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Throughput
Throughput Calculation
If the throughput or dosing time (optional) is specified together with the dosing volume (mandatory) in the process parameters, then the throughput is not calculated. Instead, all subsequent calculations are performed using the specified throughput.
The following section describes the melt-dominated throughput calculation. If a grooved bushing is used in the process, it determines the throughput (see Regular calculation and Grooved bush).
The Solids conveying is carried out in conjunction with the melt-dominated throughput calculation.
Two melt-dominated throughput models are distinguished: the equivalent zone model and the interval-based model, which will be introduced with REX 19.0 / PSI 17.0.
Initial temperature and pressure profiles are assumed prior to the throughput calculation. This is necessary to determine the viscosities required for throughput estimation. For more information, see Iterate Process.
For both models, degassing screws form a special case: the throughput is calculated solely based on the geometry of the first screw stage (before degassing), with the backpressure at the degassing location physically set to 1 bar. Consequently, the backpressure at the screw tip has no effect on the mass throughput. Additionally, for both models, pressure losses across shearing and mixing elements are calculated analytically. This also applies to all sections computed using network theory/matrix model (node method for barrier screws, always for wave screws). See also Pressure Calculation.
Melt-Dominated Equivalent Zone Model
First, a reference section is determined—this is the screw section (excluding shearing and mixing elements) with the smallest channel depth. For degassing screws, the reference section must be located before the first degassing section. This reference section is used to determine the average shear rate, which in turn is used to determine the viscosity using the given melt temperature at the end of the screw.
Based on the screw geometry and the temperature profile, factors for throughput calculation are determined. The throughput is then calculated using these geometry-dependent factors only for the reference section and the associated pressure drop. In the first step, the entire pressure difference (specified backpressure minus hopper pressure) is assumed to apply across the entire screw (excluding shearing and mixing elements), and an initial mass throughput is calculated. This value is then used to calculate pressure losses across all shear and mixing sections and, in the case of PSI, the non-return valve. These pressure losses are subtracted from the pressure difference to yield a simplified linear pressure gradient across the feeding, compression, metering, and barrier sections. With this pressure gradient, an iterative process is used to determine the throughput, which in turn affects the pressure losses across the shearing and mixing elements. Additionally, this iterative process can take into account a throughput-dependent backpressure (see Definition of a Die geometry (REX) or Input of a die characteristic (REX)).
Das schmelzedominierte intervallweise Modell
Das schmelzedominierte intervallweise Durchsatzmodell benötigt keine Bezugszone.
Im Gegensatz zum Äquivalenzzonenmodell werden keine geometrieabhängigen Faktoren bestimmt, welche das Durchsatzverhalten der gesamten Schnecke zusammenfassen, sondern es wird stets die lokale Geometrie der Berechnungsintervalle verwendet. Ebenso werden die lokal vorliegenden Temperaturen, Drücke sowie der eventuell berechnete Abbaugrad des Kunststoffes in der intervallweisen Viskositätsberechnung berücksichtigt. Dies wird durch eine allgemeingültige Regressionsgleichung erreicht, welche bei gegebenen Durchsatz den Druckverlust in abhängigkeit der Schneckengeometrie und der Viskosität berechnet. Da in über die gesamte Schnecke der Durchsatz lokal identisch ist, wird ein Durchsatz vorgegeben und der Druckverlust über jedes Berechnungsintervall bestimmt und die Gesamtdruckdifferenz über die gesamte Schnecke bestimmt. Die berechnete Druckdifferenz zwischen Trichter und Schneckenspitze kann mit der Solldifferenz verglichen werden und der Durchsatz iterativ bestimmt werden, bei welcher sowohl der Druck an der Trichtervorderkante sowie an der Schneckenspitze den Solldrücken entspricht.