Several equations have been compared for the modelling of semi-cristalline polymer crystallization on cooling from the melt and on heating from the glass. The temperature dependence was described with the Hoffman–Lauritzen (HL) equation, where U* and Kg were first determined by fitting the activation energy (Eα) dependency with temperature computed with an advanced isoconversional method (AIC) from DSC data. Several models were used to describe the extent of crystallization dependence, such as Sestak-Berggren and Avrami models. Simulations using Ozawa and AIC methods were performed for comparisons. It was shown that the model-free approach is able to take into account additional crystallization phenomena that are not considered in the classical HL or Avrami equations. Then, a new equation has been proposed to simulate additional phenomena occurring at the end of crystallization on heating from the glass, which are not described by the Hoffman–Lauritzen theory. The experimental finding that application of an isoconversional method to crystallization data following a HL mechanism, should lead to positive decreasing values for crystallization from the glass and to negative increasing values for crystallization from the melt was confirmed by simulations.