Cooling shrinkage

Usually called injection-compression molding (ICM). Details are in Chapter 4. The essential difference when compared to IM lies in the manner in which the thermal contraction in the mold cavity that occurs during cooling (shrinkage) is compensated. With conventional injection... 

At the temperatures existing within mass concrete the hydration of MgO gets under way relatively quickly, and expansive stresses develop continuously within half a year (Wang and Lou, 1992). They thus keep pace with the cooling shrinkage that is taking place in the mass concrete. At an MgO addition of abont 5 wt% the ultimate linear expansion amounts to about 1 mm/m, and this has no harmful effect on the strength of the hardened concrete. 

Unlike MgO-based binders, expansive cements (or expansive additives) based on ettringite formation or Ca(OH)2 formation from CaO are of little benefit for controlling cooling shrinkage in mass concrete, because they would undergo complete hydration before the maximum temperamre in the hardening concrete mix was reached. Consequently, the compressive stresses produced by these expansive reactions relax too soon to prevent crack formation in the concrete body in the course of cooling. 

Wang, Y., and Lou, Z. (1992) Retarded expansion for cooling shrinkage compensation in mass concrete, inProceedings 9th ICCC, New Delhi, Vol. 5, pp. 129-135. 

The pressure of the extmder forces uniform plastic distribution throughout the mold. Cooling the mold solidifies the plastic with slight shrinkage. The mold is maintained closed by mechanical or hydraulic pressure while the thermoplastic is injected and solidified. 

CompoundShrinka.g e. In its simplest form (Fig. 8a) compound shrinkage consists of machining the inner radius of an outer component I, (Qp so that it is smaller than the outer radius of an inner component II, The difference between the two is known as the radial interference 5. To assemble the cylinders, outer component I is heated and/or inner component II cooled so that the outer component can be sHpped over the inner as shown in Figure 8b. When the temperature of the assembly returns to ambient, a compressive stress (pressure) is generated across the interface which simultaneously compresses the inner and expands the outer component and, in so doing, displaces radius (r/j by Uj and radius ( jj by U, Unfortunately, it is difficult to carry out this operation without setting up stresses in the axial direction (32). 

Crystalline polymers undergo a discontinuous decrease in volume when cooled through (Fig. 4). This can lead to nonuniform shrinkage and warping in molded objects. On the other hand, it also causes the polymer to "lock on" to reinforcing fibers, eg, glass (qv), so that crystalline thermoplastics benefit much more than amorphous thermoplastics from fiber reinforcement. 

Sohd ammonium nitrate occurs in five different crystalline forms (19) (Table 6) detectable by time—temperature cooling curves. Because all phase changes involve either shrinkage or expansion of the crystals, there can be a considerable effect on the physical condition of the sohd material. This is particularly tme of the 32.3°C transition point which is so close to normal storage temperature during hot weather. 

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