FORMING AND SEALING OF PRODUCTS FROM MIXTURE
Freshly prepared mixture (mass) has a certain workability, which is expressed in its real ability to perceive technological operations for molding and sealing products.
Mixtures with very low viscosity (called cast) practically do not require compaction when forming products or coatings, which is a significant technological convenience. To reproduce the injection molding technology, often suitable plasticizers or even superplasticizers are introduced into the mixture. Introduced even in small quantities, they contribute to a sharp decrease in the viscosity of the mixture, making it easier to mold products even when their outlines are more complex. The same goals are achieved by an additional increase in the amount of liquid medium in the mixture (mass), which should be every time
Fig. 2.7. The rheological curve, or the flow curve in the coordinate system, is the shear stress-gradient ( duldx)of the strain (P)
is justified from general positions of optimization of structure and requirements to specific types of optimal structures.
When mixtures with higher viscosity are used that have a conditional dynamic yield point and shear stress on the rheological curve, it is important not to allow stresses capable of destroying the continuity of the product when forming. For example, in some cases, the formation of structure defects (blisters) is noted if the stress in the mass is allowed to exceed A r (see Figure 2.7). Experience shows that for well-formed mixtures, the ratio of P to , 1d) should be at least 2-10 s. The specific and precise limits of rheological characteristics depend on the variety of the mixture and the technological method of molding - plastic, vibrating without a load or with a load, etc.
Molding of products is, as a rule, associated with a dense packing of the mixture, granular or other kind of aggregates. The highest density is sought at the stage of preparation of raw materials - powders, suspensions, coarse-grained mixtures and other molding systems, especially in the production of firing ICS. The preliminary compaction of the mixture reduces the disconnection of the particles, transferring the bonds from the point to the interphase bonds along the contact boundaries. At subsequent stages of the technology (for example, during firing), the consumption of thermal energy is reduced due to a decrease in temperature and a decrease in the duration of exposure.
Depending on the type of mixture (mass), the molding is performed using stackers, presses (eg tape), extruders, calenders and other machines. The choice of the optimal method of molding and compaction depends on the nature of the raw material and the mass production, the required properties and the type of products. But in all ways, it is important to ensure the cohesiveness and initial strength of the products, and then harden them at other stages of processing. Initial connectivity arises under the influence of molecular (van der Waals) forces. They are of an electrical nature and are capable of developing the attraction of particles as they approach each other. The strength of the interaction of two particles (conditionally taking them to be spherical) is calculated by the formula
where D and r 2 are respectively the radii of two neighboring particles; c is the surface energy at the interface.
As the particles approach very small distances, repulsive forces arise and increase. Ultimately, the resultant force acts, which at a certain optimal distance of the particles from each other ensures the initial connectivity of the raw product.
The consolidation of molded or molded articles is an important stage in the formation of a macrostructure, since granular and other components of the conglomerate filling part are relatively stable in the binder medium during this period. Fixation can occur as a direct abutment of components, including possible fusion (for example, crystals), and through interlayers of fully cured or gradually hardening binder. Contact through the interlayers at the compaction stage is more typical in conglomerate materials than direct contact or fusion of particles under the influence of surface energy, chemical bonds or other, including complex, factors.
Owing to the approach of the particles of the mixture (mass), the molecular force field is redistributed and aligned, heat and mass transfer, in particular, migration of the medium to lower stress zones. The volume of the mixture (mass) decreases both in compaction and after it, and the polydisperse system gradually transforms into a state of relatively stable equilibrium with a given condition for molding the articles. Depending on the workability of the finished mixture (mass), some specific features of molding the macrostructure of the ICS are possible. Thus, for highly plastic and mobile mixtures (masses), the macrostructure is established very quickly and practically without the application of sealing forces, but under the influence of gravity or fluidity (in the case of liquid or casting methods of production). When sealing low-mobility and rigid mixtures containing, as a rule, a reduced amount of binder or a reduced amount of liquid medium in it, much more work is expended than when compacting plastic, mobile or cast mixtures (masses). Different techniques have forcedly bring together polydisperse grains, displacing some of the astringent substance in the intergranular pores and voids or in the pores and grooves of aggregate grains. Most of the aggregate in the volume of the monolith contacts through thin or thin layers of binder. With insufficient amount of binder, the interlayers become discrete, which increases the porosity and air content (or other gas phase) in the ICS.
In firing conglomerates, the methods of semi-dry hydrostatic pressing, vibroforming, and also hot pressing are common.
To achieve the required density, various methods are used to reduce the rheological resistance of the moldable mixture: the introduction of surfactants, plasticizers, and
superplasticizers; preheating; vibration influence; evacuation, etc. With particularly intensive compaction, it is advisable to increase the rheological resistance to a maximum. With the optimal technology of each consistency of the mixture (mass), certain parameters of the mechanical seal correspond. Each method and each intensity of the mechanical seal also has its own specific consistency, and then the placement of the solid particles as a result of compacting the mixture becomes compact.
In many technologies, the molding and compacting of the mixture are combined in one operation, as a result of which the chemical and physico-chemical processes that provide structure formation at the micro- and macrolevels also occur simultaneously. These include thixotropic liquefaction and hardening, mass and heat transfer, displacement of the filling and binding parts with respect to each other to form a dense structure at the end of such a combined operation. Naturally, during this period, the main structure-forming processes-sorption, dissolutions, and others-which, like it was at the stage of mixing the mixture, end with the emergence of new compounds and phases, although in relatively limited quantities. Much in large sizes, they are allocated at subsequent stages of the technology, for example, in the heat treatment of molded and compacted products.
