Will plexi-glass work?

[Adam Y.]

I agree, if you absolutely must use a plastic, use lexan.

I disagree there are few other plastics that are better suited for a frame than lexan. Polyethylene is probably one of the better plastics to use for frame materials. It is easy to machine and very durable. The names to look for are HDPE (same stuff used in the fields) and UHMW. My best guess is that the gearbox and mounts are some form of polyethylene.

[petek]

I'm sure that if you've gotten this far in this thread, you know that plexiglas (acrylic plastic) is not a good structural material. What might be missed is why you would use one material over another, and how you'd choose the "best" one for your application.

[A little disclaimer here: I am not a mechanical engineer or materials expert - but I do have a tattered degree from the school of hard knocks.]

First - the term "plastic" tells you something right off about these materials: they can deform, move, flow and/or creep by nature. This may be a good thing if you want low friction, impact resistance or high toughness. It also means that in general, their strength may not be the greatest. (There are exceptions to every rule, but we're talking generalities here.)

So, going back to a couple of plastic materials mentioned in this thread, what's so good or bad about acrylic and polycarbonate? Acrylic is great for clarity - you can polish it so it looks like crystal glass. It has pretty good machineability with the right tooling, too, but those tools require a different profile than what you use for metal or wood. It is pretty hard and stiff, but does break easily (notice I compared it with glass earlier). Personally, I really like working with the stuff, but I can't think of anywhere I'd use it on a robot!

Polycarbonate is what they make bullet-proof windows and jet canopies out of. You can bounce a chicken off it at very high speeds. Or another robot at somewhat lower speed. Great flexibility, too, in thin sections. Not so nice to machine, as it tends to grab the cutter and needs coolant to keep from melting, but sheets can be sheared and bent just like sheetmetal. You shouldn't expect holes tapped in it to hold a lot of force, either in tension or shear - use through bolts, nuts and washers. So this is good stuff for the bumpers, feelers, side covers and things that go bump on a robot.

Polyethylene (HDPE, UHMW, LDPE) is great as a sacrificial wearing surface, chain guides for example, and for chemical resistance. It's okay for lightly loaded bearings, but it's a pretty lousy structural material.

One way to look at materials for structures (like robot frames) is their strength to weight ratio. If we compare the ultimate tensile strenght divided by the density we get a picture of why people use steel for fasteners and really strong structures, aluminum for weight-sensitive structures, and plastics for the rest. (The numbers are the ratio of the UTS, in MPa, to Density, in g/cm3)

ASTM A574 Steel: 160
6061 Aluminum: 122
Polycarbonate: 58
Acrylic: 51
Delrin: 48
HDPE: 29
Teflon: 14

[End of lecture]

[MathewSmith]

well thanks for all your suggestions! i beat it into my team that we needed some kind of metal. we used alumminum and we are still using the plexi glass for walls and the base and the top. I suggested using the stuff that you guys told us to use...but we diddn't. and we are getting our electrical stuff mounted and all that. so i think we can have this done by thursday! i hope we can get it done! Thanks for your help!

Mathew Smith
Team 1352

[maclaren]

Plexi-glass I.E. Acrylic is EVIL it shatters even before you are finished machining it. Therefore it should be avoided like the plague. If you are put into the circustance when someone has a knife to your throat threatening to kill you if you don't use acrylic then the only answer is to use brad point drill bits. But that's about all you can do; Don't mill it, turn it, cut it with a saw, or do anything else because it will break.

However do not dispare there is an answer.

LEXAN, POLYCARBONATE is the answer!!

Tough, Strong, Clear, some varieties are bullet proof. But all varieties of lexan are better than plexi-glass.

[Aignam]

This is slightly tangential, but....
I heard a story that one team used lexan on their robot, and spray-painted it. And the paint and varieties of temperatures caused it to become very brittle and it cracked during their first match. Is there any truth in this at all?

[GregT]

Originally Posted by Aignam

This is slightly tangential, but....
I heard a story that one team used lexan on their robot, and spray-painted it. And the paint and varieties of temperatures caused it to become very brittle and it cracked during their first match. Is there any truth in this at all?

I'm not sure about "more brittle", but I'm pretty sure spray paint contains a solvent that will damage polycarbonates. I believe ketones (acetone) are the main problem (I know these are damaging to acrylics like Plexiglass). In general, polycarbonates are much more resistent to solvents then acrylics.

In general, keep organic solvents away from polymers (I believe there is a thread about this in the Control System forum).

There are spray paints specifically made for polycarbonates. Latex based paints would work (if they stick, I'm not sure).

Greg

[golf_cart_john]

Don't forget that wood is a very good option. Over the past 4 years, my team has made 2 robots with aluminum frames and 2 robots with wooden frames. The 2 with aluminum frames lost miserably. The 2 with wooden frames were our pride and joy, and played very well. The success level may just be coincidence, but we definitely did well with wood. Our team found some 9-layer plywood that works real well.

[ahecht]

190's 2k3 robot was almost entirely lexan, and it held up pretty well. The only structural metal we had in the chassis was the two pieces of channel that held our crab drive. If you use a combination of Lexan Glue (which chemically welds pieces together), bolts, and some aluminum angle on the outside corners, you can end up with a pretty strong frame.

[maclaren]

I have heard the same thing. But as for the specific case I don't know of one. It would make since to me since spray paint is very caustic. A good acrylic paint or latex paint should have the same proplem. The best guarentee would be to print of over head projection pages and adhere them to the inside of the lexan.

So be warned of compromising materials with chemicals.