The mixing ability of the standard single stage and two stage extruder screw is limited. Plastics melts are not capable of the turbulent motion that is exhibited in low viscosity liquids. Lower viscosity liquids have smaller and shorter molecules. Water, gasoline, juices are
examples of low viscosity liquids. These materials are called Newtonian fluids. As they flow, the molecules move past each other without much interference. The more interference the thicker the fluid. As plastic molecules begin to flow, they entangle with each other forming layered groups. Larger and longer molecules produce a higher viscosity melt that flows in layers. These materials are called non-Newtonian fluids. All most all plastic materials are considered non-Newtonian. The exceptions
are, silicones, polycarbonates, and polyethylene terephthalate, which are considered almost Newtonian (2). This non-Newtonian flow requires a convective mixing action. Convection means to heat a liquid by circulating currents from one region to another. Convective mixing action in the melt occurs by shearing the laminar flow and at the same time elongating (stretching) the layers. In the standard extruder screw, the plastic does not reach complete melt until the ends of the metering
section. This restricts the uniform mixing of additives throughout the metering section. To overcome this problem, mixing section have been designed and added to the metering section of the screw. A mixing screw is a modification of a standard screw that incorporates one or more special mixing sections. Figure 5-17 illustrates a specially designed mixing screw for linear low destiny polyethylene (LLDPE). Figure 5-10 shows several variations of mixing screws.
There are two classifications of elements added to form a mixing screw: (1) dispersive elements and (2) distributive mixing elements. Dispersive mixing elements are used when agglomerates or gels need to be broken down. This type of mixing element is especially important in the extrusion of small or thin gauge tube, fiber spinning, and thin film. There are two main types of dispersive mixing elements. The Maddock mixing element shown in Figure 5-18 and the Egan mixing elements
shown in Figure 5-19. The Maddock mixing elements are added to the screw geometry at the end of the metering section. In these elements the material enters an inlet channel which is blocked at the end. This restricts the flow of the plastic melt forward and forces it to flow, at 90o over a barrier that runs parallel to the barrel wall. The material is subjected to stretching, smearing and a high shear rate as it flows from the inlet channel over the lower flight and into the outlet channel.
The shear flow over the Maddock flight is large enough to break down particles of plastics that may be within the melt. Past the lower flight, the melt enters the outlet channel and continues on to the die. All material is forced over the barrier flight yielding a uniform homogenous melt. The problem with the Maddock mixing element is that pressure flow in the metering section is significantly reduced which reduces the output of the extruder. Additionally, the design of the Maddock elements
tends to collect stagnant material in at the back of the entry and exit flute. To overcome this problem, the flute angle of the Maddock elements was redesigned so it's less than a 90 degree angle; this resulted in the Egan mixing section. The helical fluted mixing of the Egan design reduces the pressure drop of the Maddock element and eliminates the stagnant regions. The Egan element also tapers to the inlet channel and the outlet channel. This change in design contributes to both an
improvement in extruder output and better mixing. Distributive mixing elements are added to the extruder screw when to or more plastics are blended together with similar viscosity's. The mixing of theses materials is relatively easier with distributive mixing. The three most common distributive mixing elements are pin, Saxton, and pineapple illustrated in Figure 5-20. These are used in the mixing of color concentrates where the breakdown of the pigment below a critical size is crucial and
in the mixing of polymer blends (6).