The key to laser wire stripping is finding the type of laser that strongly interacts with the layer you want to remove and yet is reflected from the underlying layer. With combinations of the color of the light (the laser wavelength) and the duration of the laser pulses, an incredible range of different processes can be achieved. This includes:
- cutting / vaporizing insulation from metallic conductors or shields
- cutting metallic shields from on top of insulated wires
- cutting wires
- cleaning surfaces.
Polymers strongly absorb light in the deep-infrared part of the spectrum - no matter the material or the color. They also absorb strongly in the deep-ultraviolet part of the spectrum. In between different polymers behave differently, but on the whole they are largely reflective or transmitting. Although polymers absorb strongly at both ends of the spectrum, they do so in very different ways. At the deep infrared wavelength the light shakes the polymers molecules until they break apart. The laser is effectively a very controllable form of heat. In the deep UV spectrum, the atoms' electrons absorb the light and eject them causing the atoms to be ripped from the wire surface. There is very little heat involved so adjacent material is largely unaffected.
Metals all reflect very strongly in the deep infrared part of the spectrum, absorb strongly in the near-infrared and visible regions and partially absorb in the UV. For wire stripping of polymer insulations from metals, the deep infrared is a perfect laser source as it is strongly absorbed by the plastic and then reflects from the conductor or shield. For most applications, the "hot" process is not a problem. For the most critical medical applications, thermal damage can be a problem. For these applications the more expensive ultraviolet laser sources are used to allow "cold" vaporization of the insulation.
Table of machine type versus laser type
||Metal cutting / enamel removal
||Fine wire enamel stripping / fine wire cutting
In order to precisely cut the part, the laser light needs to be concentrated to the exact point that it is needed. Apart from very large lasers, this is mainly achieved by focusing the light onto the wire with a lens. This is exactly the same way you would burn paper by magnifying the sun's rays with a magnifying glass. To burn a line, you would need to move the lens relative to the paper.
When stripping a wire, it is necessary to cut right around the wire circumference. This is usually achieved by making a cut from opposite sides. For very thick wires it may be necessary to cut right around the circumference by spinning the wire or the laser beam - but for thinner wires (<5 mm / 0.2" diameter) - it is enough to cut from two sides and have the two cuts join up.
When stripping the wire from two sides the light is strongly absorbed by the outer layer before reflecting from the substrate. This forms a stable self-limiting process with no damage to the substrate.
There are 3 main methods for moving the focused laser spot relative to the substrate:
- Fix the laser spot and move the part
- Move the lens relative to the wire (moving optics)
- Change the angle of the laser through the lens (galvanometer scanner).
The first method is simple, but has the major drawback that the product must move. For larger and longer cables this is clearly not possible. Even for smaller cables it can be undesirable as any movement of the part can result in reduced quality stripping.
< Laser nozzle focuses the laser light onto the wire.
Two axes machine with two laser nozzles >
The second method is perhaps the most common method used when making a laser wire stripper. The laser beam is directed by a series of mirrors to a laser "nozzle". The nozzle contains a lens which focuses the light onto the wire. The nozzle is moved in one or two axes to cut across with wire, or in some cases, along it (see below). In order to strip from two sides of the wire the laser beam needs to be split to be directed to the 2 nozzles. This can either be achieved by splitting the light 50:50 wire a partially reflecting mirror, or by having a mirror "shuttle" into and out of the beam, sequentially stripping the wire from one side and then the other. This method has the advantage that overlap of the two laser beams is relatively stable - although can be difficult to set up for a dual axis machine. A disadvantage is that the relatively large mass of the stages and optics severely limits the maximum acceleration of the laser spot - which limits maximum speeds. On the other hand, it is easy to achieve large process areas without compromising the laser spot size.
In the third method, a device called a galvanometer scanner (or "galvo" for short) is used to direct the beam. A galvo is a mirror mounted onto an electromagnetic galvanometer. A current passed through the galvo coil causes a fixed deflection. The laser mirror can be tilted with high accuracy and high acceleration. This allows the laser spot to be deflected at very high speeds (>100 times the moving optics method).
