Bubble in Canard skin

[Phil Kriley]

I just did the lower skin on my canard yesterday, and have a question that I hope someone can help me with. We did the layup, all llooked nice a smooth, we put on the 2 inch peel-ply on the leading edge, built a little heat tent around the work and then went bowling.

 

A couple hours later we checked the curing glass (about 80 degrees in the tent) and found two large bubbles had developed. One was about 18 inches in from the end, was about 4 inches long, 1 3/8 wide and 3/16 high, oblong in shape. The other was on the opposite end, approx. the same area, but a little smaller in size. Bother were over the foam behind the spar cap. Since the glass was still curing, I stipled the two bubble down and worked the air out as best as I could and put the tent back in place.

 

A couple hours later I checked on it aagin and the two bubbles had reappeared, although not as big. So again I stippled them down, but this time I also put on peel ply, covered with freezer paper and weighted with shot bags. Looked OK this morning, except that it's not as "pretty" as I would have liked being that there is a "patch" of peel-plied skin at each end.

 

Any thoughts as to why this happened, and what I could do differently when I do the top skin?

 

Should I let it cure at a lower temperature and then post-cure in the heat tent?

[TMann]

This problem goes away if you vacuum bag it.

[Phil Kriley]

This problem goes away if you vacuum bag it.

Hopefully there is a simpler and less expensive solution.

[michaelj]

Your layup is okay structurally as long as you caught it in time to get a good bond between the glass and foam. Contour it with micro or West and press on. I have experienced the same problem and therefore only post cure. Vacuum bagging is great if you have the equipment and technique down.

[Wayne Hicks]

How hot was your heat tent when you went bowling? If the heat tent is too hot, the heat can force air out of the foam, causing bubbles to form. Yes, our foams are "closed cell" foam. But if you get the foam too hot the air will expand and find a way to migrate out to the surface.

 

When using a heat tent, do the overnight cure first, then do the post cure later. In other words, do the overnight cure at the lowest temperature that will result in a hard, 24-hour cure. I use 80-90 degrees F. The low temperature will help prevent air migration.

 

In the ideal world, you want all of your core materials to be at working temperatures roughly 4 hours before you start the layups. This little gold nugget of info is in the education section or in one of the earlier CPs.

[steve]

i think you had dried finger prints in the glass before you started

i get them because my gloves are covered with epoxy when i play with the roll

or you mite have had the roll taped in that spot pulling on the cloth, its hard to see after you lay it out with epoxy.....just guessing here.

i have gotten over 100 bumps and whatnots from top to bottom, all were easy to fix and or cover with micro-primer-paint.

[Big Steve]

Phil:

 

I think Wayne is onto something. When I built my Longeze in Calif. I never had to post cure or use heaters to cure parts it did not get that cold. I had no bubble problems. Since I have moved to Idaho and started the Cozy I have had lots of bubbles in the curing glass problems. We are working with cooler foam and epoxy in a cooler air tempt. when we are finished we put it in a really hot environment air expands when it get hot it has to go somewhere whether the air be in the foam or just under the layup. I have had success getting rid of bubbles in my lay-ups by letting it cure at shop temperature instead of throwing it in a super hot environment. After it gets past the bubble stage I will post cure it in the summer time. No more bubbles. My .02 Steve build on

ps. vacuum bagging is nice for those who like to use it but is not necessary for the construction of these planes it sometimes just complicates things.

[Neverquit]

Something else to think about: When I do large or complex layups the epoxy will get kind of thick as it cools off to room temp. Whether I do the layup with plastic over it to squeegee excess epoxy out or stipple, I'll sometimes hit it with the heat gun to get the epoxy flowing again. I've noticed I get a lot more bubbles with it as it also heats the foam and I go back and look for bubbles a few hours later as well. I peel ply EVERYTHING which sometimes complicates the problem. I learned early that peel ply will shrink when heat is applied. You think you have a nice tight layup when it shrinks but it will continue to make problems as it pulls at the layup and causes bubbles. I still use the heat gun when the layup and peel ply is complete so the excess epoxy bleeds through the peel ply. Just not so much heat now.

