With the carbon and glass fiber prepreg composites used in aerospace, part quality depends largely on two things: using the material before its life runs out and running the cure cycle in line with the specification. Most composite defects cannot be seen by eye afterwards, so the records are everything.

Material life: shelf life and out-life

• Shelf life: The time the prepreg can be stored in a freezer.
• Out-life: The total time the material has spent at room temperature after leaving the freezer. Every removal and return must be recorded.
• Thawing: The roll must come to room temperature before the packaging is opened; otherwise moisture condenses on it.

A part made with material whose out-life has expired does not conform to the specification, even if it passes the test.

Related reading: What is a special process? · Supplying aerospace and industry in the Gulf · ERP for special-process companies

Cure cycle

In a cure cycle run in an autoclave or oven, the following are usually checked:

1. Heating and cooling rates
2. Hold temperature and time
3. Pressure and vacuum values
4. All thermocouples on the part staying within the limits

The values depend on the material and the customer specification; what applies is your specification, not typical examples.

Common mistakes

1. Tracking and calculating out-life by hand, or not at all
2. Not following the slowest-heating (lagging) thermocouple
3. No record of the vacuum leak check
4. Not recording the layup sequence and ply orientation
5. Test coupons not cured in the same cycle as the part

Which of these is in your records?

Most of these mistakes slip past when records are read one by one by eye, and auditors look for them first. The real question is which one sits in your own records. Upload your cure record to NTB SpecPro and quickly see which points may carry risk, the likely reason and a suggestion.

Keeping the records simple

The NTB SpecPro composite module tracks material shelf life and out-life, compares your cure record with the specification, gives suggestions for layup records and prepares the quality report. Final evaluation stays with your authorized quality personnel.

See the modules →

Prepared by NTB SpecPro process quality engineering; reviewed by an engineer holding the European Adhesive Engineer (EAE) qualification. Questions: info@ntbspecpro.com