CFRPs’ make-up makes them tough to machine

January/February 2013 Volume 6 Issue 1

By Ronald D. Schaeffer
CEO, PhotoMachining, Inc.

Carbon-fiber-reinforced polymers are composite materials that offer numerous benefits as workpiece materials. Reaping those benefits, however, requires effort. This is because CFRPs are non-homogeneous, which makes them tough to cut.

Carbon nanotubes represent the ultimate in carbon-fiber toughness. These reinforced polymers—used in the manufacture of Lockheed’s F-35 Lightning II warplane—are several times stronger and more costly than regular carbon fibers.

And although carbon-fiber materials cost more than metals, they outperform metals, especially in applications requiring a high strength-to-weight ratio and high rigidity.

Besides carbon, other fibers often are added to a CFRP’s weave, such as Kevlar, aluminum and glass. This mix can present processing problems by any method, including lasers.

Laser Machined Epoxy-Carbon-Fiber

Figure 1: Edge quality when cutting carbon-fiber-reinforced materials varies depending on the laser’s wavelength and pulse width. Shown are 75µm-wide cuts made in CFRP by various lasers. All images courtesy PhotoMachining.

Lasers for CFRPs

One-micron lasers work well on CFRPs if the material is thick and the ultimate in edge quality is not needed. Ultraviolet lasers with 355nm and 266nm wavelengths and diode-pumped solid-state lasers (nanosecond pulse widths) cleanly cut and drill CFRPs as well. However, the thickest material UV and DPSS lasers typically can process is on the order of a few hundred microns.

The use of ultrashort-pulse lasers to process composites is growing because they can ablate almost any material while generating minimal thermal side effects. This offers distinct and unique advantages.

Figure 1 shows 75µm-wide cuts made in 250µm-thick CFRP using various laser systems, all of which incorporate galvanometer scanning heads. The cuts were made without co-axial gas assist, and the parts were not cleaned.

laser machined carbon fiber ribs

Figure 2: High-resolution features laser-cut in 250µm-thick CFRP.

The long-wavelength CO2 laser (10.6µm)cut the material, but edge quality was poor. The 1,064nm fiber laser cut the material cleanly but left a charred edge. The 355nm UV laser left a relatively clean edge, but there was evidence of local charring. The shorter-wavelength 266nm laser and the shorter-pulse-width 355nm laser (ps) yielded the best edge quality.

If a CFRP part’s design calls for microscopic features, it’s crucial that edge quality be very high and that no residual heat develops during processing. Heat can distort the workpiece material.

Material issues

The most common non-homogeneous fiber-composite material is FR-4 (Flame Resistant Class 4), an epoxy resin embedded with glass fibers. It is used to manufacture printed circuit boards and a host of other electronic and mechanical devices. FR-4 offers many advantages, including relatively low cost, mechanical rigidity, excellent dielectric properties and water resistance. It can be drilled with far-infrared and UV lasers.

A number of difficulties arise when laser machining FR-4. Glass fibers ablate at a much higher energy density than epoxy, and this higher energy can sometimes cause epoxy-related problems, such as undercutting or burning.

A bigger challenge, though, is an outgrowth of the fibers being distributed in a non-uniform way. If a large number of holes must be laser-drilled, for example, there will be areas of the workpiece material where there is no glass, areas where there may be a single glass fiber and areas where there are crossed fibers or even bundles of crossed fibers. All of these contingencies must be taken into account before lasing can begin. Plus, laser drilling will need to be performed at the slowest speed—specifically, the speed required to cut crossed-fiber bundles, not pure epoxy.

Carbon-fiber epoxy up to several millimeters thick is used in engine components to reduce noise. It presents some of the same problems as FR-4, but the carbon fibers also tend to propagate any heat generated by the laser process. This requires that extra care be taken to avoid burning the epoxy around holes.

Large numbers of holes with diameters around 1mm are laser-drilled in these engine components. IR lasers are the best choice for the job, as the material is relatively thick. Internal stresses that may develop within the hole won’t pose any issues.

When carbon-fiber materials are thin and feature sizes tiny, a whole new set of problems arises due to material non-homogeneity. Internal stresses are relieved when cutting fine features, which can lead to distortion. Distortion, in turn, can be exacerbated by residual heat generated during cutting.

Because an ultrashort-pulse laser doesn’t put heat into the part, it offers definite advantages when cutting small features.

Internal stresses and physical distortion sometimes cause the workpiece material to “lift” during processing. This creates an effect (“shadowing”) that can impede completion of a through-cut.

For thin materials, it is sometimes helpful to epoxy the material to a flat substrate, preferably one that is transparent to the laser wavelength used for processing. This prevents localized distortion while lasing. After the glue dissolves, internal stresses are relieved in a more uniform fashion when removing the material from the carrier.

Figure 2 shows thin ribs cut by a picosecond UV laser in CFRP. The ribs are 30µm wide and 200µm apart. The beam diameter was 25µm and material thickness was 250µm, giving an aspect ratio of 10:1.

The ribs were cut cleanly and exhibit neither distortion nor obvious effects of heat—proof that though CFRPs and other non-homogeneous materials are tough to process, the job can be done when the right laser and techniques are applied. µ

— R. Schaeffer