ISO 20457 (DIN 16742) Plastic Molding Tolerances: TG1–TG8 Tables

Independent buyer guide 9 min read Updated November 2026

One of the most expensive errors on B2B plastic part drawings is leaving a default metal CNC machining title-block tolerance (ISO 2768-m or ±0.10 mm blanket block) on an injection-molded polymer component. Unlike CNC-milled aluminum—where thermal contraction after cutting is negligible—injection-molded thermoplastics shrink by 0.2% to 3.5% as they cool, and semi-crystalline resins shrink anisotropically between flow and cross-flow directions.

To replace the obsolete DIN 16901 standard and internationalize the German DIN 16742 standard, the International Organization for Standardization published ISO 20457:2018 (Plastics moulded parts — Tolerances and acceptance conditions). This guide provides the complete Tolerance Group (TG1 to TG8) selection matrix, the Tool-Bound (NW) vs Non-Tool-Bound (W) dimensional tolerance tables, and practical DFM rules for specifying achievable Cpk tolerances.

1. Why ISO 2768-m Metal Tolerances Fail on Molded Plastics

Under ISO 2768-m (Medium) for machined metal parts, a 100 mm dimension carries a general tolerance of ±0.30 mm. If you mold that 100 mm part in unfilled Polypropylene (PP) or Acetal (POM) (nominal mold shrinkage 1.8–2.2%):

  • A standard ±3 °C cooling water variation or ±5 MPa packing pressure fluctuation alters linear shrinkage by ±0.15% (±0.15 mm).
  • Add parting-line lock clearance (±0.05 mm) and post-mold ambient conditioning, and the natural 6σ process spread of that 100 mm PP feature is ±0.45 to ±0.65 mm.
  • Specifying ISO 2768-m forces the molder either to no-bid the RFQ, add a secondary CNC post-machining fixture (+$0.40–$1.20/part), or take exception to 80% of your drawing dimensions during PPAP.

2. Step 1: Assign the Resin & Process to an ISO 20457 Tolerance Group (TG1–TG8)

ISO 20457 classifies polymers by their mold shrinkage magnitude and shrinkage anisotropy, and then maps them to three production precision levels:

  • Series 1 (Standard Production — Low Cost): General commercial molding without cavity-pressure control or post-mold conditioning fixtures.
  • Series 2 (Precision Production — Industrial / Automotive Standard): Scientific molding with strict barrel/mold temperature control (±1–2 °C), desiccant drying, and optimized cooling layout.
  • Series 3 (High-Precision Production — Premium Cost +25–50%): Cavity-pressure transducer switchover, multi-circuit variotherm or conformal cooling, tight-tolerance ground tool steel (±0.005 mm), and iterative steel-safe grooming loops.
Polymer Family & Filler LoadingTypical Mold ShrinkageSeries 1 (Standard)Series 2 (Precision)Series 3 (High-Precision)
PC, ABS, PC/ABS, SAN, ASA, PSU, PEI, PMMA, PVC-U (Unfilled Amorphous)0.4–0.8%TG5TG4TG3
PA6-GF30, PA66-GF30, PBT-GF30, PET-GF30, PPS-GF40, PEEK-GF30 (Glass/Mineral Filled)0.2–0.7% (Anisotropic)TG5TG4TG3
PA6, PA66, PBT, PET, PEEK (Unfilled Semi-Crystalline, Low-to-Medium Shrink)0.8–1.8%TG6TG5TG4
POM (Acetal), PP (Unfilled), HDPE, LDPE, PVDF, PFA (High-Shrink Semi-Crystalline)1.5–3.5%TG7TG6TG5
TPE, TPU, TPV (Shore 50A–90A) (Soft Thermoplastic Elastomers)1.0–2.5%TG8TG7TG6

Note: TG1 and TG2 are reserved for ultra-precision micro-optics and fine-pitch LCP connectors (<10 mm) requiring multi-loop diamond-turned or wire-EDM cavity compensation.

3. Step 2: Distinguish Tool-Bound (NW) vs Non-Tool-Bound (W) Dimensions

Like DIN 16742 before it, ISO 20457 splits every dimension on a drawing into two physical categories depending on how the mold steel forms it:

  1. Tool-Bound Dimensions (NW — Nicht Werkzeuggebunden / Formed Within One Mold Half):
    • Dimensions formed entirely within the solid A-side cavity or entirely within the solid B-side core (such as hole-to-hole center distances on the same face, internal pocket width, or boss diameter).
    • Variation comes only from steel machining error plus polymer shrinkage.
  2. Non-Tool-Bound Dimensions (W — Werkzeuggebunden / Crossing the Parting Line or Moving Slides):
    • Dimensions formed across the main A/B parting line (such as overall part thickness or height parallel to mold opening) or between a stationary core and a moving side-action slide or lifter.
    • Must absorb parting-line flash/compression variation, guide-pin/interlock clearance, and slide wedge deflection, adding roughly +0.05 to +0.20 mm to the allowable tolerance band.

