The additive feeding systems in the plastics industry have at least two separate control programs, one for injection molding – for dosing precise amounts in part of the production cycle, and another for extrusion – for continuous feeding. In this post we’ll discuss a third program for extrusion blow molding that’s present in a number of leading feeders in the field, among them the ColorSave 1000.
Extrusion blow molding (EBM) can be viewed as a hybrid of injection and extrusion. On the one hand, it produces individual products, one each cycle, using a mold, like injection. On the other hand, the feed is continuous, like in regular extrusion. Accordingly, the EBM program is also a hybrid between injection and extrusion. It feeds the additive continuously and yet allows the treatment of production cycles individually. We’d like to show that it’s preferable to use an EBM feeder for an EBM system.
Since the feed is continuous, it’s possible in principle to use a feeder with an extrusion program (continuous feed) with an EBM system. This feeder would do the job well, however for the machine operator a feeder with a specific EBM program is easier to operate and generally preferable.
In a feeder with an extrusion program, the operator can enter a constant additive throughput in kg/hour, or if the feeder is synchronized with an extruder the operator can enter the extruder’s maximum throughput in kg/hour and the desired percentage of the additive. In each of these options, the operator must enter data in terms of throughput in kg/hour. These terms are the “language” spoken in classic extrusion factories, such as those that produce pipes, profiles, films, etc., however in a factory with extrusion blow molding machines, the machine operators’ terms are not throughput in kg/hour, but rather product weight.
The product weight is a given that the operator knows or can easily find out by weighing the final product and adding the weight of the material from that cycle that was not used and is to be recycled. As soon as the product weight value is known, it can be entered along with the desired additive percentage and that’s it, you can start working.
When working with a feeder with an EBM program that’s synchronized with the production machine, one can always enter the desired percentage for the production process. As a rule, working with synchronization with the production machine, which allows the operator to enter the desired additive percentage, is always preferable to working without synchronization, in which case constant throughput must be entered.
So how can the feeder work with a percentage of the system’s throughput? How does the feeder know what the extruder’s throughput is at any given moment? In order to know the extruder’s momentary throughput, you need a synchronization value and a reference point, and then the extruder’s momentary throughput can be calculated with simple math. The mathematical calculation is usually multiplication or division of two values – the synchronization value, which is variable, and the reference point, which is constant.
In an EBM production line, a feeder with an EBM program has an advantage regarding synchronization with the production machine. A feeder with an extrusion program can be synchronized with the production machine by means of an analog sync signal, usually 0-10V or 4-20mA, which is synchronized in direct proportion with the speed of the extruder’s motor. Sometimes it’s not possible to obtain an analog signal, mostly in old extruders. With these extruders, it’s still possible to send the feeder a sync signal by way of a digital tachometer operated by an encoder installed on the extruder’s motor, but since this entails a somewhat complicated installation and the expense of additional equipment, in most cases of an extruder without an analog output, synchronization is abandoned and the work is carried out with a constant throughput.
In a feeder with a special program for EBM the synchronization of the production machine and the feeder is made simpler by a digital signal, usually dry contact, that is received from the mold once per cycle. The time lapse between signals is the cycle time. When the weight of the product being made doesn’t change, as the cycle time increases the extruder throughput decreases accordingly and vice-versa. That is to say, the correlation between the cycle time and the throughput is an inverse ratio.
Through the synchronization of the feeder and the mold one can achieve synchronization with the system’s throughput faster and more simply than with synchronization of the extruder and a feeder with an extrusion program.
Illustration: throughput as dependent on an analog signal vs. throughput as dependent on cycle time
In an extrusion program the reference point is the maximum throughput of the extruder in kg/hour. This is an apparently measurable value, but in practice it’s not simple at all to measure. Even if the maximum actual throughput for a specific material has been measured, the value could be different for different materials and even the same material from different batches might yield a different maximum throughput of the extruder.
In the EBM program the reference point is the product weight, which is a value the operator knows or can easily find out by weighing the final product and adding the weight of the material from that cycle that wasn’t used and is to be recycled. The operator enters the product weight and from that moment, together with the cycle time, the extruder’s throughput can be calculated and synchronization can be carried out. The synchronization will remain stable for every batch of raw material.
We’ve seen that operationally, extrusion blow molding is a sort of hybrid between injection and extrusion. Additive feeders usually have a program for injection and another for extrusion. We’ve presented a dosing system with a program specific to EBM that is preferable from the machine operator’s point of view to a system with an extrusion program because it speaks his language, using terms of product weight as opposed to terms of kg/hour that are foreign to him.
In all matters of synchronization, which is important to the system operator, we’ve seen that a feeder with an EBM program is always in sync with the production machine and that carrying out synchronization is easy, as is the calculation of the reference point for synchronization.
Now it’s easy to understand why EBM operators prefer additive feeders with programs specific to EBM.
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