Custom Silicone Molds for Concrete and Cement Casting

Don't pour wet concrete into an unsupported Shore 10A–20A skin. Hydrostatic load from a full cavity will balloon the walls, flatten stone texture, and throw the finished thickness off before the mix even sets. The mold you want is a platinum RTV cavity from a resin master (photo, sketch, or CAD) sitting inside a rigid support shell, with a peel path for undercuts and a Shore chosen for release—not for “holding shape.”

RUUIPON has poured this stack in Bao'an, Shenzhen since 2018. Prototype is commonly 3–7 days after CAD lock; production MOQ is 20 molds per design. If you actually need thousands of molded silicone *parts*, that's a different quote—HCR compression or LSR with a cold-runner: HTV product RFQ or ask us. This page is the casting *tool*.

Wet mass, undercut, jacket, Shore — pick these four first

Wet mass. Write the filled kilograms, not the dry tile weight. A 400 × 400 × 40 mm paver in a wet cementitious mix is often in the 12–15 kg band; a 600 mm stepping stone is more. Soft RTV without a jacket sags about 0.5–1.5 mm at the unsupported edge under a loaded tray-scale span, and a deep vessel can bulge far more. Design the jacket for the heaviest pour you will actually make, including vibration.

Undercut. Stone texture, undercut letters, and a geometric lock are three different problems. A shallow open-back paver peels as a one-piece cavity. Deep negative draft, a wrap-around rim, or a closed vessel needs a keyed split, a removable plug, or an inner core—see split lines and keys and inner cores. Stretching a glove mold around a locked undercut is how the skin tears at the first demold.

Jacket (mother mold). The rubber copies texture; the jacket carries the wet load. Typical stack: thin high-tear platinum skin against the master, then plaster, fiberglass, or a printed shell with its own split so you can still peel the casting. A solid block of RTV “to make it stiff” just makes an operator-unliftable mold that still creeps. Details: mother mold / support shell.

Shore. Cavity Shore for concrete texture is typically 10A–20A platinum RTV so the skin can peel without ripping the aggregate print. A thick 25A wall can feel stiffer than a thin 40A lip—don't pick hardness from a catalog number alone. Raising Shore to stop sag is the wrong first move; add the jacket.

Platinum linear shrink is typically 0.1–0.3%. Tin-cure keeps moving in storage and is a poor match if you recast from the same master months later. If someone wrote “precision ±0.1 mm” on a floppy cavity, that's not ISO 3302-1 (that's a rubber-*part* tolerance class on a drawing). Measure the mold seated in its jacket, unloaded, and say so on the RFQ.

Common mix-up

Buyers send a photo of a finished paver and ask for “a silicone mold.” What they often receive is a thick one-piece glove with no jacket and a 40A Shore “for durability.” That mold demolds the first cast, then slowly grows a belly. The mix-up is treating Shore as structure. Structure is the mother mold.

A second mix-up: quoting this as if it were an LSR cavity tool. Cold-runners and heated plates don't dump 12 kg of wet cement.

Worked example: 400 mm textured paver

Geometry: 400 × 400 × 40 mm open-back paver, weathered-stone face, 2 mm max undercut in the texture, no wrap-around rim.

Cast: wet cementitious mix, ~13 kg filled, vibration table, 80–120 casts per mold before a recast is likely (abrasion and handling, not a guaranteed cycle count).

Tool family: CNC or textured master → open-back platinum RTV skin, Shore 10A–20A, vacuum-degas pour, brush/detail coat into the stone → two-piece GRP or plaster jacket with registration keys → 20 production molds after a full-weight prototype.

Acceptance: one full-weight pour; edge sag of the assembled mold under load stays in the 0.5–1.5 mm typical band; texture copies without trapped air at the deep pits; operator can open the jacket without prying on the silicone.

If you later add a 15 mm wrap-around rim, this stops being open-back. That's a split or a plug, and the jacket split has to follow.

