Amada Weld Tech
Many electronics and medical device applications require stripping outer layers of polymers from small diameter wires, and the laser is well suited for this material removal task. Lasers provide a non-contact process that is very repeatable and tailored to removal strategies for wire ends, mid-wire sections and windows. They are easily automated and can therefore transform a key step in the manufacturing process, helping to ensure consistent part quality.
Advantages of using lasers rather than chemical-based processes include reduction or elimination of chemical use and its associated handling and disposal costs, and support for a company’s ISO 14001 environmental management systems programme.
Wire stripping methods
In medical device manufacture, many cardiac rhythm management, neurological and radiofrequency (RF) ablation products require material removal to expose a wire’s underlying metal conductor. The diameter of wires used for these devices is constantly decreasing, making other stripping methods simply untenable. At the same time, wire stripping requirements are constantly increasing, with both end- and mid-span parts requiring selective removal.
Similarly, in electronics, size reduction of wires and increasing wire density in cables aligns well with laser wire stripping. The laser process imparts no physical force on the wire during the process, so delicate wires with diameters as small as 50 μm can be stripped. The material is typically removed by directing a focused beam around 25 μm or less in diameter by galvanometers, which are small, fully programmable X- and Y-axis mirrors. This enables highly tailored removal, so parts or sections of wire insulation can be removed as needed. Changes to the size and location of the removed sections can be made on the fly by calling up pre-programmed recipes.
As shown in figure 1, the material can be removed by one of two methods, namely ablation or cut and peel. In the ablation method, the polymer simply absorbs the light energy and is vaporised, effectively ejecting it away from wire. The laser does not affect the wire beneath the insulation, because the power levels needed to remove the insulation are much lower than those that affect the wire. This advantage can be augmented by selecting a laser wavelength that is readily absorbed by the polymers but reflected by the wire.
Methods for wire stripping: a) ablation, b) cut and peel end and c) cut and peel mid-section/window.
In the cut and peel method, a series of helical cuts are made in the polymer to mechanically free it from the wire, which is removed post-process by automated or manual means. This method is typically used if the cycle time is critical and post process material removal is acceptable.
Laser versus other wire-stripping technologies
There are many advantages to using laser processes over chemical and mechanical processes.
The medical device industry has relatively lower part volumes than the electronics industry, therefore the most common material removal process is manual. It involves dipping each wire into a solvent individually for a certain amount of time, then scraping away any remaining coating material deposits using a sharp X-ACTO knife. Quality and repeatability can hardly be assured using this method.
Moving away from technicians wielding X-ACTO knives to automated equipment increases production process control, ensures quality and repeatability, and increases throughput. For example, one large medical device company recently transitioned from the manual process to a laser-based process for producing stainless steel guidewires used in intravascular interventional devices. The wire, which has a diameter similar to that of a human hair, is coated with an organic material that makes it compatible for use in humans. This organic coating material must then be stripped away from the microscopic metal core wire to enable connection to the guidewire’s distal end.
The laser process consistently and precisely strips away the organic material coating from the component’s metal core wire, which enables subsequent assembly operations performed on the unit in downstream processes. Far less operator-dependent than the method it replaces, the new process takes only seconds to complete, whereas the previous process took about eight minutes. Throughput has increased by 250 percent, with an additional increase in yield.
The higher part volumes in the electronics industry dictate that material removal processes are automated, but the basic premise is the same, namely that they are either chemical, mechanical, or a combination of both. However, as wire diameters decrease, these processes afford less control of material removal, potentially giving rise to issues such as wire deformation and reduced conductivity.
Choosing the right laser for wire stripping
A number of different lasers can be used for wire stripping, depending on the particular wire diameter, insulation material (fluoropolymers, nylon, Pebax, polyimide, polyethyleneterephthalate (PET), polyvinyl chloride (PVC)), and feature requirements. The table below lists the lasers most commonly used for wire stripping in suggested order of consideration. For each combination of material, wire diameter and required features, there is a suitable laser that matches the desired criteria.
