Keying Systems for Alignment in Two-Part Silicone Molds

A two-part mold that closes 0.3 mm off is not a flash problem. It is a keying problem.

Mismatch shows up as a step across the parting line, crushed keys, torn RTV lips, and LSR parts with a ridge that no amount of polish on one half will fix. Alignment is a designed interface. A “flat butt and some clamps” is a wish on anything larger than a coin.

RTV two-part molds (platinum or tin blocks and jackets) and steel two-part LSR/HCR tools both need keys. They do not use the same hardware.

Alignment is not a lip

A parting lip or a flash land is there to shut. If you ask it to locate as well, you grind it. Location and shut-off are two surfaces.

Location (keys): features that only fit in one XY (and often one rotation). They should touch before the cavity lands crush.

Shut-off: the land around the cavity that meets last, or meets on a designed crush for RTV.

Clamps / presses: the force. They do not translate a walking half into the correct place; they freeze the error.

If you ignore this: the operator “wiggles it until it looks even,” the keys never actually seat, and every part has a 0.2 mm step that migrates around the split.

Keys that work in RTV

Soft molds need keys the rubber can find with the hands, that survive opening, and that do not undercut so hard the mold tears.

Hemispherical buttons and cups. The classic. Pour or clay male hemispheres on one half; the other half copies them. They center. They do not clock rotation unless you use three and a non-equilateral layout. Four buttons in a square can still assemble 90° off if the cavity allows it. Use odd counts or a unique extra key.

Tapered cones / pyramids. More locating, more tear on peel if the taper is too shallow and too deep. Draft them like small cores.

Fences and stepped perimeters. A raised wall around the cavity that drops into a moat. Excellent against XY slide. Machine or clay it generously; a 1 mm fence on a 300 mm mold is a suggestion.

Puzzle tabs (one unique shape). Cheap rotation lock. Put it outside the pour so it does not print on the part.

External jig. Pins in a wooden or printed jacket, the RTV halves sitting in pockets. The rubber can be almost keyless if the shell is keyed (mother-mold article in this cluster). This is often the right production RTV pattern: sloppy rubber, strict jacket.

Do not key with a few leftover sprue bumps. They crush and then you have less alignment than nothing.

Tin-cure vs platinum does not change key geometry. Hardness does: 10A buttons squash and stop locating. Put serious keys in the jacket, not in the glove.

Keys that work in steel

Leader pins and bushings. Standard. Length so the pin engages before cavity steel can clash. Two pins locate; more pins fight (over-constraint) unless they float. A typical pattern is two pins plus a residual rotation stop, not eight tight pins.

Tapered interlocks / side locks. Take the side load when the press is not perfectly square. Useful on LSR tools that see opening force from a sticky part.

Cavity insert keys. The insert must only go in one way. A single flat or a unique corner beats a round insert that a night-shift can rotate 180°.

Slide keys are a separate system. A two-part article still mentions them because a two-part tool with a slide that is not timed will “look two-part” and mismatch like a three-part.

Steel keys are wear items. Bushings are cheaper than recut cavities. Design for replacement.

How many keys, and where

Minimum that usually works:

• Small RTV (hand-size): 2-3 buttons plus a unique rotation key, or a full perimeter fence
• Large RTV: fence + 4+ buttons + jacket hardware (bolts). Bolts are clamps, not locators, unless they sit in tight steel sleeves in the jacket
• Steel: 2 leader pins (different diameters or offset so the plate cannot go on backwards) + interlocks as the load needs

Place keys outside the flash land, far apart (leverage), not in a cluster on one edge. A cluster is one key that pretends to be four.

Height: keys should register while there is still air between cavity faces. If the cavity steel kisses before the pin has entered the bushing, you will get the nick that starts flash.

What a mismatch looks like (so you stop blaming silicone)

• A step along the split, constant around the part: plates shifted in XY
• A tapered step: one corner high - dirt on a land, a crushed RTV key, or a bent pin
• Flash on one side, crush on the other: rotation or a missing unique key, assembled 180° off
• Torn RTV at the same button every cycle: that button is doing shut-off duty or is too sharp; it is not “bad silicone”

Measure the closed empty tool (or the closed RTV with talc) before you blame shrink. Talc or Prussian blue on steel; a thin smear of contrasting clay on RTV. Open. Transfer should be even on keys first, then on lands. If the cavity paints before the keys, the keys are too short.

The dry-close test is the end of the argument. No compound. Close, open, inspect. Then pour or inject. A keyed tool that only “fits” with rubber in it is already flashing; the rubber is the gasket for your alignment error.

RFQ line: *Two-part [RTV/steel], key type [buttons+fence / leader pins+interlocks], unique rotation, keys engage before cavity, jacket yes/no.* If the quote is silent on keys, the first article will write that section for you in scrap.

FAQ

Why shouldn't the flash land also locate the two halves?

A land is there to shut. If it also locates, you grind it. Keys should touch and register in XY (and rotation) before cavity faces crush. Clamps freeze whatever error is already there; they do not translate a walking half into place.

Can four hemispherical buttons in a square still assemble wrong?

Yes. Four equal buttons can still clock 90 degrees if the cavity allows it. Use an odd count, a non-equilateral layout, or a unique extra key. Leftover sprue bumps crush and then locate worse than nothing.

Should 10A glove keys live in the rubber or the jacket?

Hardness locates, not cure family. Soft buttons squash and stop working. Put serious keys in the rigid shell and let the glove sit in keyed pockets. Tin versus platinum does not change that geometry.

How many leader pins belong on a steel LSR tool?

Two pins that engage before cavity steel can clash, often different diameters or offset so the plate cannot go on backwards, plus interlocks as side load needs. More tight pins fight each other. Bushings are cheaper than recut cavities; design them as wear items.

What does a dry-close test show that a flashed shot hides?

Close empty, with talc or Prussian blue on steel (contrasting clay on RTV). Transfer should hit keys first, then lands. If the cavity paints before the keys, the keys are too short. A tool that only "fits" with rubber in it is already using the shot as a gasket for alignment error.