What GS1 says about the mark itself. GS1’s technical considerations for direct part marking ask for at least a 20 % difference in reflectance between the mark and the surface, warn against finishes that cause heavy shadowing or glare, and keep marks off sealing surfaces, wear surfaces and surfaces in direct air or water streams. Marks should go on surfaces no rougher than 250 and no smoother than 8 micro-inches; rougher surfaces need larger cells (GS1 General Specifications, 5.12.4.3).
GS1: minimum Data Matrix cell size by surface roughness (selected rows)| Average surface roughness | Minimum cell size (GS1 Table 5-61) | Typical surface |
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| 0.51 µm (20 µin) | 0.19 mm | Ground or fine-machined faces |
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| 1.52 µm (60 µin) | 0.23 mm | Typical turned or milled faces |
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| 3.05 µm (120 µin) | 0.38 mm | Rougher machined or blasted faces |
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| 5.08 µm (200 µin) | 0.51 mm | Near the GS1 upper roughness limit |
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GS1: marking depth and material thickness by method (selected rows of Table 5-62)| Marking method | Minimum material thickness | Maximum marking depth |
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| Dot peen | 1.016 mm | 0.102 mm |
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| Laser etch | 0.762 mm | 0.076 mm |
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| Laser engraving | 1.27 mm | 0.127 mm |
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| Electro-chemical etch | 2.54 mm | 0.254 mm |
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| Laser bonding (additive) | 0.025 mm | Surface mark |
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For GS1 DataMatrix marked directly on items other than medical devices, GS1 symbol specification table 7 sets a cell size of 0.254 to 0.615 mm with a target of 0.300 mm, a one-cell quiet zone and a minimum quality of 1.5/06/660 (GS1 General Specifications, table 7). Internal part marks that do not use GS1 data follow the customer’s own specification, but the same physics apply.
Worked example: how big is the mark, and can the reader resolve it? In Data Matrix, two digits or one letter take one codeword; our GS1 DataMatrix guide gives the capacity table. The rows below are arithmetic on the stated data and cell sizes.
Example: data, symbol size and cell size for direct part marks| Data | Codewords | Smallest square symbol | Cell size | Mark size |
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| Serial AB12345678901234 (2 letters, 14 digits) | 9 codewords | 16 × 16 (holds 12) | 0.30 mm (GS1 target) | about 4.8 mm, 5.4 mm with quiet zone |
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| Same serial on a cast face, Ra about 3 µm | 9 codewords | 16 × 16 | 0.38 mm (GS1 minimum for that roughness) | about 6.1 mm, 6.9 mm with quiet zone |
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| GS1 element string: FNC1, (01) GTIN, (21) 8-digit serial | about 14 codewords | 18 × 18 (holds 18) | 0.30 mm | about 5.4 mm, 6.0 mm with quiet zone |
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On the reader side, assume a 30 mm field of view on a sensor 1,280 pixels wide: each pixel covers about 0.023 mm, so a 0.30 mm cell spans about 13 pixels and a 0.38 mm cell about 16. A wider field of view to cope with loose part positioning lowers that quickly, which is why DPM stations locate the part first.
Light for marks
HIKROBOT’s glossary explains why dot-peen and laser marks need different light: below 45°, flat areas look dark and rough or raised areas bright, which lights up peened dots; coaxial light turns a flat polished face into a mirror on which marked cells show dark; shadowless light weakens grain and tooling marks (HIKROBOT light catalogue, glossary).
How a DPM station is accepted
- Read rate: good decodes divided by read attempts, on parts from every marking machine, material and surface condition that reaches the station.
- No-read rate and its bound: a trial of 1,000 parts with no no-reads bounds the no-read rate at about 0.27 % with 95 % confidence, using the Wilson method in NIST’s handbook (NIST e-Handbook, 7.2.4.1); it does not prove zero.
- Content and duplicates: every decoded serial checked against the expected format and the record, with duplicates flagged.
- Mark quality trend: where the reader reports a quality value, its trend shows a worn stylus or a drifting laser before reads fail. A formal grade to ISO/IEC 29158 needs a verifier.