Abrasion Problem of Polypropylene and ABS

[David_ps]

Dear All

 

I have a project to produce two injection-molded parts. One molded part was made of ABS. The other one was made of PP. These two parts contacted with each other. The ABS part rotated on the PP part (10 rpm). So, the ABS part abraded the PP part. After 500 times of rotation, the result was that the PP got serious wear.

 

The rockwell hardness:

 

ABS: R-Scale 110

PP : R-Scale 80

 

If I use an ABS material with Rockwell Hardness at R-scale 100, does the PP part get less wear?

 

Thank you.

[Enthalpy]

Hi David-ps, welcome here!

 

Probably not. Abrasion doesn't relate much with hardness.

 

Polypropylene is known to resist abrasion rather well, including when paired with metals, preferably hard metals. Possible scenarios are that rubbing overheated the plastics, or that these two are not compatible. ABS is not used as a rubbing material, supposedly because it has bad properties.

 

My first suggestion would be to get a doc from a supplier of polymer plain bearings and check the pressure and speed they give for their highly optimized materials. You'll see it's not much. Then check your design, keeping in mind that your materials are bad.

 

In case the design uses reasonable pressure and speed, consider other materials. Especially, replace ABS with something better, preferably a metal. Not all alloys are equal to preserve the plastics; almost smooth surfaces are better. If the design's pressure and speed resembles even remotely what plain bearings do, you need these special materials. If they exceed, make a new design.

[Enthalpy]

I've checked recently for other purposes what plain bearings can do or not, here and there

http://www.ggbearings.com/

http://www.igus.eu/iglidur

and:

• None wants plastic to rub against the bushes. All suppliers demand metal with a minimum hardness and a grinded surface.
• PP is not a material for bushes suppliers. Though, I know it does work, with less good performance.
• The best plastic bearings accept 120MPa or 2m/s but not both: their product must be P*V<1.5MPa*m/s.
• The best metal bearings accept more pressure but less P*V product.

This is for contact bearings. Hydrostatic and hydrodynamic bearings offer other possiblities.

 

The P*V limit is very low and it's a hard limit. Expect quick destruction if exceeded. So I'd recommend to check:

• If P*V>1.5MPa*m/s, redesign. Forget plain bearings, it's just impossible. Most frequent case.
• If P*V<1.5MPa*m/s, retry with the best plastic bushes againt metal.
• If <<1.5 you may try with PP and metal. ABS is probably a bad choice whatever the speed and contact pressure.

I know it's annoying... Many designs start with a P*V that no material can accept and must later be improved.