CO2 Lens Failure Causes Part 2: Machine Power and Cooling System Problems
In my previous blog post, I discussed lens failures due to problems associated with buildup on the lens. In this blog, I’ll review machine power and cooling system problems as possible causes of lens failure.
Output Coupler Degradation – The output coupler is a partially-transmissive mirror in one end of the resonator. Typically it transmits about 60% of the laser energy, which travels “downstream” through the mirrors to the lens and cutting head. If an output coupler gets too old it may transmit more energy through the system. Sometimes it’s enough energy to actually burn the coating on a lens. If you see reddish looking dust present in the lens chamber, it’s often evidence of a bad output coupler that has burned the lens coating.
Operation in Pulsed Mode for Piercing – Some lasers can operate in a “pulsed mode” in addition to a constant wave (CW) mode. Pulsed mode is often used to pierce through thicker plate materials because of the additional power required to penetrate the thicker material. Simply put, pulsed mode activates the laser for short durations, yielding higher momentary power. With some lasers, pulsed mode operation can result in peak wattages well above the upper limits of the power rating of the lens, causing them to fail.
Over-Collimation – A collimator is an optical device that “sizes” the laser beam. For instance, a specific resonator may generate a wide or “fat” beam that is too large to develop the required power densities, so a collimator is used to reduce the beam size. In some newer machines adaptive optics may adjust the beam collimation based on the material being cut. If the beam is overly collimated or reduced, lens life can suffer.
Cooling System Problems – In many applications the laser is water-cooled through an external chiller or refrigerator unit. If the cooling is impeded, the optics may overheat, causing thermal deflection of the lens and ultimately more severe lens damage. If normal tap water is used in the chiller instead of de-ionized water or a water glycol mixture, it will result in calcify in the cooling line, causing overheating and destruction of the lens.
Resonator Variations/Power Spikes – Aging resonators may product inconsistent laser beams, especially if subjected to variations in electrical power.
In the last blog of this series, I’ll discuss gas pressure issues, improper nozzle selection and lens issues.