Laser cutting benefits
The laser is ideal for medical cutting, because the laser can be focused down to a 0.001-inch diameter spot size that offers a fine non-contact “tool-less” cutting process at high speed and high resolution. As the laser cutting tool does not rely on touching the part, it can be oriented to make any shape or form, and used to make unique shapes. Table 1 lists the technology’s key benefits.
|Table 1 – Laser cutting benefits|
|0.001” diameter “tool” width|
|Non contact process|
|High precision and quality|
|Highly controllable and flexible|
For example, laser cutting is an excellent tool for small tubes, like those used for cannula and hypo tube applications that require an array of features such as windows, slots, holes and spirals. With a focused spot size of 0.001-inches (25 microns), the laser offers high resolution cuts that remove the minimal amount of material to enable high speed cutting according to the dimensional accuracy required.
Also, since laser processing is non-contact, no mechanical force is imparted onto the tubes – there is no push, drag, or other force that might bend a part or cause flex that would have a negative impact on process control. The laser can also be precisely set during the cutting process to control how hot the work area gets. This is significant, because the size of medical components and the cut features is shrinking, and small parts can heat up quickly and might otherwise overheat.
Laser cutting mechanism and equipment
Figure 1 shows the key elements of the fiber laser with gas assist typically used for fine cutting for medical components. A highly focused laser is used to melt a thin sliver of material. While the material is still molten, a 0.02-inch diameter gas jet nozzle that is coaxial with the laser blows away the molten material. The desired features are produced using this continual cycle of melt, then melt ejection. Distance between the laser and the material needs to be maintained precisely. Figure 2 is a closer view of the laser cutter.
Laser cutting is very well suited for the vast majority of materials relevant to potential medical cutting applications, including stainless steels (300 and 400 series, 17-4, 17-7), MP35N (cobalt-chrome steel alloy), and Nitinol. Other materials, such as titanium for implants and platinum iridium (Pt-Ir) commonly used for electrodes, can also be laser-cut, though these materials require a little more care in the setup. With titanium, thermal build up and runaway are concerns that can be mitigated by using pulsed laser output that finely controls part heat input and using a semi or fully inert assist gas. For Pt-Ir, excessive dross on the underside of the cut is the main issue. This can also be improved with optimized pulsed laser cut parameters. Figure 3 shows a comparison of laser cut quality between stainless steel and platinum iridium tubes. The tube cross sections clearly show the dross adhering to the underside cut edge on the Pt-Ir material.
The laser can cut materials with a wide range of thicknesses and can be operated in a variety of cutting conditions to achieve cuts a different material thickness. In virtually all medical cutting applications, the laser is operated in pulse mode due to the requirement for fine feature sizes and high tolerance parts. Considering a typically sized laser of 100W, the duration and frequency of the pulses determine the speed of the cut and also the maximum thickness of the cut. This is also related to the feature size – the finer the feature size, the slower and less thick the cut can be, though this is application specific. A 100W laser can comfortably cut a range of stainless steels up to 0.03-inch thick simply by altering the pulse width and frequency. The thinner the material, the shorter the pulse duration and the higher the pulse frequency. For instance, for 0.008-inch thick material, a pulse width of 50 microseconds and a frequency around 3000 Hz is a good starting point.
The laser excels in tube cutting single sided features such as holes and slots, off-axes geometries, and spirals that are typical features for cannula and hypo tubes. When coupled to an appropriate motion system and integrated laser control, the laser cutting provides a high throughput system for many standard cutting profiles as well as unique profiles. With an effective cut width of around 20-30 microns, it is possible to achieve very high dimensional control, routinely to ± 0.001-inch. When cutting accuracy is less fine, cutting speeds can be rapid.Figure 4 shows a variety of tube cut geometries and features possible with laser cutting.
Cutting an edge that is a 90 degree angle to the part may be sufficient for many medical tube applications, but some applications require cutting parts at an angle other than 90 degrees. Laser cutting offers such off-axes cutting, where cut edges are angled to the surface, and can also accommodate compound angle cutting. Figure 5 shows examples of off-axes cutting of tubes.
