HDPE vs aluminum for my robot?

[electrofan]

Hello all.

I'm designing a robot which has arms which can extend. Here is an image:



For reference, the robot base is 7 inches by 5.5 inches, and the large arms are 1 inch by 1 inch tubes (not finalized yet). The image above has frame-->cylindrical attachment-->large rectangular tube arm-->smaller rectangular tube arm-->wheel.

I know aluminum of 1/16" thickness for the arms will do the trick, but weight is a major concern due to the need to climb inclines, and an aluminum bot will weigh about  1-1.2 kg. That's why I'm considering using HDPE for the entire frame. But I'm worried about the rigidity of the arms. Will a 1/8" thickness HDPE tube of 1 inch diameter and ~5 inch length be rigid enough? How about 1/16"?

Another problem is aesthetics. It's not as major as weight, but still I'd like the robot to look nice. I'd be fine with black HDPE, but I can only get sheets of black from mcmaster.com, not round tubes. Anyone know of a place which sells black HDPE tubes? Can I paint HDPE easily?

Another idea is to use HDPE for the base, and aluminum for the arms. This might be nice too since HDPE tubes have more limited sizes than aluminum.

Thanks

[electrofan]

Bump...

At this point, I'm going with 1/8" HDPE for the frame (as it will be essentially completely closed and rigid), held together by G/flex epoxy. And 1/16" aluminum for the rest.

[Admin]

HDPE bends a lot . . . You want to do a stress analysis on it (basic beam bending equations). Its the only way you'll know for sure.

Also, use screws not glue to hold your robot together

[electrofan]

Hey thanks for the reply.

Yeah I've pretty much settled on using aluminum for the arms at this point, but HDPE for the frame. I assume a rigid box-frame will hold itself together fairly well - don't know any way of testing this other than making one. It will have a few (at least 2) internal diaphragms for supports as well.

Regarding the glue vs. screws, the problem is the HDPE for the frame is only 1/8" thick, and I don't really want to add in other metal parts to hold it together by screws (added cost, complexity, I want to keep the design as simple and cheap as possible) From what I've read, HDPE is notoriously difficult for bonding/gluing, but this G/flex stuff seems to be a fairly new product made especially for stuff like this. It dries flexible, so it won't crack/weaken as the plastic bends and morphs.

http://www.epoxyworks.com/26/pdf/Gluing_plastic_Gflex.pdf

Edit: Alternatively, after doing some price and weight calculations, aluminum would be suitable for the main frame as well. In that case, I can use aluminum for everything, either welding the main frame or using screws to hold it together.

[Admin]

Its possible to bend 90 degree angles into HDPE - just heat up the head with a small torch and bend it using metal bending techniques. With the 90 degree angle and holes drilled in each corner, you can get a very strong very reliable box out of it. And you can use locknuts so nothing will shift lose over time.