Chamfers and fillets are the most misunderstood features on injection molded part drawings — a chamfer where a fillet should be creates a stress riser that can crack under load, while an unnecessary fillet adds mold complexity and cost. Understanding when to use which directly impacts part strength, moldability, and tooling cost.
The Fundamental Difference
A chamfer is a flat beveled edge — simple to machine into the mold, costs nothing extra, but creates a sharp internal corner where the chamfer meets the wall. A fillet is a radius — it requires a ball-end mill to cut into the mold, costs slightly more to produce, but eliminates the sharp corner that concentrates stress.
| Feature | Chamfer | Fillet |
|---|---|---|
| Stress Concentration | High (sharp corner) | Low (smooth transition) |
| Mold Machining | Simple end mill | Ball end mill (slower) |
| Assembly Guide | Excellent (lead-in) | Poor (no lead-in) |
When to Use Each
Use fillets at all internal corners — the intersection of a rib and a wall, the base of a boss, and anywhere two walls meet. A minimum radius of 0.5mm is standard for nylon; 0.25mm for acetal. Fillet radius should be at least 0.5x the wall thickness for structural parts. Use chamfers on external edges — the perimeter of a housing cover, the lead-in for a snap fit engagement, or removable covers. Chamfers aid assembly and eliminate sharp burrs that cut assembly workers and end users.
Frequently Asked Questions
Why do my injection molded nylon parts crack at rib-wall intersections?
Sharp internal corners (no fillet radius) create stress concentrations 3-5x higher than the nominal stress. During ejection or under load, cracks initiate at these sharp corners. Adding a fillet with radius >=0.5x wall thickness eliminates the stress concentration.
Can I use chamfers everywhere to save mold cost?
You can use chamfers on external edges freely — the mold only needs standard end mills. But for internal corners, fillets are non-negotiable. A sharp internal corner in nylon will crack. The small additional cost of ball-end milling fillets into the cavity is far cheaper than field failures and warranty claims.
What’s the minimum fillet radius for glass-filled nylon?
1.0mm minimum, 1.5mm recommended. Glass fibers create natural stress concentration points, so the radius needs to be larger than for unfilled material to distribute stress around the fiber ends effectively.
Do I need fillets inside the cavity or on the core?
Both. Internal corners on the cavity (external part corners) need fillets for aesthetics and mold filling. Internal corners on the core (internal part corners like rib bases) need fillets for structural integrity. The core-side fillets matter more structurally.
Need injection molding DFM review?
We check every corner on your design — fillet where needed, chamfer where appropriate — before the mold is cut.
