Page 305 - Plastics Engineering

288 Processing of Plastics

eject. A compromise is a trapezoidal section (cut into one half of the mould)
or a hexagonal section.

(c) Sprues: The sprue is the channel along which the molten plastic first enters
the mould. It delivers the melt from the nozzle to the runner system. The sprue
is incorporated in a hardened steel bush which has a seat designed to provide
a good seal with the nozzle. Since it is important that the sprue is pulled out
when the mould opens it is tapered as shown in Fig. 4.35 and there is a sprue
pulling device mounted directly opposite the sprue entry. This can take many
forms but typically it would be an undercut or reversed taper to provide a key
for the plastic on the moving half of the mould. Since the sprue, like the runner
system, is effectively waste it should not be made excessively long.

(d) Venting: Before the plastic melt is injected, the cavity in the closed mould
contains air. When the melt enters the mould, if the air cannot escape it become
compressed. At worst this may affect the mould filling, but in any case the
sudden compression of the air causes considerable heating. This may be suffi-
cient to bum the plastic and the mould surface at local hot spots. To alleviate
this problem, vents are machined into the mating surfaces of the mould to allow
the air to escape. The vent channel must be small so that molten plastic will not
flow along it and cause unsightly flash on the moulded article. 'I)lpically a vent
is about 0.025 mm deep and several millimeters wide. Away from the cavity
the depth of the vent can be increased so that there is minimum resistance to
the flow of the gases out of the mould.

(e) Mould Temperature Control: For efficient moulding, the temperature
of the mould should be controlled and this is normally done by passing a
fluid through a suitably arranged channel in the mould. The rate at which
the moulding cools affects the total cycle time as well as the surface finish,
tolerances, distortion and internal stresses of the moulded article. High mould
temperatures improve surface gloss and tend to eliminate voids. However, the
possibility of flashing is increased and sink marks are likely to occur. If the
mould temperature is too low then the material may freeze in the cavity before
it is filled. In most cases the mould temperatures used are a compromise based
on experience. In Chapter 5 we will consider ways of estimating the time taken
for a moulding to cool down in a mould.
Example 4.5 The runner lay-out for an eight cavity mould is illustrated in
Fig. 4.37. If the mould is to be designed so that the pressure at the gate is the
same in all cases, determine the radius of the runner in section A. The flow
may be assumed to be isothermal.
Solution Although this runner system is symmetrical, it is not balanced. If
the runner had the same diameter throughout all sections, then the mouldings
close to the sprue would fill first and would be over-packed before the outermost