Edge Radius and Fillets in Silicone Mold Design

Sharp geometry concentrates stress.

In a flexible silicone mold, appropriate radii and fillets can reduce repeated stress at cavity lips, web junctions, parting roots and other flex points.

The correct radius is not one universal number. It depends on scale, geometry, surface detail, material, mold section and release path.

Common locations:

• cavity lip
• floor-to-wall transition
• inter-cavity web root
• split/parting root
• registration-key root
• logo/text base
• plug/core transitions

A sharp lip can become a repeated peel/stress point.

Where product appearance allows, a small transition can reduce concentration without visibly rounding the finished part too much.

Deep cavities often benefit from a smooth internal transition.

The final geometry still needs to match the product.

Thin webs can fail where they meet a thicker base.

Review local section and radius together.

A sharp slit or parting transition can become a tear origin.

Plan the root instead of cutting a random sharp notch after molding.

Keys should not use sharp roots that act like tear starters.

Do not blur branding simply to add radius.

Small text/detail may need a different local compromise than structural mold edges.

My mold always tears at the same corner. Should we add a radius?

Possibly. First confirm that the corner is the true stress origin and that a radius will not change the required product geometry.

Can a larger radius solve an undercut?

Not always. Deep mechanical locks may still need a split/core.

Can you recommend the exact radius from a picture?

A preliminary recommendation may be possible, but final geometry should be reviewed in CAD or from the physical sample.

Inspect:

• high-stress corners
• local deformation
• repeated peel path
• casting release
• logo/detail loss

Is a fillet always better?

Only when it supports the product/design requirement.

Should every corner be rounded?

No.