Laser cutting limitations
As with any process, laser cutting is subject to certain limitations, primarily cutting certain angles in certain orientations. This relates to part thickness, cut quality and the mechanical interference of the equipment. For example, certain designs of arthroscopic surgery shaver blades require a cut angle of up to 70 degrees to the distal part of the tube, as shown in Figure 6. The wall thickness can be up to 0.015-inch, which requires a through cut penetration of 0.044-inch. Perhaps more importantly, increasing the angle makes the focus spot more of an ellipse shape, reducing the concentration of laser power. This power density reduction, coupled with the requirement to cut a thicker section, leads to a breakdown of process due to insufficient power density required for the increased thickness.
Figure 6 also highlights how the laser head can become impeded due to mechanical interference; in some instances the cutting nozzle may interfere with the tube for high angle cuts. Cut angles of up to 30 degrees to the materials surface are possible; with certain material thickness conditions and required cut quality it may be possible to attain angles up to 45 degrees, but anything above that is not attainable at this time.
Active and inert gas cutting
The laser cutting process utilizes a coaxial gas jet that aids the cutting process. There are two gas types that can be used; active and inert. Active gases, such as oxygen and air, not only provide a shearing force on the molten material to remove it from the cut face but also react exothermically with the heat metal to increase the heat into the cut. The use of oxygen can add as much as 50 percent more heat into a cut. This results in faster cutting speeds; in fine cutting, the gas also facilitates dross-free cutting and fine dross particulate that does not adhere to surrounding material. Inert gases, including nitrogen (very inert) and argon (100 percent inert) are used to prevent oxidation of the cut face. In this case, the gas acts in shear only. Used at high pressures (up to 20bar), the gas cools the cut, so additional laser power is needed to maintain cutting speed. Visually the inert gas edge appears shiny to the eye because there is no oxide layer present. Oxygen is typically used for stainless steel cutting applications while inert gases are mainly used for titanium and platinum iridium. As noted earlier in the material selection section, thermal buildup and runaway on titanium can be mitigated by using a semi or fully inert assist gas. For Pt-Ir, use of inert gases can mitigate the excessive dross issue.
The cutting process does create particulate matter; based upon the amount of material being cut and the cut throughput an active extraction system is needed. The style and placement of the extraction system is primarily determined by the part being cut and the stage configuration. In some instances, the system can be something as simple as a removable tray that collects the particulate and is periodically emptied.
The laser is focused to a point at the surface of the material, and some amount of laser power passes through the cut. Past the focus point, the laser diverges and will strike the internal surface opposite the cut when cutting tubes. When cutting increasingly small diameter tubing, a point is reached when the laser starts to cause thermal damage to the internal wall that will not be removed through the electro polish process. At this point, using a low flow of water through the inside of the tubing eliminates the thermal effect by diffusing the laser in the water and also keeping the internal surfaces cool. This water flow is typically needed for tubes with diameters of less than 0.1-inch. The use of the water has no detrimental effect on the cutting process and usually increases cut quality. It also acts as a transport for the fine cut particles. In terms of system throughput, one must consider the added time of connecting the water to the tube.
Equipment and setup
Assuming the laser and motion control are fully integrated, the nozzle stand-off distance is an important part of the setup, ensuring consistent cut. Maintaining the nozzle strand-off distance in relation to the part is essential. For the assist gas to function most efficiently, this distance should be minimized. In most instances, it is around 0.01-inch. The tolerance of this distance depends upon a number of factors including; part-to-part dimensional repeatability, accuracy of the part to the solid model used to generate the tooling path, and the method used to datum the nozzle tip to the part.
For some critical applications that require high dimensional accuracy, the positional tolerance of the nozzle could be around ± 0.004-inch. Figure 7 provides an illustration of this stand-off distance. The arrows exiting the nozzle represent the flow of the coaxial assist gas.
Laser cutting tools are excellent for many of the feature and dimensional accuracy requirements needed in medical tube components. With a fine cutting tool diameter, high resolution cuts can be made at relatively high speeds when compared with other cutting technologies. The laser also offers unique cut features, such as off-axes cutting, which enables product design for functionality without compromising for manufacturability. The laser offers extreme control, precision and stability that align well with the requirements of medical device manufacturing.