Just how small can an overmolded feature really be?
In medical device design, overmolding enables smaller, cleaner, and easier-to-assemble parts. Handles with built-in seals, strain reliefs that don’t need adhesives, and catheter components that eliminate manual assembly steps.
But as clinical demands push devices to become smaller and more integrated, one question naturally arises: what’s the smallest feature you can overmold?
The answer isn’t simple. There’s no single limit — it depends on material flow, adhesion chemistry, and tool design. Understanding those variables early prevents wasted iterations, failed bonds, and unreliable seals.
Overmolding Basics in Miniature Medical Parts
Overmolding joins two or more materials. Typically a soft elastomer molded over a rigid thermoplastic substrate.
In miniature medical components, that second shot might be a 0.5 mm layer of TPE sealing against polycarbonate, or a micro strain relief bonded around a thin catheter tube.
At these scales, overmolding challenges include:
- Flow hesitation — molten elastomer cooling before it fills a cavity.
- Flash formation — thin gaps allowing leakage of the second-shot material.
- Bond variation — inconsistent adhesion due to surface energy or contamination.
These limits usually appear when wall thicknesses fall below 0.4 mm or feature widths approach the mold’s machining limits.
The Real-World Lower Limits
| Feature Type | Typical Reliable Minimum | Failure Risk Below Limit |
|---|---|---|
| Overmold thickness | 0.4–0.6 mm | Incomplete fill or delamination |
| Soft seal lip or edge | 0.3 mm | Flashing, tearing during demold |
| Bond interface radius | 0.25 mm | Weak mechanical lock or voids |
| Overmold around wire/tube | 0.15–0.2 mm encapsulation | Air entrapment, bond gaps |
| Undercut / grip detail | 0.3 mm | Sticking, tearing, incomplete fill |
Below those numbers, success becomes tool-dependent. The part might fill in one shot and short the next.
For reference, most medical-grade TPEs and TPUs begin to exhibit unstable flow when section thickness drops below 0.25 mm, even with optimized gating. This doesn’t always mean its impossible to overmold, but starts to require some serious trial, error, and iteration to fine tune.
Factors That Define Your Minimum
Material Rheology
- Soft durometers (25–40 A) flow more slowly and cool faster.
- Stiffer elastomers (60–80 A) can fill thinner walls but bond less effectively.
- Materials with low melt viscosity (like certain TPUs) can fill as thin as 0.2 mm if the tool is polished and heated correctly.
Tooling & Gating
- Direct gating near thin sections improves fill consistency.
- Heated inserts or localized temperature control reduce premature freezing.
- Micro vents (<0.005”) allow trapped air to escape without flash.
Substrate Design
- Maintain 0.25 mm minimum bonding surface radius.
- Avoid sharp transitions or undercuts that obstruct flow.
- Roughen or texture bonding surfaces for mechanical interlock if chemical adhesion is marginal.
Processing Conditions
- Higher mold temperatures (80–100 °C for many TPUs) improve flow and adhesion.
- Short residence time prevents material degradation and color variation.
Practical Design Guidelines
- For catheter strain reliefs, target 0.4 mm minimum encapsulation around tube OD.
- For handle seals or soft grips, keep edge lips ≥0.5 mm to avoid tearing.
- For micro-overmolded features, design a controlled shutoff — not a knife edge.
- Validate with short-shot studies early to confirm flow and bond consistency.
Having difficulties designing your smallest features for overmolding? Let us help!
The smallest feature you can overmold reliably isn’t defined by the material data sheet — it’s defined by how you design for flow, adhesion, and demolding.
For most medical-grade TPUs and TPEs, 0.4 mm is the realistic lower limit for consistent, repeatable production. Go thinner, and you’re relying on tool magic — not process control.
Understanding those thresholds early helps teams design confidently, minimizing rework and achieving production-ready overmolds on the first iteration.