Laser transmission welding of ABS: Effect of CNTs concentration and process parameters on material integrity and weld formation
Introduction
Over the past few years, welding of thermoplastics has attracted a considerable interest from a number of different industrial sectors such as automotive, packaging and healthcare [1], [2], [3], [4], [5], [6], [7]. Depending on the application and technical requirements of the component, joining of thermoplastics can be carried out by mechanical fastening, adhesive and fusion bonding [1], [2], [7], [8]. Fusion bonding refers to the generation of localized heat in the interface of the materials to be joined. The technologies behind this heat generation can be divided in thermal, electromagnetic radiation and friction force propagation. Thermal fusion bonding is currently the most prevalent bonding method considered in different applications like sealing microfluidic chips, where adhesive bonding showed several drawbacks [1], [2], [7]. Some advantages such as relatively high bond strength, reduction of the processing time and simplicity of the approach make this technology attractive for packaging of miniaturized components. One major challenge of this technology is the proper control of the temperature, pressure and heating time in order to achieve a high quality joint [1], [2], [7]. Welding technologies under this category include the following: self-bonding, hot-plate, hot-gas, infrared, extrusion and laser transmission welding (LTW) [7], [9], [10], [11], [12].
Focus on laser transmission welding, it is an emerging technology in applications where the hermetic adhesion and the minimization of the surrounding affected area are critical requirements, such as hermetic enclosure of medical devices, assembly of microelectromechanical system devices (MEMS) and packaging [6], [8], [13], [14], [15], [16]. CO2 laser systems (λ=10.6 µm) have been traditionally used in welding applications of thin plastic films, as a consequence of the high absorption coefficient of these materials to medium infrared radiation [8], [16]. The radiation produced by diode lasers, however, is less readily absorbed by plastics and, together with the fact that these systems show high electro-optical efficiency, this makes them a very good alternative for plastic welding. Laser transmission welding evolves localized heating at the interface of two pieces of plastic to be joined. One of the plastics needs to be optically transparent to the laser radiation whereas the other part has to be absorbent. This way, LTW process can be carried out by means of different approaches. One considering an intermediate absorbing layer at the interface of both plastic parts. The second one involving the use of laser absorbing additives, according to the laser wavelength considered and the final application of the component. In this particular case, depending on the optical and thermal properties of the absorbed part, the incident radiation is absorbed over a certain depth of the absorbent material. Finally, the last approach is focused on laser welding of two or more transparent plastics. It is based on the natural absorption which some thermoplastics present in the higher NIR wavelength ranges (1500–2000 nm). Therefore, absorbers or special additives are not required, which enable completely new process concepts for polymer welding that eliminate additional costs related to the drawbacks linked to plastics modification in order to make them laser-weldable [17], [18].
The research presented here is focused on the second approach mentioned above. This technology has been successfully applied in the assembly of bio-microfluidic devices or automotive taillights, among others, leading to well-defined welded joints without the use of any chemical products or adhesives [1], [2], [13], [18], [19]. The laser energy that is absorbed in the material causes vibration of electron bonds, followed by heat transfer to the surroundings through convection and radiation. When heated to temperatures above the melting point, the thermoplastic begins to flow and a weld is formed while applying a pressure to keep the intimate contact during welding and cooling processes. The molten plastic that forms improves the heat contact between both parts and involves an internal joining pressure to build up through volume expansion [20], [21]. Hence, not only the process parameters like the laser power, welding speed (irradiation time), clamping force and spot size on the absorbent part (focus plane of the laser head) [10], [13], [17], [23], [24], but also the thermal and optical properties of the material would affect the weld quality of the final component.
During the last decade, laser transmission welding experiments involving a wide range of thermoplastics, such as, polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), polymethylmetracylate (PMMA), poly-acrylonitrile–butadiene–styrene (ABS), polycarbonate (PC), poly-ether–ether–ketone (PEEK) and polypropylene (PP) have been reported based on the effect of laser parameters (power density and material interaction time) and experimental setup (optical system and clamping device) on weld quality [13], [14], [19], [22], [23], [25], [26], [27], [28], [29], [30], [31]. However, very few studies in the literature have investigated the influence of the pigment/additives concentration in the laser-absorbent part on the welding process and weld quality. Studies made by Visco et al. confirmed that the CNT filler are suitable absorbed to ensure the absorption of the laser radiation. Also, this study showed that the filler amount must be low (0.1 wt%) to avoid defect formation and to achieve high joint strengths [14], [29]. Abed et al. investigated diode laser transmission welding process of polypropylene with four percentages of pigment content (0.05 wt%, 1 wt%, 2 wt% and 5 wt%). The report focuses on the study of the great influence on seam weld properties depending on that process parameters and optical properties of absorbing polymer [32]. Potente et al. studied the joining of PEEK to PEEK utilizing quail-simultaneous transmission laser welding, where the bottom layer was pigment with varying contents of carbon black [29]. Additionally, Aden et al. analyzed the scattering produced by the additives on weld seam properties [33]. Similar studies have been done on different materials. However, different content of CNTs were not considered in these investigations.
To the best of our knowledge, no previous study has examined the impact of pigment concentration on laser welding of ABS. This polymer offers chemical resistance, heat stability, toughness and high impact strength. Consequently, ABS has a wide range of applications and is widely used in many industries such as aircraft [34], automotive [35], [36] and electronics [37].
The focus of the research reported in this paper is on investigating the response of ABS when performing laser welding with a diode laser system operating at 808 nm. The effect of laser parameters and material properties (thermal and optical properties) will be extensively analyzed in order to identify the different factors affecting material integrity and weld formation. Different concentrations of carbon nanotubes (0.01% and 0.05% CNTs) were considered as absorber of infrared laser radiation. This additive component would modify the thermal and optical properties of the material and, hence, the process window where the laser welding process would provide high quality welds.
The purpose of the research presented here is to analyze the laser weldability of ABS depending on the laser welding parameters and filler concentration. The analysis will be carried out by thermal and optical characterization of natural and doped ABS sheets. Microscopic characterization and mechanical shear tests of the joining will also be carried out in order to study the influence of process parameters on structural changes on the polymer and mechanical resistance of the welded joint.
Section snippets
Materials and methods
The polymer employed in this work was BASF Terluran® GP-35 ABS (ABS), a complex mixture consisting of styrene–acrylonitrile copolymer, a graft polymer of styrene–acrylonitrile and polybutadiene and some unchanged polybutadiene rubber [26]. The experimental investigation involved two types of ABS plates: the first one is a natural ABS, transparent to the infrared laser radiation, and the second one is an infrared radiation (IR)-absorbent ABS, where different weight percent of carbon nanotubes
Results and discussion
Compared with other traditional welding techniques, laser welding efficiency is strongly dependent on the material's properties: thermal and optical properties of the materials play an important role on the success and the understanding of the laser welding process. Thus, both physical properties are investigated as a function of the additive concentration in order to identify the process window required to obtain a high-quality weld.
Conclusions
In this paper, we have reported experimental data corresponding to laser transmission welding of ABS as a function of line energy for ABS with two selected percentages of CNTs.
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The glass transition temperature for doped ABS did not change significantly with the CNTs concentration. On the contrary, the absorption and reflection coefficients strongly depend on the CNTs content. Hence, the laser energy required for producing a certain welding seam decreases when the CNTs content increases.
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Results
Acknowledgments
This work was funded by Ministry of Industry of the Basque Country through EMAITEK project along with the Ministry of Science and Innovation of Spain though DILAPLAS project.
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