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Aug 05, 2019, 06:54 PM
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Can Post Cure Cause Delaminations?


Took center section out of the bag (in the bag 20 hours at 25C) and moved it to my Ďhot boxí. The Ďhot boxí is the bed of my pickup truck which is at 55-60C. Left it there for about 2 hours - until an impending thunderstorm dropped the temperatures.

When removing the mylar, I find 4 locations where there are delaminations; 2 of them are rather large at about 20 cm2.
I did a fairly wet layup and the outside is completely smooth. The bag was fully pulled down (~22in Hg) throughout the initial cure.

Iíve never experienced this type of delamination before, and it is my first time doing a post-cure. Are the two related?

Regarding repair....the center section looks great otherwise so Iím going to repair it. My thoughts for this are:
1 - cut out the flaps, which will cut through all the delaminations . Use a syringe to inject epoxy and weight it down while it cures.
2 - poke holes through the wing skin and inject epoxy. Rebag the wing while the epoxy cures.
3 - slice through the lamination, apply epoxy underneath, and then add another layer of carbon as reinforcement.

#1 is my first choice. Any other suggestions?

Thanks,
-Keith
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Aug 06, 2019, 06:42 AM
Barney Fife, Vigilante
tom43004's Avatar
Pictures....

Some thoughts:

1. Vacuum pressure and bleeder effectiveness... I know you've done other wings before but is it possible you had a section of the wing that sealed itself off from vacuum?

2. I assume since the hingeline will go through all the delams that they're all near the kink in the airfoil. If they're all on the top surface see #1 above since strong vacuum is required to pull down the mylars there.

3. If the delams are on the bottom surface, is there something you added to the layup to facilitate hinging (kevlar, polyester, etc) and if so, have you used it before and are you sure it's sufficiently epoxy bondable?

4. Did you wax your mylars and how long did you leave the layup in the bag prior to removal? Did the mylars come off easily? I have a video in the "Savage" thread of how it SHOULD look when you remove mylars.

5. Did you post cure without mylar removal? Heat / cooling cycles can occasionally cause XPS (or paint if painted) to off-gas and if this happens after the initial cure, this gas has nowhere to go effectively making your wing skin into a "balloon" of sorts. What temp did you post-cure at?

6. Foam prep... do you sand your cores after hotwiring or just go au naturale? If you sand them (I do) then do you remove sanding dust from the cores by vacuuming before the layup?

7. Time to bag... from the time to mix epoxy to the full vacuum application, how much time elapsed and what temperature is your shop at? Was the epoxy starting to green before you got to full vacuum? This is probably more common than you think. If it's starting to get sticky then you're probably pushing the limits of your epoxy / hardener / temperature choice.

8. (less likely) If you wax your mylars etc on your main working surface, ie where you cut materials and prepare your layups, it's pretty important to make sure you don't ever have surface contamination. I always use painter's paper from a home improvement store and tear off a new piece for each job. I cover my work surface with it, and wax my mylars etc with one side. After the wax is buffed and I'm done, I toss those gloves and turn the paper over so I have a fresh side for the layup activity. It doesn't take a lot of wax to ruin things or give delaminations a start.

These are some places to start considering. I know I'm spraying a lot out but one of them is likely to blame.
Aug 06, 2019, 06:54 AM
Red Merle SJ VIII
Curtis Suter's Avatar
One thought I had this morning... I use US Composities epoxy and it doesn't like any heat applied till at a minimum of half day after the initial mixing. I assume this time would change depending on the mix temperature as I've done a bit of bagging this summer and things sure set up quicker than my 55 degree shop in the winter.

Curtis
Aug 06, 2019, 08:58 AM
The Junk Man
I have post-cured a lot of stuff, but never by leaving them in the original mylars or release paper. So I really have no idea if that was the problem. But I have never had post curing cause delamination or poor lamination... if that ever happened it was my fault in the initial layup, and was obvious out of the bag, not the post cure.

Tom
Aug 06, 2019, 02:36 PM
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Thread OP
Thanks folks for the comments & suggestions! Let me take these in order:

1. - Isolated part of bag - This is something I check quite closely so I don't think this is an issue. Incidentally, my bagging layers outside the mylar are:
a. breathable peel-ply over the entire wing section (top and bottom) and beyond the end ribs
b. a layer of 4mil polyethylene over all the pee-ply (top and bottom)
c. a layer of flannel fabric over all of the polyethylene (top and bottom) and extended to the pump breather point.
d. a piece of rope that runs the full length of the panel and is directly under the pump breather point

2. - Kink in airfoil - Interesting point. I do use 20+ inHg with a continuously running vacuum pump (it's in the garage ). I really don't think there were any isolated portions of the bag.

