Repeatable datums
Use stable planes and functional holes, not textured cosmetics.
GD&T for molded parts
GD&T for Molded Parts: Datums Inspectors Can Repeat
Pick datums the inspector can hold. Position on hole patterns must live with shrink and warp. DFM review comments on frames you put on CAD.
Design guideline
Datums first, then frames
GD&T on plastic should define functional datums, not copy a machined-metal scheme. Primary datums belong on stable planes, not on cosmetic texture. This page covers the method, not the commercial band.
Mixing metal GD&T habits with plastic shrink causes false failures on inspection reports. Standard tolerances own the commercial band. Tight tolerances own which mates get extra control. DFM review reads the frames on CAD.
This page covers the method. Tolerance stack-up owns the assembly chain. The manufacturing standards page holds the published size-class bands. The design guide lists every topic.
Roles
Which page answers which GD&T question
Stay here for datums and frames. Open a related guide for the band, the exception, or the stack.
| Page | Use it for |
|---|---|
| GD&T for molded parts (this page) | Datums and frames inspectors can repeat |
| Standard tolerances | The commercial plus-or-minus band |
| Tight tolerances | Which mates get extra control |
| Tolerance stack-up | The assembly chain |
| Manufacturing standards | Published size-class bands |
| Shrinkage compensation | Why plastic moves after the shot |
| Gate location | Fill that can move a hole pattern |
| Wall thickness | Uneven walls widen the effective band |
| Support surfaces | Physical rest pads. Datums may share a pad. |
| DFM review | CAD check before steel |
Checks
What DFM review looks for on molded GD&T
Qualitative checks. No figure on this page is a NetProto specification.
Plastic, not metal
Shrink and warp move features. A CNC habit will fail the report.
Hole patterns
Position frames need gate and warp in mind. Open gate location if fill drives it.
No new numbers here
Published bands live on manufacturing standards. This page covers the method.
Decision
Coordinate decimals only versus datum-referenced GD&T
Wrong: a drawing full of tight decimals on every edge. Right: datums on functional faces, frames on mates.
What goes wrong
- A datum on cosmetic texture
- Metal GD&T copied onto a warped cover
- Tight decimals with no datum scheme
What to try first
- Land datums on stable functional faces
- Frame mates, not every contour
- Open tight tolerances for the exception list
RFQ
Send these notes with CAD for a GD&T review
The quote can start from CAD alone. These notes make the DFM pass specific.
Proposed primary, secondary, and tertiary datums
Mates that need position or profile frames
Inspection method if you have one
Resin family
Hole patterns that cannot drift
Whether metal-style frames were copied on purpose
Related
Related design guidelines
Related guides for the next question, plus a CAD check when you are ready.
Manufacturing standards
Published size-class bands.
Manufacturing standardsDFM review
How NetProto reads frames before steel.
DFM reviewFAQ
GD&T for molded parts FAQ
Is GD&T required on RFQs?
No. It helps when mates and inspection matter. Upload frames if you have them. DFM review still runs from CAD alone.
Can flatness be tight on large covers?
Large covers warp. A metal-style flatness call often fails. Use functional datums and open tight tolerances for the real mates.
Who picks datums?
The designer who owns the assembly. Pick faces the inspector can repeat. This page does not invent a datum scheme for you.
How is GD&T different from a standard tolerance?
A standard tolerance is a plus-or-minus size band. GD&T controls form, orientation, and location from datums.
Should I copy metal GD&T onto plastic?
Usually no. Shrink and warp move features. Start from functional datums, not a CNC default.
How is this page different from standard or tight tolerances?
This page covers the method. Standard owns the commercial band. Tight owns exceptions. DFM review checks CAD.
Next step
Upload CAD for DFM review of molded GD&T
Share datums and frames on mates so review can check whether inspectors can repeat them before quoting.