Welding plastics with near-infrared lasers (infrared heat welding of plastics)

Improvements in laser performance and cost-effectiveness have led to their wider use for welding thermoplastics. The increasing use of bonding plastics in commercial and consumer products is expected to provide many years of continued growth for this application.

Infrared laser weldingInvolves joining two parts by locally melting and fusing the materials at their common interface. Like metals, thermoplastics can be softened or melted by heat, however, there are a number of important differences between welding metals and thermoplastics. Metals absorb the most common laser wavelengths, but are excellent conductors of heat and require very high local temperatures before they will melt and weld. In contrast, many plastics are transparent at these same laser wavelengths and are notoriously poor conductors of heat. Furthermore, thermoplastics soften or melt at relatively low temperatures. Furthermore, plastics are best welded by reaching a highly softened state rather than a true liquid melt.

These differences affect how laser welding is done in plastics. Most plastic welding applications involve transmitting a laser through one of the parts to be joined. The second part then absorbs the light, producing intense localized heating and softening only at the interface between the parts.

In order to develop 鈥渢hrough鈥?welding applications, it is necessary to understand the transmission/absorption characteristics of the plastics to be joined. Since cost is often a driving issue, most plastic welding is done in the near-infrared region, where economical lasers with the required power levels (a few watts to tens of watts) are available. The near infrared (IR) absorption properties of a material can be measured with a spectrophotometer, but it is usually best to just use a laser and a power meter. Since absorption does not always scale linearly with power, it is important to measure absorption at the expected power level. Furthermore, most job shops and factories simply do not have spectrophotometers on site.

Ideally, one material would be completely transparent at the laser wavelength, while the other would exhibit very high absorptivity. In practice, most clear plastics have an absorptivity of less than 10% through an inch of material. As a rule of thumb, absorption levels of up to 20% per inch of thickness can usually be tolerated. This amount of absorption can produce modest overall heating but will not significantly affect the overall process.

Once the light is transmitted to the weld interface, how do we achieve high local absorption given that the two parts must be of similar composition to obtain a strong weld? The most common approach currently used is to dope the second part with carbon black filler, which produces very high absorptivity without significantly affecting overall strength.

The Practical Problem

During the welding process, pressure must be applied to the joint to hold the parts in place and limit the expansion of the heated polymer. Intimate contact also allows heat to flow from the absorbing material to the non-absorbing material, allowing both to soften sufficiently. Typically, a clamping force of between 40 and 60 psi is sufficient, which is fairly modest compared to other welding processes. This clamping pressure is applied by using a block of transparent material, such as glass or polycarbonate, or by using an opaque tool with a through hole for the laser beam. The weld is then created by physically moving the parts and/or scanning the laser beam.

Other practical considerations are part size and welding speed. For lap joints, the laser typically penetrates thin material and, in theory, there is no clear limit to part size. A modest focus (800 渭m spot diameter) is typically used and an extended weld area is produced by simply translating the beam or the part. However, for butt welds, the laser must penetrate the entire width of the transparent part. Furthermore, for butt welds, the laser must weld the entire edge thickness of the part simultaneously. Additionally, laser beams can easily pass through polycarbonate that is several inches thick, but for other plastics such as PTFE, polyethylene, and polypropylene, transmittance is typically limited to part thicknesses of a few fractions of an inch. Depending on the laser power, weld width, and material thickness, welding speeds can be as high as tens of meters per minute.

Although robots and translation stages can weld larger parts, in practice laser welding is best suited for parts with a maximum dimension of several inches. In addition, larger parts require higher-powered lasers to achieve acceptable weld times, but the cost of these lasers offsets some of the advantages of laser welding.

Typically, the laser beam is delivered to the welding fixture via an optical fiber. This offers an alternative to scanning the beam in situations where identical parts are being produced in batches. Specifically, the laser light is delivered via a beam that is shaped to directly create the contour of the intended weld. Circular, rectangular, and even delicate, curved contours can all be produced this way.

The picture shows two clear polycarbonate-type plastic sheets (coated with Clearweld film) welded using a 30W, 810nm diode laser system. The picture below shows an acrylic business card holder sealed using the output of a 50W multimode Nd:YAG laser. The top piece is clear and the other is black.

Advantages of Laser Welding

Laser welding of plastics offers many advantages over other welding methods and even adhesives. First, there is no physical contact between the heat source and the parts, so the product is clean and free of debris or overheated material. Laser welding only softens the plastic, and the weld area is completely surrounded by the two parts being joined. Therefore, the process does not produce fumes, which can be an important safety advantage. In contrast, many adhesives produce toxic fumes during the curing process. Laser welding also offers the advantages of speed, flexibility, and controllability 鈥?regulating the energy reaching the weld area is very simple.

Small static welds are typically completed in less than a second, with thin films welded at speeds of up to tens of meters per second; therefore, laser welding is as fast or faster than any alternative process. Because it is an optical technology, it offers the greatest design/redesign flexibility. In addition, the laser energy can be directed precisely to the weld site with little peripheral heat damage, making this technology an excellent choice for delicate parts.

Laser welding also offers flexibility in the type of weld that can be created. By using tightly focused, short focal length optics, the process can create fine welds up to 100 渭m wide. Alternatively, with higher powered lasers, the beam can be defocused to produce welds as wide as 10 mm.

Choice of Laser Technology/Wavelength

A wide variety of thermoplastics can be laser welded, including polyethylene, acrylics, and polycarbonates. All of these materials exhibit low absorption throughout the near-infrared spectral region. This means that they can be welded equally well with direct diode or 1064 nm Nd:YAG and Nd:YVO 4 lasers at wavelengths of 810鈥?50 nm. In both cases, it is best to operate the laser in CW mode rather than pulsed mode. This is because welding speed is ultimately determined by average power and total energy, both of which are higher in CW operation.

As a rule of thumb, diode-pumped solid-state lasers at 1064 nm offer higher beam quality than direct diode lasers. Therefore, they are the best choice for 鈥減recision鈥?applications where the beam is scanned directly using a galvo mirror. Here, the beam can be scanned at speeds of up to 6 m/sec. This is faster than the thermal hold time of the plastic and therefore allows a shaped weld to be created simultaneously - the time average of the plastic experiencing the motion of the laser beam.

Direct diode systems have a lower cost per watt than diode pumped systems. In addition, these lasers are typically available in very small packages, which is an advantage for integration. However, direct diode lasers output from multimode fiber offer relatively poor beam quality. This can then be re-imaged into the weld area using re-collimation and focusing optics. Using a 25mm focal length lens, this produces a typical spot size of about 600 渭m in diameter. Alternatively, the fiber can be coupled into a shaped beam as previously described.

In Summary

In summary, there are many advantages to using lasers to weld plastics. As the cost per watt of solid-state infrared lasers continues to improve, we expect this to become an important application as demand and user awareness grow.