Molding Custom Wheels?

[EricH]

Originally Posted by JesseK

Team 357 (The Jesters) has some experience molding smaller parts. They've made their own custom mecanum rollers for years now. If you have a copy of the FIRST: Behind the Design books, they're in 2007 (I think) and describe how they made quick molds for things like their battery holder, mecanum rollers, and even a 80/20 UHMW plastic linear slide.

They make the whole wheel... And they're one of the few teams to be in both Behind the Design books. 2006 details their wheels; 2007 goes after everything else. As I recall, they also went for a patent on the linear slide (which isn't UHMW; it's urethane, polyester, and a molybdenum compound molded into shape).

[JesseK]

Originally Posted by EricH

They make the whole wheel... And they're one of the few teams to be in both Behind the Design books. 2006 details their wheels; 2007 goes after everything else. As I recall, they also went for a patent on the linear slide (which isn't UHMW; it's urethane, polyester, and a molybdenum compound molded into shape).

I thought the patent was for their battery holder.

Meh, it's in the book which is at home.

[EricH]

Originally Posted by JesseK

I thought the patent was for their battery holder.

Meh, it's in the book which is at home.

Double whammy. They were going for both, I think. *goes to the other room for family copy of 2007's book* Yep, that's 2 patents, one for the battery system, one for the linear bearings.

[JesseK]

I spoke with some guys at a conference yesterday and had the pleasure of discussing robotics design with a guy from American Plastics Manufacturing based out of southern IN. They specialize in injection molding. There are a few rules of thumb he described and this particular wheel appears to break two of them: Keep a UNIFORM wall thickness, and have NO sharp corners.

Injection molded plastic has a flow direction, and all polymer compounds shrink 0.5%-3.0% depending on the compound. Most likely you'd go with a crystalline-type plastic material for the wheels, which also has unpredictable shrinkage. This shrinkage causes a variable wall thickness injection to shrink at different rates, thereby warping whatever item you're trying to make. When the variance is too great, many times the thicker area of the part will crack since the plastic is still trying to shrink after cooling down. In addition, a sharp corner will not have plastic flow smoothly through it, which will cause a non-uniform shrinkage at the corner when it cools. This severely weakens the corner.

He had some equations up, and depending on the strength of the plastic material you choose for the wheel I'd estimate that you'd want a solid 1/4" thickness all the way around, with (and this is critical) no lower than a 1/2" radius on the fillets. If you use a non-crystalline plastic, then you can have a slight variance for your thicknesses, but not much. AM's wheels are ingeniusly designed and have the right material composition. Though as a warning, since polycarb is non-crystalline, I now think that teams should take the weight of the bot off the wheels while it's in shipment, since the shipments can go through various extremes of heat which will cause the polycarb to creep.

Hope this helps if you still decide to do molding. -- edit -- As a followup, if you do want to do the molding, follow the link above to the conference and get the contact info of the APM guy. He was glad to give me a couple of pointers, and probably wouldn't mind spending a few minutes looking at your design.

I'm the one who voted other.

Do both. Make your design so it can be CNC'd, then rapid prototype some lovely little molds if you feel like doing mold work. If not, you now have a design that is either easily interchanged, or is dual function.

[FourPenguins]

So my post just got eaten. I will try again.
Pros of molding (using Urethan and silicone as Brandon suggested):
-Reproduceability
-Can be done in a garage once the molds/dies are made
-Doesn't tie up machining resources
-Is a fun job for freshmen
Cons:
-Lacks strength of aluminum wheels
-Less material flexibility
-Improperly mixed resin will have very poor structural properties (worst case is putty)

A quick overview of the process: two parts are mixed, a resin and a hardener (as in an epoxy). This is poured into a premade mold and allowed to harden. You then break it out of the mold and debur with a sander/file. Brandon's idea of molding silicone right onto the wheel as tread sounds pretty awesome. For mold design and process questions, hit up Brandon, me, or Mike. Mike and I worked with this stuff back in 2005 and apparently Brandon now gets paid for this
For material ideas see SmoothOn's website (The same company that makes the urethane foams ubiquitous in MORT robots.)

[Akash Rastogi]

Thanks for all the suggestions and information so far everyone.

[rajikurbaj357]

Originally Posted by EricH

Double whammy. They were going for both, I think. *goes to the other room for family copy of 2007's book* Yep, that's 2 patents, one for the battery system, one for the linear bearings.

I know I'm a little late, but we (357) have a patent on both, the battery base patent was just approved a few weeks ago.