3. Thermoplastic Processing
3.1. Injection Moulding
3.1.3. Polymer Flow
Understanding the polymers rheology and flow is essential in correctly designing the mould tool and choosing the processing parameters. The polymer flow is
predominantly affected by the temperature, although can be affected by other factors. When the polymer contacts the mould surface, it immediately starts to freeze,
creating a frozen polymer skin with a molten core of polymer. The polymer then starts to flow in what is known as a fountain flow. H. Mavridis, et al. [62]report that
“Fountain flow describes how liquid particles decelerate as they approach a slower moving interface and spill over towards the region vacated by the advancing
interface”. This essentially describes a layer of polymer which flows from the centre outwards and becomes stationary, whilst another layer of polymer flows over the top and stops. A diagram based on an illustration by A.G Gibson [15] shows the polymer flow through a simple channel along with the polymer velocity and shear rate at a low injection rate. The fountain flow has a high orientation effect on the polymer creating different regions of alignment to the flow direction. This is because the flow stretches
the polymer before solidifying against the mould surface. This can be especially noticed when fibre filled polymers are used.
Figure 3.3 - Polymer flow of filling stage by injection moulding [15]
Figure 3.3 shows the velocity and shear rate profile of the polymer melt as it flows along the mould surface, leaving a solid layer behind. The velocity of the polymer is at its greatest in the centre of the melt. The greatest shear rate is towards the edges adjacent to the solid polymer layer. The non-isothermal conditions cause the highest shear rates to be a small distance from the solid layer [15].
Figure 3.2 illustrates a typical filling pattern for an injection moulding in a basic deep cavity mould tool and to what position in the mould each section will flow. The drawing shows how the initial section flows outwards, towards the exterior of the mould in what is known as die swell. Die swell is where the polymer expands after
fifth and final section is the packing stage, compensating for any shrinkage of the polymer and ensuring a full moulding.
Polymer flow is not however always laminar and predictable. Like any fluid material, it can be subject to turbulent flow, which can cause processing problems.
In 1883, O. Reynolds conducted an experiment where water was drawn through a thin tube and observed a streak of die in the water at different velocities. At low velocities, the die remained smooth and stable (Figure 3.4 (a)). However when the velocity reached a critical value, the flow became unstable and eddy currents formed (Figure 3.4 (b)). The turbulent state moved towards the tube opening as the velocity increased, but would not go past the opening (Figure 3.4 (c)). Reynolds derived a dimensionless number called Reynolds number, and this equation was used to derive a non-slip condition formula (Equation 3.1). Where U is the average velocity, v is the kinematic viscosity and L is the length, over which the characteristic happens, which in the case of injection moulding, would be the height of the channel.
Figure 3.4 - Drawings of Reynolds die experiments showing stable flow (a) and unstable flow (b,c) due to higher flow rates [63]
A polymer‟s flow is directly influenced by the shear rate, which is the rate at which a layer of polymer flows over an adjacent layer. At shear rates within the limits of the polymer, it has an even, laminar flow similar to the flow demonstrated by O. Reynolds in (Figure 3.4 (a)). Outside of these limits, the material can become unstable and show turbulent flow [64]. Turbulent flow is where the linear flow starts to break up creating small eddies. Turbulent flow should be avoided as it can result in poor surface finish and weakened structure due to the random organisation of the
polymer. Turbulent flow also causes a rapid cooling effect on the polymer, because the rotating eddy currents in turbulent flow transfer polymer from the hot centre of the flow to the cooler regions next to the wall surface, whilst at the same time,
transferring cool polymer at the surface into the centre of the hot polymer melt [64]. Hesitation marks occur when the polymer flow front temporarily stops and the surface
and resulting in a short moulding. Correct mould design is important in preventing secondary flow fronts starting in areas that would have frozen off, such as small section areas near the injection point. The narrow section would prevent the polymer from flowing into the gate until sufficient pressure had built up from the rest of the part being filled. Since the narrow section would be near the start of the flow, the polymer in this area would have cooled off already and the increase in pressure would result in polymer breaking through, causing a hesitation mark.