Mold filling chemical reaction

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. 

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. 

In a more sophisticated analysis these functions can be found as the solutions of the dynamic and energy balance equations for filling a mold. 0m is the dimensionless temperature of the mold To is the initial temperature of the reactive mixture co = (H2kr,T)/a is the dimensionless factor characterizing the ratio of time scales for heat transfer and the chemical reaction. Other dimensionless variables are as follows ... 

The Reynolds number in mixing operations must be > 200 The ratio of the mold filling and gelation time must not exceed 0.5 No volatile products must appear shrinkage must be compensated chemical reactions and phase transitions (crystallization) must proceed quickly, regardless of the size of the article... 

For known values of the parameters in the kinetic equation for a specific reactive mix, it is easy to calculate the dimensionless factors y and v. Then the flow pattern in the mold filling process is completely determined by the dimensionless Da and Gz Numbers and the boundary conditions. The Damkohler Number characterizes the ratio of the rates of chemical reaction and convective heat transfer and the Graetz Number is a measure of the ratio of the convective heat flux due to a moving liquid to the heat flux due to the conductivity of the liquid. 

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

The polyurethane elastomer itself is a reaction polymer, created by the mixing of prepolymers and chain extenders or hardeners. This reactive mixture, which is created in the desired mixing ratio by special casting machines, is cast into a mold built around the roll shell with the base layer The casting procedure can be carried out horizontally or vertically. Bonding to the base layer is done chemically with adhesives or reactive layers. The polyurethane material itself is not normally filled, thus providing outstanding elastomeric material properties. 

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