re: polystyrene buildup

[jwillson]

I have an application where tight fitting polystyrene plugs are driven down a tube of anodized aluminum by air pressure, but the plugs leave residue on the tube and bind after a few hundred cycles. The surface speeds involved are moderately high, about 300ft/sec.

I am not familiar with the physics involved in plastic/anodize interaction, and resources with plastic information don't seem to be readily available (that I could find). I was also unsuccessful finding useful resources for coatings.

Can anyone point me toward a good resource to learn about the tribology involved, or offer some pointers for how to eliminate the problem? Unfortunately, the plug material isn't available to change, but the tube is and coatings are fair game as well.

Thanks

J

 

[Compositepro]

Yes, it is well known that high speed friction will melt plastic at the surface and leave deposits. This is a common problem when transporting plastic pellets pneumatically in pipes casing "angel hair".

 

[3DDave]

Give the plugs a spritz of silicone oil or cooking spray before stuffing them in.

Who thought that Polystyrene was a good idea? It's soft, has a low melting point, and isn't thermally conductive. It's like the choice is intended to fail. Filled teflon would have been a good choice - high melting point, filled to stiffen it up, and to make it thermally conductive so that heat from friction doesn't build up so badly.

You might also send a cleaning pig down the tube to scrape off the polystyrene build-up from time to time. There is a big hardness gap between polystyrene and anodized aluminum. Perhaps a fistful of scotchbrite?

Example calculation - if it takes 1 pound of force to push a plug, then at 300 feet/second (Mach .3 for STP) that is 300 foot-pounds/second or 1/2 horsepower/400 watts in frictional heating. You didn't say what values for pushing resistance is. Make a guess at the distance of 300 feet and you have one second at 400 Watts, for 400 joules. The thermal transient analysis is that you have 400 Watts at the interface between a plug that starts at room temp (however warm your room is, and the tube which will effectively be forced to be room temp. The area it is put across may be as little as 10% of the nominal contact area of the plug. You can find this by getting some metal bluing and putting it on the plug and sliding it in the tube a bit to see where the blue rubs off. That's the max contact area. Maybe half of that is under 90% of the load, the rest just tapers off to zero pressure at the edges.

Chilling the pipe to -50F could also work to prevent the polystyrene from melting, but then you have other problems. You could pre-chill the polystyrene to -50F or so to prevent it from warming up and melting. Plus it would cut the interference amount.

 

[romke]

what you experience would be called dry friction in tribological terms - which obviously generates so much friction that one of the components involved starts to melt and adhere to the other surface.

there are various things that can be done - change the surface structure of the aluminium tubing in such a way that less friction occurs is one of them, for example by coating it with something that reduces the friction. i do not think that can solve the problem completely and also might be rather difficult to carry out. you could also try to change the surface structure of the plugs, but again that may be rather difficult to carry out and may also interfere with the later use of the plugs.

your best bet would be to use some kind of lubricant, effectively reducing the amount of contact between the plug and the tube, resulting in far less friction and a much lower temperature rise - hopefully to the extend that no melting of the plugs occurs anymore.

there are two types of lubricant that may be suitable - low viscosity fluids that adhere well to both the tube and to the plugs and solid materials with a layered structure like graphite, MoS₂ and the like. standard lubricants based on mineral oil most likely will not be suitable because the polystyrene may soften and swell when in contact with those (some may even dissolve completely, as done on purpose in certain types of engine oil....).

your best bet may be a lubricant based on a mix of water and some polypropyleneglycol. the polypropyleneglycol will add some extra "lubricity" to the water, is non toxic and can be easily washed of from the plugs later on when needed. a standard antifreeze (monoethyleneglycol) might also work, but is a toxic substance.

a alternative may be found in some metal working fluids. the synthetic types may be your best bet, but even standard metal working emulsions with less then 3% mineral oil might be suitable.