Question about post curing molds & parts

[20_rc51_00]

The molecules that are wihtin the resin are all rotating and jiggling around. Eleveating the temperature allows these rotations/jiggling to happen at a higher level of energy. Allowing the temps to be ramped slowly gives these molecules enough energy that will result in a greater degree of probability of crashing into another and allow those that are still free to react to do so. This reaction and crosslinking is what results in a more stable and less reactive cured resin. If you crank the heat too aggressively the resin doesn’t have the inherent stability (because it has not been allowed to crosslink anough yet for that given temperature) to keep it from becoming soft and liquidy, thus the internal strain within the fibers is released, and since the resin is soft the fibers will relax and assume the shape and position they actually want to be in.

Gradual tmep curing is a balance between building the crosslinking gradually enough vs putting too much heat into the resin and turning it liquidy.

Hope this makes sense.
If I have explained this incorectly or omitted anything others please feel free to step in, thanks

[herman]

Just to add another explanation:

The Tg of a resin is about 15 degrees C (30F) above the highest temperature it has seen.

Exposing a laminate to a temperature above its Tg causes print. So if you toss a fresh laminate in a hot oven, it will print.

The purpose of a postcure is to have the resin crosslink further, so the Tg gets higher (up to a point that the highest Tg for that specific resin is reached, the ultimate Tg).

It takes a while for the Tg to develop. For instance, I have a resin which has an ultimate Tg of 82 degrees C. If I toss a sample in an oven at 60 degrees C, after some 3 hours the Tg reached 60, after 12 hours it is 72, after 18 hours 75, after 24 hours it is 76.

This means if you want to prevent print and distortion, you never should postcure above the current Tg of the part. This is where ramp rate jumps in.

Imagine an epoxy resin, cured at 20C. It has a Tg of some 35-40 degrees C. The correct way of postcuring is to place it in an oven, and have the oven rise the temperature slowly, so the actual temperature never overruns the Tg which is developed in the laminate.
Say the oven is 30C, the Tg will slowly rise to 45.
When at 45, you can put the oven at 40, to see the Tg rise to 55

And so on, until the ultimate Tg has reached. (no matter what you do, it never gets higher. The ultimate Tg is what is stated on the datasheet.

2 remarks:
It is not easy for us mere mortals to measure Tg, so there is some guesswork and a safety margin involved when postcuring. When in doubt, go slow.
It is important to have a means of controlling your oven.

You can also postcure in the mould, preferably with vacuum on, in which case print is less of an issue. Of course your mould should be able to withstand the temperatures as well (perhaps by doing a slow postcure?)

The speed at which Tg develops can be vastly different for different resins. Some are fast, and ramp rates can be high. Some are slow, and ramp rates should be adjusted.
Add to that the size of your part, and laminate thickness. Also (insulating) cores can do a lot of damage in this respect.

Some ballpark figures:
For smaller, thin parts, 20 degrees C per hour is usually acceptable
For larger, thick and / or cored parts, 10 degrees C per hour
For parts in heavy moulds, 5 degrees C would be better.

Composites are pretty good insulators, a 2cm thick (3/4") laminate, heated from one side with moderate heat (10C above ambient) takes 2 hours to reach an equilibrium.