Core-Related Mechanical Properties

Bowman [192] conducted a systematic study of the structure-property relations of injection molded polyacetals (polyoxymethylene or POM) and observed correlations between process conditions, structures and mechanical properties. Barrel temperature effects were studied as they are known to influence both microstructure and mechanical properties [193]. Increased barrel temperature was shown to reduce the outer skin layer while increasing the extent of the equiaxed, unoriented core, resulting in a decreased tensile yield strength parallel to the injection direction. 

Birefringence provides a measure of the local orientation of a material (i.e., the mean orientation of monomer units). The relation between orientation and birefringence was known from early studies of polystyrene filaments, which described both the theory and measurement [23, 25]. They showed that the orientation was greater at the surface than in the core. Mechanical properties, such as tensile strength and elongation at break, have been shown to... 

As indentation is performed at coating/core interface (I), hardness and elastic properties of core (S) and coating (R) are concerned during loading. The following relation was proposed to take into account the latter mechanical properties ... 

It is known that bulk or average density is the major parameter controlling the mechanical properties of polymer foams. Nevertheless, density profile has also a major impact especially for structural foams where an unfoamed skin is enclosing a foamed core. In this study, the effect of density profile on the flexural modulus of a structural foam is discussed in terms of skin thickness and taking into account the local variation of density (related to void fiaction) with position. 

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