Extrusion screw profile

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... 

Primarily used in profile extrusion Screw speeds from 10-40 rpm... 

Screw profiles were adapted to the control of the extrusion temperature. The temperature increase along the screws have been calculated for three different profiles. Screw 2 of the Figure 5 has been chosen for production. 

Steps can be taken to minimise this problem through attention to screw profile (in extrusion compounders), the location of addition (preferably by feeding through a downstream port into the melt in extruders and continuous internal mixers), and the melt temperature and shear rate experienced. 

During extrusion processing, the structural modification of starch, which is directly related to the viscosity, depends on the specific mechanical energy, feeding rate, screw speed, screw profile, and barrel pressure in the extruder. 

Vergnes y Berzan (2010) founded that starch transformation can be predicted with an error of less than 10 %. They have showed that starch transformation during twin screw extrusion can be accurately predicted using dedicated software in which viscosity and SME are coupled. This may allow the processing conditions or screw profile to be optimized and scale-up problems to be solved. 

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 twin-screw extruder has proved to be a versatile tool for continuous treatment of vegetable matter, either for food or non-food applications. Trials for the direct alkaline extraction of xylans from wheat bran in a twin-screw extruder were unsuccessful. Bran impregnation with sodium hydroxide in the twin-screw extruder was very efficient, but it was necessaiy to make the separation between the hemicellulosic gel and the lignocellulosic matrix in another apparatus and remained difficult without a dilution to a L/S ratio of 50. Bran and straw co-extrusion was therefore investigated to be able to reduce the L/S ratio. Straw fibres form a dynamic plug in the restrictive elements of the screw profile just after the filtration zone. The pressure induced in the extruder sheath by the cellulosic fibres enabled the liquid/solid separation. 

Extrusion of compact and expanded (foamed) PE takes place on the same type of extruder using the operational parameters given in Table 6.4. The recommended screw profile for the feed zone is 2D, for the compression zone is 13D, and for the homogenisation zone is 5D LID > 18 (L is the length and D the diameter). The compression ratio is 2.5-3. A laminar flow through the die is essential for good-quality foam. The expansion degree is inversely proportional to rotation speed and residence time. 

Most of the compounds were extrusion compounded in a conical, partially intermeshing, counter rotating twin screw extruder (Haake Reomix TW-lOO). The extruder speed was set at 50 rpm and the barrel temperature profile was set to produce a melt temperature of 260°C at the die. Samples were injection molded in a 31.8 MT Battenfeld press with a 59 cc shot size. Where noted, samples were compounded in a 60 cc Brabender internal mixer and compression molded. 

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. 

The baseline extrusion process was numerically simulated using the processing conditions in Table 9.4 and the method described in Section 9.2.1, that is, with a rate of 77 kg/h, a screw speed of 27 rpm, and a discharge pressure of 10.6 MPa. The iterative calculation process was used to estimate a bulk temperature of 160 °C and a pressure of 13.1 MPa at the entrance to the meter section. The axial pressure and temperature profile for the simulation is shown in Fig. 9.5. 

Axial screw temperature profiles have been measured before, but they have been limited by the number of sensors and the quality of the data. The data presented here expand the existing knowledge of extrusion and also provide a new method for determining the ending location of the solid bed. 

Keum, J. and White, J. L., Heat Transfer Coefficients and Screw Temperature Profiles in Modular Twin Screw Extrusion Machines, SPE ANTEC Tech. Papers, 50,108 (2004)... 

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