The higher the crystallinity, the lower the tensile and tear strengths.

but for example HDPE has got high crystallinity,but the film that is made of it has goft good tensile strenght

doesnt it mean that orientation in HDPE causes its crystality and thus higher tensile strenght ?

their exact relation with crystality

When you talk about crystality, are you talking about Work hardening?

not at all

i mean crystality in polymers

i missed your second post.

i totally mixed up !!!The polymer engineering is extremley confusing

The more specific the question, the better the answer.

Crystallinity in Polymers

this is the schematic of film blowing process

process can be controled to be long neck

Operation mode: long neck

" film of HDPE (high density PE ) produced by long neck method,cause if we produce it with short neck method its tear strength will be severly reduced"

i know that with long neck method ,we have Biaxial oriented film ..i think that this orientation causes high crystality in HDPE,and i do not know how it causes low tear strength... and if our polymer has got high tensile strength how is it possible not to have high tear strength? for example according to my studying, films of HDPE tear easily but they have good tensile strength

It's all relative.

Are you a film blower? Or, a student?

student...

Student. OK.

Polymers are different than metals or solids in lots of ways.

Crystallinity may have a different meaning, depending.

Are you asking if stretch blow molding is better?

Thank you so much for all your help I really appreciate it
"but comparing tear resistance of the two cannot be done directly because the stretched film is now thinner than the unstretched film "
I did not pay attention to this point at all
and about your example,it was awesome
Thank u again and also others for great help..

You're welcome Melody.

P.S. We do have a post rating system here ;)

Sounds like you are describing true stress true strain as the material elongates and thins. Milo