Page 310 - Plastics Engineering

Processing of Plastics 293

Cavity Large runnel Cavity Large runner
I ,

(b) Distributed runner system
Fig. 4.40 Insulated and distributed runner systems

Mould Clamping Force
In order to prevent ‘flashing’, i.e. a thin film of plastic escaping out of the
mould cavity at the parting line, it is necessary to keep the mould tightly
closed during injection of the molten plastic. Before setting up a mould on
a machine it is always worthwhile to check that there is sufficient clamping
force available on the machine. To do this it is necessary to be able to estimate
what clamping force will be needed. The relationship between mould area
and clamp requirements has occupied the minds of moulders for many years.
Practical experience suggests that the clamping pressure over the projected area
of the moulding should be between 10 and 50 MN/m2 depending on factors
such as shape, thickness, and type of material. The mould clamping force may
also be estimated in the following way. Consider the moiilding of a disc which
is centre gated as shown in Fig. 4.41(a). The force on the shaded element is
given by
i
Force, F = Pr2nrdr (4.23)
o
The cavity pressure will vary across the disc and it is necessary to make some
assumption about this variation. Experimental studies have suggested that an
empirical relation of the form
P, =Po (1 - (i)“) (4.24)

is most satisfactory. Po is the pressure at the gate and rn is a constant which
is usually between 0.3 and 0.75. It will be shown later (Chapter 5) that ‘tn’ is
in fact equal to (1 -n) where ‘n’ is the index in the Power Law expression for
polymer melt flow.