A single trapped air bubble the size of a pinhead can become the exact spot where a bond line cracks under load — because voids don’t just look bad, they concentrate stress and block cure at the point they occupy.
Dispensing Issues
The method used to apply the adhesive is the most frequent source of air entrapment in production.
- High dispensing pressure or speed: Too much pressure or too fast a dispensing rate can inject air directly into the adhesive stream or create turbulence that traps air within the fluid.
- Improper nozzle design: A dispensing tip that’s too narrow, or a syringe and reservoir with excess headspace, allows air to be incorporated during application before the material even reaches the substrate.
- “Tailing” or lifting the dispenser: Pulling the dispensing needle away from the adhesive too quickly creates a localized vacuum that pulls air back into the material at the point of separation.
Substrate Issues
Air can also be introduced from the components being bonded rather than the dispensing equipment itself.
- Porous or rough substrates: Ceramics, composites, and lightly abraded plastics can carry microscopic surface pockets that trap air, which is then released into the adhesive as the substrates are pressed together.
- Mating technique (air vents): When two flat substrates are brought together directly and squarely, air becomes trapped in the center of the joint with no path to escape.
Chemical Reaction (Outgassing)
In less common cases, the adhesive itself generates gas during cure. If UV light intensity is too high, the polymerization reaction becomes extremely rapid and exothermic — heat-generating — and this rapid chemical change can cause volatile components within the adhesive to outgas, forming bubbles from the inside out rather than from trapped ambient air.
Solutions for Eliminating Voids
Reducing bubbles and voids requires attention at three stages: before dispensing, during application, and during cure. Incure’s technical data sheets specify a recommended dispense pressure and nozzle geometry for exactly this reason — deviating from either is a common, easily overlooked source of intermittent void complaints.
Pre-process adhesive handling. Degas the adhesive in a vacuum chamber before dispensing, particularly for large-volume applications — this removes air that is dissolved or trapped within the liquid itself, before it ever reaches the nozzle. If the adhesive has been refrigerated, allow it to reach room temperature before use; cold adhesive is thicker and holds entrapped air more stubbornly than material at its intended working viscosity.
Dispensing and mating techniques. Lower the dispensing pressure and speed to maintain smooth, laminar flow rather than turbulent flow. Apply adhesive to only one of the two mating substrates so it wets out properly as the parts come together, and bring substrates together slowly from one edge — like closing a book — so trapped air is pushed out the sides instead of concentrated in the center. Building in a short dwell time of a few seconds after dispensing, before curing begins, gives small entrapped bubbles a chance to rise to the surface and burst on their own.
Curing optimization. A step-cure or ramped-intensity profile — starting at low UV intensity and gradually increasing it — slows the polymerization rate enough to prevent the rapid exothermic spike that drives outgassing. For high-reliability, void-free bonds in optics or aerospace sensor assemblies, curing the assembly briefly under vacuum pulls any remaining dissolved or trapped air out of the liquid adhesive before the cure locks it in place.
When Voids Show Up After the Fact
If voids appear in finished parts despite a controlled dispensing process, the fastest way to isolate the cause is to section a failed sample and note where the voids sit. Voids concentrated at the substrate interface usually point to trapped surface air or a mating-technique problem; voids distributed evenly through the bulk more often indicate dissolved air that wasn’t degassed before dispensing; voids clustered near the light source with a scorched or discolored appearance suggest exothermic outgassing from excessive UV intensity rather than an air-handling problem at all. Cross-sectioning a handful of rejected parts this way, rather than guessing from the surface appearance alone, usually narrows the fix to a single process variable instead of a full requalification. For guidance on matching UV intensity to bond-line thickness without triggering an exothermic spike, Email Us with your current dose parameters.
Void-free bonding is achievable with ordinary process discipline — vacuum degassing, controlled dispensing, and a ramped cure profile address the overwhelming majority of cases without requiring a chemistry change. The equipment generating the cure dose matters here too: a UV LED spot lamp with adjustable intensity ramping gives finer control over the exotherm than a fixed-output source, and for wider flood-cure areas the same ramped-intensity principle applies to UV LED flood lamp selection as well. Adhesive viscosity also plays a role worth checking against your material’s TDS — thicker, more thixotropic formulations resist bubble migration differently than thin, free-flowing ones, which is one more reason a documented chemistry comparison between UV adhesive and epoxy is worth reviewing before committing to a dispensing setup. Contact Our Team to review your dispensing and cure profile if void rejects are a recurring line issue.
Visit www.incurelab.com for more information.