Features

Many medical device applications require stripping outer layers of polymers from small diameter wire, and a laser is well suited for this material removal task. Offering a non-contact process that is very repeatable, lasers can selectively remove wire layers or areas. Easily automated, the laser transforms a key step in the manufacturing process into a lean operation, reducing and optimizing human-dependent processes and providing consistent part quality.

Fig. 1 – CO2 laser used for wire stripping.
Additional advantages of using lasers over manual chemical-based processes include safety benefits from eliminating chemical use, reduction of chemical handling and disposal costs, and support for a company’s ISO 14001 sustainability program.

Lasers Well Suited for Wire Stripping

Many cardiac rhythm management, neurological, and radio frequency ablation products require material removal to expose a wire’s underlying metal conductor. The diameter of wires used for these devices is constantly decreasing, which can make other stripping methods simply untenable. At the same time, wire stripping requirements are constantly increasing, with both end-span and mid-span parts requiring selective removal.

The laser process imparts no physical force on the wire during the process, so delicate wires with diameters as small as 50 microns can be stripped. The material is removed by directing a focused beam (around 25 microns in diameter) by galvanometers, which are small fully programmable x and y 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.

The material is removed by one of two methods: ablation or cut and peel. The ablation method simply removes all the material from the wire as the polymer absorbs the light energy and is vaporized—effectively ejected 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 would damage the metal wire. This advantage can be augmented by selecting a laser wavelength that is readily absorbed by the polymers, but reflected by the wire. With the cut and peel method, a series of helical cuts in the insulation are made that mechanically free the insulation from the wire (not always possible), which is then removed post process by an automated or manual means. This is typically done if the cycle time is critical and post process material removal is acceptable.

Benefits vs. Other Available Wire-Stripping Methods

Fig. 2 – 355nm laser wire strip of polymer coating.
The benefits of using the laser’s highly controlled direct removal approach for wire stripping must be contrasted with manual processes currently being used. Most frequently employed are mechanical knives and chemicals.

The most common manual process includes dipping each wire individually into a solvent for a certain amount of time, and then manually scraping any remaining coating material deposits with a sharp knife, known as the X-ACTO® method. Quality and repeatability is hardly assured using this process. Moving away from technicians wielding X-ACTO knives to automated pieces of equipment increases production process control, ensures quality, and increases throughput.

For example, one large medical device company recently transitioned from a manual to a laser process for producing stainless steel guidewires used in intravascular interventional devices. The wire, with 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 new laser process consistently and precisely strips away the organic material coating from the component’s metal core wire, which enables subsequent assembly operations performed to 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 rose by 250 percent, with an additional increase in yield.

Picking the Right Laser for the Job

A number of different lasers can be used for wire stripping, depending upon the particular wire diameter, insulation material (polyimide, Pebax®, PET (poly ethylene terephthalate), nylon, and fluoropolymers), and feature requirements. Table 1 shows the lasers most commonly used for wire stripping, listed by suggested order of consideration, from top to bottom. For each combination of material, wire diameter, and required features, there is a suitable laser to match the desired criteria.

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