Application of disc lasers in Medical device manufacturing

Size: px

Start display at page:

Download "Application of disc lasers in Medical device manufacturing"

Benjamin Pope
5 years ago
Views:

Application of disc lasers in Medical device manufacturing Ramasamy Elavarasan, Peter Jepsen Jan Scheftlein * Prosint, 2220 Oakland Road, San Jose, CA 96131, USA * Prenovatec GmbH, Nachtigallenstr,

Once lasers gained acceptance as reliable and necessary tools for medical micromachining, new lasers with improved performance were developed.

1 Application of disc lasers in Medical device manufacturing Ramasamy Elavarasan, Peter Jepsen Jan Scheftlein * Prosint, 2220 Oakland Road, San Jose, CA 96131, USA * Prenovatec GmbH, Nachtigallenstr, 13 D Meinigen, Germany. Abstract When lasers were introduced in medical micromachining industry, there were not many choices available. Once lasers gained acceptance as reliable and necessary tools for medical micromachining, new lasers with improved performance were developed. As of today, there are many different types of lasers available in the market. They vary in wavelength, power, pulse duration, and price and most importantly, the way they interact with the material. The right choice, most of the time, depends on the type of material used, the nature of the application and the required process speed. As a laser system integrator, we often came across various laser sources for use in the medical industries. In this paper we present some of our observations and provide guidance for better medical device manufacturing. Introduction In the early periods of laser usage, lamp pumped lasers were used widely in the micro machining industries. The less efficient pumping mechanism, high operating costs and thermal instabilities forced researchers to develop new lasers. The unwanted, negative, so called thermal lensing effect will be reduced if the temperature over the volume of the active material is more homogeneous. The first step in this direction was made by replacing the flash lamp with diode bars that emit light in a narrow spectral range so that they pump more efficiently with minimal heat dissipation in the pumping media. The next step was to change the geometry of the pumping media. The aim was to modify the surface to volume ratio. Initially this was done by increasing the length and reducing the diameter as in the case of Fiber lasers. The other technique is to increase the diameter and reducing the length, which lead to the invention of disc laser. The low power disc lasers are one of the most interesting developments in recent time. The geometry and lasing mechanism yield a highly stable beam. It is rapidly finding its way in the medical device manufacturing especially in stent cutting applications. The excellent beam stability, beam quality, low divergence, short pulses (in the nano second range), reduced Heat Affected Zone (HAZ), low taper angle cuts, reduced debris level and soft slag are some of its major advantages. With the combination of these characteristics and proper process control and parameters, disc lasers become an ideal choice for delicate and intricate medical device manufacturing. The best beam quality alone does not guarantee the best result. The associated processing technique is as important as the

2 beam itself. There are many literatures available to describe and quantify the disc laser properties. So the main theme of this paper is to discuss the application range of disc laser and provide guidelines for using it effectively in medical device manufacturing. Laser Choice Successful micromachining starts with the choice of the ideal laser. The laser should have a stable and high quality beam, be able to process a wide range of materials, fast processing, low HAZ and minimum maintenance requirements. Understanding the basic physics involved in laser material interaction will help one to choose the right laser for the right application. The important laser parameters that decides the quality of the machining are the energy applied on the material (laser power), method of interaction (pulsed or continuous mode), time of exposure and the frequency of the interaction. During the laser machining process, when the first laser pulse hits the material surface, the optical energy from the laser is not directly absorbed by the molecules of the material being processed. The energy is initially absorbed by the free electrons. The absorbed energy set the electrons in to an oscillatory state. In this process the optical energy is converted in to thermal energy over a time scale of ~100fs. The converted thermal energy is slowly transmitted to the neighboring molecules. The time scale involved in the transfer process is of the order of few nano seconds. This is long in comparison to femto seconds. The temporal and spatial distribution of the energy transfer dictates the cut quality. The time aspect of the energy transfer process is dictated by the duration of the laser pulse (pulse width), delay between the pulses (Pulse repetition rate), the cut velocity and acceleration. Temporal modulation of energy has been proved to be effective way to improve machining quality. Equal importance is given to the spatial distribution of the energy. This affects the extent of the heat affected zone (HAZ). The size of HAZ strongly depends on the pulse duration of the laser and material properties. It is very obvious that HAZ will be wider with CW laser than in pulsed laser process. Also, for a given pulse energy and pulse width, different materials interact in different ways. If the pulse duration is longer than the energy transfer time, then the amount of heat generated by the process accumulates near the point of interaction. In majority of the cases, peak power of the laser is sufficient to raise the temperature on the surface and initiate the phase change. This means the material changes from solid to molten state before vaporizing. If the molten material is not removed immediately from the location, the thermal effects start affecting the surroundings. The rise in temperature induces thermal shear stress in the material. Depending on the magnitude of the laser induced stress level, some important structural changes, such as micro cracks, occur. In addition, the extent of the heat affected zone (HAZ) also changes the physical condition of the material and reduces the tensile strength. None of these

