Laser advances improve links between plastic polymers

Lasers are still considered an exotic energy source, but the rapid emergence of a new type of cost-effective laser - the fiber laser - may gradually change this. We define fiber laser technology as technology where the laser beam is actually generated or amplified in the fiber component itself, rather than simply transmitted from free-space optics to the workpiece via a fiber optic cable.

These lasers are simply viewed as black boxes due to their ease of use and reliability. As a result, they do not have the mystique of other laser types that require a lot of complex optical components. New laser types and wavelengths are also emerging that are finding some of the less seemingly exotic applications. The subject of this article is fiber lasers for welding transparent polymers.

Laser Basics

Going back to the basics, a laser beam is simply a beam of light energy that can be focused onto a very small spot. This property is the reason for many high-powered industrial applications of lasers, such as cutting and welding thick steel. However, aside from this focus, there is another property of the laser beam that may be more responsible for its profound reputation: the wavelength of light produced by most laser beams is fairly distinct.

Fiber Lasers

Until recently, significant average powers for industrial laser materials processing were available only from very limited laser types鈥攅ither solid-state lasers emitting in the near-infrared 1.07渭m wavelength range or CO2 gas lasers emitting in the longer 10.6渭m wavelength range. New versions of the standard industrial fiber laser are appearing in the middle wavelength range called short wavelength infrared, producing up to 120 watts of power. This 2um wavelength is achieved by using another rare earth element, called a dopant, in the optical fiber that produces the laser beam.

Basic laws of physics tell us that photon energy decreases as wavelength increases. This means that different materials will react differently when they are irradiated with them. This longer wavelength is absorbed in different ways by many different molecules, as specific photon energies are absorbed by specific molecular bonds through a resonance mechanism.

Of particular interest to those in the medical polymer field is improving the absorption of the CH molecule, which is the background chain of all organic polymers. The end result is a significant increase in the absorption of this laser beam in transparent polymers, allowing for highly controlled melt thickness through optically clear polymers.

Why not use a CO2 laser?

For older technology CO2 lasers, the emission wavelength is much longer and the absorption is typically close to 100%. The wavelength has many advantages if you want to cut polymers with a laser, but for welding polymers where controlled melting through the thickness of the material is required, this high absorption is a serious disadvantage. Since the absorption occurs on the top surface of the part, it takes a long time to melt into the part to create a deeper weld.

Laser Cutting of Polymer Films

Recent experiments have also shown that while the absorption of most thin materials may not be sufficient to produce effective ablation and cutting, some slightly opaque films in the 50-200 micron thickness range can be easily cut using this technique.

Weldable Materials

Because the laser beam is converted into heat when absorbed by the polymer, any polymer or combination of polymers that can be thermally welded by ultrasonic or RF can also be laser welded.

Applications Real World

There are many real-world applications, including medical devices, typical microfluidic devices; and consumer products.

Softer hose materials such as PVC and many of the new non-PVC alternatives such as TPE can also be welded. These materials are difficult to weld any other way. While the rules of chemical compatibility cannot be completely changed, the laser process provides good local temperature control when joining polymers with limited compatibility.

Microfluidic devices that require fine joins. Single-mode, high-focus fiber lasers can provide very narrow melt lines and limited heat input to minimize deformation of microchannels while providing an airtight seal. However, the strength limitations of any 100-micron-wide polymer weld joint need to be considered when designing these components.

In Summary

The availability of this new wavelength at high average power brings great improvements and simplifications to the laser welding of transparent polymers in medical device and other industries. For the polymers of most interest to medical devices, most thermoplastic polymers (if not polyacrylates) do not require additional absorbers to produce nearly invisible welds.