The sealed CO2 laser should always be considered first. It has a wavelength of 10,600 nm, which is: 1) readily absorbed by every polymer, so it will work to a certain degree no matter what insulation material is used, and 2) not readily absorbed by metals, so when all the insulation material is removed and the laser impinges on the exposed wire, it has little effect for a relatively long time, thus allowing the completion of the process to the required tolerances on the insulation thickness and providing a large processing window. In addition, the CO2 laser is the most cost effective in terms of US dollars per watt power. Figure 2 shows a polyimide wire that has been stripped using a CO2 laser.
Geoff Shannon
A polyimide wire that has been stripped using a sealed CO2 laser.
The removal of the material is done more by thermal degradation, so heat input can be an issue if the wire diameter is small. This may result in wire distortion and cutting, or the insulation can be overheated, causing discolouration and burr formation. A burr develops when the material bulges or is raised and can significantly increase the overall outer wire diameter.
If a CO2 laser cannot be used for heat input control reasons, a nanosecond neodymium-doped yttrium orthovanadate (Nd:YVO4) laser should be considered, specifically one with a 532 or 355 nm wavelength. A nanosecond laser produces pulses of around 20 nanoseconds, removing insulation material with much less thermal interaction than the CO2 laser. It can be used on smaller diameter wires and where the removal edge must be well-defined with little or no burring. Figure 3 shows a wire that has been stripped using a nanosecond laser with a wavelength of 355 nm.
A small gauge wire that has been stripped using a nanosecond laser with a wavelength of 355 nm.
The choice between the 532 nm or 355 nm wavelengths is typically made based upon the insulation material, with the 355 nm being better absorbed by more polymers. If the CO2 laser is likened to a large oxyacetylene blow torch, the nanosecond laser would be analogous to a smaller, more delicate torch that might be used for finishing off a crème brûlée. Note, the popular fiber laser operating at 1070 nm is not well absorbed by most of the typical wire insulation materials, and so is rarely used or considered.
If extreme quality or minimal heat input is needed, ultrashort pulse (USP) picosecond and femtosecond lasers should be considered. These two laser families produce extremely short pulse widths. For picosecond lasers, they are 10-12 seconds (s), and for femtosecond lasers, they are 10-15 s. The pulses are so short that the insulation material does not have time to conduct any heat from the process area into the surrounding material.
This is known as cold processing and enables the best quality results, but at a premium price. A USP laser costs about eight times more than a CO2 laser, and about four times that of a nanosecond laser. Although the price difference is not so great if one is looking at buying a laser wire stripping system, since picosecond and femtosecond laser-integrated systems are around twice the price of CO2 and nanosecond laser integrated systems. Picosecond and femtosecond lasers may be appropriate for very high-value products or those that have extremely small (50 μm diameter) wires where very fine control is needed.
Laser wire stripping systemsIn medical device manufacturing, the wires are typically part of a production line. They are not usually processed in reel-to-reel machines; rather, they are processed in either a manual or automated load machine that handles the wire pieces one at a time at the required length.
Essentially, the wire stripper can either rotate the wire or use multiple heads to remove the insulation from the stationary wire. Sometimes, the process, rather than the manufacturing environment, dictates which of these techniques is used. As always, the best solution is based on a clear understanding of both the application and production needs.
Figure 4 shows a laser ablation wire stripping system from Amada Weld Tech, which incorporates:
- galvanometer mirrors for high-speed beam steering;
- a custom wire feed and rotating mechanism for accurate and repeatable wire positioning; and
- several proprietary features that manage heat balance in the part during the ablation process, facilitating clean removal of the insulation material while fully protecting the delicate metal wire substrate.
A laser ablation wire stripping system from Amada Weld Tech.
In addition, the system affords a dual self-cleaning process for removing sticky debris from the ablation process area. This is performed by a vacuum that is focused on the laser and a high-tech toothbrush that goes over the tooling after every operation. This dual self-cleaning process prevents the contamination of tooling and allows tens of thousands of wires to be run with minimal scheduled maintenance.
Conclusion
Lasers transform the wire stripping process, making it a lean operation. Key to the success of wire stripping processes is the development of the process itself. To make the right decision as to which laser source and removal methodology will work best, it is essential to test possible options in an application laboratory with a range of lasers. The resulting system solution will then be optimal in terms of both process and implementation.
Amada Weld Tech