2渭m lasers: Revolutionizing welding of transparent plastics

Until now, laser plastic welding has had one serious drawback: it cannot bond transparent parts. Although laser plastic welding has been widely used in commercial applications for decades, significant advantages over other joining methods, such as particle-free seams, precision, and high strength, have driven the technology's adoption. Bonding transparent components for medical devices and consumer products not only requires higher laser welding precision, but also often requires completely transparent components. With the new innovation of laser technology, applications ranging from microfluidics and catheters to water bottles and earplugs have benefited from laser welding.

How does a 2-micron laser change Laser Plastic Welding?

Table 1. Differences in Laser Wavelengths Between 1渭m and 2渭m

To understand how this new laser technology can help in bonding transparent applications, it is first important to understand how the original 1渭m laser welding worked and how it differs from the newer 2渭m laser welding methods. Instead of using lasers with wavelengths closer to the 1渭m spectrum, this new technology uses lasers with wavelengths closer to the 2渭m wavelength.

The initial development of laser plastic welding is called penetration laser welding, often referred to as 1渭m or "one micron" laser welding. When welding at 1渭m, 95% or more of the laser radiation enters the optically transparent thermoplastic and will pass through the plastic. Although optically translucent plastics (such as PBT or plastics containing glass fillers) have lower transmittance, they can still be welded if a portion of the laser energy can pass through the upper layer.

Because most 1-micron lasers pass through or through plastics, the optical energy of the upper layers cannot be converted into thermal energy to produce a melt or bond. Establishing a bond with a 1渭m laser requires the absorption of the transmitted radiation at a certain point to generate thermal energy. This absorption is achieved in the base component layer. The base component must have absorption properties, which can be achieved by adding carbon black or special laser-absorbing additives to the resin. Once absorbed by the substrate, the optical radiation is converted into heat, fusing with the upper (transmitting) layer and the lower (absorbing) layer.

The main difference between 2-micron laser welding and 1-micron welding is that the higher wavelength laser interacts with transparent lasers or natural plastics without additives. The shift to a 2-micron wavelength laser significantly changes the way the laser interacts with thermoplastics. When using a 1-micron laser, most of the light energy in the transparent plastic still passes through the 2-micron laser to transfer energy, but more energy is naturally absorbed - even though the plastic is optically clear. That's where all the differences lie.

The transmission of common transparent plastics at different wavelengths was experimentally verified. The vertical blue bar covers the transmission rate at or near the 2-micron wavelength. Note that the transmission in this area is lower than that at or near the 1-micron wavelength. In 2-micron welding, the Goldilocks band is fully absorbed, resulting in a strong bond and good production speeds.

For most natural or transparent thermoplastics, a 2渭m laser can deliver about 70% to 85% of the laser energy, and the remaining 15%-30% of the energy will not require any additives. This absorber causes volumetric heating and melting of the entire joint interface.

Transmittance of common optically transparent plastics at different wavelengths

Why 2渭m?

By far, the main advantages should be obvious. 2渭m allows for the bonding of transparent plastics without any additives, but it is important to understand this. Technically, 1渭m laser welding can weld transparent plastics, but it requires expensive and difficult-to-use absorbers, which increases costs and raises concerns about biocompatibility and color changes. By switching to a 2渭m process, companies can avoid additional material costs, maintain perfect clarity, and reduce the number and complexity of steps in the production line.

In addition to being able to weld transparent plastics without additives, 2渭m laser welding has all the other advantages that are very desirable for standard penetration laser welding:

1.      Ultra-fine welding: a spot size of less than 0.3mm can be achieved.

2.      Repeatability: +/-5 microns.

3.      Connection Form: Very suitable for connectors, bends, or radial joints (such as pipes, tubes, etc.)

4.      Sensitive Use: Non-contact or light contact processing.

In addition, the innovation of laser technology in recent years has significantly increased the available power of 2 micron lasers. At the beginning, the output power of 2 micron lasers was only double digits or even watts. At present, 200W stable power supply has greatly shortened the cycle time and improved the throughput.

Which application is best for 2 micron welding?

Although 2 micron lasers have applications in many industries, including automotive, general industry, film and packaging, most of the demand for 2 micron welding comes from the medical industry. Common uses include microfluidic devices, pipes, bags, connectors, catheters, body-fitting devices, diagnostic boxes, fluid chambers and tanks, filtration devices, etc.

Polarized light microscope view of microfluidic channel (COC: upper layer thickness 0.38 mm, weld width 0.2 mm)

Microfluidics, diagnostic boxes and detection usually require components with visible or UV spectrum transmission for analysis. Therefore, it is necessary to avoid the use of absorbers or pigments, which usually lead to unqualified 1 micron welds.

