Page 307 - Plastics Engineering

290 Processing of Plastics

Pressure loss in runner 2 =

Pressure loss in runner 3 =

Thus, equating pressure losses after point J

Substituting for x and rearranging to get R2

For the dimensions given:
R2 = 3.8 mm

In practice there are a number of other factors to be taken into account.
For example, the above analysis assumes that this plastic is Newtonian, ie
that it has a constant viscosity, 11. In reality the plastic melt is non-Newtonian
so that the viscosity will change with the different shear rates in each of the
three runner sections analysed. In addition, the melt flow into the mould will
not be isothermal - the plastic melt immediately in contact with the mould will
solidify. This will continuously reduce the effective runner cross-section for the
melt coming along behind. The effects of non-Newtonian and non-isothermal
behaviour are dealt with in Chapter 5.

Multi-Daylight Moulds
This type of mould, also often referred to as a three plate mould, is used when
it is desired to have the runner system in a different plane from the parting line
of the moulding. This would be the case in a multi-cavity mould where it was
desirable to have a central feed to each cavity (see Fig. 4.38). In this type of
mould there is automatic degating and the runner system and sprue are ejected
separately from the moulding.

Hot Runner Moulds
The runners and sprues are necessary in a mould but they are not part of the
end-product. Unfortunately, it is not economically viable to discard them so
they must be re-ground for subsequent reprocessing. Regrinding is expensive
and can introduce contamination into the material so that any system which
avoids the accumulation of runners and sprues is attractive. A system has been
developed to do this and it is really a logical extension of three plate moulding.
In this system, strategically placed heaters and insulation in the mould keep the