As a plastic melt starts to flow in a mould which is typically 200ºC cooler than the melt, a frozen skin forms as the melt touches the mould surface. The melt flows within this frozen skin. The velocity of the flow is fastest at the centre with the result that flow at the melt front is like that of a fountain with material being predominantly supplied to the flow front by that in the centre of the flow. The shear rate measured from the outer wall to the centre decreases. This differential velocity in a flow can be seen in many circumstances where the flow remains laminar.
If two injection units are configured to supply polymer into a mould through a single orifice such that one flow is completely contained within the other then it is possible to produce a moulding where the first material forms all of the external surfaces of the part but which the interior is entirely of the second material. There is a need to control both the viscosity and the volumetric flow rates in order that the best distribution of the two materials is achieved.