Layup and autoclave curing

How it works

1. 1

   Cut and lay up the plies. Prepreg (fiber cloth or tape already soaked in partly cured resin, kept frozen until use) is cut to shape and stacked on the mold. Each ply's fibers point in a planned direction; robots (automated fiber placement) lay the largest aircraft parts.

2. 2

   Bag it and pull a vacuum. A peel ply (a fabric that tears away later), a breather cloth (a felt that lets air travel) and a plastic bag film go on top. Sticky sealant tape seals the bag to the mold. A pump then sucks out the air, so outside air presses the stack together and trapped air escapes.

3. 3

   Cure in the autoclave. The autoclave heats the part, typically to 120–180 °C. The gas inside is pressurized to about 3–7 bar, three to seven times normal air pressure, squeezing the bag. The resin softens, flows and wets the fibers, then cures (cross-links into a hard solid) over a few hours.

4. 4

   Demold, trim and inspect. After a slow cool-down, the bag layers are stripped and the part lifts off the mold. Edges are trimmed and holes drilled. Critical parts are scanned with ultrasound for voids (trapped bubbles) or plies that failed to bond.

Key facts

Materials

Carbon, glass or aramid (a tough synthetic fiber) prepregs, mostly with epoxy resin; honeycomb or foam cores for sandwich panels

Typical parts

Wing and tail skins, fuselage panels, helicopter blades, Formula 1 chassis, satellite panels

Part size

From palm-sized brackets to wing skins tens of meters long; limited by the autoclave's inside size

Precision

Shape typically within about ±0.2–0.5 mm of the mold; each ply is about 0.1–0.3 mm thick

Surface finish

Mold side smooth and often glossy (a copy of the mold); bag side lightly textured by the peel ply

Batch size it suits

One-offs to a few thousand parts a year

Tooling and machine cost

Molds from about $1,000 (small parts) to $1 million or more (large aircraft tools); autoclaves from hundreds of thousands to many millions of dollars

Speed

Layup takes hours to days per part; one cure cycle typically takes about 3–8 hours, heat-up and cool-down included

Strengths and limits

Where it's used

• AerospaceWing and tail skins, fuselage panels, flaps, helicopter rotor blades
• SpaceSatellite structures, antenna reflectors, launcher fairings
• MotorsportFormula 1 chassis (the driver's survival cell), wings, body panels
• SupercarsCarbon roofs, hoods, aero parts, some chassis tubs
• MedicalX-ray table tops (carbon lets X-rays through), prosthetic feet

How to recognize it

• One very smooth, often glossy face (the mold side) and one slightly textured face printed by the peel ply.
• On cosmetic parts, a woven checkerboard of carbon visible under clear resin.
• Thin stacked layers at cut edges, and edges and holes that were machined after curing.
• Thin shells or sandwich panels, sometimes with honeycomb visible at an open edge.

Compare with

Watch it

Autoclave Curing Process - Introduction

PolyU IC · The basic idea of autoclave curing, step by step

From Layup to Flight-Ready: The Autoclave Process Behind Aerospace Parts

Pressure Equipment Academy · The whole route from layup to a cured aircraft part

Carbon Fiber Prepreg PART MAKING (Tutorial) PART2

Matthieu Libeert (Mat's Prototyping) · Ply layup and vacuum bagging, up close

Automated Fiber Placement Machinery by MTorres

Composites Central · Robots laying fiber tape on big aircraft molds

See real photos

• Wikipedia: Pre-preg — prepreg materials, storage and autoclave curing, with photos
• Wikimedia Commons photo search