Inner Core Design for Hollow Silicone Molds and Planters

A hollow casting is controlled by the relationship between outer mold and inner core. If the core moves 2-3 mm, the finished wall becomes thinner on one side and thicker on the other even though both mold surfaces were manufactured correctly. Core design is therefore a positioning, buoyancy, demolding and venting problem-not simply "put a smaller shape inside the cavity."

Plan the target casting wall first, then design how the core will stay there under real fill load.

Define the final wall thickness before designing the core

Start from the finished part:

• outside dimensions;
• inside dimensions;
• nominal wall thickness;
• local thick/thin zones;
• rim geometry;
• bottom thickness;
• drainage or through-holes;
• acceptable variation.

Only then offset or design the core geometry.

For decorative planters, resin vessels and concrete shells, the internal surface may not need the same cosmetic finish as the outside-but it still needs enough draft and access for core removal.

Decide whether the core should be rigid, flexible or segmented

Rigid core

Good when the internal geometry has a clear pull direction and dimensional control matters.

Flexible silicone core

Useful where the inner geometry needs some deformation for release. It may need to be harder or more supported than the outer skin to resist displacement.

Segmented / multi-piece core

Needed when the internal geometry itself contains undercuts, a neck or shape that cannot be removed as one piece.

The core material should be chosen from both dimensional stability and extraction path.

Why cores float or shift

When liquid casting material enters the gap between core and outer mold, it creates pressure around the core. Depending on density, geometry and trapped air, the core can:

• rise;
• tilt;
• move laterally;
• touch the outer mold;
• rotate;
• flex inward/outward.

The resulting casting may show:

• off-center opening;
• uneven wall thickness;
• one side too thin;
• base thickness drift;
• warped rim;
• blocked drain hole.

Do not assume the core is "heavy enough." Locate it positively.

Support the core at the top, bottom or both

Possible strategies include:

• top bridge/crossbar;
• bottom pin or socket;
• upper shoulder stop;
• keyed connection into the mother mold;
• through-pin controlling two ends;
• removable spacers that define the wall gap.

A long core may require support at both ends to resist lateral movement.

The locating features should be accessible after casting and should not create impossible demolding.

The mother mold can become the core reference frame

On large vessels, the rigid outer support shell is often the best place to locate the inner core because it is dimensionally more stable than the flexible RTV skin.

This creates a stack:

FRP shell → outer silicone skin → casting gap → inner core

The rigid shell controls the relationship between skin and core. This is usually more repeatable than letting both flexible components float relative to each other.

Drainage holes and through-features need their own pin strategy

A planter may require drainage holes through the base. These can be formed by:

• rigid removable pins;
• core extensions;
• separate plugs;
• post-drilling if the casting material/process makes that more practical.

A molded pin must be strong enough to resist fill pressure but removable without breaking the casting.

The drainage feature should also be positioned so it does not block the mold's venting or core-removal path.

Pour and vent paths must work around the core

The core occupies the center of the cavity and can split the flow into narrow channels. This can create last-to-fill pockets around the rim, base or decorative details.

Before tooling, trace:

• where material enters;
• how it flows around both sides of the core;
• where fronts meet;
• where air is trapped;
• whether the core blocks a vent route.

For fast PU resin, pot life can make this more critical than for slow plaster or concrete.

Demolding sequence is part of core design

Possible sequence:

1. remove outer rigid shell;
2. open/peel outer silicone skin;
3. free the casting outside surface;
4. remove top/bottom core locators;
5. extract or segment the core;
6. remove drainage pins/plugs.

Some products require the core to come out first. The correct order depends on geometry and casting stiffness.

Write the sequence during DFM; do not discover it after the first casting cures.

Example: large concrete planter

A large planter with a thick wet concrete load may use a flexible outer RTV skin in an FRP mother mold and a rigid inner core located from the same support frame.

The system must control:

• outside shape;
• inside shape;
• rim thickness;
• base thickness;
• drainage-hole position;
• core buoyancy;
• opening sequence.

A water-volume test alone cannot prove wall-thickness consistency. Section or measure the first full casting at defined points.

What should change on the existing RUUIPON page

/silicone-mold-inner-core-design/ is already the correct canonical URL and should be rewritten in place. The existing page has the right intent but should move away from generic repeated DFM blocks and fixed shop values toward a core-centric engineering narrative.