Vacuum Bagging Complex Geometry: Joining a Carbon Beam to a Hull
Vacuum bagging a flat panel is straightforward. Bagging a structural carbon tape that has to bond a beam to a hull — transitioning from vertical to horizontal, over compound curves, into tight internal corners — is where it gets interesting.
The prep comes first, and it matters. The overlap staggers for each layer of carbon are marked directly onto the hull before anything else is cut — each layer stops at a different point so the laminate tapers gradually rather than dropping off a cliff. Hard edges in a laminate create stress concentrations, and stress concentrations are where things fail. The tacky tape goes onto green masking tape rather than directly onto the substrate, which sounds fussy but saves a lot of grief at cleanup when the gray tape would otherwise be a nightmare to remove.
The consumable stack is the sequence of materials that sits between your laminate and the vacuum bag. Each layer has a specific job. Peel ply goes on first — it stays against the carbon and comes off after cure, leaving a resin-rich textured surface that’s ready to sand for secondary bonding without cutting into the carbon fibres themselves. Over the peel ply goes a perforated release film — a thin plastic with tiny holes that lets excess resin squeeze through under vacuum pressure while stopping the next layer from bonding to the part. The breather felt goes over that, a geotextile matting available from any hardware store. Its job is to carry air from anywhere in the bag to the vacuum port — if the breather fills with resin and seals off, you can end up with sections of the bag not under proper vacuum, which means sections of laminate not properly consolidated.
The frog’s tongue is an extension of breather felt that runs away from the laminate to the vacuum port. If any resin bleeds heavily, the tongue gives it somewhere to go before it reaches the valve. If resin gets into the valve, you have a problem.
On a flat job you can use one large piece of each consumable. On a job with this much geometry change — flat here, curved there, internal corners, external corners, the whole shape of the beam-to-hull junction — you cut everything in sections. Peel ply doesn’t stretch and bend the way carbon does, so it needs joins at every geometry change, small wedge pieces tucked into corners with overlaps to allow it to lie flat without pulling away from the laminate. Same with the breather — butt jointed at corners rather than one piece bridging across, because a bridge means no contact, and no contact means no vacuum in that spot.
The bag itself needs to be oversized. You can’t stretch a vacuum bag into a corner — the atmospheric pressure is pushing inward at fourteen pounds per square inch and if the bag is tight it will bridge rather than conform, leaving the laminate unpressed in the corner. The technique is pleating — the same principle as a dart in a garment, folding excess material so it can expand into a corner when the vacuum pulls. Pleats go at every geometry change, especially inside corners. The bag gets pinned at the corners first, then the middle of each span, then pleated in between.
Once the bag is roughly positioned the vacuum pump goes on and you work fast — the bag starts evacuating quickly and you need to be pressing tacky tape down and eliminating air leaks before the resin starts to move. Any resin that gets onto the tacky tape stops it sealing, so those sections of tape have to come off and be replaced with fresh. The last thing you want is the sun coming out and warming the resin while you’re still chasing leaks — a warmer resin has less working time and a lower viscosity which means it bleeds faster.
Once the bag is down and sealed, you can see the resin bleeding through the perforated film into the breather — that’s what good consolidation looks like. Check every corner for bridging, press the bag in firmly, then get heat on it as fast as possible. The sooner the resin kicks off, the shorter the window for something to go wrong.
Vacuum bagging is one of the core techniques in the members’ technical library — if you want to go deeper on how we approach composite work on Paikea, that’s where the detail lives. Join here.