Taper screw

Conical mixers are also known as Nauta mixers (Fig. 18-53). Material placed in the conical bin is lifted by the rotation of the helical screw, which in turn is rotated around the wall of the cone. The lifting actions of the screw combined with motion around the cone provide bulk mixing for flowable dry powders, paste materials, and even viscous fluids. The specific energy input is relatively small, and the large volume of the mixers can even provide storage capacity The mixers may have multiple screws, tapered screws, and high-speed dispersers for different applications. At constant speed, both the mixing time and power scale up with the square root of volume. Sizes from 0.1 to 20 m3 (3.3 to 700 ft3) are available. 

A screw conveyor was used originally to convey the cake, but this has been replaced with a chain-type conveyor. The first prototype used a tapered screw to form a plug before discharge into atmospheric pressure this has been replaced by compaction in a vertical pipe. 

Screw presses have been used extensively in North America, but are not popular in Europe. The mash is added at the top of a vertical rotating screw. As the mash moves down it is compressed in the taper and juice flows out through slatted conical walls. Compression of the pomace blocks channels for juice flow and iuhibits juice extraction. Extensive use of press aids is necessary for efficient operation. 

Cranberry juice, too acidic to be consumed as a 100% juice drink, has been sold since 1929 as cranberry juice cocktail. Juice extraction usually involves pressing the juice from thawed cranberries in a tapered screw press, which affords a 60—64% juice yield. The juice is diluted with two volumes of water and sugar is added to raise the °Brix to 15 to produce a juice cocktail. Under the Federal Food, Dmg and Cosmetic Act, cranberry juice cocktail must contain not less than 25% single-strength cranberry juice with soluble soHds content of 14—16 °Brix, vitamin C content of 30—60 mg/177 mL (6 02), and... 

Eig. 1. Parts of an extmder A, screw B, barrel C, heater D, thermocouple E, feed throat E, hopper G, thmst bearing H, gear reducer 1, motor , deep channel feed section K, tapered channel transition section and L, shallow channel metering section (15). 

The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

A combination of tapered shaft diameter and increasing pitch is shown in Figure 10a. This allows a length-to-diameter ratio of about 6 1 instead of 3 1. A half pitch screw is used over the tapered diameter. This approach results in an exceUent mass flow pattern provided that the hopper to which it attaches is also designed for mass flow. 

Fig. 10. Mass flow screw feeder designs, (a) Combined tapered shaft and variable pitch screw feeder where A represents a conical shaft and constant pitch (feed section) B, constant shaft and increasing pitch (feed section) and C, constant shaft and constant pitch (conveying section), (b) Stepped shaft screw feeder where A represents a stepped diameter shaft and constant pitch (feed section) and B, constant shaft and constant pitch (conveying section).

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. 

FIG. 18-48 Mixing enhancers for single-screw extruders a) Maddock (straight), (h) Maddock (tapered), (c) pineapple, (d) gear, (e) pin,... 

Screws into the female thread in the fitting or hydraulic component. This thread also is tapered. 

Hub bores are usually straight, although for some special applications taper bores are sometimes specified. For smaller diameters, bores are designed for clearance fits and a set screw is used over the key. The major advantage of a clearance fit is that hubs can be easily assembled and disassembled. For larger diameters, the bores are designed for interference fits without setscrews. For rapid-reversing applications, interference fits are required. 

Sikora, J.W., The Effect of the Feed Section Groove Taper Angle on the Performance of a Single-Screw Extruder, Polym. Eng. ScL, 41, 1636 (2001)... 

The restriction was mitigated by modifying the screw as outlined in the case study in Section 11.10.1. That is, the depth of the melt channel of the barrier section was increased to that of the solids channel at the entrance, and it was tapered into the depth of the melt channel over 2 diameters. The barrier flight was removed for the first 2 diameters and blended in with the melt channel, the shallower of the two channels in this region. The barrier flight for the next diameter was blended into the original undercut. With this modification the restriction still existed but it was spread over a three-diameter length of the screw instead of over half of a diameter. 

The commercial extruder was modified by tapering the entrance to the new barrel such that a ledge did not occur between the feed casing and the barrel. As predicted by the experimental research, the commercial line was able to operate stably and at a high specific rate of 4.2 kg/(h-rpm), and at a screw speed of 150 rpm the plant was operating at a rate of 630 kg/h. Plant personnel ordered a new feed casing and planned to Install the casing at the next planned shutdown. 

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