Introduction — The Unexpected Truth About Tolerance Stack-Up for Plastic Assemblies
Every engineer learns to calculate tolerance stack-up for plastic assemblies. Few stop to ask if they really need to.
The easiest way to solve a stack-up problem? Redesign your assembly so you don’t have one.
In many medical plastic components, the root cause of stack-up issues isn’t tolerance — it’s too many parts. The more interfaces you design, the more opportunity for dimensional variation, assembly misalignment, and functional drift.
Why “One Part” Is the Best Tolerance Strategy
When you consolidate multiple parts into a single molded component, you eliminate:
- Interface gaps and accumulated dimensional variation.
- Secondary assembly steps (and their associated fixtures, adhesives, or hardware).
- Alignment dependencies that shift over time.
In DFM terms, every interface adds uncertainty and every uncertainty requires detailed analysis that often results in costly overhead and yield. By rethinking an assembly as one part, you game the system.
When Consolidation Isn’t Possible
Sometimes, one part isn’t realistic. You might need:
- Different materials (rigid + flexible, clear + opaque).
- Insert molding or overmolding for hybrid components.
- Access for assembly, wiring, or actuation.
In those cases, tolerance stack-up analysis becomes essential. But it’s still guided by the same DFM mindset: simplify where possible, control what you can’t.
How to Approach Tolerance Stack-Up the Right Way
- Start from function, not from geometry. Define which dimensions actually affect performance.
- Reference everything to a single datum — reduce chained tolerances.
- Use mold-based GD&T thinking: your real limits come from steel precision, not CAD idealism.
- Account for shrink variability — especially across different resin families.
- Build in adjustability (drafts, snaps, press fits) to absorb variation where possible.
For many medical device plastics, practical tolerance control in production ranges from ±0.05 mm on precision features to ±0.2 mm on cosmetic or non-critical geometry.
The Real Goal Isn’t Zero Tolerance — It’s Predictable Fit
Tolerance stack-up analysis isn’t just about numbers; it’s about designing confidence into the assembly.
The best designs don’t fight variation, they accommodate it through robust geometry and smart feature control.
When stack-ups become complex, it’s usually a design signal: your part count or feature alignment is working against manufacturability.
Don’t believe you can do it in one part? Let us help.
We’ve redesigned assemblies that dropped from five parts to one, removed adhesives, and eliminated tolerance stack-ups entirely — all through thoughtful DFM and molding strategy.
The easiest tolerance analysis is the one you never have to do.