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Injection moulding of plastics: Some ideas about the relationship between mould filling and birefringence

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Summary

Under practical mould filling conditions one always observes an interaction between polymer flow and heat transfer. A rather rough description of this process is given in the form of a dynamic model for the growth of the solidifying layer. A consideration is added concerning the usefulness of the birefringence pattern, which is observed in the moulded sample, as a source of information about previous melt flow during mould filling. The rubberlike stress-optical behaviour of polymer melts is advocated as a powerful means in finding the thickness of the so-called freeze-off layer and in tracing back the development of shear stresses in the vicinity of the wall during injection.

Zusammenfassung

In der Praxis des Spritzgießens beobachtet man immer ein Zusammenspiel zwischen Fließvorgang und Wärmeübertragung. Eine ziemlich grobe Beschreibung dieses Vorgangs wird in der Form eines dynamischen Modells zur Berechnung des Anwachsens der erstarrenden Schicht gegeben. Hinzugefügt wird eine Betrachtung über das Doppelbrechungs-Streifensystem, wie man es im Spritzling findet, und seine Verwendbarkeit als Informationsquelle für den Fließvorgang während des Füllens. Das gummiartige spannungsoptische Verhalten der Polymerschmelzen wird als sehr geeignetes Hilfsmittel zur Bestimmung der Dicke der während des Einspritzens erstarrten Schicht anempfohlen. Ebenso sollte sich hieraus die Möglichkeit ergeben, den während der Injektion auftretenden Verlauf der Schubspannungen in Wandnähe zurückzuverfolgen.

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Abbreviations

a :

heat diffusivity of polymer melt m2 s−1

b :

temperature coefficient of viscosity K−1

B :

breadth of mould channel m

c :

heat capacity of polymer melt j kg−1 K−1

C :

stress-optical coefficient of melt m2 N−1

D :

reduced flow rate in rectangular duct s−1

h :

heat-transfer coefficient j K−1 s−1 m−2

Gz:

Graetz-Number (4H2 \(\bar v\))

H :

half height of mould channel m

L :

length of mould channel m

Nu:

Nusselt-Number (2H h/λ)

n :

refractive index tensor (deviatoric components)

Δn :

flow birefringence

p :

Cauchy stress tensor (deviatoric components)

P :

pressure at the mould wall N m−2

Q :

volumetric flow rate m3 s−1

T :

temperature K

T i :

injection temperature

T l :

temperature at which flow virtually stops in the channel

T m :

mould temperature

T g :

glass-transition temperature

t :

time s

t i :

injection time

\(\bar v\) :

average velocity of flowing polymer m s−1

x :

coordinate perpendicular to large surface of sample (of mould)

z :

coordinate in the flow direction

δ :

thickness of stagnant layer m (atT l)

δ ∞ :

steady state thickness of stagnant layer (atT l,∞ )

δ 0 :

optically detectable freeze-off thickness (atT g )

λ :

heat conductivity of polymer melt j K−1 s−1 m−1

ρ :

density of the polymer melt kg m−3

τ :

dimensionless time (eq. [2])

ψ :

dimensionless layer thickness (eq. [3])

χ :

extinction angle (eq. [17])

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  1. Laboratory for Technology of Macromolecular Compounds Department of Chemistry, Delft University of Technology, Julianalaan 136, Delft 8, The Netherlands

    H. Janeschitz-Kriegl

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Janeschitz-Kriegl, H. Injection moulding of plastics: Some ideas about the relationship between mould filling and birefringence. Rheol Acta 16, 327–339 (1977). https://doi.org/10.1007/BF01534086

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