3 conditions are acceptable in the medical industry. So it is highly desirable that the laser pulse duration should be comparable to the time scales of the material energy transfer process. However there should be some compromise between the extent of the HAZ and the machining efficiency. Ultra short pulse durations might eliminate the HAZ, but the process time increases five to ten fold. Such increased processing times, in most cases, cannot be accepted in a production environment. Efficient material removal and acceptable HAZ is the ideal combination. Depending on the nature of the application, the spatial and temporal modulation is varied to achieve the desired results. For example, long pulses and exposures are desired in welding applications. Short pulse and exposure are desired in delicate cutting application where low HAZ is important. However not all lasers are capable of all applications. Some long pulse lasers are used effectively in welding and marking applications. The ultra short pulses are good for soft materials such as polymers, biodegradable materials etc. The Long pulse lasers cannot be used in delicate applications and in the same way ultra short pulses are not suitable for certain applications such as welding and high speed cutting applications. Figure 1 shows a generalized chart that provides a rough guide for selecting the appropriate time scale and the laser power for particular application Hardening Irradiance W/sqcm Trimming Glazing Material Removal Heat ing Melting Plasma Ignition Drilling Cutting Welding Heat treating Duration of interaction - Sec Figure 1. Laser energy and interaction duration scale for various applications Based on our experience with various lasers and applications we derived a general view of the class of the lasers and their operating range (Figure. 2). As mentioned above, based on the application, a suitable laser is to be selected to achieve the best result.

4 Welding Micro welding Coarse Cutting Fine Cutting Soft/Delicate material Cutting Long pulses CW - ms high µs Medium range pulses Low µs to high ns Short pulses Low ns -ps fs Flash Lamp lasers Fiber lasers Disc lasers DPSS lasers Pico/femto second lasers Figure 2. An overview of the various classes of lasers and their working envelope. As can be seen in Figure 2, the disk laser overlaps the domains of long and short pulse lasers. This makes disk laser an attractive choice for wide variety of applications. The disk lasers are operated typically in Q switched mode with a pulse width of about nsec. This provides a peak power of 125MWatt per pulse from a 50W average power laser. These pulses not only yield a clean cut with minimal HAZ but also provide very high process speeds. Also the initial pierce point into the material and the following kerf size are almost of the same size. This allows for the machining of complicated patterns especially in stent applications and avoids any unwanted notches in the struts. The disc lasers are also capable of operating at CW mode. With CW and a proper set up the disc laser can very well be adopted in micro welding and marking applications. Based on these observation it is very clear that the low power disc lasers (<100W) seem to be a perfect fit for wide range of medical industry applications. Applications A variety of experiments have been carried out to prove the performance of disc laser over a wide range of applications such welding, cutting and a broad array of materials ranging from stainless steel to gold. We used Prenovatec 50W laser in all the experiments in combination with a Precitec fine cutting beam delivery unit. A cold form copper nozzle from Laser Mechanism is used with the beam delivery. The ability to control the power on demand made it possible to easily change the process parameters during the cutting process. Aerotech Laser Turn5 with A3200 controllers are used as motion control system. Water cooling is used in all tube cutting applications. Most of the applications presented here are actual medical devices that are currently in production. Figure 3a shows the fine micro welding accomplished with the disc laser operated in CW mode. The nitinol wire diameter is about 50 microns. It is very clear that the HAZ is minimal and the 100 micron diameter wires are very well intact.

