Operating extruder

Example 2 Flow Calculations for a Properly Operating Extruder... 

Thus, the expected maximum rate of the extruder using the analytical analysis for the 0.8 MI resin Is 320 kg/h at the maximum screw speed of 130 rpm. A similar calculation was performed for the 2 MI resin, and the rate due to the imposed pressure gradient is 27 kg/h, and the expected maximum rate of the extruder is 310 kg/h. The specific rate should always be calculated by dividing the rate by the screw speed In rpm. For the 0.8 and 2 MI resins, the specific rate Is calculated at 2.46 and 2.38 kg/(h-rpm). The pressure gradient here caused the rate to decrease by about 5 to 8% from the rotational rate. This type of decrease is typical for a properly operating extruder. 

Diagnosing and eliminating a problem that occurs due to an improperly operating extruder can be difficult and time consuming. This section contains several case studies where improperly designed processes created contamination defects in the final articles. The axial pressure profiles of the metering channels for these case studies and other studies in the next chapters were calculated using the method described in Section 9.2.1. The simulated axial pressure profiles are shown with solid lines while the estimated pressure profiles are shown by dashed lines. 

Other wire coating operations extrude a tubular sleeve which adheres to the wire via stretching and vacuum. This is called tube coating. 

An example of an exception to the use of a screw operating extruder occurs when a very low melt plastic such as PTFE (polytetrafluoro-ethylene) is to be used where its specialty properties are needed. PTFE does not easily melt, particularly in the production of heavy wall PTFE wire coatings, so ram extrusion is used. Ram extrusion is used for these wire and cable jobs and also for producing rods, tubes, and large round solid cakes that are later skived to make PTFE film. 

Intermittent accumulator EBM machines use a normal conventional axially fixed continuously operating extruder to prepare the melt. The accumulator is a heated reservoir where the melt is temporarily stored in the intervals between parison extrusion (similar action of a two-stage injection molding machine (IMM) as reviewed in Chapter 4. Also used is a conventional reciprocating IMM that delivers melt through a die into a blow mold. 

The first mixer was an annular container with a spiked rotor for rubber compounding [Hancock, 1823]. The calender/two roll mill was patented by Chaffe in 1836 and manufactured by Parrel Co. A counter rotating twin shaft internal mixer with elliptical rotating discs or sigma-blades was developed by the end of the last century [Freyburger, 1876 Pfleiderer, 1880]. The first hand-operated extruder was a ram press, used for forming NR or GP, then later NC. 

Most blown film operations extrude the resin in an upward direction. However, blown polypropylene film is generally extruded downwards and water or mandrel quenched. The extruded tube is then reheated, to a point still below its melt temperature, before it is blown. The collapsed bubble can be fed over a series of heated rollers to reheat it and relieve thermal stresses if a heat-stabilized film is wanted or it can be heated and reinflated in what is known as the double bubble process, which will be discussed in Section 7.3.7. In either case, the film is restrained until cooling is complete, to keep it from shrinking. 

In an operating extruder, however, it is not very likely that the screw and barrel are at the same temperature. The largest difference between the screw and barrel tern-... 

FEA can be very helpful in analyzing details of flow and heat transfer inside extruders that are very difficult to determine on operating extruders. FEA can predict three-dimensional velocity profiles in screw extruders. In addition, the pressure and temperature fields can be determined. 

In an operating extruder the beginning of the screw can be ftilly described by the solids conveying model... 

Hot melt machines require a much smaller capital investment than extruders, and narrow width coaters are used by label manufacturers and for smaller scale tape operations. Extruders are justified only in the case of a single large-volume product. 

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