Vacuum bagging, or squeezing a part with the weight of the sky

Using atmospheric pressure as a uniform clamp during cure.

Vacuum bagging clamps a curing composite part with the weight of the sky because sealing a laminate inside an airtight bag and pumping the air out of it leaves nothing pushing back against the ordinary atmospheric pressure pressing in from every direction outside, and that everyday pressure, normally too evenly balanced to notice, becomes a firm, uniform clamping force squeezing the part from every side at once.

Vacuum-sealing coffee at home

Vacuum-sealing food, coffee or clothes in a plastic bag at home relies on the same effect. Drawing the air out from around the contents lets the surrounding atmospheric pressure clamp the bag down tightly onto whatever is inside, following every lump, fold and gap and pressing it flat with an even grip that no amount of careful hand-pressing could match, because a hand can only push where it is placed, while the atmosphere pushes everywhere at once. The shrink-wrapped bag looks as though it is doing the clamping. In fact it has no clamping force of its own and is simply a flexible skin passing on a pressure that was already there, pushing on everything, all the time, before the bag was ever sealed.

Taking away the push from inside

The air at ground level presses on every surface with about ten tonnes of force on each square metre, which works out at roughly a kilogram resting on every square centimetre, an area about the size of a fingernail. Nobody feels it because it pushes equally from every direction, cancelling itself out on anything solid enough not to be crushed.

Sealing a wet composite layup inside a flexible, airtight bag and drawing the air out from underneath removes the pressure that used to push back from inside, while the atmosphere outside keeps pushing in exactly as before. The bag has nothing left to resist with, so it collapses onto the laminate and presses evenly across every part of its surface, following every curve and change of shape in the mould. A rigid tool would press at a handful of clamping points, whereas the atmosphere presses uniformly on every direction the bag's surface faces.

Scaled up to a real part, that becomes a very large force. A bagged panel half a metre wide and a metre long is being pressed down with the weight of about five tonnes, roughly three family cars, spread perfectly evenly over its surface. Pressing the same panel by hand, or with a stack of weights, would be hopeless, and even a sheet of plywood laid over it and loaded with bricks would bear down hardest wherever the bricks happened to sit and barely at all over any dip in the mould.

That is why a bag squeezes a curved wing skin or a compound-curved body panel just as evenly as it would a flat sheet. Getting the same result with mechanical clamps would need a matching curved caul plate built specially for the job, and even then the clamps would push hardest near where they were tightened.

A leak gives the clamp back

Because the clamp comes from removing an opposing pressure, the seal around the edge of the bag matters as much as the pump. Even a small leak lets outside air refill the space under the bag, and every bit of air that gets back in pushes outward again and cancels part of the squeeze. A bag that looks taut at the start of a cure but slowly softens over the following hour is showing a leak letting the vacuum bleed away, and a laminate cured under a slowly failing vacuum ends up with the same uneven, poorly consolidated result as one that was never clamped evenly in the first place. For that reason a bagged part is usually left on the pump for a while and watched on a gauge before the cure begins, so a leak can be found and sealed while the resin is still workable.

One atmosphere is the ceiling

A vacuum bag can never clamp harder than one atmosphere's worth of squeeze, however thoroughly the air is pumped out or however good the seal is. The best any pump can do is remove all of the pressure inside, which leaves the full weight of the outside air as the limit, and that limit is set by the weather above the workshop, with nothing in the workshop able to raise it.

Thicker structural laminates, or parts where the resin has to be forced hard through many stacked layers of fibre, sometimes need more than that. An autoclave is the next step up. It cures the same bagged part inside a heated, pressurised chamber, often at around six or seven atmospheres, adding its squeeze on top of what the vacuum was already providing. For most of the panels and brackets a small composites shop produces, though, the one-atmosphere ceiling is already more even and more generous than anything a scattering of mechanical clamps could apply across a curved surface, which is why the vacuum bag remains the default long before an autoclave is ever needed.