Adding Threaded Metal Inserts into Silicone Parts During the Molding Process

A thread cut into 40 Shore A silicone is a suggestion. A brass or stainless insert molded into the rubber is a fastener.

Heat-staked or ultrasonic inserts that plastic designers love do not exist here: there is no thermoplastic melt pool. You either insert-mold the metal in LSR or HCR, or you bond/pot it after, or you use a clamp plate the rubber never pretends to be. Tapping the elastomer is for temporary jigs and toys. If the joint must take wrench torque or repeated assembly, design a metal thread and a rubber lock around it.

Primer versus mechanical lock on metal overmolds is covered elsewhere in this library. This article is the threaded case: bosses, weld nuts, standoffs, and fittings that must accept a machine screw after the shot.

Why the rubber cannot be the nut

Silicone creeps under load. A thread form in the elastomer will relax, especially in heat, oil, or a 10-30 Shore A grade. Pull-out becomes a torn helix. Torque to strip is a tear number, not a steel property. Fine pitches are worse. Coarse “threads” in hard 70A HCR can work as a cheap lamp-holder once; they are not an M3 in a device that sees service.

Metal inserts exist so the thread is metal-to-metal. The silicone’s job is to retain the insert (torque-out, pull-out, rotation) and to seal if that is required.

Insert geometry that actually stays

Buy or machine inserts with something for rubber to grab:

• Knurls, grooves, hex collars, undercuts, through-holes the LSR can rivet.
• A flange that the rubber cannot pull through.
• Length enough that the insert is not a washer sitting on a 0.8 mm skin.

A smooth OD with primer only is a peel waiting for the first reverse torque. Primer (silane systems, cleanliness, sometimes blast) still helps against spin in shear. Geometry does the wrench.

Materials: brass is common and machines well; stainless for corrosion and medical; aluminum if you like galling and CTE fights. Coatings and oils on catalog inserts are inhibition and bond killers. Degrease. If the insert is passivated stainless, say so; some films fight primer.

Load path: put the metal on metal when you can. A screw that clamps a housing through the insert into a *metal* jack on the other side does not ask the silicone to carry compression. A screw that only squeezes rubber under the insert head will embed and loosen.

If you ignore the flange, the defect is an insert you can pull out with the screw still attached.

Multi-insert parts (a pad with four standoffs) need a load sequence the cell can repeat. A single missed pin in a 4-cavity HCR tool ships a blind hole that looks like a mold problem. Fixtures, magnets, or spring-loaded pins that make the insert sit before close are cheaper than a rework cell. Do not leave insert loading as an unwritten tribal step.

Tooling: keep rubber out of the thread

LSR will fill an M3 like a grease gun. The tool must shut off on the insert and pin the thread.

Typical hardware:

• A locating pin or screw that occupies the female thread (or a male-threaded core if the insert is a stud).
• A land that pinches a designed face of the insert so LSR cannot run the helix. Gaps on the order of 0.005 mm are enough to flash. Insert OD tolerance plus pin runout is the stack.
• Nests that hold height. An insert that sits 0.3 mm low becomes a flash path and a proud/sunken boss.
• For multi-cavity, poke-yoke the orientation of hex collars.

HCR compression: load inserts by hand onto pins, then the biscuit. Pins walk; operators miss a cavity; you ship a blind boss. Poka-yoke and a vision check belong on that cell as much as on LIM.

Heat: LSR cavities are hot. Thin aluminum inserts can grow and then sit loose after cool-down if the rubber lock was marginal. Brass and stainless still need the mechanical grab.

Demold: the pin must unscrew or pull without rotating the insert in green rubber. A robot that yanks a core pin out of a freshly shot 20A part can spin the insert and tear the lock. Time, pin polish, and a slight hold on the part.

Process and quality

• No mold release on the insert OD.
• Preheat if the insert is a heat sink that would otherwise steal cure from the interface (large stainless fittings).
• Self-bonding LSR grades that list metals: still clean, still knurl.
• Torque-out and pull-out tests on the *production* durometer and thickness, after post-cure if you post-cure. Day-one green rubber lies.
• Electrical: the insert can be a live terminal. Creepage through a flash path of carbon-filled silicone is a different product. Name the grade’s conductivity.

FDA 21 CFR 177.2600 and ISO 10993 apply to the rubber (and to what the insert is made of, if it contacts). A medical-grade LSR around a mystery zinc insert is an incomplete file.

RTV benches can glue a nut into a prototype. That torque number is not the LIM number.

RFQ notes for threaded inserts

Insert drawing (knurl, flange, material, coating), thread, whether the pin is supplied, allowed flash on the helix (ideally none), rubber thickness around the insert, torque and pull tests, process (LSR insert mold or HCR compression). If the screw must seal, add the gasket function. If you ignore pin shut-off, the first article will be a tap full of silicone.

FAQ

Can I tap threads directly into molded silicone instead of using an insert?

Only for low-duty, low-cycle uses in harder compounds, and even then the thread will creep. For machine screws, assemblies, and service parts, insert-mold metal. Tapping 20-40 Shore A is how you strip a hole on the second assembly.

How do I keep LSR out of an M3 thread during injection?

Occupy the thread with a pin or sacrificial screw and shut off on a designed insert face. LSR flashes through about 0.005 mm. A loose pin plus a dirty helix is a filled thread. Spec the insert tolerance and the pin together.

Is primer enough to stop an insert from spinning?

Primer helps shear on a clean OD. A wrench is a peel and a torsion on a smooth cylinder. Add knurls, hex, grooves, or a through-rivet of rubber. Primer plus a smooth bushing is a spin-out after heat aging.

Should inserts be brass or stainless in medical LSR?

Stainless is the usual corrosion and documentation choice. Brass is common in industrial and electrical. Either must be degreased and called out in the biocompatibility and materials file if there is patient or fluid contact. Do not swap alloys after you qualified pull-out and bond.

Why did torque-out pass at T1 and fail after post-cure?

The rubber shrank and the network finished. An insert that was jammed in green LSR can sit looser, or the lock can tear at a flash nick, after the oven. Test pull and torque after the full thermal history you will ship, on the production thickness, not on a warm first shot.