Feeding Polymer to the Extruder
The proper method of feeding polymer to a twin screw extruder depends on: the twin screw extruder design, the material being fed, and the feed location. In parallel counter and corotating twin screw extruders, single or multiple resin feed streams and additives are starve fed to the extruder feed section. Feeders deposit the formulation directly onto the extruder screw with the screw
speed set to process the formulation at a higher rate than it is deposited on the screw. There is no material build-up in the extruder feed section, and the throughput rate is determined by the total feed streams, and not the extruder screw speed. Additives, other resins, reinforcements, etc., can be added to the extruder melt at several downstream locations by either gravity feeding or with a side feed extruder. Liquid additives are introduced downstream with the aid of a liquid feed pump and liquid injection system. Each component is metered in the correct ratio to the total extrusion rate. The advantage of starve feeding is that all the formulation ingredients are fed in the proper ratio directly onto the extruder. Feed problems due to bridging or slippage in the extruder feed hopper are typically eliminated. Feeders are normally set up directly above the feed opening or on a mezzanine above the feed opening to deposit materials directly onto the screw.
Conical counterrotating twin screw extruders are normally starve fed, similar to other twin screw extruders. Additives, reinforcements, fillers, etc., can be fed downstream into the polymer melt using a stuffer or side feed extruder. The side feed extruder rate is normally run in a starve-fed mode for additive rate control. Side feeders are either single or twin screw extruders with zero compression screws that positively convey ingredients into the melt stream. This positive material conveying in side feed extruders provides higher downstream addition rates than by gravity feeding into a downstream barrel. When using high secondary feed rates downstream, a proper screw design is necessary to handle the high rates and accomplish the extrusion objectives of melting (if required) and mixing.
extruder feeding ( sample )