The calculator
Runs in your browser. The defaults are an example cycle, not a recommendation.
How the plan is built
The FAA Airframe Handbook (FAA-H-8083-31, Chapter 7) describes an elevated cure in at least three segments. Ramp up: the heating device rises at a set rate, typically 3 °F to 5 °F per minute. Hold or soak: it keeps the cure temperature for the specified time. Cool down: typically below 5 °F per minute, and when the part is below 125 °F it can be removed.
Each ramp takes the temperature change divided by the rate: from 70 °F to 250 °F at 3 °F/min is 180 / 3 = 60 minutes. The total is ramp + hold + cool-down to the bag-opening temperature.
The Handbook's example cycles differ: its autoclave cure ramps at 1–5 °F per minute and holds 120–180 minutes at 355 °F; its wet-layup repair ramps at 2–5 °F per minute and holds 90–150 minutes at 260 °F. That spread is the reason the figures here are inputs: the material decides them.
Worked example
A heat-blanket repair: part at 70 °F, cure at 250 °F, ramp 5 °F/min, hold 120 minutes, cool at 5 °F/min, open the bag below 125 °F, started at 08:00.
- Ramp: (250 − 70) / 5 = 36 min.
- Hold: 120 min, counted from the last thermocouple in range.
- Cool: (250 − 125) / 5 = 25 min.
- Total: 36 + 120 + 25 = 181 min = 3 h 01 min. Done at 11:01.
3 h 01 min from switch-on, if the blanket tracks the controller. Plan the shift for longer.
Limits — when NOT to use this
- Material data governs. Cure temperature, hold time, ramp limits, pressure and any intermediate dwell come from the prepreg, resin or adhesive data sheet or the SRM. This tool does the arithmetic on them.
- Part temperature, not air temperature. The Handbook warns that the oven probe can differ substantially from the part when the part or tool acts as a heat sink. Control on thermocouples at the repair.
- Multi-step cures with an intermediate dwell are not modelled. Plan each step separately.
- Real equipment lags the set rate, especially on thick laminates and large tools.
Questions people actually ask
When does the hold time start?
When the last thermocouple reaches the specified cure range. The Handbook's autoclave figure states it in those words. A cold corner of the repair delays the start for the whole part.
Why not cool faster to save time?
Fast cooling builds thermal stresses into the laminate and the bond line. The Handbook's figures cap cool-down at 5 °F per minute; your data sheet may be stricter.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 7, Curing of Composite Materials, Figures 7-53 and 7-62 — the three segments, ramp rates of 3–5 °F/min (figures: 1–5 and 2–5), cool-down at 5 °F/min maximum, removal below 125 °F (52 °C), and hold time counted from the last thermocouple.