Processing via the Viscoelastic State

Welding and sintering can be cited as types of processing carried out in the viscoelastic state. Cutting, bracing, and adhesion presuppose the solid state. 

Sintering is used for the treatment of surfaces, the production of porous materials, or the manufacture of large, hollow bodies. The material is compressed to a sinter under pressures of 500 MN/cm and then carefully heated in such a way that only the surface layers begin to melt. The particles adhere, creating bodies with open, porous channels. These permanently porous bodies are used as filter supports, as bodies with large surface areas for physical processes such as thermal exchange, or for ventilation surfaces. Poly(ethylene), poly(propylene), poly(tetrafluoroethylene), poly(methyl methacrylate), and poly(styrene) are used as raw materials. 

In the double-rotation centrifuge process, the heated grains are sintered together in a kind of centrifuge. Hollow bodies of poly(ethylene) with capacities of up to 10,000 liters are manufactured by this technique. 

Fluidized bed sintering is not really a type of sintering. Here, heated metal parts which have previously been roughed by sandblasting or treated with primers are dipped into a fluidized bed of the plastic powder. The plastic powder, with particle sizes of 200 jum melts on the warm surfaces and 

The metal parts for treatment must also be roughed or pretreated with primer for flame spraying. The granulated thermoplasts are then melted in a flame-spraying gun and sprayed onto heated metal surfaces. The method is particularly suitable for small numbers of pieces. Poly(ethylene), PVC, cellulose esters, and epoxy resins are processed in this way. 

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Welding, sintering, fluidized bed sintering, flame spraying, and hot blast sprinkling are types of processing via the viscoelastic state. 

The modeling and control of movements in this chapter relates to external control of muscles via so-called functional electrical stimulation. Macroscopic viscoelastic models started from the observation that the process of electrical stimulation transforms the viscoelastic material from a compliant, fluent state into the stiff, viscous state. Levin and Wyman [35] proposed a three-element model— damped and undamped elastic element in series. Hill s work [36] demonstrated that the heat transfer depends upon the type of contraction (isometric, slow contracting, etc). The model includes the force generator, damping and elastic elements. Winters [37] generalized Hill s model in a simple enhancement of the original, which... 

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