-polarized light used in this study, the reflection decreased with increasing angle of incidence for the range of angles studied. It was also found that when the surface roughness was close to zero and when it was close to the wavelength of the input laser beam (i.e. 940nm), the reflectance values were close and reached a minimum between these two roughness values.

Experimental investigation on laser transmission welding of PMMA to ABS via response surface modeling

In this paper, an experimental investigation on diode laser transmission welding of dissimilar thermoplastics between PMMA (polymethyl methacrylate) and ABS (acrylonitrile butadiene styrene) has been carried out. The effect of the laser welding parameters such as laser power, welding speed, stand-off distance and clamp pressure on weld strength and weld width is investigated using response surface methodology (RSM). Planned experiments and subsequent analyses are carried out to develop the mathematical models to establish the correlation between the process parameters and the responses. The adequacy of the developed models is tested using the sequential F-test, lack-of-fit test and the analysis-of-variance (ANOVA) technique. A numerical multi-objective simultaneous optimization technique, in which the RSM is incorporated, is used to find the optimum solutions, according to the desired optimization criteria. In addition to that, a graphical optimization technique is, also implemented which allow identifying a region in the graphic where optimal conditions lay on.

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

AbstractThe 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

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

AbstractVibration 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

Application of artificial neural network for predicting weld quality in laser transmission welding of thermoplastics

The present work establishes a correlation between the laser transmission welding parameters and output variables though a nonlinear model, developed by applying artificial neural network (ANN). The process parameters of the model include laser power, welding speed, stand-off distance and clamping pressure; whereas, the output parameters of the model include lap-shear strength and weld-seam width. Experimental data is used to train and test the network. The present neural network model is used to predict the experimental outcome as a function of input parameters within a specified range. Linear regression analyses are performed to compute the correlation coefficients, to measure the relationship between the actual and predicted output values, for checking the adequacy of the ANN model. Further, a sensitivity analysis is performed to determine the parametric influence on the model outputs. Finally, a comparison is made between the ANN and multiple regression models for predicting laser transmission weld quality. The same data set, which are used to develop the ANN model, are also used to develop the multiple regression models. The simulation data obtained from the neural network confirms the feasibility of this model in terms of applicability and shows better agreement with the experimental data, compared to those from the regression models.

Effect of carbon black on light transmission in laser welding of thermoplastics

It is important to describe the laser energy transmission and absorption in polymers properly when modeling heat transfer in laser transmission welding (LTW) of thermoplastics. This paper uses the Bouguer–Lambert law and an apparent absorption coefficient to describe the total laser energy attenuation in scattering polymers with and without carbon black (CB). The model was validated using the measured transmittances as a function of part thickness for unreinforced polyamide 6 (PA6), glass fibre reinforced polyamide 6 (PA6GF) as well as amorphous polycarbonate (PC). A linear relationship between the (apparent) absorption coefficient and the CB level in polymers was observed experimentally for PA6, PA6GF and PC.

Residual stresses in the quasi‐simultaneous laser transmission welding of amorphous thermoplastics

AbstractLaser transmission welding is frequently being used increasingly for joining complex, assembly‐oriented components, thanks to its small heat affected zone. As a result of the cooling processes, residual stresses can develop inside the components, potentially leading to stress cracking and premature component failure. One subgoal of process design should therefore be to reduce the level of residual stresses as far as possible when laser welding is employed. This work looks into experimental testing for residual stresses, as a factor for the weld parameters in quasi‐simultaneous laser transmission welding, with the objective of determining the optimum welding parameters. The ARAMIS optical measuring system for deformation analysis was used to this end, in conjunction with a modified hole‐drilling method. POLYM. ENG. SCI., 50:1520–1526, 2010. © 2010 Society of Plastics Engineers

Preliminary study on the feasibility of friction spot welding in PMMA

In this work the feasibility of friction spot welding of thermoplastics was investigated on poly (methyl methacrylate) plates. Preliminary results have shown that the weld strength is comparable to other available welding techniques, while joining times are equal or shorter. Light optical microscopy and Vickers microhardness measurements showed the presence of a heat affected zone and a thin, consolidated stir zone, where physical–chemical transformations related to thermo-mechanical processing led to changes in local mechanical strength. The work has demonstrated for the first time that the welding of thermoplastic materials by friction spot welding is feasible.

Optical characterization of polycarbonate: Influence of additives on optical properties

AbstractIn laser transmission welding of thermoplastics the optical properties of the joining parts determine the quality of the welding result. Especially, the scattering of laser radiation in the transparent welding part has an impact on weld seam properties. This scattering is caused by additives. For polycarbonate (PC) with different additives the transmittance, the reflectance and the collimated transmittance are measured with a UV‐VIS‐NIR spectrometer. From this data, the optical properties, such as scattering coefficient, absorption coefficient, and anisotropy factor are calculated. The calculations are made with the aid of the four‐flux model of radiation transport in the diffusive approximation. The results show that the additives have a significant influence on the scattering coefficient. For most additives under consideration the scattering is forward directed, which means that most of the radiation is transmitted into the absorbing welding part. However, the power density distribution of the transmitted radiation may differ significantly from PC without additives. So, the weld seam may also differ due to different additives. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 451–455, 2010

Weld line transverse energy density distribution measurement in laser transmission welding of thermoplastics

While laser beam profiling techniques are available for the measurement of a laser beam power profile, such techniques are difficult or impossible to use in order to characterize the power profile of the beam which has passed through the light-scattering semi-crystalline polymer (such as nylon). This paper presents a technique for measuring the scattering of the laser beam caused by the laser-transparent part in laser transmission welding. The technique is easy to implement and does not require specialized or expensive equipment. It relies on measurements of line widths for a sequence of lines left on the polymer surface by a rapidly moving laser beam at progressively increasing powers. The line width and power information is then used to obtain the distribution of the laser energy across the weld line. An application for a case of a high-power diode laser and (glass-reinforced and natural) polyamide 6 is presented.