Plastics
 

Hi all,

We're looking into using plastics for FRC this year - we've used sheetmetal of various aluminum alloys in the past, waterjet and bent to specs, for our robot.

This year we're interested in making some flat parts out of sheets of plastic (with patterns waterjet cut).

We were wondering what sorts of plastics are good for this - one main application we envision is using plastic for the our drivetrain bed - that is, the electronics bed.

We're obviously trying to move in this direction to minimize weight, so we'd like to hear about strong plastics (w/ associated recommended thicknesses) that are also lightweight.

Thanks!

Re: Plastics
 

G-10/FR4 is a fiberglass composite that is light weight and stiff (a poor man's carbon fiber). I believe 973 used it one year for an electronics bedpan

Re: Plastics
 

It is very hard to prescribe a good type of plastic. Every plastic has it's pros and cons. You need to find the one that best fits your application. Please give us more information and we will be able to help you loads more ;)

Re: Plastics
 

Plastic for a bellypan is a bad idea. It isn't very rigid. I know that our bellypan is really important in our drive performance and would not work if it was plastic. However, I've heard that garolite is also a good Al replacement for bellypans too and is quite light, however it is difficult to machine.

We really like using polycarb. We use mostly 1/16" and a bit of 1/8". It waterjets fine and if you get smoked polycarb it looks great. We also usually bend it if we need a bit of rigidity. Bended polycarb makes a great replacement for AL as it can be really rigid, yet is light.

How thin al are you using?(Specifically on for the bellypan. Is the bellypan pocketed?) If you have some good sheetmetal capabilities using thinner aluminum(.06 and .09) could possibly be your best bet.

Re: Plastics
 

Delrin is nice. It is a great plastic and acts like a bearing because it is so smooth. We used delrin for many different parts, especially within our shooter, where we have high speed objects

Re: Plastics
 

We've used ABS fairly successfully for non-structural electronics boards in the past. However, at the thicknesses that are easy to work with (1/16" is what we usually do), it's essentially useless as a structural member.

We waterjetted some Delrin slides for an elevator this year, which was fairly successful (in terms of the machining quality). ABS and polycarb waterjet nicely too. In fact, it's hard to find something solid that doesn't machine nicely on a waterjet, so that shouldn't be your first concern in terms of your choice of plastics.

You might want to look into high-quality plywood bellypans. They can be made quite thick (and hence rigid) for very low cost. They're also really easy to machine, and can be used with self tapping screws to hold down components. Of course, a nice powdercoated and waterjetted aluminium bellypan might look nicer, but (especially if you're paying for waterjet time) it's a heck of a lot more expensive.

Re: Plastics
 

ABS is nice because it can be 3D printed!

Re: Plastics
 

http://www.chiefdelphi.com/forums/sh...390#post984390

Re: Plastics
 

I cant recommend using this for a bellypan though, that stuff is heavy. We used hdpe on year and man did it weigh in.

Re: Plastics
 

We've used Polycarbonate as a substantial part of our robot construction for several years now, without any major issues.

Initially we used it only for the belly-pan of the robot to mount electronics on, but the past few years we've made drive bases, gearboxes, shooter frames, and more (We've also used it as a frame for a grabber arm, but I don't recommend this because it's not the most stable).





These systems work great for us and are quite a bit lighter than the aluminum-frame robots we used to build.

As far as thickness, we've used a variety ranging from 1/16" to 3/8" depending on the application. Keep in mind that thinner material can usually be made fairly ridged if supported in the right places (like the belly-pan in the first image above).

Re: Plastics
 

Yep. That is a drawback of delrin, however, if you need a smooth surface, delrin will cut it, most of the times!

Re: Plastics
 

We had good luck with a composite material that was 2 thin sheets of aluminum (about .010" thick), bonded to either side of a corrogated plastic core. The trade name is "Alumalite" and it is used in the signmaking industry. It was very light, and had good stiffness. The only downside was that the corrogated core would collapse under concentrated loads. We used this material for our belly pan, so the solution to the point loading was to use c'sink head screws, and they would self-countersink as we tightened them up, being careful not to over-torque them. Velcro also worked well to avoid the issue.

