Extrusion screw feed extruders

Figure 12 Influence of the amount of water on the extrusion forces (gravity feed extruder) and power consumption (twin screw extruder) on extrusion of MCC. , gravity feed extruder (N) , twin screw extruder (W). Source From Ref. 37.

A gravity feed extruder with L/D = 2 recorded (28) higher forces, compared to power generated from a twin-screw extruder with L/D = 0.9, upon extrusion of mixtures of MCC with either lactose or DCP. The extrudate from the basket extruder was denser with a smoother surface than that from the screw extruder. Similarly, material extruded through a screen with L/D = 4 in a basket extruder was denser than that with L/D = 2 (37). However, a smaller L/D ratio is preferred in an axial extruder. An improper L/D ratio could result in a loosely bound extrudate with large surface defects (37), resulting in a formulation less amenable to pelletization, as seen from Figure 14. Since the mean diameter of the pellet often approximates the diameter of the die used in extrusion, choice of the latter is also dictated by desired pellet size. 

Extrusion of tubes, hose, and profiles is done on standard extruders for rubber. The usual temperature pattern is a gradual increase of temperature from the feed zone to the die. The die temperature is typically 100°C (212°F), and the screw temperature is approximately the same as the temperature of the feed zone [44]. Processing aids are almost always required to improve the surface appearance and to increase the extrusion rate. Extrusion represents only a small proportion (about 10%) of the total consumption of fluorocarbon elastomers [52].Cold feed extruders typically used for extrusion of rubber stocks are also suitable for the extrusion of fluorocarbon elastomers. Extrusion should be carried out at temperatures below 120°C (250°F) to avoid scorch. In most cases a breaker plate and screen pack are used to generate backpressure on the screw and to remove foreign particles from the stock. A straight head is used for the extrusion of profiles or tubing, whereas a cross-head die is used for... 

Screw-fed extruders have screws that rotate along the horizontal axis and hence transport the material horizontally they may be axial or radial screw extruders (Fig. 5). Axial extruders, which have a die plate that is positioned axially, consist of a feeding zone, a compression zone, and an extrusion zone. The product temperature is controlled during extrusion by jacketed... 

Depending on capital investment limitations, extruders may be heated by passing steam through the barrel jacket and/or by direct steam injection. Low-cost autogenous machines, which create heat by friction between the feed material and the surfaces of the barrel, screw and steam locks, and do not require steam-generating equipment, are also used (Wiejratne et al., 2004). This process is referred to as dry extrusion, and the extruder can be powered by a farm tractor and was originally developed to cook whole soybeans on-farm to prepare feeds. 

The cold-feed extruder had shallow screw channels and a much longer length/diameter ratio. The original cold-feed extruder produced extrudates which had large temperature distributions and became distorted as they emerged from the die. The issue in cold-feed extrusion of rubber was to produce a thermally homogenized product in the extruder screw which would yield thermally uniform extrudate that does not distort. From the 1960s, there have been several improvements in cold-feed extruders. First, Maillefer [M6]... 

Single-screw extruders are commonly used for the conversion of polymers into prodncts such as pipe, cables, film and so on. It is used for compounding with a modification of the screw, feed zone and mixing zone. The compounding operation can be synchronised with product extrusion in a single operation. 

It relies on conical extrusion screws that mesh together to provide a series of separate material flow paths. Each rotating conical core provides two material paths, through screw feeds along the outer and inner surfaces of the core. By adding additional cones in series, additional cylindrical extrudate paths are built up. 

A feed hopper and a mixer continuously feed resin pellets (and carbon black batch pellets if necessary) and a certain amount of granulated edge trimming from the running geomembrane production into the extruder (Fig. 2.5). The main part of the extruder is the extrusion screw, which rotates in a heated barrel (Fig 2.6). Typical process temperatures for HDPE... 

Unlike axial and dome extruders, radial extruders extrude the wet mass perpendicular to the plane of transport. Material is transported to the extrusion zone where it is wiped against the screen die by an extrusion blade. The mass is forced through the die by pressure generated at the nip. A screw feed radial extruder is shown in Figure 6(C). 

Principles of extrusion Description of single screw extruder Smooth bore and grooved-feed extruders Blown film process Cast film process Extrusion coating Profile extrusion Materials for extrusion Auxiliary equipment Die design... 

The extruder was run with the pellet feed maintained by hand at a level just covering the screw. Individual black masterbatch pellets were added at timed regular intervals. After a suitable overall extrusion time the extruder was stopped, rapidly cooled and the screw ejected with a hydraulic ram. A pipe grade low-density polyethylene (LDPE) was used with masterbatch pellets containing 40% carbon black. 

There is also the siloxane process, which simply mixes the various additives (silane chemicals, cross-linking agents, antioxidants, catalyst, colorants, etc.) along with the polyethylene base resin using a high-intensity mixer prior to feeding the mixture into the pipe extruder. The siloxane process requires a special type of extrusion screw. 

Figure 13.11. Screw of extruder (a) Feeding section for pellets (b) compacting section for the matter and meiting (c) extrusion section.

As the screw turns, it conveys the pellets from the feed zone towards the melting zone. A combination of external heating and mechanical work melts the polymer as it is transported towards the metering zone. The metering zone pumps a uniform stream of molten polymer to a forming device, such as a profile die. Other types of extruders that employ two or more screws are commonly used for compounding polymer blends. The principles of twin screw extrusion will be discussed in Chapter 12. 

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. 

---