Screw feed extruders, design

There are two types of cold feed extruders, the standard screw design type (Fig. 15) and one with a screw feed using a ram action to feed the extruder. In both... 

Wortberg, J., Screw and Barrel Design for Grooved Feed vs Smooth Bore Extruders, SPEANTEC Tech. Papers, 48, 80 (2002)... 

For example, if a two-stage vented extruder has a screw that was designed for resins with an Mi range of 2 to 20 dg/min, and plant personnel are asked to manufacture a resin with an MI of 100 dg/min, the extruder may not process the resin acceptably. For this case and flood feeding, the extruder is likely to have resin flowing into the vent because the first stage can deliver more resin than the second stage can pump. One practical solution to this problem is to operate the slide valve in a partially open manner such that it controls the rate of the extrusion process. This will allow the extruder to operate at a lower specific rate and allow operation without the flow of resin into the vent. 

The factory system based on internal mixers, screw extruders, calenders, and vulcanization presses has remained basically unchanged in the past half-century. Internal mixers have had major improvements, e.g., intermeshing rotors proposed by Francis Shaw and Company [C16] and Werner and Pfleiderer [L3], and variable intermeshing clearance rotors [PI] proposed by Pomini-Farrel SpA. Sophisticated computer control systems have been introduced. The early single hot-feed extruders have been replaced by cold-feed extruders with increasingly sophisticated design including pin barrel extruders [G7, H12, H13, M18, W16] as well as complex control systems. 

The process is illustrated schematically in Fig. 8.3. Polypropylene granules are fed into a continuously rotating screw of special design to permit nitrogen gas to be introduced to the melt. The screw feeds melt and gas into an accumulator where they are maintained under high pressure to keep the gas in solution . When sufhcient volume of melt has been accumulated (limit switch) a valve in the mould nozzle is opened and melt is forced into the cavity where, under reduced pressure, the nitrogen gas is free to expand. The nozzle valve closes and the accumulator continues to be charged with melt from the extruder barrel. 

Principles of extrusion Description of single screw extruder Smooth bore and grooved-feed extruders Blown film process Cast film process Extrusion coating Profile extrusion Materials for extrusion Auxiliary equipment Die design... 

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