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Table 1 – Selecting the Right Laser for Wire Stripping.
The sealed CO2 laser should always be considered first. With a wavelength of 10,604 nanometers (nm), the CO2 laser is readily absorbed by every polymer, so it will work to a certain degree no matter what insulation material is used. Also, the CO2 laser is not readily absorbed by metals, so when all the insulation is removed and the laser hits the exposed wire, it has little effect for a relatively long time. This allows the completion of the process to the required tolerances on the insulation thickness and provides a large processing window. In addition, the CO2 laser is the most cost effective in terms of dollars per watt power. Figure 1 shows a polyimide wire that has been stripped using a 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 potential cutting, or the insulation can be overheated, causing discoloration and burr formation. (A burr develops when the material bulges or is raised, and can significantly increase the overall wire outer diameter.)

If a CO2 laser cannot be used for reasons of heat input control, the nanosecond laser should be considered next, specifically those with 532nm and 355nm wavelengths. Nanosecond lasers produce pulses of around 20 nanoseconds, removing wire insulation material with a much less thermal process than that of the CO2 laser. It can be used on smaller diameter wires, or where the removal edge must be welldefined with little or no burr. Figure 2 shows a wire that has been stripped using a nanosecond laser with a wavelength of 355nm.

The choice between the 532nm or 355nm is typically made based upon the insulation material, with the 355nm being better absorbed by more polymers. If the CO2 laser is likened to a large oxyacetylene blow torch, the nanosecond later would be analogous to a smaller, more delicate torch that might be used to create the crispy top of a crème brûlée. Note the popular fiber laser operating at 1,070nm is not well absorbed by most of the typical wire insulation materials, and so is rarely used or considered.

When extreme quality or minimal heat input is needed, the options to consider are the ultra-short pulse picosecond and femtosecond lasers. These two laser families produce pulse widths that are extremely short: picoseconds is 10-12 seconds (s) and femtosecond is 10-15 s. The pulses are so short that the material does not have time to conduct any heat from the process area into the surrounding material.

This so-called “cold processing” enables the best quality results, but such a high quality level comes at a steep price. Ultra short pulse lasers cost about 25 times more than CO2 lasers, and about 5 times that of a 532/355nm laser. They may be appropriate for very high-value products or for those with extremely small wires (50 microns diameter) where very fine control is needed.

Laser Wire Stripping Systems

Fig. 3 – Laser ablation system for wire stripping
In 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 a 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 3 shows a recently developed laser ablation system that includes high speed galvo beam steering and a custom wire feed and rotating mechanism that achieves accurate and repeatable wire positioning. Several proprietary features needed to manage heat balance in the part during the ablation process are included in this system. Its features facilitate clean removal of the insulation material, while fully protecting the delicate metal wire substrate.

The approach also includes a self-cleaning mechanism that removes sticky debris from the ablation process area that might have contaminated tooling. In effect, the system has a dual cleaning process: the vacuum on the laser itself, along with a high-tech “toothbrush” that mechanically cleans the tooling after every operation. This self-cleaning feature allows tens of thousands of wires to be run with minimal scheduled maintenance.

Use of lasers for wire stripping transforms a key step in the process to a lean operation. The key to the success of wire stripping processes is the development of the process itself. To make the right decision on which laser source and removal methodology works best, it is absolutely essential to test possible options in an application laboratory with a range of lasers. The resulting system solution will then be optimal in both process and implementation.

This article as written by Geoff Shannon, Laser Technology Manager at Miyachi America, Monrovia, CA. For more information, Click Here.

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