[aviator_edb]

Would 'pre-heating' the foam a little bit possibly help this? I'm not talking about heating it way up but perhaps tossing it in the heat tent for a bit prior to micro and glass?

[Neverquit]

yep, wouldn't hurt

[Steve Innova]

Hopefully there is a simpler and less expensive solution.

For the canard, vacuum bagging is a very simple and relatively inexpensive solution.

 

Canard, Wings, winglets, bulkheads, roll-bar; are all parts that turn out great when vacuum bagging.

[argoldman]

Something else to think about: When I do large or complex layups the epoxy will get kind of thick as it cools off to room temp. Whether I do the layup with plastic over it to squeegee excess epoxy out or stipple, I'll sometimes hit it with the heat gun to get the epoxy flowing again. I've noticed I get a lot more bubbles with it as it also heats the foam and I go back and look for bubbles a few hours later as well. I peel ply EVERYTHING which sometimes complicates the problem. I learned early that peel ply will shrink when heat is applied. You think you have a nice tight layup when it shrinks but it will continue to make problems as it pulls at the layup and causes bubbles. I still use the heat gun when the layup and peel ply is complete so the excess epoxy bleeds through the peel ply. Just not so much heat now.

greetings Never,

 

It's not that the epoxy gets apapreciably thicker as it cools off (unless you are building at 50 degrees), it is that the material is polymerizing and getting thicker.(going-or getting off) (that's what it's supposed to do) The heat gun does soften the polymerizing material and bring some of it back to it's liquid form.

 

Peel ply will only shrink if you over heat it too much for the purpose for which you are using it. When you are covering a ragwing, the shrinking is a positive. It will not shrink below a certain temperature and that is the max temp you want to use if you choose to use the heat gun for peel ply application. It is possible that the heat volatilized some of the uncured epoxy and thus the bubbles.

[Marc Zeitlin]

It's not that the epoxy gets apapreciably thicker as it cools off (unless you are building at 50 degrees), it is that the material is polymerizing and getting thicker.

That's incorrect. In the early stages, while still within it's "pot life", the viscosity of the epoxy is highly dependent upon its temperature. As it cools, the viscosity rises substantially. Using the hair dryer lowers the viscosity (and ACCELERATES the cure), but is not changing the "polymerization" from one state to another.

[Phil Kriley]

Thanks for all the advise! I think that the problem is exactly what Wayne and others have pointed out. When I work on a layup, the shop and the work itself is at about 65. I use warm (90 degree) epoxy, but applied over the cold foam. Then when I put a heat tent over it, raising the temp to 80 or more, the cold gas expands and causes the bubbles.

 

When I do the top skin, I'll warm up the canard under the heat tent FIRST, then do the layup with warm epoxy and then let it cure at room temperature - or if I do still need to use the heat tent I'll use a lower setting so as not to cause the gas expansion. Then when the part is cured I'll do the post-cure heat treatment.

 

THANKS FOR THE REPLIES!

[argoldman]

That's incorrect. In the early stages, while still within it's "pot life", the viscosity of the epoxy is highly dependent upon its temperature. As it cools, the viscosity rises substantially. Using the hair dryer lowers the viscosity (and ACCELERATES the cure), but is not changing the "polymerization" from one state to another.

 

 

While what you say is most likely correct, Marc, Sometimes clinical experience differs from the basic studies that are done.

 

I have used various epoxies at both elevated temperatures, from the heater, and also at room temperature.

 

Using the stuff, at room temperature, as well as at elevated temperatures, I find that after a while that heat "rewets" the material. If the thickening were due only to the loss of heat, where does that heat go. If my hanger is 70 when I start, and the epoxy is 70, and the substrate is also 70 when I start, and the hanger is 70 when I finish presumably the epoxy and the substrate is 70, or perhaps a little warmer due to exotherm, how can the stuff thicken due to heat loss?? At what point does the polymerization and attendant thickening start??? In a pot that answer is easer because of the sometimes rapid exotherm.

