Undercut Depth Limits in Silicone Molds: When Soft Shore Still Tears

A 4 mm hook lip on a resin figurine cavity tears on pour 35 when your tray runs Shore A 35 with 2 mm webs - soft silicone does not forgive geometry that fights demold. RUUIPON builds custom silicone molds at MOQ 50 with prototypes in 3-7 days. This guide sets undercut depth limits by Shore A so your CAD survives vacuum-degassed pour and ~0.1% shrink without re-tooling.

Quick answer: On production trays with walls 2.5-5 mm and spacing 8-20 mm, plan undercut depth ≤1.5 mm per side on Shore A 30-40 resin molds, ≤2.5 mm on Shore A 20-25 food molds, and zero undercut on stiff Shore A 45+ unless you split the mold. Deeper hooks need two-part parting lines or collapsible cores - not "just use softer silicone."

Undercut Depth in One Table

Undercut depth is how far a cavity feature extends past the widest demold plane - measured perpendicular to pull direction, not along the sloped face.

Shore A	Max undercut depth (single pull)	Typical application
20-25	2-3 mm	Chocolate, gummy - warm demold
25-30	1.5-2.5 mm	Soap, wax melt
30-35	1-2 mm	General resin, candle
35-40	0.8-1.5 mm	Stiff resin, sharp logos
40-50	0-0.8 mm	High-detail chocolate logos only

What Counts as an Undercut on a Silicone Mold

Any cavity surface that blocks straight-line removal of the cast part is an undercut. Silicone stretches - that is the advantage over rigid steel - but stretch has a limit before the mold web or cavity lip tears.

• External undercut - bulbous midsection on a pillar; peel sideways to release.
• Internal undercut - re-entrant corner, hook, or dovetail inside the cavity.
• Negative draft - wall angled inward >0° from vertical counts as progressive undercut over height.
• Parting-line undercut - feature crosses the split plane without relief. See draft angles guide.

Master model undercuts copy 1:1 into silicone. A CNC master with 3 mm hook becomes a 3 mm hook in every cavity - master materials do not reduce undercut risk.

Depth Limits by Shore A Hardness

Softer silicone elongates more before tear - but tear strength and wall thickness still cap depth. We vacuum degas every production pour; shrink ~0.1% slightly tightens fit on deep hooks after cure.

Feature type	Shore A 25	Shore A 35	Shore A 45
Smooth external bulb	3 mm	2 mm	1 mm
Sharp internal hook	2 mm	1 mm	Split required
Embossed logo (0.5 mm line)	0.4 mm depth OK	0.4 mm depth OK	0.3 mm max - logo limits
Thread or rib detail	1.5 mm	0.8 mm	Two-part only

Full hardness selection: Shore hardness guide. Tear at demold: tear strength failures.

Draft Angle & Undercut Pairing

Draft reduces effective undercut over cavity height. Rule of thumb: 1° draft removes ~0.17 mm undercut per 10 mm of wall height.

Cavity height	1° draft relief	3° draft relief	5° draft relief
20 mm tall	0.35 mm	1.0 mm	1.7 mm
40 mm tall	0.70 mm	2.1 mm	3.5 mm
60 mm tall	1.0 mm	3.1 mm	5.2 mm

Pair minimum 2° draft on resin cavities with undercut depth under 1.5 mm. Zero-draft vertical walls with hooks fail regardless of Shore A if webs are thin - walls stay 2.5-5 mm. Wall thickness.

When to Split: Two-Part Mold Lines

Undercut depth beyond single-pull limits needs a parting line through the feature - not thinner walls.

1. Place split at widest section - each half pulls perpendicular to its face.
2. Registration keys - 3-5 mm keys on flange; registration key spec.
3. Flash control - clamp pressure on thin lips creates flash at split. Flash QC.
4. Seam on candle/resin - hide on rear edge. Seam line placement.

Compare one-part vs two-part economics: one-part vs two-part.

Tear Failures We See at MOQ 50 QC

Failure	Root cause	Fix before production
Web snap between cavities	Undercut + 2 mm web	Increase spacing to 12-20 mm; thicken web to 3 mm
Cavity lip rip	3 mm hook, Shore A 40	Split mold or reduce hook to 1 mm
Progressive tear after pour 50	Stress at sharp undercut root	Radius ≥0.5 mm on hook root; soft Shore A 25-30
Cast part stuck	Negative draft 2° over 30 mm height	Add 3-5° draft or two-part

Layout density makes undercuts worse - cavity spacing 8-20 mm gives silicone room to flex during peel.

Production Spec at MOQ 50

PO line	Example
Pull direction	Arrow on drawing - Z-axis demold
Max undercut per side	1.5 mm @ Shore A 35 - or split noted
Draft minimum	2° resin / 1° chocolate
Silicone	Platinum cure, vacuum degassed, ~0.1% shrink
Walls / spacing	2.5-5 mm walls, 8-20 mm cavity spacing
Sample	5 test demolds on prototype in 3-7 days

Send STEP with pull direction and undercut callouts to [email protected] | Contact RUUIPON | WhatsApp +86 135 3006 2969.

Frequently Asked Questions

Can soft Shore A 20 fix a 5 mm undercut?
No - the cavity lip and webs still tear. Split the mold or reduce hook depth.

Do undercuts affect cycle life?
Yes - stress concentrates at hooks; expect 30-50% shorter life vs clean-draft cavities. Cycle life by application.

Will vacuum degassing help demold?
Degassing removes bubbles - it does not add stretch. Vacuum degassing guide.

Chocolate undercuts?
Warm demold allows ~2 mm on Shore A 20-25; deeper logos need tapered walls. Chocolate Shore A.

Can RUUIPON DFM my undercuts?
Yes - we flag red lines on quote. DFM red flags.

MOQ with complex undercuts?
Still MOQ 50; two-part molds may add 3-5 days to prototype lead time.

Prototype before MOQ?
Single-cavity sample in 3-7 days - test demold 10× before production approval.