3. - Live Hinge - there is one delam (bubble) on the bottom. I did not incorporate a live hinge (e.g., kevlar) as I'm planning to use Curtis' sealant process.

4. - Waxing - these are new mylars so I waxed them three times with Partall 2. They removed easily, just as they always do. No need to try to put my hand between the mylar and wing.

5. - Post cure with mylars - THIS sounds like the issue. I left the mylars on during the postcure and they do look like bubbles. I did not realize the XPS has an outgas issue. Paint was water based Createx airbrush paint so there shouldn't be many solvents to create an outgas.

6 - Core prep - My process is to brush the foam hairs, then vacuum, then brush, then vacuum, then sand (in one direction), then vacuum, then work with the core as needed (including sanding), then vacuum (... you get the idea; I really dislike those hairs especially when they get under my sander...)

7 - Epoxy setting up - I took about 90 minutes from start of epoxy to pulling vacuum. I know, that's a very leisurely pace. I'm used Adtech 820/824 in my air-conditioned shop at 25C; which gives 50-60 minutes pot life. My largest batch was 115g kept spread out on a paper plate and used within maybe 10 -15 minutes of being mixed. The other 4 batches were smaller and all used in a similar time. I've had epoxy heat up in a cup and do not want to experience that again.

8 - Contamination - I don't use painters paper, but I do have a roll of newsprint. Two layers on my work bench, one layer on the floor in case I need to move something off the work bench, and a piece for 'just in case'. I also use a couple layers when applying paint on the mylars. Note to self - my 500' roll is about half gone...


Some great comments; and I think doing the post-cure with the mylars might be the problem. I'll give the post-cure a second try with one of the tip panels, without the mylars, and see what happens.

Incidentally, the center panel is back in the bag. I chose to go with my second choice repair:
1 - painters tape over the bubble
2 - cut a small cross through the tape and wing skin
3 - inject epoxy under the skin
4 - work epoxy throughout the bubble; wiping up whatever comes out of the injection cut
5 - remove painters tape
6 - tape down a small piece of waxed mylar over the repair
7 - put peel-ply over the repairs and extend to the pump connection point
8 - pull full vacuum; currently 22inHg.

Fingers crossed!

Thanks again
-Keith
Aug 06, 2019, 02:42 PM
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Quote:
Originally Posted by Curtis Suter
One thought I had this morning... I use US Composities epoxy and it doesn't like any heat applied till at a minimum of half day after the initial mixing. I assume this time would change depending on the mix temperature as I've done a bit of bagging this summer and things sure set up quicker than my 55 degree shop in the winter.

Curtis
I'm using Adtech and there's nothing noted about that as a concern.
In this case, it was in the bag at 25C for 20 hours before I attempted the post cure.

Resin Research is in Tucson. I stopped by their office and had a really good discussion with the owner. He really made me appreciate just how much science is involved in epoxy. He 'dumbed it down' for me, but advocated doing a post cure, even if it is slow and at a relatively low temperature -which could just be leaving the wing in my non-airconditioned garage (~120 F).

That's the opposite problem of winter in Montana
-Keith
Aug 06, 2019, 02:43 PM
Registered User
Thread OP
Quote:
Originally Posted by T_om
I have post-cured a lot of stuff, but never by leaving them in the original mylars or release paper. So I really have no idea if that was the problem. But I have never had post curing cause delamination or poor lamination... if that ever happened it was my fault in the initial layup, and was obvious out of the bag, not the post cure.

Tom
Tom - I think you (and the other Tom) have identified my problem. I'll do a test without the mylars.
Thanks,
-Keith
Aug 06, 2019, 05:43 PM
supreme being of leisure
ZAGNUT's Avatar
surfboard shapers have all but given up on XPS because of delamination issues. off gassing is usually blamed but ive never seen it. what i have seen is PP or PE added to the XPS in varying amounts, some stuff i have here is so "greasy" that nothing sticks to it other than hot glue and even that doesn't really hold.
Aug 06, 2019, 06:54 PM
Registered User
Thread OP
I've wondered why surfboards weren't using XPS anymore...

