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-   -   Polycarbonate vs. Aluminum (http://www.chiefdelphi.com/forums/showthread.php?t=154495)

pmangels17 31-01-2017 15:22

Re: Polycarbonate vs. Aluminum
 
Just a reminder, if your intake is made of polycarb, it will probably deflect some. And if that deflection makes your robot exceed the maximum size constraints (of which there are two possible combinations), you theoretically could get flagged for violating G04. I'm not advocating for aluminum over polycarb, but do be aware that they each come with their own set of challenges, and this is one of them.

JamesCH95 31-01-2017 16:16

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by pfreivald (Post 1638706)
We commonly bend polycarb on a break to 90 degrees and have never had a failure. Not that it *can't*, of course--but thus far it hasn't.

As to the OP, we generally treat 1/8" and 1/16" polycarbonate as if it's indestructible in terms of impact damage, and it has yet to let us down.

I've been tempted to prototype a "sheet metal robot" made from poly instead of aluminum and take it to an off-season competition, just to see how well it would stand up to full gameplay. ...maybe some day.

I would love to see pictures of how these bends turn out!

Bonus points if you can show the residual stress fields in the plastic.

WSiggs 31-01-2017 19:22

Re: Polycarbonate vs. Aluminum
 
Thank you everyone for the input thus far, all of this has been helpful for us and we will take this information into account.

Quote:

Originally Posted by pfreivald (Post 1638706)
As to the OP, we generally treat 1/8" and 1/16" polycarbonate as if it's indestructible in terms of impact damage, and it has yet to let us down.

Is this to say that anything thicker (or thinner) would not be "industructible"? Someone mentioned earlier that the bending radius for PC is about 100x the thickness, so would this mean a 1/4" plate for example would be more prone to breaking at a certain impact speed as opposed to 1/8" if it bends away? Or would the extra strength in the plate cancel this out?

pfreivald 31-01-2017 21:04

Re: Polycarbonate vs. Aluminum
 
Okay, so, guess what snapped in my hand this evening?

A piece of 1/16" polycarbonate that we'd bent on the break. We've been doing it for years with no problems, and have played some awfully aggressive games with pieces made like this, without any problems.

It took almost no effort to break. It was, mind, bent much farther than 90 degrees, and I'd warned my students that this might be a problem.

So as with most things, you might want to try things for yourself before implementing them based on advice from some internet blowhard (i.e. me).

Skyehawk 01-02-2017 16:49

Re: Polycarbonate vs. Aluminum
 
Another poly-card intake from 2014, MORE Robotics.
https://www.thebluealliance.com/team/1714/2014

This one freaked me out a little when I first saw it, not because of the design, but I had never seen a polycarb intake of that scale before, it had significant wobble but worked just fine.

Chris is me 01-02-2017 16:52

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by Skyehawk (Post 1639191)
Another poly-card intake from 2014, MORE Robotics.
https://www.thebluealliance.com/team/1714/2014

This one freaked me out a little when I first saw it, not because of the design, but I had never seen a polycarb intake of that scale before, it had significant wobble but worked just fine.

1714 has routinely built entire robots out of polycarbonate; often thick plates lightened up a bit. It takes a bit more weight to get the same level of rigidity as aluminum but it fits well into the team's resources / sponsorship.

Quote:

Originally Posted by JamesCH95 (Post 1638748)
I would love to see pictures of how these bends turn out!

Bonus points if you can show the residual stress fields in the plastic.

It's just a matter of having a large enough bend radius. It behaves almost exactly like sheet metal, just with a bit more danger of cracking if you are too sharp on the radius.

JamesCH95 01-02-2017 17:01

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by Chris is me (Post 1639193)
It's just a matter of having a large enough bend radius. It behaves almost exactly like sheet metal, just with a bit more danger of cracking if you are too sharp on the radius.

People are saying that, but it doesn't jive with my experience, and no one has posted a pic or provided bending parameters for cold-bending... I'd really like to see something solid.

s_forbes 01-02-2017 22:04

Re: Polycarbonate vs. Aluminum
 
On 1726 we had access to a large brake for several years and made a lot of polycarb pieces in it. Mostly 1/8" thick material. I don't recall the parameters we used for the bend, but it had to have a larger bend radius than we'd normally use for aluminum (it was one of those things where we'd adjust the brake to "right about there").

Example, battery holder (click for larger).


cadandcookies 01-02-2017 22:33

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by JamesCH95 (Post 1639197)
People are saying that, but it doesn't jive with my experience, and no one has posted a pic or provided bending parameters for cold-bending... I'd really like to see something solid.

