RTV silicone is the default when a joint has to flex, tolerate heat, and shrug off weather. What it has never been is fast. Incure Pyra-Sil™ 904 is a one-part RTV silicone rubber that cures primarily under UV light, with a secondary moisture cure for shadowed areas, so it fits a production takt time.

RTV silicone with a fast fixture

A conventional room-temperature-vulcanizing silicone develops strength over hours and days as atmospheric moisture works into the film. That is fine for a field seal and a problem for an assembly line, where the part has to be handled and moved within seconds of dispensing.

904 fixes under UV in seconds. The exposed silicone gels and holds parts in position immediately; the moisture cure then completes any material the light could not reach. The finished joint has the flexibility, ozone resistance, and temperature range of an RTV silicone without the line dwell.

What Pyra-Sil™ 904 is

904 is a one-part UV/moisture-cure silicone adhesive usable as a form-in-place gasket, a conformal coating, or a sealant. The cured rubber resists ozone, harsh weather, and elevated temperature, making it a fit for outdoor and engine-bay environments.

Bonding and sealing dissimilar substrates

Because silicone stays compliant, 904 is useful where two materials with different expansion rates are joined, a metal bracket to a plastic housing, a glass window to a metal frame. A rigid adhesive concentrates thermal-cycling strain at the interface and eventually cracks; a silicone rubber joint stretches and relaxes with each cycle. The general mechanism is described in how CTE mismatch causes adhesive bond failure.

Where it is used

• Consumer and industrial electronics: bonding and sealing components and enclosures.
• Automotive: securing sensors and connectors, sealing covers and housings against weather and vibration.
• Marine and offshore: sealing electrical enclosures and deck hardware against salt spray and UV.
• General industrial manufacturing: flexible bonding and sealing across a wide temperature range.

Application and cure control

904 is applied by manual brushing for low volume, or by automated dispensing or spray for production. The UV step needs a measured dose; verify with a radiometer and allow for lamp aging. Batch work cures under Incure L-Series UV LED flood lamps at a set intensity, and inline work runs on an Incure CDM UV conveyor.

For help matching Pyra-Sil™ 904 to a substrate pair, seal geometry, or environment, Email Us with the part details.

Failure modes to design against

• Treating the UV fix as a full cure: shadowed silicone needs its moisture-cure time before the joint carries load or pressure.
• Low-humidity cure environments: the secondary cure slows in dry air, so fully shadowed sections take longer.
• Bead gaps in a FIPG seal: control the dispense path and inspect before closing the housing.
• Bonding an untreated low-energy plastic: clean the surface and, for difficult substrates, use a silicone primer.

Surface preparation for dissimilar substrates

A silicone joint between two different materials only lasts if both surfaces are properly prepared. Metals should be degreased and, for demanding service, lightly abraded to remove oxide and increase surface area. Glass bonds well after a solvent clean but is sensitive to surface moisture, so it should be dry before the adhesive goes on. Plastics vary widely: polycarbonate and ABS bond after a solvent wipe, while polyolefins need plasma or flame treatment to raise surface energy.

For joints that will see moisture, temperature extremes, or sustained load, a silicone primer on one or both surfaces improves adhesion durability. The primer deposits a thin coupling layer that chemically links the substrate to the cured silicone, which matters most on glass and on painted or coated metal where the adhesive would otherwise be bonding to a weak boundary layer rather than the substrate itself.

Frequently asked questions

Q: What is the difference between 904 and a standard RTV silicone?
A: The cure path. A standard RTV cures on moisture alone over hours. 904 adds a UV fixture that takes seconds, so the part is handleable immediately while the moisture cure completes hidden areas.

Q: Can it bond glass to metal?
A: Yes. Its compliance suits it to joining materials with different expansion rates, including glass-to-metal and plastic-to-metal.

Q: Does it need primer?
A: Clean, dry metal and glass usually bond well without primer. Low-energy plastics and critical joints benefit from a silicone primer.

Q: Is it paintable?
A: Silicone surfaces generally do not accept paint. If a painted finish is required over the joint, mask and paint the surrounding area, not the silicone.

Getting support

Incure can help select a silicone grade, define the cure process, and troubleshoot bond or seal issues. Contact Our Team to discuss your application and request technical data.

Visit www.incurelab.com for more information.