Better Laser Polymer Welding

The unique properties of polymers – lightweight, corrosion resistant, biocompatible, electrically and thermally insulating, and low cost – are driving their increased use in medical devices, automobiles, consumer electronics, and more. Assembling polymer components often involves welding parts together. For demanding applications, the welding process must achieve high mechanical precision, no part distortion, minimal particulate debris production, and excellent bond strength.

Laser welding delivers on all these requirements. And one type of laser welding in particular – quasi-simultaneous (QS) through transmission laser welding (TTLW) – offers the best results in applications where part distortion must be completely avoided, or those involving complex weld seam geometries.

But putting a consistent, high-quality QS welding process into production requires a couple of steps. First, careful process monitoring, usually including closed loop clamping force control and thermal imaging. Second, a part design which lends itself to the process. 

Collapse control

A popular form of QS welding for precision applications is the “collapse rib” method. This technique can produce strong welds with good cosmetics while avoiding the need for post-processing to trim or remove flash. Plus, this method can yield good results even if the parts aren’t perfectly flat or tightly toleranced.

In the collapse rib process, one part has a thin ridge which sticks out and mates into a corresponding (and slightly wider) groove in the other part. To weld the parts, the ridged part is placed in a holder, and the grooved part is put on top of it. A laser heats the rib to partially melt it while clamps actively press the two parts together. Molten material flows and fills some of the gap between the parts. The resolidified material forms the weld joint. 

One key element in successfully implementing this process in production is “collapse control.” This means actively monitoring and controlling how much the top part moves down during welding (called the “collapse height”). 

Coherent has developed its own unique form of collapse control that consistently delivers superior results. To implement it, our ExactWeld 230 P and ExactWeld IP polymer welding tools all incorporate force-sensing transducers. These continuously measure the pressure the clamps are applying to the top part. The collapse height (the distance the top part has moved) is measured, as well. These signals are used as feedback to control the servomotors which drive the clamping. 

No other manufacturer employs this method. Some use the servo motor current as a feedback signal, but this doesn't deliver the same measurement accuracy and degree of dynamic control. Some just measure the collapse height, rather than the clamping force. And some use pneumatic actuators for clamping; these can’t provide the precision or response speed needed to accurately control the clamping, even if they did measure the force.

Measuring the true clamping force enables Coherent welders to control it as needed to respond to the exact conditions of the part being welded. It can correct for part-to-part dimensional variations, changes in material absorption characteristics, fluctuations in the ambient environment, and much more. This is the key to consistent results. It also delivers flexibility because it enables the clamping force to be purposefully varied, if desired. For example, sometimes it’s better to apply less pressure during the cooling phase in order to control part stress. 

Monitoring both the actual clamping force and the amount the part has "collapsed" during the process is key to getting consistent welds, even when there are part-to-part variations.

The overall result is improved yields, quality, and consistency – because it eliminates process variations caused by changes in the machine, environment, or parts themselves. Another benefit is that a welding system can be configured in one location – and then shipped to another – and still perform the same. Additionally, it compensates for machine-to-machine variations. This allows a process to be configured on one system and then transferred to others with consistent results and makes processes scalable.

Thermal imaging is cool

Another critical tool for successful polymer welding is thermal imaging using an infrared camera system. After welding is complete, the thermal vision check images the entire weld seam. Any breaks in the image along the path reveal gaps in the weld seam, and variations in line thickness indicate weak spots in the weld. 

This allows the manufacturer to immediately identify and reject bad parts. It saves the manufacturer money because it prevents building more value into a bad part. And, of course, it prevents shipping a bad part to a customer.

Design for manufacturing

The other part of successful polymer welding happens before production – during product development. It’s important that the part design allows enough space at the right locations for the clamps to engage and deliver the required downward force during welding. Plus, the weld tooling must be configured so that the clamps don’t block the laser beam from reaching the weld path at any point.

The dimensions and shapes of the ridge and groove must provide enough molten material for the welding process. They must also have enough space to contain the molten material that is produced. Additionally, the part design must permit a sufficient collapse height. The goal is to achieve a strong weld with good cosmetics, while eliminating the need for post-processing to trim or remove flash.  

With extensive experience in polymer welding, applications engineers at Coherent Labs can provide guidance in the design of both the parts and the welding tooling. Plus, Coherent can assist in developing the process recipe. This is valuable because there is often a fairly large “parameter space” which must be explored for laser welding. Prior experience enables our applications development staff to quickly find the optimum approach. 

Finally Coherent can aid in installing equipment, getting the process running, and training staff. Utilizing this combination of expertise, equipment, and support available from Coherent provides the best way to realize all the benefits of laser polymer welding in production.

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