We describe a mathematical model to evaluate the critical heat flux required to ignite thermoplastics in radiative ignition experiments. The concept of a critical mass flux from the solid phase into the gas phase is used as the criticality condition. Using this criterion, the solid temperature corresponding to the critical heat flux is related to the 'characteristic temperature' determined in Thermogravimetric experiments (TG). We investigate the dependence of the critical heat flux and critical surface temperature on the value taken for the critical mass flux. The specific application we have in mind is piloted ignition in the cone calorimeter.