Cure in Heated Molds

The thermal energy balance for this case may be written as 

The rate at which conversion increases at any point is given by 

Equation (9.2) simply states that the rate of heat accumulation in a differential volume (first term) is the difference between the heat flow that enters and leaves the volume element by thermal conduction (second term) plus the rate of heat generation by the polymerization reaction (third term). 

To illustrate the system s behavior it will be assumed that p, Cp, ky, and (—AH) are constant, and that the polymerization rate may be described by a simple second-order equation  

To complete the mathematical description it is necessary to state initial and boundary conditions. For illustration purposes, it will be assumed that the initial temperature (Tq) is uniform in the mold, the wall temperature 

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The use of empirical rules is very often misleading for example, it is frequently stated that the cycle time is directly proportional to the specimen thickness or that the maximum temperature in the part is limited by the adiabatic temperature rise. Both statements have no general validity, as discussed in the analysis of the cure in heated molds. 

The injection molding regimes are defined proceeding from the material composition and correlation of ingredients. A peculiarity of liquid-filled compositions is slow curing in the mold and gate channel due to their high heat conductivity. To prevent the formation of blisters or pits on the surface of products the time of their endurance in the mold under pressure should be extended. 

The process of cure results from the heating stage in the reservoir followed by the stage of cure in the mold parts of these two stages are simultaneous. 

The Equations 5.5 (or 5.2) to 5.7 describe the process and allow the calculation of the final value of the state of cure obtained in the mold when the sample is spherical in shape. Calculation is made by using the above three equations, when the process of heating and cure starts from the temperature of injection to the mold temperature for the rubber sample. The total time necessary for the process of molding injection is thus the time t, in addition to the time necessary for the rubber to be cured in the mold, called t . However, it should be said already that the stage of heating in the reservoir for the nth material is done simultaneously during the stage in the mold of the (n-l)th rubber previously injected... 

Figure 14.2 The compression molding process as described by a simplified, four-step procedure. (A) Loading of charge into mold. (B) Compaction of charge in heated mold. (C) Curing of part inside mold cavity. (D) Removal of solidified part.

In other areas, POD has been used to improve the wear resistance of a mbber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this appHcation a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

Films from prepolymer solutions can be cured by heating at 150°C. Heating the prepolymer in molds gives clear, insoluble moldings (38). The bulk polymerisation of DAP at 80°C has been studied (35). In conversions to ca 25% soluble prepolymer, rates were nearly linear with time and concentrations of bensoyl peroxide. A higher initiator concentration is required than in typical vinyl-type polymerisations. 

There are many ways to measure these properties and some of them are proprietary. However, most laboratory tests are standardized by American Standard Testing Methods (ASTM). Many of them are interactive to various degrees. The rate and state of vulcanization is especially important to consider for components of heavier and thicker tines. The heat used to vulcanize the tine in a mold under pressure requites time to penetrate from both sides of the giant tine to the innermost portions. Securing a balanced state of cure, ie, the maximizing of physical properties in all the components, results in the innermost components having a faster rate of cure. The peripheral compounds should have a cure system which holds its physical properties well when overcured. 

These three cure systems have in common the need for a two-step cure cycle to generate the best cured properties. The first step is the appHcation of heat and pressure in a mold to shape the article (press cure). The second step is a high temperature oven cycle at atmospheric pressure to obtain the final cured properties. 

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