Laser Transmission Welding Of Thermoplastics Research Articles

Study on Effect of Laser Process Parameters on Laser Transmission Weld Parameters using ANSYS

In recent years, a mode of welding that has garnered a considerable amount of interest is the laser transmission welding of thermoplastics. Laser transmission welding is now being used as an alternative to adhesives to join two thermoplastics. In this study, a finite element model has been developed to simulate the laser transmission welding of polypropylene. The movement of the laser beam was done using a Moving Heat Source in Ansys®. Process parameters namely laser power, welding speed, and the number of passes have been studied in order to investigate their effects on the temperatures and the weld widths achieved during welding. It was found that an increase in the laser power had a positive effect on the maximum temperature at the weld interface as well as the weld width. Similarly, an increase in the welding speed had a negative influence on the maximum temperature at the weld interface as well as the weld width.

A review on laser transmission welding of thermoplastics

A review on laser transmission welding of thermoplastics

A new finite element model accounting for thermal contact conductance in laser transmission welding of thermoplastics

As the traditional laser transmission welding (LTW) generally ignores the influence of real interfacial contact status on welding width, which is not accurate. For this problem, the Thermal Contact Conductance (TCC) was introduced to establish a new LTW prediction model so as to investigate the TCC at different LTW parameters. Then, a mathematical equation between welding process parameters and TCC was proposed by Response Surface Method. At last, the mathematical equation was verified by a series of experiments. The results showed that the TCC increases with the increases of laser power, clamping pressure, and decreases with the increases of welding speed. The simulated weld width considering TCC was consistent with the experimental weld width, which verified that the error of this prediction model was within 3.3%. Therefore, TCC can be a key factor of the finite element model in laser transmission welding.

3-D FE heat transfer simulation of quasi-simultaneous laser transmission welding of thermoplastics

Laser transmission welding of thermoplastics is a non-contact welding process for producing aesthetically and qualitatively high-grade joints with low thermal and mechanical stresses. In this research work, a three-dimensional heat transfer model is developed to simulate the quasi-simultaneous variant of laser transmission welding of thermoplastics. The thermal analysis is implemented to acquire the temperature history in this multi-pass welding process. The transient temperature field is computed using ANSYS® finite element code. The multi-pass movement of laser beam is realized by implementing a subroutine in ANSYS® Parametric Design Language (APDL). The effect of process variables namely laser power, welding speed, number of welding passes and line energy on temperature field during welding is investigated. The predicted outcomes of the temperature profiles, weld pool and time–temperature history are useful for optimizing the process parameters in the quasi-simultaneous laser transmission welding process.

Modeling of the thermally induced residual stresses during laser transmission welding of thermoplastics

Laser transmission welding is one of the various welding techniques used to join thermoplastics. Low heat introduction into the welded parts and a high welding speed are the reasons why laser transmission welding established itself in the plastic processing industry. In all welding processes for thermoplastics, residual stresses occur due to the thermal expansion during heating and its resetting during subsequent cooling. These thermally induced residual stresses impair the mechanical properties of the welded components. To estimate the residual stresses and therefore to improve the process setup and understanding of the welding process, a thermo-mechanical simulation model for the laser transmission welding is developed at the IKV. In a linked simulation, the temperature distribution and mechanical stresses are calculated parallelly for each time step. The material, a polyamide 6.6, is modeled with elastic, elastoplastic, and linear viscoelastic behavior. After cooling, residual stresses of a realistic magnitude are calculated. However, as the yield point of the polyamide is exceeded in the welding process, the tensile/compression ratio in the welded component is only mapped correctly for the elastic-plastic material model.

Simulation-based investigation on the temperature influence in laser transmission welding of thermoplastics

Because of several advantages, e.g., an exact and high energy input, laser transmission welding has become more and more important in the last few years. Due to the contactless energy input, a sufficient process control is a challenge. In industrial production, the process parameters for a good weld seam are qualified by the energy input, which describes the process parameters laser power, laser velocity and irradiation time. These process parameters lead to the welding temperature, which influence the weld seam quality. The question remaining is whether the energy input describes the weld strength sufficiently or whether the welding temperature has a higher influence on the weld quality. In this study, the influence of the energy input on the weld quality is determined for an industrially relevant material combination (PBT ASA-GF20 and PC) in experimental examinations for quasi-simultaneous laser transmission welding. The welding temperature for every design point is calculated and the influence of the temperature on the weld strength is analyzed in an FEM model. In order to compare the influences of the two factors, welding temperature and energy input, a correlation analysis is performed. The correlation analysis shows a higher influence of the welding temperature on the weld strength compared to the energy input. But the energy input is also able to describe the weld strength.

