REX calculates the temperature in the melt vortex and in the melt film for each interval after the location of the melt vortex formation (OSW). As melting progresses, the constantly growing melt film becomes thicker and at the same time is scraped off by the screw flights. This not only makes the melt vortex warmer due to barrel tempering and shear dissipation, but also takes into account the mixing with the generally cooler melt film. As a result, the temperature increase per unit length of the screw during melting can be lower than after melting.

The temperature calculation is based on the gutter model. The following conditions are assumed for the temperature curve calculation:

* The screw channel is considered a flat channel, i.e. $b \gg h$. The influence of the webs can therefore be neglected

• The melt adheres to the wall
• The temperature of the melt at the cylinder corresponds to the cylinder temperature
• The flow is laminar creeping and incompressible
• The flow behaviour of the melt should follow the power law $\tau = K * \dot \gamma^n$
• All material values with the exception of viscosity are considered (interval-wise) to be temperature-independent. Provided the temperature range is not too large, this assumption is permissible for plastic melts to a reasonable approximation. This applies in particular to thermal conductivity and thermal diffusivity

The resulting differential equation can now be applied interval by interval and the temperature of the plastic melt in the melt vortex can be calculated over the channel height. The melt film is taken into account using the first law of thermodynamics.

In addition, a proportional downtime is taken into account for each interval in PSI. The procedure is very similar to the consideration of downtimes during melting.

For the calculated downtime weighted per interval, heating is calculated purely by heat conduction through the cylinder temperature control. The more downtime the plastic experiences between entering the injection moulding machine and injection, the closer the temperature curve comes to the heating zone profile.