Page 88 - Engineering Plastics Handbook

62 Introduction

thermoforming, and blow molding. Observing the melt flow through a
capillary is used to measure apparent viscosity, and it offers information
on melt flow during various processing conditions [20].
ASTM D3835/2000 test method measures rheological properties of
thermoplastic (and thermosetting) melts by using a capillary rheome-
ter [4]. The test method includes measurements of viscosity, shear rate,
shear stress, swell ratio, and percent of extrudate swell. Assuming a
newtonian fluid, to calculate melt viscosity µ, use
4
µ= Fr t Pa ⋅s
2
8 rLI ′
where F = force on ram, N
r = radius of capillary, m
t = extrusion time, s
L = length of capillary, m
3
I′= volume extruded, m
To calculate shear rate S ,
r
S = 4 Q = 4 I′ s −1
r π π rt
r 3 3
2
where Q = ram speed (m/s) × barrel cross-sectional area (m )
3
= flow rate (m /s)
Die swell ratio and melt shear modulus exhibit melt viscoelasticity.
Rheological behavior can be determined with small-amplitude sinu-
soidal shear, using the cone-and-plate steady-shear test to determine the
linear viscoelastic shear strain. A sinusoidal curve is charted to repre-
sent the viscous (loss) modulus (out-of-phase segment) and the elastic
(storage) modulus (in-phase segment) [2].

G′′ = viscous modulus
G′= elastic modulus
δ= phase angle, rad

G ′′ = tanδ
G ′
Viscous and elastic moduli with small-amplitude sinusoidal shear can
be determined by using an orthogonal rheometer [2]. Small-amplitude
sinusoidal shear, using cone-and-plate or parallel-plate test methods, can
determine rheological behavior for normal stresses in shear flow, as well
as for shear strain.