Some technologies use an intermittent, stepped seal, for example, with a time interval between two vibrations or pressings. The repeated compaction promotes, as it were, secondary - plastic deformation of the conglomerate with the release of the medium from its micro- and macropores, and ultimately - additional compaction in conditions when the amount of binder remains within the permissible deviations from the recommended one. The repeated compaction, especially in the case of vibration molding processes, promotes the relaxation of stresses that occur during structure formation, reduces the size and concentration of structural defects.
From molding and compaction, to a certain extent, only the character of the structure formation depends on maintaining the same compactness of the particle packing, but also the texture features of the product. For example, reorientation of particles is possible, as a result of which a wide cross section of particles and pores is often located in planes parallel to the plane of pressing, with the appearance of anisotropy. It is also possible to partially re-grind the grains of an elongated shape or change (decrease) in volume.
With semi-dry pressing, the volume of the resulting product can be 1.5-2 times less than the volume of the freely poured mixture (mass). Naturally, then the porosity is reduced. Thus, from the formula AS. A careful P = a - in lgp (where P is the total porosity,%; a, in are constant coefficients, and the constant and as the porosity of the initial mass before pressing is 50%, and in reflects the ability of the mass to compact, p is the pressing pressure, MPa) > p * 100 MPa, in many cases in = 15 and II 20%, i.e., the porosity decreased by 2.5 times (50:20). The pressure distribution along the vertical decreases from the stamp, which leads to heterogeneity in the porosity of the molded raw product (raw material). The inhomogeneity does not depend only on the height of the product, but also on the hydraulic radius R ~ 2F/H, where F is the area; And - the perimeter of the product. The unevenness of porosity is also fixed in horizontal sections: the highest density is formed in the upper horizontal sections of the raw material near the walls of the mold, decreasing to the center. In the lower sections, the opposite phenomenon is observed, and in the average cross sections, the zone of equiporosity. The type of press is also important, but the density of the raw material always depends on the pressure, the technological properties of the mass and the time of action of the press pressure; sometimes it is also important to determine the rate of increase of the maximum pressure during compaction.
In addition to conventional presses (mechanical, hydraulic), some technologies use compression molding with the formation of ultrahigh pressures of instantaneous action with a change in the crystal-chemical structure of the substance. In the method of plastic molding, the compaction is carried out in belt presses (most often vacuum ones) with subsequent prepressing.
Articles made of plastics have an increased porosity compared to semi-dry products. The properties of such a mass are estimated by the methods of rheology. As already noted, the main parameters of plasticity are: 1) the viscosity of the largest unassembled structure (t | o), the structural viscosity (t |), the viscosity is the smallest with a completely destroyed structure (ct). Therefore, t | o & gt; l & gt; Fri! 2) yield strength: conditional static p m , conditional dynamic p n , yield strength p to (see Figure 2.7). There are other characteristics of the rheological properties of the mass in its plastic state-the instantaneous elastic modulus, the elastic modulus of elasticity, the relaxation period, etc. A set of such characteristics makes it possible to determine the amount of permissible stresses during the molding and compaction process. For example, it is necessary that the shear stress in the mass does not exceed p k at which there is still no complete destruction of the structure, since this will lead to a rupture of the mass tape in the belt press, which in the case plastic clay gives a defect in the structure, depriving it of optimality by this criterion.
The most characteristic in technologies with vibrational shaping is the imparting of velocities and accelerations to the particles of mass and, as a result, the weakening of the forces of internal friction and molecular bonds, as well as the thixotropic destruction of the primary structures (Figure 2.8). The particles move relative to each other with a dense packing. Surface, hinged, deep vibrators, vibrating tables, vibration tampers, etc. are used. The intensity of vibration is expressed by means of vibration cm/s:
Fig. 2.8. Dependence of the coefficient of internal friction of the vibrational mass on the average velocity of the particles, cm/s: v-A2f, where A is the amplitude of the oscillations;/- oscillation frequency, Hz
where A is the amplitude of the oscillations (half the largest span); ω is the angular velocity, rad/s,/is the oscillation frequency, Hz.
The product of the quantities A and ω gives the mean velocity of motion of the particles during vibration. There are optimal values of the amplitude, vibration acceleration, which depends on the depth of the mass layer. Vibration with an acceleration exceeding the optimum, is accompanied by loosening and differentiation of particles in size. The loosening is removed by immersion in vibration, for example, up to 8-10 MPa. The optimal vibration time is determined by experiment.
In the case of vibration pressing, a predetermined porosity of the products is achieved at a significantly lower energy cost than in static pressing. In addition, the quality of products increases, there is no anisotropy of properties, a more even structure is formed. The effectiveness of vibrocompression in a number of technologies is enhanced by alignment with evacuation.
In the technology of firing ICS, so-called slip casting is widely used to produce thin-walled products or products of large size and complex shape. Slicker is an aqueous suspension of clays, kaolins, other refractory substances with particles about 1CI cm in size, bearing a certain ionic potential PI = Z/r, where Z is the charge of the cation; r is its ionic radius. At values PI = 65-100 (suspension from acidic materials), higher densities of castings are obtained, i.e. with relative
with a density of 0.8-0.9. The method of slip casting is more dependent on the nature of the raw materials than other methods of compaction.
When forming and compacting, guncrete is often used with the transfer of the mixture to the surface using compressed air. Such a method makes it possible to obtain a very dense layer of a moldable substance. However, it should be borne in mind that an unavoidable loss of the used mixture occurs in connection with particle rebound.
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