Canopy Heat Treating

My friend who is an engineer and in the industrial plastics business guided me with heat treating my canopy to reduce stress.
I carefully measured the frame when installed on the airplane track and installed temporary bracing on the frame to keep things exactly where they need to be. We placed the canopy in an oven which is designed to anneal plastics. We brought the temperature up slowly to 190 degrees, maintained it for 2 hours and brought the temperature down very slowly.
I removed the clecos and the plastic fits the canopy frame perfectly with zero stress.
After going through this process, I believe that a simple homemade oven can be made up with some basic materials to accomplish this without spending a ton of money.
I was just lucky to know a guy who knows what he is doing and has a set up already.
I thought that it may help someone who is worried about too much stress on the plastic when riveting it to the frame, which may invite cracks.

It is indeed quite easy to make an oven to do this kind of thing.

We made an oven for post-curing carbon-epoxy composite parts, primarily wing tips made with epoxy that requires a 150F post-cure.

The "oven" is just a box made from 2" styrofoam insulation available at home improvement stores. We pin the panels that form the box together with bamboo shish kabob skewers. Then we tape all the seams with 2" wide aluminum foil tape.

The heat source for the oven is an electric hot plate from Goodwill, and a 6" desktop circulating fan. We use a couple of oven thermometers stuck through the foam to the interior to monitor the temperature.

With the big L-39 "CEllipse" racing tips in the oven, it takes about 2--3 hours to get up to 150F, which is what we want for the post-cure cycle. Adding another hot plate would bring the temperature up faster obviously.

I wonder if a completed canopy assembly could be heat treated this way after it was already attached to the frame, or if the differential expansion would just make the assembly more likely to crack during the process?

EXCELLENT thread. Canopy cracks is one of the things i worry about the most. Especially in cold weather. I thankfully live in the south. I would love to fly up to Massachusetts to visit my son sometimes in the winter but the super cold temps keep me from it.

I would almost rather have a canopy that's in smaller sections or something to eliminate cracking issues. Thankfully none in the 11 years mine has been flying.

I wish I had known about this "stress relief" before installing mine.

Revisiting the canopy heat treating thread:

I was talking about this process with a person with extensive Plexiglass experience. He had these tips:

"1) Before doing anything, calibrate your thermometer. The easiest and BEST way to do this is to check the thermometer in ice water and boiling water (preferably distilled water). If the thermometer reads 32 F and 212 F (or O C and 100 C), then you're good to go. Check both points!

2) Stabilize the temperature of the EMPTY oven for a couple hours before placing the Plexi inside. This will help you avoid melting the Plexi by having a hot spot while the oven is warming to temp. Better safe than sorry. At this point, you're making sure that the oven is at the correct temperature and staying there, not varying up and down, especially up.

3) You can anneal the Plexi at temps from 175 F (takes longer) or up to 190 F (takes about 2 hours. Using temperatures above 190 F increases the chance of ruining the Plexi. Anything above 195 F increases the chance that you'll turn your Plexi bubble back into a flat sheet. (Um, that's bad!)

4) The Plexi should be clamped or strapped to the frame securely. However, it is better to NOT use clecos, screws, or other "point" attachments as the Plexi may pucker between those points. This likely means that you would not want to try to anneal a canopy that is already installed.

Annealing your Plexi can help your bubble last longer, but it isn't always necessary. It can be most helpful in the case where areas of the Plexi are being drawn into position by the clecos, screws, etc. In other words, the Plexi will be happier if it isn't forced into position. So, either fix the canopy frame or consider annealing the Plexi (if the frame isn't too bad!).

Last tip: Unrelated to annealing the Plexi, but very important. The better job you do deburring and smoothing the edges of the Plexi, the longer your Plexi is likely to last. If you rush through this task and leave tool marks, jagged screw holes, and other rough spots...well, you may regret it."


Here's link to a PDF that gives his recommended annealing temps:
http://f1aircraftforum.com/wp-content/uploads/2023/06/Temperatures-for-annealing-cellcast-acrylic.pdf