Why Atmospheric Pressure Does the Work

If you have ever hand laid a laminate and been quietly disappointed by the result, vacuum bagging is the technique that tends to change your mind. It is not a mysterious industrial process reserved for aerospace factories; it is simply a way of using the roughly one bar of atmospheric pressure pressing down on everything around us to squeeze a laminate while it cures. The trick is to remove the air from one side of a sealed film, so the atmosphere does the clamping for you, evenly and continuously.

The practical payoff is measurable. A well-bagged wet lay-up will typically climb from around 50–60% fibre volume fraction to somewhere nearer 60–70%, because excess resin is drawn out into the breather rather than sitting between the fibres doing nothing useful. Voids drop sharply too, since trapped air and volatiles have somewhere to go and the consolidating pressure closes the gaps between plies. The result is a part that is lighter, stiffer and far more consistent from one batch to the next.

The Consumables Stack, Layer by Layer

Getting the stack order right removes most of the guesswork. Over the wet laminate, you build up the following, working outwards:

• Peel ply — a tightly woven nylon or polyester fabric. It leaves a bondable, slightly textured surface once peeled away, which saves a great deal of sanding later.
• Perforated release film — allows excess resin through the holes but stops the peel ply bonding to the breather. Non-perforated film is used when you want to keep resin in place.
• Breather fabric — a thick, lofty polyester felt that gives resin somewhere to collect and provides a continuous path for air to travel to the vacuum port.
• Bagging film — the sealed membrane itself. Choose a grade rated for your cure temperature; standard nylon film is fine for room-temperature and low-temperature epoxies.
• Sealant tape — the sticky, putty-like ribbon that sticks the film to your mould or tooling board and forms the airtight boundary.

Two small items make a disproportionate difference: a spiral wrap or resin break at the vacuum port to stop neat resin being sucked into your pump, and a pleat folded into the bag at corners so the film can follow the shape without tearing or bridging.

Hardware for a Modest Budget

You do not need a laboratory-grade pump. A single-stage rotary vane pump rated around 3–5 cubic metres per hour will comfortably handle a bag up to a couple of square metres. A catch pot between the bag and the pump is non-negotiable — it protects the pump from resin and lets you see how much has been drawn off.

Add a vacuum gauge you trust, a manifold with several ports if you are bagging more than one part, and plenty of good-quality vacuum hose. For very small workshops, a compressed-air venturi generator is a legitimate alternative: it uses less electricity, tolerates resin ingress better and reaches roughly 0.8 bar, which is plenty for most wet lay-ups. A vacuum pump with a proper gauge will still pull harder and hold pressure more reliably on long cures.

Laying Up and Bagging: A Practical Sequence

• Prepare and release the mould thoroughly, then apply gel coat or surface ply if your process calls for it.
• Wet out each ply on a flat table where possible, rather than in the mould. It is cleaner, faster and gives you better control of resin content.
• Build the laminate, then apply peel ply, release film and breather, trimming each layer so it sits inside the sealant tape line.
• Run sealant tape around the perimeter, leaving a generous margin — at least 50 mm — between the breather edge and the tape.
• Lay the bagging film over the stack, press it down onto the tape in sections, and work out the wrinkles as you go.
• Fit the vacuum port through the film, seal it, and connect the hose via the catch pot.
• Pull vacuum slowly. Watch the bag draw down, then check the gauge reading and listen for leaks.

Do a drop test before you commit: pull full vacuum, isolate the bag, and watch the gauge for ten minutes. A loss of more than about 0.05 bar over that period means you have a leak worth chasing. Sealant tape edges and the vacuum port are the usual culprits.

Leaks, Bridging and Resin Starvation

Most problems in small workshop bagging fall into a handful of familiar categories. Bridging happens when the film stretches straight across a corner or recess instead of following it, which starves that area of pressure. Pleats and careful film layout fix it. Resin starvation occurs when too much resin is pulled into the breather, leaving dry patches — reduce vacuum slightly, use less breather, or increase the resin content of your wet-out.

If the gauge will not hold, walk the sealant tape line with your hand, listening and feeling for the tell-tale hiss, and press the tape down again. A cheap stethoscope or a length of hose held to your ear works surprisingly well. Temperature matters too: a cold workshop makes resin viscous and slow to move, so warm the space to around 20°C if you can.

Getting Repeatable Results

Vacuum bagging rewards discipline more than expensive equipment. Keep a simple log of resin batch, ambient temperature, vacuum level and cure time, and note anything that looked odd during the bagging. Within half a dozen parts you will have your own reference points and your own favourite consumables.

Work cleanly, cut your consumables to size before you start wetting out, and keep a spare roll of bagging film and tape within reach. The technique is patient and unglamorous, but it turns a variable hand lay-up into a dependable engineering process — and that is exactly what a small workshop needs.