Melting Point of Plexiglass?

[Mike]

Does anybody know the melting point of plexiglass? Or the point where it would get deformed?<p>What I want to do is build a subwoofer plate amp out of plexiglass. I already have the amp built, using two OPA549 chips bridged into an 8ohm speaker. They put out well over 200W (or at least should), so the final heatsink (once I find one) would get quite warm (the small testing heatsink gets VERY hot without a fan, then it stays completely cool, no matter how much I've got the thing cranked). The heatsink would mount to the plexiglass.<p>I want to do this for a few reasons. First, it would be neat to see all the parts. Second, I wouldn't have to worry about isolating the chips from the heatsink (though I might in case an input cable comes in contact with it). And finally, I can't find a piece of aluminum thick enough at the size I need. I can find thick pieces that are about 4" x 8". The transformer alone is about half of that piece of aluminum. The large pieces are way too flimsy and would rattle. Thats another reason to use plexiglass. It's not metal so it won't rattle.<p>Thanks,
Mike

[Mike6158]

Found a couple of forums on the web about bending plexi (google search). The only one that made a reference to temperature said a 300 degree oven works well.<p>I have no idea...

[jwax]

From a thermal design standpoint, mounting a heat sink to plastic is asking for trouble. If a fan fails, you not only get a meltdown, you get a potential fire, smoke- ugly.
For cosmetics (and "coolness") if you insist, go with a polycarbonate like Lexan- stronger and higher temp tolerant, and available in static-dissapative to keep the dust accumulation at a minimal. Plus, please use a thermal switch to shutdown during overtemp! Two even better! Look at Pepi thermal snap switches.

[Chris Smith]

Most 1500 watt blow dryers work for the job.

[jollyrgr]

I don't know about large sections of plexiglass but for small project type boxes I've softened it over a gas stove burner.<p>For your heatsink issue, should you be overly concerned, I suggest that you mount the transistors to regular circuit board material with the heatsinks as normal. Cut a hole in the plexiglass that is about the same size as the heatsink and put the heatsink to the outside. Mount the circuit board to the plexiglass as it should be real cool. <p>In all honesty I think that if your heatsink is getting hot enough to melt plexiglass you have a problem with too much heat in your semiconductors anyway and need a bigger heatsink. You should be able to mount the transistors to the heatsink then mount the heatsink to the plexiglass and not melt it.<p>Have you seen those thermoelectric coolers? The heatsink is right up against the plastic and never causes a problem. If you are overly concerned get a normally closed thermal cutout switch and mount it to the heatsink. Something like a 150 degree F device should do. Put this in series with your power supply. Should the heatsink hit 150 F it will shut down the amp until the heatsink cools.

[peter-f]

Commercially, strip heaters are used to bend along a line... similar to plumbing anti-freeze heaters, about 300 deg F.
I'd agree, the application should be reviewed... plastic will smolder long before it burns... and the smoke can be acrid.
See modern Plastic Encyclopedia. (McGraw Hill) for the Acrylic manufacturer's specs... DuPont or Rohm & Haas.
Also, Lexan (polycarbonate) is more resistant to burning and is almost the same to machine.
You may want to 'play' a bit with (real- not similar to..) silicone adhesives to 'insulate' the heat sink From the acrylic. Mount heat sink along 4 edges (assuming it's rectangular) with an air gap between surfaces.
Beware of additives in such products... GE 100% silicone caulk is what I'd think is most available over the counter.

[dacflyer]

i used to form plexiglass with a simple blow torch.. just do not hold it in one spot long.. wave it back and forth fast to it all gets warm evenly.. a heat gun works great as well..i made a few dozen large dust pans for the shop , made them out of old scratched up video game monitor protectors.. 1/8" plexi-glass. i marked the area that i wanted to fold..put the line on edge of a table..heated it to a point where it slowly drooped,,then i continued to heat it untill it was all relaxed.. then i let it cool..
it takes a lot of heat to melt plexi glass.. 300F sounds about right..

[Deal]

You have to be nuts to even think about heat sinking into a flammable material. I have designed and fabricated using all these materials including thermoelectrics. That you cannot "find a piece of aluminum large enough" shouts that your post is freak. Congrats, you wasted time of helpful people. Freak parasite loser posts like this are the reason public community help and instruction won't continue. If I am wrong, then post here. Loser

[jwax]

Deal- You are way out of line.
Unless you know everything about everything, I'd recommend you keep your flaming retarded outbursts to yourself.
Any inquiries are invited on this forum, and if you think they are "freak", keep it to yourself.

