Introduction to Minimum Feature Sizes in Injection Molding
Engineers love to ask: How small can we make it?
When talking about minimum feature sizes in injection molding, the question isn’t just about tool precision — it’s about flow, cooling, and stability. Every micron you shave from a feature comes with consequences for manufacturability and yield.
For medical and precision plastics, “how small” really means “how reliably repeatable” a feature can be across hundreds of cycles. There’s no single cutoff, but there are predictable boundaries where the process — and physics — start pushing back.
Why Minimum Feature Size Matters
Tiny features look great in CAD. In production, they:
- Trap air and fail to fill.
- Warp or flash under pressure.
- Disappear after polishing or texturing.
Understanding these limits early keeps you from chasing designs that can’t be tooled or replicated.
Typical Safe Minimum Feature Sizes by Material
| Material | Practical Minimum Feature (mm) | Pushing the Limit (mm) | Notes |
|---|---|---|---|
| ABS / PC-ABS | 0.25 – 0.4 | ~0.15 | Standard materials for housings and handles; sensitive to venting. |
| Polycarbonate (PC) | 0.3 – 0.5 | ~0.2 | High strength but higher viscosity; needs high injection pressure. |
| Polypropylene (PP) | 0.2 – 0.4 | ~0.1 | Excellent flow; can mold fine ribs and living hinges. |
| PEEK / PPSU | 0.3 – 0.5 | ~0.2 | Stiff, heat-resistant; flow limited by viscosity. |
| Thermoplastic Elastomers (TPE / TPU) | 0.3 – 0.6 | ~0.2 | Flexible and slow-flowing; venting and gating critical. |
| Liquid-Crystal Polymer (LCP) | 0.1 – 0.2 | ~0.05 – 0.075 | Extreme flowability; used for micro-molded and thin-wall parts. |
With all that said, going tighter than above isn’t impossible, but goes deep R&D territory. At that scale, you’re not just molding parts; you’re engineering the process itself. Tool steel finish, gate geometry, resin rheology, even ambient temperature start to matter. That’s where teams stop following design rules and start breaking them (which we love to do).
What Limits Feature Size
Flow Length-to-Thickness Ratio (L/T)
When the ratio exceeds 200:1, flow fronts hesitate and trap gas. For ultra-thin features, every 0.01 mm matters in maintaining pressure and temperature uniformity.
Tooling and Polishing
EDM and micro-machining can hold ±0.005 mm, but tool life shortens as features shrink. Polishing or texturing can easily erase 0.01 mm of detail — enough to kill a micro-rib or vent.
Venting and Gas Management
Air trapped in micro features burns resin or prevents fill. Vent depths must scale with feature size — typically 0.005–0.02 mm for micro-molding.
Material Rheology
High-flow grades of PP, PC, or LCP can extend thin-section capability, but only if the gate, runner, and cavity temps are precisely balanced.
When “Small Enough” Becomes “Too Small”
Signs you’ve gone past the practical limit:
- Short shots on identical parts across cavities.
- Flash or splay near gates.
- Unstable part dimensions between lots.
If you see those, your feature isn’t just small — it’s unpredictable.
How to Design at the Edge
- Keep flow paths short and direct.
- Add radii instead of sharp corners to maintain flow.
- Use uniform wall thicknesses — uneven cooling magnifies distortion.
- Test mold flow simulation at full fill speed.
- Partner with a molder experienced in micro-tooling and vent control.
Need help pushing the limits of injection molding?
Short answer: It depends. Every geometry, resin, and gate tells a different story. For most medical plastics, 0.2–0.3 mm is reliable. That said, devices are getting smaller and rules are made to be broken.
Let’s find that balance together. Design small — but design smart.