A Practical Engineering Guide for Buyers and Designers

1. Why Tolerances Are the #1 Source of Tooling Conflict

Over 60% of tooling disputes originate from:

• Unrealistic part tolerances

• Confusion between tool tolerance and part tolerance

• Ignoring material behavior (shrinkage, fiber orientation, creep)

• Mixing CNC accuracy with molding process capability

Injection molding is not CNC machining. The mold is rigid. The plastic is not.

2. Three Different Tolerances (Often Confused)

Type	Definition	Governing Standard
Machining tolerance	Accuracy of steel manufacturing	ISO 2768, ISO 286
Tool tolerance	Cavity/core geometry	ISO 8015, ISO 1101
Part tolerance	Final molded part variation	ISO 20457, DIN 16742

Only the third one defines customer acceptance.

3. What CNC Toolmaking Can Achieve (Steel)

Feature	Realistic Tolerance	Standard Reference
Simple dimensions	±0.003–0.005 mm	ISO 2768-f
Complex geometry	±0.005–0.010 mm	ISO 8015
EDM features	±0.008–0.015 mm	ISO 1101
Large tools (>800 mm)	±0.010–0.020 mm	ISO 286 IT6–IT7

4. What Injection Molding Can Achieve (Plastic Parts)

Typical capability (CpK ≥ 1.33)

Feature Size	Typical Tolerance	Standard Class
< 10 mm	±0.03–0.05 mm	DIN 16742 Class F
10–50 mm	±0.05–0.10 mm	DIN 16742 Class M
50–150 mm	±0.10–0.20 mm	ISO 20457 Medium
>150 mm	±0.20–0.40 mm	ISO 20457 Coarse

5. Key International Standards to Reference in Drawings

Plastic Part Tolerancing

• ISO 20457 – Plastics molded parts: tolerances and acceptance conditions

• DIN 16742 – Dimensional tolerances for injection molded parts

• SPI / SPE Mold Standards – North American reference

Geometrical Tolerancing

• ISO 1101 – GD&T (form, position, flatness, parallelism)

• ISO 8015 – GPS fundamental rules (independency principle)

• ISO 5459 – Datums and datum systems

Process Capability

• ISO 22514 – Process capability and performance (Cp, Cpk)

• AIAG SPC Manual – Automotive capability requirements

Surface & Form (Optical / Sealing)

• ISO 10110 – Optical component tolerances

• ISO 3601 – O-ring grooves and compression

• DIN 5480 / ISO 1328 – Gear quality grades

6. Material Impact on Achievable Tolerances

Material	Stability	Relevant Standard Notes
PP, PE	Poor	ISO 20457 coarse class
ABS	Medium	DIN 16742 M
PC, PC/ABS	Good	DIN 16742 F
PA6/PA66	Medium	Moisture conditioning per ISO 1110
GF-reinforced	Directional	ISO 20457 anisotropy note
POM	Very good	DIN 16742 F
PPS	Excellent	Automotive Class F possible

7. Realistic Capability Matrix

Requirement	Feasible?	Standard Reality
±0.01 mm molded part	❌	Outside ISO 20457
±0.02 mm	⚠️	Local features only
±0.05 mm	✅	DIN 16742 F
±0.10 mm	✅	ISO 20457 M
Flatness 0.05 mm on 200 mm	❌	Violates form stability limits
Hole-to-hole ±0.03 mm	⚠️	Requires steel inserts + stable resin

8. Optical, Sealing, and Precision Parts

Optical (ISO 10110)

• Size: ±0.03–0.05 mm

• Form more critical than size

• Steel: S136 ESR or M300

Sealing (ISO 3601)

• Groove tolerances dominate, not free-state part size

• Compression zone more important than nominal dimension

Gears (ISO 1328 / DIN 5480)

• Molded gears: Quality 8–10

• Machined gears: Quality 6–7

9. Cost Impact of Tightening Tolerances

Tolerance Class	Relative Tool Cost
ISO 20457 Coarse	1.0
ISO 20457 Medium	1.3
DIN 16742 Fine	1.8
Optical / ISO 10110	2.5–4.0

Rule: Each halving of tolerance ≈ doubling of total tooling and validation cost.

10. Recommended Drawing Practice (TTH Standard)

Add a general note:

“Plastic part tolerances according to DIN 16742 Class M unless otherwise specified.
Functional features according to ISO 20457 Fine.
Geometrical tolerances according to ISO 1101.
Process capability: Cpk ≥ 1.33 per ISO 22514.”

11. Buyer vs Toolmaker Responsibility

Item	Standard	Owner
CNC accuracy	ISO 2768	Toolmaker
GD&T compliance	ISO 1101	Toolmaker
Part tolerance class	DIN 16742 / ISO 20457	Buyer
Capability (Cpk)	ISO 22514	Shared
Functional fit	Application standard	Buyer

12. Summary

Injection molding tolerances are limited by:

• Polymer physics

• Cooling uniformity

• Tool stability

• Process capability per ISO 22514

Not by CNC resolution.

Using the correct standards (ISO 20457, DIN 16742, ISO 1101) in RFQs and drawings:

• Aligns expectations

• Reduces change loops

• Prevents acceptance disputes

• Lowers total tool cost