13-07-2008 22:29

JESTER357

Re: pic: Team 2614's Cracked Plexi

what an amazing time. i remember u bringing that up to me and saying there is no way that happened

13-07-2008 22:43

Andy Baker

Re: pic: Team 2614's Cracked Plexi

One of the ways to tell the difference between acrylic (aka: Plexiglas) and polycarbonate (aka: Lexan) is the color of the edges.

Acrylic has a white edge.
Polycarbonate has a blue edge.

The producers of polycarbonate resin call this a "blueness index", if I recall correctly.

On this picture, many of the edges appear white. I only see blue edges where there are 2 layers of plastic. My guess is that we are looking at an acrylic sheet. And, of course, acrylic is much more brittle than polycarbonate.

Look at these two pages on www.matweb.com:

Extruded Acrylic
Extruded Polycarbonate

If you check out the "mechanical properties" of these two materials, they look somewhat similar in their strengths and properties, until you see the "Izod Impact, Notched" rating:

Acrylic has a rating of 0.3-1.1 ft-lb/in
Polycarbonate has a rating of 0.9 - 18.0 ft-lb/in

The higher value of the polycarbonate means that it can take an impact hit much better than acrylic.

Andy B.

14-07-2008 10:44

IKE

Re: pic: Team 2614's Cracked Plexi

Yeah the plexi(acrylic) vs. lexan(polycarb) mistake is a bad one.

We had some high stress pieces of 3/16 lexan on or robot this year that began to break after the first regional. We replaced them with 3/8" ??? Clear stuff. They snapped in the first match at the second regional. Turns out they were plexi.

The solution ended being going back to the thinner lexan and paying closer attention to surface cracks. A bandsaw will leave gouges perpendicular to the material thickness where-as a dremel sanding drum will make the gouges parallel to the thickness and thus slightly tougher.

Noticing that this looks like a non-structural body panel, here is a little secret:
McMaster Carr sells polycarbonate film (0.02-0.04"). Attached with velcor, this makes very puncture resistant "armor" without adding much weight. Be careful if you cut sharp inside corners into it as this can initiate a tear quite easily. Also, going much less that 0.020" is pretty thin stuff and thus not terribly strong.

14-07-2008 13:44

Dan Petrovic

Re: pic: Team 2614's Cracked Plexi

One of my friends was at a hardware store when someone asked an employee what the difference between Plexiglass and Lexan was.

The employee didn't say a word. He grabbed a hammer, a sheet of Plexiglass and struck it. The plexiglass shattered. He grabbed a sheet of Lexan and hammered on it repeatedly with no effects.

That was an amusing story.

14-07-2008 15:15

EricH

Re: pic: Team 2614's Cracked Plexi

One year, 330 had a plastic-encased robot. Lexan wedges (pre-no-wedge rules), and a tower that had thinner Lexan on 3 sides and acrylic on the fourth. We replaced one panel all season. One good hit from a robot at our first regional of the year, going up a wedge, broke it. After that, there wasn't acrylic on the robot...

14-07-2008 16:03

IKE

Re: pic: Team 2614's Cracked Plexi

That is the exact procedure I recommended to another mentor on our team on how to distinguish acrylic from LEXAN.

14-07-2008 18:24

smurfgirl

Re: pic: Team 2614's Cracked Plexi

We had a plexiglas cover over our electronics board (mounted vertically at the back of the robot) on our 2006 robot. In that position, it naturally took a lot of hits, and it looked like that after the first few matches... we learned our acrylic vs. polycarbonate lesson that way, too.
Everyone else gave you good advice, so I won't repeat it.

14-07-2008 18:58

R.C.

Re: pic: Team 2614's Cracked Plexi

Our Shop teacher laughed and put each one through a table saw. The acrylic shot out and cracked and polycarbonate was cut with no problems.

14-07-2008 20:30

Akash Rastogi

Re: pic: Team 2614's Cracked Plexi

umm...don't do that ever again. lol

14-07-2008 21:13

R.C.

Re: pic: Team 2614's Cracked Plexi

Yeah we weren't planning to, but we had safety glasses on =).

Thanks for the reminder

15-07-2008 07:49

Daniel_LaFleur

Re: pic: Team 2614's Cracked Plexi

This was the exact test I showed my team this past year.

1 shot with a hammer and they were convinced that polycarbonate is the way to go.

15-07-2008 11:28

Dan Petrovic

Re: pic: Team 2614's Cracked Plexi

...are there any situations where acrylic is more useful than polycarbonate?

unless the part is designed to shatter, of course.

15-07-2008 13:29

Francis-134

Re: pic: Team 2614's Cracked Plexi

While I'm not an expert on plastics, I have found that the only advantage to plexi over Lexan is cost (a cursory inspection of Wikipedia confirms this). Plexi tends to cost much less than Lexan.

For example, the large tank's viewing screen at the Atlanta Aquarium is made of acrylic not Lexan because it's never going to come under any significant shock loads and will never risk fractuing. If it were made of Lexan, it would probably cost MUCH more money.

15-07-2008 15:18

SlaminSwimster

Re: pic: Team 2614's Cracked Plexi

Yeah we definitely learned our lesson about plexi. But a broken shield in exchange for winning the Pittsburgh Regional- totally worth it!!!!

15-07-2008 15:39

Tom I

Re: pic: Team 2614's Cracked Plexi

Yeah my team tried weighing this benifit against the disadvantages in the 2007 build season. we needed 3 large shields for our robot sides, so we decided that we would go with 1/8" Acrylic to make them for cost benifits. Luckily we were able to fix this mistake before ship, when the shields started cracking while simply practicing! We replaced them with 1/8" Lexan, and we never had a problem durring the season.

Our team has a motto... although it is not true, it is still funny... "Lexan NEVER breaks!"

15-07-2008 17:55

Akash Rastogi

Re: pic: Team 2614's Cracked Plexi

Congrats on that btw. highly impressive for a rookie team to win a regional.Come to the NJ regional next year

15-07-2008 19:25

R.C.

Re: pic: Team 2614's Cracked Plexi

Yes it is, congrats again from 1323

17-07-2008 17:14

SlaminSwimster

Re: pic: Team 2614's Cracked Plexi

Thanks 4 the congrats guys!!!

18-07-2008 00:10

Cuog

Re: pic: Team 2614's Cracked Plexi

We also played a fun prank on some of the freshmen on my team. I brought out two pieces of clear plastic, handed one to them and told them to try and break it, I took the other and bent and snapped it. They were all afraid of me for a little while till someone explained the different between the piece I gave them(lexan) and the piece I had(acrylic)