How Quality Assurance Laboratories Test Silicone Molded Part Dimensions (CMM & Optical Measurement)

A CMM stylus on a Shore 40A seal measures how far the probe sank, not the part. Silicone is an elastomer. It yields under a touch probe, under a clip, and under its own weight. A QA lab that treats an O-ring like a steel bushing will pass a thick, squashed ring and fail a correct one that was standing free.

This is how dimensional labs should approach molded silicone parts: HCR and LSR seals, grommets, medical components, overmolds. It is not a Gage R&R spreadsheet and not a machine brochure.

Free-state is a method, not a mood

Decide, in writing, the state of the part at measurement:

• Free-state, no squeeze. Most O-rings, gaskets, and flexible housings. The part sits, supported so it does not sag into a banana, but is not clamped to a drawing that assumed metal.
• Defined compression. Only if the drawing is a gland or a compressed thickness. Then the fixture must reproduce that squeeze with stops, not with a technician’s fingers.
• On a mandrel or in a nest that copies the use orientation (a tube ID, a strap on a wrist form). Say so. A strap laid flat and a strap on a form are different lengths.

Temperature matters. Silicone expands more than steel. A lab at 23 °C and a press-side check on a 40 °C part will disagree. Condition parts, then measure. Do not argue about 0.1 mm on a 50 mm LSR part across a 15 °C swing without doing the arithmetic.

Contact CMM: where it still belongs, and where it lies

A tactile CMM (touch probe on a moving bridge) is excellent on rigid datums: overmolded metal frames, plastic carriers, and the steel mold itself. On the rubber:

• Probe force dishes the surface. Low-force probes and large-radius styli reduce the dish. They do not make it zero.
• A scanning probe dragged on a soft lip will plow.
• Fixtures that “hold datum A” on a gasket often are the measurement.

Use contact CMM on silicone when you need a 3D point that vision cannot see (a deep undercut, a hole axis in a stiff-enough boss) and when you have a fixture that supports the part the way the drawing meant. Record probe force and tip radius. If two labs disagree, those two numbers are the first suspects, not the molder’s shrink factor.

Do not accept a CMM report on an O-ring CS that was taken by touching the torus at the top of a ring lying on a granite plate. You measured squash plus gravity.

Optical and vision: the default for flexible parts

Vision measuring machines, optical CMMs, profile projectors, and calibrated camera systems take edges without a dent. They are the right first tool for:

• O-ring ID/OD and cross-section on a glass stage, ring lying naturally, no clip.
• Gasket outlines and hole positions in free-state.
• Flash width, gate vestige, pin dishes (the defect is optical).
• Thin membranes and slits that a probe would open.

Lighting is the new fixture. Backlight for a silhouette. Raking light for a step. A shiny LSR lip can fake an edge. Telecentric optics help on height-sensitive silhouettes. A human with a steel caliper is also an optical method with an uncontrolled squeeze; it is not “more real” than a vision system.

Laser or structured-light 3D scans can map a surface, but the part still must not sag, and shiny or translucent silicone can drop data. A light dusting that the customer forbids is not a method. If you scan, prove the setup on a retained part.

Fixturing that does not become the dimension

Support long, floppy parts on a bed of pins or a printed nest that matches the free-state curve, or measure in sections. Do not tape a gasket flat unless the drawing says the part is used flat and the tape does not pull.

For compressed measurements (installed O-ring height, a pad under a lid), use hard stops at the designed gap and measure the remaining gap or the load. Do not “press until it looks seated.”

Never use a vise.

What a competent silicone FAIR looks like

• Drawing with datums that make sense on rubber (a sealing face, an ID, not six points on a floppy wall).
• Measurement state: free or compressed, temperature, orientation.
• Method per dimension: vision / optical, low-force contact, mandrel, go/no-go of a defined design.
• Shrink applied to the steel, not corrected in QA by “the rubber always measures small so we add 0.2.”
• Thickness of seals by a non-crush method (optical, or a drop-gauge with a large foot and a specified force, used with a correction you have demonstrated).
• Flash called as a maximum, measured, not ground off before FAIR unless the process includes that destage.

If the lab only reports a CMM printout of 40 points on a 20A gasket, ask how the part was held. If they cannot answer, the numbers are a fixture study.

Do not fake capability theater

Gage R&R on silicone is sensitive to operator squeeze and to part relaxation between trials. It can be done, with a fixture that removes those knobs, on a defined dimension. Publishing a table of made-up %GRR values does not make a lab. Run the method, on the actual part, with the actual fixture, or do not claim it.

A shop-floor check can be a nest and a vision snapshot, or a mandrel and a go/no-go, aligned to the same state as the lab. Two methods in two states will never match. Pick one state and put it on the drawing.

When a dimension is chronically “off,” look at fixturing and probe force before you recut steel. A surprising number of silicone tools are innocent. The CMM was measuring rubber as if it were a block.

FAQ

Why does a contact CMM on a Shore 40A seal measure the fixture and the dent?

The elastomer yields under the stylus, under a clip, and under its own weight. A lab that treats an O-ring like a steel bushing will pass a squashed thick ring and fail a correct free one. Record probe force and tip radius; those are the first suspects when two labs disagree, not the molder's shrink factor.

What does "free-state" have to mean on the drawing?

A written measurement state: no squeeze for most rings and gaskets, or a defined compression with stops if the drawing is a gland height, or a mandrel that copies use. A strap laid flat and a strap on a form are different lengths. Condition temperature; silicone expands more than steel.

When does a tactile CMM still belong on a silicone job?

On rigid datums: overmolded metal frames, plastic carriers, and the steel mold itself, or a deep undercut vision cannot see in a stiff-enough boss. Do not touch-probe O-ring cross-section on a ring lying on granite. You measured squash plus gravity.

Why is lighting the fixture on optical measurement?

Vision avoids the dent and is the default for ID/OD, outlines, flash, vestiges, and membranes a probe would open. Backlight for silhouette, raking light for a step; a shiny LSR lip can fake an edge. A steel caliper is also an optical method with uncontrolled squeeze. Never use a vise.

What should I check before recutting steel for a chronic "off" dimension?

Fixturing and probe force. A surprising number of silicone tools are innocent; the CMM was measuring rubber as if it were a block. A competent FAIR names state, method per dimension, and non-crush thickness. Shrink is applied to the steel, not corrected in QA by adding a constant. Two methods in two states will never match.