Ejector Pin Mark Prevention on Flexible Liquid Silicone Molded Parts

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.

This is an LSR injection problem. HCR compression ejects differently. RTV skins are peeled. Do not solve a glove-mold tear with a pin layout.

Why LSR shows pins so readily

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

A pin that is hotter or colder than the cavity prints a cure halo as well as a dent. That halo is not dirt.

Change the push before you polish the pin

Eliminate pins where you can. Stripper plates, stripper rings around a hub, air poppets, and a robot pull on a cold-runner vestige (if you still have one) or on a designed tab. Grommets and valves want a ring push on a thick shoulder, not three Ø1.5 mm pins in a 0.8 mm web.

If pins stay, put them 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, or a skin-contact cosmetic face.

Increase diameter and count so the pressure drops. A few small pins are a high local stress. More, larger faces on a rib reduce the dish. Knife-edge pins on flexible LSR are a cut.

Keep the pin face flush or a defined 0.01-0.03 mm proud only if you need to break vacuum, and only on a non-cosmetic face. A proud pin on a gloss pad is a deliberate witness. A sunk pin collects flash and then stamps a raised ring.

Vent the pin bore so you do not trap air that burns or leaves a bubble over the pin. LSR will find that clearance.

Timing is half of the mark

LSR has to be vulcanized through the section before it can take a push. A short cycle that still demolds will show pin craters on the thick section last to cure. Raise cavity temperature, add delay on open, or lengthen cure until a cut-through of the thickest boss is fully elastic, not gummy. Then eject.

If you speed the pins to “get it off the core” because the part is sticking, you are treating stick with impact. Fix polish, draft, undercut, or a collapsing core. Faster pins on a sticky, green part tear the land and the rubber.

Synchronize slides and cores so a pin never fires into a closed steel feature. That is a tool-life event and a unique mark you will not polish out of the part.

Steel 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 SPI on cosmetic-adjacent pins.
• 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 LSR is a fish-eye, then a stick, then a harder eject, then a mark. Dry-running or a clean-room-compatible micro-lube, wiped.
• Uneven pin length. One long pin takes the whole load. Blue the plate.

A tryout sequence that isolates the cause

1. Mold a shot and demold by hand with air only, pins locked back, if the geometry allows. If the mark is gone, the pin is the cause. If a texture from the cavity remains, it was never a pin.
2. If you need pins, double the delay-on-open. If the crater shrinks, you were green.
3. If the crater stays, increase pin area or move the push to a stripper. 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 steel.

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

Drawing language that prevents the default pin farm

“No ejector contact on Face A (seal/optical/skin). Ejection by stripper and air. Pins, if any, on Face B pads, Ø and count to be agreed, flush.” That sentence on an LSR RFQ is worth more than a general “minimize witness marks.” Flexible parts will always copy what pushes them. Choose what pushes them.

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. A pin hotter or colder than the cavity also prints a cure halo. This is an LSR injection problem, not a glove-mold peel.

Should I polish the pin before I change what pushes the part?

Change the push first. Stripper plates, rings around a hub, air poppets, or a robot on a tab beat small pins in a thin web. If pins stay, put them on a non-functional island, never on a seal, optical window, or skin-contact face. Knife-edge pins on flexible LSR are a cut.

Why is delay-on-open half of the mark?

The section must be vulcanized before it can take a push. A short cycle craters the thick boss last to cure. Speeding pins because the part sticks treats stick with impact; fix polish, draft, undercut, or a collapsing core. A pin firing into a closed slide is a tool-life event.

How do I isolate pin marks from cavity texture in tryout?

Demold once with air only and pins locked back if geometry allows. If the mark vanishes, the pin is the cause. Then double delay-on-open; if the crater shrinks, you were green. If it stays, increase pin area or move to a stripper. Do not sand the dish out of the rubber.

What drawing sentence prevents the default pin farm?

No ejector contact on the seal, optical, or skin face; ejection by stripper and air; pins if any on named pads, diameter and count agreed, flush. Flexible parts will always copy what pushes them. Oil weeping up a pin bore is a fish-eye that then forces a harder eject.