Vacuum feed hopper

One method to overcome this air entrapment problem is to use a vacuum feed hopper. In principle, this is quite simple however, in practice, a vacuum feed hopper is not a trivial matter. The first problem to occur is how to load the hopper without losing vacuum. This has led to the development of double hopper vacuum systems, in which material is loaded into a top hopper and the air is removed before the material is dumped in the main hopper (see Fig. 3.14). 

In this design, the feed port is combined with the vent port. The volatiles are removed through the annular space between the feed pipe and the feed port housing. The system is used in conjunction with a vacuum feed hopper as described by... 

Franzkoch [21], Such a system allows devolatilization to occur in the feed hopper section of the extruder. This type of devolatilization is successfully used with powders, particularly when the material in the hopper can be heated. The vacuum feed hopper system and some of the problems associated with it are discussed in Section 3.4. Hopper devolatilization of pellets is generally unsuccessful because the larger particle size reduces the surface area and thus the devolatilization effectiveness, as discussed in Section 5.4.1. 

Fig. 7 Extruder and extrusion. 1 Drive unit, 2 barrel inlet, 3 temperature control, 4 feed hopper and dosing unit, 5 vacuum vent, 6 barrel, 7 screw. (Reproduced with permission from Reifenhauser)...

A specially designed compression feed screw and upper arm assembly are shaped to closely conform to the inner wall of the feed hopper. The feed screw (usually non-cylindrical) has uniquely pitched edges. The feed screw design features and gravity aid to drive the powder-stock into the compaction area. There is no vacuum deaeration system. Predensifica-tion and deaeration occur in the screw feed region during operation. 

Trapped air in extruder] unvented extruder/wrong screw design/pressure too low/rear-barrel temperature too high/screw speed too high/vacuum too low in feed hopper/powder feed instead of pellets. 

Usually part of the resin drying system that also can be an independent unit, to convey material to the machine s feed hopper using vacuum or pressure. 

Feed hopper Extruder Gear pump Spinneret Cooling air Draw roll Air gun Porous belt Bonding oven Vacuum exhaust Fabric inspection Wind up... 

While negative pressure conveying systems have been covered in some detail in Chapter 4, there is a small sub-set that will be considered here. Small volume batch conveyors, or simply vacuum conveyors , generally consist of a small tank and a relatively short conveying line in which a vacuum is generated, and this is connected to a material feed hopper. Once the required negative pressure has been reached using the attached vacuum pump, a valve is opened on the feed hopper and the material is conveyed into the vacuum tank. The system... 

For fine powders that tend to bridge or stick and are of low bulk density, some form of forced feed, such as the tapered screw feeder shown in Figure 9, must be used to deaerate, precompact, and pressurize the feed into the nip. Large machines are available with up to five screw feeders to spread the flow across the roUs, and vacuum hoppers are also used to remove air when densifying low density feeds. 

Against this, it may be necessary with a batch vacuum filter to provide holding vessels for feed, and possibly filtrate and wash. The discharge of solids is intermittent and this may lead to complications in feeding the solids to a continuous dryer or to some other continuous process, and cake hoppers and a feeding arrangement may have to be provided. 

The equipment feed design consisted of a funnel-shaped powder hopper (with no vacuum deaeration system) that was located directly above the rolls. The powder hopper was retrofitted with vacuum deaeration capability. A high compression feed screw, fitted inside the funnel hopper, fed the powder directly into knurled rolls. The compaction trials were conducted with and without vacuum deaeration. The compact was carefully collected directly on a 10-mesh screen. Powder particles, that were not compacted (i.e., those particles which were not attached to the compact, for example, fines bypassing roll compaction and the non-adhering compacted powder particles) were weighed and separated. The compact was not milled. The parameters are noted in Table 4. 

Hopper dewaterer Top feed Scroll-discharge Tilting-pan Belt-discharge Continuous vacuum disk Continuous vacuum Precoat Continuous pressure Precoa... 

Fig. 6.2-61 depicts two typical designs and arrangements of roller presses for pharmaceutical applications. As mentioned above, small presses in particular are sometimes equipped with rollers one-above-the-other (alternative a2). In this case, the force feeding screw is installed horizontally. It is often said that this design allows for better de-aeration as shown in Fig. 6.2-62. In this diagram, (1) is the raw blend, (2) is a special hopper equipped with a rotating flow stimulator, a separate de-aeration chamber, which is also used for feeding recycle (3) and additives (if applicable), and air removal port (4), (5) is a vacuum pad option for further direct de-aeration from the horizontal feed screw, and (6) is the recirculation of powder leakage [B.13b, B.48, B.97].

Solid Feeders. A solids feeder is any device that will maintain a reasonably uniform flow of bulk material, implying a metering function. It deals with solids in bulk, solid-liquid, and solid-gas mixtures which may be free-flowing, lumpy, sticky, corrosive, erosive, fluidizable, hot, plastic, or pasty. It deals with feeding against pressure or vacuum, with feeding where precision is required, as well as with more usual conditions. Finally, since many materials tend to hang up in bins and hoppers, and... 

Using this combination, the compaity says the Feed-Vac can eliminate the need for a separate feeder and hopper while optimising efficiency. Tlie final component included is an adaptable vacuum conveying sy stem capable of transporting most dry powder or granule products through a dedicated pipe system to the Accu-Feed and separator. 

The pellets of plastic are first transferred from storage to the hopper on the extruder, which serves as a reservoir to feed the solid particles into the barrel. The hopper can be a simple one like the one shown in Fig. 7.1. Most extrusion operations are automated at least to the point of having a vacuum hopper loader to bring material to the hopper. Specialized equipment is available to mix in color and other... 

Changing the initial melt temperature in either direction may solve the problem. With a barrel and screw, it is important to study the effects of temperature changes. Another approach is to increase the pressure in processes that use process controls. Particle size, melt shape, and the melt delivery system may have to be changed or better controlled. A vacuum hopper feed system may be useful. With screw plasticators, changes in screw design may be helpful. Usually, a vented barrel will solve the problem. 

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