The same conditions must be met for the calculation of the pressure profile as for the throughput or dosing time calculation. This applies to all input data (geometry, material data and process parameters). In addition, the calculation position for which the pressure profile is to be calculated over the screw length must be specified.

When calculating all curves in the REX/PSI (pressure, melting, temperature, power and wall shear stress curve), the screw is divided into at least 150 irregular intervals and the corresponding variable is then calculated for each of these intervals. The following figure schematically illustrates the allocation of interpolation points.

Platzhalter Abbildung 9.12: Verteilung der Stützstellen

The following locations are initially assigned a support point:

• Start and end of a screw zone
• Start and end of a heating zone
• Location of the melt vortex formation (OSW), if specified
• Location of the isothermal throughput calculation, if specified

The remaining interpolation points are distributed in such a way that the largest interval is divided in each case until 150 interpolation points have been allocated.

The pressure curve is calculated back to the location of the melt vortex formation. If it was not possible to determine this value due to missing data, REX/PSI calculates up to the end of the feed zone. The progression from the front edge of the funnel to the last calculated point is represented by a logarithmic approach.

The pressure curve calculation starts at the screw tip. The calculated flow rate is used to determine the pressure loss between two calculation points and added to the previous absolute pressure. The pressure in the screw vestibule (at the screw tip) results from the set dynamic pressure.

The pressure curve is calculated isothermally at the melt temperature in each step and then multiplied by a correction factor. From a physical point of view, this correction factor ensures that the calculation is performed at a temperature that allows the pressure and flow rate behaviour to match. This is necessary because the flow rate must be calculated according to a mixed isothermal/non-isothermal approach, but the pressure curve must be calculated either isothermally or non-isothermally.

If the location of the isothermal throughput formation was specified by the process parameters, the calculation is performed from the screw tip to this location isothermally at the melt temperature.

In order to calculate a barrier as described above, the assumption of an equal pressure gradient must be made (see chapter Barrier rezone). However, as each additional assumption increases the inaccuracy of the simulation, the so-called nodal point method was implemented to eliminate this assumption, whose calculation process generates a grid in the flow area similar to the FEM calculation method.

Individual intervals are formed for the barrier web, solids channel and melt channel and for each section between the individual balance points, the change in flow is described by the linear pressure-throughput equation. These equations form a linear system of equations. The equations only describe the flow development along the channel direction, so that the established network corresponds to the subdivision of the geometry into the required intervals of constant conditions (see following figure).

In order to be able to solve the resulting system of equations with the known solution algorithms, it must be reduced by taking boundary conditions into account (see following figure).

For the closed barrier zone, the mass flow pointing towards the first node or away from the last node in the channel direction can be set to zero. If, on the other hand, an open barrier zone is calculated, it is sufficient to change the boundary condition in order to be able to calculate this design. In this case, the pressure difference between the first and last nodes of the solid and melt channels is set to zero due to the lack of geometric separation.

The mass flow must be known for a clear solution of the system. This is therefore calculated in a first step in which the screw geometry is simulated without taking the barrier zone into account. The result is then used to determine the pressure curve in the barrier zone and used in the next iteration step when calculating the entire screw geometry. the calculation of the entire screw geometry.

In addition to the analytical method, the numerically corrected method can also be selected, which adjusts the calculated values using a regression model in order to achieve a better approximation to the test results.