Uneven Wear 1

[kunc]

I have two plastic round plates (disks)in contact. One is rotating against the other one. (A type of brake application). The wear of the thickness of the plates is uneven. A cross section would show a wavy pattern from the inside radius to the outside radius. Any idea what could cause this? thanks.

 

[patprimmer]

Possibly differential surface speeds as the plastic near the outside is travelling a greater distance per revolution. This generates more heat if the pressure is the same.
Another possibility is variations in crystalinity due to differential cooling in the mould 9or elsewhere for that matter.
Another possibility is variations in orientation due to turbulence in the flow during moulding.
It's very hard to say without more details of the test and the method of manufacture of the plates and the make up of the plastic.
Regards
pat

 

[kunc]

The differential surface speed is an interesting idea, although the wear looks like a sine wave from inside to outside. I learned that the part was molded cold, which for this semi-crystaline material would result in less crystallinity and lower wear resistance. I think this will be retested after being molded in a hot mold. I also posted this in the Tribology section.

 

[patprimmer]

Depending on the polymer, and other requirements, a few tricks to improve wear resistance are
1) Increase crystalinity (as you have indicated), but by useing neucleating agents. Titanium Dioxide generally works well, so long as a white colour is OK, and Molybdenum DiSulphide works great in nylons, so long as black is OK
2) Fillers such as carbon fibre or Aramid fibre pulp also work well, but are costly

Fibres may reduce overall wear, but exagerate the uneven of it if the cause of it is variations in flow direction dureing fill. Look at progressive short shots to see how it fills Regards
pat

 

[chaving]

Because you have not said otherwise I asume your two rotaing disks are the same material, this is going to give all sorts of problems because they will be trying to weld themselves together when they get warm.

Try to use 2 different hardnesses of the same material (maybe add a filler to one) or evan use 2 different materials.