Lasers shine for thermoplastic welding

Tom Kirkland
Business Unit Manager
Dukane Corp.
Intelligent Assembly Solutions
St. Charles, Ill.

Unlike laser welding of metals that focuses light on a joint edge, thermoplastics use a through-transmission technique. Here, laser light passes through a part that is transparent to the laser wavelength (typically near infrared) and is absorbed by a part filled with carbon black or other colorant. The absorbent part melts and conducts heat to the transmissive part, welding the two together.

There are two kinds of through-transmission laser welding: collapse and contained. In the collapse mode, laser light illuminates and welds the entire joint width. Clamp force applied to the parts squeezes molten material from the immediate joint area as flash. No particulate forms in most cases, making the technique suitable for high-cleanliness applications. Clamp force must be carefully controlled. Too little and the parts may not fully touch one another during welding, which can hurt weld quality. Too much clamping force pushes excessive material from the joint, causing a thinning of the bond line and poor molecular orientation, both of which lower weld strength. The collapsemode process tolerates air gaps to about 100 m or more. The process better compensates for surface inconsistencies than contained welding, though not as well for part warp.

Contained welding shines laser light on an area smaller than the total wall section of a weld. Molten material stays within the part edges, hence the name. The contained approach eliminates both flash and particulates, though gaps exceeding about 75 m compromise joint strength. A minimum amount of clamp force holds parts for welding, the upper limit of which is set by part structural strength. Clamp force, therefore, is not considered a process variable. Because contained welding does not produce flow-induced molecular orientation, it is theoretically capable of weld joints that are nearly as strong as the parent material.

There are four ways to introduce laser light into a thermoplastic assembly: contour, mask, simultaneous, and quasi-simultaneous.

Contour welding trains a single point of laser light on the workpiece and is limited to contained-welding mode. Systems move the beam, the part, or both. They are energy efficient and highly programmable, have low tooling costs and quick setup, but relatively long cycle times.

Curtain-mask systems drag a line of high-intensity laser light — from banks of diode lasers — across a glass mask. A coating on the glass shades the workpiece-from the laser light except where it is needed for welding. Much of the laser light is wasted in most cases, though process simplicity makes up for low efficiencies. The method works only for near-planar joints and contained-mode welding. Tooling costs more than for contour welding and masks need regular maintenance and replacement. And setup typically takes longer for curtainmask systems, though cycle times are somewhat shorter.

Simultaneous welding systems use an output lens shaped specifically for the part to be welded. Multiple diode lasers deliver light (through bundles of optical fiber) to the lens. Such systems are typically designed for collapse-mode welding, though they can accommodate 3D geometries in contained-welding mode. Short cycle is a key advantage of the technique. However, lenses and fiber bundles need regular maintenance, and balancing diode-laser output can be a challenge. Tooling costs tend to be high and setup times, long.

A quasi-simultaneous system uses a pair of servocontrolled mirrors (galvo head) to steer a single laser beam on several trips around a joint. Each trip raises joint temperature so an entire part perimeter melts at about the same time. High heat loss associated with the technique requires extra laser power to overcome. Quasi-simultaneous systems typically work in collapse mode and joints must be nearly planar. Galvo heads limit beam travel to their field of illumination, typically about 200-mm diameter. Cycle times can be about equal to that for mask welding, but are longer than simultaneous welding. Tooling costs are on par with mask welding, but galvo-head systems are more programmable and take less time for setup.

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Dukane Corp.
St. Charles, Ill.
www.dukcorp.com