Failures you will actually see

Symptom	How to confirm	Fix
Finished paver is thicker in the middle, thinner at the rim	Compare empty-cavity depth to a full-weight pour; look for jacket gap	Seat the skin in a rigid jacket; don't raise Shore first
Stone pits print smooth or with pinholes	Raking light on the first cast vs the master	Brush a detail coat, add vents or vibration; degas the RTV itself
Skin tears at a deep undercut on demold	Mark the tear against the master undercut	Split, plug, or soften locally; stop stretching a one-piece glove
Aggregate scours letters after a few dozen pours	Check max stone size vs letter depth	Thicker detail coat, finer mix at the face, or recast schedule
Jacket won't come off around a rim	Dry-fit the empty assembly	Split the jacket on the same logic as the rubber; keys on both

A photo that would prove the claim: assembled jacket with the RTV skin seated, a scale under a full wet pour, and a caliper on rim vs center thickness. We don't invent that photo here; ask for 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
Cavity 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); tin keeps moving in storage
Loaded-edge sag, unsupported	About 0.5–1.5 mm; worse on a deep vessel without a jacket
Prototype	3–7 days after CAD lock
Production MOQ	20 molds per design

What to ask when you're buying

What's the filled wet mass, and is the jacket designed for that load or for the empty mold on a desk?
Open-back, keyed split, or core—and why? Show the peel path on the master.
Which Shore, and will you still add a jacket at 10A–20A?
How is the mold measured: seated in the jacket, or floppy on the bench?
Who keeps the master and the jacket after you pay the tooling charge? Paying NRE is not unrestricted ownership unless the PO says so.
Is a recast of the RTV skin in the price after abrasive wear?

We would not recommend a one-piece unsupported glove for a paver or vessel whose wet mass is measured in kilograms. We would not raise Shore A first to “make it stiffer.” We would not pour a solid RTV block as a substitute for a skin-plus-jacket. We would not copy a worn, already-bellied sample as the new master. We would not quote LSR steel with a cold-runner because you “might scale later” while you still need a cavity to dump cement into.

RUUIPON Mold Structure Decision:

Flat open-back paver? - no wrap or vessel -> Deep undercuts?
Deep undercuts? - texture peels safely -> Open-back RTV skin
Deep undercuts? - locked rim or core -> Two-part / plug / inner core
Wet mass in kilograms? - tiny shallow sample tile -> Open-back RTV skin
Two-part / plug / inner core -> Mother mold / jacket required
Open-back RTV skin -> Wet mass in kilograms?

When this page is the wrong quote

A tiny 100 mm sample tile you will cast twice can skip the jacket. A closed architectural panel better served by a rigid plastic or steel form, or a chemically aggressive mix that attacks platinum RTV, should not be forced into this stack. High-volume molded silicone *parts* belong on an HCR/LSR RFQ, not here. Geometry can override quantity: one large wet vessel still needs a jacket even if you only want two molds.

Planters that are hollow (wall + core) belong on planters and vessels, not this open-back paver path.

Concrete mold RFQ — copy this

Master: CNC, textured print, or physical sample (who keeps it)
Finished L × W × H and filled wet mass in kg
Mix family and max aggregate size
Open-back vs wrap-around rim vs hollow vessel
Texture target (stone, wood, logo depth)
Undercuts, drain holes, or inserts
Jacket material if you already care (plaster / GRP / printed)
Shore if already validated; otherwise “peel vs sag” in words
Quantity: prototype + production (MOQ 20)
Recast of the RTV skin: included or extra
Current failure, if any: belly, tear, air in pits, too heavy to lift

Send the master photos or STL, the wet kilograms, and the one failure you most want to avoid. Email [email protected] or WhatsApp +86 135 3006 2969. Factory: Licheng Technology Industry Park, Xinhe Blvd, Shajing, Bao'an, Shenzhen.

Matched CTA: Send wet mass, mix, and undercut photos for a jacket-and-Shore review — not a generic free quote. Contact / RFQ · prototype validation · mother mold.

FAQ

Do I need a mother mold for a concrete paver? If the pour is measured in kilograms, yes. A 10A–20A skin copies texture; the jacket stops the 0.5–1.5 mm (and worse) sag. Tiny sample tiles can skip it.

Can I just order a harder Shore instead of a jacket? Don't. A thick 25A wall still creeps under wet cement. Raise Shore only after the jacket exists and you still can't peel a locked undercut.

Will abrasive aggregate kill the mold? It will wear a thin letter or a sharp arris. Name max stone size on the RFQ, use a detail coat, and budget a recast of the skin against the same master.

Is platinum shrink a problem for a 400 mm paver? Typical platinum linear shrink is 0.1–0.3%. That's millimeters on a large tile, so lock the master oversize if the finished OD is CTQ. Tin-cure is a worse recast partner.

Does paying for the mold mean I own the jacket and master? Only if the PO says so. Tooling charge is not unrestricted ownership. Write who keeps the master, who keeps the jacket, and whether a recast is included.