(A) Figure 3. A) Micro welding of fine wire mesh.

Two stainless tubes of about 5 mm diameter are welded together in this medical device.

These two examples prove that disc laser is capable of handling extreme cases in the same operating range.

Compressed air is used as a process gas (pressure 3 psi (<0.3 bars). (A) (B) (C) Figure 4. Thick wall Nitinol (0.

switch mode) in cutting thick walled Nitinol in a very impressive manner.

It is obvious that the features are cut with minimal distortion and reproduces the geometry to its specification.

5 (A) Figure 3. A) Micro welding of fine wire mesh. (B) B) Seem welding of heavy tubes Figure 3b shows an example of macro welding. Two stainless tubes of about 5 mm diameter are welded together in this medical device. The results were achieved simply by adjusting the power and operating mode. These two examples prove that disc laser is capable of handling extreme cases in the same operating range. In both cases the laser was operated in CW (Continuous Wave) mode. Compressed air is used as a process gas (pressure 3 psi (<0.3 bars). (A) (B) (C) Figure 4. Thick wall Nitinol (0.65mm) cutting (A) Top side (B) Bottom side Figures 4 A and B show the ability of the disc laser (40 W output power in Q switch mode) in cutting thick walled Nitinol in a very impressive manner. Figure 4A shows the top side view of a sharp feature cut. Figure 4B shows the same feature from the bottom side. It is obvious that the features are cut with minimal distortion and reproduces the geometry to its specification. In most of the laser cutting applications, usually the bottom side, especially at the sharp edges, gets distorted because of the large taper angle. Typically most of the laser cutting results in a taper angle of about 6 degrees in cutting a 0.5mm thick wall. With the disc laser the taper angle obtained was only around 1-2 degrees. Figure 4C shows clear evidence of the very small taper angle. Disc lasers are also suitable for machining a broad variety of materials. In the medical device industries stainless steel, Nitinol, cobalt chrome, tungsten, alloys of tungsten, platinum, platinum alloys and other exotic materials such as gold and silver are widely used.

(A) Nitinol (B) CoCr (C) Tungsten alloy (D) Platinum alloy Figure 5. Medical devices made of various materials.

These pictures are clear evidence of the capability of disc laser in machining wide range of material very efficiently.

The main issue is the soft slag formed during the cutting process that is difficult to remove and requires extensive post cleaning.

Unlike the slag formed with long pulse laser processes, the slag formed with the disc laser process does not stick to the material and thus the removal can be achieved with minimal

6 (A) Nitinol (B) CoCr (C) Tungsten alloy (D) Platinum alloy Figure 5. Medical devices made of various materials. Disc lasers machine a wide variety of materials Figures 5 A-D show medical devices made of various materials machined with disc lasers. These pictures are clear evidence of the capability of disc laser in machining wide range of material very efficiently. Soft materials such as CoCr, Tungsten and platinum are in general difficult to machine. The main issue is the soft slag formed during the cutting process that is difficult to remove and requires extensive post cleaning. However, as can be seen from the above pictures, the disc laser not only provides clean cuts with such soft materials but also results in reduced slag formation. Unlike the slag formed with long pulse laser processes, the slag formed with the disc laser process does not stick to the material and thus the removal can be achieved with minimal effort. Another feature that can be achieved only with disc lasers is shown in Figure 6. The machining was done on a Nitinol tube of about 0.3 mm wall thickness. There was no processing done after the machining except cleaning OD surface with 1600 grid paper to remove the water marks and debris. These features are cut all the way through which can be seen from the letter O in Prenovatec. A size analogy is done by comparing the cuts with a human hair. The main point to be noted here is the cleanliness of the

characters machining on a relatively thick wall material. It is also to be noted that the initial pierce point and the following cuts (kerf widths) are of same size. Human Hair Through Cuts Figure 6.