[Mike]

Thanks to all so far:<p>Guess this is just one of those things I need to try and see what happens. If it gets too hot, I could always try and find a piece of aluminum thick enough.<p>Ever notice how jerks like this "Deal" guy never even read, nor think before they speak? If this post is a waste of time to him, then don't spend time posting in it. And, I never said I couldn't find a piece of aluminum large enough, it's that I couldn't find one thick enough at the size I need to not rattle when the speaker is on.<p>Anyway, I think as long as I have a good enough heatsink, it should be fine. Besides, when watching movies, it barely works except when something explodes, so it won't get that hot anyway. I could also get spacers to have the heatsink stick 1/4 inch or so off of the plexiglass.<p>What I'll do is look around and try to find a decent heatsink for the amp (and my wallet), and try it without being put on the plexiglass. If it doesn't get too hot, then I'll put it on and see.<p>and jwax, in regard to your first post, I wasn't planning to use a fan, as it is annoying when watching a movie. Do you think I'd really need one as long as the heatsink is large enough. And, what temperature would you suggest I get a thermal switch for?<p>Thanks agan for your replies<p>edit: rereading my first post, I noticed a typo. I meant to say that with a fan on it the heatsink stays completely cool.<p>[ November 26, 2004: Message edited by: Mike ]</p>

[dyarker]

Plexiglass(TM) should be okay, peter-f's idea of Lexan(TM) is better. Jolly Roger is right about thermal cutout, luckily the OPA549 has built in thermal cutout at 160°C junction temperature. That makes case temperature about 90°C (194°F). If the heatsink is large enough, it will not go above 150°F (probably less). The fuse in the power supply protects against catastrophic(sp?) failure of OPA549s causing a fire.<p>Page 11 of the OPA549 spec sheet is about heatsinking. The formulas for heatsinking are similar to electronic formulas. Replace Volts with temperature in °C, Amps with Watts, and Ohms with °C/W.<p>Where for electronics the formula is:
Volts = Amps * Ohms
for heat it is:
°C = Watts * (°C per Watt) ((where Watts is power dropped in the IC, not power delivered to the speaker.))<p>Normal room temperature is 25°C; call it 30°C because maybe near other equipment, poor air circulation under desk, whatever. Spec sheet says design to 125°C max junction temp.<p>125°C - 30°C = 95°C temperature difference to push the Watts from the junction to the air. Just for example lets say the max power one IC drops is 30W. The max total thermal is:
TRtotal = 95°C / 30W = 3.16°C/W<p>Junction to case thermal resistance is 1.4°C/W, so:
3.16°C/W - 1.4°C/W = 1.76°C/W<p>A good thermal connection from case to heatsink is 0.5°C/W, so thermal resistance of heatsink to air should be less than:
1.76°C/W - 0.5°C/W = 1.26°C/W<p>Calculating from spec'ed heatsink in the Mouser catalog I get 65°C/W per square inch of heatsink surface.
HeatSinkSurface = (65°C/W/sqin) / (1.26°C/W) = 52sqin or more for each IC.<p>Don't worry that's not as big as it sounds. A 4in by 4in heatsink with six fins 5/8in tall is 46sqin.<p>((possibly interesting - recalculate the heatsink size for 50°C air (summer afternoons in some deserts, a few factories), and you start to see why industrial and military equipment are more expensive than the exact same equipment built for home use.))<p>-----------------------------------
Where did this "Deal" guy come from? Totally un-called for! Right-on jwax!<p>I've corrected Mike before (and been embarrased by being overly harsh once), but there is absolutely no need for name calling here.<p>Deal - Constructive critisizm or useful suggestions, or keep it to yourself.
--------------------------------------<p>Cheers,

[Mike]

Thanks, Dale<p>right now the heatsink I have now is very small (for what it's cooling), so that's why it probably gets so hot. I'll just look around and find one that would work.<p>edit: rereading your post, Dale, I noticed you said to use 30W dissipation as an example. Do you really think that it would be that low? So both chips would put out only 60W total in most cases?<p>[ November 27, 2004: Message edited by: Mike ]</p>

[paulrevelcet]

Deal this is the first time since I have been posting on this site that I have heard such a asinine post, this site should be keep free of this kind of crap, its one thing to have a difference of opinion but its another to try and hurt others on purpose.
Mikes question is perfectly valid as all questions are, please deal, try to be more constructive in the future, besides it would be ashame if due to some freak accident you had all your fingers cut off for being a mean little shit to people on the internet, you know, divine intervention, it could happen
At any rate if we tolerate this kind post then before you know it we will be overrun with this kind of behavior, I love the atmosphere of this forum and I hate to see it change and Im sure others do to.<p>[ November 27, 2004: Message edited by: paulrevelcet ]</p>

[dyarker]

Nope, 30W was a WAG so I'd have a number to put in the example calculations.

[upsmaster]

hi

Try www.mcmaster.com for raw just about anything u need. try aavidthermalloy.com for hearsink info and free software Cool-Cat down load this may help.<p> joe