Today's KNOWLEDGE Share : Extensional Flow

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Today's KNOWLEDGE Share

Why should we avoid branched polymers in Injection Molding and always keep thickness constant when possible ?



In a transition from thick to thin, the polymer undergoes substantial extensional flow. This aspect of flow is challenging to measure in a lab and difficult to implement correctly in Flow Analysis. When polymers are non-linear in their chain structure a strong "strain-hardening" will appear, resulting in unexpectedly high pressure drop at these thickness transitions. Runner to gate transitions may present significant pressure drop due to this elongational viscosity effect (possibly hundreds of bar, thousands of psi).


We have of course many other good reasons to keep thickness as uniform as possible, first of which, the attempt to have uniform freezing rates everywhere, get uniform packing and minimize differential shrinkage.


sourece:Vito leo

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