We also used it for our shooter deck sandwich. It was light and lent itself well to waterjet cutting. Later, we found that there is a solid polyethylene core version called "Alupanel". It solves the point-loading problem, but is a good bit heavier than the corrogated core. Both are available through sign shops (always ask for donations, they may have cutoffs) or online.

Re: Plastics
 

As others have said so far, Delrin (Acetal), Lexan (Polycarbonate), and ABS are all pretty common plastics that are fairly easy to work with. Composites like Fiberglass, Garolite, etc are also well within the range of most FRC teams but require some special considerations when being machined and used, specifically ventilation, etc.

My personal method for deciding whether or not a part can be made out of plastic is based on a bit of previous experience and some guessing. One of the biggest considerations is the purpose of the part and how strong it needs to be. If it's something that needs to hold relatively precise geometry, but doesn't necessarily need to be strong, Plastics are a pretty good option and can be easier to work with than Aluminum. The common trade off is that you're going to need more (thicker) material to get a part that is strong enough, which means that you could get into situations where a plastic part is the same weight as a part made of Aluminum and is as strong.

One of the other nice things about plastic is that they're really useful when something needs to be a specific thickness for one reason or another. We've done bearing blocks and things along those lines from 1/4" Delrin before since they didn't need to be strong, but did need to keep the bearing flush with a surface.

There's also the added bonus of elasticity, should you need to have a part that has a known amount of 'give' or impact resistance before it permanently yields. Both lexan and delrin are pretty handy for this purpose, since they're fairly good at taking a beating if they're outside of the frame perimeter.

Re: Plastics
 

Also, pay attention to the possibility of ESD when using plastics for electronics mounting. A fellow inspector and I witnessed a team last year at Chesapeake having strange issues caused by their mounting board.

A sheet of 0.063" aluminum with some dimple dies may be lighter and thinner than a plastic of the same stiffness and load capacity.

Re: Plastics
 

Like many of the posters above have stated the exact application drives the plastic would want to use.

For electronics this year we are looking at HDPE and similar sheet plastics that we can mill easily. We also 3D print small parts in ABS.

In the past we have used corrugated fiberglass sheets for many applications including drive train pans, electronics, and one year ramps for other robots to drive onto. It has good stiffness but one needs to be very careful in mounting it using washers and other things to make sure stuff does not pull through.

Re: Plastics
 

1370 used Alumalite for our shooter deck and some other portions this year. It worked very well, but, as HumblePie noted, the corrugations will crush if you get aggressive with the fasteners. Of course, we had some students crushing our 1x1x1/6 aluminum tubing in our frame, so a little plastic was no problem at that point. :yikes:

I would not recommend Garolite (G10) or FR4 because of the health hazards and precautions recommended for machining them. If you are going to work with them correctly, you need ventilation for any dusts and masks/respirators for the operators. At the very least, that was why my former employer had outside machine shops make our G10 parts.

1370 used 1/8" thick High-Strength PVC (perforated) for our electronics deck this year and it worked out very well. We got ours from McMaster-Carr and it is p/n 92985T51. We got the idea from 1089 in 2012 who used perforated polycarbonate on their robot that year.

Re: Plastics
 

We use .125 aluminium composite material (ACM) for belly pans. The material is two sheets of aluminum sandwiched around a sheet of plastic. The plastic center allows the material to be easily routed and bent to our specifications. If you wanted to use plastic I would go with .125in thick lexan or uhmw polyethylene. AndyMark sells some perforated material that always looked cool to me as well for bellypans if you wanted to go that route. Of course if you really wanted to get fancy they do make honeycombed aluminum and plastic sheet material as well.

Re: Plastics
 

I've used approx. 3 mm Alupanel in an FRC robot for the structural belly pan. It was alright, but ultimately needed lateral ribs that the quasi-monocoque design didn't provide for.

