Cured rubber, heating stage

Another problem should be considered in detail with the process of curing the rubbers in the mold this occurs when the cure reaction takes place, and is associated with the heating stage. 

In the vulcanisation of thick rubber articles the temperature may be stepped up in stages to ensure adequate heat penetration to the interior before the external surface begins to cure. Such a cure schedule is termed a step cure. Step post cures are used with thick section rubber articles to ensure that volatiles are released gradually without causing damage, such as porosity, due to an over rapid release. 

As the viscosity of the rubber compound increases with the stage of cure, it is important to know exactly the temperature at which the cure takes place, as heat increases viscosity, so as to inject into the mold the rubber with an appropriately low viscosity. 

FIGURE 5.1 Uniform value of the state of cure of the rubber compound as a function of time during the heating and storing stage in the reservoir, for the injection temperatures 80, 100, 110,120,130, 140°C. 

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... 

Resitol n. A phenol-formaldehyde resin in the transition state of the curing process. Under heating it softens to rubber-like consistency but without melting. It swells when it is immersed in alcohol or acetone, but does not dissolve. Alternate term for B-stage. 

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