This discussion started in another thread, but I think it’s interesting enough to warrant its own thread, so I’m kicking it off here…
The question is, what plastic sheet stocks are best for robot chassis applications? I have noticed that this community is decidedly pro-lexan, but there are many, many other options out there, so let’s talk about them. To keep cost comparisons apples-to-apples, let’s assume 1/8" x 12" x 12" sheets.
When looking at the different plastics out there, keep a few things in mind. First, some plastics are better at some things than others, meaning many of them are engineered to be good for a specific purpose or purposes. Often when a plastic is really good at one thing, it compromises somewhere else. Of course some are general purpose and work well in a lot of applications. Cost is also a factor - hobby roboteers are not going to spend $300 on a small sheet of PEEK, but any of the plastics in the $5 - $20 a sheet range are in play.
So let’s talk specifics. Below are the plastics I have direct experience trying to build with and my opinion on them.
MDS-Filled Nylon 6/6: This is a nice looking dark gray plastic with a metallic shimmer to it. On paper it has good properties for the price (about $11). It’s very hard and strong.
Unfortunately, it is prone to warping. My sheet was warped, and when I complained, I got a slightly less warped sheet. They said it’s all warped. Maybe it could be straightened, but I wouldn’t trust it to stay that way. Tis a shame because it’s otherwise a great material. I’m going to stay away from nylon-based sheets from now on; however, I have had good experience with molded nylon small parts and will continue to use them (like the Lynxmotion hex standoffs).
Polycarbonate (Lexan): This is the plastic used in the Lynxmotion kits, and it’s used for RC car bodies (because it’s strong, and comes in clear, so that painting the underside gives a durable shiny finish). It’s quite stiff, is straight and true as an arrow, and is cheap ($6). It’s shiny and scratches easily, so you have to random-orbit sand it to get a more durable satin finish.
My personal opinion is that lexan is somewhat over-hyped due to its use as “bulletproof” glass, which means it has a very great point-impact strength for a clear plastic. Remember, when it’s great at one thing, there is usually a weakness, and lexan is not without limitations. It’s well known that lexan is damaged by common chemicals used in hobby projects, including thread lock and super glue. This is a problem for me since I’m addicted to Loctite. Also, lexan is prone to cracking and chipping under stress (power tools), but that can be true of any hard plastic if you are not careful. Despite these criticisms, the combination of strength, rigidity, stability and light weight make lexan a great choice for a robot chassis.
Acrylic (Plexiglass): This material is used in a lot of the same ways as lexan, and it has similar properties. It’s straight, stiff and strong. It’s inexpensive. It is more brittle than lexan, and also stratches easily. Acrylic could be a good option for someone wanting the good properties of lexan without the chemical interactions.
PVC Type 1 (vinyl): This a low cost plastic that’s merely ok. It’s dense and it feels heavy. This stuff is everywhere, so most people have probably used it for one thing or another at one point. But my feeling is that if you can afford a $4 sheet of plastic, you can afford a $6 or $10 sheet of plastic that will be lighter and stronger. I can’t see any reason to use this over some other commonly available options unless you have it lying around.
PVC foamboard (Sintra): This is a very light weight and stiff but weak plastic, which melts if you look at it the wrong way. It’s not at all suitable for a robot chassis in my opinion unless the robot is tiny and weight is extremely important, but even then I would just go with a thinner stock of a more durable plastic.
Acetal resin (Delrin): This is a strong and stiff plastic, but it’s noticably heavier than the others (except PVC Type 1). No warping, and it holds its shape. It cuts and drills really smooth, probably due to its density. I think this is a great choice for a larger bot chassis that mixes metal and plastic. This is what I’ll use for my tracked vehicle chassis. Cost is a bit more than lexan ($9).
ABS: This is a light weight plastic that’s relatively strong. It’s flexy and warps a bit. Cost is about $2 more than lexan. It’s a decent choice for small lightweight bots. I may end up using this for a mini-sumo bot.
Polyethylene UHMW: From EddieB, who used some surplus in his ROV project: “I wouldn’t recommend it really as it is somewhat soft and really hard on tools. On the positive side it’s easy to self-tap threads and it doesn’t tend to chip when you’re cuting/drilling it either.” Looking at the specs on it, it’s not very strong and costs more than 3 times as much as lexan in small quantities. Methinks it’s not designed for this application.
I wanted to also include this good advice from spacejunk on working with plastic: “Try a plastic drill bit for drilling plastic. They have a negative rake angle and scrape rather than cut. Also drill at a slower speed and make sure the bit doesn’t melt the sheet.”
Please feel free to share your wisdom and experiences with any of the above or talk about other plastics you have used or plan to use.
Yes, you need to slow down the speed a bit, but there’s no trouble. No I have gotten it to melt with a reciprocating saw once. But it’s not the tool of choice for robot builders anyway.
it holds long enough usually for the cut to begin to heal shut, depending on the cleanness of the cut.
