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How to prevent vacuform canopies yellowing?


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Hi Dale,

I’m afraid you won’t like my answer : « Make new ones ! »

Old vac-form canopies were made of PVC, which yellows and becomes brittle with age. This is due to ultra-violets, and there is not much you can do about it, but for keeping your models, including when finished, in the dark ...

More recent vacs are made of PETG (one commercial name is « Vivak ») which is more tolerant of UVs, and does not seem to yellow with age (so far)

Hubert

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To complete my answer. Since you have an old, yellowed, canopy, and need to vac-form a new one in PETG, you have two options:

1) Make a female mold around the old, yellowed canopy. This is the best option for vac-forming, whether the part is transparent or opaque. But it has some constraints, like needing a hole to suck the heated PETG in the female cavity. The diameter of this hole should be about 50% fo the thickness of the material formed to avoid any dimple. One way to get it is to drill a same diameter hole in the old part, insert a piece of monofilament in it, and pour the mold material around the part and the monofilament. When the model is set, remove the monofilament, et voilà, you have a hole for succion !

2) Make a male mold by pouring your mold material in the old canopy, than vacform, or plug-form the heated PETG around this male mold.

Whichever way you go, remember that PETG loves humidity, and will store it between its molecules. Thus, when you heat it, if you go above a certain limit (170° C), bubbles will appear within the plastic :wallbash:, created by water vapour. To avoid this, one way is dehydrate the PETG sheet for two hours at 50° C. Another way is to heat the PETG by plunging it in boiling oil (160° C). And Peter from Airscale seem to have been successful by plunging the sheet in boiling water.

HTH

Hubert

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