Effect of processing parameters on meltdown in quasi-simultaneous laser transmission welding

Meltdown is the term used to describe the collapse of two thermoplastic parts during welding. It is a critical process parameter in the laser-transmission welding of thermoplastics. This study examines the effect of quasi-simultaneous (QS) laser transmission welding (LTW) processing parameters including input power (P), number of passes (N), scan speed (V), total scan length (LS) and weld pressure on the meltdown behavior of polycarbonate (PC) and polypropylene (PP), using a T-shaped test assembly. The total meltdown is shown to depend linearly on the total line energy (LET) defined as the product of P and N divided by V. Increasing LS increases the critical LET for meltdown to begin. A simple model is presented that captures the main elements of meltdown in this thermoplastic welding process.

Laser transmission welding of polymers – Irradiation strategies for strongly scattering materials

In laser transmission welding of thermoplastics the optical properties of the transparent part have great influence on the weld seam quality. Especially scattering, for example at crystalline structures or additives, can significantly affect the intensity distribution of the radiation in the joining zone. Since scattering is a strongly wavelength dependent process the right choice of the laser wavelength can enable laser transmission welding even for challenging materials. It is shown that the necessary optical material properties needed to assess the weldability of scattering materials can be directly obtained from NIR-spectroscopy.

Matching of laser intensity distribution for laser transmission welding of thermoplastics

Laser transmission welding of polymers is becoming more and more important in industrial applications. In contrast to other polymer welding processes, for example hot plate welding or vibration welding, the exact definition of the major process tool, the laser beam, is challenging. The comparison of investigation results with each other can be very difficult, because of, for example, the different intensity distribution and the specimen geometries. In this study, a method to match the laser power profiles of two different laser systems was developed. Shear-test results confirm that statistically equal weld strengths can be achieved when laser power profiles and test geometries are matched.

Modeling of the heating process during the laser transmission welding of thermoplastics and calculation of the resulting stress distribution

At present, various welding techniques are used in many different process chains and steps. Moreover, when joining metallic materials, welding processes are used to create integrated material bonds of organic as well as inorganic materials. One of the methods used to join thermoplastics is laser transmission welding. In addition to the low heat introduced into the welded parts, high welding speed is one of the reasons why this process is used in industry. Highly accurate simulation of the heating process would improve understanding of the process, facilitate and reduce the time required for process installation in the long run, and provide a significant contribution for computer-aided component design. The quality of the weld line in laser transmission welded plastic parts is affected by material properties, machine parameters, and their interactions with each other. For these reasons, a thermal simulation model was developed as part of a research project due to support the operator of the laser transmission welding process in setting up the process faster and more reliably. First, the presented model considers the various interferences of the welding process by the material, the thermal, and optical properties which are often not known exactly enough. Second, interaction between the laser beam and the material was determined and implemented. The developed calculation model allows for highly reliable determination of the size of the heat affection zone. The exact process window can be identified and predicted. Furthermore, the heating and cooling process can be reproduced using the spatial temperature distribution, which allows cycle time to be optimized on the basis of these results. In addition to the impact on weld quality, residual stresses can be found in welded thermoplastic parts as a result of local thermal expansion and shrinkage. Especially, tensile stresses are undesired effects of the welding process because they can have an unfavorable impact on the local mechanical properties of the welded components. Mainly, in laser transmission welded parts, these tensile residual stresses occur in the weld seam. These tensile stresses can overlap with permitted operating stresses, and this may result in early failure of the component. This can take place, in particular, when the structural part is exposed to oscillating and/or highly corrosive stresses. These are the reasons why it is important to know the residual stresses introduced into the component, in order to minimize the distribution of these stresses by means of suitable process management. To calculate the mechanical behavior in the structural simulation of a welded part, precisely mapping the temperature field is of fundamental importance. Residual stresses and warping are directly based on the temperature field. Based on thermal calculations, the paper presents thermo-mechanical simulations for the determination of residual stress distribution in the weld seam, depending on the welding parameters.

