Custom Silicone Molds for Planters, Bowls and Hollow Vessels

A planter or hollow vessel is not one cavity. It's an outer RTV skin, a registered inner core that sets wall thickness, and a rigid jacket that carries the wet mass of concrete, plaster, or resin in the annulus. If you skip any of those three, the usual result is a walking core, a thin wall on one side, and a bellied outer face. Shore 10A–20A is for peeling the undercut and the drainage geometry—not for holding 8–15 kg of wet mix.

RUUIPON builds this stack in Bao'an, Shenzhen (since 2018): photos, sketch, or CAD, then a 3D-printed pattern, often a temporary silicone flip, a resin production mother, then batch RTV with a registered inner core and a rigid support shell. Prototype 3–7 days after CAD lock; MOQ 20 molds per design. LSR cold-runner tools that mold silicone *parts* are a different RFQ - ask us. This page is the casting tool.

Wet mass, undercut, jacket, Shore

Wet mass. The load is the annulus: (outer volume − core volume) × mix density. A 280 mm tall, 12 mm wall planter in wet concrete is often in the 8–12 kg band; bigger bowls climb fast. Hydrostatic pressure tries to bulge the outer skin *and* lift a buoyant core. Design the jacket and the core locks for that filled kilograms number, including vibration. Unsupported sag of 0.5–1.5 mm is the tray-scale number; a tall vessel without a jacket can go well past that.

Undercut. Organic outer texture usually peels. What locks you is the inner draft, a rolled rim, an integrated saucer, or a drainage hole that the core punches as a through-feature. Drainage that undercuts the wall needs a plug or a drafted pin, not hope. See inner core design and split lines / keys.

Jacket. Outer skin + wet concrete without a mother mold is how you get a pregnant planter. The jacket splits so you can still peel; keys on the jacket and on the core keep wall thickness. Details: mother mold / support shell. The core itself often needs to be rigid (or a rigid-backed RTV) so it doesn't oval.

Shore. Outer skin typically 10A–20A platinum so texture and a slight rim undercut peel. Core Shore can be firmer if the core is the demold problem, or the core can be a rigid plug. Don't jack the outer Shore to 40A to “hold the wall”—that's the jacket's job. Platinum shrink ~0.1–0.3%; on a 280 mm OD that's not nothing, so if the saucer has to fit a shelf, put the shrink on the master. ISO 3302-1 is a rubber-part class, not a slogan for a floppy RTV skin. Measure wall thickness on a real pour with the core seated.

Common mix-up

A buyer sends an outer STL and no inner surface. The shop is then guessing wall thickness, drain geometry, and whether the core can extract. The other mix-up is ordering a solid-cast “planter mold” (open-back like a paver) when the product is hollow. You'll get a 15 kg paperweight, not a 12 mm wall.

Worked example: 280 mm concrete planter

Geometry: 280 mm tall, 220 mm OD at the rim, 12 mm nominal wall, 18 mm drain with 2° draft, light stone texture, no integrated saucer.

Cast: wet concrete, ~10 kg in the annulus, vibration, 50–100 casts before a recast is a conversation (abrasion + handling).

Tool family: outer master + inner core master → platinum RTV outer skin Shore 10A–20A → rigid or RTV-faced core with foolproof keys (can't assemble 180° off) → two-piece jacket → prototype one set, then MOQ 20.

Acceptance: three wall-thickness sections 90° apart inside a stated window; drain punches clean; full-weight pour doesn't bulge the outer face past the agreed sag; operator can extract the core without cracking a green casting.

Add an integrated catch basin under the drain and this is no longer a simple core pull. Budget a plug and a jacket split that doesn't trap the basin.

Failures you will actually see

Symptom	How to confirm	Fix
Wall 8 mm on one side, 16 mm on the other	Section the first pour; check core keys	Register the core; stop using a floating mandrel
Outer face bellies	Compare jacketed vs unjacketed full-weight pour	Mother mold; don't raise Shore first
Drain hole locks or tears the casting	Dry-fit core pins; check draft	Drafted pin or removable plug
Core assembled backward	Look for asymmetric keys	Foolproof bosses; can't mate 180° off
Jacket too heavy to flip	Weigh the empty assembly	Skin + shell, not a solid RTV block

A photo that would prove the claim: core seated in the outer skin, jacket closed, three caliper hits on the wall of a cut pour. Request it on the prototype.

