Edge Radius on Silicone Molds: Small Fillets That Save Tear-Outs

A sharp 90° cavity lip on a Shore A 35 resin tray tears on pour 42 - not because the silicone is bad, but because stress concentrates at a zero-radius corner with no room to stretch. RUUIPON builds custom silicone molds at MOQ 50 with prototypes in 3-7 days. This guide sets edge radius minimums so your fillets survive vacuum-degassed pour, ~0.1% shrink, and daily peel cycles.

Quick answer: Spec ≥0.5 mm radius on every cavity lip and web corner; ≥1.0 mm on external tray edges and split-line roots. Pair with walls 2.5-5 mm, spacing 8-20 mm, and Shore A matched to your demold temperature - sharp CAD corners copy 1:1 into silicone and fail regardless of platinum grade.

Edge Radius Minimums in One Table

Edge radius is the fillet where two surfaces meet - measured at the mold cavity, not the cast part. Silicone stretches at radius; sharp corners tear.

Location	Min radius	Why
Cavity lip (demold edge)	0.5 mm	Peel stress spreads; prevents lip rip
Web-to-cavity junction	0.5-1.0 mm	Stress between adjacent cavities
Tray outer perimeter	1.0 mm	Handling + shipping flex
Two-part split root	1.0 mm	Flash trim without nicking silicone
Embossed logo root	0.3 mm	Detail retention - see logo limits

Where Radius Matters on a Production Tray

Buyers focus on cavity shape; tear-outs start at geometry transitions the CAD hides in wireframe view.

• Cavity lip - where cast part exits; zero radius = knife edge on silicone.
• Floor-to-wall junction - resin and epoxy shrink pull inward; sharp root cracks on demold.
• Inter-cavity webs - thin bridges between cells; radius at both ends doubles flex life.
• Registration key roots - sharp key corners chip on repeated clamp. Registration keys.
• Pour spout bases - resin flash builds at sharp spout roots. Pour spout design.

Undercuts with sharp roots fail faster than rounded hooks - see undercut depth limits.

Radius by Feature Type & Application

Application	Cavity lip R	Wall junction R	Notes
Resin geode / coaster	0.5 mm	1.0 mm	Epoxy shrink pulls - resin design tips
Chocolate / gummy	0.3-0.5 mm	0.5 mm	Warm demold; softer Shore A 20-25
Cold process soap	1.0 mm	1.0 mm	Caustic + long cure - soap mold guide
Candle pillar	0.5 mm	0.8 mm	Hot pour wax; seam pairing - seam lines
Wax melt clamshell	0.8 mm	1.0 mm	Flex hinge needs radius at fold root

Shore A Hardness & Radius Pairing

Softer silicone tolerates smaller radii because elongation is higher - but tear strength still caps how sharp you can go.

Shore A	Min lip radius	Typical cycle before tear
20-25	0.3 mm	300-500 demolds (food)
25-30	0.4 mm	200-400 demolds
30-35	0.5 mm	150-300 demolds
35-40	0.5-0.8 mm	100-200 demolds
40-50	0.8-1.0 mm	80-150 demolds - stiff resin

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

CAD DFM: How to Spec Fillets Before Quote

1. Apply uniform fillet - use 0.5 mm minimum on all internal cavity edges in STEP/STL export.
2. Mark pull direction - radius on demold path matters; radius against pull creates hidden undercut.
3. Pair with draft - 2° draft + 0.5 mm radius beats either alone. Draft angles.
4. Wall thickness - radius does not replace thin walls; keep 2.5-5 mm. Wall thickness.
5. Flag logo exceptions - text roots may need 0.2-0.3 mm for legibility; accept shorter cycle life.

We flag zero-radius red lines on every quote - DFM red flags.

Production QC at MOQ 50

PO line	Example spec
Silicone	Platinum cure, vacuum degassed, ~0.1% shrink
Cavity lip radius	≥0.5 mm all edges - or noted exception on drawing
Walls / spacing	2.5-5 mm walls, 8-20 mm cavity spacing
Shore A	Locked at sample - e.g. Shore A 30 ±3
Sample	5 test demolds on prototype in 3-7 days
Production	MOQ 50 trays after sample sign-off

Send STEP with fillet callouts to [email protected] | Contact RUUIPON | WhatsApp +86 135 3006 2969.

Frequently Asked Questions

Will a chamfer work instead of a radius?
Chamfers help on outer tray edges but concentrate stress at the cavity lip - use a true fillet on demold paths.

Can CNC masters hold 0.3 mm radius?
Yes on aluminum or resin masters - master materials hold fine detail when toolpath stepover is ≤0.1 mm.

Does radius affect cast part dimensions?
Yes - fillet adds material at corners. Account for ~0.1% shrink plus fillet volume in fill weight calculations.

My logo needs sharp corners - what then?
Accept 0.2-0.3 mm root radius minimum; deeper emboss needs Shore A 40+ and shorter cycle life.

Will vacuum degassing fix sharp edges?
No - degassing removes bubbles, not stress concentration. Vacuum degassing guide.

MOQ with radius DFM changes?
Still MOQ 50; fillet revisions on approved CAD add 2-3 days if master re-cut is needed.

Prototype lead time?
Single-cavity sample in 3-7 days - we test peel 10× at lip before production approval.