⚡ Quick Answer: Readable vs. Verifiable DPM
What is the difference between a Readable code and a Verifiable code?
- Readable:Means a scanner can extract the data. It is a simple “Pass/Fail” check used in retail. A code can be readable by a smartphone but still fail industrial standards.
- Verifiable:Means the mark is graded (A through F) against specific quality standards like ISO 29158 (AIM DPM). It measures technical parameters like Cell Contrast, Modulation, and Quiet Zone to ensure long-term scannability.
How do you achieve Grade A Verification? To move from Grade D to Grade A on metal parts, you must:
- Switch Standards:Stop using ISO 15415 (Paper) and use ISO 29158 (DPM).
- Optimize Hatching:Use Cross-Hatching (0.05mm spacing at 45°/135°) to create solid, dense cells.
- Enhance Contrast:Use MOPA Fiber Lasers to create high-contrast black annealed marks or white foam marks depending on the substrate.
- Protect the Quiet Zone:Ensure a clear margin of at least 4 modules around the code.
The Standard: What is ISO 29158 (AIM DPM)?
For decades, the world used ISO 15415. This standard was designed for high-contrast labels—black ink on white paper.
But in industrial manufacturing, we aren’t printing on paper. We are blasting grey craters into grey steel, or burning dark oxides onto aluminum. The contrast is low, and the surface is shiny or rough. ISO 15415 fails almost all industrial marks.
Enter ISO 29158 (formerly known as AIM DPM). This is the global standard for Direct Part Marking. It adjusts the grading rules to account for:
- Lighting: It uses specific lighting angles (30°, 45°, 90°, Dome) to create shadows in the marked dots.
- Threshold: It calculates a specialized “binary threshold” to distinguish the mark from the background noise of the metal texture.
The Anatomy of a Grade: Why Did You Get a “C”?
A DPM Verification system (like a Cognex or Keyence verifier) analyzes several parameters. If any one of these fails, the entire code fails.
📉 A. Cell Contrast (CC)
This measures the difference in brightness between the “marked” cell and the “quiet” background.
The Problem: On dark metal, your laser mark might not be dark enough (or light enough) to stand out.
HeatSign Fix: Use a MOPA Fiber Laser to create “Black Annealing” (black mark, no depth) or “White Foaming” on plastics to maximize contrast.
💠 B. Cell Modulation (CM)
This is often the grade-killer. It measures the consistency of the cells. Are some dots darker than others? Is the background uneven?
The Problem: Uneven surface texture or a laser that is fluctuating in power.
HeatSign Fix: Use “Wobble” settings in EzCad software to widen the beam path, creating a more solid, uniform fill for each cell.
🚧 C. Fixed Pattern Damage (FPD)
This checks the critical “Finder Pattern”—the L-shaped solid line and the timing dots on the edges.
The Problem: If your “Quiet Zone” (the blank space around the code) is cluttered with scratches or the part edge, this grade drops.
HeatSign Fix: Ensure a 4-module Quiet Zone (white space) around every code.
📏 D. Axial Non-Uniformity (ANU)
Is the square actually a square? Or is it stretched into a rectangle?
The Problem: The part was moving on the conveyor, or the laser lens was distorted at the edge of the field.
HeatSign Fix: Ensure the focal distance is perfect (use the Red Light Pointer). For cylindrical parts, use a Rotary Axis (HeatSign HS-FL30-R) to prevent stretching.
How to Improve Your Grades: Tuning Your Machine
You cannot simply press “Start” and expect Grade A. You must engineer the mark. Here is how to adjust your HeatSign equipment for ISO compliance.
⚙️ Optimizing Fiber Lasers (HS-FL Series) for Grading
- The “Hatch” is Everything
The most common cause of Grade C/D marks is poor hatching.
Bad Practice: Using a single hatch with wide spacing (e.g., 0.1mm). This leaves gaps inside the black squares.
The Grade A Secret: Use Cross Hatching (two passes at 90-degree angles).
Hatch 1: 45 degrees, 0.05mm spacing.
Hatch 2: 135 degrees, 0.05mm spacing.
Result: A solid, dense black square with high modulation scores.
- Spot Compensation (Bleed)
Lasers burn material, and heat spreads. If you tell the laser to make a 0.5mm dot, it might actually make a 0.55mm dot due to melting.
The Issue: The dots swell and touch each other (“Cell Overlap”), ruining the grade.
The Fix: In EzCad, adjust the “Spot Diameter” setting. If your cells are touching, reduce the fill percentage or enable “Spot Compensation” to shrink the vector slightly.
- Cleaning Pass
If marking on cast aluminum or rusty steel, the background noise will kill your grade.
The Fix: Run a high-speed, low-power “cleaning box” pass over the area before marking the code to create a smooth, uniform canvas.
🔨 Optimizing Dot Peen (HS-DE Series) for Grading
Dot peen is harder to verify because it relies on depth/shadows rather than color change.
