mixing screws for injection molding

Almost all processes using a single screw require some degree of mixing, whether it’s just to homogenize regrind with virgin resin or to mix in some color concentrate.
There are three kinds of mixing: dispersive, distributive, and extensional. Extensional mixing occurs predominantly in the twin screw extrusion process. Dispersive mixing is similar to using two materials to be mixed on two plates and rotating one plate. The shear stress developed in the polymer between the plates would be proportional to the distance between the plates and the speed at which the plate was rotated. Distributive mixing would be similar to putting the two materials in a bowl and mixing with a spoon. The number and path of the spoon strokes would be proportional to the degree of mixing. Screws inherently provide both dispersive and distributive mixing. Generally, the shallower the screw the more dispersive mixing occurs, and the more disruptions in the flow path the more distributive mixing occurs. dispersive mixing occurs as the polymer and additives pass through the narrow gap between the screw flight and barrel, where the shear stress is very high. Distributive mixing occurs as the polymer leaking over the flights is continuously blended into the polymer in the adjacent channels dispersive mixing occurs as the polymer and additives pass through the narrow gap between the screw flight and barrel, where the shear stress is very high. Distributive mixing occurs as the polymer leaking over the flights is continuously blended into the polymer in the adjacent channel dispersive mixing occurs as the polymer and additives pass through the narrow gap between the screw flight and barrel, where the shear stress is very high. Distributive mixing occurs as the polymer leaking over the flights is continuously blended into the polymer in the adjacent channels. Worn screws would be an easy solution to many mixing problems, but the downside is you get decreased output from them. As the polymer leakage over the flights increases with increasing barrel/screw clearance, both dispersive and distributive mixing increase significantly. The above mentioned information is for illustration of the principles and are not necessarily the only recommended designs.