Feeding flood

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 equipment can be designed to stop and restart under load, accommodate a controlled or flood feed at the inlet, be reversible, and accept multiple inlets and outlets. Extended inlets may be provided to collect from long slots, which can be either of a flood feed nature with live extraction along the whole length, or of a type which accepts an erratic in-feed and is only required to clear eventually. 

Screw feeders are normally subjected to flood feed of a bulk material over the exposed inlet section of the screw. The volume of material moved by the feeder is then dependent upon the speed of rotation of the screw. In a wider sense the term screw feeder applies to screw conveyors that normally operate with their feed rate controlled by prior equipment, but act as feeders by controlling the rate of discharge when material in the supply channel completely covers the screw at the inlet point. These circumstances apply in hoppers subjected to surge loads, batch fills, or products accumulated in the supply hopper if the screw is stationary for a short period. Such applications are here termed coUectmg screw feeders , and screw conveyor feeders , but considerations relative to screw feeders of all types bear strongly upon the design of such equipment and their feed channels. 

Screw feeders require a flood-feed supply of bulk material in order to control the rate of discharge. Apart from exceptional duties, such as extracting from a parallel supply chute, a bulk storage container provides... 

A flooded feed screw should never be reversed to clear a blockage, without provision for clearing the end of the screw towards which the material is conveyed. Flight damage or drive stalling is inevitable. 

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. 

Single screw typically operated 100% filled. Usually flood feed. 

This perhaps could be as a result of flood feed conditions due to which the movement of powder bed gets impeded. As a result, the same particles might come in contact with the hammer that could result in attrition type conditions. From the simulation data the particle size obtained at 5,10,15, and 20 s was compared to experimental results for the three different feed rates at 600 rpm. As the parent panicle size in simulation (2imn) differs from experimental (1.1 mm), a dimensionless size was computed aud plotted against time. The simulation illustrated a similar treud (Fig. 14.12) to the experimeutal observations. 

At 1140 rpm lower feed rates generated a much narrower size distribution (400-580 pm). As the feed rate was increased there was some decrease in breakage rate however, no flood feed conditions were observed (as shown in Fig. 14.14). This suggests that due to greater centrifugal forces movement of powder bed is not obstructed, and hence there is no much accumulation of particles. 

The hopper is usually maintained at a near-full level since most single screw extruders are flood fed (Fig. 2.5). Flood feeding occurs when the throat area is kept full, allowing the screw channels to completely fill with each screw rotation. Occasionally, single screw extruders are starve fed, an arrangement where solids are trickled into the throat at a metered rate that doesn t completely fill the screw channels. This may be necessary for difficult to feed materials, such as powders. 

Flood feeding vt. The usual way of feeding an extruder or screw-injection molder, in which the feed material flows from the feed hopper by gravity and completely fills the feed section of the screw. The actual throughput is thus controlled by screw design, die resistance and temperature conditions within the screw, in contrast to what occurs in starve feeding. 

When a single screw extruder is used for demanding mixing and compounding operations it is often preferred that the extruder is starve fed rather than flood fed. Flood feeding often results in higher pressures inside the extruder and this can lead to an agglomeration of powdered fillers. When this occurs it may be difficult or... 

Most single screw extruders are flood fed this means that the bottom section of the feed hopper is completely filled with material and the screw will take in as much material as it can handle. When an extruder is flood fed the output is determined primarily by the screw speed. Flood feeding is illustrated in Fig. 7.34. 

Always operated by starve feeding, flood feeding not possible... 

An extruder consists of a hopper, a heated barrel, a screw, and a die. The hopper is used to flood-feed the colorant and plastic premix into the feed section of the screw. An alternative to flood-feeding is to starve-feed the screw by metering in a controlled flow of material. Metering is not required for a single screw and has been found to have no impact on pigment dispersion. 

Sta rve Feed i ng. Starve feeding is a method of feeding the extruder where the plastic is metered directly into the extruder at a rate below the flood feed rate. This means that the plastic drops directly into the screw channel without any buildup of plastic in the feed hopper. With starve feeding, the screw channel is partially empty in the first few diameters of the extruder. Thus, if one looks into the feed opening, one should be able to see the screw flight and part of the root of the screw. 

Small-scale manufacturing using 16-20 mm diameter screws is often the easiest, because the 1-10 kg/h production rate is easy to handle, and the equipment size is on par with the size of those operating the equipment. Operating conditions can be quickly tested, but care must be taken that the conditions chosen for production have analogues at larger scales, a topic that will be covered later in the chapter. As with the micro scale, machine torque or flood-feeding limits are often what restrict overall production. 

There are a number of ways to feed an air-assisted gravity conveyor. The most common feed method is simply a flood feed from a mass flow hopper. A non-mass flow hopper may cause problems with inconsistent feed which may result in blockages. That is, if too much material is fed onto the conveyor, then there will be insufficient air to fluidise the material, and if too little material, or none at all, then conveying may halt due to a maldistribution of air from the plenum chamber. While an air-assisted gravity conveyor does not work independently as a feeder, if the bulk solid is metred onto it by the use of a screw feeder or rotary valve, it will then convey this material at a constant rate. Another method of controlling the flow of material is to use a gate or baffle at either end of the conveying duct. 

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)...

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