- Dot Spacing (DPI)
The Issue: If dots are too close, the metal deforms and merges them into a trench.
The Fix: Ensure clear separation. The ideal dot peen Data Matrix is discontinuous (dots do not touch). The Verifier uses “Dot Center” algorithms to connect them virtually.
- Stylus Angle
The Fix: Use a 90-degree stylus (sharper) rather than a 120-degree stylus. This creates a steeper crater wall, which casts a better shadow for the verifier camera to read.
Comparison Table: Readable vs. Verifiable
Feature | Standard “Readable” Code | ISO 29158 “Verifiable” Code |
Scanner | Smartphone or Handheld Barcode Scanner | Certified DPM Verifier (e.g., Cognex/Keyence) |
Criteria | Can the data be extracted? | Does the mark meet geometric & contrast math? |
Lighting | Ambient light | Controlled 30/45/90° + Dome Lighting |
Longevity | Uncertain | Guaranteed for part life (10-20 years) |
Industry | Retail, Logistics, Marketing | Aerospace, Automotive, Medical (UDI) |
Pass/Fail | Yes/No | Grade A (4.0) to F (0.0) |
HeatSign Equipment Recommendations
Not all machines can achieve Grade A. It requires precision.
🏆 For Strict ISO 29158 Compliance (Grade A/B)
Recommendation: HeatSign HS-FL20 Fiber Laser Marker
Why: The galvo scanner precision (0.001mm) allows for perfect cell modulation.
Key Feature: Support for MOPA sources. MOPA lasers allows you to adjust pulse width. Short pulses (2ns) can mark plastics without burning; long pulses (200ns) can create deep black marks on stainless steel. This flexibility is essential for “tuning” contrast for verification.
Price Range:$1800 ~ $2500
🚜 For Deep Marking (VINs / Chassis)
Recommendation: HeatSign HS-DE01 Dot Peen Marker
Why: When the requirement is durability over contrast (e.g., painting over the code later).
Note: Dot peen usually achieves Grade B or C due to the nature of the indentation. This is acceptable for most automotive chassis applications if the “Dot Peen Algorithm” is used in verification.
Price Range:$1900 ~ $2900
🩺 For Medical UDI (Corrosion Resistant)
Recommendation: HeatSign HS-FL30M (MOPA Laser)
Why: Surgical tools must pass passivation tests (acid baths). Standard laser marking can rust. A MOPA laser creates a Black Annealed mark that is smooth to the touch and does not trap bacteria, meeting FDA UDI Class I/II standards perfectly.
Price Range:$3500 ~ $4500
Conclusion: Quality is Not an Accident
Achieving Grade A verification isn’t about luck; it’s about physics. It requires the right wavelength, the right pulse width, and the right geometry.
At HeatSign, we don’t just ship machines; we help you configure them. If you are struggling with DPM grading, send us your technical requirements. We will mark sample parts and provide a verification report to prove our capability.
FAQs
What is the minimum grade required for Automotive (Tier 1)?
Most OEMs (Ford, GM, Toyota) require a minimum of Grade B (3.0) or Grade C (2.0) at the time of marking. Grade D or F typically results in a rejected shipment.
Can I verify a code using my phone?
No. There are apps that claim to “grade” codes, but they are not ISO compliant. They cannot control the lighting angle or distance strictly enough. You need a dedicated hardware verifier.
My code fails on "Quiet Zone." What is that?
The Quiet Zone is the blank margin around the barcode. It must be at least 4 times the width of a single cell. If you mark too close to a screw, edge, or another logo, the verifier cannot find the code boundaries.
Is it easier to grade Square Dots or Round Dots?
For lasers, Square Dots generally yield higher grades because they fill the grid pixel 100%. Round dots leave empty space in the corners of the grid cells, which can lower the “Modulation” score. HeatSign EzCad software allows you to choose “Square” filling.
Why does my mark grade well at 90° lighting but fail at 30°?
90° (direct top-down) lighting is good for high-contrast printed labels. 30° or 45° lighting is designed to catch the shadows of Dot Peen or engraved marks. If you are laser engraving deep, you need to check which lighting angle your customer specifies in their engineering drawing.
Does marking speed affect the grade?
Yes. If you mark too fast, the laser mirrors may drift (wobble), causing “Axial Non-Uniformity.” Slowing down the speed by 10-20% often instantly improves the grade from a C to a B.
What is the difference between Validation and Verification?
Validation: Checking if the data is correct (e.g., “Does this GTIN match the product?”).
Verification: Checking if the symbol quality is good (e.g., “Is the contrast sufficient?”).
Can HeatSign machines interface with Verifiers?
Yes. Our systems can be integrated with inline verifiers (like Cognex). If the verifier detects a Grade C or below, it can send a signal to the HeatSign machine to stop production or re-mark the part.
