Effect of glass reinforcement on impact & wear properties of Nylon? Need help with this applicat 4

[anuragsidh]

Dear all

I wish to know what is the effect of glass reinforcement on the impact & wear properties of nylon.

In picture 1, the part movement is described. It is a hollow part of nylon, weighs around 8 grams, takes a stroke of around 8 centimeters @ 10 times/sec.

http://files.engineering.com/getfile.aspx?folder=c63df498-ea62-453b-ac7a-919844e5157e&file=1.JPG

http://files.engineering.com/getfile.aspx?folder=33067269-27c6-4613-988f-930fac3d5f6f&file=2.JPG

The gap between the plastic part & the steel pins is nearly 1 mm, due to which it hits & cuts/wears the plastic at point of contact with the steel pins, as shown in picture 2. Same happens at the bottom of this part.

The other surface where the part is sliding is also steel but we don't have problems there. Also, it has been observed that if the nylon is harder (may be by adding glass, not sure), it wears out even faster. Softer material runs better.

Queries for you -
1. Is this damage due to abrasion/wear or impact?
2. Do you think Zytel ST801 is the right choice for this application? It has the highest imapct rating.
3. What wear resistance additives would you recommend for Zytel ST801? I have chosen Fluoroguard
4. As I mentioned, harder product wears out faster, would you still recommend Glass reinforcement?
5. will the impact strength decrease or increase by glass reinforcement?
6. will the wear resistance increase or decrease by glass reinforcement?
7. Is there any suggested optimum glass reinforcement percentage for this application which will make the best compound?

Any suggestion is welcome as I don't have a plastics/ material background & am about to start with the above mentioned products using my best possible understandings of the data sheets.

Thanks all

 

[Demon3]

1. My best guess is that it's wear because impact is normally a brittle response where you'd get shards and not a smooth surface like that (if e.g. Pat or Pud disagree then take their opinion over mine)

2. As this doesn't seem to be an impact problem I'd go for a reinforced nylon, for example 10% glass fiber and add in some oil to improve slip

3. Fluoroguard sounds fine (fluorinated oil). You could also try ultrahigh molecular weight silicone (likely to last longer because it will stay where you put it. It's polymeric so it can't move around whereas oil will be lost over time. You can get the UHMW silicone from Wacker or Dow Corning Multibase

4. Yes, that or a slippery additive like graphite or hexagonal boron nitride

5. Increase (check properties for free at

http://www.matweb.com)

6. Increase

7. At a guess 10%, usually low levels of filler work well for wear

All the best,

Chris

Chris DeArmitt - PhD FRSC CChem
Plastic & Additives Webinars
Instant Downloads & Inexpensive

http://www.plastictraining.com

 

[anuragsidh]

Hi Demon

Thanks a lot for your detailed answer. Which, according to your knowlwdge is the most wear resisting nylon ( or nylon+aditive combination). How can I get a review from Pat or Pud?

Best Regards

 

[patprimmer]

I resisted review of this as I could not make much sense of your drawings and I was not prepared to make suggestions based on the info posted.

Tribology with plastics is a bit of a stranger beast and the added complication of fibre reinforcement makes it even more complex.

Due to not having a good picture of the part in my mind, my reply should be considered as a broad generalisation.

GF nylon makes a good bearing for a while, but as the polymer wears away, the glass fibre becomes exposed. It can in most circumstances, them matt and ball up and become extremely abrasive on the matching surface. Carbon fibre on the other hand tends to behave a lot better, but at a much higher cost. Aramid pulp can help with reduced wear of the bearing, but sometimes can be to abrasive of the mating surface.

If you have impact damage that is one thing. If you have a bearing failure from friction, that is another. There are four conflicting requirements that I currently see to solve what it appears that maybe this might be (at a guess)

If it is brittle failure due to impact you need a tougher material.

If it is ductile failure from impact, you need a harder material.

If it is fatigue, you need a material that has better fatigue resistance at that strain and frequency. Yes rankings to fatigue do vary with both strain and frequency.

If it is abrasion related failure, then you need to consider a bunch of things dependant on the environment. What works in a flowing water lubricated environment, is different to dry, which is different to clean vs dirty which is different to high load vs high velocity. It also varies greatly depending on the nature and finish of the mating surface.

Generally nylons make very good plastic bearings and generally, a strong nucleating agent like MoS2 or TiO2 combined with a lubricant like graphite powder work best, BUT it does depend.

For instance.

Molly/graphite nylon 6.6 is widely used for timing chain tensioners in car engines in a low load, high speed hot oil environment.

Unfilled nylon 4.6 works better if the oil is even hotter like in high performance air cooled engines, but it's cost is higher.

Acetal works quite well for boat propeller shaft skeg bushes as the water keeps it cold.

Acetal, nylon, PTFE and HDPE all work best at different locations in race car suspension bushes.

If the point loads are high, acetal is best because it is hard and does not pound out.

If the surface speed and load are high, nylon might be best because it better resists the heat generated by the friction.

If the load is moderate but the environment is dirty, HDPE or PTFE might be best as they are slippery and soft and allow grit to become embedded rather than sit at the surface and grind.

I know in gear trains, alternating nylon and acetal works a LOT better than all in one or the other.

I know PTFE is bad if it stops for a while as it is under load as it cold flows and conforms to the iregular surface, causing resistance to start up again.




Regards
Pat
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[Demon3]

Brilliant Pat!

Chris DeArmitt - PhD FRSC CChem
Plastic & Additives Webinars
Instant Downloads & Inexpensive

http://www.plastictraining.com

 

[IRstuff]

Why is it necessary to run the pins over these plastic parts with that severe a motion? From the photo, it appears to be a fairly deep stroke into the plastic, which would suggest that the operation severely deforms the surface of the plastic.

Your comment w.r.t. the lower surface, which is presumably flat, demonstrates that even though the stroke count is on the order of 315 million per yr, the excessive deformation at the top is the issue. The harder the material the more damage you do to it when you deform it.

The expectation, therefore, is if you reduce the penetration in the plastic by even a small amount, your problem would be greatly diminished.

TTFN
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