Distinguishing laser welding of plastics with and without colour additives

Key Highlights:

• The laser technique is frequently applied in applications with higher quality requirements like medical technology, electronics, or automotive, which can also afford slightly higher investment costs for machinery.
• Laser welding is widely used in industries with strict quality requirements such as medical technology, electronics, and automotive due to its ability to introduce energy locally and precisely into materials without affecting surrounding areas.
• An alternative application involves welding plastic parts without any colorant, particularly crucial in medical devices where colorants may require material re-qualification.

For welding plastic parts, the approach to using laser light as an energy source to melt the plastics has become increasingly established in many industries. The main advantage when compared to other techniques is that the energy is introduced locally and precisely into the material without influencing areas near the weld seam. Only static clamping is necessary to bring the two parts in physical contact, but no motion relative to one another like in ultra-sonic or vibration welding, which can create particles or damage nearby sensitive items like electronics or reagents. Consequently, the laser technique is frequently applied in applications with higher quality requirements like medical technology, electronics, or automotive, which can also afford slightly higher investment costs for machinery.

Laser welding of plastics can be subdivided into several different process types like contour welding, quasi-simultaneous, simultaneous, or mask welding. In this article, we will distinguish by the wavelength of the employed laser in the classical laser welding process with a laser-transparent and laser-absorbing part and a newer adaptation, which allows also for clear-to-clear welding.

Classical laser welding process: laser-transparent on laser-absorptive

In typical laser welding of plastics, one part must be transparent to the laser, while the other part must absorb it. The laser wavelengths used are in the near-infrared wavelength range, typically between 800 and 1100 nm. Historically, diode lasers at 808 nm were more common, but nowadays, 980 nm lasers are preferred due to their better energy efficiency, allowing for air-cooling instead of water-cooling. 

Since the vast majority of polymers are transparent at these wavelengths, a colourant must be added for the laser to be absorbed. This colourant can simply be carbon (carbon black) to colour it black or other colours with a suitable masterbatch. Almost any colour can be formulated laser-absorbent with the right additives.

For the laser-transparent part, it is ideal if no colourant is added and the polymer remains in its natural form. Depending on the polymer, the part can be clear like glass for amorphous polymers, or translucent and milky for semi-crystalline polymers. While the laser beam can penetrate centimetres of amorphous polymers for welding, the crystallites in semi-crystalline polymers scatter the beam, limiting the possible thickness. Typically, the achievable thickness is in the range of millimetres, but can also be less than one millimetre for highly crystalline polymers like PEEK or PPS. Similar to crystallinity, additives like glass fibres or mineral fillers also scatter the laser and limit the possible thickness of the translucent part.

As the visible wavelengths (400-700 nm) differ from the laser wavelengths, the laser-transmissive part can be formulated in nearly all possible colours to the human eye and still be transparent for the laser. A frequent example in the electronics and automotive industry is a black-to-black combination. The laser-absorptive part is doped with carbon black, and the laser-transmissive part is coloured with a special black pigment, which is only black to the eye but is transparent to the laser, as shown in the image (right).

Clear-to-clear welding process: special long wavelengths

Another application case involves welding plastic parts that do not contain any colourant. This is particularly relevant for medical devices and consumables where adding colourants could necessitate re-qualification of the material. The alternative is to change the laser wavelength to a range where many polymers naturally absorb eliminating the need for colourants. In the 1700-2000 nm range, most polymers absorb through vibrational overtones of their molecular bonds. Depending on the type of polymer, these absorption bands are differently strong and shifted in wavelength.

Within the interesting wavelength range, two types of lasers primarily offer enough power to melt plastics: the standard fibre laser at about 1070 nm shifted by the Raman effect to longer wavelengths at 1725 nm, and the Thulium fibre laser at 1940 nm. While absorption is generally higher for all polymers at 1725 nm, making it more efficient, the 1940 nm wavelength can be more suitable when dealing with thicker upper parts. This is because, at 1725 nm, the laser energy might be absorbed too readily before reaching the weld seam. In these cases, the laser beam is strongly focused on the weld seam plane. Still, most energy is absorbed at the front face of the first part, albeit over a larger area compared to the focal region. The highest energy density is present at the focal point leading to the melting required for welding only around the laser focus.

Since the upper part absorbs part of the laser beam at these longer wavelengths, the melting zone becomes significantly deeper than with the standard process. The laser penetrates deeper into the material and is not solely converted into heat on the top surface of the lower part. Consequently, the spatial resolution of the weld seam is not as precise as with the classical standard process.

As an example, the image below shows an inflatable balloon welded to a tube both based on soft PVC for artificial respiration. It is achieved without additional colour additives using the special long wavelengths. The inflatable balloon requires an airtight connection with the tube to ensure proper sealing within the trachea when inflated. During welding, the tube is rotated beneath the optics, which focuses the laser into a spot, and simultaneously moved along the rotation axis.