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Basic welding principles
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Laser transmission welding technology is being increasingly employed in medical engineering. It is suitable for hermetic sealing of electronics and parts with high demands in terms of appearance, strength and diversity in size. The characteristic principle of laser transmission welding is that a focused laser beam penetrates a component that is transparent to the laser beam and meets a component that is absorbent to the laser beam. Both components are under pressure during the welding process. The absorbing component converts the laser energy into thermal energy, which enables the melting zones to be fused together. The simultaneous exertion of pressure allows the melt in the joining zones to mix together. The subsequent resolidification or cooling phase after the laser energy effect has dissipated forms the last part of the laser joining process. The main advantages of laser joining techniques are
- hermetically tight weld seams
- good appearance of the joining zone (invisible seam)
- no residual dirt as found in seams created by ultrasound or vibration
- suitability for a variety of part sizes.
Using infrared absorbers
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Figure 1: Transmission of different NIR absorbers used as a coating (LD types) or as an additive for resins ( LWA types).
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Most thermoplastics have low absorption in the near-infrared (NIR) wavelength range (700–1500 nm). Using infrared absorbers in the form of coatings/absorbing films or compounded resins adapts those absorption properties to allow welding of thermoplastic substrates using NIR laser systems. To match the wavelength of commonly used lasers with the application, various absorber materials with different absorption properties are available (Figure 2). Decisions on the best absorptive material to employ are dependent on the laser utilised and other application requirements, including material properties and required colour of the final part.
Infrared absorber coatings. These contain infrared absorbing organic dyes that are designed for use with lasers in the wavelength range of 940–1090 nm. When an infrared absorber coating is applied to the surface of a plastic part by liquid dispensing or spraying, a thin, uniform layer of NIR absorber will be deposited. If NIR energy is applied to the area that has been coated, the infrared absorber material absorbs the energy and converts it to heat; localised melting of the plastic then occurs and a weld forms.
Custom-compounded resins. These contain infrared absorber additives (organic dyes) that convert laser energy into heat to facilitate the welding process. The weldable resin is designed for use with lasers in the wavelength range of 808–1090 nm. Infrared absorber resin systems can be injection moulded or extruded and can be colour matched to meet specific requirements.
The benefits of infrared absorbers include
- precise, colourless welds
- design flexibility (joint design and component colouration)
- hermetic seals
- simultaneous multilayer or complex geometry welds
- transparent, light coloured or opaque welds
- biocompatibility (absorber is tested according to US Pharmacopoeia Class 6 and for cytotoxicity).
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Figure 2: PMMA medical devices welded using an infrared absorber coating.
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By using various coating techniques such as dispensing or spraying, the infrared absorbers can be applied to the specific weld area of parts without influencing the appearance of surrounding surfaces. Applications extend from microfluidic parts (lab-on-a-chip) for medical analytics with channel dimensions in the submillimeter range to filter housings, tubes and large-sized components.
Figure 2 shows a laser welded medical device, which is in series production. Before laser welding, the production method involved gluing and extensive preparation of each component by mechanical cutting and excess trimming. Using absorbing coating, simultaneous laser welding of the top and bottom lid to the tube leads to production time being reduced by 50% for each part. Increased product quality is achieved via hermetic seals and fully transparent product design without visible weld seams.
Stiff plastic materials such as polycarbonate, polymethylmethacrylate and acrylonitrile butadiene styrene are weldable with lasers using the infrared absorber process. Transparent and translucent soft plastics such as thermoplastic elastomer (TPE) can also be laser welded. Figure 3 shows laser welded TPE-polypropylene (PP) samples employing an infrared absorber additive in the TPE material.
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Figure 3: The graph shows the increasing tensile strength of the laser joints as a result of increasing absorber concentration.
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The graph in Figure 3 shows the increasing tensile strength of the laser joints that is achieved by increasing the absorber concentration. As the images of tensile tested samples show, most of the welded samples break beside the weld in the weaker TPE material. This demonstrates that the strength of the weld is higher than the strength of the parent TPE. Only one sample failed in the weld (bottom left image) and this failure is caused by lower absorber concentration in combination with higher welding speed (5.0 mm/s). The fracture behaviour of the tested samples demonstrates the ability to use TPE with an infrared absorber additive for laser welding TPE components.
Checks for series production
If a company decides to investigate laser transmission welding more closely, it would be well advised to follow these research and design processes:
- Feasibility study. Analysis of the material planned to be used to determine its compatibility with the corresponding infrared absorber and feasibility with the laser source considered for the absorber.
- Basic test. If the feasibility study delivers a positive result, a basic test can be performed by laser welding plastic plates. The plastic plates should be made from the material that will be used for series production. The purpose of the test is to analyse the behaviour of the coating or additive in terms of its impact on the quality of the weld seam and to thereby allow an optimal welding geometry to be designed.
- Small batch production. Following a successful basic test and with the weld seam optimised, small batches of approximately 100–200 parts can be laser welded.
Only once these three research and development steps have been completed and positive results achieved is it advisable to initiate series production.
For more information, contact Markus Bleher, bielomatik Leuze GmbH + Co. KG, Daimlerstrasse 6-10, D-72639 Neuffen, Germany, tel. +49 7025 12 340, e-mail: markus.bleher@bielomatik.de or Dr Rolf Klein, Gentex Corp., D-64823 Gross-Umstadt, Germany, tel. +49 6078 93 8487, e-mail: rklein@gentexcorp.com.