4. ISO 20457 (DIN 16742) Dimensional Tolerance Table (TG3 to TG7, in mm)

Values below show the total tolerance band (± half of total band for symmetrical bilateral tolerances) in millimeters (mm) across nominal dimension steps:

| Nominal Dimension Range (mm) | Condition (NW vs W) | TG3 (High-Prec Amorphous/GF) | TG4 (Precision Amorphous/GF) | TG5 (Std Amorphous / Prec PA/PBT) | TG6 (Precision POM/PP/PE) | TG7 (Standard POM/PP/PE) | |---|---|---|---|---|---|---|---| | 1 to 3 mm | NW (One mold half) | ±0.025 | ±0.040 | ±0.060 | ±0.090 | ±0.140 | | 1 to 3 mm | W (Across parting/slide) | ±0.040 | ±0.060 | ±0.090 | ±0.140 | ±0.210 | | > 3 to 6 mm | NW (One mold half) | ±0.030 | ±0.050 | ±0.075 | ±0.120 | ±0.180 | | > 3 to 6 mm | W (Across parting/slide) | ±0.050 | ±0.075 | ±0.120 | ±0.180 | ±0.270 | | > 6 to 10 mm | NW (One mold half) | ±0.040 | ±0.060 | ±0.095 | ±0.150 | ±0.220 | | > 6 to 10 mm | W (Across parting/slide) | ±0.060 | ±0.090 | ±0.145 | ±0.220 | ±0.330 | | > 10 to 18 mm | NW (One mold half) | ±0.050 | ±0.075 | ±0.115 | ±0.180 | ±0.270 | | > 10 to 18 mm | W (Across parting/slide) | ±0.070 | ±0.110 | ±0.175 | ±0.270 | ±0.400 | | > 18 to 30 mm | NW (One mold half) | ±0.060 | ±0.095 | ±0.145 | ±0.220 | ±0.340 | | > 18 to 30 mm | W (Across parting/slide) | ±0.085 | ±0.135 | ±0.210 | ±0.330 | ±0.500 | | > 30 to 50 mm | NW (One mold half) | ±0.075 | ±0.120 | ±0.185 | ±0.280 | ±0.440 | | > 30 to 50 mm | W (Across parting/slide) | ±0.105 | ±0.165 | ±0.260 | ±0.400 | ±0.630 | | > 50 to 80 mm | NW (One mold half) | ±0.095 | ±0.150 | ±0.235 | ±0.360 | ±0.560 | | > 50 to 80 mm | W (Across parting/slide) | ±0.130 | ±0.210 | ±0.325 | ±0.500 | ±0.780 | | > 80 to 120 mm | NW (One mold half) | ±0.120 | ±0.190 | ±0.300 | ±0.460 | ±0.720 | | > 80 to 120 mm | W (Across parting/slide) | ±0.160 | ±0.260 | ±0.405 | ±0.630 | ±0.980 | | > 120 to 180 mm | NW (One mold half) | ±0.155 | ±0.245 | ±0.385 | ±0.600 | ±0.940 | | > 120 to 180 mm | W (Across parting/slide) | ±0.205 | ±0.330 | ±0.515 | ±0.800 | ±1.250 |

5. How to Call Out ISO 20457 on Engineering Drawings

  1. Title Block Callout: Replace ISO 2768-m in your CAD title block with the appropriate ISO 20457 Tolerance Group for your material—for example, General Tolerances: ISO 20457 - TG5 (for standard PC/ABS or precision PA66) or ISO 20457 - TG6 (for precision POM or PP).
  2. Draft Angle Compensation (Reference Plane Specification): Under ISO 20457 Section 5.3, tapered walls with draft (1°–2°) will measure differently at the top vs bottom of the wall. For any critical-to-quality (CTQ) slot or bore with draft, explicitly define the Inspection Measurement Plane (e.g., “Dimension Ø12.00 ±0.06 measured 2.5 mm below Datum A”).
  3. Inspection Conditioning Atmosphere (ISO 291): Always note on the drawing that dimensional CMM/optical inspection applies minimum 16 to 40 hours after molding at 23 ± 2 °C and 50 ± 10% Relative Humidity (ISO 291). For hygroscopic PA6 and PA66, explicitly state whether tolerances apply in the Dry-As-Molded (DAM) state or after moisture conditioning (+0.3% to +0.7% linear dimensional growth at 50% RH).