UPDATE - Repair is out of the bag and the skin is well adhered. However, there is now a slight depression in the area that the bubbles occurred. I'll be curious to see what it looks like when I cut the flap, but the issue of some type of reaction occurring (outgassing?) seems plausible.

-Keith
Last edited by kablair; Aug 06, 2019 at 09:01 PM. Reason: Decided I won’t be doing a postcure on the tips.
Aug 10, 2019, 10:22 PM
Registered User
Where I had worked the guidance was that if a part was raised to within 50F of its cure temperature it had to be supported and bagged.
Problems I see is that resin is fairly slow to cure at 25C, (7 days). So it would have been no where near fully cured when it was unbagged. Then it was raised to post cure temps in an uncontrolled way by placing it into the truck. It probably became soft due to the temperature and lack of cure. The uncontrolled heating could have ramped the temperature too fast and caused some gassing. Not being vacuum bagged its not surprising that problems came up.

I could not find a ramp rate for that resin. But did find some cure info here in their product data sheet https://advanced-resins.sika.com/usa...arine-820.html
Aug 10, 2019, 11:59 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Roto Rob
Where I had worked the guidance was that if a part was raised to within 50F of its cure temperature it had to be supported and bagged.
Problems I see is that resin is fairly slow to cure at 25C, (7 days). So it would have been no where near fully cured when it was unbagged. Then it was raised to post cure temps in an uncontrolled way by placing it into the truck. It probably became soft due to the temperature and lack of cure. The uncontrolled heating could have ramped the temperature too fast and caused some gassing. Not being vacuum bagged its not surprising that problems came up.

.........................
That is just what I have been thinking. I meant to post this myself.
+1!
Aug 11, 2019, 07:26 AM
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Thread OP
Thanks for pointing that out about it not being a full cure; although I think thatís true of many/most products. Or at least they publish that data as part of a standard industry practice.

My reason for doing a postcure was to raise the Tg.
The idea being that, now that I live in Arizona, the wings could get soft at the elevated ambient temperatures.
Actually, I havenít had any problems in the past 3 years here, and I never had any problems in Houston. Although humidity is the real killer there

So, while there wasnít a pressing need for a postcure, I thought that if it could be done easily (in the back of my pickup at 50-60C), then it might be a good thing to do.

Now learning that there are some meaningful downsides to the process, and if it isnít actually necessary, I have stepped back from doing a postcure on the wing tips. No surprise - they have both come out of the bag just fine.

And - like concrete - epoxy never really stops curing; so my wings will get some postcuring while being stored in the 45C garage

Thanks to everyone for your comments and suggestions!
-Keith
Aug 11, 2019, 09:50 AM
Red Merle SJ VIII
Curtis Suter's Avatar
Grab your favorite beverage....

https://www.rcgroups.com/forums/show...S-vs-RR-vs-MGS
Aug 11, 2019, 04:13 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by kablair
Thanks for pointing that out about it not being a full cure;
So, while there wasn’t a pressing need for a postcure, I thought that if it could be done easily (in the back of my pickup at 50-60C), then it might be a good thing to do...............
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.

Quote:
Now learning that there are some meaningful downsides to the process, ..............................
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!..........................

Quote:
And - like concrete - epoxy never really stops curing; so my wings will get some postcuring while being stored in the 45C garage
-Keith
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.
Last edited by Jim.Thompson; Aug 11, 2019 at 04:19 PM.
Aug 12, 2019, 12:53 AM
Registered User
Quote:
Originally Posted by kablair
Thanks for pointing that out about it not being a full cure; although I think thatís true of many/most products. Or at least they publish that data as part of a standard industry practice.

My reason for doing a postcure was to raise the Tg.
The idea being that, now that I live in Arizona, the wings could get soft at the elevated ambient temperatures.
Actually, I havenít had any problems in the past 3 years here, and I never had any problems in Houston. Although humidity is the real killer there

So, while there wasnít a pressing need for a postcure, I thought that if it could be done easily (in the back of my pickup at 50-60C), then it might be a good thing to do.

Now learning that there are some meaningful downsides to the process, and if it isnít actually necessary, I have stepped back from doing a postcure on the wing tips. No surprise - they have both come out of the bag just fine.

And - like concrete - epoxy never really stops curing; so my wings will get some postcuring while being stored in the 45C garage

Thanks to everyone for your comments and suggestions!
-Keith
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).


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