P. J. Bates

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.

A 3-D Thermal Model for Spot Laser Transmission Welding of Thermoplastics

This paper presents a three-dimensional (3-D) transient thermal model of spot Laser Transmission Welding (LTW) solved with the finite element method (FEM). Unreinforced nylon 6 temperature-dependant properties were employed for a lap joint geometry. The heating and cooling stages in a spot laser welding process were addressed. The spot weld dimensions obtained from the model were compared with the experimental weld dimension data, and were in good agreement. It is shown that the thermal model is capable of accurately predicting the temperature distribution.

Estimating Contour Laser Transmission Welding Start-up Conditions using a Novel Non-Contact Method

This study presents a simple technique that can be used to estimate the power and speed required for optimal contour laser transmission welding as a function of material, part and laser characteristics. The technique involves rapidly scanning across a pair of laser transparent and absorbent parts in close proximity using a range of scan speeds and powers. The power and scan speed required to initiate visual degradation in the absorbent part are closely related to the conditions required for maximum shear strength. The technique has been tested successfully on four different polymers over a range of part thicknesses and carbon black levels.

A two-dimensional thermal finite element model of laser transmission welding for T joint

Recent years have seen wider application of laser transmission welding (LTW) as a means for joining of plastic components. Advantages of LTW arise from it being a contact-free method for delivering precisely controlled energy to the surfaces of the welded components and from flexibility with regard to welding geometry afforded by the laser being under computer control. LTW involves a laser beam passing through a laser-transparent component being absorbed by the laser-absorbent component at the weld interface. Heat generated at the interface melts a thin layer of plastic in both components and thus forms a joint through molecular interdiffusion. To form a strong bond, it is important that the weld interface is exposed to sufficient heat to melt the polymer without degrading it. Delivery of the thermal energy by the laser beam is affected by process parameters, such as laser power, scan speed, beam spot size, and material properties, such as absorptivity, presence of reinforcements, and other additives. Dev...

Thermal degradation of PC and PA6 during laser transmission welding

Laser transmission welding (LTW) causes a temperature rise at the weld interface which leads to melting, molecular diffusion and ultimately joining of the two components. Weld temperatures increase with laser power at a given scan speed. However, at higher temperatures, it has been observed that weld strength starts to decline due to material thermal degradation. Thermal degradation is a kinetic phenomenon which depends on both temperature and time. Thermal gravimetric analysis (TGA) is used to study the thermal degradation of two commonly used thermoplastic materials: polycarbonate (PC) and polyamide 6 (PA6). Each material was studied at several levels of carbon black (CB). The TGA data were then used to obtain the kinetic triplets (frequency factor, activation energy and reaction model) of the materials using a non-linear model-fitting method. These kinetic triplets were combined with temperature-time data obtained from a finite element method (FEM) simulation of the LTW process to predict material degradation. The conditions predicted to cause thermal degradation were then compared with experimental data. It is found that the predicted onset of material degradation is in reasonable agreement with both the onset of experimentally observed degradation and the onset of weld strength decline for PC and PA6.

A Three-Dimensional Thermal Finite Element Model of Laser Transmission Welding for Lap-Joint

Abstract Recent years have seen wider application of laser transmission welding (LTW) as a means for joining plastic parts. This paper presents a three-dimensional (3D) transient thermal model of LTW solved with the finite element method (FEM). A modified T-like joint geometry was modelled for unreinforced polyamide (PA6) specimens. This thermal model addressed the heating and cooling stages in a laser welding process with a moving laser beam. In a previous study, the authors have showed that the two-dimensional (2D) model has the potential to predict the molten zone depth as well as transient temperature distribution along the weld line. This paper eliminates some simplifying assumptions made in the previous studies and takes into account the heat conduction along the beam travel direction.

Optical Properties Characterization of Thermoplastics Used in Laser Transmission Welding: Scattering and Absorbance

A better understanding of the optical properties of plastics at IR wavelengths is important for optimizing the laser transmission welding process. In this study, the optical properties, scattering and absorbance, of polypropylene (PP), polycarbonate (PC) and polyamide (PA) were investigated by using spectrophotometer with integrating sphere. The influence of thickness, surface roughness and filler content on optical properties of thermoplastics was evaluated. The results could contribute to a better understanding of the process itself and to success in the practical applications.

3D Finite Element Modelling of Contour Laser Transmission Welding of Polycarbonate

A three-dimensional finite element model was developed for the simulation of heat transfer in contour laser transmission welding of amorphous polymer — polycarbonate. By introducing mass flow in the model, a time-dependent, contour welding process was solved as a time-independent heat transfer problem (Quasi-static model). The short solution convergence times (under three minutes for a set of process and material conditions) and model output agreement with the experimental data indicate that the Quasi-static approach is convenient for use in solving heat transfer problems in laser contour welding. Combined with experimental data, the simulation results indicate that, in contour welding of PC, welding initiates when temperature at the weld interface reaches the maximum of 200 °C. The simulation results also provide further insight into the effect of proportionally increasing laser scan speed and power (i.e., while maintaining constant line energy).

Experimental study on sheet metal bending with medium-power diode laser

In an experimental study of laser sheet bending, a 160 W diode laser is used for two-dimensional sheet bending of low-carbon steel. The variables investigated include metal sheet thickness, laser scan speed, laser power, laser beam width, and laser scan pass number. Bend surface appearances are also analysed. The laser sheet bend results demonstrate that a 940 Nm diode laser is an effective tool for laser forming of carbon steel sheets. No additional surface coating was required. The buckling mechanism may be the main source contributing to the large angle of bend found for the laser-beam-width to sheet-thickness aspect ratio close to 4; for a laser-beam-width to sheet-thickness aspect ratio of less than 2, both temperature gradient and buckling mechanisms contributed to the lower bend angles. The laser beam width study showed that, for the given material thickness range and laser beam profile, the maximum bend angle depends mainly on the material thickness, not the power intensity distribution across the bend line. However, a more evenly distributed laser beam gave the same bend angle with less material property and surface appearance changes. For obtaining the same bend angle, less laser line energy was required if a higher laser scan speed was applied, except for the extreme high-line energy level. Also, multi-path bend strategies may be preferred for maximizing the total bend angle as well as reducing bend surface morphology changes.

Finite volume model for laser-soot interaction for a laser transmission welding process

Laser transmission welding, a technique to join thermoplastic components, involves a laser beam passing through a laser-transmitting part being absorbed by a laser-absorbing part at the weld interface. The heat generated at the interface melts a thin layer of the plastic in both parts and forms a joint. Laser-absorbing agents such as dyes or soot particles are added to the laser-absorbing part to make it absorbing to the laser beam. Thermal and optical interaction of the soot particles and polymer with laser beam determines heating, melting, and, consequently, welding of plastics. To form a strong bond, it is important that the weld interface be exposed to sufficient heat to melt the polymer without degrading it. This paper investigates the thermal response of soot particles to a diode laser heat source. A thermal model is presented herein for a soot particle that is surrounded by a semicrystalline material (PA6) and solved using finite volume technique. The results are then compared to the ones obtained from a finite element analysis solved with a commercial software (ANSYS®). The microscale model predictions for the peak temperature of the soot particle appear to be reasonable when compared with the results of the macroscale finite element models for the same process parameters and set up developed in the previous work of the authors.