Questions about different types of plastics?

[mike96]

Currently I am in the process of getting my shoptask 3in1 back up and running because I have a new found re-interest in hobby-machining but I need some help with a couple of things. One of the many projects I am looking at trying is taking one of my radio controlled cars and increasing the size of the vehicle. While it would look very nice all done in aluminum I don't really don't have the money to spend experimenting with that grade of material. there for I have been looking at some lesser expensive alternatives for prototyping and mocking up the build to see if I have the patients and talent to take on this project.

My questions...

Does anyone have any experience with Polyethylene? I was looking on McMaster-car at some low cost alternatives to aluminum and this particular plastic caught my eye. According to the web site it is a high tensile strength plastic with good wear characteristics but how is it in the mill? Would it require any special tools or techniques to get a good quality part from. At around $35 for a 12" x 24" x 1/2" thick piece it would be more that enough for over half of the parts in the car but. with this I hope that it would be a strong enough part that I might even be able to drive the car with the parts that I made until I can get them re-machined out of a good grade aluminum.

Keeping low cost as being a "main characteristics" are there any other materials that I can look into as an alternative? Teflon for example... While not exactly cheep in its 12 X 24 X 1/2 sheet from McMaster-car I have found a supply in relatively smaller more affordable sizes. My Wife recently picked up a 12" X 16" X 1/2" thick cutting board from the store for about $15. While it would take a few more of these to complete the car it is still cheaper than the hefty $75 plus price tag from McMaster. Again I would like to know what machine-ability this material is and what special precautions I should take if necessary.

Thanks for any and all inputs.

Mike.

[DMF_TomB]

Polyethylene comes in at least 3 main types
LDPE is soft and often used for plastic bottles you squeeze to get liquid out. it is more rubber like and the least brittle or least likely to break.

HDPE is moderately harder and is used for milk bottles. More rigid but still moderately flexible

UHMWPE or ultra high molecular weight polyethylene is a hard plastic that is much less flexible. some types with graphite fillers to make them electrically conductive or static dissipative do not bend far before breaking. so this plastic is rarely bent 90 degrees in a sheetmetal brake. it is often used as a teflon substitute as it has almost as much chemical resistance to most chemicals that teflon will resist. we use UHMWPE to make parts before making them of metal for testing purposes. plastics that are moving near other objects can develop an electrical static charge. this charge can be dangerous and the reason for additives mixed in the plastic to make them electrically conductive to prevent charge buildup.
....... you should be aware that Polyethylene does not glue easily because it is chemical resistant. it can be welded with plastic hot air welders.
.
cutting tools. i use tools made for cutting aluminum with a high helix and positive rake. the sharper the better. i use tools like end mills that have never been used to cut metal because dull tools will cause a poorer finish and raise more burrs. in general cut as fast as possible but not so fast as to melt the plastic. we cut sheets on a woodworker table saw but be aware that a dull blade will overheat plastic and cause binding when cutting.

[mike96]

I think the UHMWPE is what I was looking at on line the other night. In fact i looked it back up again and yea, that is what I am looking at using. McMaster-Carr Luckily I am not looking at having to glue anything together, most of the cat has either alignment pins and screws holding it together. the hinge pin areas are what I am the most worried about. With up scaling this thing to the size I want to, I am looking at the over all dimensions will probably about 2 feet wide 3-3.5 feel long weighing in the neighborhood of 20-30 pounds and running off the the equivalent of an RC car version of a weed eater engine. I would love to know what the manufacture actually casts the parts from because if I could use the same materials as them I would have no worries what so ever. I have seen mine ran into a curb at full throttle doing well over 40mph and it bent the metal hinge pin before it did any damage to the plastic suspension components. There will be some machining challenges but I think I have a few ideas that will make it easy enough.

Just to give you an overall idea of the construction on this monster I am planing.

Gallery

Gallery

[stewi]

I had a hard time to machine the parts in polyethylene and went on casting parts in epoxy and other casting resins.
Look here:
Mold Making Videos - Silicone Rubber, Urethane Casting Resin, & more
Casting Resin and Silicone Mold Making Rubber
Urethane Plastic Resin for Casting into Rubber Molds by Smooth-On