Frequently Asked Questions (PAA)

Is DIN 16742 the same as ISO 20457?

Yes—ISO 20457:2018 is the direct international adoption of DIN 16742:2013, which itself replaced the obsolete DIN 16901:1982 standard. The Tolerance Groups (TG1 through TG8), the tool-bound (NW) vs non-tool-bound (W) numerical tables, and the polymer shrinkage classifications are identical between DIN 16742 and ISO 20457.

What is the difference between NW and W dimensions in ISO 20457 / DIN 16742?

In ISO 20457 (retaining the German abbreviations from DIN 16742), NW (Tool-Bound) refers to dimensions formed entirely inside a single solid mold half (either the A-cavity or B-core), where steel components do not move relative to one another. W (Non-Tool-Bound) refers to dimensions measured across the parting line or between a fixed mold half and a moving slide/lifter, which carry a 30–50% wider tolerance band to account for mold breathing, flash clearance, and slide lock play.

Can injection molding hold ±0.025 mm (±0.001 in) tolerances?

Yes, ±0.025 mm (TG3) is achievable on small tool-bound (NW) dimensions under 6–10 mm when molding low-shrinkage amorphous or glass-filled engineering resins (PC, LCP, PEEK-GF30, PPS-GF40, PA66-GF30) in a single- or low-cavity hardened mold using cavity-pressure sensor switchover and a 1-loop Steel-Safe Grooming correction after T1 tryout. However, holding ±0.025 mm across a 100 mm Polypropylene part is physically impossible without secondary CNC machining.

Scope Note

PlasticsTechnologyAlliance.com is an independent buyer resource and does not manufacture parts, build molds, or certify suppliers. Confirm final tooling architecture, resin grade datasheets, and validation protocols directly with your contract molder and material manufacturer.

Buyer-side engineering explainer

Warpage in T1 Samples: Dimensional Approval Explainer

A bowed or twisted T1 sample becomes an approval risk when it affects critical dimensions, assembly surfaces, sealing, or function. Visual flatness alone does not establish dimensional readiness.

PTA buyer review mode
Buyer review: connect the finding to evidence and a decision Warp map CTQ map Fixture method Cavity data Functional test DECISION Evidence before release Static fallback: review the full evidence package before releasing the next buyer decision.

Static review summary: map distortion to critical dimensions and assembly function, compare representative measurements, and delay approval until repeatable dimensional evidence is reviewed.

Step 1: What the buyer sees

Record the distortion direction, cavity, time after molding, conditioning state, and repeatability.

Step 2: Why it matters

Focus on whether distortion affects fit, sealing, appearance, or critical dimensions rather than on visual flatness alone.

Step 3: Map CTQ impact

Connect the observed distortion to drawing requirements, fixtures, mating parts, and functional criteria.

Step 4: Patterns to verify

Warpage often reflects a system of interacting patterns; do not treat one visible direction as proof of one cause.

Step 5: Compare measurements

Ask for a consistent measurement method and compare cavities, process conditions, and relevant conditioning states.

Step 6: Request evidence

Require evidence that the proposed correction produces repeatable dimensional and functional results.

Step 7: Approval decision

Delay approval when distortion affects CTQs or assembly and the corrected condition is not demonstrated across representative samples.

This buyer-side explainer presents a dimensional review framework, not a confirmed warpage diagnosis. Verify significance and possible contributing patterns against the actual drawing, measurement method, material, geometry, mold, process, samples, and assembly requirements.

Sources and references

  1. Plastics moulded parts — Tolerances and acceptance conditionsISO 20457:2018Plastics moulded parts — Tolerances and acceptance conditions · Accessed November 2026
  2. Plastics moulded parts; tolerances and acceptance conditions (Superseded by ISO 20457)DIN 16742Plastics moulded parts; tolerances and acceptance conditions (Superseded by ISO 20457) · Accessed November 2026
  3. Guideline for Applying ISO 20457 in Injection Mold Design and Quality AssuranceVDI 2006 / VDWFGuideline for Applying ISO 20457 in Injection Mold Design and Quality Assurance · Accessed November 2026

Figures quoted from these sources are reproduced as published. Where this guide describes a range or a rule of thumb without a citation, treat it as general orientation and confirm the number against your own part, resin, and supplier. Corrections: [email protected].

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