How to Design Self-Demolding Geometry Features in Flexible Silicone Molds

Design the peel path before you design the cavity.

A flexible silicone *mold* (RTV glove or soft two-part block) can release a rigid casting that a steel tool would trap. A flexible silicone *part* can strip off a steel core that would need a slide in ABS. Those are both “self-demolding,” and they are not the same CAD exercise. Mixing them is how a glove mold gets drafted like an LSR insert, or an LSR boot gets asked to invert like Shore 10A.

This article is the RTV mold case first (the mold stretches off a master or a casting), then the LSR/HCR part case (the part stretches off steel). Geometry features overlap. The hardness sits on opposite sides of the interface.

Peel path first

Draw the motion, not the pretty section.

1. Where does the rubber start to leave? (a lip, a corner, a cut edge)
2. What feature does it have to climb over next?
3. What is the extra width of that feature versus the opening, and how thick is the rubber that must stretch?
4. What is the first place a tear would start? (a knife corner, a thin fin of rubber, a 90° inside corner in the mold)

If you cannot sketch that sequence on a section, you do not have self-demold. You have hope.

If you ignore this: the shop “just uses 10A” and the tear still starts at the same square undercut because the path asked a 2 mm rubber wall to double its length at a sharp edge.

Negative draft you can still strip (RTV molds)

RTV self-demold works when the mold is the elastomer. Typical pattern: Shore 10-20A platinum skin, often with a rigid shell, peeled or inverted off a resin, plaster, or metal master - or off the casting.

Features that usually self-release in a high-elongation, high-tear skin:

• Rounded beads and barbs with gentle entry angles
• Recessed logos with filleted letter sidewalls
• Modest return flanges if the flange thickness and the skin thickness are both honest
• Through-holes in the casting that are formed by pins the rubber can stretch around

Features that pretend to self-release and then do not:

• Square, deep undercuts (a dovetail in the master)
• Thin rubber webs that must stretch over a wide lock (a 1 mm skin over a 8 mm bulb)
• Sharp internal corners in the mold (stress risers)
• Hard jackets (30-40A) asked to invert like 10A
• One-piece blocks with no peel start (you need an edge you can get a fingernail under)

Fillet the lock. A R1 mm on a bead changes the strain concentration more than dropping 5A of hardness. Tear grades matter; a soft rubber with poor tear will split on the first climb.

Tin-cure vs platinum does not change the geometry rule. It changes shrink and inhibition. Do not pick tin so the mold “releases better.”

Holes, lips, and return flanges

Holes in the casting, formed in a one-piece RTV mold. The rubber fills the hole as a peg. On demold the peg has to stretch out. Draft the peg (the hole), fillet both faces, keep the hole short. Long, parallel holes in a stiff casting will tear the peg out of the mold and leave a rubber scrap in the part.

Lips and wiper edges. A thin lip pointing against the pull is a blade. Give it a peel chamfer on the trailing side, or split. Self-demold lips want to be short and radiused.

Return flanges. Treat as a calculated stretch. If the flange returns 3 mm on a 40 mm opening, a 10A glove with a 4 mm skin may roll off. A 30A block of the same geometry will take the flange with it or tear. Shell the soft skin so you are rolling rubber, not lifting a floppy tub.

When the part is silicone and the tool is steel

LSR and HCR can self-strip small undercuts: a retention bead on a tube, a shallow groove, a snap lip. That is elongation of the part, at the hardness you shipped.

Rules that keep it from becoming a tear:

• Keep the undercut height small relative to the local wall and to the opening
• Round the steel (the part will copy the radius; a sharp steel bead cuts the rubber on strip)
• Put draft on everything that is not the lock
• Air-eject or peel; do not expect a long zero-draft core plus a barb
• Harder LSR (60-80A) strips like a stiff boot. Do not copy a 20A baby-nipple undercut onto a 70A industrial cap

If you ignore this: the bead fills, then shears on eject, and you add draft to the OD while the barb was the problem.

Steel does not invert. If the ID return is a full bulb, you are back to cores and splits, not self-demold.

Draft versus stretch: pick one primary trick

Self-demold is not “zero draft everywhere plus vibes.” A clean self-demold feature usually has:

• Draft on the long walls
• Stretch reserved for one named lock
• Radii on that lock
• A peel start
• A rubber thickness that can take the strain
• A hardness chosen for that strain (10A skin vs 50A part)

Two named locks in two axes is a split. One named lock with a knife corner is a tear. Zero named locks and zero draft on a 40 mm-deep RTV block is a stuck master.

The test: print or machine a coupon of the worst lock at real size, real fillet, real hardness, real skin thickness. Ten demolds. If the tenth still has no white stress mark and no nick, you may skip the extra mold piece. If the first demold needs two people and a plastic spatula, you do not have self-demolding geometry. You have a fight.

RFQ line: *Self-demold on [feature] only; peel direction [axis]; mold [or part] hardness; skin/wall thickness; all other walls drafted. Coupon required before deleting the split.*

FAQ

Can I copy an RTV glove undercut into an LSR steel cavity?

Usually not. In an RTV mold the rubber stretches off the master; in LSR the part stretches off steel. A lock that peels from Shore 10-20A skin will shear a 60-80A part. Sketch the peel path, then coupon the worst lock at production hardness before you delete a split.

Does tin-cure RTV self-release better than platinum?

Tin versus platinum does not change the geometry rule. It changes shrink and inhibition. Do not pick tin so the mold "releases better." Fillet the lock; a small radius on a bead cuts strain more than dropping a few points of hardness.

How do I know a return flange will actually strip?

Treat it as calculated stretch: extra width versus the opening, skin thickness, and hardness. A 10A glove with honest wall may roll off a modest flange; a 30A block of the same shape will take the flange with it or tear. Shell the skin so you roll rubber, not lift a floppy tub.

Will 70A industrial LSR invert like a baby-nipple undercut?

No. Harder LSR strips like a stiff boot. Keep undercut height small relative to local wall and opening, round the steel so it does not cut on strip, and put draft on everything that is not the named lock. A full ID bulb is cores and splits, not self-demold.

What coupon lets me skip an extra mold piece?

Machine or print the worst lock at real size, fillet, hardness, and skin thickness, then do ten demolds. If the tenth still has no white stress mark and no nick, you may skip the split. If the first pull needs two people and a spatula, you do not have self-demolding geometry.