Preventing Pin and Peel Witnesses on Flexible RTV Faces

An ejector witness on an LSR *part* is usually designed in, then blamed on the press. Flexible liquid silicone copies the pin face, then yields under the pin force. If the rubber is still green, the mark becomes a crater. If the wall is a membrane, the pin becomes a punch.

That is an LSR injection problem on a parts OEM. On this site you peel an RTV cavity. Do not solve a glove-mold tear with a pin layout. The analog defects are: registration pins and screws in the master or box that print into the pour face, keys on a gloss split, and ice-white stretch marks from a stick-slip peel.

If your drawing says “no ejector contact on Face A” and the quote returns a cold-runner, you are on the wrong factory.

Why flexible silicone shows witnesses so readily

Platinum RTV and LSR both copy micro-gaps. A pin-to-bore clearance that would hide on a rigid thermoplastic can flash a ring or print a step. The rubber is an elastomer, so even modest force locally compresses the face; the compression can remain as a glossy dish, a matte bruise, or a hole. Optical, chocolate, and skin-contact faces make that dish a reject. Sealing lips make it a leak.

A master pin that is proud of the pattern prints a crater in every dump. A key on a lipstick gloss face is a silver streak. A peel that yanks a 10A–20A skin puts yield marks that look like dirt and are not.

Change the push before you polish the pin

Eliminate pins where you can — on the master and the box. Stripper analog: peel from a thick shoulder, air at the edge, jacket opening on stops. Grommet casts and valves want a ring push on a thick shoulder, not three Ø1.5 mm master pins in a 0.8 mm web.

If hardware stays, put it on a non-functional island. A pad, a lettered recess on the back, a tab you destage. Never on a sealing bead, an optical window, a chocolate highlight, or a skin-contact cosmetic face.

Increase diameter and count so the pressure drops if you truly need locating pins in a rigid master. A few small pins are a high local stress. Knife-edge pins on flexible silicone are a cut.

Keep pin faces flush on the master. A proud pin on a gloss pad is a deliberate witness. A sunk pin collects rubber or media and then stamps a raised ring.

Vent the box so you do not trap air that leaves a bubble over a locator. RTV will find that clearance when you pour the skin.

Timing is half of the mark

LSR has to be vulcanized through the section before it can take a push — on that OEM. RTV castings have to take their own weight before you peel. A short cycle that still demolds will show stretch craters on the thick section last to set. Wait. Then peel.

If you speed the peel to “get it off the core” because the part is sticking, you are treating stick with impact. Fix polish on the master, draft, undercut, or a removable core. Faster yank on a sticky, green casting tears the skin and the copy.

Synchronize cores so a locator never fires into a closed jacket feature. That is a tool-life event and a unique mark you will not polish out of the rubber.

Steel and master details that print even when force is low

• Pin steel and polish. A rough pin face copies as a matte coin. Match or exceed the surrounding finish on cosmetic-adjacent locators.
• Wear. A mushroomed pin is a crater waiting to happen. Replace on PM, do not tap it back.
• Lubricant rings. Oil that weeps up a pin bore onto platinum is a fish-eye, then a stick, then a harder peel, then a mark. Keep oil off food and skin faces.
• Uneven pin length. One long pin takes the whole load. Blue the plate — or, on a box, seat the master so it cannot rock.

A tryout sequence that isolates the cause

1. Pour a skin and peel by hand with air only, locators locked back or masked, if the geometry allows. If the mark is gone, the pin is the cause. If a texture from the master remains, it was never a pin.
2. If you need locators, wait longer to peel. If the crater shrinks, you were green or over-stretched.
3. If the crater stays, increase pin area or move the push to a shoulder. If a tiny dish remains but no tear, decide whether that face is cosmetic.
4. If the part tears at the pin, stop increasing force. You have an undercut or a membrane. Change the master or add a core.

Do not sand the dish out of the rubber. You will matte a larger zone and change the thickness you just specified.

Common mix-up: a LIM pin farm on an RTV RFQ

“Minimize witness marks” on a parts drawing means stripper and air, pins on Face B. On an RTV drawing it means keys and pour off Face A, peel path drawn. Same buyer sentence. Two tools.

