Extrusion continued screw

Continuous screw extrusion is another way to form PVC plastisol propellant into the desired size and cross-sectional shape. Simultaneous curing of the propellant is also accomplished. This process has been described fully by Rossen and Rumbel (9) and is only summarized here. Mixed propellant is fed from a casting pot into the feed end of a continuous worm-screw extruder which is fitted at the discharge end of the screw with a device for constricting propellant flow. The action of the... 

Continuous extrusion presses As shown earlier (Figure 310), the operating point of a continuous (screw) extrusion press is defined in the extrusion rate/pressure diagram by the point of intersection between lines representing nozzle and screw characteristics. Control of such equipment is accomplished by maintaining the pressure at the optimum level while changing the extrusion rate. 

In continuous processing, a fixed transversal screw, comparable to extrusion, continuously forces out a plasticized strand onto a heated discharge belt. 

Extrusion. The forcing of clay or other material through a die this is done by a continuous screw on a shaft rotating centrally in a steel cylinder (an auger ), or by a series of knives obliquely mounted on such a shaft (a pug ), or, occasionally, by means of a piston (a stupid ). Ceramic material may be extruded either to mix and consolidate it (as in the extrusion of pottery body prior to jiggering) or to give the body its final shape (as in the extrusion of bricks and pipes). 

Continuous Solvent—Extrusion Process. A schematic for a typical continuous process, widely used for making solvent propellant for cannons, is shown in Figure 7. This continuous process produces ca 1100 metric tons of single-base propellant per month at the U.S. Army Ammunition Plant (Radford, Virginia). Continuous processes have also been developed for double- and triple-base propellants and for stick as well as granular geometries. A principal aspect of these processes has been the extensive use of single- and double-screw extmders instead of the presses used in the batch process. 

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. 

Fig. 2.15. Reactors for producing these materials include batch, continuously vented, tray reactors, twin-screw extruders, and vented single-screw extruders. These production devices will not be covered in this text because they are of more interest to the manufacturing engineer than the extrusion process engineer.

Rotational flow and pressure flow rate calculations for the screw geometry and process conditions are performed for the injection-molding process in the same manner as for an extrusion process. Since the plasticator of an injection-molding process is not a continuous process, the instantaneous rate must be calculated based on the time that the screw is actually rotating. The instantaneous rate is the rate that is compared to the calculated flow rates for the screw. The instantaneous specific rate is calculated as follows ... 

The disadvantage of ram extrusion is that it is a batch process and requires mixing of the components beforehand. On the other hand, screw extrusion is a continuous process. 

Extrusion accounts for about 30% of nylon produced and is used in various processes (24). Nylons can be extruded on conventional equipment having the following characteristics. The extruder drive should be capable of continuous variation over a range of screw speeds. Nylon often requires a high torque at low screw speeds typical power requirements would be a 7.5-kW motor for a 30-mm machine or 25-kW for 60-mm. A nylon screw is necessary and should not be cooled. Recommended compression ratios are between 3.5 1 and 4 1 for nylon-6,6 and nylon-6 between 3 1 and 3.5 1 for nylon-11 and nylon-12. The length-to-diameter ratio, L D should be greater than 15 1 at least 20 1 is recommended for nylon-6,6, and 25 1 for nylon-12. 

The twin-screw extrusion (TSE) technology is utilized extensively in the plastics and food industries to complement continuous manufacturing of products. Ghebre-Sellassie et al. [37] and Keleb et al. [36] have cited the application of this technology in the continuous production of pharmaceutical products. Commercially available TSE provide the great Lexibility required for effective continuous mode of operation. The schematic of such a system is shown in Figure 23.6. The twin-screw extruder... 

The theoretical analysis of the injection unit involves all the facets of steady, continuous, plasticating screw extrusion, with the added complication of a transient... 

The extrusion blow molding process can be continuous or intermittent, as shown in Fig. 14.16. The former, employed commonly for parts less than 1 gal, has a continuously rotating screw extruder, extmding parisons through one or more dies. The latter may use either an accumulator head with a piston-driven extrusion forming of the parison, or a reciprocating screw, such as the one used in injection molding. 

Fig. 14.16 Schematic view of (a) continuous extrusion blow molding (b) reciprocating screw blow molding and (c) ram-accumulator blow molding. [Reprinted by permission from Modem Plastics Encyclopedia, McGraw-Hill, New York, 1976-1977.)...

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