Radii and Corner Sharpness Guidelines for Custom Silicone Mold Longevity

Knife-edge corners do not look premium in a silicone tool. They look like the first place the steel or the rubber will tear.

Longevity arguments usually start at cycle counts and compound abrasion. Those matter. The cavity still dies at a corner: a chipped letter, a torn RTV skin at a square undercut, an LSR lip that nicks on a dead-sharp steel bead and then tears the next thousand shots. Radii are a maintenance spec. Cosmetic “crispness” is a separate, named exception.

Unmix the tools. Steel LSR/HCR cavities chip and wear. RTV molds tear. The fillet rules rhyme. The repair methods do not.

Internal versus external, on the part versus in the tool

On the part:

• An internal corner (a valley) is a steel external edge in the tool - a male knife that chips.
• An external corner (a part edge) is a steel internal corner - a female sink that is hard to polish and a stress riser in the rubber as it strips.

On an RTV mold the mapping flips with the impression, but the physics is the same: rubber hates being asked to fill and then peel off a needle.

LSR design notes commonly put inside fillets on the order of the local wall thickness, and they warn that radii much larger or smaller can encourage porosity. That is a flow/cure note. Longevity adds: even a small break (0.2-0.5 mm where the wall is thin) beats a theoretically sharp mill leftover.

If you ignore this: T1 looks jewelry-sharp. By a few thousand LSR shots the letter serifs are gone and the parting knife is dented. The quote’s cycle life assumed you did not design a chisel.

Steel tools: the edges that actually fail

Watch these in review, in this order:

1. Parting-line knives. A 0° shut-off on a thin lip is a razor. It nicks, then flashes, then you weld and recut. Give shut-offs land and a few degrees (see draft notes in this cluster).
2. Core pins and hole intersections. A pin meeting a face at a sharp shoulder cuts the rubber on strip and mushrooms the pin. Chamfer or radius the pin shoulder.
3. Logo and texture peaks. Raised-on-part letters are steel valleys (durable) or steel peaks (fragile) depending on raised vs recessed. Recessed letters on the part = steel standing type = chip city unless sidewalls have draft and a tiny tip radius.
4. Bead and seal knives. A “G9-sharp” sealing edge can be required. Then it is a named wear item with a polish/repair plan. A decorative ring does not get that privilege.
5. EDM recast on internal corners. A burned cusp is a crack starter in the steel and a tear starter in the rubber. Blend it.

Polish does not replace a radius. Mirror on a knife is still a knife, and it is a sharper one.

HCR compression tools see more sliding at shut-off than a good LSR cold-runner tool. Their knives die faster. Do not copy an LSR “we run sharp” habit onto a compression gasket tool.

RTV tools: corners are tears, not chips

A platinum or tin mold fails at:

• Square undercuts (self-demold article in this cluster)
• Thin rubber fins that fill a sharp master groove and then rip
• Inside corners of the mold where peel strain concentrates (white stress marks, then a split)

Masters should carry fillets the production part is allowed to have. Sharpening an SLA master with a blade so the silicone “looks crisp” is how a 10A glove lasts twelve pulls. If the product needs a crisp logo, that logo may belong in steel, or as a separate insert, not as a knife in a soft mold.

Mother-mold shells (fiberglass, SLA jackets) need plastic-style radii on *their* splits so the jacket does not crack. That radius is not on the part.

Sharp logos versus sharp structural corners

You can have one true sharp. You should not have twenty.

Artwork. Specify which edges are optically sharp, the minimum tool radius you will accept (even “sharp” steel has a real number), draft on letter sidewalls, and whether the shop may add a 0.05-0.1 mm break the camera will not see. Deep, skinny serifs at 0° draft are not a longevity problem. They are a no-fill and chip problem on day one.

Structure. Fillets on bosses, rib roots, tube/wall junctions, and anything a demold strain will see. Here the wall-thickness-scale fillet is the default. A structural knife is almost never a product requirement. It is leftover CAD.

Seals. A sealing lip may be intentionally fine. Then hardness, support, and shut-off land are the longevity package. Do not add a random R2 on a 0.4 mm wiper and ruin the function. Do break the *tool* edges that are not the lip.

The radius that pays for itself

A useful drawing note:

> Unless otherwise specified: internal part radii ≥ 0.3 mm or 0.5× local wall, whichever is larger. External part edges 0.2 mm min break. Letter sidewalls 1-3° draft; letter crest radius 0.05 mm min. Named seal edge [ID] is sharp; tool land per shut-off spec. No knife-edge parting.

That note is what you should not skip in order to look “designed.” A second note belongs on the RFQ: *which edges are wear items, and whether recut inserts are allowed.* If every corner is theoretically sharp, the first recut is not a failure of the shop. It is the drawing arriving on schedule.

Do not specify sharp as a cosmetic default and then argue cycle life. You picked the failure location when you left the R0.00 in the model.

FAQ

Why do raised-on-part letters chip first in LSR steel?

Recessed letters on the part are standing type in steel. Those peaks nick, then the serifs disappear. Give letter sidewalls draft and a tiny crest radius unless the edge is a named wear item with a recut plan.

Can a sealing lip stay sharp if everything else is filleted?

Yes, if it is a named exception. Hardness, support, and shut-off land are then the longevity package. Do not drop a random large radius on a thin wiper and ruin function, and do break the tool edges that are not the lip.

Do RTV molds fail the same way steel cavities fail at corners?

No. Steel chips; RTV tears. Square undercuts, thin fins in sharp master grooves, and inside corners that concentrate peel strain are the RTV killers. Sharpening an SLA master with a blade so the copy "looks crisp" is how a 10A glove lasts a handful of pulls.

Will a mirror polish save a knife-edge parting?

Polish on a knife is still a knife, and a sharper one. Parting knives nick, then flash, then you weld and recut. Give shut-offs land and a few degrees. HCR compression knives also die faster than a good LSR cold-runner shut-off; do not copy an LSR "we run sharp" habit onto a gasket tool.

What drawing note actually protects cycle life?

Call default internal and external breaks, letter draft and crest radius, and which seal edges are allowed to be sharp. Add which edges are wear items and whether recut inserts are allowed. Leaving every corner at theoretical sharp is choosing the failure location in CAD.