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Unread 07-06-2017, 04:51 PM
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Fear of Plastic

I've heard from many teams that they try to avoid plastic parts on principal. When I ask why, most reply "because we don't trust plastic", or something along those lines. In my experience, teams tend to avoid plastic parts for non-engineering reasons.

There are a variety of plastic parts that serve to replace metal in many places on a robot. Plastic wheels and hubs are among the first that come to mind, but VEX also has plastic VersaFrame stock, and plastic gears are available from many vendors (although I haven't seen plastic gears used widely on FRC robots).

I can understand the "engineering reasons" why teams shy away from plastic, because there are obvious design tradeoffs with durability being the most significant in most cases. But are there legitimate non-engineering reasons to avoid plastic parts?

Does your team make a point to avoid plastic parts? If so, why?

If you do use plastic parts, what parts, and what has been your experience?

Would your team consider using plastic parts more widely (such as for power transmission with plastic gears) if they were demonstrated to be effective?

What other uses for plastic parts do you think would be effective? Plastic hardware? More plastic structural options?
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Unread 07-06-2017, 04:59 PM
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Re: Fear of Plastic

Ive seen plenty of cool 3d printed parts on robots, from camera mounts to minor mechanisms to claw style grabbers ect.

Plastic has its uses if you're smart about it. Pretty much for me if I expect it to take a load I shy away from plastic, because having something break mid event is no fun and why take the risk

"Plastic" itself is a bit of an umbrella term though, as there are many different kinds, some stronger then others and some have properties better suited for certain uses. I've used a slicker low friction "plastic" on a floor intake once as a skid of sorts.
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Unread 07-06-2017, 07:47 PM
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Re: Fear of Plastic

Most wheels are plastic. Polycarbonate is used by all the cool teams. The roborio has lots of plastic. It is easily fabricated. Lite weight. Metal has advantages in high stress places, but a lot of places plastic is a better choice.
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Unread 07-06-2017, 07:57 PM
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Re: Fear of Plastic

Quote:
Originally Posted by Ginger Power View Post
I can understand the "engineering reasons" why teams shy away from plastic, because there are obvious design tradeoffs with durability being the most significant in most cases. But are there legitimate non-engineering reasons to avoid plastic parts?
Most? I'm not so sure. A polycarbonate part is more likely to bend, but it's also more likely to spring back. That's a good thing when a mechanism is expected to take a whack, like when they extend beyond bumpers or interface with field elements closely.
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Unread 07-06-2017, 08:05 PM
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Re: Fear of Plastic

I like plastic.
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Unread 07-06-2017, 08:13 PM
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Re: Fear of Plastic

Actually, HDPE is REALLY good at being a strong plastic,and it has more give than metal. 254 Used something like HDPE (not exactly sure) for their fuel intake, for it allowed the mechanism to 'bounce back' into shape. Metal would bend and get dented in these kinds of applications.
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Unread 07-06-2017, 08:14 PM
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Re: Fear of Plastic

Quote:
Originally Posted by FrankJ View Post
Metal has advantages in high stress places, but a lot of places plastic is a better choice.
Exactly. Shooter hood? Probably polycarbonate. Camera mount? 3D printed. Electronics board? Polycarb, wood, sheet metal. Climber? Metal. Drivetrain gearbox? Definitely metal.

We don't intentionally avoid plastics as far as I know but I don't think we would use them very extensively yet. We need more experience.

This year, we used 3D printed camera mounts and polycarb for the sides of our ball hopper and gear mechanism. We also 3D printed spindles for out climber hit they never had to bear weight, only guide the rope to the right spot. Shooter hood was sheet metal but easily could have been polycarb. Electronics board was some non conductive material (forget the name off the top of my head) with sheet metal supports. I think that most of the other stuff was metal.

EDIT: Electronics board was masonite with 1/8" aluminum supports.

We would probably consider using more plastics since they are much lighter than metals but we don't have the experience or resources to do so (yet). For example game piece manipulators would benefit from plastics.
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Unread 07-06-2017, 08:17 PM
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Re: Fear of Plastic

Quote:
Originally Posted by messer5740 View Post
Actually, HDPE is REALLY good at being a strong plastic,and it has more give than metal. 254 Used something like HDPE (not exactly sure) for their fuel intake, for it allowed the mechanism to 'bounce back' into shape. Metal would bend and get dented in these kinds of applications.
Almost certain they used poly carb. In my limited experience with HDPE, it likes to bend instead of "bouncing back".
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Unread 07-06-2017, 08:35 PM
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Re: Fear of Plastic

Is it too early for the 2018 FRC Parody competition?...

