Dump extruders

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. 

Reasonably constant throughput can be obtained from most dump extruders, as long as the extruder speed is set so that one batch is just running out as the next one is dropped from the mixer. The extrudate is not good enough to form an accurate finished product directly. 

Twin screw versions of dump extruders are available which are claimed to overcome some of the problems of single screw machines, i.e., high energy input, sticking and accelerated wear. 

Roller die systems can combine a dump extruder with a two bowl calender to produce good quality sheet for application in such products as conveyor belting. Roller head extruders are more integrated with the calender bowls mounted at the front of the extruder cylinder. 

A post-mixer dump extruder has a large feed hopper which is often equipped with a pusher to force the material into the screw. The mixer discharges directly into this feed hopper. The mix is then taken into the screw and extruded down the screw flights to the discharge end of the extruder (head end). 

Fig. 3. Section through dump extruder with slabber head. Parrel Bridge...

Twin-screw dump extruder with gear pump and strainer head... 

In another case where the twin-screw extruder was used, the rubber and plastic were melt mixed with all ingredients in a similar manner as described in blend compositions for static vulcanizations. The product was then dumped, cooled, and granulated. The premixed granules were then fed into a twin-screw extruder where a very narrow temperature profile was maintained with a relative high compression (2 1), and the screw speed was adjusted depending on the final torque and the flow behavior of the extruded stock. The stock was cured by shear force and temperature enforced by the twin-screw extruder. The dynamically crosslinked blend was taken out in the form of a strip or solid rod to determine the... 

The question remains, as to whether or not the gel and long branch formation resulting from the reaction between TESPT and NR may be responsible at least as a part of the extrusion problem. This question may be answered by what is shown in Eigures 12.11 and 12.12 that when the dump temperature is higher, the waviness of the extrudate surface is more extensive. 

To produce a PPVC dry blend, the PVC polymer and other ingredients (with the exception of the plasticizer and lubricant) are fed into the chamber of a high speed mixer and blended at high speed until a temperature of between 90 to 95° C (194 to 203° F) is reached. The plasticizer is then added to the mix and when 110 to 125°C (230 to 257°F) is reached, lubricant may be added the resultant mix is dumped into a cooler/blender. A large cooler/blender, for example, a ribbon blender, may be used. This cools the mix, improve the batch-to-batch consistency and also reduce the electrostatic charges on the dry blend. The dry blend may then be used as it is or, pelletized using an extruder. 

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