For decades, lasers have been examined as an alternative heat source for automated fiber placement (AFP) of composites. Until recently, the belief was that lasers were too expensive and inefficient to get any real benefits over hot gas or infrared lamps. Modern manufacturing techniques and demand have driven the price of high powered lasers down while driving their capabilities up and up.

In order to be useful in processing thermoplastic composites there were several issues (beyond cost) with lasers that had to be overcome. The first issue was that lasers had to be small enough and hardy enough to be mounted on a robotic processing head. Earlier lasers had many moving, delicate parts. Newer lasers use a solid state design, eliminating the moving parts and significantly reducing the size of the required equipment. This increase in toughness and reduction in size allowed the laser to be mounted to Automated Dynamics' robotic platform.

Image #1. Automated Dynamics prototype laser heating system with thermal camera temperature control

The next serious issue with earlier lasers was that they tended to damage the composite matrix. This was primarily brought on by several different laser characteristics: One was that, historically, to reach the energy required to melt engineering grade thermoplastics, lasers had to pulse their output. This leads to spikes in energy intensity that tend to blast away the outer layer of resin on the prepreg rather than gradually raise its temperature. Newer laser designs offer a continuous wave (CW) power output which minimizes this issue.

Graph #1:Pulsed Output vs. Continuous Output Laser Intensity

Another source of matrix damage is that in addition to pulsing, most laser applications (cutting, welding metal, drilling holes, etc.) require a very small beam size on the order of a few microns. If such a beam were used to heat a 1" wide strip of prepreg in a typical AFP process, it would need to be scanned back and forth across the area. Even with a CW laser, this results in the same peaks in intensity that tend to ablate the surface of the composite. Automated Dynamics has developed specialized optics can be used to create a beam that is matched to the geometry of the raw tape. This creates a large zone of applied energy that can be used to evenly heat, rather than ablate, the prepreg.

Image #2: Laser Heating of a Thermoplastic Structure

Image #3: IR Image of Laser Processing. Note the even energy
distribution.

Finally, early lasers heated the matrix itself. It is very difficult to find the balance between emitting enough energy to melt matrix, but not too much that would cause matrix damage. New lasers produce light at a different wavelength. This wavelength passes through the matrix and heats the composite fiber. Then the heat from the fiber melts the matrix. This indirect energy transfer significantly minimizes any matrix damage.

With a properly designed laser heating system, huge gains can be realized in processing speed, reliability, energy efficiency, and even performance of the finished part. The laser is capable of applying much more energy to the composite more quickly than other methods so the part can be fabricated faster. Furthermore, the laser can be turned on, off, or change intensity almost instantly allowing for improved control of the AFP process. Other types of heat sources often require many minutes to be slowly ramped up or down in temperature to avoid damaging their heating elements.