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Throughput
Throughput
If the throughput or the dosing time (optional) is specified together with the dosing volume (necessary) in the process parameters, the throughput is not calculated, but all further calculations are carried out with the specified throughput.
The following content describes the melt-dominated throughput calculation. If a grooved bushing is used in the process, this determines the throughput (see Regular calculation and Grooved bush).
The calculation of the Solids conveying is coupled with the melt-dominated throughput calculation.
As a standard feature, a linear temperature profile beginning at the front edge of the hopper $T_G$ / $T_K$ and progressing to the point of the isothermal throughput calculation with a specified melt temperature is assumed (see also necessary process parameters).
Next, the reference zone is determined; this is the screw zone (no shear or mixing section) with the lowest channel depth. The reference zone is used to determine the average shear rate there. The mean shear rate for this section is obtained from the circumferential velocity, the screw back speed and the channel depth. This shear rate is used to establish the current rheological material data. The following approach is adopted here:
If the flow curve is available in the form of the Carreau data, a tangent is drawn to the Carreau curve at the appropriate shear rate and temperature. The power law parameters are then established for this tangent. After these pre-calculations the actual throughput respectively metering time calculations are conducted.
only vented screws **</summary>
The degassing screws are an exception here, where the location of the specified melt temperature is already set at the start of the degassing zone. The temperature curve then remains constant.
In degassing machines, the throughput is calculated solely on the basis of the geometry of the first screw stage, whereby the back pressure at the degassing point is set to 1 bar in accordance with the physical conditions. The back pressure at the screw tip therefore has **no influence on the mass throughput. For degassing screws, the reference zone must be located BEFORE the first degassing zone.
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In the first step, the total pressure difference (specified back pressure minus hopper pressure) is assumed for all screw zones (excluding shearing and mixing parts) and an initial mass flow rate is therefore determined. Based on this mass flow rate, the pressure loss across all shearing and mixing parts as well as the non-return valve (PSI only) is calculated and subtracted from the pressure difference so that a simplified linear pressure gradient is obtained across all feed, (de)compression, metering and barrier zones. This pressure gradient is used to calculate the throughput in an iterative process, which in turn influences the pressure loss across the shearing and mixing parts present. A throughput-dependent back pressure (see Input of a tool or Input of a die characteristic can also be taken into account via the iterative calculation.
only PSI
At the beginning of the calculation of the metering time the screw is in the front screw
stage and will be retracted by 0.1* diameter. Thus, the length of the feed section is
cut by this amount and a new temperature profile is assigned to each screw stage.
The characteristic values of the throughput equation are determined for the screw
stage and the throughput is calculated afterwards. Using the cross sections of the
barrel the proportion between the screw back speed and the melt conveyed in the
screw vestibule enables the determination of the screw back speed. The required
metering time of 0.1 d for the shortening of the effective screw length can be
obtained by dividing the screw shift through the determined screw back speed.
In order to determine the entire metering time the single times for the metering path
divided into 0.1 D steps are added up. Besides the metering time the throughput for
the calculation point is defined, which is the basis for further calculations for this
screw stage.
The prerequisite for the throughput calculation is:
- A defined screw geometry that begins with a feed zone, i.e. a zone with a constant flight depth. The length of the feed zone must be at least equal to the metering stroke (PSI only).
- A defined cylinder whose length is identical to the length of the screw, or at most twice the length of the screw.
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- The glass transition temperature $T_g$ for amorphous thermoplastics or the crystallisation temperature $T_k$ for semi-crystalline thermoplastics.
- The density or volume function for the melting range must also be entered.
- The process parameters: (i) speed, (ii) mass temperature at the screw tip (REX) at the start and end of dosing (PSI), (iii) pressure at the screw tip (REX) or in the screw vestibule (PSI), (iv) dosing path (PSI). If only one of the two melt temperatures (PSI) is known, both entries can be assigned the same value.
Further topics
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only PSI
At the beginning of the calculation of the metering time the screw is in the front screw stage and will be retracted by 0.1* diameter. Thus, the length of the feed section is cut by this amount and a new temperature profile is assigned to each screw stage. The characteristic values of the throughput equation are determined for the screw stage and the throughput is calculated afterwards. Using the cross sections of the barrel the proportion between the screw back speed and the melt conveyed in the screw vestibule enables the determination of the screw back speed. The required metering time of 0.1 d for the shortening of the effective screw length can be obtained by dividing the screw shift through the determined screw back speed. In order to determine the entire metering time the single times for the metering path divided into 0.1 D steps are added up. Besides the metering time the throughput for the calculation point is defined, which is the basis for further calculations for this screw stage.