Why does acrylic warp near saltwater?

[Pat H.]

I think its called memory its in the process when they make it ok question answered next one please .steve stop down to plastic's unlimited they can tell you more bring the piece with you.

Then why does acrylic always seem to curl away from the water. Even when I've flipped the curled cover over, in a week it looks the same.

 

[Clam Man]

I don't think it matters I think its in the way it formed streatched.so are you telling me you flipped the piece over and it curled up on two different ends if so I have never seen that before.

 

[aweeks95]

the cover on mt HOB skimmer is 1/4" thick. you can't bend it by hand but it does have a slight bow to it from the water....if it can absorb water i guess that would be a good explanation. other than that i have no idea.

 

[Pat H.]

I don't think it matters I think its in the way it formed streatched.so are you telling me you flipped the piece over and it curled up on two different ends if so I have never seen that before.

I flipped the cover so that the bow arched up, then a week later the bow arched down again.

As far as I know acrylic is either extruded (hot plastic pushed, not streached, through a die, like a play-doe pump) or cast. I believe that stress memory could (and probably does) add to the warping of extruded, but not cast, which is what I have used for covers in the past.

 

[Clam Man]

I flipped the cover so that the bow arched up, then a week later the bow arched down again.

As far as I know acrylic is either extruded (hot plastic pushed, not streached, through a die, like a play-doe pump) or cast. I believe that stress memory could (and probably does) add to the warping of extruded, but not cast, which is what I have used for covers in the past.

ok it bowed up on the same 2 sides when you flipped it im asking did it dow up in the other 2 sides or is it always the same 2 ends.

 

[Pat H.]

ok it bowed up on the same 2 sides when you flipped it im asking did it dow up in the other 2 sides or is it always the same 2 ends.

Sorry, I don't understand what your asking. I'll try to explain myself better. The whole piece curls not just the ends. When it curled up away from the sump, I flipped it over so that it arched up. Then a week later it looked the same as it did before I flipped it.

 

[Rick M]

Anti-Gravity

Well, if it was sitting on eggcrate and curled up, that shoots down my gravity explaination. I have found that if you take a flat sheet and heat and bend it, then go back and heat it, it will "remember" (memory refered to in other posts) that it was once flat and go back to being flat. But I think the extruded sheets are extruded flat, so that would not explain the curling up either. You may be onto something with the salt water absoroption and/or reaching for the heat from bulbs ?!?!

Rick

 

[smithcreek]

I think it's a combination of gravity and absorbing water. I have stored sheets of acrylic leaning against the wall at a bit of an angle and they always bow toward the wall. I have to flip them around for a week or so to straighten them out.

I have an AquaC remora simmer on my holding tank. One time I took the top off and flipped it over for some reason. Forgot to flip in back and when I looked at it a day later it had bowed from the moisture in the collection cup. It wasn't gravity since the ends were bowed up, not touching the cup.

In most cases both forces are acting to make it sag into the tank. Gravity pulls the center down and the side facing the tank expands. The acrylic on the eggcrate isn't affected by gravity, but shows that even just expanding due to water will make it curl.