61 - What needs to be considered when designing dosing systems?
Throughput is not equal throughput – this is especially true for throughput details of injection moulding machines. Even experts can get their wires crossed designing or choosing a suitable synchronous dosing unit with free intake of the main component. This is because the maximum possible throughput of a dosing unit needs to be significantly higher than that of the injection moulding machine.
The background: Injection moulding machines operate cyclically, and plastification is one of the process steps. Synchronous dosing units with free intake of the main component operate only during the plastification time of the injection moulding machine. A signal from the machine indicates when plastification begins and the main component starts to flow. Synchronically, the (or all) additive(s) are then dosed in the set proportions.
In order to now determine the necessary dosing rate, the current plastification rate of the machine’s injection unit needs to be calculated first – this must not be confused with the machine throughput. The following example makes the difference between the two very clear:
- shot weight [g] 100
- cycle time [s] 20
- plastification time [s] 6
- ration additive [%] 5
The necessary values for calculation the current plastification rate of the injection unite are the
- shot weight [g] and the
- plastification time [s]
The additive ratio can be calculated using these values. Plastification rate [g/h] can be calculated as follows
From this 60 kg/h total dosing power, that the dosing unit must deliver, the minimum throughput rate of the additive station can then be calculated – in this example 5 % additive ratio 3 kg/h.
When calculating the machine throughput, the cycle time is also a factor:
This example makes clear, that a dosing unit needs to be chosen that can guarantee the necessary throughput rate. The most important value in this context is the shot weight, because the amount of mixed material available must be at least the same value as is needed for one shot.
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