RTV Undercuts: Split, Plug, or Inner Core | RUUIPON

An undercut on a custom RTV mold is a peel problem, not a hardness problem. If the rubber has to stretch around a hook, a bail, a closed letter, or a hollow wall, you decide structure before anyone weighs A/B. We use one house sequence: the RUUIPON Mold Structure Decision. Flat back? Open-back tray. Not flat? Can 10A–20A peel that undercut without tearing the cavity or the dump? If it cannot, you leave one-piece and you pick a keyed two-part, a removable plug, or an inner core. Large wet mass still wants a mother mold behind a soft skin, whatever you chose.

This page is for buyers pouring into a tool — resin figurines, chocolate shells, soap stamps, planter walls, textured pillars. Molded silicone *parts* with steel lifters go to wydsilicone.com.

Does not apply when the geometry is an open-back chocolate bar or a shallow gummy pocket, when the only “undercut” is a 0.3 mm logo well we will draft, or when you already own a rigid PC tool and you want us to copy that shut. Those jobs skip this page.

Split, plug, and core are three exits, not three upsells

Start with the back of the part. If every face you care about sits on one side and the back is a flat dump face, we pour an open-back RTV tray. No keys. No split flash. That is the honest structure for a logo bar, a gummy bear, a wax-melt pocket.

If the part is two-sided, a hook closes, or a neck is smaller than the body, the next question is peel, not “can silicone stretch.” Platinum RTV at 10A–20A will stretch a surprising amount. It will also tear at a thin lip in the third week you run it. We mark KEEP / SPLIT / PLUG / CORE on the 2D and we say why.

Keyed split. The dump has to leave a closed envelope and both halves of the cavity are working faces. Flash lives on the parting land. Keys — bumps, sockets, a fence — find the halves every cycle. Two-sided chocolate, a resin mascot, a soap loaf with a side stamp.

Removable plug. One local lock would force a full split you do not want: a closed letter counter, a small side pocket, a texture island that would tear a one-piece lip. Pull the plug, pour, replace it. Extra handling every dump, cheaper than a three-piece jacket you never needed.

Inner core. The dump is hollow and wall thickness is a CTQ — planters, vessels, nested cocoa-bomb halves. The core registers so it cannot float, and it drafts so the cured dump releases. A floppy 10A core inside a 10A outer is how walls wander. Named walls in millimetres live on inner cores.

Jacket is a separate line. A split without a jacket will balloon on a wet pour. A jacket without a split will not save a hook you cannot peel.

The photo that would prove this is the master on the bench with the undercut taped, the same geometry as a one-piece glove torn at the lip, then the recast as a keyed two-part with the flash land off the show face. Until that set exists on your SKU, treat the flowchart as the decision.

Typical shop values

Caption: typical shop values, not a named PO.

Common mix-up: raising Shore so a one-piece “still works”

RFQs arrive as “make it 40A so we don’t need a split.” Harder rubber grabs an undercut more, then tears the dump or the lip. Shore is peel-feel and tray stiffness. Structure is how the part leaves. The purchasing version of the same mix-up is lining a one-piece RTV glove against a steel side-action *part* tool because both PDFs say “undercut.” Paying a tooling charge does not give you unrestricted ownership of master, jacket, or CAD.

Worked example (anonymized)

A craft brand sent a 68 mm resin figurine, arms folded, a 4 mm bail at the neck, about 120 pours a month. They asked for a one-piece platinum glove. We marked the bail as a tear site and the folded arms as a lock. CAD lock used a keyed two-part with the parting land down the side seam, plus a small neck plug so the bail would not force a third plate. Jacket outside because the dump was full-volume epoxy. Prototype 3–7 business days after lock; they signed the second dump. Production at MOQ 20 of that design. Simple 3D was USD $50 because they had photos plus one caliper height, not a watertight STEP.

We would not recommend

We would not recommend stretching a one-piece glove around a closed hook and calling the weekly tear “consumable tooling.” Recast the structure.

We would not recommend a three-piece mother mold on a shallow open-back bar because a sales PDF listed “split mold” as an upsell.

We would not recommend tin-cure for a food-contact cavity you handle as a plug every dump. Name 21 CFR 177.2600 extraction on the finished article and/or LFGB / BfR XV — not a factory stamp.

flowchart TD
  A[Part on the drawing] --> B{Flat back, all detail on one face?}
  B -->|yes| C[Open-back RTV tray]
  B -->|no| D{Can 10A-20A peel without tearing cavity or dump?}
  D -->|yes| C
  D -->|no| E{Is the lock local?}
  E -->|yes one pocket or letter| F[Removable plug]
  E -->|no closed envelope| G[Keyed two-part]
  E -->|hollow wall is a CTQ| H[Inner core, registered]
  C --> J{Large wet mass or soft skin?}
  F --> J
  G --> J
  H --> J
  J -->|yes| K[Mother mold / rigid jacket]
  J -->|tiny shallow tray| L[Skin only]

What to ask when you are buying

• Will this dump peel as a one-piece, or do you mark SPLIT / PLUG / CORE on the 2D before rubber?
• Where does flash land — show face, logo, or sealing rim?
• How do the halves find each other, and can the operator assemble them backwards?
• If there is a core, how is it keyed against buoyancy, and how do you measure wall on the first dump?
• Who keeps the master after the tooling charge, and may the shop keep a copy for recast?

Related: DFM before tooling, inner cores, contact. Resin jobs that already know they need a split can start from custom resin molds.

RFQ copy-pack — undercut structure

Paste into [email protected]:

Object: RTV mold I pour into, not an LSR/HCR part File: STEP / STL / photos with a millimetre scale Pour material and peak temperature: Undercuts I can see (hook, bail, closed letter, hollow wall): Preferred structure: open-back / split / plug / core / you mark it Jacket: yes / no / you tell us Target size or grams, first qty, monthly dumps Food-contact path: 177.2600 / LFGB / BfR XV / craft-only CAD: mold-ready STEP OR $50 3D if that scope applies Prototype 3–7 business days after CAD lock, then MOQ 20 per design Mark KEEP / SPLIT / PLUG / CORE on the 2D before you pour

Send the drawing and the pour. We return the Structure Decision marked on your 2D — not a blank “free quote.” Phone +86 135 3006 2969. Licheng Technology Industry Park, Xinhe Blvd, Shajing, Bao’an. Shop since 2018.

FAQ

Can a very soft Shore replace a split? Soft rubber peels a shallow undercut. A closed hook still locks. Structure first; Shore after the peel path is honest.

Is a plug the same as a core? A plug kills a local lock. A core sets a hollow wall and has to register against float. Do not order “a core” when you mean a letter plug.

Will split flash sit on a chocolate or resin face? Flash lives on the parting land. Place that land off the show face and off the logo. Keys belong in the jacket or on a fence.

Do I own the split mold after I pay tooling? A tooling charge is not unrestricted ownership of master, jacket, CAD, or the right to have a second shop recast from our files. Write who keeps what on the PO.

Can you copy a one-piece YouTube glove that already tears? We can recast it as a split or a plug. We would not copy a torn glove as production structure. Send the part or a scaled photo pack.