Its stiffness was better than plastic of equivalent thickness, but quite substantially less than aluminum—and although the solid core made it easy to rivet down, it meant that the weight savings were quite modest.

It cuts decently well on a CNC router, which is what we used.

If I had to do it again, and I really wanted to optimize the part for performance, I'd find a suitable sheet of fibreglass composite sandwich panel (carbon fibre is perhaps too expensive for FRC), or make one from raw material. (There's a fair bit of up-front expense to make these, but you can always make tooling and refine the process in the off-season, and those costs don't count against the limits.) If you make your own, you can lay it up with a lightweight honeycomb core in the centre, and rigid plywood or foam cores where you need to attach mechanical fasteners.

More ideas:
http://www.chiefdelphi.com/forums/sh...d.php?t=121911
http://www.chiefdelphi.com/forums/sh...ad.php?t=98779

Re: Plastics
 

You need to contact More Robotics in Milwaukee. They build the entire robot from plastics since a mentor/sponsor, Mike Whitman owns a plastics firm.

Re: Plastics
 

I'm curious, what kind of loading dues the bellypan support in a traditional WCD? If I say the X-direction is the direction of forward travel, Z is up and Y is the cross product of X and Z. Am I looking at torsional loads in the Z axis or the X axis? (Or heck, even Y?) what kind of loads are typical?

I ask because we're exploring a WCD and I'm contemplating material choice for a bellypan. I've heard Travis say they use 3/16 Aluminum (I think it was a 5052 alloy). It looks like 254 just uses an 1/8" sheet of presumably 7075-T6?

We've got a couple sheets of .050" 2024-T3 and .090" 6061-O. I'm contemplating sandwiching them into a flanged isogrid, but now I'm wondering if that's even necessary. I would use the 2024 as my flanges because of the additional shear strength. The stack up would be 0.19" thick, but now that seems like a bit too much.

Re: Plastics
 

Not only does 1714 build their robots entirely out of plastics, they are also perennially successful. Their 2012 robot was incredible, and their 2013 robot was one of the more successful climbers in Archimedes this year.

Re: Plastics
 

We use our bellypan for shear and torsional strength. We used 1/8" 6061, which worked well, but 5mm baltic birch should work just as well if you are looking to use less resources.

Re: Plastics
 

We used 1/8" 6061 aluminum sheet that we routered out in a rather minimal diamond pattern, leaving mounting locations for electronics and things, and it came out to about 3.7 pounds if I recall. It was quite stiff.

I like doing this because the bottom pan serves as the welding jig for the frame rails and crossmembers. It helps line everything up. We rivet the side rails and crossmembers to the bottom pan, then weld the joints.

In 2012 we used 1/8" ABS which was okay, but a little soft and flexible. We had to brace it.

Re: Plastics
 

3/16" for a bellypan would be a bit overkill..

Also a 1/8" sheet of 7075 would be quite expensive (well maybe not crazy expensive, but you could probably get two 6061 pans for a 7075 pan price wise) and overkill. 254 and many of the westcoast teams use 6061-T6 bellypans pocketed @ .125" Thick.

971 does a cool job of utilizing a turret punch to make their belly pan:

https://picasaweb.google.com/1177698...001343255 282

I believe its .060 or .090 5052 AL. Travis/Austin/971 can clarify.

-RC

Re: Plastics
 

Thanks, I just assumed they were using 7075 because they use it for so many other things

Re: Plastics
 

Torsional strength about which axis?

Re: Plastics
 

I would not advice anybody to use/machine G-10 unless they really had a need and that it is the best material. For FRC robots poly carbonate works for most applications.

From the G-10 MSDS:
“Dust generated during grinding, cutting, or drilling fiber glass reinforced plastic produces respirable fiber shaped plastic (organic) particles whose concentration increases proportionally with dust concentration. These particles are not classified as carcinogenic by IARC or NTP. However, prolonged inhalation of dust can produce lung disease.”

Re: Plastics
 

7075 isnt used that much. Mostly in places that steel would be used in industry like gears and shafts.