Problems of laser cutting machines in medium and thick plate processing，Processing medium and thick plates (typically defined as 10mm to 25mm+) presents a unique set of physical challenges compared to thin sheet metal. The primary difficulty is managing the immense heat accumulation and ensuring the assist gas can effectively flush molten metal out of a deep, narrow “kerf” (cut slot).
Here are the common problems encountered in thick plate laser cutting, along with their causes and solutions.
1. Perforation (Piercing) “Blowouts”
The most dangerous part of the process is the initial hole.
Unlike thin sheets where the laser punches through instantly, thick plates require a “drilling” time.
- The Problem: Molten metal explodes upwards (“popping”) instead of flowing down. This spatter can ruin the nozzle, crack the protective lens, or create a crater that ruins the start of the cut.
- The Cause: Applying continuous high power (CW) instantly creates too much pressure in a blind hole.
- The Solution:
- Pulse Piercing: Use a pulsed frequency (e.g., 100Hz) with low duty cycle to gently melt the material layer-by-layer.
- Multi-Stage Pierce: Modern machines use a 2-step or 3-step pierce (e.g., blast hole high → pulse mid → low power low) to penetrate safely.
- Oil Spray: Coating the pierce point with oil before cutting prevents slag from sticking to the surface.
2. Striations and Rough Edges
Thick plate cuts often exhibit vertical lines (striations) along the edge, which worsen towards the bottom of the plate.
- The Problem: The top of the cut is smooth, but the bottom 1/3 is rough or serrated.
- The Cause:
- Gas Drag: As the gas blows down deep into the cut, it loses velocity and becomes turbulent, causing the molten metal to “wobble” as it ejects.
- Incorrect Focus: If the focus point is too high, the beam diverges before reaching the bottom, losing energy density.
- The Solution:
- Lower the Focus: For thick stainless (Nitrogen cut), focus should be deep inside the material (negative focus). For carbon steel (Oxygen cut), focus is typically on top or slightly above to widen the kerf.
- Increase Nozzle Size: Use a larger nozzle (e.g., 3.0mm – 4.0mm) to allow a higher volume of gas to support the cut column.
3. Dross / Slag Buildup (Bottom Burr)
- The Problem: Hard, molten metal solidifies on the bottom edge of the part (dross).
- The Cause:
- Speed Too Fast: The beam moves on before the cut is fully cleared, dragging metal behind it (drag lag).
- Speed Too Slow: The metal overheats, melting the bottom corners excessively.
- Gas Pressure Too Low: Not enough force to eject the heavy molten column.
- The Solution:
- Adjust Speed: Find the “sweet spot.”
- Oxygen Purity: For thick carbon steel, oxygen purity is critical. Dropping from 99.95% to 99.5% purity can reduce cutting speed by 50% and cause massive dross.
4. “Over-Burning” of Corners
- The Problem: Sharp corners or intricate details melt away, turning a 90-degree corner into a rounded blob.
- The Cause: Heat accumulation. When the laser head slows down to take a corner, the heat builds up in that spot faster than it can dissipate into the surrounding thick metal.
- The Solution:
- Power Ramping: The machine must automatically lower the laser power and frequency as it decelerates into a corner.
- Cooling Points: Program the machine to pause or add a “cooling ring” (micro-joints) at corners.
- Nitrogen vs. Oxygen: Oxygen reacts exothermically (creates its own heat), making burning worse. Nitrogen is cooler but more expensive.
5. Thermal Distortion (Warping)
- The Problem: The part bends or bows upwards after cutting, sometimes causing the cutting head to crash into the tipped-up part.
- The Cause: Thick plates hold massive residual stress. The heat of cutting releases this stress, causing the metal to spring back.
- The Solution:
- Common Line Cutting: Reduce heat input by sharing cut lines.
- Stitch Cutting: Leave small tabs (micro-joints) to keep the part locked to the heavy skeleton sheet, then break them later.