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  #11  
Old 11-29-2011, 04:10 PM
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swisseagle swisseagle is offline
 
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Default SIKA thicknes?

Hello,

as Charly mentioned, you should have at least a 1/4" bead thickness, so that there is some "flex" area.

But for sure also, a canopy as long as a RV-8 one will streach quite an amount under heat/cold.

Interestingly how many builder build there canopys in warm condition. This is ok for trimming and drilling it. But for the final position, when it is screwing or riveting time, it maybe better to do this when it is not so cold, lets say in the lower half of the temp-range of usage. So the stress should also be eaqual for cold/warm wether.

Regards, Dominik
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  #12  
Old 11-29-2011, 05:44 PM
Captain Avgas Captain Avgas is offline
 
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Location: Melbourne, Australia
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Default

In the great Sikaflex debate of 2008 on VansAirforce I stated the following:

In actual fact polymethyl methacrylate (PMMA) or "acrylic" as it is commonly known is notoriously difficult to obtain a good bond to. Sikaflex itself will not bond to PMMA. An interlayer (or adhesion promoter) needs to be used. In the case of Sikaflex its 209 primer contains 25% MEK and 20% Ethyl Acetate. Both of these are solvents known to break down the cross links of the long chain molecules in PMMA. In the longer term that might very well lead to edge cracking (one case already reported).... the very thing builders were anxious to avoid.

I am providing a link to the thread if anyone is interested. The comments above were in Post #6

http://www.vansairforce.com/communit...ad.php?t=29804
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  #13  
Old 11-29-2011, 06:29 PM
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Av8torTom Av8torTom is offline
 
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Default Chloroform

Is anyone using chloroform to "weld" these cracks? I used to use it all the time to assemble plexiglass parts. Created a joint that was stronger than the material itself. I don't know how readily available it is, but it worked amazingly well. Besides its performance as an "adhesive" is has such low surface tension that it wicks into the crack by capillary action so there is no need to open the crack to get the glue in there.
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Last edited by Av8torTom : 11-29-2011 at 06:39 PM.
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  #14  
Old 11-29-2011, 07:04 PM
Bavafa Bavafa is offline
 
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I have tested Weld-on 3 on Plexi with great success. It does create a great bond and wicks into very tinny cracks. Fortunately, I have not had to repair any canopy and l will keep my fingers cross that I won't ever have to.
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  #15  
Old 11-29-2011, 08:49 PM
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Snowflake Snowflake is offline
 
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Default

Sikaflex vs. Rivets, Alternative vs. Lycoming. It's amazing how similar the arguments look.
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  #16  
Old 11-30-2011, 07:38 AM
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Rick6a Rick6a is offline
 
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Location: Lake St. Louis, MO.
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Default Peer Pressure

Quote:
Originally Posted by Snowflake View Post
Sikaflex vs. Rivets, Alternative vs. Lycoming. It's amazing how similar the arguments look.
Amazing similarities, that's for sure. Technically, we truly are "experimenters" and we enjoy great flexibility with the liberty to go down the wrong road if we so choose. But speaking directly to the sometimes hotly debated arguments you mentioned in your post, I would add that thanks in part to the ever-growing body of information contained within the VAF archives, it would appear the passage of time and operational experience does have a way of sometimes exposing persuasive or cleverly crafted opinion to the cold hard realities and glaring light of day. If anything, The Great Sikaflex Debate clearly demonstrates the intellectual minefield we sometimes deal with and an objective review of that lively discussion can be a fascinating and instructive exercise in Monday morning quarterbacking complimentary to your favorite cup of joe:

http://www.vansairforce.com/communit...ad.php?t=29804

Kudos to Captain Avgas:
Quote:
.....it is my observation that those who desperately WANT to believe in the merits of the Sikaflex solution are often quite hostile to any poster who puts forward any information or data that suggests there might be intrinsic problems....
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Last edited by Rick6a : 11-30-2011 at 11:44 AM.
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  #17  
Old 11-30-2011, 07:56 AM
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Default

I'd be willing to wager that there are a lot more canopies that have cracked that have been installed by conventional means. The noises my canopy made on my -6 when warming up the hangar made me nervous about getting a crack.
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Last edited by rocketbob : 11-30-2011 at 07:58 AM.
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  #18  
Old 11-30-2011, 08:19 AM
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Andy Hill Andy Hill is offline
 
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Quote:
Originally Posted by N282S View Post
I have two cracks. One, aft right side - about two inches. Happened on a very cold winter morning.
The second is right dead center (top) aft about two inches.

Heaven knows how many cracks I would have had I riveted the thing together.
I've never seen anything to suggest that Sikaflex actually reduces cracks occurring? Lots seem to think it will though...
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  #19  
Old 11-30-2011, 11:59 AM
Lars Lars is offline
 
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Location: Davis, CA
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Default

Part of the issue with Sikaflex (or, more generically, adhesives) is that the material, installation requirements and limitations need to be understood in order to use it properly. Which I suspect doesn't always happen. One hazard of using an adhesive is that it can possibly fasten the acrylic too well, which could make for some big localized stresses, depending on part geometry.

Confession: I used Sika products to bond my canopy (RV-7 slider). I was attracted to the process because I have some background with bonding and casting with urethanes (Sikaflex 295UV is a single part, moisture-curing urethane) to a variety of substrates. Most of that experience was in a marine environment, with equipment that had fairly severe environmental exposure. One thing that I learned was that urethanes worked very well for various chosen applications if used correctly. If not, it could be pretty frustrating. Sometimes the process was pretty fussy.

Because of a dumb, careless mistake, I had the pleasure of doing two Sika canopy installations: I dropped pliers on the first one a couple of months after completing it, making a nice star-shaped crack above the pilot's head. Duh. There was one advantage to the Sika process: I got the broken canopy off the frame with only a razor blade, in about 10 minutes. Well, not completely off. I left it attached to part of the forward bow, then tried tearing it off. Might as well see how strong the stuff is. Obviously a purely qualitative test. Still, I was pleased. I wasn't strong enough to pull it free.

Bob Barrow pointed out that the 209D primer contains substances known to attack acrylic. That could lead to some interesting stresses at the primer line edges. I didn't see any evidence of failure along the primer line on my first canopy when I was tugging on it. Sika does represent the system as being suited for bonding acrylic windows into boat hulls, but the jury is still out.

Please don't take the above as an argument in favor of using Sika. I was interested in using due to my aforementioned previous experience. So far I'm happy, but I'm not flying yet. I too have noted that the canopy shape is temperature dependent. The skirt on mine hugs the fuselage a lot more tightly when it's cold.

There's one other thing I thought of, maybe it's been mentioned in canopy threads, but I haven't seen it. The side-by-side canopies are formed in a mold- I believe they are vacuum-formed. The tandem canopies are blown. Would the difference in the two processes lead to different surface conditions on the plastic, possibly making one or the other more susceptible to cracking?
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  #20  
Old 11-30-2011, 01:02 PM
RV8R999 RV8R999 is offline
 
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Default Post #3

I think Craig (post #3) is spot on.

I used Sika for my -8 canopy after the usual careful cut and fit. There is very little risidual stress if the canopy has been cut and fit with care. It more or less lays exactly in place.

I noticed after it was all cured that the entire frame/canopy assembly would deform significantly when engaging the canopy latch by pulling the left side further forward than the right side. This resuts in a noticible torsion applied to the entire canopy against many compound curves - not good.

Filing the latch to the point where it just engages the lock pin prevents any deformation and in my opinion will significantly reduce the chance for cracking.
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