However, selection of the laser alone does not guarantee an acceptable result.

7 characters machining on a relatively thick wall material. It is also to be noted that the initial pierce point and the following cuts (kerf widths) are of same size. Human Hair Through Cuts Figure 6. Demonstration of fine feature machining and no Splash pierce points. All these results confirm that the choice of laser is very crucial especially in medical application. However, selection of the laser alone does not guarantee an acceptable result. A correct choice of other process parameters such as process gas, process head, the distance from the work piece and the process head are also equally important. There are numerous reports available to describe the contribution of individual parameters. However it is time consuming for an individual to go through all the details and select the appropriate parameters for his/her application. A consolidated report of fundamental process details would be beneficial for both laser manufacturers and end users. In the following section a selection guide for main process parameters is presented. Process Parameter Selection. The laser material process is a complex process and there are number of variables that determine the quality of the end product. The process gas management is the one of the important factors next to the choice of the laser. The gas jet removes the debris and molten material from the cut zone and cools the proximity by forced convection. It not only determines the cut quality but also the machining efficiency. The choice of the gas mainly depends on the application and the material. When reactive gas such as oxygen is used, it also delivers the additional exothermic reaction to aid the cutting process. However there are some side effects in using the reactive gases. Even for a short period of time, when exposed to high temperature, the reactive gas reaches the critical ignition point suddenly and results in violent explosions. When there is no explosion, the reactive gas leaves a trace of oxide layer on the cut surface. This oxide layer, in general, is an unwanted side effect. The removal of the oxide layer, in many occasions, requires

8 extra effort in terms of post process. Inert gases such as Argon, Helium are widely used in medical applications. The inert gas cuts generally appear dry. If one of the process parameters such as the pressure, nozzle stand off distances or laser power settings is not selected properly, the cuts might produce rough edges at the exit side. In some occasions, a mixture of argon (95%) and oxygen (5%) is also used. The 5% oxygen provides necessary additional energy to cut thick wall materials and the inert gas keeps the oxide layer formation at a minimum level. Such gas mixtures are commercially available. However, the cost of cutting increases by two folds in using such complex mixtures. For welding and marking applications, in addition to oxygen, compressed air and nitrogen are also used. The usage of inert gas increases the laser power requirement by double when compared to reactive gas. As far as the process gas pressure is concerned, Figure 7 gives an over view of the limits for various material thicknesses. The data in the table are gathered from actual medical device production environment. All this data is verified and used in our application lab. Material Wall Thickness (mm) Pressure psi (bar) Nozzle size (3.4 to 5.5) 0.3mm (6.9 to 10) 0.3mm (10 to 14) 0.5mm (14 to 17) 0.5mm/1mm >250 (> 17) 1-1.5mm Figure 7. Overview of process gas pressure range and nozzle diameter for various wall thicknesses. The parameters may have to be fine tuned based on the material and application. For welding and marking applications, generally larger nozzle (>5 mm) and low gas pressure (typically <3 psi/0.3 bar) are desirable. Also, the nozzle stand off distance can be increased to 5 times the nozzle opening size. At some occasion the The process gas is delivered through a well designed nozzle in the form of a jet stream. Nozzle tips are available in various shapes and design. The main aim of the nozzle tip is to provide an uninterrupted flow with out any unwanted effects on the material. Hence the choice of the nozzle shape and its standoff distances from the material are critical.