Many microfluidic devices, such as the one on the upper left, use COC due to its low fluorescence, which improves hyperspectral imaging and analysis.

Other applications, including bags, tubing, and connectors (lower left and page 52), often require transparency so that the end user can see the fluid or fluids flowing through these devices.

The consumer sector is also beginning to use 2-micron laser welding. Major uses include: water bottles, kitchenware, earbuds, microphones, battery housings, sensor housings, wearables, ink cartridges, and hearing aids.

Although consumer products have less stringent requirements than medical devices, products are getting smaller and aesthetic requirements are becoming more stringent. In addition, the marketing team often has as much, if not more, say in the choice of plastic as the product color) as the engineering team, so it becomes increasingly important to be able to provide plastics that can be used in all colors and styles while maintaining clarity. Laser Plastic Welding Advantages.

When is it appropriate to use 2渭m laser welding?

There are many factors that influence this decision, but the main reason is that if the application requires all the benefits that laser welding brings (clean joints, precision, repeatability, high throughput), and both joined parts are clear or optically transparent, then the application is on track for 2渭m laser welding.

There are a few other things to look for in the search for a clear laser weld, transmittance and optical clarity.

It is important to note that laser transmittance and optical clarity are not the same. For 2渭m welds, what is needed is transmission in the infrared spectrum of ~2000nm rather than the visible spectrum. That is, the part does not have to be optically clear; however, this is often the color format that is desired.

Colored plastics, even black plastics, can be bonded if the material does not contain adsorptive compounds such as carbon black. In general, most colored inorganic dyes have sufficient transmittance to bond with 2 micron laser welding.

Choosing the Right Plastic:

Most natural thermoplastics have some reasonable transmittance and are suitable for 2 micron laser welding. However, some plastics are difficult or impossible to weld. This function is primarily the transmittance of the plastic at these wavelengths.

Remember that 2 micron welding relies on a small amount of laser energy being absorbed by the plastic, and generally 15%-30% absorption of the energy is ideal. What is needed is a "Goldilocks zone" where the transmission and absorption are good enough to bond securely and maintain good production speeds. If the transmission rate is too high, the absorbed energy will not be enough to form a bond quickly in most production processes. Additionally, if the transmittance is too low, it will be difficult to weld through thicker parts and get enough energy in the joint.

Most amorphous and/or transparent plastics, such as ABS, COC, COP, PET, PP, PMMA, PS, PC, can be used for fabrication. Even PC, due to its high transmittance at 2,000 nm, can cause some problems, although these can usually be overcome with proper part design. Less transparent plastics, such as PA, TPU/TPE, and PE, can also be used, but they have lower transmittance, which can affect part design.

Semi-crystalline polymers are more difficult to weld because of the way light energy is scattered, making absorption difficult. Some semi-crystalline materials, such as PP, can be welded at 2渭m, but materials such as PBT, PEEK, LCP, etc. are extremely difficult or even impossible to weld.

Part size.

Most laser welding relies on galvanometer scanners to control the trajectory of the laser. This allows for flexibility in weld patterns and part shapes. A major drawback of the 2-micron process is the small working envelope of the scanner when the scanner outputs the laser wavelength. To achieve a very small beam (<1mm), the working area is usually limited to less than three to four inches. A larger working envelope can be achieved, but at the expense of beam size and accuracy, and is generally not suitable for larger microfluidic boxes or applications.

To address this issue, Weld Systems has developed a scanner with a servo-driven XY stage. With patented software, the scanner and XY stage can synchronize the beam delivery to the part, while also adding the flexibility of the galvanometer scanner and the working envelope of the high-precision XY stage. In this way, a working area of 490mm2 (19.3in2) can be achieved, allowing large sizes or more parts to be processed in a single cycle.

Is 1渭m or 2渭m welding better for multiple applications?

The laser plastic welding market has matured quite a bit over the past few decades and has been growing rapidly, in part due to innovations like 2渭m welding and new color additives from material suppliers and compounders, but not entirely because of them. So which process is better?

Honestly, it鈥檚 up to you. While clear requirements from medical devices and consumer products have led to new requirements for laser welding in these industries and opened new doors for 1渭m applications that were previously unattainable, the 1渭m market still represents the largest share of applications.

Every application is different and has its own unique set of requirements, optical clarity being one of them. The real benefit is that the limitations of transmissive plastics in laser bonding applications are rapidly decreasing.