MATERIALS – HDPE FOR ROBOTS TUTORIAL

Plastics

It should not be hard to convince anyone the benefit of using plastics instead of metals for so many applications. Cars, clothes, food containers, toys – these are all items that have been revolutionized by the invention of plastics.

Plastic is cheap, light, strong, easier to cut, shape and drill, plus it has a higher strength to weight ratio than metals. So why isn’t your robot made of plastic already?

It is probably because you do not know where you can get large, cheap quantities of the stuff, nor have you actually realized the advantages of it.

HDPE, or other plastics? :

The type of plastic I will talk about is HDPE. The reason why I choose this particular plastic over any other is because it is the cheapest, and yet still has all the good properties you want for a small-ish sized robot. HDPE is often used in pipes, packaging, and vehicals too. I really like it! (can you tell?)

I used it most recently on a robot called pure evil, here is a video of it in testing:

As you can see it is very artistically shaped, showing how easy it is to cut with basic tools. I used steel brackets to mount my wheels for high stiffness, using screws between the HDPE and the steel. At the rear of the robot is the castor, this stops the rear of the robot from dragging against the ground, it just required a few screw holes drilled into the HDPE to attach it. In the front, if you look carefully, is a small hole cut out to allow the axe to fit. I cut and filed it down so the bent piece of aliminium that protects the axe gearing could fit. it can be cut with anything from a bandsaw to your average woodsaw.

HDPE is the perfered choice in material for lightweight robots such as featherweights due to its affordability, high strength, and easy machinability (power tools not required). A £20 or so sheet is all you need for every structural component on your robot, and you will still have some left over! So where can you get this stuff from? Go to http://www.directplasticsonline.co.uk/ and do a search for HDPE. Then just choose the shape and dimensions you want. You probably want a thickness big enough that you can put screws into it, although I have found uses for thinner sheets (such as for curved parts).

Where to Use HDPE:

HDPE is a flexible material, so if you plan to have not very wide parts, you may want to get a thicker piece of it. I tend to use it for the main robot base and side plates. This is because the base is the largest single part on your robot, and is a significant percentage of weight . it is also where you would attach your motors and electronics and whatever else. I recommend a 1/4″ thick sheet for your base, unless you reinforce it  (I will talk about this later).

Designing Your HDPE

As I stressed above you should plan out your robot before building it. This is very important for when you cut your material, as making a mistake could cost you more money. Whenever I use HDPE  I first design the shape and mark every hole that gets drilled into it. This is the design I used to cut the HDPE for pure evil and my other robots.

HDPE Construction Techniques:
There are some instances where you can use really thin sheets of HDPE, such as when you use layers or reinforcement. This below image is of my robot ministrom:

The bottom layer is a thin sheet of flexible aluminum, and the top layer is a thin sheet of HDPE. Joining them together are four threaded spacers. these where then bolted to a box section frame/chassis, it is really tough and should stand up to the rigors of combat!

Tools For HDPE
Because HDPE is a highly machinable material, you can use around-the-house tools to work with it. A Dremel tool would be ideal for drilling and making small cuts. A fine-toothed hack saw or bandsaw/woodsaw can easily cut out shapes. Sandpaper or a  sander would work for smoothing out the cuts and for rounding off corners. Any tool that you would normally use for wood can actually be easier to use with HDPE. Even more amazing, the thinner sheets of HDPE can be cut with every day scissors and clippers. And for bending HDPE (for thinner sheets only), just place it on the edge of a table, and use a flat heavy item (like a book) to push on the part hanging off the edge.

Lastly, Disadvantages in Using HDPE
Yes, plastic is not as magical as we wish. There are several occasions you should not use HDPE.

. Do not use it when you expect a lot of wear applied to it. HDPE is a soft material and can easily be worn away with abrasive contact. However I have seen HDPE used quite well for chain tensioners on a robot called thants, because of it’s smoothness. Chain tensioners are just a big block of material that pushes against a moving chain to make the chain tight around the sprocket

. Do not tap HDPE. This is when you put a screw thread into a material. HDPE is very weak when it is made thin, so the tapped threads will end up being lose and will fail after only a few uses. However what is great about HDPE is that is a great self tapping material. Instead of tapping, just screw a screw into a tight hole and the screw will stay in really tight and firm. I use this method very often as it is very reliable.

. Do not use HDPE if you need highly rigid strength. HDPE, unless the dimensions are chosen carefully, will bend and flex. This is likely to happen if you use long thin pieces to hold heavy weights at the end. It is possible to mathematically calculate how it will bend so you can design around the problem, but I will not go into it as it is somewhat complex theory.

and thats really all you need to know about hdpe!