The pressure of the gas stream coming off the nozzle is in general much higher than the atmospheric pressure and improper nozzle shape and stand off distance can create unwanted shock structures in

9 The pressure of the gas stream coming off the nozzle is in general much higher than the atmospheric pressure and improper nozzle shape and stand off distance can create unwanted shock structures in the flow (Figure 8). These shock structures appear prominently when the distance between the nozzle and the material (stand off distance) is more than three times the nozzle diameter. The shock structures slow down the flow velocity significantly and reduce the effectiveness in removing the debris during the cutting process. Shock structures Standoff shocks Figure 8. The formation of shock structures in a round impinging gas jet when the exit pressure is much higher than the atmospheric pressure (nozzle pressure ratio). One of the main purposes of using process gas is to remove the molten material immediately from the side wall. The effective removal yields a clear side wall and best results. This can be achieved if the process gas flow remains attached to the wall until the laser completes the penetration. Nozzle Flow Stand-off Distance Laminar Flow Region Separated Flow Transition Figure 9. The cross structure during the cutting process. section of flow

However, in actual practice, this situation is very difficult to achieve. Figure 9 shows a cross section of the flow structure during the cutting process.

The best results are accomplished by extending the laminar region as long as possible and minimizing the transition and separated regions to as short as possible in the flow.

10 However, in actual practice, this situation is very difficult to achieve. Figure 9 shows a cross section of the flow structure during the cutting process. There are typically three prominent regions that exist. They can be classified as initial attached laminar region, middle transition region and separated flow region. The best results are accomplished by extending the laminar region as long as possible and minimizing the transition and separated regions to as short as possible in the flow. Laminar Region Transition Region Separated region Figure 10. Cross of a machined sample shows the various flow regions and the cut quality at those locations. Figure 8 shows the existence of these three regions and the cut quality in these locations. At the laminar region, the surface is smooth with no pulse marks. At the inside of the sample where the flow separated, the material removal became very irregular and resulted in a very rough cut. Well adjusted flow conditions can result in best possible results. Figure 11 shows the result of such a condition. This was possible by adjusting the stand off distance between the nozzle and the work piece properly and choosing a right nozzle design. Improper nozzle stand off distance can trigger the flow separation much earlier.

11 Figure 11. Cross section of a machined part with properly adjusted flow parameters. A typical stand-off distance between the nozzle tip and the work piece is half the size of the nozzle diameter. For example, with 0.5 mm size nozzle, the stand off should be around 0.25mm. There have been many research papers compiled on various nozzle designs and their effects. The best and easy to use nozzle design is the simple round conical cold form nozzles (from Laser Mechanism) made of tellurium copper material. The cold form nozzles have very smooth mirror like inner walls which avoid any flow separation before the flow exits from the nozzle. In most medical device manufacturing applications, a 0.5 mm opening nozzle is used. In some thick wall applications (wall thickness more than 0.5 mm), 1mm opening nozzle tips are used. The larger opening nozzle, however result in increased kerf width by about 25%. One more process parameter that also needs to be taken in to consideration is the focus point location. For thinner walls (<0.250mm) the focus point is kept on the surface of the material. As the thickness increases, the focus point should be moved inside the wall. While adjusting the focus point, one should not alter the stand off distance. The focus adjustment should be independent of the nozzle stand off distance. Conclusion and Future of Disc laser Low power disc lasers (<100 Watts) are swiftly finding their way in the medical device manufacturing. The high efficient, stable and fine quality beam disc lasers are unquestionably suitable for a wide range of medical applications. The low operating cost, stability, reliability, ease of operation, high efficiency cutting and remarkably high quality results are attracting more and more medical device manufacturing. There are many developments currently being undertaken with low power range disc laser technologies. A 515nm green wavelength is already in the market. Other future development work in disc laser technology includes the 3 rd harmonic output at 355nm wavelength and ultra short pulse lasers. With this wide range of operating wavelengths being offered, the disc laser technology will surely dominate the laser industry in the near future.

Dr Jack Gabzdyl Product Line Manager Pulsed Lasers

AILU PHOTONEX 08 16 th October 2008 Fiber Lasers for Medical Applications Dr Jack Gabzdyl Product Line Manager Pulsed Lasers General Advantages of Fibre Lasers Beam Quality & Stability Diffraction-limited