HDPE took an bend, but polycarb Bounce Back
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Unread 07-06-2017, 09:43 PM
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Re: Fear of Plastic

Plastic is a material. Just like aluminum, steel, carbon fiber, cardboard, and balsa wood. They all have applications and properties; get it wrong and things break or otherwise not work as intended.

A lot of it is design as well. For example, sharp cuts/edges on brittler types of plastic are just asking for cracking. Rim failures on some types of wheels/pulleys/etc., especially in 2016, were a good showing of this limitation. If you need sharp cuts and are expecting any significant impact or strain, pick a less brittle material or else find a way to chamfer/smooth your design.

Also keep in mind that many materials have a grain of some sort (including metals in some cases, which is why forging is usually more robust than plain machining). In the context of plastic, 3D printed parts come to mind, especially extruded parts (but also anything built layer-by-layer). The grain can work for you, but also against you. It's just another design factor that can be overlooked and cause problems.

TL,DR: Plastic things can work great, if they're designed right and are not pushed beyond their limits. They can also fail miserably if the design is poor and/or the item is abused.
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Unread 07-06-2017, 09:46 PM
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Re: Fear of Plastic

Not sure if it quite counts as "non-engineering reason", but we avoid 3D printed parts (perhaps not plastic in general) because they're a pain. Our 3D printer is quite slow (any reasonable part will take >8 hours), and by school policy, requires that someone be in the room with it at all times. Many times it's faster for us to machine it ourselves, or in the case of bigger parts, to send it off to one of our sponsors to get laser-cut.

Our bots are very heavy because of this.
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Unread 07-07-2017, 01:56 AM
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Re: Fear of Plastic

I'm just waiting for 1714 to weigh in--anybody see them lately?

Now, who's that team that 3D-prints most of their robot? I know it's not 5817 (though they did 3D-print significant portions of their drivetrain as I recall), it's someone on the East Coast/Smoky Mountains area.


We use plastic when appropriate to do so. Just remember: Before using the random clear piece, clamp it to the table and hit it a good one with a hammer, just to make sure it's not acrylic.
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Unread 07-07-2017, 06:48 AM
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Re: Fear of Plastic

Excluding shrouds (of the roborio, enoders, control board covers), sponsor signs, insulation/connectors, labels, internal gears of PG and NR gearboxes, pneumatic tubing and some fittings, and wire management (panduit, cable chain, zip ties), and bumper fabric:

Wheels: 2012, 2014-2017
Rollers: 2012-2014, 2016
Belts, drive train: 2015, 2017
Belts, manipulator: 2012-2013, 2017
Manipulator Structure: 2012, 2017
Air tanks: 2013
Gearbox housing: 2013-2017
Control board: 2013-2017
Velcro and/or Dualock: 2012-2017
Spacers: 2012-2017

We have not yet used plastic for the robot structure, but most other places where it makes sense, we have. I did not include the year we had a manipulator made of steel with a plastic coating (truck bed liner).
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Unread 07-07-2017, 07:15 AM
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Re: Fear of Plastic

More teams and designers should design parts where their strength is derived from their cross section as much as their material. All too often people draw something, it breaks, and then they go "well, let's remake the exact same part, except out of steel!"

Almost any practical material can be a part of a successful FRC robot, if properly designed for. Various plastics are no exception.
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Unread 07-07-2017, 07:28 AM
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Re: Fear of Plastic

Quote:
Originally Posted by Ginger Power View Post
I can understand the "engineering reasons" why teams shy away from plastic, because there are obvious design tradeoffs with durability being the most significant in most cases. But are there legitimate non-engineering reasons to avoid plastic parts?
(Emphasis Mine)

Allow me to introduce you to the magic that is the 2008 Bomb Squad Intake. https://www.youtube.com/watch?v=vtMPpETrRNM

I've seen that thing rotate 180 degrees around, bounce back and pick up a ball. I saw the same system in 2014 get pounded with minimal failures (JT also nearly broke my nose with that bot, but that's unrelated) Imitated it on 79's intake this year, despite being rammed into walls and other robots, it was never swapped out during 4 events.

I think there's a discussion to be had in the precision of language - Durability to me means the part retains functionality over time, to me plastics do this better in a lot of cases. Perhaps you meant, retains shape which I will give you plastic parts tend to deform sooner than aluminum parts of similar weight. They also tend to return to their original shape which is beneficial in some circumstances.

There's legitimate engineering reasons not to use plastics in some applications. I wouldn't use plastic axles in FRC (though I've investigated it in non FRC applications) I wouldn't use them in places where you need the thing to retain it's shape during an impact, such as gear teeth in an 2 jointed arm (otherwise known as how got sprayed with gear teeth in 2007) But even in those circumstances you could make them work, but it's probably easier to just use metals in those cases.
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