“Flange sealant” covers two very different ways of replacing a cut gasket, and picking the wrong one is why some flanges weep at the first thermal cycle while others seal for a decade. One cures in the dark between tightly mated metal faces; the other cures in seconds under UV light as a shaped, compressible bead. Here is how to tell which your flange needs.
Q: What is a flange sealant, and how does it differ from a form-in-place gasket?
A: A flange sealant is usually an anaerobic liquid: it stays fluid in air and cures only when confined between close-fitting metal surfaces, filling microscopic voids so the two faces seal metal-to-metal under clamp load. A form-in-place (FIP) or cure-in-place (CIP) gasket is a dispensed bead cured on the part — under UV light in Incure’s Uni-Seal™ line — into a compressible elastomer that seals by being squeezed between the faces like a rubber gasket. Anaerobics suit rigid, precisely machined metal flanges with near-zero gap; FIP/CIP gaskets suit larger gaps, non-metal or dissimilar flanges, and joints that will be opened for service.
How Anaerobic Flange Sealants Seal
Anaerobic chemistry needs two things: exclusion of oxygen and contact with metal ions, which catalyze the cure. That is why it works between two steel or aluminum faces bolted together and stays liquid on a plastic flange or in an open gap. The cured film is thin and rigid, holds clamp load without creep, and resists most fluids. Its limits follow from its chemistry: gaps beyond roughly 0.25–0.5 mm cure slowly or not at all in the center, inactive surfaces (stainless, plated, anodized, plastic) need a primer or cure very slowly, and disassembly means scraping the cured film off both faces. Application practice for this class — surface prep, bead placement, bolt-up timing — is covered in Incure’s flange sealant application guide.
How a UV-Cured Form-in-Place Gasket Seals
A CIP gasket bead is dispensed onto one flange face and cured under UV light before the joint is assembled — Incure’s Uni-Seal™ grades cure in seconds at 365–405 nm. The result is a permanent, compressible elastomer bonded to one face and simply compressed by the other, which changes what the joint can tolerate. The Uni-Seal™ UV gasket line spans Shore A5 elastomers to Shore D85 rigid sealants: the soft Shore A grades — 1366, 3301, 3339, and the 3368 family — compress under clamp load like a rubber O-ring and deliver IP68-rated sealing, with 3339 at A5–A15 built specifically as a form-in-place gasket replacement. Because the bead is cured before assembly, the mating face can be plastic, painted, anodized, or stainless without affecting cure, the gap can be whatever the bead height bridges, and the joint opens and re-closes on the same gasket.
Email Us with the flange materials, the gap or flatness tolerance, the fluid, and whether the joint will be serviced, and Incure’s engineers can confirm which sealing approach — and which grade — fits.
Choosing Between Them: Five Questions
- What are the two faces? Steel-to-steel or aluminum-to-aluminum, machined flat: anaerobic is the default. Any plastic, composite, painted, anodized, or stainless face: a cured-in-place gasket seals regardless of surface chemistry.
- How big is the gap? Under 0.25 mm: anaerobic. Larger, or a stamped/cast flange that is not flat: a UV gasket bead sized to the gap. Uni-Seal™ 6322TC (12,000–24,000 cP) bridges gaps up to 5 mm and holds on a vertical face; 3339 (63,000–126,000 cP) forms a true gasket bead that will not run.
- Will the joint be opened? Service covers, inspection ports, and housings that are disassembled favor a reusable compressible gasket over a film that must be scraped and reapplied each time.
- What does it move with? Bolted rigid flanges under thermal cycling need a seal that flexes: an anaerobic film relies on the clamp holding the faces together; a Shore A gasket absorbs the movement itself. The choice between sealant and gasket-maker chemistries under vibration is compared further in Incure’s guide to flange sealant vs. gasket maker.
- How is it inspected? A CIP bead is inspected before assembly — visually, by height, or under blacklight for fluorescing grades such as Uni-Seal™ 3718F and 6243F. An anaerobic film is inspected only by leak test after bolt-up.
Where Each One Fails
Anaerobic sealant fails when the gap is too wide, the surface is inactive, or the flange was bolted before the sealant wet both faces — all of which produce a liquid pocket that never cures and weeps under pressure. A cured-in-place gasket fails when the bead is under-compressed (too little clamp travel), over-compressed past the elastomer’s set limit, or dispensed onto a contaminated face so the bead peels rather than bonds. Neither failure is the product’s fault; both are specification errors that the five questions above are designed to catch.
Where Incure Fits
Incure does not offer an anaerobic flange sealant; where a rigid machined-metal flange calls for one, that is the right product class to specify. Where the flange is plastic, dissimilar, gapped, serviceable, or needs a compressible seal, the Uni-Seal™ UV-cured gasket line is Incure’s answer — and the faster one to process, since a bead cures in seconds rather than waiting hours for an anaerobic to set in the dark.
Contact Our Team to match a Uni-Seal™ grade to your flange geometry, gap, and service requirements.
Visit www.incurelab.com for more information.