Undercut Geometry and the Demold Direction | RUUIPON

An undercut is not a yes/no on the CAD. It is a feature that blocks a peel vector — the direction the dump (or the glove) actually leaves. A bar that flexes straight off a flat back can keep a shallow logo well. A figurine whose arms hook *across* that path cannot. We mark KEEP / SPLIT / PLUG / CORE on the 2D after we name the vector, not after we guess Shore A. Soft 10A–20A stretches; it also tears a lip on week three if you asked it to peel the wrong way.

RUUIPON Mold Structure Decision, Bao'an since 2018: Flat back? Open-back tray. Else can the peel happen without tearing cavity or dump? If no → keyed two-part, removable plug, or inner core. Large wet casting? Mother mold. Prototype 3–7 business days after CAD lock. MOQ 20 pieces per design. USD $50 when we must model the parting. Steel lifters on LSR *parts* are wydsilicone.com.

Does not apply when the geometry is already an open-back chocolate bar with a 0.3–0.6 mm brand, when you own a rigid PC tool and want that shut copied, or when the only “undercut” is a draft miss we will fillet. Those jobs skip this page.

Name the vector, then classify the hook

Open-back peel (dump lifts off the floor). Logo wells and gummy pockets sit on this vector. They are undercuts *in the rubber*, but they strip if depth and draft are honest. Wrong Shore is not the first lever when a well is a hook.

Glove peel (skin rolls off a one-piece master). Clay and some prints survive this. A rigid PU dump of the same shape may not. Confirm with the dump you will actually run, not the master material.

Split peel (halves open on a parting land). The vector is normal to the shut. Flash lives on that land — keep it off the show face. Keys find the halves every cycle.

Plug pull (local lock, rest stays one-piece). Closed letter counters, a small side pocket. Extra handling; cheaper than a three-piece you do not need.

Core withdraw (hollow dump, wall CTQ). Planters, vessels, nested shells. Core drafts along *its* vector, which may not match the outer peel. Two vectors, two notes on the 2D.

If you cannot draw an arrow on a screenshot without crossing a hook, stop asking for 10A. Change structure.

The photo that would prove the call: master on the bench with an arrow of peel, tape on the hook, a failed one-piece lip, then the keyed split with flash off the show face.

Typical shop values

Caption: Typical shop values.

Worked example (anonymized)

An 80 mm mascot, arms across the chest, buyer STL, “one-piece like the print.” Print peeled. First PU dump locked at the elbows — glossy well, no etch. Vector was along the feet; arms crossed it. We modeled a keyed split on the side seam ($50), 20A, plaster jacket. Second lock, 3–7 business days. Dump released with a thin flash we had placed on the seam. MOQ 20 of the split. 40A was never poured.

Keys are not decoration on a split. They are how the peel vector repeats every cycle. Bumps, sockets, a fence — placed so the operator cannot close the halves 180° off. A split without keys looks cheaper in CAD and flashes everywhere on dump three. Put key talk on the same RFQ line as the arrow.

Draft lives on the vector, not “on the part.” A 2° draft that points into the hook still locks. We would rather you mark 1–3° along the peel and a fillet at the root than paste a generic draft note from a steel handbook. ISO 3302-1 M2 on the dump 2D still does not open a hook.

Food vs resin changes how aggressive we are about one-piece. A chocolate shell that contracts can leave a mild well. A PU brick will not. Do not copy a chocolate vector onto a resin PO and call it DFM.

One procurement mix-up

“Silicone stretches so undercuts are free” skips the vector. Stretch is a property. Peel is a path. Tooling charge on a one-piece glove is not unrestricted redesign into a split when the dump is rigid.

If the first sample locks, send the empty-well photo before anyone types 50A. Glossy well plus a hook is structure. Matte cooked well is exotherm. Strings on the face is inhibition. Those three are different next steps; only the first belongs on this peel-vector page. Keep the other two off this RFQ line.

We would not recommend

We would not recommend raising Shore to force a one-piece past a hook.

We would not recommend a one-piece glove because the SLA print came off the build plate.

We would not recommend a full three-part when a plug would free one letter.

We would not recommend leaving flash on a show face to “save a split.”

flowchart TD
  A[Part has a possible undercut] --> B[Draw peel vector on the 2D]
  B --> C{Does the hook cross that arrow?}
  C -->|No, well is along peel| D[Open-back or glove — draft the well]
  C -->|Yes| E{Local lock or whole envelope?}
  E -->|Local| F[Removable plug]
  E -->|Whole envelope| G[Keyed two-part]
  E -->|Hollow wall CTQ| H[Inner core — second vector]
  D --> I{Large wet dump?}
  G --> I
  F --> I
  H --> I
  I -->|Yes| J[Mother mold / jacket]
  I -->|No| K[Pour after CAD lock]

What to ask when you're buying

• Can we mark the demold arrow on the first screenshot?
• What actually dumps — clay, chocolate, PU — along that arrow?
• Split, plug, or core if the arrow is blocked?
• Who pays CAD if the first glove proves the vector wrong ($50 when scope applies)?
• Jacket for a wet dump even if the split is right?

RFQ copy-pack

[email protected] · +86 135 3006 2969 · Xinhe Blvd, Shajing, Baoan.

RUUIPON peel-vector RFQ STEP or STL in mm + screenshot with peel arrow Dump: PU / epoxy / chocolate / soap / other Hook locations: arms / bail / closed letter / hollow Preferred structure: open-back / glove / split / plug / core / not sure Jacket: yes / no / ask Qty: sample / MOQ 20 CAD: lock from file / need $50 3D for parting

Matched next step: send the file and an arrow. We will mark KEEP / SPLIT / PLUG / CORE before A/B. Prototype 3–7 business days after CAD lock. MOQ 20 per design.

Related: undercuts and split molds, draft angle on RTV, parting line placement, contact.

FAQ

Is every undercut a two-part mold? No. If it sits along the peel vector and the dump can strip, keep one-piece. If it crosses the arrow, split, plug, or core.

Can 10A peel a hook a 30A cannot? Often, for a few cycles. Then the lip tears. Structure first, Shore second.

What if I change dump from wax to PU later? Wax can forgive a vector PU will not. Say the dump you will run, or budget a modification after the first sample.

Does a mother mold change the peel vector? It supports the skin. It does not open a hook. You still need the split or plug.

Is the split land $50 CAD? USD $50 when that CAD scope applies. A STEP that already has a shut may skip it. A figurine STL usually does not.