I think this might be one of those things where things that work in FRC are not necessarily the things that are acceptable in industry. Sure, 100x bend radius may be what's necessary for retaining 100% strength, but in many (perhaps even most) cases in FRC, you don't actually need to retain all of the material properties. Success with more aggressive bend radii almost certainly depends on the use case. Last year 2667's robot had polycarbonate tabs that held the ball in on the bottom and got banged up a bit traversing the field. The students (without my knowledge) bent them on a sheet metal break with essentially no radius. Between the competition and practice robots, we didn't have a single tab fail.

Of course, this is a very different use case than an outside-frame intake, but my point is mainly that what you can get away with depends on use context and runtime. FRC teams can and do get away with doing a lot of objectively awful things to our robots.

JamesCH95 02-02-2017 06:37

Re: Polycarbonate vs. Aluminum
 
Thanks s_forbes!

Quote:

Originally Posted by cadandcookies (Post 1639310)
I think this might be one of those things where things that work in FRC are not necessarily the things that are acceptable in industry. Sure, 100x bend radius may be what's necessary for retaining 100% strength, but in many (perhaps even most) cases in FRC, you don't actually need to retain all of the material properties. Success with more aggressive bend radii almost certainly depends on the use case. Last year 2667's robot had polycarbonate tabs that held the ball in on the bottom and got banged up a bit traversing the field. The students (without my knowledge) bent them on a sheet metal break with essentially no radius. Between the competition and practice robots, we didn't have a single tab fail.

Of course, this is a very different use case than an outside-frame intake, but my point is mainly that what you can get away with depends on use context and runtime. FRC teams can and do get away with doing a lot of objectively awful things to our robots.

I understand the point(s) you're trying to make and I appreciate them.

However, I reserve the right to be skeptical. I'd rather see one example in practice than a dozen people insisting something is okay without any tangible proof.

Sperkowsky 02-02-2017 08:18

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by JamesCH95 (Post 1639369)
However, I reserve the right to be skeptical. I'd rather see one example in practice than a dozen people insisting something is okay without any tangible proof.

We have a large amount of pieces on our robot that were cold bent. In fact only on one bend did we use any heat.

Here is the only picture of one of the Cold bent parts I have on my phone.
https://drive.google.com/file/d/0B5V...ew?usp=sharing

I apologize for the google drive upload. I'm at school ;).

If you want any more pictures of parts Ill take more when I can get into the lab after school.

MrBasse 02-02-2017 10:28

Re: Polycarbonate vs. Aluminum
 
We have a few scraps in the shop. I'll bend some up and post a picture today.

We do cold bends on non crucial parts. Things we want to have survive for a long time we heat bend.

amesmich 02-02-2017 11:50

Re: Polycarbonate vs. Aluminum
 
Don't confuse polycarb with Acrylic. People do it all the time because they look the same but drastically different properties. Acrylic will shatter and crack. Poly is the same stuff bulletproof windows in banks are made from.

Skyehawk 02-02-2017 11:56

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by amesmich (Post 1639461)
Don't confuse polycarb with Acrylic. People do it all the time because they look the same but drastically different properties. Acrylic will shatter and crack. Poly is the same stuff bulletproof windows in banks are made from.

:deadhorse: That has been stated a lot so far, not trying to push any buttons, but read the thread first.

EDIT: apologies, it was only explicitly stated once, now I look like a hypocrite...

Donut 02-02-2017 12:41

Re: Polycarbonate vs. Aluminum
 
Quote:

Originally Posted by Skyehawk (Post 1639191)
Another poly-card intake from 2014, MORE Robotics.
https://www.thebluealliance.com/team/1714/2014

This one freaked me out a little when I first saw it, not because of the design, but I had never seen a polycarb intake of that scale before, it had significant wobble but worked just fine.

I'm going to play a little bit of devil's advocate here since I remember that intake from 2014 very well. 1714 always builds beautiful polycarb robots, but their intake that year flexed too much, and it affected their ability to securely grab the ball. When we faced them in the quarterfinals at Wisconsin our alliance's defensive strategy revolved around specifically targeting them on any attempts to pass them the ball since we knew there was a higher probability of the ball being knocked loose from their intake (and it's largely why we were able to win as the 6 seed).

I think polycarbonate intakes are great, but you need to ensure it is functional under defense with the added flexibility (though this is really applicable to any intake).


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