Owing to their very short pulse durations - sometimes less than the target material's conduction time - and very high peak powers, ultrafast (femtosecond and picosecond) laser systems offer unique material processing possibilities such as “cold machining” of parts with no/negligible heat affected zone. In fact, this short pulse width/high peak power, combination enables processing capability of almost any material, including metal, plastics, ceramics or glass.
The machining method works by sublimation, directly vaporizing the material from a solid. This offers a long list of advantages that simply cannot be produced by any other processes. We start with a near zero heat affect and then add minimal burring and debris and say "goodbye, post processing!." As if those two were not enough, add high dimensional accuracy and the ability to produce very high quality small features.
The down side? Ultrafast or ultra-short pulse lasers systems are expensive – typically in the range of $400,000 and up. So be sure to consider whether the return on investment (ROI) from cost reduction and/or unique processing capabilities makes it worthwhile.
How short is ultra-short?
Let’s start by defining what we mean by ultrafast or ultra short pulse lasers. The term is divided into two main categories. A picosecond laser emits optical pulses with a pulse duration of around 10 picosecond – just over one trillionth (10-12) of a second, or one millionth of a microsecond. A femtosecond laser emits pulses that are around 400fs, less than one trillionth of a second in duration.
One more thing to keep in mind: the term “ultrafast” does not refer to the material removal rate. In fact, quite the opposite is true. These lasers excel at processing material thicknesses of less than 0.01-inch (250 microns). Thicker materials can be processed, but cycle time may be affected.
Which one is right?
Processing differences between picosecond and femtosecond lasers are sometimes subtle and sometimes more obvious. Here’s what I mean: when used to process metals, the difference is subtle. The femtosecond laser will give you zero topside burr, with slightly better defined features and lower surface roughness. When drilling or machining small features, it laser provides more efficient processing and less cycle time, because there are no material conduction losses. The femtosecond laser can also process a greater range of plastics more efficiently. Quality comparison between picosecond and femtosecond is material-dependent. When the absolute best quality is needed, femtosecond is the clear choice. However, picosecond lasers tend to machine faster, so ask yourself, “How good is good enough for the process?”
Figure 1 compares the two lasers machining a 100-micron wide channel in metal. Using a similar material removal method and cycle time, the FS laser produces cleaner edges and a smoother base.
As shown in Figure 2, shorter pulse durations can more efficiently process plastics, specifically shown is the scribe depth in polytetrafluoroethylene (PTFE) as a function of pulse width for same pulse energy of 100mJ and 1064nm wavelength.
Ultrafast wavelength choices
Both lasers offer wavelength choices of infrared (IR), green (GR), and ultraviolet (UV). Certain wavelengths work best for specific materials and/or one can also select the wavelength based on a particular feature size required. For example, use picosecond IR for glass cutting and picosecond or femtosecond GR for medical plastics like Pebax®*.
In the final analysis, understanding which laser works best for the application can only be determined through parts testing. As part of defining the application and system, AMADA MIYACHI AMERICA typically runs samples on both femtosecond and picosecond lasers and on multiple wavelengths. The final decision is made after an iterative process, which usually includes a number of short part runs.
*Pebax is a registered trademark of the Arkema Group.