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Aug 12, 2019, 07:45 AM
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Originally Posted by Jim.Thompson
That is not a proper "Post cure". That will only heat up one side unequally with the other. Also, it is not controlled or "ramped up" and down suitably. Many things wrong with it. This is a recipe for all kinds of faults including warping.



You might have missed the point here. Post curing, when done properly, does not have any downsides in terms of our model construction. Your process (in the back of the truck) has downsides and is severely flawed.
Don't conflate the two!..........................



There is some validity in this. Much depends upon the caveats mentioned in previous posts. For example, leaving the laminate constrained etc. (Mylars on etc).

Edit: I have just opened the thread that Curtis linked above. I recall reading it years ago. Any such threads written by Adam are well worth your while to read through. He has done some excellent material over the years and I owe much to him.

Jim.

Yup - that thread has been one of my Subscribed Threads for several years. It was also the impetus for me to go talk with the owner of the local epoxy manufacturer (Resin Research).

Curing in the truck - it is a fully enclosed bed, and the wing was supported (fully) at about 6” off the bottom and located centrally relative to temperature gradient (numerous measurements were taken to identify any ‘hot spots’ - there were a couple places where the temperature varied by 1C, but no more than that). You’re right that it was not a controlled postcure in terms of ramp-up and ramp-down. And the mylars were still attached. So - “yes” - I did it wrong; but I was experimenting with something that might be ‘easy’.

Regarding ‘downsides’ - I’m a mechanical engineer so cost/benefit is second nature to me. If it is easy and provides some benefit, then it’s worthwhile. If it is difficult for little perceived benefit, then it’s not worthwhile. In this case, there is little benefit for my circumstances, and the process is not easy so I would consider that amount of effort a ‘downside’. Also, there does seem to be some evidence (e.g., Zagnut & surfboards) that there have been issues with XPS even with proper postcure. Again - with little or no benefit, non-trivial effort, and potential for damaged parts - the cost/benefit ratio just doesn’t appear to line up in this case.

Great discussion here so thank you all for your comments. I worry that my tone may unintentionally communicate displeasure with some of the replies, especially when there are translation or local cultural differences - so please read these ‘generously’

Thanks,
-Keith
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Aug 12, 2019, 07:47 AM
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Originally Posted by Roto Rob
The guidance on getting to within 50F of the cure temperature applies to old parts also. So if you put it in a hot storage that is actually above the cure temperature you are asking for problems. I have seen many an old part soften up as it gets near the cure temperature. In fact I have used heat to help remove faulty repairs (done by others).
Interesting!
For 25 years I have been curing at 25C and storing at 40-45C without any issues of parts softening up. Are you seeing something different?
Or are you referring to something different than the actual temperature used during the cure?
However - I have heated parts (mostly fuselages) with a hot air gun to twist things into alignment.
-Keith
Aug 12, 2019, 07:57 AM
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Quote:
Originally Posted by kablair
Interesting!
For 25 years I have been curing at 25C and storing at 40-45C without any issues of parts softening up. Are you seeing something different?
...............................
-Keith
Keith,

With respect and no intention to offend unnecessarily; you still have not understood what post curing achieves and how it is correctly carried out!
Of course they will not "soften up". They have been exposed to the temperatures you quote progressively.
Aug 12, 2019, 10:59 AM
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Quote:
Originally Posted by Jim.Thompson
Keith,

With respect and no intention to offend unnecessarily; you still have not understood what post curing achieves and how it is correctly carried out!
Of course they will not "soften up". They have been exposed to the temperatures you quote progressively.
You're probably right and maybe you can help me here...

I thought that post curing:
- increased strength by creating a higher crosslink density
- in product shops, it allows for a longer pot-life hardener to be used (and heated for lower viscosity) as the cure time is reduced by a higher temperature cure
- and, the issue I was most interest in, an increase in Tg or heat deflection temperature

To implement the postcure, I thought that procedure would be to:
- utilize a proper autoclave/oven that can ramp and hold temperature
- allow the part to set, generally at room temperature for 12-24 hours
- start oven, ramping temperature at 8-10C per hour, with an hour 'hold time' after each ramp
- repeat the ramp process until the target temperature is reached
- hold the target temperature for 8 hours
- reduce temperature at about 10C per hour; with a couple hour hold at ambient+10C before the last hour decrease to ambient

I realize this is grossly simplified, but is the concept about right? And is that how it would be done when there is an XPS foam core???

Back to my original post - and it looks like I have unintentionally offended the true Subject Matter Experts who do this for a living - but I was looking for a small increase in Tg for a minimal amount of effort. I clearly have erred by calling it 'post curing'; and I apologize for improperly using a term-of-the-art.

-Keith
Last edited by kablair; Aug 12, 2019 at 11:07 AM.
Aug 12, 2019, 03:33 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by kablair
.......................
I thought that post curing:
- increased strength by creating a higher crosslink density
- in product shops, it allows for a longer pot-life hardener to be used (and heated for lower viscosity) as the cure time is reduced by a higher temperature cure
- and, the issue I was most interest in, an increase in Tg or heat deflection temperature

To implement the postcure, I thought that procedure would be to:
- utilize a proper autoclave/oven that can ramp and hold temperature
- allow the part to set, generally at room temperature for 12-24 hours
- start oven, ramping temperature at 8-10C per hour, with an hour 'hold time' after each ramp
- repeat the ramp process until the target temperature is reached
- hold the target temperature for 8 hours
- reduce temperature at about 10C per hour; with a couple hour hold at ambient+10C before the last hour decrease to ambient

I realize this is grossly simplified, but is the concept about right?
..............................................
-Keith
That is about right.............details will vary with the particular resin and the service expected of the part.
However, the understanding you demonstrate here is not consistent with your comment in post #17.
Aug 12, 2019, 09:23 PM
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The only info I found at Sika was the specifications they provided of, resin cured 16 – 24 hrs at 77°F (25°C) + 4 Hours @ 212°F (100°C)
Ramp rates I have found to work with many resins without specs are 2 to 8F per minute. With a maximum cooling rate of 5F per minute. This was based on rates given for other resins and tried on many that I did not have ramp specs on.
So I would vacuum bag the part. Ramp it to 77F if needed and hold for 16 to 24 hours. Then ramp it up to 212F and hold for 4 hours. Then cool at 5F per minute or less until room temp.

There are no additional steps in the ramp during the process for this resin. It goes straight from room temp up to the final cure temp. Cure cycle would look something like the below picture.


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