How to Select the Lower Protective lens of the Laser Cutting Head?

Among laser equipment accessories, lower protective lens is a consumable with a very high replacement frequency.

It is a “double-sided fence” in the external light path of the laser.

It should not only resist the harsh environment of external processing, prevent the invasion of dust and splash, but also resist the high-power laser transmitted in the internal forward direction. Even when there is a problem in the gas path, it is also affected first, so the consumption is huge.

Customers often ask how to choose protective lens.

In order to answer a series of puzzles, we specially interviewed the R & D and after-sales engineers of Raycus to ask your concerns.

1. Is there a big difference between the lower protective lens of different manufacturers?

There is a big difference.

Because the lower protective lens not only protects the cleanliness of the internal light path of the cutting head, but also needs to have high-power laser bearing capacity and high laser transmission.

In order to meet these conditions, the requirements for the substrate, processing, coating and packaging of the lower protective lens are very strict.

lower protective lens

Generally speaking, the original lower protective lens of the cutting head is expensive, which also makes the lower protective lens sold by the third party very popular, but not every third-party manufacturer has the production capacity of this kind of consumables.

In recent years, when the price war of fiber lasers is hot, the price of laser accessories is getting lower and lower.

Blindly pursuing low prices and ignoring quality will not pay off.

2. What consequences will there be if the selection is improper?

The unqualified lower protective lens absorbs light and heats violently, which will cause the focus to drift significantly, resulting in poor cutting.

Serious heating will also lead to the breakdown of the lower protective lens, and even reverse breakdown of the focusing and alignment of the cutting head.

Once the high-value parts are damaged, the maintenance cost is as high as 20%-50% of the price of the new cutting head, and the loss is huge.

Lower protective lens breakdown

The unqualified coating has weak light transmittance, which will lead to insufficient transmission and strong reflection, which will cause the return light of the lower protective lens to reverse through the collimating lens and focusing lens of the cutting head, and then converge near the end cap of the output head, resulting in the burning of the protective window of the output head.

The protective window cover and protective end cap of cutting head are burned

More seriously, when the end cap of the output head is burned, the forward laser cannot be transmitted and is suffocated in the output head, which directly leads to the destruction of the metal structure of the output head, water leakage, slag spraying and other phenomena.

In this case, it will generally cause irreversible damage to the cutting head.

Cutting head shell burned

3. How to choose protective lens?

Generally speaking, for small and medium-sized end customers, the demand for consumables is relatively small, and the bargaining power is weak.

They can purchase through the channel of equipment suppliers.

For informal channels or when the price deviates significantly, they should be cautious.

Large terminal or integrator customers can purchase from integrators, consumable manufacturers or first-class agents because of their large demand and strong bargaining power and quality inspection ability.

When purchasing protective lens, you can distinguish them according to the following points:

Base material: select mature materials in the industry.

Manufacturers: try to select optical cold processing manufacturers with high-power laser window, lens and other optical parts production qualifications.

Coating: the antireflective film is customized according to the laser output wavelength.

Test report: provide transmission wavelength report of protective lens, qualified finish report, material model, weak absorption report (not necessary), coating adhesion report, size and shape report, etc.

4. What kind of protective lens must not be used?

There are four kinds of protective lenss that must not be used.

One is the protective lens with film coating and exposure of quartz substrate.

This kind of protective lens can be recognized by naked eyes;

Lower protective lens coating stripping

The second is the protective lens with poor coating quality.

The appearance of this protective lens is correct, but the transmittance of some areas of the coating is tested. The transmittance of some areas is low and the reflection is high.

This type of protective lens cannot be recognized by naked eyes and microscope, and can only be measured by professional instruments;

Poor coating quality of lower protective lens

Poor coating quality of lower protective lens

The third is a protective lens with foreign matters or bubbles between the film and the substrate.

This type of window is used under high power, with short service life and easy to burn;

Bubbles or foreign matters at the bottom of the film

The fourth kind of protective lens with poor finish.

This kind of service life is short.

Poor finish of lower protective lens

5. How to judge if the lower protective lens is abnormal?

How to accurately identify the system instability and risks caused by the abnormal protection is an urgent need in the current cutting market.

At present, most high-power cutting heads have protective lens temperature and scattered light monitoring, but some cutting head monitoring deduces the current temperature indirectly, which cannot achieve high accuracy and high sensitivity response.

Sensor packaging position and abnormal points that can be monitored

In view of the above situation, Raycus has fully introduced a new return light monitoring system into the medium and high-power lasers.

The most important return light monitoring sensor is encapsulated near the end cap of the output optical cable, forming an approximate coaxial monitoring.

When the optical elements in the cutting head are damaged and generate return light, it can respond quickly to it.

Model of laser power section and output optical cable with return light monitoring

Under the condition that the water cooling system is normal, the return visit optical monitoring of Raycus laser will quickly send an alarm signal when it monitors the following three conditions:

(1) The lower protective lens is abnormal;

(2) Abnormal upper protective lens;

(3) Processing high reflective materials.

Taking Raycus 20kW model as an example, the return light monitoring has been introduced since 2019, and the following problems have been successfully avoided:

  • Identify the improper use of the client and protect it from the burning of the output head;
  • Identify the bad design of the cutting head manufacturer;
  • Determine that the cutting head of the customer is in poor condition;
  • Identify customer high reverse cutting.

At present, the quality of protective lenses in the market is uneven.

It is recommended to carefully identify them according to the above contents and purchase them through formal channels.

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