What to ask when you're buying

• Object: RTV cavity or LSR part?
• Face A: no locators, no keys, no pour scar?
• Peel from a thick shoulder, air, jacket on stops?
• Wait until the casting can take its weight?
• Who owns the master if we recut a proud pin?

Worked example: chocolate highlight vs LSR strap

12-cavity chocolate bear, platinum 10A–20A, jacket. Face A is the highlight: no box screws through the snout, keys on the back, slow peel. A sister PO for a flexible LSR strap with pin dishes on the skin-contact face is a LIM problem: stripper and air, delay-on-open — different quote.

When this article is the wrong quote

If you need ejector layout on a hot LSR tool, send the parts OEM. A tiny open-back ice tray can skip locators. Geometry can override: a membrane grommet still needs a core, not smaller pins.

Related reading: peel vs yank, beauty gloss faces, grommet membranes, custom RTV molds.

Drawing language that prevents the default pin farm

“No locator or key contact on Face A (seal/optical/food/skin). Peel by jacket and air. Pins, if any, on Face B pads, Ø and count to be agreed, flush.” That sentence on an RTV RFQ is worth more than a general “minimize witness marks.” Flexible rubber will always copy what pushes it. Choose what pushes it.

Send Face A keep-outs and undercut photos. We will put keys on the back and tell you when the pin farm belongs on a LIM RFQ.

We would not recommend three Ø1.5 mm pins in a 0.8 mm web

We would not recommend sanding a dish out of the rubber. We would not raise Shore A first to “take the pin.” We would not speed peel because it sticks. We would not paste a hot-cavity ejector map onto this jacket.

RUUIPON Mold Structure Decision: show face stays clean. Flat back? open-back, locators off the pour. Deep undercut that cannot peel? core / two-part, not a punch. Soft wet dump? mother mold registered during peel.

Typical shop values

Reference values only. Confirm material-specific limits against the supplier’s technical data and agree project acceptance criteria before production.

| Item | Typical shop value | | --- | --- | | RTV skin | Platinum 1:1 or 10:1, 10A–20A, jacket | | Platinum shrink | About 0.1–0.3% after full cure | | If a pin must stay (master) | Flush; never on Face A | | Contrast: LSR pin proud (parts) | Sometimes 0.01–0.03 mm to break vacuum — not on this pour face | | Loaded-tray sag vs a peel bruise | Sag ~0.5–1.5 mm is jacket; white stretch is yield |

FAQ

Why do flexible LSR parts show ejector witnesses more than rigid plastics? LSR copies pin-to-bore clearance as a flash ring or step, then yields under pin force into a glossy dish, matte bruise, or hole. If the rubber is still green, the mark becomes a crater. That is an LSR injection problem. On RTV, the same physics prints master locators and peel stretch. Do not solve a glove with a pin layout.

Should I polish the pin before I change what pushes the part? Change the push first. On RTV: peel, air, jacket stops, locators on a non-functional island. On LIM: stripper plates, rings around a hub, air poppets. If pins stay, never on a seal, optical window, chocolate highlight, or skin-contact face. Knife-edge pins on flexible silicone are a cut.

Why is wait-to-peel half of the mark? The section must be set before it can take a pull. A short cycle craters the thick boss last to cure. Speeding peel because the part sticks treats stick with impact; fix polish, draft, undercut, or a core. A locator firing into a closed jacket is a tool-life event.

How do I isolate pin marks from master texture in tryout? Peel once with air only and locators locked back if geometry allows. If the mark vanishes, the pin is the cause. Then wait longer; if the crater shrinks, you were green. If it stays, increase pin area or move to a shoulder. Do not sand the dish out of the rubber.

What drawing sentence prevents the default pin farm? No locator or key contact on the seal, optical, food, or skin face; peel by jacket and air; pins if any on named pads, diameter and count agreed, flush. Flexible rubber will always copy what pushes it. Oil weeping up a pin bore is a fish-eye that then forces a harder peel.