Press Fits 2

[Agitan]

I have developed a new line of products that utilize a press fits between similar plastics (Nylon 6/6).
Over time retention force decreases significantly.
We have tried Dupont Crastin and ST-801AW NC010.
In this application there must not be any distruction between the mating parts. Any suggestions?
Thanks
Agitan

 

[patprimmer]

Steel.

If it must be plastic, depending on price and a whole host of other properties that you have not mentioned, acetal or 60% GF PET or 50% GF nyon 6.6.

ST801 was a poor choice as the impact modifier reduces creep resistance. All plastics creep under constant load, but some a lot more than others. Aromatic nylons like Reny or Grivory are also better than most, but are quite expensive.

Maybe an exotic like PPS o polysuphone might work, but I don't have creep data for them at hand.

At a significant extra cost a slight improvement is obtainable with long fibre, but it can only be use effectivly in some moulds.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Agitan]

patprimmer,
Thanks for your response.
In our case the material selections have been suggested by the plastic manufacturer. It is worrisome that they could be taking us down the wrong path. They seemed to focus on moisture as the culprit eventhough, products are deployed in a strictly controlled environment. If you have the time I am attaching our website that gives a good view to our product and how it is applied.
Regards,
Agitan

 

[Pud]

Could you just use a thermoplastic elastomer (TPE)?

You would have to try various hardnesses to get the push on/pull off forces required, but a few guesses - e.g. medium, hard and v.hard should be worth a trial in your tooling to see what end of the range you need.

They can be pretty cheap too - especially compared to ST801/Crastin.

btw, I have found that the "Big Boys" are generally pretty poor at suggesting the optimum material. Resellers who sell more that one maker's product have been the best.

Here is a link to a TPE selector ( I have no connection with them)

http://tinyurl.com/6zstpw

Cheers

Harry

 

[Agitan]

Pud,
Thank you.
Any recommendations on reputable/technical savvy resellers to contact?
US contact would be preferred.
(I hope this request is within the rules of the forum)
Regards,
Agitan

 

[patprimmer]

I would try 55 or 60% GF PET. It's fairly chap, very rigid, good to creep, but difficult to mould unless you have very good equipment and knowledge.

Dupont's Rynite i a bit easier to mould, but sacrifices a touch of creep and rigidity to obtain the easier processing.

Nylon softens with moisture absorption, but that s absolutely known and predictable.

ALL thermoplastics creep, so even the best will lose some pull out force with time.

I think the part is to small to take real advantage from long fibre.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Agitan]

patprimmer,
Thanks once again. I am running your recommendations through our molder as we speak.
Since, our background is mechanical/electrical and this is our first attempt at plastics manufacture we have been taking the plastic manufacturer's recommendations as gospel.
We are trying to save our current design but, if I knew before what I know now I would have designed for some type of snap fit as opposed to a press fit. It will be interesting to see the response to your recommendations. I will be sure to post throughout the solution and hopefully save someone else some time and money.
Regards,
Agitan

 

[Pud]

Agitan,

Assuming your caps fit on standard receptacles, a snap fit will not be possible as you cannot put a corresponding feature on them...
As Pat stated, PET (or any polyester) requires stringent pre moulding drying, otherwise you get parts with a great finish but the strength of biscuits (cookies)!
Unfortunately, I am in the UK and cannot recommend anyone in the US, but I guess there must be hundreds, if not thousands there.
As (I guess) there are no particular environmental conditions to consider, you might investigate polypropylene or polyethylene based elastomers - but they also require careful processing! (Polyurethanes)

Cheers

Harry

(I wish you lot could spell 'Moulder'!!)

 

[MacGyverS2000]

(I wish you lot could spell 'Moulder'!!)

Isn't that the guy on X-Files? ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Pud]

I thought it was Scully and "Molder" - oh well..

We really need Demon3 to respond to this I think. Heellpp!

H

 

[patprimmer]

We down under sill know who invented this language.

Moulder moulder moulder moulder.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Agitan]

Et al,
Status on the recommendations is that the molder and the plastics manufacturer are looking at each other to make a recommendation. Should it be this complex? I would find this issue interesting if it wasn't delaying production.
Please accept my apologies for my Colonial spelling of "mould". Would you believe that our programs do not even recognize the "correct" spelling?
Regards,
Agitan

 

[Pud]

Apologies for the diversion... this is a "running joke" - civilisation vs. the rest! Yes, I do do believe M$ spell incorrect. But Bill does not give a s*hit!

Do not talk to a "manufacturer" - they will try to sell you what they make (obvious eh?)

Find your nearest reseller of plastics with a wide portfolio. Give them a call. Google "TPE" "thermoplastics"

Where is Demon3 (man in white coat - boffin- Help!)

H

 

[patprimmer]

Pud.

I disagree to a point. Talking to a number of manufacturers can help so long as you put the filters in to eliminate the sales pitch from the fact. They do have access to an unparalleled data source on "THEIR" products. Some (not many) even have the sense and integrity to walk away if they know their product won't do the job. You need to talk to most of the manufacturers of engineering and exotic polymers with this problem, as it is a difficult problem.

ALL THERMOPLASTICS CREEP UNDER LOAD, and this is probably why you see the situation with your moulder and your material supplier.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.