Automation speeds clear-plastic welding

EFD's stainless-steel dispensing valve accurately deposits the solvent-based Clearweld liquid.

Manufacturing advances, including inkjet and tabletop dispensers, now make this a viable alternative in low and high-production applications.

In laser welding, a part absorbs electromagnetic energy and converts it to heat. However, plastics do not absorb near-infrared (NIR) light, so NIR lasers normally pass directly through them. In the past, the most common method to laser-weld polymers had been to disperse carbon black throughout the bottom substrate. It absorbs light near the interface and generates enough heat to produce welds. Unfortunately, this renders the plastic opaque.

The Clearweld process, on the other hand, lets lasers weld clear plastics without opaque colors or additives. The key is a series of virtually colorless materials that absorb NIR radiation. A thin layer of Clearweld liquid on the plastic joint concentrates laser energy at the interface. The material absorbs light, produces a localized melt of the substrates, and creates an instant weld requiring no cure time and with no particulates or visible color. Invented by TWI, a U.K.-based R & D organization, the system is available from Gentex, Carbondale, Pa. (www.gentexcorp.com).

Making the process commercially viable requires a means to accurately place NIR-absorbing material on the joint interface. It is imperative that the liquid is precisely deposited on the substrate because the laser heats only the light-absorbing material. Gentex partnered with several manufacturers to meet these requirements.EFD, East Providence, R.I. (www.efd-inc.com), for example, makes tabletop systems that accurately and repeatably dispense Clearweld liquids. The machines can be configured to meet a wide range of manufacturing processes.

EFD three and four-axis dispensing systems follow compound arcs and patterns on different planes. Controls are built into the robot base and include a user interface. A microdot-dispensing valve works with the positioning system to accurately deposit liquid. The valve's stainless-steel construction stands up to the solvent-based Clearweld material. The compact systems are suited for process development and low-to-medium volume production with limited space requirements.

A dispensing system based on commercial piezoelectric-inkjet printing technology is available from Xennia, Royston, Hertfordshire, U.K., (www.xennia.com). An industrial PC with touchscreen menu controls dispensing in virtually any pattern. The print head's 500 nozzles have a printing swath of 70 mm. Up to four heads can be stacked for wider coverage. Maximum recommended line speed is 100 mm/sec, and an integrated IR lamp quickly dries the parts. The system integrates into a variety of industrial processes and production lines for higher-volume applications.

Other factors affecting the Clearweld process include contact between the substrates at the weld interface. The process requires a certain amount of clamping pressure that varies with material and surface condition. This ensures sufficient melt flow to weld components.

And it takes high-power diode lasers to provide the electromagnetic energy necessary for generating heat. Typical wavelengths range from 940 to 1,000 nm. Power levels and configurations depend on the application. It is critical, however, that laser wavelength match the Clearweld material's absorption wavelength.

Fine-tuning these parameters produces welds stronger than the parent material, at processing speeds that equal or exceed other joining techniques. The process is suitable for a range of materials, such as ABS, PEEK, polyurethane, and polyester. Welding dissimilar materials and some thermosets is also possible. Applications include medical products, packaging, automotive components, consumer products, electronics, and textiles.