Weldon 16 is cracking my canopy. Why???

[Captain Avgas]

Originally Posted by rv9builder

Bob,

What grade of sandpaper do you recommend for the final sanding? 400 wet? Or maybe something else?

Thanks,

I?d use wet and dry throughout. I?d start off with something quite coarse...say 80 grit. I?d use this to completely shape the edge into a continuous radius and to remove enough material to ensure that there are absolutely no little recesses left in the surface. All cutting techniques will leave little discontinuities (pitting) in the acrylic edge and you need to sand down beyond the depth of the deepest of these. Machining and cutting of acrylic also induces localised stresses so if you sand off enough material with the 80 grit you should also be reducing those local stresses. Then come over that surface with progressively finer grit...say 120, then 400, then 800. If you?re really fussy go to 1200. The more glass-like the surface of the edge is the safer it will be. It?s a little bit painstaking but not nearly as painstaking as getting a crack in your canopy and having to replace it.

[Captain Avgas]

Originally Posted by dwranda

Played around with some samples. Bent it with clamps so it was under stress. Put 2 drops of weld on 3 on the surface. Within a minute the surface cracks showed up under the drops. After that dried I heated the piece with a heat gun. I moved the gun back and forth about an inch from the sample for about 45 seconds to a minute. The surface was quite hot to the touch. After it cooled I placed 2 more drops on the piece. NO cracking. I must have heated it enough to relieve the stress and there is no distortion visible in the piece.
I will do 1 more sample then if that is also successful I will heat the corner of the canopy which should relieve any stress in that area. Then I should be able to fix the holes with weld on with no further cracking.
Has anyone else taken a heat gun to their canopy?

David, annealing of large thermoformed complex acrylic shapes such as glider canopies is common practice and is probably the only way to reduce internal stresses created by the forming process to reasonable levels. I have no idea if Vans canopies are annealed....and it wouldn’t surprise me if Vans don’t know either. At any rate the annealing of aircraft canopies takes a number of hours in an oven at elevated and very controlled temperatures. You will not be annealing your canopy by hitting it with a heat gun for a minute. In fact it is possible that by heating up only a small part of the canopy to a high temperature you might in fact be locking in further stresses.

[Captain Avgas]

Originally Posted by Aviaman

Here are some thoughts, admittedly purely speculative.

Internal stresses in materials are often relieved with ?annealing?, which is the application of heat, allowing the molecular structure to ?flow?. As the temperature rises, eventually the acrylic will melt. Before that temperature is reached, there may be a point where a slow stress relief occurs, without large scale distortion. If this is true, you would think it would be part of the manufacturing process. But even if so, the NEW stresses caused by installation of the canopy will not be neutralized by the previous annealing. So I?m wondering if an application of heat might anneal the new stresses caused by the installation process. I?m not recommending any particular procedure. Experimentation with scrap materials might shed some light on this. Even if positive results are obtained with experiments, scaling it to a whole installed canopy introduces the problems of temperature control, distribution of air temperature, duration of heating, etc. Perhaps relatively high ambient temperatures should exist during drilling, and installation.

John, see my post #21 re annealing. You would think that Vans canopies would be annealed but annealing is expensive because it involves a number of hours of oven time at elevated temperatures....and Vans are very price driven. Maybe Scott can clarify this issue. He has been following and posting on this thread so he will read this.

[rv9builder]

Thanks for the info, Bob!

[Captain Avgas]

I note that Scott did not respond as to whether Vans acrylic canopies were annealed or not after thermoforming to minimise intrinsic stresses that can lead to cracks. As I suggested, they probably don't know if their canopies are annealed or not. Or maybe they looked into it and found out that they have not been annealed. At any rate the silence is a bit deafening.

[Captain Avgas]

Originally Posted by jcarne

As far as I know the Sika method is 100% as well if it was properly done but I haven't been around long enough to know this for sure.

The Sikaflex method is only as good as the weakest adhesion link in the chain....and the weakest link on the sliding canopies could very easily be Vans powder coated steel. I don't see how builders can control that critical aspect of the adhesion. All I know is that some of the powder coating on my slider frame lifted off because it was not applied properly by Vans subcontractor. I was in Australia and returning the frame to the US was prohibitive so I had to have it chemically stripped and re-powdercoated locally. I believe the Vans powder coat lost adhesion because there was no proper passivation of the steel. Either that or the baking ovens were not running at a sufficient temperature to properly cure the powder coat batch...or the canopy frames were run through too fast and the base metal did not come up to proper temperature. There's lots of ways you can end up with powder coating with poor adhesion. When I asked Vans for a copy of their powder coating specification they told me they did not have one. For that reason alone I would be very reluctant to use Sikaflex on Vans powdercoated steel canopies.