Plastication rate

Table 1.8 indicates the service temperatures which can be used with a range of plastics. It may be seen that there are now commercial grades of uiweinforced plastics rated for continuous use at temperatures in excess of 200°C. When glass or carbon fibres are used the service temperatures can approach 300°C. 

Whereas heat capacity is a measure of energy, thermal diffusivity is a measure of the rate at which energy is transmitted through a given plastic. It relates directly to processability. In contrast, metals have values hundreds of times larger than those of plastics. Thermal diffusivity determines plastics rate of change with time. Although this function depends on thermal conductivity, specific heat at constant pressure, and density, all of which vary with temperature, thermal diffusivity is relatively constant. 

Plasticating single-screw extruders will typical range in size from 15 mm up to 250 mm in diameter. Larger diameter plasticating machines are very rare and were likely built years ago. Plasticating rates above about 4000 kg/h are more economically processed using twin-screw machines. 

The parts produced were in specification and did not have the splay defect. When the press was switched between different colored resins, the purge time was very short and typically required less than 20 parts. The higher compression ratio and compression rate screw combined with channels that had large radii provided a constant plastication rate and no detectable degradation of the resin. 

Since the materials have a very soft plasticity rating, or inches of flow on spiral mold, the materials are all molded in special transfer molding presses and specially designed transfer molds. Because an acid catalyst and glass or mineral fillers are used to make the compound, these materials may have an adverse effect on the hardened steel mold with the result that there may be excessive runner and gate wear. 

NEB reaction rate model system containing sorbitol or no added plasticizer. Rate against moisture (a) and water activity (b) (Labuza et al., 1977). 

The size of injection molding machines is usually specified in terms of injection capacity and clamp force. These factors are not directly related. Other important variables are the L D (length to diameter) ratio, barrel size, plasticizing rate, injection rate, and injection pressure. Major components of injection molding machines include the injection unit, the clamp unit, and the machine base which contains the power and control units. 

This is the well-known Considere (1885) criterion for impending necking for a rigid plastic, rate-insensitive material. It is expressed graphically with a simple construction in Fig. 10.1 showing the intersection of the stress-strain curve with the curve of the decreasing slope of that curve, where the point of intersection is at u, gives the uniform extension strain. In Fig. 10.1, for strains e < u,dr/d > t e)... 

This equation shows that both plasticizer type and type of carbon affect performance. On the plasticizer side, its density, affinity, saturated vapor pressure and its actual concentration determine adsorption. On the carbon side, its porous stmcture and surface area available for adsorption determine the capacity of a particular filter. The data for the parameters of the equation can be found in chettucal handbooks and therefore the equation can be used for predicting the adsorption capacity of a filter. The capacity of coimnercial filters is normally rated by an experimental method in which the amount of the plasticizer adsorbed by filter is determined under conditions which specify concentration of the plasticizer, rate of flow, and time. Vapor pressures of plasticizers are very low, therefore calculated service life of filters is usually very long. 

The back pressure ranges from 0 to 100 Ib/in (0 to 0.69 MPa). TPU elastomers usually require very little or no back pressure. When additives are introduced by the processor prior to molding, back pressure will enhance mixing, and when the plastication rate of the machine is insufficient for shot size or cycle time, a back pressure up to 200 Ib/in (1.4 MPa) can be used. ... 

Thermal diffusivity determines plastics rate of change with time. Although this function depends on thermal conductivity, specific heat at constant pressure, and density, all of which vary with temperature, thermal diffusivity is relatively constant. 

The range of material behavior considered next is broadened significantly by appeal to the notion of a plastic rate equation as a model for any possible physical mechanism of deformation that may be operative. The ideas will be developed for general states of stress, but will be applied primarily for the case of thin films in equi-biaxial tension. Constitutive relationships that serve as models for inelastic response of materials for a wide variety of physical mechanisms of deformation have been compiled by Frost and Ashby (1982). These constitutive equations are represented as scalar equations expressing the inelastic equivalent strain rate /3e in terms of the effective stress (Tm/ /3 and temperature T. These strain rate and stress measures are denoted by 7 and as by Frost and Ashby (1982), and the rate equations representing models of material behavior all take the form... 

In the present discussion, it will be assumed that the plastic response is dominated by the thermally activated glide of crystal dislocations past discrete obstacles in the lattice. With reference to (7.78), the plastic rate equation proposed by Frost and Ashby (1982) has the form... 

Fig. 7.20. The stress-temperature history for a thin film of elastic-plastic material on a relatively thick substrate during a temperature excursion is shown. The dependence of plastic strain rate on stress is prescribed by the plastic rate equation (7.81) and is based on the assumption that resistance to plastic determination is due to thermally activated motion of dislocations past obstacles. After one temperature cycle, the response adopts a repeated cyclic behavior for fixed temperature limits.

The process of combined surface diffusion and grain boundary diffusion was discussed in detail by Thouless (1993) who developed a plastic rate equation to describe the process. In the present notation, this rate equation has the form... 

PE pipe is used for hot water heating in concrete floors. Temperatures of up to 38 "C are common for foot comfort. For this nse, however, the best brand of virgin plastic rated at 46 "C should be used. Also, mixing valves for water temperature control should be installed. 

We present an elasto-plastic rate-independent model of solid bodies, based on the existence of local, current, relaxed, isoclinic configurations (Lc.r.i.c). V e consider that the model adequately describes the behaviour under moderately large deformation. We prove that the model represents a realization of the theory of materials with elastic range. 

In this paper we present an axiomatic approach to the elasto-plastic rate-independent behaviour of the metals. The approach is based on mechanical and physical considerations given by Lee, Liu [1], [2], Teodosiu, Kroner, Sidoroff [3]-[6], Mandel [7], Kratochvil [81, [9], llalphen [loj. 

W e shall consider that a continuous body B is made up of an elasto-plastic rate-independent material at any particle X (X is fixed and it will be not mentioned in what follows) if the following consitutive assumptions are met ... 

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