autoclavable, machinable plastic that won't crack @ tapped holes

[bschena]

I'm looking for an autoclavable resin that can be purchased in standard shapes (e.g. for billet-machining fabrication, not injection molding). However, I'd really like it to be able to take a threaded metal fastener (e.g. stainless steel screw) without cracking.

It needs to be able to survive multiple (say, 50-100) steam autoclave cycles at 134°C and 2 bar without cracking.

Conventional wisdom is that Ultem is a poor choice as it tends to fracture at the sharp root of any tapped hole.

Any alternative suggestions? Any experience with filled (e.g. glass) resins - better? worse?

Thanks!

Bruce @ Intuitive Surgical

 

[patprimmer]

That is a big ask.

The hard part is the autoclave bit.

Are you talking about self tapping screws or threads that are cut with a tap.

Glass filled materials are stronger, but they have lower elongation at break and the interface between the resin and the glass can be susceptible to attack from hot water.

My best guess would be the special radiator header tank grade of glass filled nylon 6.6, however I am not confident it can be purchased in semi finished form and I am not confident it will work.

Polysulphone may work as may poly ether keytones, but they are VERY expensive.



Regards
Pat
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[Pud]

PEEK - maybe with a s/s female insert to take your fastener, if possible.

I am assuming that as it's medical and maybe low volume, cost is not too much of a consideration...USP approved grades will be available - even more $$$$!

H

http://www.tynevalleyplastics.co.uk

 

[bschena]

Thanks Pat & Pud - yes, this is a low volume medical application, so cost is not my primary consideration. I had a feeling I was asking a lot on this.

I've been reading that PEEK seems to be better than Ultem in regards to cracking. Do either of you have any experience with tapped PEEK? I'm not planning on using threadcutting screws (e.g. Plastites) on this as I figured the sharp cutting action would only make things worse.

Inserting is a good thought. I assume you are thinking of heat-staked inserts rather than threaded ones?

-Bruce

 

[patprimmer]

Some inserts can be fixed in undersized drilled holes by ultrasonics. If you can access an ultrasonic welder that might be the best option.

If cost is not an issue, I agree, PEEK has excellent high temperature and hydrolysis resistant properties.
The correct inserts greatly reduces problems from notch sensitivity.

A company called Victrex in the UK (Welyn Garden City last time I looked) should be able to provide technical data and material or at least a source of material.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[Pud]

I was thinking more like top hat type inserts from the rear of the part if possible. PEEK is pretty tough stuff - I would have thought correct plastic screws would be fine in it. It's a completely different material to PES.

Type of screws here:

http://www.aspltd.co.uk/screws-for-plastics.php

Obviously others.

Cheers
Harry

http://www.tynevalleyplastics.co.uk

 

[HDS]

As you said with whatever plastic you choose thread forming screws are better than thread cutting. Defiantly needs to be designed for plastics. Inserts are usually stronger but ad expense.

Camcar makes plastite brand screws that are thread forming or thread cutting. Their website should help you

http://www.acument.com/northamerica/deltapt.asp

. It has recommendations for different resins based on material properties.

 

[bschena]

Great information - thanks all. I think I'll move forward with the Victrex PEEK, thread forming screws and/or inserts. Thanks for all the help.

Cheers!