Extrusion - Basic Requirements

Two basic requirements must be satisfied for plastic forming to be successful. First, the mixture must flow plastically (above a certain yield stress) for formation of the desired shape. Second, the shaped article must be strong enough to resist deformation under the force of gravity or under stresses associated with handling. These two requirements are particularly important in extrusion, where the powder mixture contains a limited amount of binder and the shaped article must be dried after forming. As in casting methods, the... 

The basic requirements of the extrusion blow-moulding process are to extrude a molten plastic tube (the parison), clamp it in a cold split mould, expand the parison by means of compressed air, cool the shaped moulding, remove from the mould and trim as necessary. Interest in the present work centres on the example of the Acitainer, which is produced by this process. 

Irogran from Huntsman is a trade name of thermoplastic polyurethane elastomers, which are easily processed by injection molding or extrusion techniques. It is used when a maximum of mechanical strength is a basic requirments. Irogran materials combine an extreme wear resistance with high tear strength and mechanical reliability. Table 19 shows their typical properties. 

In the two typical shaft seal manufacturing processes shown in Fig. 2, the extrusion process is only required if the compression molding process is being used. There are basically two types of extruders in general use, the cold feed and the hot feed. In both cases their role is to produce the preforms for use with compression molding. The critical issues with the preform are the shape, the dimensions, and the weight. 

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

The time required to cure the materials is dependent entirely on the method of molding, mold temperature, and material temperature when introduced into the mold cavity. There are now four basic methods of molding thermoset molding compounds (1) Compression, (2) Transfer, (3) Injection, and (4) Extrusion—with the method most commonly used as rated. Very small quantities are processed by extrusion. All methods of molding may be done automatically or semi-automatically. In most cases, the injection method is practically all done automatically. 

Films and sheets are produced in several ways, including extrusion, calendering, and casting. Method used involves the properties required of the basic plastics and finished products as well as cost usually based on quantity. The following classification can be helpful as a guide to film and sheet thicknesses (1) film is generally less than 0.010 in. (0.003 mm) and (2) sheet at 0.010 in. or more. In turn sheet can be classified as ... 

Some of the basic factors involved in screw and pipe head design for chlorinated PVC extrusion are discussed. It is shown that extrusion of small-diameter CPVC pipe requires specially-designed equipment and that use of a Y-block should be considered. Some examples of successful applications are described. 4 refs. 

The interpretation of these two schematic views confronts some difficulties. Option (1) requires the extrusion of a hydrogen from a methylene hardly active in acidic medium (acetic acid). Under basic conditions, it would be necessary first to convert the sulfide to a sulfoxide such as V to achieve sufficient activation of this methylene. This oxidation may be accomplished by LTA as indicated previously. Nevertheless, the ensuing cyclobutane fragmentation in VI— the actual oxidative step—by concurrent attack of acetate and departure of Pb(II) diacetate (X = PbOAc2 in Scheme 27.1) has no precedent in LTA-amine chemistry, although it is electronically balanced. Besides, the final product of this sequence would be sulfoxide VII. Having no reductive work-up procedure, this sulfoxide should survive until the isolation step. Since this is not the experimental fact, option (1) must then be discarded. 

High Pressure Axial Screw and Ram Extruders Extrusion is possible by low, medium, and high pressure. In all three cases the same underlying basic principle is responsible for the agglomeration process. Tab. 8.8 lists the requirements. 

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