How do I interpret the curing pressure of a composite from this data sheet?

Question

A question asked about the necessary curing pressures for a composite resin in the autoclave. Looking through the corresponding datasheet (HexPly F155), I'm confused:

Cure Procedure

1. Apply vacuum of 22 inches (74 kPa) Hg minimum.
2. Apply 85 + 15 – 0 psig (586 + 103 – 0 kPa) pressure for laminates.
3. Apply 45 + 15 – 0 psig (310 + 103 – 0 kPa) pressure for sandwich.
4. Vent vacuum bag to atmosphere when pressure reaches 20 psi (138 kPa).
5. During cool-down when the part temperature falls below 140°F (60°C), pressure can be released and the test panel removed from the autoclave and debagged.

The first step is clear, but not the next steps, for instance 85 + 15 – 0 psig (586 + 103 – 0 kPa) is causing me a headache and I can't make sense of it.

I'd guess that one is the external positive pressure and the other is negative vacuum pressure under the sheet, but I couldn't for instance have a 103 kPa vacuum.

Answer 1

Cure Procedure

1. Apply vacuum of 22 inches (74 kPa) Hg minimum.
2. Apply 85 + 15 – 0 psig (586 + 103 – 0 kPa) pressure for laminates.
3. Apply 45 + 15 – 0 psig (310 + 103 – 0 kPa) pressure for sandwich.

I believe Steps #2 and #3 are providing you a tolerance for the applied pressure.

So, for Step #2, it's telling you to apply 85 psig. You can go +15 psig higher (for a total of 100 psig) but should not go lower than 85 psig. Normally, this is written as $85^{+15}_{-0}$ psig or 85 +15/-0 psig.

For Step #3, apply 45 psig, with an upward tolerance of +15 psig (total 60 psig) but go no lower than 45 psig total pressure. Normally, this is written as $45^{+15}_{-0}$ psig or 45 +15/-0 psig.