Today's KNOWLEDGE Share:FOUNTAIN FLOW AND FREEZING

-

Today's KNOWLEDGE Share:

FOUNTAIN FLOW AND FREEZING:

I often get questions in my seminars about molecular orientation in molded specimens.

People often get a bit confused about the layers that could possibly show some degree of molecular orientation, potentially observed as birefringence for transparent grades.





- The very skin is coming directly from the fountain flow and freezing, so it is not very oriented at all since it never experienced much flow in the main direction.


- The "frozen skin" below it is extremely oriented since, by definition, it freezes UNDER FLOW. As a result it is very oriented, almost regardless of the polymer relaxation time / molecular weight.


- Other layers below, approaching the center line can show orientation induced during the second stage of the molding process, the packing phase. This flow stress is active at later times, thus acting on deeper layers.


- Although thick parts should show little orientation below the frozen skin, high molecular weight materials could still show significant orientation if their relaxation time exceeds the overall cooling time of the specimen. Try molding a blow-molding grade and see for yourself !


- Strictly speaking, the very center line should not show orientation since the shear-rate and shear-stresses are zero at all times during the process.


A quick test to assess the degree of average orientation (which really works perfectly for opaque crystalline polymers) is to examine the degree of (post-)shrinkage in the flow direction when annealing the specimen at the appropriate temperature (just below Tg for amorphous, just below Tm essentially for semi-crystalline).


Orientation is not strictly speaking a "residual stress" given the weakness of the entropic force at play. Upon reheating or softening with a solvent (PVC and acetone for instance reveal flow induced orientation), such tiny force can produce this significant post-shrinkage.


Source:VITO LEO

Visit MY BLOG http://polymerguru.blogspot.com


#plastics #injectionmolding #amorphous #semicrystalline #relaxationtime #molecularorientation #freezing #orientation #flow

Comments

Post a Comment

Popular posts from this blog

Today's KNOWLEDGE Share:PLA BOTTLE

-
Image
Today's KNOWLEDGE Share Ever wondered how we can reduce plastic pollution in India? The Mysuru-based Defence Food Research Laboratory (DFRL), affiliated with the DRDO, has launched an eco-friendly remedy with the introduction of biodegradable water bottles. Crafted from sustainable Polylactic acid (PLA) material, these bottles not only combat plastic waste but also contribute to reducing the overall carbon footprint. The best thing about this bottle is its all-encompassing sustainability approach – the bottle, cap, and label are all compostable. This innovation, derived from 100% bio-based and renewable sources, sets us on a path towards environmental conservation and fosters a greener and more sustainable future. Source:The Better India #polymers #bioplastics #pla   #compostable   #ecofriendly   #sustainability
Read more

Today's KNOWLEDGE Share:High and low shear rate changes in Rheology

-
Image
Today's KNOWLEDGE Share If a relatively high Mw grade flows from thin to thick, as shown in the picture, and if the time needed to fill the thick section is way shorter than the average polymer relaxation time, the polymer will not be able to reach its higher viscosity (corresponding to higher thickness/lower shear-rate). I am assuming here the polymer reached equilibrium low viscosity in the thin section, say for instance because it is much longer. Any pressure prediction not accounting for transient rheology would then overpredict the pressure to fill.Of course the diverging flow will further complicate the situation and mess with the molecular orientation created in the thin section, but to what extent ? Since this is more of a reasonable "conjecture", don't take this for granted, and discuss or share your opinion in the comments. There are hundreds of Rheology papers on the "4:1 contraction flow". Not sure I have ever seen any on the..."1:4 expansio
Read more

Today's KNOWLEDGE Share : biobased content of polycarbonate diols

-
Image
Today's KNOWLEDGE Share MCG increases biomass content of polycarbonate diols Mitsubishi Chemical Group (MCG) has introduced grades of its Benebiol biomass-based polycarbonate diols that have a higher biomass content. The product is primarily used in the manufacture of polyurethane resins for coatings in sectors such as automotive, furniture and outdoor products. It is also used in the production of artificial leather. Existing Benebiol grades had a bio-based content of 20-50%, but the new HSS and NLDS grades have a biomass content in excess of 80%. Some of its previous petroleum-derived grades have also been converted to biomass, as the NLS grades. This, the company said, allows it to offer bio-based grades of all its Benebiol products. The shift to biomass-based raw materials while maintaining high-quality functions will contribute to our customers’ efforts to achieve sustainable manufacturing. source:MCG/www.utech-polyurethane.com
Read more