Silicone Mold Undercuts: When to Use a Split, Plug or Core

What Creates an Undercut?

An undercut is geometry that resists a straight release from the mold.

Flexible silicone can temporarily deform around some undercuts that would lock in a rigid tool, but flexibility does not eliminate every mechanical lock. The right structure depends on undercut depth, local mold section, casting material, release direction, support and repeated demolding stress.

Common undercuts include:

• return lips
• hooks
• necks
• closed holes
• geometry wider below than above
• deep texture behind a narrow opening

A feature only becomes an undercut relative to the intended release direction.

When a One-Piece Mold Can Work

A one-piece mold may work when:

• the silicone can peel around the feature
• the local section is supported
• the casting is strong enough
• the operator has a practical release path
• repeated deformation does not create severe stress

Prototype uncertain geometry.

When to Use a Split or Slit Mold

Use a controlled split when:

• geometry exists on both sides
• the casting is fragile
• the undercut is deep
• cosmetic surfaces need a planned seam
• forced release would overstretch the mold

A slit mold can keep one main mold body while opening locally.

Plan:

• slit location
• closing/sealing
• alignment
• cosmetic surfaces
• stress at the slit root

Removable Plugs and Cores

A plug is useful when a localized opening, neck or recess creates the lock.

It should:

• locate repeatably
• release in a clear sequence
• avoid leaking or trapping casting material
• remain practical for production operators

A removable core can create:

• internal cavities
• hollow sections
• deep recesses
• through-features

The core removal sequence must be part of the design.

Support Shells and Release Geometry

A rigid support shell helps large/soft flexible molds hold shape while filled.

It does not remove an undercut by itself, but it can stabilize the mold during casting.

Where the product design allows, helpful changes can include:

• taper
• softer internal corners
• changed feature orientation
• relocated parting line

Select draft and radii for the geometry, material and expected demolding load, then verify alignment, release and durability with the prototype.

Frequently Asked Questions

Can softer silicone solve the undercut?

Sometimes, but not if the geometry creates a true mechanical lock.

Can you recommend the split from photos?

A preliminary recommendation is possible from clear multi-angle photos. Final structure may require CAD or the physical sample.

My plaster casting breaks before the silicone tears. What should change?

Review the release path, split/core strategy and fragile features. Softer silicone alone may not solve it.

Does flexible silicone eliminate draft requirements?

No. Flexibility helps, but release geometry still matters.

Is a split mold always two completely separate halves?

No. A planned slit can be another option.