Curing liquid, flow

Haberstroh et al. (2002) modelled the injection moulding of liquid silicone rubber via the use of curing kinetics, flow models and the pressure-volume-temperature behaviour. 

A step change of viscosity at critical conditions to infinity (3) is a convenient model for investigating the flow of a curing liquid. The simplest model employs the most important physical property of the process. In a number of papers [83-87], the dependence of the induction period t, the time during which a reactive substance retains the ability to flow, on the rate of shear y is studied. 

Therefore, it is necessary (1) to calculate real temperature fields in the production equipment during flow of curing mixtures with an allowance for specific rheological properties and real hydrodynamic situation (2) the tempCTature fields in the technological process are to be strictly controlled. These results stress the role and importance of meclmnical sources of heat and the necessity to take them into account in all production proceses where high-viscosity polymers or polymerizing liquids flow. 

Consideration of a two-dimensional flow pattern was a fundamental step in the analysis of the flow of rheokinetic curing liquids, as well as in the case of a sharp but limited viscosity growth. The loss of the possibility to flow leads to a stronger distortion of all characteristics over the section of Ae channel as compared to a usual pattern and to new qualitative regularities. This was demonstrated by Vaganov [91], where the ana-... 

The problem on the flow of a curing liquid in a flat slit was solved more strictiy in two-dimensional problem [KW]. The concept of the gel time, t, reached for a certain critical conversion — was used (an analogue of the induction period t [83-87]). Other authors solved the most different versions of flows and carried out specific calculations of the hydrodynamics of the process for unit production equipnrent and various moulds. From the point of view of the present paper, of some interest is the theoretical analytis of the flow in a rectangular channel [101], where the viscosity variation during the process was described by the following formula ... 

Fig. 17a-c. Profiles of flow velocities of a curing liquid in the presence of a wall slip (a, b, c are the different conditions of flow, dashed line is the Hquid-sotid boundary)... 

After reading this chapter, some readers may get the feeling that designing a slurry flow is a combination of science and art. Slurry dynamics may appear to be an exercise of examining each mixture for its properties, much as the physician must examine each patient before administering the cure. The flow of coarse particles in water or mixtures of coarse and fine solids in a liquid is complex. When data is not well accumulated, it is recommended to conduct slurry tests in a pump test loop. 

Polymers can be evaporated, deposited as a thin film, and cured in a vacuum system to provide a basecoat. For example, acrylate coatings can be deposited and cured with an e-beam. The deposited liquid flows over the surface and covers surface flaws, reducing pinhole formation. This technique can be used in vacuum web coating and has been found to improve the barrier properties of transparent barrier coatings. 

For commercial use a partially condensed furan resin is normally prepared which is in the form of a dark free-flowing liquid. Final cure is carried out in situ. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

The rate of bleeding is dependent on several factors, including the permeability of the fiber bed, both vertically and horizontally, and the viscosity of the liquid resin. The permeability of the fiber bed will depend on the weave of the fabric, the fiber diameter, and the fiber volume fraction. The resin viscosity is determined by the chemistry of the resin and the thermal profile of the cure cycle. The cure cycle greatly affects resin viscosity and the flow process, both directly through the pressure application and indirectly through the effect of the thermal profile on resin viscosity. 

The cure of thermoset resins involves the transformation of a liquid resin, first with an increase in viscosity to a gel state (rubber consistency), and finally to a hard solid. In chemical terms, the liquid is a mixture of molecules that reacts and successively forms a solid network polymer. In practice the resin is catalyzed and mixed before it is injected into the mold thus, the curing process will be initialized at this point. The resin cure must therefore proceed in such a way that the curing reaction is slow or inhibited in a time period that is dictated by the mold fill time plus a safety factor otherwise, the increase in viscosity will reduce the resin flow rate and prevent a successful mold fill. On completion of the mold filling the rate of cure should ideally accelerate and reach a complete cure in a short time period. There are limitations, however, on how fast the curing can proceed set by the resin itself, and by heat transfer rates to and from the composite part. 

Torque rheometers are multipurpose instruments well suited for formulating multicomponent polymer systems, studying flow behavior, thermal sensitivity, shear sensitivity, batch compounding, and so on. The instrument is applicable to thermoplastics, rubber (compounding, cure, scorch tests), thermoset materials, and liquid materials. 

The construction of a mold-filling model has been considered in the theory of thermoplastics processing. A rapid increase in viscosity also occurs in the flow of these materials, but the effect is different than in flow during reactive processing. The increase in viscosity of thermoplastic polymer materials is due to physical phenomena (crystallization or vitrification), while the increase in viscosity of reactive liquids occurs due to chemical polymerization reactions and/or curing. This comparison shows that the mathematical formulation of the problem is different in the two cases, although some of the velocity distributions may have similar features. 

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