Page 316 - Plastics Engineering

Processing of Plastics 299

nozzle valve rotates so that the skin material is injected into the sprue thereby
clearing the valve of core material in preparation for the next shot. In a number
of cases the core material is foamed to produce a sandwich section with a thin
solid skin and a cellular core.
It is interesting that in the latest applications of sandwich moulding it is the
core material which is being regarded as the critical component. This is to meet
design requirements for computers, electronic equipment and some automotive
parts. In these applications there is a growing demand for covers and housings
with electromagnetic interference (EMI) shielding. The necessity of using a
plastic with a high loading of conductive filler (usually carbon black) means
that surface finish is poor and unattractive. To overcome this the sandwich
moulding technique can be used in that a good quality surface can be moulded
using a different plastic.

4.3.6 Gas Injection Moulding
In recent years major developments have been made in the use of an inert gas
to act as the core in an injection moulded plastic product. This offers many
advantages including greater stiffnesdweight ratios and reduced moulded-in
stresses and distortion.
The first stage of the cycle is the flow of molten polymer into the mould
cavity through a standard feed system. Before this flow of polymer is complete,
the injection of a predetermined quantity of gas into the melt begins through a
special nozzle located within the cavity or feed system as shown in Fig. 4.45.
The timing, pressure and speed of the gas injection is critical.
The pressure at the polymer gate remains high and, therefore, the gas chooses
a natural path through the hotter and less viscous parts of the polymer melt
towards the lower pressure areas. The flow of gas cores out a hollow centre
extending from its point of entry towards the last point of fill. By controlling
the amount of gas injected into the hollow core, the pressure on the cooling
polymer is controlled and maintained until the moulding is packed. The final
stage is the withdrawal of the gas nozzle, prior to mould opening, which allows
the gas held in the hollow core to vent.
The gas injection process overcomes many of the limitations of injection
mouldings such as moulded-in stress and distortion. These limitations are
caused by laminar flow and variation in pressure throughout the moulding.
With the gas injection process, laminar flow is considerably reduced and a
uniform pressure is maintained. The difficulty of transmitting a very high pres-
sure uniformly throughout a moulding can also cause inconsistent volumetric
shrinkage of the polymer, and this leads to isolated surface sink marks. Whilst
cycle times are comparable with those of conventional injection moulding,
clamping forces are much lower. Also, by using gas to core out the polymer
instead of mixing with it, gas-injection overcomes a number of shortcomings
of the structural foam process. In particular there are no surface imperfections