EFFECTS OF EXTRUSION BLOW MOLDING INTERNAL COOLING ...

“Crystallization is useful in blow molding because (1) it freezes [the container] in
the stretch orientation and thus gives the oriented structure permanence; and (2)
it improves many end-use properties of particular importance in food packaging,
including rigidity, dimensional stability on reheating, and impermeability. On the
other hand, crystallinity tends to harm some useful properties such as ultimate
elongation, impact strength, transparency, and environmental stress crack
resistance” (Rosato et. al., 2004).
If we assume that ∆H
f
is proportional to the % crystallinity of the test
specimen and if we know the ∆H
f
of the test specimen in pure
crystalline form (100% crystallinity), we can compute the %
crystallinity as follows:
∆H f
% Crystallinity = x100%
∆H f *
Where:
∆ H f
∆ H f
= heat of fusion of semi-crystalline polymer, J/g
* = heat of fusion of 100% crystalline sample, J/g. For PE, this value is
286.2 J/g (Selke and Xiong, 2003).
The Fasti unit’s internal cooling technology is likely to affect crystallinity of the
finished container due to the quicker cooling and therefore shorter period of time
during which the bottle is at its crystallization temperature.
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