Silicone Overmolding: Multi-Material LSR Design Guide [2026]

Silicone overmolding is a manufacturing process that bonds a flexible silicone layer to a rigid plastic part or metal substrate, all in one shot of the overmolding process. Two materials-the ease of a thermoset silicone on the “soft” side plus the “hard” side of thermoplastic behavior-solve into a single assembly. The trick is how many ways to get both parties to stay married through heat, vibration and version 2.0 of rubber-to-go, as most designs fail silently.

This primer is focused at engineers specifying multi-material liquid silicone rubber (LSR) parts-such as medical connectors, automotive gaskets, or consμmer-electronics seals-who need to filter for the right substrate, bonding architecture, and design tolerances before understanding each trade-off instead of paying to learn it. We depend on up-to-date research, FDA and NIST data, and trademarked industry partner diagnostics instead of vendor boilerplate. Mono- and multi-material silicone parts out of the mold perform best when chemistry and geometry are designed in tandem.

What Is Silicone Overmolding? Process, Mechanics, and When It Makes Sense

The multi-material silicone molding sequence for high-performance overmolded parts looks like this. You select a substrate, mold it, or place a pre-molded part into the second cavity. The tool closes and LSR is injected hot onto the core, where it cures into a single sealed component joined. To produce with proprietary elastomer and ingredient controls, high-end processes brands talk ease with: 149-204 C mold temps, 121 C minimal LSR interface. Lower than that and the interface drops down, trapping bubbles and dust in a question able interface that is only discovered in production.

The bonded interface is not a press fit or glued joint. According to Eric Bishop, application engineer at Shin-Etsu Silicones, “The bonding of LSR to the thermoplastic is a chemical reaction. It needs a combination of time, temperature, and pressure.” Today’s advanced technology compounds need mid-range 149-204 C (300-400 F) injection mold temps and 121 C (250 F) or more to cure reliably, with curing time calibrated to silicone rubber overmolding chemistry and part thickness. A cold thermoplastic surface halts the reaction, yielding a weak interface that unfortunately is only revealed as field service failure.

Silicone overmolding is used in integrated components combining two different operational senses as rigid structures plus soft seals, damping, or complex shapes where design flexibility matters. Trials to bond two separately molded structures in one shot with glue require extra assembly hours and open the door to joint-line failure. In exchange, overmolding offers elimination of assembly and joint-line failures and, when the interface is engineered right, bonds succeed laboratory vibration and high-temperature sterilization testing — a cost-effective path compared to bonded assemblies for plastic substrates and plastic part families exposed to repeated flex or thermal cycling.

Two-Shot Molding vs. Overmolding: What’s the Difference?

The two can be used synonymously in casual conversation but they refer to very different machine configurations: Two-shot (or multi-shot) molding is preformed on one injection molding press with two separate barrels, a rotating or shuttle tool, and the two component materials are processed continuously once in a single shot. Overmolding, more generally, covers any injection molding process where a second material is molded onto a first, including insert molding via loaded pre-formed substrate (often done by other suppliers) into the tool before the LSR is injected. Two-shot offers intimate, maximal adhesion because any residual heat in the molded substrate accelerates LSR set. Insert-molded overmolding, on the other hand, provides more sourcing options and, depending on economy of scale, it might be cheaper to produce.

Material Compatibility Matrix: Which Silicone + Substrate Pairs Actually Bond

Overmolding compatibility is not black-and-white. A certain silicone grade that bonds consistently with polycarbonate may not bond well on polypropylene, while an LSR designed for self-bonding use on PEEK may require a primer for ABS. Below, we summarize six different commercially available LSR strategies against typical various thermoplastics and metals, based on published supplier datasheets, and the Shin-Etsu principles of mechanical bonding and chemical adhesion shown in Plastics Technology.

Lumping together different thermoplastics, supplier-posted datasheets, and metal substrates this table summarizes commercial LSR offers matched against common plastics and metals; this is merely a starting point to help guide the process designer who is considering multi-material components. LSR injection molding services that handle overmolding should be able to tell you which self-bonding grade they’ve qualified on your chosen substrate.

Design Rules for Multi-Material LSR Parts

The key in designing multi-material LSR components is not the geometry but the chemistry. A molding service that can do overmolding should be readily able to offer two things: what self-bonding grade they have qualified with your chosen plastic or metal and if there are any primer requirements or other preprocesses.

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  Silicone layer thickness 1.5–2 mm minimum. Areas below 1 mm are possible but unreliable; tear-out during demolding becomes the limiting factor.
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  Draft 0.5°–3° on all vertical silicone surfaces. LSR is flexible but demolds cleaner with draft — critical for textured or matte finishes.
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  Design shrinkage 2–3% into the LSR zone. Substrate shrinkage is typically under 1%; differential shrink pulls the interface into tension — a silent bond-failure driver.
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  Mechanical interlocking at the interface. Through-holes, undercuts, or a textured bonding zone add bond strength redundancy even when chemical bonding is the primary strategy.
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  Parting line placement away from sealing surfaces. Flash at a seal land breaks the seal; locate parting lines on cosmetic or non-sealing faces, targeting ±20–50 μm tolerance.
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  Specify a PTFE mold coating and controlled surface finish. Per Shin-Etsu guidance, PTFE coating reduces LSR adhesion to mold surfaces, gives a smooth surface on the cured silicone, and cuts demolding and trimming damage.

