Viscous plastic processing

Piezoceramic materials are chemically inert and physically strong. In fibre form, they have anisotropic structures. Usually, ceramic fibres are produced by the spinning of an organic or mineral precursor fibre, followed by heat treatment and pyrolysis (Hearle, 2001). Piezoceramic fibres comprising lead zirconate/lead titanate exhibit better sensitivity in terms of piezoelectric activity and elevated operating temperatures (Swallow et al., 2008). PZT fibres can be manufactured by various processes, such as sol-gel, viscous suspension spinning, extrusion and viscous plastic processing, some of which are already commercially available (Strock et al., 1999 Meyer et al., 1998 French and Cass, 1998 Meister et al., 2003 Bowen et al., 2006). 

Plastic processing is primarily the flow and shaping of viscous liquids. The scientific study of this flow is called rheology. Assuming laminar shear flow, viscosity is defined as the ratio of shear stress to shear rate. 

No functional relationship between the flowability index according to Rashig and the other indices of the viscous-plastic properties has yet been establkhed. The flowability index determined by this method serves as a preliminary assessment of the suitability of the material for processing. 

Plastic materials are those that are formed from synthetic compounds e.g., polymers or natural compounds that have previously been modified, for example hydrocarbons refined from crude oil, natural gas or derivatives of ethane, methane and naphtha. By definition the manufacturing of a plastic component should include a viscous flowing process that usually requires heat and pressure, for example extrusion or injection moulding. 

For use as plastic in bulk form, cellulose nitrate is plastidzed with camphor. The product is known as celluloid. In a typical process alcohol-wet cellulose nitrate is kneaded at about 40°C with camphor (about 30%) to form a viscous plastic mass. Pigments or dyes may be added at this stage. The dough is then heated at about 80°C on milling rolls until the alcohol content is reduced to about 15%. 

The main heat transfer processes that are come across in plastics processing are conduction, convection and viscous heating, with radiation only playing a role in thermoforming. Most products are much thinner than they are wide, so only one-dimensional heat flow (Fig. 5.2) will be considered. The heat flow direction is along the x axis, perpendicular to the surface of the product there are planar isotherms perpendicular to the x axis. The heat flow Q is considered across an area A of the isothermal surface. 

A thermoplastic material can exist in a linear or branched structure. On heating a thermoplastic, a highly viscous liquid is formed that can be shaped with plastic processing equipment. 

Disk extruders. There are several types of screwless extruders. These machines employ a disk or a drum to plasticate, mix, and extrude. Most designs are based on viscous drag flow. They include Maxwell s elastic melt extruder, Westover s stepped disk and drum extruders, and Diskpack [39]. The Diskpack extruder has the capability of performing all the elementary steps of plastics processing by combination of differently shaped rotating... 

The most common use of additives are to aid in specific areas of processing. Many of these additives are hidden from sight, as it were, as they are designed to affect the viscous properties of the molten resin, to help it flow better, or to be more homogeneous, or to cool and solidify (and shrink) in a more consistent manner. Some additives are intended to preserve the equipment used in plastic processing, to prevent corrosion of the metals, or enhance the release of the solidified plastic from the mold... 

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