What matters
- Tighten the few dimensions that protect function; leave the rest producible.
- A tolerance is incomplete without datums, measurement method, and conditioning assumptions.
- Use process capability data from the intended tool and resin for critical characteristics.
Use a plastics-specific framework
ISO 20457:2026 covers geometrical and dimensional tolerances and acceptance conditions for plastic molded parts. It explicitly recognizes that plastics show more dimensional, form, and location variation than metals because of shrinkage, stiffness, anisotropy, cooling, warpage, and processing conditions.
The standard is a framework, not a promise that every tolerance class is achievable on every part. The designer defines functional need; the manufacturer confirms whether the selected material, tool, and process can achieve it economically.
Features that deserve an early capability discussion
| Feature | Why it moves | Useful control |
|---|---|---|
| Long flat panel | Differential cooling and orientation | Profile to functional datums |
| Dimension across parting line | Tool alignment and flash | Datum strategy and tool construction |
| Hole-to-hole pattern | Shrinkage and local cooling | Position relative to datums |
| Snap or seal | Creep, material and local strain | Functional gauge or assembly test |
| Post-mold dimension | Conditioning and time | Defined measurement condition |
A drawing that supports molding and inspection
- Choose stable, functional datums that can also be fixtured and measured.
- Identify critical-to-function characteristics instead of applying a tight title-block tolerance everywhere.
- State resin, color/additives, measurement temperature, conditioning, and time after molding when relevant.
- Separate cosmetic acceptance from dimensional acceptance.
- Agree sample size, gauge method, and capability statistic before production validation.
Why unnecessary precision costs money
A tighter number can require mold-safe steel, correction loops, longer cooling, narrower process windows, more inspection, sorting, or secondary machining. Sometimes that is the correct trade. Often an assembly can be redesigned with clearance, compliance, a locating feature, or a functional gauge instead.
Do not loosen a requirement simply because it is difficult. Connect it to the failure it prevents, then choose the lowest-cost control that still protects that function.
Use this as a design review, not a production release
These guides support early planning. The selected resin supplier, toolmaker, and molder still need to confirm the final geometry, process window, tolerances, safety factors, validation plan, and commercial assumptions for your application.