Nylon and wear resistance - ptfe additives necessary? 1

[DDengr]

I'm working on a design for a motorcylce chain guide. This application won't see much loading, but it will experience friction and some heat from constant interaction with the chain. It's basically a plastic block through which the chain travels to keep it from slapping against the swingarm.

Engine size I'm dealing with is 125 to 450cc, so rear wheel hp should be between 30 and 50, give or take a few ponies.

I would like to use 13 - 15% glass filled PA6 or PA66 for this application due to the additional impact resistance and rigidity offered via the GF.

However, I'm unsure of two things. The first is whether glass filled nylon would even be appropriate here - I don't want to cause undue wear on the chain, but I also don't want a part that's going to bend/tear/break easily when it hits rocks or other obstacles.

Secondly, would a ptfe additive be necessary here to ensure decent part longevity? I know a lot of factors are at play and a definitive answer will probably not be gained w/out actual testing, but would appreciate any input regardless.

 

[Pud]

I would suggest an unfilled, impact modified nylon. Should do the job.

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[IRstuff]

What's the probability of the chain being misadjusted, or whatever, enough to be constantly rubbing against the block?

TTFN
faq731-376

 

[patprimmer]

Glass filled is not suitable. It will wear then expose glass fibres that will ball up and become an abrasive contaminant. Also although glass greatly increases energy at break in certain so called impact tests, it is very poor in the other direction, kinda like a piece of wood when testing along or across the grain. Also even in the good direction, glass fibre increases energy at break and reduces notch sensitivity, but also dramatically reduces elongation at break and hence reduces the ability to deflect to share the load. This results in a brittle nature in the real world.

The universal material of choice for automotive timing chain tensioners and guides in engines that use long metal chains is graphite and molybdenum disulphide filled nylon 6.6. Teflon does not help nylon much re friction and wear.

If a std grade lacks impact you can improve that by increasing the molecular weight of the base polymer used for making your compound. The higher molecular weight improves impact and wear resistance without reducing temperature resistance or melting point.

If localised melting from friction is not a problem, an impact modifier will help toughness but can reduce abrasion a bit under some conditions.

If you really need a reinforcement, which I very much doubt you do, carbon fibre although expensive helps re bearing properties and is gentle on the mating part.

If the part in high molecular weight graphite and molly filled nylon 6.6 wears out to fast, plain old unfilled nylon 4.6 will do better due mainly to it.s substantially higher level of crystallisation and higher melting temperature.

I very much doubt you really need anything fancy. Plain old unfilled, unmodified nylon 6.6 will probably work OK. Heck, if it's what I think it is, even a block of plain old HDPE would work.

Regards
Pat
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[DDengr]

Thanks for the replies.

Even with a well adjusted chain, there will be constant contact with the block, albeit very light contact, more so with a poorly adjusted chain. If memory serves me correctly, the oem uses a TPU block. I did consider HDPE Pat, but it's a little too prone to showing mars/scratches and would likely look like crap after one ride, especially if you frequent rocky areas.

I'm attempting to go about this venture/hobby as economically as possible, so would like to utilize one of the resins my molder stocks instead of purchasing 55lbs of the stuff. I'll probably need less than ten pounds to run an initial sample lot and see how things go.

The only two non-glass filled nylons my molder offers are Zytel® 103HSL NC010 and Zytel® ST801AHS NC010. The latter includes an impact modifier and is described by Dupont as a "super tough high performance polyamide 66 resin." The 103HSL is your general, lubricated nylon 66.

My molder has a "rubber" classification with the ST801AHS, so I need to get a chip to see exactly what it looks like. My decision will also depend on how well these accept colorant, as I'll be using a master batch for color.

Pat, do you have any comments on the Zytels? I do have a chip of the 103HSL and its natural color is quite green, so I'm not sure how easy it's going to be to color, may require a high let down ratio.

 

[patprimmer]

Zytel is a top notch nylon. ST801 is the industry std for toughened nylons.

103HSL NC010 is a pretty basic nylon 6.6

The 103 is basic injection moulding grade polymer

The HS = heat stabilised.

The L = lubricated

The NC 010 is natural colour

The heat stabiliser is copper iodide based hence the green colour.

The lubricant in this case is nothing to do with end use bearing properties. It is about waxes added to improve flow in the mould and release from the mould.

If you want to add colour, the HS will affect the colour on pastel shades. To complicate it, the green discolouration varies from batch to batch and even with time so it is real problem with regard to colour match of light shades. One advantage of the HS material is that it also has much better UV light resistance.

If the difference between 1/2 a bag and a full bag matters to you, then your stuck with what the moulder has and will be hard pressed to get much support for lack of commercial value.

Black is nice for several reasons.

Regards
Pat
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[patprimmer]

Oh

I should have mentioned, any change of colourants can have a much larger than imagines influence on cycle time, mould shrinkage and impact strength.

Do not be fooled by the argument that there is so little in there it cannot matter. It surely does matter, to the extent that on pressure pack can lids, they often use different moulds for different colours just to get the size correct.

Regards
Pat
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[DDengr]

Thanks again Pat.

Yes, black is a possibility, but I'm hoping to match the oem plastics. I have another product line made in PP that was matched and it came out excellent. I do think think the nylons are more challenging in the color dept. though.

Fortunately, the molder will run parts at their expense until the processing is dialed in. Another reason why I'd rather use their resin than purchase my own.

 

[patprimmer]

What colour are you trying to match and how accurate are you trying to hold the colour. Yes I know, very subjective questions

Regards
Pat
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[DDengr]

I would like to hold color to a fairly consistent benchmark, within a few shades of the oem at least. I'm not going after a scientific measure or expecting a miracle, but I would like it to be close.

The color I'm matching is in the darker spectrum, not pastel at all, so it should prove a little easier.

 

[patprimmer]

Medium to dark shades and dull shades will be OK.

Regards
Pat
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[Pud]

How hot do you think it's going to get?

Heat stabilisation is for when parts are at sustained high temps (+100C).

H

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?