Typical shop values

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

Item	Typical shop value
Outer-skin Shore	10A–20A platinum RTV, jacket behind it
Mix	Platinum 1:1 or 10:1 by grade; vacuum degas until the foam collapses
Linear shrink	~0.1–0.3% (platinum)
Loaded sag, unsupported	About 0.5–1.5 mm on a tray-scale span; tall vessels bulge more without a jacket
Prototype	3–7 days after CAD lock
Production MOQ	20 molds per design

What to ask when you're buying

Outer OD, inner ID, and target wall—all three, not just a pretty outer.
Filled wet mass of the *annulus*, not the solid envelope.
How the core registers, and can it be assembled backward?
Drain / saucer / through-holes: drafted pins or plugs?
Jacket split vs rubber split—do they coincide?
Who owns the outer master, the core, and the jacket after the tooling charge?

We would not recommend a one-piece glove with a loose bottle as a “core.” We would not skip mother-mold support because the outer texture “needs to be soft.” We would not raise Shore A first to hold wall thickness. We would not punch an undrafted drain and hope green concrete survives. We would not copy a already-bellied production sample as the new master.

RUUIPON Mold Structure Decision:

Hollow planter or solid paver? - hollow -> Inner core required
Deep outer undercuts? - locked rim / saucer -> Two-part / plug
Wet annulus in kilograms? - tiny dry sample -> Open-back outer RTV skin
Inner core required -> Deep outer undercuts?
Two-part / plug -> Mother mold / jacket required
Open-back outer RTV skin -> Wet annulus in kilograms?
Inner core required -> Mother mold / jacket required

When this page is the wrong quote

A solid stepping stone or paver belongs on concrete and cement. A tiny 80 mm succulent pot you will cast twice can skip a full GRP jacket. A thin-wall plastic planter you actually want injection-molded is not an RTV job. Aggressive PU systems that attack the rubber belong on a chemistry-first PU resin review before anyone pours a core.

Planter / vessel RFQ — copy this

Outer and inner geometry (STL or OD/ID/height)
Target wall thickness and tolerance window
Filled wet mass and mix (concrete / plaster / resin)
Drain, saucer, through-holes
Texture (stone, wood, smooth)
Core: rigid, RTV-faced, or both
Jacket preference if any
Shore if already validated
Prototype acceptance: wall map on a real pour
Quantity: prototype + production (MOQ 20)
Ownership of outer master, core, jacket; recast of the RTV skin

Send OD, ID, wall, mix, and kilograms. [email protected] · +86 135 3006 2969 · Licheng Technology Industry Park, Xinhe Blvd, Shajing, Bao'an, Shenzhen.

Matched CTA: Send outer/inner CAD, wall thickness, and wet kilograms for a core-and-jacket review. Contact / RFQ · inner cores · mother mold.

FAQ

Why isn't a planter just a bigger concrete paver mold? Paver molds are open-back. Planters need a core to make the wall, keys so the core doesn't walk, and a jacket for the wet annulus. Skip the core and you cast a solid block.

Will the core float in wet concrete? If it isn't locked, yes. Buoyancy plus vibration is how walls go 8 mm / 16 mm. Register the core to the jacket; don't rely on the operator holding it.

What Shore should the outer skin be? Typically 10A–20A platinum so texture peels. The jacket holds the OD. A 40A outer skin still needs the jacket and will fight a rim undercut.

How do drainage holes get molded? Drafted pins or removable plugs. An undrafted hole in green concrete is a crack starter. Say whether the hole must be through-cast or drilled after.

If I paid for the mold, do I own the core and jacket? Only if the PO says so. Tooling charge is not unrestricted ownership. Recasts of the RTV skin need the same masters and keys.