Manufacturing methods vs mechanical properties 1

[georgacus]

How much does the mechanical properties of polymers like PTFE or PEEK change depending on the process it was formed from? For example a billet formed from extrusion as opposed to a billet formed from moulding.

We buy in valve seats to an internal spec which says that the raw material can be either extruded or moulded. However, we aren't sure if the forming method makes a huge difference.

If someone could point me to some reading material on this topic or explain, that'd be great. Thanks

 

[Demon3]

That's quite complex to answer.

For unfilled polymers the two would have similar properties assuming the polymer grade used is the same.
There could be some differences in alignment of the polymer chains giving anisotropic properties but that should be minor.

If a spherical (very low aspect ratio) filler is used then there should be no big differences.

If you use a reinforcement like talc, mica or glass fiber then the differences become large because of how that type of particle aligns in the flow. The flow pattern for extrusion is very different to injection molding.

It is important to note that injection molding grades of plastics usually have an MFI of 10-30 or so whereas extrusion grades often have an MFI of 1g/10min or less. That means that extrusion polymers have much higher molecular weight which means more strength, better environmental stress cracking resistance and better impact resistance.

PTFE is pretty famous for not being moldable because it doesn't melt. There is a trick to overcome that but it involves adding low molecular weight material to improve the flow and again that means lower strength etc..

Dr. Chris DeArmitt
President

Expert plastic materials help

http://www.phantomplastics.com

 

[georgacus]

Thanks Demon3. The orientation of the glass fibres makes sense in terms of strength.

What do you mean by PTFE not melting?

 

[Pud]

What do you mean by PTFE not melting?

It degrades* before it gets fluid enough to process.

H

* Gives off of HF when it degrades - not nice...

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.