Progress in Lasers Improves Polymer Joining
by Tony Hoult
Lasers still are often perceived as exotic energy sources, but the rapid emergence of a new type of highly cost-effective laser – the fiber laser –gradually may change this. We will define fiber laser technology as the technology where the laser beam is generated or amplified actually within a fiber optic component itself, rather than simply being delivered to the workpiece from free space optics via a fiber optic cable.
The ease of use and reliability lead to these lasers being thought of simply as black boxes; therefore, they do not have the mystique of other laser types which require a number of complex optical components. New laser types and wavelengths also are emerging, and these new laser wavelengths are finding uses in applications that also might seem less exotic. One of these applications is the subject of this article: fiber lasers for welding clear polymers.
Fiber lasers have been gaining market share and increasing in power dramatically, to the point at which earlier this year a 100kW fiber laser was delivered! Alongside these power increases, another recent development from IPG Photonics shows that fiber lasers now may be used economically for relatively mundane tasks such as precision welding of polymers, and this is of interest to the medical device industry.
Going back to basics, a laser beam is simply a beam of light energy that can be focused down to a very small spot, and this property alone is responsible for many of the high-power industrial applications for which lasers are used, such as cutting and welding thick steels. But along with this focusability is another property of laser beams that is perhaps more responsible for their esoteric reputation: most laser beams produce light of a fairly well-defined wavelength. Although most of us are familiar with the concept of the wavelength of light, when the discussion moves to the relationship between wavelength and energy (which is at the heart of all branches of physics) many non-scientists and some engineers will disengage. However, this important relationship is central to understanding why clear polymers now can be laser welded without the complications of using different colors or additional inks and dyes.
Short infrared thulium fiber lasers
Until recently, significant average power for laser materials processing in industry only was available from a very limited number of laser types – either solid state lasers emitting in the near infrared 1.07 µm wavelength regime or carbon dioxide gas lasers emitting at the longer 10.6 µm wavelength regime. The advent of new versions of the standard industrial fiber laser with longer wavelengths now produces up to 120 watts power at an intermediate wavelength regime known as the short wavelength infrared regime. This 2um wavelength is achieved by using an alternative rare earth element, known as a dopant, in the fiber in which the laser beam is generated.
The basic laws of physics tell us that as wavelength increases, photon energy is reduced. This means that when different materials are irradiated, the response of these materials also differs. Because specific photon energies are absorbed by particular molecular bonds via a resonance mechanism, this longer wavelength is absorbed differently by many different molecules.
Of particular interest to those in the field of medical polymers is the improved absorption in the C-H molecule, which is the background chain of all organic polymers. The end result is that absorption of this laser beam in clear polymers is greatly increased to the point at which highly controlled melting through the thickness of optically clear polymers is possible.
Why not CO2 lasers?
In the case of older-technology CO2 lasers, the emitted wavelength is very much longer and absorption is often close to 100 percent. This wavelength has many advantages if lasers are to be used for cutting polymers, but for welding polymers where a controlled melting through the thickness of the material is required, this high absorption is a severe disadvantage. With absorption occurring at the top surface of the part, melting into the part to produce a deeper weld takes an unrealistically long time.
Cutting thin polymer films
Recent trials also have shown that although the absorption of most thin materials may be inadequate to produce efficient ablation and cutting, there are some slightly opaque films in the 50-200um thickness range that can be readily cut by this technique.
As the laser beam is converted into heat when absorbed by polymers, any polymer or polymer combination that is thermally weldable by techniques such as ultrasonic or RF also can be laser welded.
Real world applications
There now are a number of areas where real world applications are developing, including medical devices; typical microfluidic devices; and consumer products.
Softer, flexible tubing materials such as PVC and many of the newer non-PVC substitutes such as TPEs also can be welded (Reference Figure 1). These materials are difficult to weld by any other means. Although the rules of chemical compatibility cannot be changed completely, the fine, localized temperature control provided by the laser process sometimes can provide surprising results when joining polymers with limited compatibility.
Microfluidic devices where fine joint lines are required (Reference Figure 2). The single-mode, highly focusable fiber laser can provide very narrow melt lines with limited heat input to minimize distortion of micro-channels when providing hermetic seals. Having said that, the strength limitations of a 100um-wide weld joint in any polymer needs to be taken into consideration when these components are designed.
Consumer products such as twin walled fluid containers, drinks bottles and baby bottles. Some of these products can be expected on the market fairly soon. The transcolors (such as the blue component shown in Figure 3) usually only absorb slightly more in the 2um spectral regime than do clear polymers. Hence, these also are highly weldable by this technique.
The availability of this new wavelength at high average power has led to a greatly improved and simplified technique for laser welding clear polymers for the medical device and other industries. For the polymers that are of most interest for medical devices, no additional absorbers are required to produce almost invisible welds in most, if not all, thermoplastic polymers used in the medical device industry.