Effect of part thickness, glass fiber and crystallinity on light scattering during laser transmission welding of thermoplastics

Abstract It is important to understand how laser energy scatters within the transparent component in order to predict and optimize the laser transmission welding process. This paper examines the influence of part thickness, glass fiber and crystallinity levels on the distribution of laser light after transmission through amorphous polycarbonate (PC) and semi-crystalline polymers such as polyamide 6 (PA6), polypropylene (PP), and polyethylene (PE). An experimental technique based on laser-scanned lines of progressively increasing power was used to assess the transmitted energy distribution. This distribution was characterized using a two-parameter model that captures scattered and un-scattered components of the laser beam. The results clearly show how the scattering is increased by increasing the numbers of interactions between laser light and phase boundaries either by increasing the particle concentration (i.e., glass fiber level and crystallinity) or increasing part thickness.

Effect of glass fiber and crystallinity on light transmission during laser transmission welding of thermoplastics

Abstract In order to predict and optimize the contour laser transmission welding (LTW) process, it is important to understand how the laser energy behaves during transmission through the transparent part. In this study, transmission measurements were made on unreinforced and glass-fiber-reinforced amorphous and semi-crystalline thermoplastics at different thicknesses. Using the ratio of transmitted power to laser power, apparent absorption coefficients and apparent reflections were calculated. The results indicate that there is a linear relationship between the glass fiber volume fraction and the apparent absorption coefficient of reinforced polymers; similar effects were also observed for crystallinity. A simple model was developed to estimate apparent absorption coefficient of reinforced polymers as a function of composition. The apparent reflection increased with crystallinity due to increased back scattering. However, for glass-fiber-reinforced polymers, the apparent reflection displayed a more complex behavior.

Multi-objective Optimization in through Laser Transmission Welding of Thermoplastics Using Grey-based Taguchi Method

The aim of this study is to analyse the effects of process parameters on joint strength and weld width of welded acrylic plastic specimens. Process optimization is also made to maximize joint strength as well as minimize of weld width of welded samples. Lap welding has been done by through transmission laser welding with varying process parameters namely current, stand-off distance and clamping pressure. Experiments have been planned as per L9 orthogonal array of Taguchi method. Grey-Taguchi method has been utilized to optimize multi-responses joint strength and weld width simultaneously. Statistical techniques like analysis of variance and signal to noise-ratio have been used to identify the significance of welding variables. The optimal parametric setting has been obtained from main effect plots of signal to noise ratio. The confirmatory test has been conducted to validate the optimized condition and optimization procedure used in the study.

Irradiation Strategy for Laser Transmission Welding of Thermoplastics Using High Brilliance Laser Source

This article describes the correlation between crystallinity degree and spatial seam homogeneity for laser transmission welding of polymers applying TWIST method, which is based on a superposition of slow regular feed and fast periodic circular or elliptical beam motion. Laser welding experiments and measurements of optical polymer properties (transmittance, reflectance and collimated transmittance) are reported for thermoplastic polypropylene PP and Polyamide PA66 as well as for thermoplastic elastomer PEBAX. The shape of the seam is visualized by measuring the heat affected zone (HAZ). Compared to the circular the elliptical motion leads to a homogeneous HAZ, which is advantageous for the welding process in many cases. Besides TWIST parameters, scattering of the laser radiation influences the welding result considerably. This is shown for high-crystallinity PA66, which needs more laser power to achieve similar weld seams than PP or PEBAX. The correspondence between homogeneous HAZ and good welding results is investigated by measuring the strength of welded T-shape samples with respect to the laser power. The elliptic TWIST motion leads to a broader processing window, if the ratio of the beam diameter and the major TWIST radius is smaller than a certain value (<1/2). For PEBAX, a local maximum of strength-vs.-power curve is obtained for all TWIST® radii. For welding PP, the maximum is observed only for big radii.

Joint Strength for Laser Transmission Welding of Thermoplastics: A Simulation Approach

Abstract For laser transmission welding of thermoplastics the tensile strength of the joint is measured and microtome cuts of the joint are taken. The joining experiments are carried out for PA66 (Ultramid A3K) for different scan velocities and laser powers. Varying the laser power the tensile strength of the joint obtains a maximum. The value of the power where the maximum is reached depends on the scan velocity. In the thermal simulation the temperature distribution is calculated. The distributions perpendicular to the scan direction are compared to the microtome cuts of the joint. The temperature distribution in a plain parallel to the joining zone is used to calculate an area where the temperature is above the melting temperature and below the degradation temperature. Calculating this area as a function of the laser power gives a maximum area approximately at the same power where the tensile strength of the joint is maximal.

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.

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.

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.

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