The finer elements of laser tube cutting

Figure 1
Ever hear of a femtosecond laser? It can mean a world of difference for those manufacturers involved in laser cutting stents and flexible tubes with intricate features.

Many adults are old enough to remember a time when heart problems were a death sentence. Thankfully, times have changed.

Today a small device such as a stent (see Figure 1), which is placed in an artery as part of a coronary angioplasty surgery and helps to restore blood flow through damaged arteries, can mean a new lease on life. With the growth of minimally invasive surgery, the medical community has an incredible need for these kinds of laser-cut stents and flexible tubing.

While legacy stent and tube cutting systems have performed well during recent decades, new cutting technologies coming onto the market offer faster and better cuts, with higher production rates and new and unique cutting capabilities.

Replacing Legacy Cutting Systems

The pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers used in the past two decades have definitely been great workhorses. Unfortunately, the original pulsed Nd:YAG lasers that remain in operation can’t match new laser capabilities and are increasingly difficult to service. While many of these systems have been upgraded to fiber lasers, they are still beset with older support systems and slow and aging controllers with legacy software.

Simply put, the laser, mechanicals, controller, software, water systems, and automated tube loader technology have all moved on. All of these components contribute to better cuts with higher production rates and less downtime.

Laser. The pulsed Nd:YAG lasers used in the past have been superseded by fiber lasers with better beam quality that does not change with pulse energy and average power. This beam provides a smaller and more consistent focused spot size, which results in tighter cutting tolerances and, with spot sizes down to 10 microns, the ability to cut much finer detail features. These lasers provide pulse frequencies over 5 kilohertz and pulse widths down to 20 microseconds to enable energy input optimization for a variety of tube materials and wall thicknesses. Higher frequencies can be used to maximize acceleration and speed for a range of part thicknesses.

From an operational standpoint, fiber lasers have a number of advantages. They are air-cooled, run off single-phase 240-volt electrical power, and have diodes with lifetimes that are greater than 70,000 hours. Figure 2 shows an example of a tube produced by new laser tube cutting technology and a close-up of laser tube cutting.

Fiber lasers use microsecond pulses and have a cutting speed and edge quality that are sufficient for many applications. The femtosecond laser offers laser pulses that are under 400 x 10-15 seconds, or about 1 million times shorter than the fiber laser. The very short pulse duration, combined with peak powers into the gigawatt level, allows special cutting capability. The fiber laser has a fusion cutting mechanism, whereby the laser pulse melts the metal, which is then ejected from the part by a coaxial high-pressure gas. The very high peak power of the femtosecond laser and a pulse duration that is shorter than the material’s conduction time create a very nearly pure vaporization mechanism. Since there is no melt creation during the cutting process, there is no burr, which is very beneficial for such materials as nitinol.

Take the example of the ubiquitous coronary stent, one of the first devices manufactured with both Nd:YAG and fiber lasers. First, the part has to be machined, honed, or cleaned on the inside with a mechanical tool and finally deburred. Then a chemical etch process must be performed to clean up around the edges, followed by an electropolishing step. These steps are quite time-consuming. They also can cause the part to become brittle or deformed and may result in microcracks. Yields tend to be in the 70 percent range, which means the loss of a considerable amount of end product, which can be a significant material cost in the case of nitinol.

By contrast, the femtosecond laser produces a burr-free cut that drastically reduces the number of time-consuming postprocessing steps. The part is machined and then undergoes an electrochemical process to round the edges. The integrity of the part is improved, and yields can be closer to 95 percent. In addition, using a femtosecond laser can be an attractive proposition for fabricators that may be looking to bring the cutting process in-house, but do not want to go through the arduous red tape of also bringing in-house the necessary chemical postprocessing materials and processes needed for fiber laser cutting.

Figure 2
Here water is used during the tube cutting to ensure that the laser does not harm the opposing wall’s interior surface.

With its minimal heat input, the femtosecond laser is suitable for cutting small features in small parts with good edge quality and feature definition. Figure 3 shows some examples of femtosecond laser cutting.

The majority of stents and tubing are metal. However, Food and Drug Administration-approved polymer stents and scaffolds are now on the market, which can be cut only with a femtosecond laser. The fiber laser does not absorb well enough in the polymer to make quality cuts. The femtosecond laser has such great photon density that it is absorbed by the polymer material through a process known as multiphoton absorption, which makes cutting possible. This cutting can be further enhanced by using a green wavelength over 1 micron, which provides better cut quality, faster speeds, and a larger processing window.

Software, Controllers, and Mechanicals. New digital motion controllers and improved system acceleration enable manufacturers to follow the programmed tooling path with fewer errors and at fast speeds. In most tube cutting applications, the limiting factor for cycle time is the motion, specifically the rotary axes, and so mechanicals and controller performance improvements are a key part of maximizing production.

As part of day-to-day operation, the interaction of the operator with the control software can optimize efficiency in setup and process monitoring. The use of large-screen monitors has facilitated single-screen, operator-oriented interfaces.

In addition, inline sensors, gauges, digital flowmeters, and valves can report on the status of all process-critical parameters, including assist gas pressure, water flow, and pressure. Not only are these vital process conditions monitored, but values also can be set with alarms and error states for low levels to avoid wasted material stock, equipment damage, and downtime.

Water System. In many legacy laser designs, the water system was a weak point, requiring constant attention and maintenance to keep the machine running. Such issues as small water tank sizes, pumps with short lifespans, and lack of internal flow monitoring contributed to systems that could be looked upon as somewhat unreliable. Compounding these issues was the fact that it was often difficult to change the water filters.

Today newer systems have a 10-gallon tank size, four-level debris filtering, intelligent programmable flow valves, multiple solenoid switches to prevent large water leaks, and drawer-mounted hardware that enables filter changing in seconds. The user interface gives the operator all the necessary information, along with precutting safeguards and go/no-go limits, to ensure that all is well.

Automated Tube Loaders. The standard stent and tube cutter is loaded manually with tubing that is typically up to 3 meters long. The cutter then cuts parts and advances the tube according to the program. When most of a tube has been cut, the remainder is removed and a new tube is loaded.

With more pressure to improve productivity and minimize labor costs, many manufacturers are now using automated tube loaders to feed the laser cutting machine. While it is not recommended that these machines be operated in a “lights-out” mode, they can be helpful in reducing labor allocated to the machine.

Open-architecture System Design

A key part of any system is making the hardware usable for the operator every day. One feature that helps to accomplish this is composite over granite for better vibration damping. Because the composite has a uniform internal structure, it can be mechanically modeled and optimized for vibration isolation, load-bearing capacity, and deflection under load. These features enable a cantilever arm to support the focus optics and movement across the axes. As a result, this type of machine has an open design, and the operator can access the work area easily (see Figure 4).

Better Technology, Better Parts

The design and cutting performance of the latest-generation stent and tube cutting systems offer significant advantages and capability over legacy machines. Fabricators that take advantage of these technology advances can look forward to increased productivity with these newer user-friendly and reliable tools.

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