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Injection Molding creates non-monotonic crystallinity gradients through the thickness, and corresponding non monotonic elastic modulus.
On one hand the rapid quench of the skin (combined with fountain flow) reduces crystallinity of the most outer layers leading to typically half the nominal modulus ( PP data).
The high shear just below (frozen skin) will produce strong "flow induced nucleation" and more crystallinity ( and oriented structures). These layers can be 4X stiffer than the skin in PP.Finally the core section undergoes a more quiescent crystallization with slower cooling and shear rates and will have "average" crystallinity, larger non-oriented crystals and pretty much the data-sheet kind of modulus.
Source:Vito leo
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