The Advanced Techniques in Laser Welding of Polymers

In this article, discover more about advanced techniques in laser welding of polymers.

A range of industries demand compact, user-friendly, hermetically sealed plastic parts. The automotive industry, chemical industry, pharmaceutical industry and especially medical institutions require these hermetically sealed plastic parts in high volumes.

Image Credit: Monocrom

Increased availability of high-power diode-based fiber-coupled laser sources has led to techniques based on plastic welding becoming commonplace across each of these fields and industries.

Companies are calling for higher throughput and smaller welding details, as well as reduced color sensitivity and clear-to-clear joints, the latter of which is often provided via Monocrom’s FLEX Series modules, able to operate in the 2 µm to 3 µm region because of an increase in absorption for most thermoplastics.

The company’s unique ‘rectified polarization beam combining’ technique allows applications to reach up to 105 W in CW.

Laser Plastic Welding

Laser plastic welding is a common contactless technique able to combine a number of custom plastic parts via laser radiation interaction. Laser plastic welding is an increasingly popular material processing application that is consistently evolving to meet changing industrial demands.

Laser plastic welding is best suited for working with thermoplastics, for example, PA6, PEEK, PMMA, PTFE and TPU. Some combinations of thermoplastics will exhibit a stronger weld strength than others, and it is advisable to look this up before commencing.

Depending on the type and temperature range in question, some forms of thermoplastics establish a liquid phase prior to decomposition. Appropriate combinations can be selected based on these temperature windows.

Welding approaches can generally be divided into two major schemes. These are outlined below.

Through-Transmission Laser Welding

Figure 1a displays a schematic of standard through transmission laser welding. In this example, energy deposition takes place in the lower part as a result of the total absorption of the incident laser light. Energy absorption is due to the use of a different plastic or through the addition of IR (~1 µm) absorbent particles.

Through transmission, laser welding is established between two different plastics or between the same plastic, though when using the same plastic, the lower plastic will use IR absorptive additive or be a different color.

Standard through-transmission laser weld

Figure 1. a shows schematically standard through transmission laser welding where the energy deposition takes place in the lower part due to total absorption of the incident laser light. Energy absorption is caused by using a different plastic or by adding IR (~1 µm) absorbant particles.

Image Credit: Monocrom

Clear joining laser weld

Figure. b is illustrating a so called clear-to-clear laser welding scheme. Here the phenomena used is different since the energy is deposited in the bulk material by means of higher intrinsic abrosption of longer wavelengths (>2 µm) laser light. This approach allows multi-layer welds. 

Image Credit: Monocrom

In through transmission laser welding, energy deposition takes place in the joint section of both parts via total absorption of radiation from the lower part (Figure 1a).

Wavelengths between 808 nm and 1064 nm are typically used. The range between 808 nm and 1060 nm can be achieved using high-power direct diode lasers, while the range between 1030 nm and 1064 nm are generally emission lines of Yb and Nd doped lasers.

Figure 1b illustrates a clear-to-clear laser welding scheme. In this scheme, the energy is deposited in the bulk material via a higher intrinsic absorption of longer laser light wavelengths (>2 µm). This approach can be used to facilitate multi-layer welds.

The majority of thermoplastics exhibit increasing intrinsic absorption in the SWIR (>2 µm) region of light, therefore allowing for bulk energy deposition. Depending on the plastic being welded, a 2 µm direct diode-based laser source can provide 20% to 30% higher adsorption than lasers operating in the vicinity of 1 µm.

Because energy deposition takes place inside the bulk material and not at the lower part, multi-layer joints can be created. These are especially useful in the manufacture of next-generation microfluidic chips and devices.

Key Challenges

This section focuses on clear-to-clear joints, whereby two parts of the same plastic are welded together. Both parts are clear in the visible range of light, with an intrinsic higher absorption in the SWIR (>2 µm) region.

The use of color or white plastics makes this process more challenging for standard laser light sources (808 nm to 1060 nm), leading to undesirable absorption or reflection.

A number of important welding parameters must be considered:

Optical Power

Optical power should be around 100 W to 200 W in CW in order to maintain a reasonable level of throughput in a production process. Monocrom offers instruments with an optical power of 200 W in 940 nm and 980 nm, and also 105 W in 2 µm and 3 µm.

Beam Quality

Source size will determine the ‘initial’ M2-value, and this will directly translate to minimum spot size and/or through-weld thickness. 200 µm fiber cores are the most commonly used options.

Beam Shape

A super-Gaussian intensity distribution can help avoid hotspots inside the weld, improving overall weld quality. A trade-off must be made in order to balance the structure and size of the weld.

Wall Plug Efficiency (WPE)

Laser diode-based solutions consistently offer higher total WPE values than other laser systems, making them an ideal solution for polymer welding. Monocrom’s FLEX Series modules working at 2 µm to 3 µm can achieve up to 10% WPE.

The set-up of a mechanical assembly must be comprehensively planned. Weld quality can also be improved by employing the most appropriate choice of beam delivery options; for example, light guides can be used to homogenize the welding spot.

Application and Process Support from Monocrom

Monocrom is able to supply fiber-coupled modules in wavelength ranges between 808 nm and 1060 nm, as well as between 2 µm and 3 µm. All of the company’s offerings are in the form of laser bar-based modules coupled into different fiber cores. These can be equipped with HP-SMA, D80 or QBH connectors.

The 2 µm – 3 µm section of wavelengths is covered by the FLEX I-2000 modules, and these modules can cover any wavelength between 2 µm and 3 µm while achieving up to 105 W in CW.

Applications requiring specific wavelengths like 2520 nm can be accommodated, even in instances where other laser technologies such as fiber or solid-state may struggle. This is made possible due to the FLEX I-2000 systems’ unique combination technology - rectified polarization beam combining.

Rectified polarization beam combining is wavelength agnostic, meaning it can follow the moving gain of semiconductor laser diodes, whether this is caused by injection current or either an increase or decrease in p-n-junction temperature.

This information has been sourced, reviewed and adapted from materials provided by Monocrom.

For more information on this source, please visit Monocrom.

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