Melt Temperature range & Transition temperature.

[Mandar]

Hello All,
If material has Melt temperature range from 230-250 deg. and its TG is 90deg. So my question is what will happen in between 90deg to 230deg, as material will get solidify below 90deg.



Thanks and regards,
Mandar.

[MTUHusky]

Is the material amorphous or semi-crystalline?

If amorphous, then nothing is really happening except cooling and some shrinking. The polymer chains are coming closer together.

If semi-crystalline, then between 230 and 90 crystals are forming. The long polymer chains are folding up into little crystals. The crystal formation increases density while decreasing the size (shrinkage).

Husky

[JayDub]

Tg is glass transition temperature. In a semi-crystalline plastic, it is the point where the bonds holding the crystallites together start to break down. The material is still solid above Tg until reaching melting range, but there are a couple of implications:

If you process a material using a mold temperature below its Tg, parts will freeze in the amorphous state. Shrink will be less than anticipated and physical properties will generally be poorer.

Annealing parts above Tg will relieve molded-in stresses – may alleviate warp.

Heating above Tg and cooling slowly will allow maximum crystallinity to develop, for best dimensional stability and physical properties.

Materials whose Tg is below room temperature (notably polypropylene) are inherently dimensionally unstable. Try to hold a tight tolerance and you may find the parts are in spec today and out of spec next week.

Hope that helps.

[Mandar]

Thank you JayDub and MTUHusky for answering .
It really useful . :o

[chrisprocess]

Just wanted to point out Tg is generally determined at 1 atmospheric temperature..
At high pressure it will likely change..

See the below graph of Tg at various temperatures and pressures for PP..
203

Here's a link of the article from which I pulled this image:
http://nvlpubs.nist.gov/nistpubs/jres/68A/jresv68An3p273_A1b.pdf

I was a little confused at first because Tg of PP at 1 ATM is 0C - but then the article describes the shift is likely from their testing method(s)/delays in measuring ?

At least it's clear as a rule of thumb Tg actually will be higher when the material is under pressure..
My guess is this would apply to most semi-crystalline materials.

-I thought was interesting