Thick Plate Laser Cutting Guide (20-40mm)

Cutting thick plate (20-40mm) requires high-power fiber lasers (12-30kW), specialized piercing techniques, and careful process control. This guide covers power requirements, parameters, piercing strategies, and quality optimization for structural steel fabrication.

Published: January 15, 2026

Last Updated: January 15, 2026

Power Requirements by Thickness

Thickness	Minimum Power	Recommended Power	Typical Speed	Gas
20mm Mild Steel	10kW	12-15kW	0.5-0.8 m/min	O₂
25mm Mild Steel	12kW	15-20kW	0.4-0.6 m/min	O₂
30mm Mild Steel	15kW	20-25kW	0.3-0.5 m/min	O₂
40mm Mild Steel	20kW	25-30kW	0.2-0.3 m/min	O₂
20mm Stainless	15kW	20kW	0.3-0.5 m/min	N₂

Higher power enables faster cutting and better pierce quality. 20kW+ lasers are standard for structural steel fabrication (20-40mm range).

Piercing Techniques for Thick Plate

1. Pulse Piercing (Standard)

Most common method. Uses pulsed power ramping to gradually penetrate material.

Process:

• Start at 10-20% power, ramp to 100%
• Pulse duration: 1-5ms
• Pierce time: 3-15 seconds
• High gas pressure (15-20 bar)

Pros & Cons:

✓ Safer for optics (less back-reflection)
✓ Cleaner pierce hole
✗ Slower (3-15 sec per pierce)

2. High-Speed Piercing

Uses full power with controlled oxygen assist. Faster but requires protective measures.

Process:

• Full power (100%) from start
• Low O₂ pressure initially (0.5 bar)
• Ramp gas pressure to 2-3 bar
• Pierce time: 1-3 seconds

Pros & Cons:

✓ Very fast (1-3 sec)
✓ Higher productivity
✗ Higher spatter risk
✗ Requires PierceGate protection

3. Edge Start (Best for Quality)

Start cut from plate edge rather than piercing. Highest quality but limited applicability.

Process:

• Ramp power from edge (0-100%)
• Gradual acceleration to cut speed
• No pierce hole needed
• Lead-in from outside material

Pros & Cons:

✓ No pierce marks
✓ Highest edge quality
✗ Requires access to edge
✗ Not for internal features

Detailed Cutting Parameters

Mild Steel with Oxygen

Thickness	Power	Speed	O₂ Pressure	Focus	Nozzle
20mm	12kW	0.6-0.8 m/min	1.5-2.0 bar	+4mm	4.0mm
25mm	15kW	0.5-0.6 m/min	1.8-2.2 bar	+5mm	4.5mm
30mm	20kW	0.4-0.5 m/min	2.0-2.5 bar	+6mm	5.0mm

Key Parameter Notes

Positive Focus: +4 to +8mm above surface for thick plate
Large Nozzle: 4.0-5.0mm diameter for debris clearance
Moderate O₂: 1.5-2.5 bar (too high = excessive burning)
Slow Speed: Quality over speed for thick material
Beam Quality: M² <1.1 critical for deep penetration

Quality Indicators

Top Edge: Should be clean, minimal rounding
Bottom Edge: Slight dross acceptable (<0.5mm)
Surface Roughness: Ra 12.5-25 μm (ISO 9013 Range 3-4)
Perpendicularity: ±0.5mm tolerance typical
Heat Affected Zone: 0.3-0.8mm depth

Common Challenges & Solutions

Challenge: Pierce Hole Blow-Back Damage

Problem: Molten material ejects upward during pierce, damaging optics

Solutions:

Use PierceGate or protective shutter (Trumpf, Bystronic)
Start pierce away from edge (>20mm clearance)
Use pulse piercing instead of full power
Increase nozzle standoff during pierce (3-5mm)
Replace protective window every 40-60 hours

Challenge: Excessive Bottom Dross

Problem: Heavy dross formation makes parts unusable without grinding

Solutions:

Increase cutting speed 10-15%
Optimize focus position (+0.5 to +1mm adjustment)
Increase oxygen pressure slightly (0.2-0.3 bar)
Check material quality (oxide scale causes issues)
Use larger nozzle (better debris evacuation)

Challenge: Cut Loss (Incomplete Penetration)

Problem: Laser fails to penetrate full thickness

Solutions:

Increase laser power 15-20%
Decrease cutting speed 10-15%
Verify beam quality (M² check)
Clean/replace focus lens if contaminated
Check material thickness uniformity (mill scale variation)

Challenge: Taper (Non-Perpendicular Cut)

Problem: Cut angle deviates >1° from perpendicular

Solutions:

Optimize focus position (critical for thick plate)
Reduce cutting speed if burning too aggressively
Check nozzle centering and alignment
Verify material is flat (max 2mm deviation)
Consider dual-focus technique (advanced)

Best Practices

Material Preparation

• Remove heavy mill scale (sand blast if thick)
• Verify material thickness consistency
• Check flatness (critical for 20mm+)
• Use heavy-duty support slats (25mm spacing)
• Preheat in cold conditions (<10°C)

Process Optimization

• Start with manufacturer parameters
• Test cut on scrap (full thickness)
• Monitor first 3-5 parts closely
• Document optimal settings by thickness
• Plan pierce locations away from features

Equipment Requirements

• Minimum 12kW for 20mm production
• 20kW+ recommended for 25-40mm range
• PierceGate or protective shutter essential
• High-capacity chiller (cooling critical)
• Robust support structure (heavy plates)

Safety & Maintenance

• Inspect protective window every 30 hours
• Monitor cutting head for spatter buildup
• Clean nozzle exterior daily
• Replace nozzles every 40-60 hours
• Heavy debris = fire risk, clean regularly

Cost Considerations for Thick Plate

High Power Requirements

20mm needs 12-15kW, 30mm needs 20-25kW. Equipment cost scales with power.

Slow Cutting Speeds

0.3-0.8 m/min vs 2-5 m/min for thin material. Longer cycle times.

Accelerated Consumable Wear

Nozzles, protective windows wear 2-3× faster due to heavy debris.

Post-Processing Often Required

Grinding bottom dross adds $5-15 per meter of cut.

Typical Cost Impact: Thick plate (20-30mm) costs 3-5× more per part than equivalent 10mm parts due to equipment, time, and consumable requirements.

Data Sources

• Trumpf High-Power Laser Guide 2024: 15-30kW cutting parameters
• Bystronic Thick Plate Application Notes: Piercing techniques
• ISO 9013:2017: Quality classification for thick plate
• Field data: Structural steel fabrication experience

Disclaimer: Thick plate cutting is challenging and requires high-power equipment (12kW+). Parameters are starting points; optimize based on material condition and quality requirements.