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Problems that may occur when laser cutting plates

In laser cutting, there are problems such as scum at the bottom of the workpiece, folds at the lower end of the cutting surface, over-burning of right angles/sharp corners, and rough cutting surface. The following will systematically sort out the causes and solutions of these problems.

Overview of troubleshooting core issues

Problem 1: scum at the bottom of the workpiece (hanging slag)

Cause analysis: after the laser melts the material, it fails to be blown away from the bottom of the incision by the auxiliary gas in time and completely, thus condensing and forming hanging slag. This is essentially a matter of energy and gas balance.

Solution:

1. Optimize gas parameters:

  • Pressure; For stainless steel, aluminum alloy, etc., when using nitrogen as a high-purity auxiliary gas, the pressure must be high enough (for example, 1.6MPa or more) to achieve oxidation-free cutting and blow away slag. When using oxygen for carbon steel, the pressure should be moderate, too high will intensify the reaction to form rough slag.
  • Purity and type: to ensure that the gas purity (especially nitrogen requires more than 99.9%), oxygen purity also needs to be more than 99.5%.

2. Calibrate beam focus:

  • The focus position deviation is the most common cause. Make a focus calibration and try to fine-tune the focus position up and down. Generally, lowering the focus slightly (deep inside the material) helps to improve the bottom cut quality.

3. Adjust power and speed

  • Too fast speed or too low power will lead to cut through, the formation of soft hanging slag; too slow speed or too high power will lead to over melting, the formation of hard and brittle hanging slag. It is necessary to find the best power-speed combination that matches the thickness of the sheet.

4. Check and replace the nozzle:

  • The use of damaged, deformed or mismatched nozzles can disrupt the airflow. Check whether the center hole of the nozzle is rounded, and select the nozzle with the appropriate diameter according to the thickness of the plate (usually large diameter for thick plate and small diameter for thin plate).

Problem 2: Cut lower pleats (stripes/overburn)

Reason analysis: This is a typical overburning phenomenon. Due to excessive heat input, the material is excessively melted or even vaporized, forming rough, wrinkle-like stripes in the lower half of the cutting section.

Solution:

  • Primary adjustment: Increase the cutting speed. Speed is too slow is the main reason, improve the speed can reduce the laser in the unit area of the role of time, to avoid heat accumulation.
  • Reduce the laser power. If the speed can no longer be raised, or the lifting leads to a cut through, the power should be appropriately reduced to reduce the total heat input.
  • Adjust the focus position. Try to lower the focus position a bit to make the energy more concentrated and reduce the heat affected zone.
  • Check and reduce the auxiliary gas pressure. Especially when using oxygen in cutting carbon steel, too high oxygen pressure will aggravate the oxidation reaction like a “combustion improver” and generate excessive heat. Lowering the air pressure properly may have unexpected effects.

Problem 3: Right Angle/Sharp Burn

Reason analysis: The speed of the laser cutting head is stable when walking in a straight line, but when cutting to a right angle or sharp angle, it needs to decelerate, turn, and accelerate again. The speed drop at the corner causes the laser dwell time to become longer, and the heat accumulates sharply, thereby ablating the sharp corner.

Solution:

1. Use rounded corner transition (the most effective method):

  • In the drawing design stage, the sharp corner is changed to a small rounded corner (R corner). Even a small fillet with a radius of 0.5mm can make the cutting head transition smoothly, avoid drastic changes in speed, and effectively prevent over-burning.

2. Use the corner process function (bright corner function):

  • Modern laser cutting systems come with a “corner power control” or “bright corner” function. This function can:
  • When a corner is detected, the laser power is automatically reduced.
  • Or use segmented cutting, briefly turn off the laser at the corner, and cut in two segments.
  • Please consult your device operating system manual to enable and debug this feature.

3. Reduce the cutting speed at the corner:

  • In the process parameter table, the cutting speed of the corner is set separately to make it lower than the linear speed, but this needs to be used in conjunction with power reduction.

Problem 4: rough cutting surface (vertical grain)

Reason analysis: the cutting surface appears obvious, not smooth vertical stripes, usually due to energy instability or parameter mismatch, resulting in uneven melting state of the material.

Solution:

  • Ensure that the focus position is accurate and stable: This is the most critical factor to obtain a smooth cutting surface. A slight change in focus will lead to a huge difference in cross-sectional lines. Clean and calibrate the focusing lens to ensure that the focus is stable.
  • Use high-purity auxiliary gas: Impurities in the gas can interfere with the flow and cooling process of the molten metal, resulting in rough sections.
  • Optimize the cutting speed: Unstable speed or improper setting will cause the molten metal to form uneven stripes during the cooling process. Find a stable speed that produces continuous, smooth chips.

Check equipment stability:

  • Is the guide rail smooth? The vibration of the equipment will be directly reflected on the cutting surface.
  • Is the nozzle coaxial with the beam? Air flow of different axes can cause section asymmetry and roughness.
  • Are the optical lenses clean or damaged? Contaminated lenses can scatter laser energy, resulting in weak cuts and rough sections.

General Foundation Checklist

Before making any fine parameter adjustments, be sure to perform the following basic checks:

1. Optical lens: check whether the focusing lens and protective lens are clean, no pollution, no water stains, no damage.

2. Nozzle alignment: Ensure that the laser beam passes perfectly from the center of the nozzle.

3. Gas and gas circuit: confirm that there is no leakage in the gas circuit, and the gas type and purity are correct.

4. material: to ensure that the surface of the plate is clean, no oxide layer, oil, and uniform material.

Summary and Recommendations

Laser cutting is a precise process with multi-parameter synergy. The key to solving the problem is to “change only one variable at a time” and keep records to pinpoint the root cause of the problem.