Discover the advantages of laser technology in plastic welding

The automotive, chemical and pharmaceutical industries, and not to be overlooked the medical sector, all demand compact, easy-to-use and sealed plastic parts in large quantities. Plastic welding is rapidly conquering all these sectors thanks to the availability of high-power diode-based, fiber-coupled laser sources. In addition to higher throughput and smaller weld details, customers are also eager for reduced color sensitivity and crisp joints. The latter can be used with laser modules in the 2 渭m to 3 渭m range due to the increased absorption of most thermoplastics. Our unique "rectified polarized beam combining" technology enables us to reach powers of up to 105 W in CW.

What is laser plastic welding?

Today, laser plastic welding is a commonly used technology for joining multiple customized plastic parts together contactlessly through the interaction of laser radiation. It is a material processing application that is constantly gaining more attention and is constantly evolving to adapt to industrial needs. Laser plastic welding works best with thermoplastics such as PA6, PMMA, PEEK, PTFE and TPU. There are many more, but this is just a selection of possible candidates. Some combinations offer stronger welds than others, which can be looked up in the table. Within a certain temperature range for a specific type, thermoplastics form a liquid phase before decomposing. Based on these temperature windows, a suitable combination can be selected.

In general, welding methods can be divided into two main schemes (simplified).

• Through-type laser welding

• Standard transmission laser welding

Figure 1: Schematic representation of standard transmission laser welding, where energy deposition occurs in the lower part due to complete absorption of the incident laser. Energy absorption is caused by using different plastics or adding IR (~1 渭m) absorber particles. Figure 2:

Illustrate the so-called transparent-to-transparent laser welding scenario. The phenomenon used here is different, since energy is deposited in the bulk material by the higher intrinsic absorption of longer wavelength (> 2 渭m) lasers. This approach allows multilayer welding.

The first scenario is established either between two different plastics or between identical plastics, while in the latter case the lower one has an infrared-absorbing additive or a different color. With through-beam laser welding, energy deposition takes place in the joining section of the two parts by complete absorption of the radiation from the lower part. This can be seen in Figure 1a. The wavelengths commonly used are between 808 nm and 1064 nm. While the range from 808 nm to 1060 nm can be covered by high-power direct diode lasers, 1030 nm and 1064 nm are emission lines of Yb- and Nd-doped lasers.

Another approach is called 鈥渢ransparent-to-transparent鈥?laser welding, where the physical principle used is somewhat different from the first approach. A schematic diagram is shown in Figure 1b. Most thermoplastics have an increased intrinsic absorption in the SWIR (> 2 渭m) light region, which makes bulk energy deposition possible. Depending on the plastic to be welded, 2 渭m direct diode-based laser sources can provide 20% 鈥?30% higher absorption compared to lasers around 1 渭m. Because energy deposition occurs within the bulk material rather than underneath it, multilayer joints can be created, which is useful for next-generation microfluidic chips/ devices.

What challenges need to be overcome?

This section focuses specifically on transparent-to-transparent joints, where two parts of the same plastic are welded together. As the names suggest, both parts are clear in the visible range and have a higher intrinsic absorption in the SWIR (> 2 渭m) region of light. Colors and white plastics pose additional difficulties for the standard light sources used (808 nm to 1060 nm) due to unwanted absorption or reflection. The important welding parameters are:

1. Optical power: Approximately 100 W to 200 W CW will provide a reasonable throughput for your production process. Leiser lasers offer 200 W at 940 nm and 980 nm and 105 W at 2 渭m and 3 渭m.

2. Beam Quality: Source size determines the 鈥渋nitial鈥?M 2 value, which translates directly to minimum spot size and/or weld penetration. A 200 渭m core is typically used.

3. Beam Shape: A top hat or super-Gaussian intensity profile will prevent hot spots inside the weld and can improve weld quality. Tradeoffs are needed to match the structural dimensions of the weld.

4. Wall Plug Efficiency: Laser diode-based solutions offer higher overall WPE values than other laser systems, which makes them an excellent solution for polymer welding. Our @FLEX modules can achieve 10% at 2 渭m to 3 渭m.

In addition to achieving these parameters, the setup of the mechanical assembly needs to be thoroughly planned, and the weld quality can be improved by the correct choice of beam delivery, for example, light guides can be used to homogenize the weld spot.