 

Respectfully submitted and awaiting answerhood.

[Marc Zeitlin]

While what you say is most likely correct, Marc, Sometimes clinical experience differs from the basic studies that are done.

As I've said many times, in theory, theory and reality are the same, but in reality, they're not.

 

But this isn't a question of theory - it's reality :-).

 

I have used various epoxies at both elevated temperatures, from the heater, and also at room temperature.

Same here.

 

Using the stuff, at room temperature, as well as at elevated temperatures, I find that after a while that heat "rewets" the material. If the thickening were due only to the loss of heat, where does that heat go. If my hanger is 70 when I start... how can the stuff thicken due to heat loss??

There are two things going on. In the case you postulate, the epoxy starts out at viscosity "X" at time zero. After most of the pot life is gone, the viscosity will increase to "X+17", due to the chemical reaction that begins occurring as soon as you start mixing it. In THIS case, the thickening isn't from heat loss.

 

In many other cases, however, where the epoxy comes out of a pump at 90F and gets spread on something that's at 50-60F within a few minutes, the thickening is (in the short term) due to heat loss.

 

In EITHER case, as long as the chemical reaction isn't TOO far along, heating the epoxy with a hair dryer (or heat tent, or whatever) will lower the viscosity (unless it's so far along that the extra heat merely "kicks" the epoxy into the "B" stage, and it gets rubbery fast).

 

At what point does the polymerization and attendant thickening start??? In a pot that answer is easer because of the sometimes rapid exotherm.

The thickening "starts" immediately upon mixing the two components, but the RATE of thickening is very slow at first, and only near the end of the pot life does it accelerate. At any time prior to that, heating the epoxy will lower the viscosity, but NOT because it's stopping the chemical reaction - just the opposite - it's accelerating it, but the decrease in viscosity due to heat is way more than the increase in viscosity due to increased reaction, until you get near the end of the pot life time (AT THE NEW, HOTTER TEMPERATURE).

 

Make sense?

 

Respectfully submitted and awaiting answerhood.

Answerhood, eh? We don't need none of that big-city talk around here, bub.

[argoldman]

As I've said many times, in theory, theory and reality are the same, but in reality, they're not.

 

But this isn't a question of theory - it's reality :-).

 

Same here.

 

There are two things going on. In the case you postulate, the epoxy starts out at viscosity "X" at time zero. After most of the pot life is gone, the viscosity will increase to "X+17", due to the chemical reaction that begins occurring as soon as you start mixing it. In THIS case, the thickening isn't from heat loss.

 

In many other cases, however, where the epoxy comes out of a pump at 90F and gets spread on something that's at 50-60F within a few minutes, the thickening is (in the short term) due to heat loss.

 

In EITHER case, as long as the chemical reaction isn't TOO far along, heating the epoxy with a hair dryer (or heat tent, or whatever) will lower the viscosity (unless it's so far along that the extra heat merely "kicks" the epoxy into the "B" stage, and it gets rubbery fast).

 

The thickening "starts" immediately upon mixing the two components, but the RATE of thickening is very slow at first, and only near the end of the pot life does it accelerate. At any time prior to that, heating the epoxy will lower the viscosity, but NOT because it's stopping the chemical reaction - just the opposite - it's accelerating it, but the decrease in viscosity due to heat is way more than the increase in viscosity due to increased reaction, until you get near the end of the pot life time (AT THE NEW, HOTTER TEMPERATURE).

 

Make sense?

 

Answerhood, eh? We don't need none of that big-city talk around here, bub.

 

Ah-- The P(l)ot thickens (as does the epoxy)... Thanks for the explanationhood:cool:

[Phil Kriley]

Ah-- The P(l)ot thickens (as does the epoxy)... Thanks for the explanationhood:cool:

Same from me, Marc - you sure have a talent for making things easy to understand! Thanks!