Today's KNOWLEDGE Share : The annoying saddle twist warpage of low MFI polyolefin grades

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 Today's KNOWLEDGE Share

147. The annoying saddle twist warpage of low MFI polyolefin grades.


If you ever molded extrusion or blow-molding grades of PP or PE, you have certainly experienced a big warpage problem, stemming from the surprisingly higher than usual IN-FLOW shrinkage.


The graph on the right (similar to my previous post nr. 146) shows that moving to a lower Melt Index increases the chance of freezing more molecular orientation in the part, as a result of the longer relaxation time of low MFI grades.


The graph in the center, from the cited article of 2006, shows the trend of increasing parallel to flow shrinkage with increasing molecular weight of a simple unfilled PP homopolymer.

Note that the three grades are all more viscous than typical in Injection Molding, and they all show a strong anisotropic shrinkage (this is measured on a dog-bone classical tensile bar sample) with larger shrinkage in the flow direction, due to strong molecular orientation.

It is the exact contrary of classical higher flow PP grades where perpendicular shrinkage is higher than parallel !


Finally, as seen on the left drawing (by Covestro), a centrally gated part will warp in a “saddle twist” fashion when the In-FLOW shrinkage is larger than the CROSS-FLOW shrinkage (perimeter wants to be larger than the corresponding radius).


When dealing with these viscous “twisty-warpy” grades one could in theory add just a few % GF that create the opposite effect of lower IN-FLOW shrinkage. Magically, you could suddenly mold a perfectly flat disc, by adding just a pinch of GF to the mix !

A higher packing (qualifying probably as OVERPACKING of the part center) would also flatten these part by creating the opposite warp trend, something illustrated in classical MOLDFLOW® literature more than 40 years ago.


What is your own experience with low MFI grades ?

Please comment and share so that we all learn more about this issue.


source : Vito leo

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