Extrusion starved feed

For extrusion compounding, if the components have different densities (e.g. polymer versus fillers) or different shapes (pellets versus regrind flakes) then prefer to use the extruder for mixing. That is, operate the extruder with starved feeding conditions with the components metered separately into the extruder, see Section 9.11. On the other hand, for the blending of polymer feedstock for extrusion, if the components have very similar properties, then use tumble, rotating drum or ribbon blenders or rotor-stator blenders of the feed upstream of the extruder and use flood feeding of the mixture to the extruder. 

Starve feeding (starved feeding) n. In extrusion, regulating, with an auxiliary feeding device such as a weigh feeder or screw conveyor, the rate at which feedstock enters the feed port of the extruder so that the screw flights are less than ftill. 

Liquid colorants can create problems in extrusion such as solids conveying problems. This can be avoided by using a porous carrier resin. Several resin suppliers (e.g., DSM, Akzo, Montell) now have porous carriers for use with liquid additives these can be colorants or other additives, such as antioxidants, peroxides, silanes, etc. Mixing can further be improved by starve feeding the extruder as discussed in Section 11.3.4.2.5. 

In fact this idea was not completely original, but a recognition that this starved feeding behaviour illustrated by Maddock [8] in screw jacking experiments could be suitable for the avoidance of agglomerates generated by normal full channel extrusion. 

Table 13.1 Summary of screw details used in starved feed zone extrusion trials by Thompson, Donoian and Christiano [10] and by Elemans and Van Wunnik [11] ...

Figure 13.8 Extrusions comparing flood-feed with starved feed by increasing screw speed at constant throughput rate. Reproduced with permission from RH.M. Elemans and J.M. Van Wunnik, Polymer Engineering and Science, 2001, 41, 7, 1099. 2001, Wiley)...

During extrusion, the blend components are fed into the feed throat of the extruder. Compounding extruders are starve fed. This means that material is conveyed away from the feed throat more rapidly than the raw materials are fed into the barrel. Therefore the volume of raw material introduced is always less than the total available volume of the flights... 

Many extruder manufacturers now produce dump extruders which are fitted beneath the discharge door of an internal mixer and receive into their feed hopper the full charge of the mixer. The speed of extrusion of the machine is governed by a series of sensors in the feed-hopper, to ensure that the process is continuous and that the screw will not be starved of compound, thus ensuring a continuous production of the mixed rubber compound. Product from such a machine can be slab or pellets. 

The feed zone is the area where the moistened formulation is first introduced into the extrusion device. It includes a hopper to channel and distribute the flow of material into the chamber containing the screw(s). Most screw extruders will be operated with only a slight excess of feed or even in a somewhat starved state. Because, for extrusion, the material must be plastic, too much feed tends to build up over the screws and bridging is likely to occur. 

Thompson, M. R., G. Donoian, and J. R Christiano, Examinations of Starve-Fed Single Screw Extrusion in Conventional and Barrier Feed Screws, 57th Annual Technical Conference of the Society of Plastics Engineers, 1999, p. 145. 

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