The increasing use of plastic parts, especially for high-precision products, is leading manufacturers to look for joining technologies that offer better welds, higher production performance and lower costs. Laser welding of plastics promises to be a boon in all of these areas. Thus, the introduction of the laser process delivers optimal results and reduced costs.
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There was a time when the word "plastic" was synonymous with "cheap". Today, however, plastics, or more precisely "polymers", are used in all sorts of high-quality and technically demanding products. These include consumer electronics such as mobile phones, tablets, laptops and fitness trackers, as well as cars, aircraft, medical devices and more.
One reason for the increasing use of plastics is weight savings, which is key in the aerospace and automotive industries. Another advantage is that plastics do not rust or corrode, which is useful in everything from automobiles to medical devices. Last but not least, the ease of manufacturing small complex shapes plays a role. Using methods such as injection moulding, small plastic parts with complex shapes that designers and manufacturers love can be produced easily, quickly and cheaply.
How to laser weld plastics
When making objects from plastic, or more correctly polymer components, it is often necessary to permanently join the parts together. In high-volume production, some form of welding is usually used for this, rather than simply gluing. This is because welding can usually be done much more quickly and accurately than gluing, and welding produces stronger and more reliable joints.
In practice, then, there are several ways in which plastics can be welded. They usually involve some kind of heat source that melts a thin layer of polymer, or mechanical movement between two parts that does the same thing by friction. In both cases, the molten polymer then re-solidifies to form the weld. Laser welding uses a laser beam as a heat source, and this method is widely used, especially in the medical and automotive industries.
Transmission-Time Laser Welding (TTLW)
One of the most commonly used techniques for laser welding of plastics is called "transmission laser welding" (TTLW). The idea is actually quite simple. One of the two parts to be welded is made of transparent plastic, while the other is opaque - by transparent here we mean transparent to light with the wavelength of the laser beam, not transparent to the human eye. The transparent part does not absorb the energy of the laser beam, while the opaque part does. The welding is done by holding the parts together with the transparent part on top. The laser is then directed down through the transparent part to the opaque part. The laser beam then starts to rapidly traverse ( = scan) the surface of the part, eventually drawing the shape of the desired weld. The opaque polymer absorbs the energy of the laser beam, heats up and melts. The heat from it also melts part of the transparent part. The parts are pressed together, while the molten plastic fills any gap between them. Finally, the laser is switched off and the molten material solidifies again, creating a strong weld joint with no gaps or weak spots.
Figure 3: Schematic of the main steps of the quasi-simultaneous TTLW method with "rib collapse".
Advantages of laser welding of plastics
One of the best features of laser welding of polymers is that it produces no dirt, dust or particles. In contrast, almost all different friction or vibration methods (such as ultrasonic welding) produce some contaminants. Dirt-free welding is particularly useful in the manufacture of medical devices and various electronic sensors. It makes laser welding compatible with cleanrooms.
The absence of vibration and low thermal and mechanical stresses in laser welding also minimizes any deformation in the shape of the parts being welded and prevents damage. This is particularly important for delicate precision parts, for example in electronics and medical applications. The weld joint itself, created by the laser, is strong and almost invisible to the eye. This is particularly important for consumer goods.
Last but not least, laser welding is fast for manufacturers and can be monitored and controlled very precisely. This maximizes yield and ensures excellent part consistency. In addition, it is relatively easy to change process parameters to accommodate design revisions or material changes.
Preparing for successful plastic welding
What do you need to do to make laser plastic welding work for you? There are three key things you should think about before you start production and perhaps before you buy any equipment.
The thermal properties of the materials
The first is choosing the right materials for your parts. The temperature range in which the two polymers (clear and opaque) will remain molten (but not disintegrate) must overlap to some degree for the method to work. In addition, a larger overlap increases the process window, which makes the production easier and more robust. The table summarizes which common polymer combinations are compatible with laser welding.
Polymer combinations compatible with laser welding
Another consideration is 'design for manufacture' issues. Consult with laser welding experts early to ensure that the part design is compatible with the laser process and to determine if any design changes could lead to improved manufacturing results.
Testing and development support
Finally, if you don't have a lot of previous experience with laser polymer welding, work with someone who can provide assistance in developing your application solution. Specifically, get in touch with a supplier who can perform tests and suggest what system configuration will yield the best results and help you determine the optimal laser parameters for the entire manufacturing process. Coherent Labs provides just these services in its application labs. The resulting report then includes the specific type and parameters of the laser setup appropriate for your application.
Source: coherent.com