How Effective Is Overmolding of Low Durometer Silicone on Polycarbonate?

Low durometer LSR (10-30 Shore A) on polycarbonate is one of the toughest overmolding combinations to execute — the softer the silicone, the less your mechanical bite at the interface, and polycarbonate’s surface energy is near the minimum for silicones to self-bond. Three things need to be held simultaneously for success: a qualified self-bonding LSR grade (Shin-Etsu KE2090 and Momentive Silopren 47X9 are the known examples), a substrate temperature above 80 C at the point of LSR injection, and a thermoplastic grade specification free of amine-based antistats, sulfur-bearing additives, or silicone mold releases — all of which inhibit LSR cure. In a controlled production environment soft-on-PC overmolding is routine; in high-mix prototyping, only a sample plaque test with the LSR supplier reliably confirms it will work. When the Amerimold Tech case cited by Plastics Technology showed a 60 percent scrap rate, the prime suspect was incompatibility of the silicone grade.

Bonding Strategy: Chemical, Mechanical, and Hybrid Approaches

There are three bonds that are possible with the silicone rubber overmold, and the vast majority of production parts use 2 of them. Chemical bond (via selfbonding LSR or primer application), mechanical bond (via the geometry interlocks) and surface-activation techniques, such as corona discharge or air-plasma all excel on certain types of applications. Up front decision making limits your mold design, LSR grade selection and cleanliness requirements.

Cleanliness is essential for adhesion so always keep the substrate clean and dry. The LSR injection point should be heated areas of the substrate. Usually, higher the temperature the better the cure (Typical mold temperature for LSR 300-400 F, minimum 250 F).

Do not use any additive which contains sulfur, aniline or amines as it prevents LSR cure.”

— Eric Bishop, North American Marketing Manager, Shin-Etsu Silicones of America (as quoted in Plastics Technology)

Does Silicone Bond to Thermoplastics Without Primer?

Yes-just with the correct LSR grade. Self-bonding (“primerless” or “self-adhesive”) LSR families have been commercially available since the late 2000s, of which include Shin-Etsu Select-Hesive, Wacker ELASTOSIL LR 3078, Momentive Silopren LSR 47X9, and KCC self-bonding grades. These contain reactive additives, which will cross-link with specific thermoplastic surface chemistries at the time of LSR cure.

Primer-Free results are proven on PC, PBT, PPO alloys, PEEK, and polyphenylsulfone; they are poor on polypropylene and polyethylene, and require primer for most metals. The question is can you get primerless?

It is simply will the grade (with its specific additive package) “play”, and the only sure fire way is trial with the LSR supplier on that particular substrate grade.

Cost Structure: Tooling, Cycle Time, and Production Economics

Overmolding tooling cost is a function of: cavity count, LSR injection system complexity, and is the substrate molded in the same tool (two shot) or an insert (two shot). A 2 cavity LSR overmolding tool $40,000-$100,000, scale up as a function of cavity count, slide complexity, and accuracy. An Amerimold Tech case published in Plastics Technology showed a molder had sunk just under $70,000 into a 2 cavity LSR tool before discovering, as you would expect, that their originally specified LSR grade was incompatible with the polycarbonate keypad substrate upfront, not after.

Cycle time for the LSR portion of a two-shot cycle can vary from about 20s (thin gaskets, small parts) to about 180s (thick-section medical parts). At volume, each second of cycle time is a COGS line, so two-shot systems (where substrate heat helps cure the LSR) usually beat insert molding in the cost model by a few tens of thousands of parts. Shifting the economics of liquid silicone rubber molding often comes down to preheating the substrate: if you need a conveyor oven to warm your inserts, you have effectively doubled your footprint for each cell.

Industry Applications: Medical, Automotive, and Electronics

Silicone overmolding is a ubiquitous solution for rigid parts that need to function with a flexible, biocompatible, or sealing surface. Product designers and engineers succeed when they match the load requirements of their application environment with an appropriate substrate – overmold silicone combination. Here are three engineered solutions for three different applications.

Common Design Pitfalls (And How Engineers Avoid Them)

Drawings from seasoned overmold engineers reveal frequent failure points caused by way less than optimal dimensioning techniques, as documented in supplier trade publications, online practitioner forums, and the Shin-Etsu diagnostic guidance in Plastics Technology. Maintaining process consistency has more to do with your choice of specification than process using exotic molds.

Frequently Asked Questions