Fixing a Vacuum Infusion Implosion: The Repair
The infusion had been running for a couple of minutes, resin pulling through the laminate exactly as it should, when the pull sped up and the whole box beam gave way at once. A week of templating, foam cutting, and careful fitting, gone in the time it took to say so.
The Autopsy: What Actually Failed
The critical structure — the unidirectional glass running along the hull and up the flange — was untouched. What collapsed was the foam former underneath it, and understanding why matters more than the collapse itself.
The core was a recycled PVC foam, built from offcuts and second-quality sheets glued back together — good enough for its intended job as low-load furniture core, but riddled with glue joints that aren’t fully airtight. One pinhole in one of those joints was all it took. Once the vacuum found it, the trapped air inside the hollow section had nowhere to go but out through that leak, leaving a vacuum inside the cavity itself. From there the maths is straightforward and brutal: a full atmosphere of external pressure, around a tonne per square metre, pressing in on a section with nothing left inside to resist it.
The beam failed exactly where you’d expect once you understand the load path — not at the strongest bond, but at the weakest one. Sections that had been both glued and laminated held. The one seam that was glued only, with no laminate reinforcing it, gave way under the shear load. It’s a clean, almost textbook failure in that sense: it found the one link in the chain that hadn’t been reinforced.
Two Options: Rebuild or Repair
Cutting the whole thing out and starting again was the safe option. The other was to reshape the foam that had caved in and rebuild the beam’s geometry from the inside — riskier, but far less wasteful of a week already spent getting the fit right. Since the fibres doing the actual structural work were still intact, repair won out.
Warming It Back Into Shape
PVC foam and the resin that had already saturated it both share a property worth knowing: heated past their heat deflection temperature, they go soft and pliable, and they’ll hold whatever new shape you push them into once they cool back down. For this foam and resin combination that’s around 65°C — not especially hot, but it takes patience to soak the heat evenly through the material rather than blasting the surface and bubbling it.
Through small access holes cut into the panel, the collapsed sections were warmed and gently pushed and pulled back toward their original shape — sometimes with a bolt from behind to press a caved section back out, sometimes with a hooked tool to draw a buckled panel back into position. It’s slow, iterative work: warm an area, nudge it into place, let it cool, come back and refine the next section. Some of the original geometry had relied on the foam being thermoformed into a tight radius in the first place, and it was along exactly that tight curve that a crack had opened up — very possibly the original source of the leak that started the whole failure.
Sealing It for Good
Once the shape was back, the gaps and hollows left behind were filled with polyester filler, sanded fair, and the whole repaired beam was hand-laminated with a layer of 300g double bias glass — something that should have gone on before the very first infusion attempt. That skin does two jobs at once: it seals every one of those glue joints airtight, and it adds a genuine layer of structural glass over the entire repair, making the beam stronger than the original plan, not just patched back to parity.
Back Under Vacuum
With the beam sealed and reinforced, the infusion ran again — this time clean, holding vacuum the whole way through. Unwrapping it afterward, the unidirectional flanges came out well-saturated and tidy, with one small cosmetic wrinkle where a squared uni plank met the beam’s internal radius — not structural, just a finish detail to note for next time. The finished beam ended up around 100g heavier than the original plan. Given what it replaced, that’s a rounding error.
The Series So Far
This repair only made sense once the original build and the reasoning behind the material choices were laid out — start from Building a Structural Box Beam for Paikea’s Hull if you’re coming to this fresh, or catch the reasoning behind hollow sections, foam density, and bladder bags in Why Did the Box Beam Implode? Your Questions Answered.
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