The Design of a TM 10 Resonant Cavity Microwave Applicator as a Preheating and Crosslinking Die for Pultruded Composites
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THE DESIGN OF A TM 10 RESONANT CAVITY MICROWAVE APPLICATOR AS A PREHEATING AND CROSSLINKING DIE FOR PULTRUDED COMPOSITES S.R.Ghaffariyan and J.M.Methven, Division of Polymer Engineering, PO Box 88, Manchester M60 IQ0, England.
UMIST,
PULTRUSION Pultrusion is
the only continuous process available for the manufacture
of fibre reinforced thermosetting composites.
It
its simplest form it
requires
that dry reinforcement in the form of continuous rovings or fabrics is pulled through a shallow bath of resin and then through a heated metal die wherein the resin crosslinks to a rigid
network.
The
rate of manufacture
(pulling
speed) depends on two factors; the rate of heat transfer to and through the composite by conduction from the (heated) die wall and the generated internally by the (exothermic) crosslinking reaction.
rate
of
heat
Heat Transfer Heat conduction is determined by the thermal diffusivity of the composite (
l-5x10- 7 mZs- ),
a squared
temperature gradient (100K
length dimension,
L,(thickness,
radius),
the
5 AT : 200K) between the die wall and the profile,
and the boundary condition(s) at the die wall(s). This is usually expressed in terms of a convective heat transfer coefficient h, where -k(dT/dL)W = h(TW-T0) and
T
is
the
nominal
wall
(mould)
temperature.
h
is
in
the
range
(200Wm- K-1 5 h : 2000Wm-2 K-). It
is
useful to describe pultrusion in terms of an equivalent
number where N
Or
Graetz
= Rate of axial convection/Rate of transverse conduction.
For a cylindrical die of length Z and radius R the numerator is (v/Z)pCp and the denominator k/R 2 where v is the pulling speed, k is the thermal conductivity
and
C
the
specific
heat
capacity.
Putting
the
thermal
p
diffusivity, o as k/(pC ) gives P
No0 = VR/ This temperature
is
equivalent T to
to
penetrate
the
ratio
to the
(1) of
centre
the
time
of the
taken
profile
for
the
(Fo=l),
mould to
the
residence time in the die. Equation 1 has no physical meaning in the context of pultrusion but by correlating profile size with pulling speed for a variety of systems, N0z is beteen 2 and 4 depending upon the type of resin and reinforcement employed. The chemical reaction rate can now be incorporated by equating the numerator of equation 1 to the sum of the heating time and the reaction time.
Mat. Res. Soc. Symp. Proc. Vol. 189. 01991 Materials Research Society
136
Moreover the reaction rate can be expressed as equivalent to the (isothermal) rate at the temperature which develops in the centre actual die exit, T . This is given approximately from
of the profile at the
T -T
e
=
= Cexp(-_ C
Fo)
(2)
V
where the subscripts refer to the (w)all and (c)harge temperature. number Fo=cxt/R 2 , and the time, t here, by (Z/v).
The Fourier
is the residence time in the die given
Values for C and ý for a cylinder are 1.6015 and 2.309 [i].
Taking an example of a cylinder
(R=6mm,q6=0.65),
charged at T=25*C and
pulled at 0.5m/min through a im die at 150 0 C, gives T =120cC.
The required
reaction
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