Mates only
Fits, seals, snaps, bearing seats, alignment pins.
Tight tolerances
Tight Tolerances on Molded Parts: Mates Only, Not Every Edge
Mark critical mates on CAD. DFM review asks which interfaces are critical before quoting inspection-heavy programs. Published bands live on standards.
Design guideline
Tighten the mate, not the envelope
Tight calls belong on mates, seals, snaps, and alignment features. A blanket of tight decimals on every edge is not a quality plan. This page owns the exception list, not the published band.
Plastic does not hold metal-like bands on every edge. Standard tolerances own the commercial default. GD&T for molded parts owns how to frame the exception. Manufacturing standards hold the published size-class bands. DFM review reads the mate list.
This spoke owns what to tighten. Tolerance stack-up owns whether the assembly still fits. The design guide hub lists every topic.
Roles
Which page answers which tight-tolerance question
Stay here for the exception list. Open a sibling for the band, the method, or the chain.
| Page | Use it for |
|---|---|
| Tight tolerances (this page) | Which mates get extra control |
| Standard tolerances | The commercial default band |
| GD&T for molded parts | How to frame those mates |
| Tolerance stack-up | Whether the assembly still fits |
| Manufacturing standards | Published size-class bands |
| Shrinkage compensation | Why plastic will not hold a metal band |
| Wall thickness | Uneven walls fight a tight call |
| Gate location | Fill that can move a mate |
| DFM review | CAD check before steel |
Checks
What DFM review looks for on tight calls
Qualitative checks. No figure on this page is a NetProto specification.
Leave the envelope
Outer contours and cosmetic edges can stay commercial.
Plastic is not metal
A CNC default on every edge is the wrong start.
No new band here
Published figures live on manufacturing standards. Do not invent a number.
Decision
Blanket tight decimals versus GD&T on mates
Wrong: tighten everything to force quality. Right: mark the mates, frame them, then check the stack.
What goes wrong
- Tight decimals on every edge
- Metal-like bands on a large warped cover
- Tighten the drawing with no inspection plan
What to try first
- List the mates that actually matter
- Open GD&T for datums and frames
- Open stack-up if several parts must fit
RFQ
Send these notes with CAD for a tight-tolerance review
The quote can start from CAD alone. These notes make the DFM pass specific.
Mating features that must be tight
Features that can stay commercial
Resin family
Inspection method if you have one
Whether GD&T frames already exist
Assembly stack sketch if more than one part fits
Related
Related design guidelines
Stay on this spoke for its question. Open a sibling for the neighbor topic or the CAD check.
Manufacturing standards
Published size-class bands.
Manufacturing standardsDFM review
How NetProto reads the mate list before steel.
DFM reviewFAQ
Tight tolerances FAQ
Can plastic hold metal-like tolerances?
Not on every edge. Tighten mates. Leave general contours on the commercial band. Standards hold the published figures.
Does mold temperature matter?
Process and steel temperature affect shrink and warp. That is why a blanket tight drawing still fails. Name the resin.
Should I tighten tolerances to force quality?
No. That adds inspection without fixing warp or shrink. Mark the mates that matter.
Which features deserve a tight call?
Mating fits, seals, snaps, bearing seats, and alignment features. The standard-tolerances page lists what can stay commercial.
Where are the published numbers?
On the manufacturing standards page. This spoke does not republish them.
How is this page different from standard tolerances or GD&T?
This spoke owns the exception list. Standard owns the default band. GD&T owns the method. DFM review checks CAD.
Next step
Upload CAD for DFM review of tight mates
Share the mate list so review can keep extra control on the interfaces that matter before quoting.