Laser welding-The researchers used spherical and roller optical elements for laser welding of foils

Techniques for soldering foil differ in how energy is used to melt plastic. It also depends on the pressure between the materials to be welded and how long it takes the plasticized plastic to cool and solidify into a strong bond. This relationship to the crystal ball and roller is shown by Probylas.

Welding foils uses different techniques. Where they differ is in how energy is introduced to melt the plastic. In addition to this energy, pressure between the materials to be welded is always necessary for the plasticized plastic to cool again and solidify into a strong connection. For large series with uniform layout, high frequency (HF) or heating element welding is usually used. In heating element welding, the tool can be heated continuously or only during the welding process, thus maintaining the pressure of the tool even during the cooling phase -- a technique also known as thermal pulse welding. While heating element welding can be applied to all foils, in high frequency welding, the plastic must have polar components that are constantly rearranged in a strong alternating electric field to generate heat. The polymer itself has dipole groups, such as polyvinyl chloride (PVC) or thermoplastic polyurethane (TPU), or the corresponding additive must be added to the plastic. Both high frequency and heating element welding require a tool capable of replicating the desired weld profile and are also used for compression foil. In high frequency welding, it is an electrode in the form of a weld that applies an alternating electric field. In heating element welding, electric heating element is usually used. For longer welds, a straight beam electrode or heating element or segment with a certain curve radius is sometimes used for stepwise welding. In the case of large volumes with the same geometry over and over again, the amount of work required for the tool is modest and the technique is very effective.

Batch 1 laser welding

Lack of flexibility becomes a problem for small batches, constantly changing layouts, and even for batch 1 personalized geometry. Tool effort and lost time becomes a problem. Laser welding is an alternative welding technique. No fixing tools are required, but the weld profile is tracked using a shaft system and CNC control. Depending on the speed, this takes more time than welding with high frequencies or heating elements, but the contours can be easily changed by loading other data into the controller. The pressure required for welding need not be applied to the entire foil surface at the same time, but can be applied dynamically where welding is also taking place at the time. Glass spheres or rollers are particularly useful for this purpose, focusing the laser beam simultaneously on the weld, where it is locally compressed and can also move freely at the same time by unrolling.

Spherical optics for 2D and 3D contours

In the case of spherical optics, the glass sphere can rotate freely in an air bearing. The pressure chamber behind the air bearing closes at the rear through the window entered by the laser. At the front of the glass sphere is a hole through which air exits and a laser beam radiates. Depending on the type of air bearing and spot size of the laser and weld, the glass beads use quartz glass or sapphire. By default, the diameter of Probylas is 12 mm. Air bearings have pressures up to 6 bar and can produce pressures up to 60 N, which is usually more than adequate for thin films. At the same time, the outflow of air through the air bearing has the expected side effect of cleaning and cooling the film. Smaller and larger ball versions are also available upon request. For special micro applications, balls with a diameter of only 3 mm have been selected, with correspondingly less pressure on the film. The advantage of spherical optics over roller optics is that it can traverse any two-dimensional geometry. You can also use small curve radii or corners. In principle, three-dimensional geometry is also possible if, for example, a spherical optical element is guided by an articulated robot. The bigger challenge is often how to cut the film into three dimensions for application and hold it in place for welding. In the case of thin, light or even elastic films, waves form in front of the glass sphere due to rolling resistance. The closed outline can no longer be completely closed because the wrinkles are opened at the ends. Using a stiffer film or sheet made of clear material that does not bond to the film to be welded can easily counteract wave formation.

Roller optical element, wider weld

In the case of roller optics, the glass roller rotates in an air bearing instead of a ball. This has the advantage of not having to balance and the glass roller can be easily replaced compared to the axle at the end of the cylinder surface. In addition, as with spherical optics, there are cleaning and cooling side effects. Instead of a point-like focus of a laser beam, short laser lines are produced. To ensure that it has the same power and is uniform over the entire length, a microlens array is used in the beam path, which fan out the beam accordingly in one direction. As standard, the Probylas glass roller is 10 mm wide and the laser beam is usually 5 mm wide. However, other widths of glass roller and laser line are possible depending on the application. Therefore, the advantage of roller optics over spherical optics is that the weld is wider. In the case of spherical optics, the maximum width of the weld is limited to about 2 mm due to point-like pressure on the glass roll. On the one hand, the glass roller has the disadvantage of greater air consumption compared to the ball, on the other hand, only direct welding or very large radius welding.

Transparent films are also possible

Like hot plate welding, laser welding is suitable for virtually all types of thermoplastic polymers. In order for polymer melts to mix, they must have similar melting points to be chemically miscible. The easiest way to do this is to weld foil sheets of the same polymer together. In the traditional laser welding method, one layer is transparent to the laser while the other layer is absorbent. For example, it is transparent due to the absence of color additives and absorbent due to the addition of carbon (carbon black). However, these color additives are not ideal for many medical technology applications because they require re-qualification of the material. Alternatively, a different laser wavelength can be selected instead of the color additive. Use 1700-2000 nm instead of the usual 800-1100 nm wavelength (at which all polymers are transparent), where most polymers have weak absorption and can be used for laser welding. However, the process is much slower, and lasers are obviously more expensive for the same performance. In addition to foils, textiles or non-woven fabrics may be welded together if the coating of the fibre or textile is thermoplastic. This is the case with most synthetic fibers such as polyester and polyamide, but not with natural fibers such as cotton, wool, or silk. A good example of strong, tight laser welding is an inflatable air cushion made from transparent and black TPU films. For the surface automatic shutoff valve, two transparent TPU films with longer wavelength are connected to each other. The app will be demonstrated live at various trade shows this year, such as Medteclive in May, the Swiss Medical Technology Fair in September, and Fakuma in October.

Source: Laser Net