Plexiglass limit - rationale

Just curious - what is the RECF’s/Game Design Committee’s rationale behind limiting the amount of plexiglass that may be incorporated into a robot? Part of me is curious to see what a team could do with unlimited plexiglass…


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My guess would be to try and even the playing field a bit between teams.

Cutting and bending polycarbonate requires tools that not all teams have (tin snips and a heat gun) and is harder than working with ordinary Vex metal. Allowing unlimited amounts of it would place teams that don’t have the knowledge, tools, or confidence to work with polycarbonate (or the money to buy all that polycarbonate) at a disadvantage. That being said, allowing a small amount of polycarbonate incentivizes teams to develop the skills to work with it, while not putting newer, less experienced teams at too much of a disadvantage.


I don’t know about the rationale, but let’s be clear that Plexiglass is an illegal plastic. Polycarbonate and other non-shattering plastics are legal, according to the list of legal plastics in the manual.


One of the main elements of VRC that sets it apart from other robotics programs is its extremely constrained environment. The limit on plexiglass is simply a manifestation of this.


also, if teams didn’t have to save their polycarb for the important stuff, you’d have teams who have more access and skill in machining build large portions of your robot with polycarb. So I do think the limit is in place for a good reason, even if it does annoy me at times. what does annoy me is the really confusing wording of the rule that makes life for inspectors hard. Why say you can only use polycarb from a single sheet, where all your components have to be able to tetris together? I understand it creates an extra level of design constraints, but wouldn’t saying 2 square feet be much easier on everyone? idk, just my thoughts.


Honestly yeah, but there would be no way to prove that you do in fact have exactly 2 square feet or less. Right now, you can at least show the inspector documentation of a template you made that fits within the space. At least with this years game, most polycarb pieces will be cut out in a rectangular shape to fit in a tray or some kind of cube holding mechanism. It will make creating templates a lot easier.

Plexiglass is a shattering plastic. There is a limit of exactly 0 square inches of plexiglass allowed on your robot, for a variety of reasons besides just danger.

The most commonly used non-shattering thermoplastic in vex is polycarbonate (brand names lexan, makrolon, etc.).

You can use any from a list of allowed non-shattering thermoplastics (Manual, R9), and the most significant reason they are limited to 12x24" in my opinion is that an unlimited amount could lead to custom polycarbonate parts being sold, which would be quite strange and unwieldy for this competition. Remember, by allowing an unlimited amount of sheet thermoplastic, you would be allowing essentially a raw material in unlimited capacity, which can lead to a whole line of possible products and services (think gears, etc. that are not even custom-made for vex) being legal for use–not a great move. The reason this is not such a possibility right now is because it would be difficult to sell a product that people are inherently limited to use. I remember a few years back when someone was selling a small number of laser-cut polycarbonate C-Channels; rest assured, they weren’t so successful.

Of course, the biggest rationale that supersedes the above reason is that it is a good constraint upon the design systems available in vex–these custom products and other derivatives would not be a good way for students to learn.


Unlimited polycarb would be crazy in VRC HS.

With Delrin I could make a whole lot of other custom parts.

Though to my eye, and maybe I’m wrong, I think some of those videos from China had robots that were using more than the 12x24 polycarbonate allowance.


I think so too. The surface area for a 3 stage tray has to be really close if not more than the allowance.


lets not assume
lets assume they use 4’’ for the width (im being generous )
does anyone have an exzact count of cubes and breaks in their system or the vevo
we can math this out

I have a 2 stage tray with lexan backing. If each strip was 2/3 the current size then yes you would have enough for 3 stages.
However, if all strips are 5.5 inches or wider and span the length of the 35 hole c channel, then they are probably using too much

lets map it out

Honestly this seems awfully like a witch hunt.

There is (supposedly) a team, somewhere on the other side of the world from where I assume most of us are, that may or may not be using slightly too much polycarbonate.

Why would you all care? You will likely never compete with this team. You will likely never have any interaction with them. “Exposing” them on the forums here does nothing to help anyone.

Let the inspectors in China (and everywhere, really!) do their jobs. Have trust in the teams to follow the rules. None of this affects you, so you have no real reason to involve yourself in this potential dispute that may have happened between a team and an inspector that are on the other side of the world from you.


I don’t think this is about any specific team at all, no one mentioned any team number. All we’re saying is that trays take up a lot of polycarbonate which can be frustrating due to the limit. To make this more productive I guess is what would be the most effective use of poly carbonate and how would you ration it effectively.

People were simply contemplating the ethics behind going over the limit even if it is slight. In general this will affect everyone even if it isn’t directly because there are teams that follow the rules exactly while there are others who don’t. Just because no one gets caught doesn’t make it okay.


That seems like it’s mentioning a specific team, or set of teams, to me. While @OscarMNOVA12’s comment wasn’t bad in and of itself, subsequent calls from other community members to “map it out” and “math this out” in reference to the amount of polycarbonate on some Chinese team robots, presumably in order to catch them breaking the rules, seems pretty witch-hunty to me.

That being said, the rest of your post I completely agree with. Going over the limit is bad. We need to find ways to build our designs within that limit. But I don’t think we should be taking it upon ourselves to try and “expose” teams over rules violations that we have nothing to do with.

I mostly just wanted to steer the conversation away from this kind of behavior, considering there’s been quite a bit of what could be considered anti-China bias/viewpoints on this forum before.


If we don’t do it then who will? At competitions, you are allowed to question the legality of a robot on the field. Maybe a team is considering this design or something; I think it should be talked about if they feel that because other teams have gotten away with it then maybe they can too. I want to discourage this kind of culture.

Also, it is not just Chinese teams though they have pioneered the three-stage trays so as of now it is mainly the chinese teams that are used as examples. I highly doubt an inspector has gone out of their way to measure how much polycarbonate is on a robot, rather they eye ball it or ask the team (at least that’s what was done for me). No one is exposing any team in particular, nor was any match specifically pointed out. I do think that it is worth calculating the area of polycarbonate on a three stage tray for future reference. This way you can build accordingly.

Assuming you use a full-length c-channel (35 holes) and go 14 holes wide (7 inches across counting the holes that’ll be mounted to the tray “rails”) (the comfortable sizing for a tray intake), the area comes out to be 119.52 square inches. Multiply that by 3 and you get 367.5 square inches. ( 17.5 in (full length c-channel) * 7 in (14 long c channel) * 3 (three stages)). According to rule the legal amount is 288 square inches (12in x 24 in).

So yes, it is over the limit assuming they used those dimensions which I believe to be reasonable. A cube is 5.5 inches across therefore this design leaves you with 12 holes to work with or 6 inches, essentially .25 an inch on either side of the cube in an ideal scenario. Correct me if I’m wrong.

A solution to this problem: zipties. I’ve seen a Chinese Finals match of a really good 3 stage tray robot that used zipties for the backing of their tray. It seemed to work well though there wasn’t any close up or anything. Regardless, zipties can be a good supplement to the polycarbonate limit. I highly suggest you look into using zipties, otherwise consider an aluminum plate or standoffs maybe. Zipties are the lightest however.


The reason I mentioned China is because that is where the majority of match videos with tray bots are from, so it’s where we get the best look.


Read A

This has been covered in the thread before, some just use “plexiglass” to collectively refer to the vex-legal plastics

But then people would search for and buy plexiglass instead of lexan so we should try correcting them even if they mistakingly say it. I have found a team or two at my competitions in the past who puts plexiglass on their robots by mistake likely because someone said Plexiglass instead of Lexan to them.