By using two galvos, the laser spot can be directed in two axes. To ensure the laser light is focused in the same plane, no matter the positions of the mirrors, a special lens shape is used called an "f-theta" lens. The main reason to use this method is to achieve high laser spot speeds which are needed if the laser spot is to be used to burn an area of insulation (area ablation - see below). This key advantage is achieved at the expense of the working area (for a given laser spot size) - although careful design can mitigate this somewhat. Note that high speed motion of the laser spot is also important for minimizing heat damage of the part.
When stripping a wire (or cutting a shield) - there are 3 main methods:
- Cross Cut: the laser spot moves perpendicular to the wire to separate a slug of insulation
- Cross Cut and Slit : after the cross cut, the laser beam moves along the wire to help peel off the slug
- Area Ablation: the spot is moved back and forth to fully vaporize an area of insulation
Cross cut only. This example is a coaxial cable. The laser beam moves perpendicular to the cable (from above and below). Two cuts are made - one for the jacket and one for he inner dielectric.
Once cut, the Operator can pull off the insulation by hand. By leaving this until immediately before termination it is possible to prevent fraying of the shield or stranded conductor.
The cross cut method works if the insulation is not bonded and if the slug (waste piece) is short so there is not too much friction/
The simplest moving optics machines only move the laser light in one axis (across the wire). To set the strip position along the wire - the wire is pushed up against a movable back stop. Changing the backstop position changes the strip length. A more sophisticated machine allows the laser spot to move in 2 axes - across and along the wire. This allows slits along the wire to be created - but also also the cables to be placed up against a fixed back stop so that the strip position is programmed.
For longer slugs (typically found when stripping the jacket of a cable) - it is helpful to also slit along the cable as it is difficult or impossible to pull off the insulation. This allows the slug to be peeled away from the cable. If two cross cuts are made with a joining slit, a window can be made in the cable jacket.
For bonded insulation - it is not possible to peel off the insulation - even when slitting the jacket. In this case the insulation must be totally vaporized. The laser beam needs to be moved at high speed perpendicular to the wire and this "ablation line", moved slowly along the wire to totally vaporize the insulation. This is not the preferred method as it is slower than the cross cut & slit and can leave a residue - but for some bonded materials there is no choice. A good example is the bonded enameled insulations found on motor windings. Careful selection of the laser can avoid residue issues - but for the best quality strip always use a pulled off slug if possible.
A hatched stripping pattern used to ablate an area of bonded insulation.
||Moving optics - 2 axes
||Cross cut & slit, windows
||Cross cut & slit, windows, area ablation
||Moving optics - 2 axes
||Shield cutting & area ablation
||Metals & enamels
Cutting the shield from coaxial cable and particularly micro-coaxial cable is a key application of laser wire stripping. There are two methods which are based on the same principle. In the first method, the shielded conductors are dipped in a solder bath to create a solid yet brittle layer on the shield. A pulsed laser with a 1 micron wavelength (near-infrared) is passed over the soldered shield at high speed. The 1 micron light is strongly absorbed by the metallic shield and a small groove is cut in it. This groove, or scribe, is enough to create a fault line and the shield can be snapped at this weakness. The process is similar to that of scribing glass.
The shield can then be pulled off to reveal the inner insulated conductors. In the second method, a similar laser is used to directly cut off the insulation. This so called "solder less scribing" has many production advantages as it skips the manual step of dipping the cable end in solder. Laser Wire Solutions is expert in both standard laser scribing and solder-less scribing. Read more about shield cutting here.
The picture above shows a micro coax cable that has been cut using the solder-less scribing method.
From left to right:
Cable is prepared by removing the outer layer of insulation. It is either laser cut and pulled back, or scanned out using a high speed laser stripper
The 1 micron laser cuts the shield from above and below.
The shield is then pulled away revealing the inner insulated conductors
The inner conductors are then laser stripped ready for joining.