high stiffness plastic

[jshugs]

I am looking for a high stiffness, high strength, light weight plastic that can be machined to tight tolerances. It isn't really going to see high temperatures. I would like to fasten this part down with screws, but I'm not quite sure yet the best way to do that... maybe a nut, or an insert of some kind. I am not that familiar with various engineered plastics, so I thought I would see what advice people give on both the various plastics I could use, and a good way to fasten it down.

Thanks!
-Jon

 

[KENAT]

How stiff, some of the filled Delrins, acetal co-polymers etc. are stiff by the standards of plastics. They also machine fairly well, you can tap them.

However, your 'requirements' are pretty vague. If you can give more detail on just how 'stiff' it needs to be, and other requirements that may help more knowledgeable folks give answers.

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[patprimmer]

Are you a student.

This sounds suspiciously like a homework question. We have been inundated lately with similar real vague and clueless questions from first time posters.

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Pat
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[jshugs]

Thanks for the replies, and no I'm not a student. Sorry for the vagueness, I was just wondering if people had good references or ideas for me to look at, which is why I was not that specific. I am familiar with Delrin, and in my experience, the threads tend not to be very durable that are tapped directly into it. One material that I found info on that will probably work for my application is PEEK, which has a Density of 1320 kg/m3, a Young's modulus of (E) 3.6 GPa and Tensile strength of 90-100 MPa. I guess I am wondering what other similar plastics I should look at. It seems like Ultem is similar as well. Are there any plastics that have a higher stiffness to weight ratio than PEEK or ultem?

Thanks again!
-Jon

 

[KENAT]

Yes repeated insertions of fine thread form metal screws into Delrin etc. will tend to wear them. So if you need to use metal screws multiple times then yes inserts or similar may be an idea.

I'd still look into filled plastics for your stiffness concerns, you can get filled PEEK on McMaster-Carr.

In fact given your vague requirements looking at the materials places like McMaster(ttp://

http://www.mcmaster.com/#plastics/=g5j281

have on the shelf might be a place to start. They give some vague material properties and it may give you a feel for cost comparisons, availability etc.

Then try to find some material properties for them - preferably in a good materials resource book but otherwise matweb or similar.

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[patprimmer]

http://www.rtpcompany.com

is another good source of data.

PEEK is just about the most expensive plastic known to man so cost may be an issue. Glass reinforcement will certainly reduce it's cost and will upgrade some properties.

Acetal is normally the best engineering plastic (as opposed to exotic plastic) for stiffness and for thread holding in the non-reinforced state. Acetal does not respond quite so well to reinforcements as some other engineering plastics. If you want heavily reinforced, nylons and PET perform better.

No plastic will hold a thread well compared to steel and it is generally considered necessary to either use threaded metal inserts or if the load is not to high, to use special thread forms designed for plastics







Regards
Pat
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[bobslo]

Try with phenolic.It is an "old" fashion material but it has high E-module , high strength, and no creep effect.
Machining reinforced phenolic is superb.
It is cheap.

Back draw: weight (starts with 1.6 g/ccm).

 

[patprimmer]

Phenolic is certainly good to high temperatures and compression and creep.

Coulors are VERY limited at has really poor elongation at break and poor impact. Urea or melamine gives more colour options.

It cannot be moulded as fast in normal thin sections as can thermoplastics. It moulds faster though in real thick sections

Regards
Pat
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