Page 306 - Plastics Engineering

Processing of Plastics 289

50
ir

rad = 8 mm rad = 6 mm

Fig. 4.37 Lay-out for eight cavity mould

cavities were filled. In a good mould design, all the cavities fill simultaneously
at the same pressure. In this case it is necessary to ensure that the pressure
drop in Sections 1 and 3 is the same as the pressure drop in Section 2.
It will be shown in Chapter 5 that the pressure drop, AP, for isothermal flow
in a circular section channel is given by

(4.22)

where q = viscosity of the plastic
L = length of channel
Q = volume flow rate

R = radius of channel
If the volume flow rate towards point J is q (ie the input at the sprue is 2q)
then at J the flow will split as follows:
Flow along runner 1 = xq
Flow along runner 2 = 1/2( 1 - x)q

Flow along runner 3 = 1/2xq
where

(A refers to the area of the relevant runner).
Using equation (4.22) we can write
8rlL1xq
Pressure loss in runner 1 = -
n@