After the activation of the Next button, the boundary conditions and the defaults for the transfer can be determined (see figure). The screw length to diameter ratio (L/D) and the boundary conditions are of particular importance for the transfer of the operating point.

Some options presented in the figure. are of experimental character. This especially concerns the exponents for lead and mass temperature. Normally, presetting should fit here. Nevertheless, a change of the mass temperature exponent and of the lead exponent is possible in SIGMA 7. Through this, the user is provided with the opportunity to assess the influence of individual geometry values on the process and to influence the simulation result by adjusting it. It has to be taken into consideration, however, that doing this is always connected with a violation of similarity theoretical assumptions.

As shown in the basics handbook, the channel depth exponent is calculated from the geometrical data of the chosen machines. The radial clearance can be considered or not in SIGMA 6. This will take direct effect on the mass flow rate and screw speed of the target machine.

SIGMA suggests keeping the L/D ratio constant for model and main machine. However the user is allowed to define an L/D ratio for the target machine. Thereby, the machine is automatically trimmed to the required length.

Furthermore it is possible to calculate a recommended L/D ratio by choosing ”Calculate”. In this case the conditions constant shear rate and constant residence time are fulfilled at the same time.

By defining an L/D ratio or using the presetting, there are five scale-up boundary conditions available in SIGMA. These are:

• Constant specific energy input
• Constant heat flow density
• Constant shear rate
• Constant barrel temperature
• Constant residence time

Among other things, the choice of the boundary condition has considerable influence on the screw speed as well as the throughput. Both are functions of the channel depth exponent, for this reason the evaluation must be done for the unique process.

With a rising throughput, however, the requirements on the heat transfer also increase. In this way, higher temperature inhomogeneity in the melt evolves and, possibly, a non-negligible increase of the melt temperature occurs.

Figure: Settings of exponents